( PDF ) Fácies, petrografia e geoquímica da formação codó, neo-aptiano, bacia de São Luís

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UNIVERSIDADE FEDERAL DO PARÁ

CENTRO DE GEOCIÊNCIAS

CURSO DE PÓS-GRADUAÇÃO EM GEOLOGIA E GEOQUÍMICA

FÁCIES, PETROGRAFIA E GEOQUÍMICA DA

FORMAÇÃO CODÓ, NEO-APTIANO, BACIA DE SÃO

LUÍS-GRAJAÚ

TESE APRESENTADA POR

JACKSON DOUGLAS SILVA DA PAZ

Como requisito parcial à obtenção do Grau de Doutor em Ciências

na área de GEOLOGIA

DATA DA APROVAÇÃO

COMITÊ DE TESE:

_____________________________________

DILCE DE FÁTIMA ROSSETTI

(ORIENTADORA)

_____________________________________

MOACIR JOSÉ BUENANO MACAMBIRA

(CO-ORIENTADOR)

_____________________________________

ALCIDES NÓBREGA SIAL

_____________________________________

ANA MARIA GÓES

_____________________________________

VIRGÍNIO NEUMANN

_____________________________________

WERNER TRUCKENBRODT

Belém

2005

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AGRADECIMENTOS

À Dilce de Fátima Rossetti, para quem eu não tenho palavras suficientes para expressar toda a minha gratidão,

afeto e admiração;

Ao Prof. Moacir Macambira, pela amizade e pela co-orientação em todas as etapas dentro do Laboratório de

Geocronologia da UFPA;

Ao CNPq pela concessão da bolsa de estudo;

Ao Grupo Itapicuru Agro-Industrial S/A pela colaboração e pela liberdade que nos foi concedida de acessar as

minas exploradas por esta companhia nos arredores do Município de Codó;

Ao Grupo Mineradora Vale do Grajaú S/A pela atenção e pela liberdade concedidas para observar as minas

exploradas por esta empresa na região do Município de Grajaú;

À hospitalidade do povo tanto de Codó quanto de Grajaú, onde sempre fomos bem recebidos. Em especial, às

Sras. D. Gonçala e D. Edite, donas das pousadas que nos receberam muito cordialmente neste trabalho durante as etapas

de campo, e à D. Fátima, cuja comida foi um alento durante estas etapas;

Ao Museu Paraense Emílio Goeldi pela infra-estrutura concedida durante os vários anos deste trabalho;

À Universidade Federal do Pará, ao Centro de Geociências e ao Curso de Pós-Graduação em Geologia e

Geoquímica, pela infra-estrutura, pelo pessoal e pela organização que me auxiliaram muito neste trabalho;

À Coordenação do Curso de Pós-Graduação em Geologia e Geoquímica, nas pessoas do seu atual coordenador

Prof. Dr. Cândido Veloso, do antigo coordenador Prof. Dr. Paulo Gorayeb e das secretarias Gladys Pimentel e Cleidiane

Caldeira, pela atenção e competência que dispensaram a mim em todas as etapas deste trabalho;

À comissão que formou a banca examinadora desta tese: à Dra. Ana Maria Góes, amiga de todos, membro do

nosso grupo de pesquisa em geologia histórica e sedimentar (GSED) e por quem nossa admiração vai além da geologia;

ao Dr. Werner Truckenbrodt, pela paciência com que tentou melhorar cada aspecto dos mais variados textos relacionados

com este trabalho, durante os anos em que trabalhei ao seu lado; ao Dr. Alcides Nóbrega Sial, cujo apoio foi fundamental

na execução da parte deste trabalho relacionada aos isótopos estáveis; e ao Dr. Virgínio Neumann, que acreditou neste

trabalho de imediato, sabendo que seria difícil mas executável no prazo exigido;

Ao Grupo de Geologia Histórica e Sedimentar (GSED) do Museu Goeldi e Universidade Federal do Pará, em

especial aos estudantes do grupo que sempre estiveram um ao lado do outro a fim de ajudar da melhor forma possível:

Emídio, Carol Miranda, Daniele, Marivaldo, Anderson, Leandro, Hebérton, Cristovam, Leonardo, Denis, Sue Anne,

Samanta e Paulinha;

Ao Laboratório de Isótopos Estáveis do Núcleo de Estudos de Granitos da Universidade Federal de Pernambuco

(LABISE/NEG/UFPE), nas pessoas da Profa. Dra. Valderez Ferreira, que além disso foi extremamente atenciosa com este

trabalho quando da minha estadia no LABISE, auxiliando-me totalmente na parte de análise elementar com fluorescência

de raios-X; à Silvana Diene Barros, pela amizade, pelo amor e pelo carinho mútuos; Roberta Brasilino, pela amizade e

carinho recebidos na sua casa e de Xuxxu; à Gilsa, pela experiência compartilhada na separação química de C e O; e, de

novo, ao Prof. Sial, pela paciência e atenção com que me recebeu em seu laboratório.

Ao Laboratório de Geocronologia e Isótopos Estáveis da Universidade Federal do Pará (Pará-Iso/UFPA), nas

pessoas do Prof. Dr. Moacir Macambira, das Químicas Rose Brabo, Roberta e da estudante Gilmara, pela atenção

despendida com meu trabalho e pelo auxílio na execução das etapas mais difíceis;

Ao Laboratório de Geoquímica Orgânica da Universidade Estadual do Rio de Janeiro, nas pessoas do Prof. Dr.

René Rodrigues, do Químico Luis Freixo e da secretária Sra. Rosalva, pela ajuda dispensada durante a minha estadia

naquele laboratório;

Ao pessoal técnico do Laboratório de Laminação do Centro de Geociências, cujos técnicos Sra. Shirley Tavares,

Sr. Eduardo Soares e Sr. Israel Tavares, eu sou muito grato pela franca amizade, e ao Laboratório de Sedimentologia do

Centro de Geociências, na pessoa do Sr. Antônio Lopes, pela amizade e paciência. Todos com atenção e cuidado no trato

deste trabalho;

Ao pessoal da Coordenação de Ciências da Terra do Museu Paraense Emílio Goeldi, nas pessoas do Chico,

Cristina, Márcia, Cléia, Paulo, Hudson, Maria Tereza, Amílcar, Josué e D. Maria, pelo companheirismo compartilhado

nestes anos;

Ao pessoal de transporte do Museu Goeldi, pela amizade nas duras etapas de campo:

Aos meus amigos Braga e Paulo Benevides e suas famílias pela amizade e cordialidade em me receber durante a

minha estadia na sua cidade, realizando etapas deste trabalho;

Aos amigos mais de perto, daqui de Belém, Leonilde, D. Siroca e Seu Joca, Leonete, Leonora, Vante, Paulinho,

Veruska, Angelina e a toda a família Aviz, pelo bem-querer como a de um membro da família;

Ao amigos próximos Erimar, Vanderlei, Cláudia e Heloísa, que são amigos de todas as horas;

E aos amigos distantes Eisner, Elias, Izaías, Naná, Roseane, Douglas e Ana Maria, que são amigos para sempre.

À minha família, mais perto, Naíde, Élson, Everton, Felipe, Chico e todos os outros mais longe;

À minha mãe...

Aos meus irmãos Jailton, Derimar e Janderson ...

À Leninha...

MUITO OBRIGADO!

iii

O Lago

Eu não vi o mar.

Não sei se o mar é bonito,

não sei se ele é bravo.

O mar não me importa.

Eu vi o lago.

O lago, sim.

O lago é grande

E calmo também.

Na chuva de cores

da tarde que explode

o lago brilh

o lago se pint

de todas as cores.

Eu não vi o mar. Eu vi o lago...

(Corrompido a partir de Carlos Drummond de

Andrade... no original “A lagoa”).

SUMÁRIO

DEDICATÓRIA .…………………………………………………………………........ ii

AGRADECIMENTOS .................................................................................................... iii

EPÍGRAFE ...................................................................................................................... iv

LISTA DE FIGURAS ..................................................................................................... vii

RESUMO ......................................................................................................................... 1

ABSTRACT .................................................................................................................... 5

APRESENTAÇÃO .......................................................................................................... 9

1. INTRODUÇÃO ......................................................................................................... 10

1.1. ARCABOUÇO GEOLÓGICO E PROBLEMÁTICA ............................................. 10

1.2. OBJETIVOS ............................................................................................................. 14

1.3. ÁREA DE ESTUDO ................................................................................................ 14

1.4. MÉTODOS ............................................................................................................... 15

1.5. GEOQUÍMICA ISOTÓPICA ................................................................................... 17

1.5.1. Fundamentação teórica ...................................................................................... 17

1.5.2. Aplicação em depósitos lacustres ....................................................................... 19

REFERÊNCIAS .............................................................................................................. 24

2. LINKING LACUSTRINE CYCLES WITH SYN-SEDIMENTARY

TECTONIC EPISODES: AN EXAMPLE FROM THE CODÓ FORMATION

(LATE APTIAN), NORTHEASTERN BRAZIL……………………………………

2.1. ABSTRACT .....................................................................................………….…... 29

2.1. INTRODUCTION ..................................................................................………….. 30

2.3. GEOLOGICAL SETTING ………………………………………………………... 31

2.4. DEPOSITIONAL ENVIRONMENT ……………………………………………... 35

2.4.1. Central lake facies association ………………………………………………… 36

2.4.2. Transitional lake facies association …………………………………………… 38

2.4.3. Marginal lake facies association …………………………………………….… 39

2.4.4. Saline pan/sabkha facies association …………………………………………. 40

2.4.5. Interpretation …………………………………………………………………. 41

2.5. SHALLOWING-UPWARD CYCLES AND DEPOSITIONAL UNITS …….…... 43

2.5.1. Description .....................................................................................………….…. 43

2.5.2. Interpretation .....................................................................................…………. 48

2.6. ORIGIN OF THE SHALLOWING UPWARD CYCLES ……………………….. 49

2.7. CONCLUSION .....................................................................................………….. 54

REFERENCES .........................................................................................................…... 55

3. PETROGRAPHY OF GYPSUM-BEARING FACIES OF THE APTIAN

CODÓ FORMATION .....................................................................................………..

3.1. ABSTRACT .....................................................................................………….…... 60

3.2. INTRODUCTION ..................................................................................………….. 61

3.3. GEOLOGICAL SETTING ………………………………………………………... 62

3.4. PETROGRAPHY OF THE EVAPORITES ………………………………………. 65

3.4.1. Chevron gypsum (selenite) .................................................................................. 65

3.4.2. Nodular/lensoidal gypsum/anhydrite ................................................................. 70

3.4.3. Acicular gypsum .................................................................................................. 70

3.4.4. Mosaic gypsum ..................................................................................………....... 70

3.4.5. Brecciated gypsum/gypsarenite ……………………………………………….. 70

3.4.6. Pseudo-nodular anhydrite/gypsum .................................................................... 72

3.4.7. Rosettes of gypsum .............................................................................................. 72

3.5. PARAGENESIS ................................................................................................…... 74

3.6. CONCLUSIONS ..................................................................................……….…... 79

REFERENCES .........................................................................................................…... 80

4. GENESIS AND PALEOHYDROLOGY OF A SALINE PAN/LAKE SYSTEM

(LATE APTIAN) FROM THE BRAZILIAN EQUATORIAL MARGIN:

INTEGRATION OF FACIES, SR AND S ISOTOPES …………………………….

4.1. ABSTRACT ..................................................................................………………... 85

4.2. INTRODUCTION ..................................................................................………….. 86

4.3. MATERIAL AND METHODS .............................................................…………... 87

4.4. GEOLOGICAL FRAMEWORK AND PALEOENVIRONMENTAL CONTEXT 89

4.5. CHARACTERIZATION OF THE EVAPORITES ………………………………. 92

4.5.1. Description ..................................................................................………………. 92

4.5.2. Depositional setting …………………………………………………………….. 94

4.6. Sr AND S ISOTOPES FROM THE EVAPORITES ………………………….…... 96

4.7. BURIAL OVERPRINT ……………………………………………………….…... 100

4.8. DISCUSSION ..................................................................................………………. 102

4.9. CONCLUSIONS ..................................................................................……………. 108

REFERENCES ................................................................................................................ 109

5. PALEOHYDROLOGY OF A LATE APTIAN LACUSTRINE SYSTEM

FROM NORTHEASTERN BRAZIL WITH BASIS ON THE INTEGRATION

OF FACIES AND ISOTOPIC GEOCHEMISTRY ………………………………...

113

5.1. ABSTRACT ..................................................................................………………... 113

5.2. INTRODUCTION ..................................................................................………….. 114

5.3. GEOLOGICAL SETTING ................................................................................…... 116

5.4. FACIES ARCHITECTURE AND DEPOSITIONAL SYSTEM ...………………. 118

5.4.1. Description ..................................................................................………………. 118

5.4.2. Interpretation ..................................................................................………….… 122

5.5. GEOCHEMICAL TREATMENT …………………………………………….…... 123

5.6. EVALUATION OF DIAGENETIC IMPRINT ……………………………….….. 124

5.7. STABLE ISOTOPES ..................................................................................………. 128

5.8. ISOTOPIC CHARACTERIZATION OF THE LACUSTRINE SYSTEM ………. 129

5.9. FINAL REMARKS ……………………………………………………………….. 138

REFERENCES ................................................................................................................ 140

6. CONCLUSõES GERAIS ..................................................................................…… 147

LISTA DE FIGURAS

Fig. 1-1: A) Corte esquemático do Sistema de Gráben Gurupi, incluindo as sub-

bacias do Grajaú, Sao Luís e Ceaté; B) estratigrafia, preenchimento e

tectônica da Bacia de Sao Luís- Grajaú; C) localização da área de estudo

nas porções leste e sul da Bacia do Grajaú; D) detalhe da figura anterior,

indicando as localidades com minas a céu aberto estudadas neste

trabalho (ver texto).

...................... 12

Fig. 1-2: Respiração do lago, consumo de matéria orgânica, soterramento de

carbono orgânico, influxo de águas fluviais com bicarbonato mais

“leve”, baixa troca de gás carbônico com a atmosfera e participação do

gás carbônico metanogênico alternam-se em importância durante fases

de lago alto e baixo e influenciam indiretamente no valor do d13C do

carbonato lacustre (Modificado de Guzzo, 1997).

...................... 20

Fig. 1-3: A) processos de enriquecimento em

O de águas lacustres ligados ao

ciclo hidrológico (cf. Lister et al., 1991); B) modelo de sinal

paleohidrológico de isótopos de oxigênio em carbonatos lacustres,

considerando bacias hidrologicamente fechadas (Lister et al., 1991;

Bellanca et al., 1992).

...................... 21

Fig. 1-4: Ciclo resumido do enxofre, enfatizando a passagem do sulfato para

sulfeto de hidrogênio por respiração anaeróbica de bactérias sulfato-

redutoras (modificado de Hoefs, 1980).

...................... 22

Fig. 1-5: Fatores influentes na composição da razão

Sr/

Sr de depósitos

lacustres e de bacias oceânicas (cf. Holser et al., 1996).

...................... 23

Fig .2-1: Location map of the study area in the eastern and southern of São Luís-

Grajaú Basin, northern Brazil, with indication. The map to the right

indicates the studied localities where the Codó Formation is well

exposed in limestone and evaporite quarries. (A=Santo Amaro; B-

C=Itapicuru Evaporite Quarry; D-E=Itapicuru D-6 Limestone Quarry;

F= Transmarenhense Road-Km 22; G=Olho D´água Quarry; H=Chorado

Quarry; I=Transmaranhense Road-Km 16.

...................... 31

Fig .2-2: a) Map displaying the structural lineaments of the São Luís-Grajaú

Basin. Note the main NW-SE, and NE-SW oriented fault traces, and the

subordinate E-W lineaments, the later representing the record of the

strike-slip phase of the basin. b) A geological section interpreted from a

seismic line and the well logs indicated in figure a, with the plot of the

main fault traces. Note that these are vertical to nearly vertical and

represent reactivations of faults derived from the crystalline and

Palaeozoic basement. The faults cut throughtout the entire basin,

particularly disrupting the Cretaceous successions, attesting the rift

development (Modified after Góes & Rossetti, 2001).

...................... 33

Fig .2-3: Stratigraphy of the São Luís-Grajaú Basin.

...................... 34

Fig .2-4: Exposure of the Codó Formation in a limestone quarry, near the town of

Codó, eastern São Luís-Grajaú Basin, with indication of the syn-

sedimentary soft sediment deformation zones formed by seismic activity

as described in the text.

...................... 34

vii

Fig .2-5: A summary of sedimentary facies described in the Codó Formation,

with their distribution arranged according to facies associations Fa1 to

Fa4, attributed to marginal, intermediate and central lake environments,

as well as saline pan/sabkha complex, respectively.

...................... 36

Fig .2-6: Sedimentary facies representative of the Codó Formation in the study

area. a, b) Central lake deposits, with massive macronodular evaporites

(Gy) and bituminous black shales (Bsh) within evaporites. The hatched

line in a, indicates the top of the evaporite beds. The deposits overlying

this surface consists of limestone and grey/green shale interbeddings,

attributed to transitional lake settings. (person in a for scale=1.6 m tall;

rod subdivision in b=10 cm). c) Transitional lake deposits, consisting of

limestone (Lm) and grey/green shale (Sh) interbeddings. (pen for

scale=15 cm long) d-g) Marginal lake deposits, illustrating: rhythmites

of ooidal/peloidal packstone (p) and ostracodal wackestone-grainstone

(g) with cryptomicrobial mats (am) in d; a detail of the ooidal/peloidal

packstone in e; fractured massive pelite in f; and photomicrography of

intraclastic grainstone in g. (pen for scale in e and f=15 cm long). h-i)

Saline pan/sabkha deposits, illustrating the layered gypsum. (arrows

indicate laminae of gypsum formed by vertical aligned chevron

crystals). Note in h the cycles formed by darker/lighter couplets, which

consist of microgranular gypsum/vertical aligned gypsum, respectively,

attributed to seasonal fluctuations.

...................... 37

Fig .2-7: Lithostratigraphic profiles representative of the Codó Formation

exposed in the study area, with the main facies characteristics and the

two ranks of shallowing-upward cycles described in the text. (See figure

1 for location of the profiles A-I). Datum=discontinuous surface with

evidences for maximum subaereal exposure within the Codó Formation.

...................... 45

Fig .2-8: Diagrams illustrating the four types of lower-rank, shallowing-upward

cycles of the Codó Formation. Thickness of individual cycles range from

0.3-5.6 m. See figure 2-7 for legend.

...................... 46

Fig .2-9: Types of shallowing-upward units present in the Codó Formation,

illustrating: a) Unit 2, represented by one complete cycle type 1 with

evaporites and shales. b) Unit 3, presented from base to top by two

incomplete cycles type 1 and one complete cycle type 2. c) The

uppermost portion of unit 2 and the base of unit 3, with the latter

showing three incomplete cycles type 1 and an upper incomplete cycle

type 2.

...................... 47

Fig .2-10: A representative lithostratigraphic profile of the Codó Formation

showing the good match between higher-rank shallowing-upward

cycles, represented by depositional units 2 to 4, and the syn-sedimentary

deformational zones described by Rossetti and Góes (2000). (See Fig. 2-

7 for legend).

...................... 53

Fig. 3-1: A) Location map of the study areas in the São Luís-Grajaú Basin,

northeastern Brazil. B) A close up map, indicating the location of the

studied sections in the Codó and Grajaú areas.

...................... 62

Fig. 3-2: A) Diagram with the sketched representation of the proposed structural

pattern of the São Luís-Grajaú Basin in the Gurupi Graben System, and

its relation with the Pará-Maranhão Basin. In this model, the Ferrer-

Urbano Santos Arch is considered as an intrabasin horst within an

...................... 63

viii

abandoned intracontinental rift system formed by combination of pure

shear stress and strike-slip deformation. The northeastward rifting

migration through time gave rise to the development of a deeper basin,

represented by the Caeté Sub-basin (cf. Góes & Rossetti, 2001). B)

Stratigraphic framework of the São Luís-Grajaú Basin (cf. Rossetti,

2001).

Fig. 3-3: Depositional model proposed for the Codó Formation in the Codó and

Grajaú areas (cf. Paz & Rossetti, 2001).

...................... 64

Fig. 3-4: Measured vertical sections representative of the evaporites studied in the

Codó Formation (see Fig. 3-1 for section location).

...................... 67

Fig. 3-5: The several phases of evaporites recognized in the Codó Formation,

with paragenesis indicated by lateral position of the boxes.

...................... 68

Fig. 3-6: Textures of the evaporites from the Codó Formation exposed in the

study areas. A) Photomicrography of chevron gypsum. B)

Photomicrography of the nodular gypsum (ng) within bituminous black

shales (sh). Note calcite relicts (arrow) within nodules (crossed nicols).

C) A close up in SEM view from the gypsum nodules shown in figure B,

illustrating their composition by micrometric, equant, lath-like crystals,

typical of anhydrite. D) Photomicrography of the acicular gypsum and,

at the base, mosaic gypsum. E) Photomicrography of relics of anhydrite

(arrow) within a large mosaic gypsum crystal.

...................... 69

Fig. 3-7: Textures of the evaporites from the Codó Formation exposed in the

study areas. A) Brecciated gypsum (upper part) evolved from mosaic

gypsum (lower part). The dark lines around the clasts in the upper part

of the photograph were formed by muds resulting from mechanical

infiltration (parallel nicols). B) A detailed of the brecciated gypsum

illustrating a crystal with several fractures (arrow). Note that the optical

continuity beyond fractures, attesting fracturing occurred within a single

large crystal (crossed nicols). C and D) Brecciated gypsum locally

displaying well rounde clasts resembling gypsarenite (C=parallel nicols;

D=crossed nicols). Note in D that several clasts are in optical continuity,

revealing they were most likely formed by fracturing of a same large

crystal.

...................... 71

Fig. 3-8: Textures of the evaporites from the Codó Formation exposed in the

study areas. A) Overlay field drawing illustrating a spot within the

pseudo-nodular anhydrite/gypsum with relics of a complex arrangement

formed by mosaic, acicular and fibrous gypsum, bounded by films of

calcimudstone. B) Pseudo-nodular gypsum, formed by fracturing. C) A

spot within the pseudo-nodular anhydrite/gypsum, with nodules of

anhydrite bound by films of fibrous gypsum (arrows). Note the

superimposed rosettes of gypsum of variable sizes (rg). D) Alabastrine

(gy) and fibrous (gf) gypsum in gradational contacts. The arrows

indicate places where the fibrous gypsum is partly replaced by the

alabastrine gypsum, recorded by numerous crystals of the later over the

fibrous gypsum, which in turn remains as diffuse relics. E)

Photomicrography of the nodules shown in C, illustrating their

composition of tiny, equant, lath-like crystals (an). Note that the edges

of the nodules were replaced by fibrous (gf) or mosaic (gy) gypsum.

...................... 73

Fig. 3-9: Textures of the evaporites from the Codó Formation exposed in the

d)ifhddlhdi/

...................... 74

study areas. A) SEM view of the pseudo-nodular anhydrite/gypsum,

with lath-like anhydrite (an) interlaced with gypsum (gy). This material

comes from a nodule of anhydrite depicted in figure 8C. B) Calcite (ca)

cementing fractures in the pseudo-nodular anhydrite/gypsum (gy). C) A

detail of the pseudo-nodular anhydrite/gypsum showing several fractures

(arrows) filled by a mixture of calcite and muds. Note larger calcite

crystals (ca) that grew sidewards from the fractures trough replacement

of gypsum (gy). D) Relics of calcite (ca) within alabastrine gypsum (gy)

from the pseudo-nodular anhydrite/gypsum. E) Rhombs of calcite after

dolomite (arrows). (Except for the SEM micrography shown in A, all the

other figures were obtained under petrographic microscope with crossed

nicols).

Fig. 3-10: Model to explain the formation of primary and early diagenetic

evaporite phases recorded in the Codó Formation.

...................... 75

Fig. 4-1: A) Location and geological map of the study area in the north of Brazil.

B) Stratigraphic framework of the São Luís-Grajaú Basin, with the

depositional sequences discussed in the text.

...................... 88

Fig. 4-2: A) A view of the unconformity between the Codó Formation and the

overlying Albian deposits of the Itapecuru Group. (Gy= gypsum;

Lsh=interbedded limestone and shale). B) Lithostratigraphic profile

representative of the Codó Formation in the study area, with indication

of the shallowing upward cycles formed by central lake (1), intermediate

lake (2) and marginal lake (3) deposits. The letters to the left locate

figures A-F. C) Bituminous shale (black areas) interbedded with

evaporite (white areas) from central lake deposits. D) Interbedded

limestones (lm) and shales (darker areas from intermediate lake deposits.

E) Interbedded lime-mudstone (Ml) and pisoidal limestones (Pl) and F)

Rhythmite of lime mudstone (white beds) and microbial mats (black

beds) from marginal lake deposits.

...................... 90

Fig. 4-3: Evaporite deposits from the Codó Formation. A) Layered gypsum.

(Person for scale=1.68 m tall). B) Detail of the layered gypsum

illustrating the alternating darker and lighter beds consisting of gypsum

crystals or nodules within a matrix of black shale (Dg), and upward-

oriented chevron/acicular gypsum crystals (Lg), respectively. C) A plan

view of the gypsarenite facies (pen for scale=15 cm long). D)

Massive/macronodular gypsum (Gy) underlying interbedded limestones

and shales (Lsh). The white dotted line indicates the discontinuity

surface between the Codó Formation and the Itapecuru Group (Albian).

(Person for scale=1.68 m tall) E) A detail of the massive/macronodular

gypsum strongly affected by fractures, resulting in almond-shaped

anhydrite nodules. Note the abundant rosettes of gypsum (rg) that are

disperse in this facies. (Lens cap for scale=10 cm in diameter).

...................... 93

Fig. 4-4: Microscopy of the evaporites from the Codó Formation. A) Gypsum

with nodular texture of the dark layered facies (parallel polarizers). B)

Layered nodular gypsum with relics of anhydrite (=an; crossed

polarizers). C) Recrystallized nodular gypsum displaying mosaic of

gypsum crystals that grew beyond the nodule boundaries (crossed

polarizers). D) Scanning electron photomicrography of the light beds in

the layered gypsum, showing chevron gypsum locally replaced by

acicular gypsum. E) A petrographic view of the gypsarenite formed by

rounded gypsum grains after overgrowth. F) Alabastrine gypsum.

...................... 95

Fig. 4-5: A summary of facies distribution and depositional interpretation

proposed for the Codó Formation in the study areas.

...................... 97

Fig. 4-6: Lithostratigraphic profiles representative of the Codó Formation in the

study areas with the plot of

Sr/

Sr, Sr (ppm) and δ

S isotope values.

...................... 99

Fig. 4-7:

Sr/

Sr and δ

S values from the Codó Formation and comparison with

the values from the Aptian seawater. Note that the strontium values for

both of the study areas are much higher than those expected from Aptian

seawaters, which support a continental-sourced brine for the evaporites.

This interpretation is further supported by the higher values of S

isotopes, and the scatter nature of both isotopes. The lower S values

from the Grajaú area are interpreted to have a facies control, and

probably respond to increased evaporation (see text for further

discussion).

...................... 104

Fig. 4-8: Model to explain the distribution of

Sr/

Sr according to the low

frequency cycles of the Codó Formation. (Inspired on Rhodes et al.,

2002).

...................... 106

Fig. 5-1: A) Location of the study area in the Codó region, eastern margin of the

Grajaú Basin. B) Stratigraphy and main tectonic stages of the São Luís-

Grajaú Basin.

...................... 117

Fig. 5-2: Diagram illustrating the proposed lacustrine depositional model for the

Codó Formation, characterized by central to marginal lake deposits. A)

General view of marginal lake deposits between intermediate lake

deposits. B) A detail showing the upward gradation from interbedded

limestones (Lm) and laminated argillites (Al; intermediate lake) to

rhythmites (Rh; marginal lake). C) Fenestral calcarenite from marginal

lake deposits. D) Rhythmite of limestones (lighter color) and microbial

mats (darker color) from marginal lake deposits. E) General view of

central lake deposits (Ev=evaporite). F) Bituminous black shales (Sh)

interbedded with evaporites (Ev). G) Layered gypsum.

...................... 119

Fig. 5-3: Shallowing-upward cycles of the Codó Formation. A) Examples of first-

and second-order cycles. B-D) Third-order cycles formed by alternations

of bituminous black shale with streaks of lime-mudstone (Bsl) and

bituminous black shales with native sulphur (Bss) (B), ostracodal

grainstone (Gro) and vadose pisoidal packstone (Pp) with microbial mats

(M) (C), ostracodal grainstone (Go) and wackestone (Wo) (D).

...................... 120

Fig. 5-4: First- and second-order cycles of the Codó Formation with relation to

soft-sediment deformation zones attributed to syn-sedimentary seismic

activity. (See Fig . 5-7 for legend).

...................... 121

Fig. 5-5: Photomicrographies of biogenetically not affected facies utilized for the

isotopic analysis. A,B) Calcimudstone (A=crossed nichols; B=scanning

electron microscopy). C) Ostracodal grainstone (crossed nichols). D)

Electron photomicrography illustrating ostracode shells of ostracodal

grainstone, formed by densely packed, columnar calcite crystals

attributed to primary origin.

...................... 125

Fig. 5-6: Distribution of the trace elements Fe (A), Mn (B), Mg (C) and Sr (D)

from samples used in the isotopic analysis presented herein (I=meteoric

...................... 127

calcite; II=marine calcite; III=diagenetic calcite). (After Tucker and

Wright, 1990).

Fig. 5-7: Lithostratigraphic profiles representative of the Codó Formation

exposed in the study area, with the stratigraphic distribution of facies

associations, first- and second-order cycles, and δ

(%o PDB)

and δ

(%o

PDB)

values.

...................... 130

Fig. 5-8: Plot and correlation of carbon and oxygen isotope data from the Codó

Formation in the eastern Grajaú Basin. The positive correlation in

profiles B and C is attributed to episodes of dominantly closed lake

system, while the negative correlation in profile A records lake opening.

...................... 131

Fig. 5-9: δ

C trend of marine-bearing Aptian carbonates (Modified from Jones

and Jenkins, 2001).

...................... 131

Fig. 5-10: Plots of carbon and oxygen stable isotope data from several marine and

lacustrine deposits throughout the world (Modified from Hendry and

Kalin, 1997), and their comparison with data obtained in the Codó

Formation. This diagram shows that the isotopic composition of the

Codó Formation is in conformity with isotope data from lacustrine

limestones.

...................... 133

Fig. 5-11: Schematic diagram illustrating the main mechanisms that contribute to

modify δ

(%o PDB)

and δ

(%o PDB)

of the carbonates in a lake system.

...................... 134

Fig. 5-12: Summary of the depositional model proposed for the origin of the

Codó lake system in the eastern of the Grajaú Basin, illustrating the

close relationship of facies development, and thus the distribution of

oxygen and carbon isotopes, with alternation between syn-sedimentary

fault displacement and uplift. A) Offset of few meters along faults

displaced along the basin margins resulted in the creation of

accommodation space along subsiding areas, where the lake system

developed, giving rise to first-order cycle represented by unit 2. B)

Uplift contributed to decrease the lake level with the consequent spread

out of marginal deposits at the top of unit 2, culminating with lake

dryness and formation of a discontinuity surface with paleosols. C) Fault

reactivation resulted in a renewed phase of lake deepening, with

deposition of central and intermediate facies deposits recorded by unit 3.

D) Renewed uplift promoted the fall in lake level and widespread

formation of marginal deposits represented by pisoidal packstone to

grainstone and rhythmite, which culminated with lake exposure and soil

development at the top of unit 3. E) Fault reactivation with renewed

deposition of laminated argillites, recorded by unit 4. F) Increasing

stability led to progressive decrease in water level resulting from the

abandonment of the lacustrine deposition in the study area, with

subaerial exposure and formation of an unconformity at the top of the

Codó Formation.

...................... 136

xii

RESUMO

A Formação Codó, objeto deste estudo, corresponde a uma unidade geológica

neoaptiana bem conhecida por ser o único registro exposto de rochas desta idade na margem

equatorial brasileira. Esta formação, constituída de folhelhos betuminosos, calcários e

evaporitos, é particularmente bem exposta nas bordas sul e leste da Bacia de São Luís-Grajaú,

MA, áreas aqui investigadas com o intuito: 1. de aprimorar o entendimento do sistema

deposicional, discutindo-se a hipótese de formação em ambientes lacustres; e 2. reconstituir as

condições paleohidrológicas com base na integração de dados faciológicos, estratigráficos,

petrográficos e isotópicos (C, O, Sr e S). Os dados de campo confirmaram sistema lacustre

para a área de Codó, onde se desenvolveram lagos salinos, estáveis, bem estratificados, e com

períodos de fechamento, quando prevaleceram condições anóxicas acompanhadas pela

precipitação de sais em subambientes de lago central. Na região de Grajaú, por outro lado,

prevaleceram condições mais efêmeras, com desenvolvimento de complexo de sabkha/saline

pan, e precipitação de evaporitos principalmente nas margens do sistema, sob condições de

salinas marginais e de planícies lamosas.

Os estudos faciológico e estratigráfico mostraram, também, que a Formação Codó em

ambas as áreas estudadas está organizada em ciclos de arrasamento ascendente, que registram

a progradação de depósitos de lago marginal sobre os de lago central. Três categorias de ciclos

foram distinguidos, designados aqui de inferior, intermediário, e superior. Os ciclos de ordem

inferior, de espessuras variando entre milímetros a poucos centímetros, são formados por

depósitos com acamamentos constituídos de um dos seguintes arranjos litológicos: a) folhelho

negro betuminoso e evaporito; b) folhelho negro betuminoso e calcimudstone; c) folhelho

negro betuminoso e packstone-wackestone peloidal; d) folhelho cinza-esverdeado e

calcimudstone; e) folhelho cinza-esverdeado e packstone-wackestone peloidal; f) folhelho

cinza-esverdeado e packstone-wackestone ostracodal; ou g) grainstone-wackestone ostracodal

e/ou calcimudstone com tapetes criptomicrobiais e packstone ooidal-pisoidal. Estes ciclos são

atribuídos a depósitos sazonais, tendo em vista as suas espessuras regulares na escala

milimétrica, típicas de depósitos climaticamente controlados.

Os ciclos de ordem intermediária têm, em média, 1,7 m de espessura e são

subdivididos por ciclos completos e incompletos. Ciclos completos são compostos de

depósitos de lago central, que gradam para acima a depósitos de lago intermediário e marginal,

sendo representados por dois tipos: ciclos com depósitos de lago central, constituídos por

folhelhos e evaporitos (C1); e ciclos com depósitos de lago central, constituídos por folhelho

cinza esverdeado (C2). Ciclos incompletos são formados por sucessões faciológicas onde pelo

menos uma das associações de fácies está ausente. São também de dois tipos: ciclos com

depósitos de lago central e intermediário (I1); e ciclos com depósitos de fácies de lago

intermediário e central (I2).

Os ciclos de ordem superior medem, em média, 5,2 m de espessura e consistem em

quatro unidades deposicionais, limitadas por superfícies de descontinuidade, sendo

internamente constituídas por ciclos intermediários, tanto completos quanto incompletos, e de

distribuição variável em direção ao topo das seções. A unidade 1, mais inferior, está apenas

parcialmente exposta, com 2,7 m de espessura em média, sendo formada por um intervalo

constituído por ciclos I1 delgados. A unidade 2 tem, em média, 5,2 m de espessura e contem

todos os tipos de ciclos, principalmente ciclos completos. A unidade 3, com 2,6 m de

espessura em média, é constituída por quase 80% de ciclos I2. A unidade 4 apresenta 2,2 m de

espessura média, inclui exclusivamente ciclos incompletos, embora a maior parte desta

unidade tenha sido destruída pela formação do limite da seqüência aptiana.

A caracterização sedimentar detalhada e o padrão de empilhamento dos ciclos de

ordens intermediária e superior suportam gênese ligada à atividade tectônica sin-sedimentar.

Isto é particularmente sugerido pela alta variabilidade de fácies, pela extensão lateral limitada,

e por mudanças aleatórias na espessura e freqüência dos ciclos de ordem intermediária. Além

disto, os quatro ciclos de ordem superior são correlacionáveis com zonas estratigráficas

apresentando diferentes estilos de estruturas de deformação sin-sedimentar, atribuídos em

trabalhos anteriores a atividades sísmicas sin-deposicionais. Portanto, os vários episódios de

arrasamento do lago, registrados na Formação Codó pelos ciclos de ordem intermediária e

superior, são atribuídos a flutuações no nível de água do lago promovidas por pulsos sísmicos

contemporâneos à sedimentação.

A análise petrográfica dos evaporitos da Formação Codó permitiu que se definissem

melhor as histórias tanto deposicional do sistema lago-sabkha-saline pan, quanto pós-

deposicional. Sete morfologias de evaporitos foram reconhecidas: 1. gipso en chevron; 2.

gipso/anidrita nodular/lenticular; 3. gipso acicular; 4. gipso em mosaico; 5. gipso

brechóide/gipsarenito; 6. gipso/anidrita pseudo-nodular; e 7. gipso em roseta. A despeito desta

ampla variedade de fases, a abundância de gipso en chevron, gipso/anidrita nodular/lenticular

e gipso brechóide/gipsarenito, registra a boa preservação de formas primárias. Esta

interpretação é suportada pela associação destas morfologias de gipso com depósitos

mostrando acamamento horizontal de natureza cíclica, que são atribuídos a flutuações do nível

de base do lago, eventualmente culminadas com períodos de exposição subaérea. Mesmo o

gipso acicular e o gipso em mosaico, interpretados como produtos de substituição do gipso en

chevron e do gipso brechóide/gipsarenito, mostram características de formação autigência

ainda sob influência do ambiente deposicional. Fases de formação de gipso sob condições

diagenéticas mais profundas são registradas somente no gipso/anidrita pseudo-nodular,

atribuídos a mobilizações durante halocinese. Além disto, gipso em rosetas, que interceptam

todas as outras fases evaporíticas, têm também origem diagenética ligada a processos tardios

por interação com água subterrânea e/ou intemperismo superficial.

A constatação de forte influência deposicional registrada em, pelo menos, grande parte

das morfologias dos evaporitos da Formação Codó (i.e., gipso primário ou eodiagenético),

além da constatação de microfácies carbonáticas com poucas modificações diagenéticas,

motivaram a aplicação de métodos isotópicos com propósitos de reconstituição

paleoambiental. Os resultados obtidos mostram que ciclos de expansão/contração do sistema

deposicional em ambas as áreas estudadas são acompanhadas por variações significativas nos

valores isotópicos. A ampla dispersão de valores dos isótopos de Sr e S dentro de cada ciclo

deposicional reforça a interpretação petrográfica de que a diagênese não modificou a

assinatura geoquímica deposicional dos evaporitos, confirmando seu valor como ferramenta

paleoambiental. Além disto, origem não marinha para os evaporitos é sugerida pelas razões

Sr/

Sr, que variaram de 0,70782 a 0,70928, consideradas mais altas do que aquelas

esperadas para evaporitos oriundos da água do mar no Neo-Aptiano (entre 0,70720 e 0,70735).

O δ

S variou nas amostras estudadas de 16.12‰ to 17.89 ‰

(V-CDT)

na região de Codó,

mostrando-se também em total desarmonia com valores marinhos do Neo-Aptiano (i.e., entre

13‰ e 16‰

(V-CDT)

). Tanto Sr quanto S foram influenciados pelas características das fácies

deposicionais, de tal forma que, durante a expansão do sistema deposicional, os valores de

Sr/

Sr decresceram devido à inibição do

Sr liberado a partir de argilominerais pela

drenagem interna de planícies lamosas. Nos picos de expansão, os valores de

Sr/

Sr eram os

mais baixos, o que é relacionado à submergência de planícies lamosas e introdução de águas

depletadas em

Sr oriundo do intemperismo de calcários e evaporitos marinhos permianos a

neocomianos, tanto quanto basaltos triássicos a neocomianos.

Enquanto o estudo dos isótopos de Sr e S observou o comportamento destes nos

evaporitos da Formação Codó, análises de isótopos de C e O foram realizadas nos carbonatos

e também revelaram uma ampla distribuição isotópica, com valores exclusivamente baixos de

C e δ

O, ou seja , entre –5.69‰ e –13.02‰

(PDB)

e –2.71‰ e –10.80‰

(PDB)

respectivamente. Adicionalmente, estas razões variam de acordo com ciclos de arrasamento

considerados neste trabalho como de origem tectônica e que, em geral, mostram razões de

C e δ

O mais leves na base, onde predominam depósitos de lago central, e

progressivamente mais pesados em direção ao topo, onde depósitos de lago marginal são mais

expressivos. Também confirmando a assinatura deposicional, este comportamento leva a

propor um modelo isotópico controlado por eventos de sismicidade sin-sedimentar. Assim,

razões isotópicas com valores mais leves parecem estar relacionados com eventos de

inundação promovidos por subsidência, que resultou no desenvolvimento de um sistema de

lago perene. Razões isotópicas com valores mais pesados estariam relacionados a fases de lago

efêmero e seriam favorecidas pelo soerguimento e/ou aumento da estabilidade tectônica. Além

disso, os resultados mostram que sistemas de lagos fechados predominaram em pelo menos

parte do tempo de evolução desses depósitos, o que é indicado pela boa covariância positiva

(i.e., +0,42 e +0,43) entre o carbono e o oxigênio, embora fases de lago aberto também sejam

registradas pelos valores de covariância negativa (i.e., –0,36).

ABSTRACT

The Codó Formation is an important geological unit in Brazil, representing the only

record of Neoaptian rocks exposed along the Brazilian equatorial margin. This unit consists of

bituminous black shales, limestones and evaporites, which are particularly well represented in

the south and east margins of the São Luís-Grajaú Basin, around the towns of Codó and

Grajaú, State of Maranhão. These areas were investigated in order to: 1. improve the

depositional system, discussing the hypothesis that the Codó Formation was produced in a

lacustrine setting; and 2. reconstruct the paleohydrological conditions with basis on the

integration of facies, stratigraphy, petrography and isotope (C, O,Sr and S) data. Hence, the

field data presented herein confirmed a lacustrine system for the Codó area, where prevailed

stable, well-stratified, saline lakes characterized by periods of closure, anoxia and salt

precipitation in the central saline lakes. On the other hand, ephemeral conditions with

development of a sabkha/saline pan complex prevailed in the Grajaú area, where salts

precipitated mostly in the marginal portions of the system (i.e., marginal saline pans and

mudflats).

Studies focusing facies and stratigraphy also revealed that in both areas the Codó

Formation is arranged into several shallowing-upward cycles formed by progradation of

marginal into central lake deposits. Three types of cycles were distinguished, referred to here

as lower, intermediate and higher rank cycles. The lower rank cycles correspond to millimetric

interbeddings of: a) bituminous black shale and evaporite; b) bituminous black shale and

calcimudstone; c) bituminous black shale and peloidal wackestone-packstone; d) grey/green

shale and calcimudstone; e) grey/green shale and peloidal wackestone-packstone; f)

grey/green shale and ostracodal wackestone/grainstone; h) ostracodal wackestone/grainstone

and/or calcimudstone with cryptomicrobial mats and ooidal/pisoidal packstone. These are

attributed to seasonal deposition with basis on their regular nature forming very thin cycles

resembling varves.

The intermediate rank cycles average 1.7 m thick and are formed by complete and

incomplete cycles. Complete cycles show an upward transition from central to intermediate

and then marginal facies associations, and include two types: C1 cycles with central lake

deposits consisting of evaporites and black shales; and C2 cycles with central lake deposits

formed by gray/green shale. Incomplete cycles are those formed by successions lacking at

least one of the facies associations, consisting of either central and intermediate lake deposits

(cycles I1) or intermediate and marginal lake deposits (cycles I2).

The higher rank cycles average 5.2 m thick and consist of four depositional units,

which display shallowing-upward successions formed by both complete and incomplete,

intermediate rank cycles that vary their distribution upward in the section, and are bounded by

sharp surfaces. Unit 1, the lowermost one, averages 2.7 m in thickness, being entirely

composed by thin I1 cycles. Unit 2 averages 5.2 m thick, and displays all of the

aforementioned intermediate cycles, especially complete ones. Unit 3, averaging 2.6 m thick,

consists of 80% of cycles I2. Finally, unit 4, which averages 2.2 m in thickness, displays only

incomplete cycles, though its uppermost part was not preserved due to erosion during the

development of the Aptian sequence boundary.

The detailed sedimentological characterization and the stratal stacking patterns of the

intermediate and higher rank cycles support a genesis linked to syn-sedimentary tectonic

activity, particularly suggested by high facies variability, limited lateral extension, as well as

frequent and random thickness changes of the intermediate-rank cycles. Additionally, the

matching between the four higher rank cycles with four stratigraphic zones having different

styles of soft-sediment deformation structures previously described in the literature as

resulting from seismic activities, is a further argument to corroborate this interpretation.

Therefore, the several episodes of lake shallowing recorded in the intermediate and higher

rank cycles of the Codó Formation are attributed to fluctuations in the lake water level,

triggered by seismic pulses alternating with sediment deposition.

The petrographic analysis of the evaporites from the Codó Formation allowed to better

defining both the lake-sabkha-saline pan depositional system and the post-depostional

histories. Seven evaporite morphologies were recognized: 1. chevron (selenite) gypsum; 2.

nodular/lensoidal gypsum/anhydrite; 3. acicular gypsum; 4. mosaic gypsum; 5. brecciated

gypsum/gypsarenite; 6. pseudo-nodular anhydrite/gypsum; and 7. rosettes of gypsum. Despite

of this large variety of evaporite phases, the chevron gypsum, the nodular/lensoidal

gypsum/anhydrite and the brecciated gypsum/gypsarenite record the preservation of primary

features. The association of these morphologies with deposits displaying cyclic horizontal

bedding, attributed to lake level fluctuations eventually culminated with subaerial exposure,

reinforces this interpretation. Even acicular gypsum and mosaic gypsum, which replaced the

chevron and brecciated gypsum/gypsarenite, respectively, formed under the influence of the

depositional surface. Burial phases of gypsum are only recorded in the pseudo-nodular

anhydrite/gypsum, attributed to salt mobilization induced by halokinesis. In addition, rosettes

of gypsum, which crosscut the other evaporite morphologies, diagenetic in origin, have

probably formed as the latest evaporite phase of the study area, under the influence

groundwater and/or surface weathering.

In the present research, isotope studies aiming paleoenvironmental purposes were

motivated by both confirmation of strong depositional influence for at least great part of the

evaporites from the Codó Formation (i.e., primary and eodiagenetic gypsum), and the low

diagenetic modification recorded for the limestones. Results of these approaches show that

expansion/contraction cycles in both studied areas were accompanied by significant changes

in isotope values. The wide dispersion of Sr and S isotope data within individual depositional

cycles reinforces the lack of significant diagenetic modification as suggested by the

petrographic analysis, and confirms the utility of these isotopes as environmental tools.

Additionally, a non-marine brine source is suggested by

Sr/

Sr ratios ranging from 0.707824

to 0.709280, which are higher than those from late Aptian seawater (i.e., between 0.70720 and

0.70735). The δ

S varies from 16.12 to 17.89 %

V-CDT)

in the Codó area, which is also in

disagreement with late Aptian marine values (ranging from 13 to 16 %

(V-CDT)

). Both

geochemical tracers were influenced by facies characteristics, and thus a model is provided

where expansion of saline pan/lake systems led to decreasing

Sr/

Sr values due to the

inhibition of the

Sr from clay minerals originated during the internal draining of mudflats.

During expansion peaks, the

Sr/

Sr values were lower due to submergence of mud flats and

introduction of external

Sr-depleted waters related to weathering of Permian to Neocomian

marine limestones and evaporites, as well as Triassic to Neocomian basaltic rocks.

Furthermore, the sulphur isotope values decrease in the southern margin of the basin from

14.79 to 15.60 %

(V-CDT)

probably due to increased evaporation in shallower water settings.

While the studies of Sr and S isotopes emphasized the evaporites of the Codó

Formation, the analysis of C and O isotopes were carried out on the carbonates. The data

revealed a wide distribution of dominantly low δ

C and δ

O values, ranging from –5.69‰ to

–13.02‰ and from –2.71‰ to –10.80‰, respectively. It was also observed that these ratios

vary according to seismically-induced shallowing-upward cycles, in general becoming lighter

in their bases, where central lake deposits dominate, and progressively heavier upward, where

marginal lake deposits are more widespread. In addition to confirm a depositional signature

for the analysed samples, this behavior led to introduce a seismic-induced isotope model.

Hence, lighter isotope ratios appear to be related with flooding events promoted by

subsidence, which resulted in the development of a perennial lake system, while heavier

isotope values are related to ephemeral lake phases favored through uplift and/or increased

stability. Furthermore, the results show that a closed lake system dominated, as indicated by

the overall good positive covariance (i.e., +0.42 to +0.43) between the carbon and oxygen

isotopes, though open phases are also recorded by negative covariance values of –0.36.

APRESENTAÇÃO

Esta tese acha-se organizada sob a forma de quatro artigos, já submetidos ou aprovados

em periódicos científicos indexados na categoria internacional A ou B. Adicionam-se a estes,

um capítulo introdutório de integração e um capítulo final com uma síntese das principais

conclusões advindas desta pesquisa.

O capítulo 1 tem por meta fazer uma introdução, com a apresentação da área de estudo,

problemática que motivou a pesquisa, objetivos, métodos, bem como um breve sumário dos

princípios teóricos relacionados com a aplicabilidade da geoquímica isotópica na reconstrução

paleohidrológica de sistemas lacustres, tema de abordagem de dois dos capítulos desta tese. O

capítulo 2 trata da descrição litológica e caracterização das diferentes categorias de ciclos

sedimentares reconhecidos na Formação Codó, com discussão dos possíveis fatores

causadores. O capítulo 3 contém a descrição petrográfica dos evaporitos da unidade

investigada, com o objetivo de: 1. reconstituir sua paragênese; e 2. distingüir entre fases

evaporíticas formadas sob influência do ambiente deposicional e aquelas geradas por

processos diagenéticos, possibilitando a escolha de amostras com melhor potencial para os

estudos isotópicos de Sr e S. O capítulo 4 apresenta os resultados da análise isotópica de Sr e S

dos evaporitos da Formação Codó, com o objetivo de fornecer elementos adicionais que

contribuam na determinação da origem da salmora. O capítulo 5 contém os resultados dos

estudos isotópicos de C e O das rochas calcáreas intercaladas aos evaporitos e folhelhos da

unidade estudada, tendo por objetivo de contribuir para no melhor entendimento do regime

hidrológico vigente durante a deposição. Por fim, o capítulo 6 compreende as principais

conclusões sintetizadas neste estudo.

A grande contribuição desta tese foi a consolidação do conhecimento a respeito da

Formação Codó, fazendo uso do estudo integrado combinando análise faciológica com estudos

petrográficos e métodos isotópicas enfocando Sr, S, C e O, como ferramenta chave para a

caracterização mais precisa de sistemas deposicionais lacustres e entendimento de sua

paleohidrologia.

1. INTRODUÇÃO

1.1. ARCABOUÇO GEOLÓGICO E PROBLEMÁTICA

A configuração atual da costa brasileira e as principais feições estruturais que

controlam a dinâmica geológica da margem continental brasileira começaram a ser

estabelecidos durante o Mesozóico, ao mesmo tempo em que as principais reservas de petróleo

do mundo se formavam. No Eocretáceo, a região nordeste do Brasil formava o último elo de

ligação entre a América do Sul e África, o que se estendeu até o período Aptiano, o qual

marca, assim, a separação definitiva entre estes dois continentes e a livre circulação das águas

entre o Atlântico Norte e Sul (Szatmari et al., 1987; Zanotto & Szatmari, 1987; Feijó, 1996).

Este processo de separação foi desencadeado por um complexo sistema de rifteamento,

marcado inicialmente pela implantação de diversos sistemas deposicionais lacustres ao longo

de toda margem continental brasileira (inclusive continente adentro), e que culminou com

transgressão marinha. A implantação dos lagos foi assíncrona ao longo da margem continental

brasileira, ocorrendo genericamente de sul para norte. Desta forma, as bacias das regiões sul e

sudeste brasileiras começaram a apresentar lagos representativos da fase pré-rifte ainda no

Neocomiano, enquanto que nas bacias da região equatorial, os lagos são registrados apenas a

partir do Aptiano.

Na Bacia de São Luís-Grajaú, localizada em grande parte do Estado do Maranhão, a

sedimentação aptiana é bastante expressiva, merecendo destaque por ser a única bacia da

margem equatorial onde a sucessão aptiana pode ser investigada na escala de afloramentos.

Esta bacia foi formada durante a separação dos continentes sul-americano e africano, nos

últimos estágios da fragmentação do Gondwana, no Cretáceo Inferior, por processos

transtensivos associados com o estabelecimento do Sistema de Gráben Gurupi (Góes &

Rossetti, 2001). Este sistema interliga as sub-bacias de Grajaú, São Luís e Caetés numa única

feição estrutural tipo rift (Fig. 1-1A). A Bacia do Grajaú, por si só, abrange uma área de

aproximadamente 130.000 km

e atinge espessuras de até 900 m, e, como continuidade ao sul

da Bacia de São Luís, tem sido redenominada por Bacia de São Luís-Grajaú (Góes & Rossetti

2001).

O embasamento da Bacia de São Luís-Grajaú é composto de rochas da Bacia do

Parnaíba, de cinturão de dobramentos Gurupi e Tocantins-Araguaia e do Cráton São Luís

(Góes, 1995). O preenchimento sedimentar é representado por três megasseqüências

deposicionais, designadas S1, S2 e S3 (cf. Rossetti, 2001; Fig. 1-1B). A seqüência S1 tem

idade aptiana superior a albiana inferior e inclui as formações Grajaú e Codó em sua porção

basal. A Formação Grajaú consiste de arenitos esbranquiçados, quartzosos, com granulação

fina, e conglomerados quartzosos de origem flúvio-deltaica com contribuição eólica (e.g.,

Rezende, 1997). A Formação Codó constitui-se em folhelhos betuminosos, anidritas, calcários

e arenitos lacustres (Campbell et al., 1949; Aranha et al., 1991). A seqüência S2 depositou-se

durante o Eo/Mesoalbiano, correspondendo à Unidade Indiferenciada descrita na Bacia de São

Luís (cf. Rossetti & Truckenbrodt, 1997), cujas características faciológicas são inferidas

principalmente a partir de dados de subsuperfície e que levam a interpretá-la como registro de

ambientes fluvio-deltaico a marinho marginal (Rossetti, 2001). A seqüência S3 depositou-se

entre o Neoalbiano e um tempo incerto no Cretáceo Superior/Terciário, sendo representada

por sucessões sedimentares atribuídas a sistemas estuarinos de vales incisos, cujas duas

sucessões mais superiores desta seqüência acham-se bem expostas na Bacia de São Luís e

correspondem às formações Alcântara e Cujupe (Rossetti, 2001). A Formação Alcântara

(Albiano superior a Cenomaniano) constitui-se de arenitos bem estruturados, de coloração

característica marrom-chocolate, granulometria fina a média e cimentação de calcita.

Subordinadamente, ocorrem pelitos, conglomerados e calcários. Estes depósitos foram gerados

principalmente em ambiente de ilha-barreira sujeito à ação de ondas de tempestade (Rossetti,

1998). A Formação Cujupe (Neocretáceo ?) é constituída por arenitos cauliníticos, de

granulometria fina a muito fina, bem selecionados, cores branca, rósea ou amarelada, com

estruturas geradas dominantemente por correntes de maré (Rossetti, 1998). Sobrepostos a estas

unidades, ocorrem depósitos terciários atribuídos às formações Pirabas, Barreiras e depósitos

Pós-Barreiras (Rossetti & Truckenbrodt, 1997).

A Formação Codó, objeto deste estudo, representa os depósitos neoaptianos da Bacia

de São Luís-Grajaú. Resgata-se a definição original de Lisboa (1914) e Campbell et al. (1949)

para o termo Formação Codó que aqui se emprega para referenciar folhelhos betuminosos

intercalados a evaporitos e calcários, aflorantes nas regiões em torno das cidades de Codó e

Grajaú, no Estado do Maranhão.

A Formação Codó desperta os mais diversos interesses econômicos e científicos.

Economicamente, esta unidade pode representar a geradora da bacia, dado o seu alto teor de

carbono orgânico associado a folhelhos betuminosos (Fernandes & Della Piazza, 1978). Além

disto, calcário, gesso e argilas nobres têm sido exaustivamente explorados, devido à alta

qualidade para uso industrial (Rezende, 1997). Cientificamente, o posicionamento

estratigráfico da Formação Codó no período Aptiano a coloca como uma chave para o

entendimento da paleogeografia, paleoclima e paleohidrologia do Cretáceo e ajuda a compor

um quadro mais completo a respeito da implantação dos sistemas lacustres ao longo da

margem continental brasileira. O conteúdo fóssil desta unidade, rico em elementos delicados e

raros do Aptiano como, por exemplo, insetos, plantas e peixes, bem preservados em ritmitos

de calcário e folhelho, ainda são atrativos importantes da Formação Codó (Pinto & Ornellas,

1974; Duarte & Silva-Santos, 1993; Popov & Pinto, 2000).

Dado o interesse despertado sob os mais diversos aspectos, vários estudos teceram

considerações sobre os ambientes deposicionais da Formação Codó, com a proposição de

modelos mais ou menos concordantes entre si (e.g., Aranha et al., 1991; Batista, 1992; Paz,

2000; Paz & Rossetti, 2001). Porém, a caracterização precisa do sistema deposicional (p.e.,

Batista, 1992; Paz, 2000) e, principalmente, a origem dos evaporitos (p.e., Rodrigues, 1995;

Rossetti et al., 2000) são temas que permaneceram por ser mais sistematicamente abordados.

Além disto, estudos petrográficos detalhados dos evaporitos nunca foram abordados

anteriormente, tendo merecido apenas uma caracterização geral em dissertação de mestrado

(i.e., Paz, 2000). Da mesma forma, apesar da abundância de calcários e evaporitos em

afloramentos, a Formação Codó havia sido investigada em estudos isotópicos somente com

base em dados de testemunho (Rodrigues & Takaki, 1993; Rodrigues, 1995). Por fim, a

organização estratigráfica desta unidade, e a caracterização e entendimento da origem de sua

natureza cíclica, são temas que foram somente superficialmente abordados em trabalhos

prévios (p.e., Paz 2000; Gonçalves, 2004).

Assim, a situação existente no início desta tese era de um panorama com informações

ainda inadequadas para sustentar solidamente o modelo deposicional dos depósitos aptianos da

Bacia de São Luís-Grajaú. Por outro lado, os estudos preliminares disponíveis sugeriam que

estes estratos tinham grande potencial para servir como modelo de integração de dados

faciológicos, petrográficos e isotópicos, com o intuito de reconstituição de seu sistema

deposicional e entendimento das condições hidrológicas dominantes durante a deposição.

1.2. OBJETIVOS

O objetivo desta tese foi aprimorar (ou redefinir) o modelo paleoambiental da

Formação Codó nas regiões de Codó e Grajaú, bordas leste e sul da Bacia de São Luís-Grajaú,

respectivamente, integrando-se informações faciológicas, petrográficas e isotópicas (C, O, Sr e

S), como ferramenta de análise do sistema deposicional, bem como das condições

hidrodinâmicas dominantes durante a sedimentação. O surgimento de novas exposições em

minas a céu aberto na região em torno da cidade de Grajaú, ainda não documentada na

literatura, mostravam grande potencial de se incrementar as informações de campo. Uma vez

superado este obstáculo (Rossetti et al., 2004), esta região tornou-se promissora para: 1) testar

o modelo deposicional lacustre anteriormente adotado para a região de Codó, confrontando-o

diante de novas evidências; 2) reconstituir os processos de formação dos evaporitos da região

de Grajaú, comparando-os com a área de Codó; e 3) auxiliar no entendimento do significado

das variações do sinal isotópico dos depósitos das áreas de estudo comparando-se ambientes

deposicionais em duas áreas distintas da bacia.

1.3. ÁREA DE ESTUDO

As áreas estudadas neste trabalho localizam-se nas proximidades das cidades de Grajaú

e Codó, Estado do Maranhão (Fig. 1-1C), onde ocorrem os melhores registros aflorantes da

Formação Codó na Bacia de São Luís-Grajaú. Na área de Codó, esta unidade aflora em 3

minas a céu aberto, uma na BR 316, ao lado da Fábrica da Itapicuru Agroindustrial S/A, a 5

Km da entrada para a cidade de Codó, uma na estrada que leva à sede do município de Codó, a

15 Km após o cruzamento com a BR 316, e outra na localidade de Santo Amaro (Fig. 1-1D).

A área de Grajaú possui inúmeras minas rudimentares, sendo que para este estudo foram

utilizados 7 perfis, correspondentes às localidades de Chorado, Barreirinho, Olho D´Água,

Olaria (ou Cerâmica), Faz. São João Oneida, Faz. Pau Ferrado e Faz. Fortaleza, além de um

ponto à beira da estrada, no Km 22 da Rodovia Transmaranhense (MA-026). Estas sessões

foram escolhidas com base na boa preservação dos estratos. Em geral, os perfis apresentam

espessuras em torno de 10 m, com continuidade lateral de até 400 m, como no caso da mina de

gesso a 5 Km da entrada para a cidade de Codó da Fábrica Itapicuru Agroindustrial S/A.

O acesso às áreas de estudo é relativamente fácil, sendo feito pelas rodovias BR-316,

BR-226 e MA-026. Esta última, entretanto, é intransitável em certos pontos como, por

exemplo, entre os municípios de Arame e Entroncamento. Embora a infraestutura seja

precária, é possível encontrar boa hospedagem e até ótima uma alimentação tanto em Codó

quanto em Grajaú.

1.4. MÉTODOS

A fim de alcançar o objetivo proposto nesta tese foram adotados os seguintes métodos:

análise faciológica/estratigráfica, análise petrográfica e análise geoquímica (elementos

principais, traços e isótopos).

A análise faciológica e estratigráfica foi feita com base em dados de afloramentos,

tendo abrangindo mapeamento vertical e horizontal das fácies deposicionais, observando-se as

seguintes feições: litologia, cor, textura e estruturas deposicionais, geometria da fácies e

conteúdo paleontológico (cf. Tucker, 2003). Os dados foram registrados pela confecção de

perfis litoestratigráficos, mapeamento de superfícies-chaves e documentação fotográfica de

seções panorâmicas. Adicionalmente, procedeu-se com a coleta orientada de amostras para

estudos de laboratório. Assim, tornou-se possível detalhar a geometria das fácies e reconhecer

superfícies de importância estratigráfica. Esta metodologia tem sido adotada com sucesso em

diversos trabalhos realizados na Bacia de São Luís-Grajaú (p.e., Anaisse, 1999; Lima &

Rossetti, 1999; Paz, 2000; Rossetti & Góes, 2000; Rossetti et al., 2000; Paz & Rossetti, 2001).

A análise petrográfica consistiu no estudo de 220 seções delgadas de carbonatos e

evaporitos. A caracterização petrográfica das lâminas foi feita pela: 1) determinação da

composição dos carbonatos, através do seu tingimento com alizarina vermelha-S, ferricianeto

de potássio e ácido clorídrico diluído em água destilada; 2) identificação e estimativa dos

constituintes da seção delgada; e 3) determinação do relacionamento entre as diversas fases

mineralógicas. Como ferramenta auxiliar à microscopia óptica convencional (i.e., microscópio

petrográfico), aplicação de microscópio eletrônico de varredura (MEV) e espectrometria de

energia dispersiva de raios-X (EDS), do Laboratório de Microscopia do Museu Goeldi,

ajudaram a solucionar problemas de definição de aspectos morfológicos, composicionais e

mineralógicos observados nas amostras e seções delgadas estudadas.

A análise geoquímica elementar consistiu na determinação de Ca, Mg, Fe, Mn, Sr e Na

das amostras de rocha total de carbonato, a fim de investigar possíveis modificações

diagenéticas. Estes elementos são os mais importantes na estrutura da calcita tanto marinha

quanto não marinha e, considerando que os teores destes elementos mantiveram-se mais ou

menos constantes através do Fanerozóico (cf. Holland, 1978), uma comparação adicional com

valores comumente esperados em águas de formação poderia detectar potenciais influências

diagenéticas. O procedimento consistiu na secagem de 1,5 g de 70 amostras a 1000

C por

duas horas, sendo fundidas com tetraborato de lítio e fluoreto de lítio, fornecendo pastilhas

fundidas que foram analisadas pelo espectrômetro de fluorescência de Raios-X do Laboratório

de Isótopos Estáveis da Universidade Federal de Pernambuco (LABISE/UFPE), aos cuidados

da Profa. Dra. Valderez Ferreira.

A análise geoquímica isotópica consistiu na determinação das razões isotópicas de C e

O em carbonatos e de S e Sr em evaporitos da Formação Codó, visando testar variações no

regime hidrológico do ambiente deposicional, que ajudassem a entender e compor um quadro

mais refinado do sistema deposicional. Esta caracterização tem se mostrado bem sucedida em

diversos trabalhos visando-se estabelecer arcabouços hidrológicos e climatológicos, tanto em

ambientes modernos, quanto em depósitos de ambientes antigos (p.e., Chivas et al., 1991; Bird

et al., 1991; Bellanca et al., 1992; Camoin et al., 1997; Chaves & Sial, 1998; Li & Ku, 1997;

Neumann, 1999; Playà et al., 2000; Ortí e Rosell, 2000; Lu et al., 2001; Wignall & Tretchett,

2002; Krull et al., 2004).

Maiores detalhamento sobre os procedimentos utilizados durante as análises

geoquímicas encontram-se inseridos nos capítulos respectivos.

1.5. GEOQUÍMICA ISOTÓPICA

1.5.1. Fundamentação teórica

A geoquímica isotópica como ferramenta para auxiliar na interpretação paleoambiental

de depósitos lacustres é uma abordagem relativamente nova (Stuiver, 1970). Este tema

representa o cerne dos capítulos 4 e 5 desta tese e prima pelo ineditismo deste tipo de

abordagem nos depósitos da área de estudo. Merece, portanto, que seus fundamentos teóricos

sejam revisados em mais detalhes ao longo deste item.

Isótopos são átomos que possuem o mesmo número atômico, mas que apresentam

diferentes números de massa, conseqüentemente de quantidades diferentes de nêutrons em seu

núcleo. Na natureza, os elementos químicos são formados por uma mistura de, pelo menos,

dois isótopos, onde um deles em quantidade abundante e o outro, subordinada. Os isótopos

podem também ser considerados estáveis ou instáveis, sendo que a estabilidade é um fator

relativo ao tempo de decaimento radioativo muito longo, da ordem de 10

anos para os

isótopos mais leves (massa atômica ≤ 34). Como ocupam a mesma posição na tabela

periódica, formam um mesmo elemento químico e compartilham das mesmas propriedades

químicas, mas por causa do número de massa distinto, apresentam propriedades físico-

químicas (p.e., densidade, temperatura de congelamento e de ebulição, viscosidade, pressão de

vapor) sutilmente distintas entre si. Esta distinção nas propriedades físico-químicas dos

isótopos que constituem um mesmo elemento químico é chamada efeito isotópico.

Por causa do efeito isotópico, durante eventos físico-químicos de transformação ou

transição dos elementos, estes acabam por se tornar mais enriquecidos ou mais empobrecidos

(=depletados) em alguns dos isótopos que os constituem. Este efeito é conhecido por

fracionamento isotópico, que ocorre fundamentalmente por troca isotópica ou por efeito

cinético.

O fracionamento por troca isotópica diz respeito aos processos em que não ocorrem

mudanças ordinárias no sistema químico, mas na distribuição dos isótopos entre diferentes

substâncias químicas, fases, ou ainda, moléculas (cf. Hoefs, 1980). Simplificadamente, a troca

isotópica pode ser entendida como um caso especial de equilíbrio químico:

+ bB

↔ aA

+ bB

onde A e B representam os elementos que contêm as moléculas leve ou pesada 1 ou 2. O

fracionamento por efeito cinético diz respeito àquelas reações químicas unidirecionais, em que

as taxas de reação entre os reagentes e os produtos são sensíveis às diferenças de massas dos

átomos. Quando o fracionamento ocorre em equilíbrio isotópico (i.e., sem modificações no

sistema físico-químico durante a reação), os isótopos são redistribuídos proporcionalmente de

acordo com um fator de fracionamento da reação. O fator de fracionamento α entre duas

substâncias A e B é a razão R

do número de quaisquer dois isótopos na substância A dividida

pela razão R

deste dois isótopos na substância B, de tal maneira que:

A-B

= R

O fator de fracionamento em equilíbrio isotópico é dependente da temperatura,

experimentalmente determinável e diretamente relacionado à constante de equilíbrio da

reação. Em termos absolutos, os valores das razões isotópicas são números muito pequenos e,

por convenção, são expressos em notação δ, que expressa o desvio (em partes por mil) da

razão isotópica medida em relação a uma amostra padrão, de acordo com a expressão:

δ = ((R

amostra estudada

– R

padrão

) / R

padrão

) * 1000

Existem diferentes padrões para cada isótopo estudado. Entre os principais, estão o

PDB (PeeDee Belemnite), usado para expressar razões isotópicas de C e/ou O em amostras de

carbonatos, o SMOW (Standard Mean Ocean Water), usado para expressar razões isotópicas

de H e/ou O em amostras de água, silicatos e, ainda, carbonatitos, e o CDT (Canyon Diablo

Troilite – um composto de FeS oriundo de meteorito), usado para expressar as razões

isotópicas do S em amostras dos mais diversos materiais. A principal fornecedora destes

padrões ao redor do mundo são a Agência Internacional de Energia Atômica (Áustria) e o

National Bureau Standards (EUA). Alguns destes padrões já se exauriram ou estão muito

perto disso. Por isso, tem sido comum a adoção de padrões alternativos àqueles originais,

reconhecidos pelo acréscimo da partícula V- (

Viena-)

ao nome do padrão (por exemplo, V-

PDB, V-SMOW e V-CDT).

1.5.2. Aplicação em depósitos lacustres

O princípio do estudo de isótopos estáveis em depósitos lacustres é que tanto

carbonatos quanto evaporitos precipitam-se em equilíbrio isotópico com a água do lago

(Stuiver, 1970; Talbor, 1990; Talbot & Kelts, 1990). Os isótopos estáveis leves mais utilizados

em estudos de reconstituição paleoambiental de depósitos lacustres são carbono, oxigênio e

enxofre.

O carbono possui três isótopos (i.e.,

C e

C). O

C é o mais abundante (~99%),

sendo que em depósitos muito antigos (acima de 50.000 anos), todo

C já sofreu decaimento

radioativo para as formas estáveis. O carbono sofre fracionamento por troca isotópica durante

a transformação do gás carbônico da atmosfera em bicarbonato dissolvido e precipitação do

bicarbonato em carbonato dentro do lago. O fracionamento por efeito cinético ocorre durante a

fotossíntese, que favorece o acúmulo de

C mais “leve” na matéria orgânica do lago e torna a

razão isotópica δ

C residual na água do lago mais “pesada”. Em sistemas lacustres, a variação

da composição isotópica de C depende da produtividade do lago, da estratificação da lâmina

de água, do tipo de degradação bacteriana, da geologia da área fonte, do tipo de vegetação da

área de captação da bacia e da troca de gás carbônico na interface água-ar (Stuiver, 1970;

Hoefs, 1980; McKenzie, 1985; Herczeg, 1988; Li & Ku, 1997; Talbot & Lærdal, 2000;

Wignall & Twitchett, 2002). Durante períodos em que o nível do lago está mais alto (Fig. 1-2),

processos como maior respiração do lago, elevado consumo de matéria orgânica, alto

soterramento de carbono orgânico, maior influxo de águas fluviais teoricamente com

bicarbonato mais “leve”, baixa troca de gás carbônico com a atmosfera e menor participação

do gás carbônico metanogênico na formação do carbono orgânico total dissolvido na água do

lago, favorecem que o δ

C da água do lago seja mais “depletado” do que em períodos em que

o nível do lago está mais baixo (cf. Talbot, 1990; Talbot & Kelts, 1990; Guzzo, 1997).

O oxigênio também possui três isótopos (

O e

O). O

O é o mais abundante,

respondendo por mais de 99% do oxigênio disponível na natureza. Em geral, a razão mais

usada é

O por causa das maiores abundâncias em relação ao

O e da maior diferença de

massa entre eles. O oxigênio sofre fracionamento por troca isotópica, durante as fases do ciclo

hidrológico (Fig. 1-3A), quando se torna mais pesado nas fases residuais, e por efeito cinético,

durante a fotossíntese, quando provavelmente contribui significaivamente com valores

bastante negativos produzidos durante a respiração (cf. Hoefs, 1980, p.122). Embora a

temperatura seja o fator mais importante no fracionamento do oxigênio, a variação da

composição isotópica do O nas águas do lago refletem preferencialmente variações no balanço

hídrico da bacia (cf. Lister et al., 1991; Fig. 1-3B). Durante períodos de nível do lago mais

alto, o δ

O tende a se tornar mais negativo dado o aporte de águas depletadas em

O. O

período de rebaixamento do nível do lago que se segue favorece a evaporação de

O e torna a

água residual do lago mais enriquecida em

O. Depósitos cretáceos lacustres da região do

Recôncavo na Bahia que registram este comportamento isotópico são interpretados neste

sentido (p.e., Guzzo, 1997).

O enxofre possui quatro isótopos (i.e.,

S e

S), dos quais os mais abundantes

são o

S (~ 95%) e o

S (~ 4%). O fracionamento de enxofre em sedimentos ocorre quase

exclusivamente por redução bacteriana de sulfato para H

S (Fig. 1-4). A troca isotópica

durante processos de redox provavelmente desempenha um papel importante ainda não bem

definido no registro antigo (Lu et al., 2001). Desta maneira, períodos em que a estratificação

da coluna d’água no lago é mantida por um longo período favorecem a proliferação de

bactérias sulfato redutoras, que por sua vez promovem o enriquecimento em

S do sulfato

residual na água do lago.

Fig. 1-4: Ciclo resumido do enxofre, enfatizando a passagem do sulfato para sulfeto de hidrogênio por respiração

anaeróbica de bactérias sulfato-redutoras (modificado de Hoefs, 1980).

Sr não é um isótopo estável leve, uma vez que surge como produto de decaimento

Rb. Entretanto, a análise da razão

Sr/

Sr tem acompanhado diversos trabalhos

orientados por isótopos estáveis, uma vez que também apresenta um tempo de meia-vida

bastante longo, comparável ao dos isótopos estáveis leves (Holser et al., 1996). Ao contrário

dos isótopos leves, o Sr não sofre fracionamento no ambiente sedimentar e é facilmente

capturado pela cristalização de evaporitos, que reflete exatamente a composição isotópica da

água da qual se precipitou. Outro aspecto curioso, é que as razões dos isótopos de Sr são

escritas diretamente de um isótopo em relação ao outro, sem necessidade de padrões de

comparação. Os principais meios de alteração da razão

Sr/

Sr numa bacia deposicional são

através de maior influxo fluvial, que favorece o aporte de Sr oriundo do intemperismo de

rochas continentais, em geral produzindo valores superiores a 0,707, e a “troca” de Sr com

basalto de crosta juvenil, cujas razões

Sr/

Sr são iguais ou inferiores a 0,706, mas que só

ocorre em ambiente hidrotermal marinho abaixo das cordilheiras meso-oceânicas (Fig. 1-5;

Holser et al., 1996). Em sistemas continentais, portanto, a variação na razão

Sr/

Sr está

diretamente ligada a variações no aporte fluvial da bacia de captação do sistema (cf. Rhodes et

al. 2002).

Este quadro mostra que a deposição de carbonatos e evaporitos em lagos reflete

fortemente as condições paleohidrológicas atuantes durante a deposição. As variações na

composição isotópica destes sedimentos revelam mudanças no balanço físico- e/ou bioquímico

na água do lago e/ou na água vadosa da partes emersas do ambiente lacustre. Eventualmente,

estas variações isotópicas refletem indiretamente perturbações de ordem climática e, quiçá,

tectônica.

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2. LINKING LACUSTRINE CYCLES WITH SYN-SEDIMENTARY

TECTONIC EPISODES: AN EXAMPLE FROM THE CODÓ

FORMATION (LATE APTIAN), NORTHEASTERN BRAZIL

2.1. ABSTRACT

The Codó Formation exposed in the eastern Grajaú Basin consists mostly of black

shales, limestones and evaporites arranged into several shallowing-upward cycles formed by

progadation of lake deposits. Three ranks of cycles are distinguished. The lower rank cycles

correspond to milimetric interbeddings of: bituminous black shales with evaporites,

calcimudstones or peloidal wackestone-packstone; grey/green shale with calcimudstone,

peloidal wackestone-packstone or ostracodal wackestone/grainstone; and ostracodal

wackestone/grainstone and/or calcimudstones with cryptomicrobial mats and ooidal/pisoidal

packstones. The intermediate rank cycles average 1.7 m thick and are formed by complete and

incomplete cycles. Complete cycles show a transition from central to intermediate and then to

marginal facies associations and include two types: C1 cycle with central lake deposits

consisting of evaporites and black shales; and C2 cycle with central lake deposits formed by

grey/green shale. Complete cycles are attributed to the upward gradation from central to

marginal environments of the lake or saline pan-sabkha system. Incomplete cycles are either

successions with central and intermediate facies associations (I1) or successions with

Authors: J.D.S. Paz & D.F. Rossetti. Geological Magazine, in press.

intermediate and marginal facies associations (I2), which represent shallowing-upward

successions where at least one facies association is lacking. The higher rank cycles average 5.2

m thick and consist of four depositional units that display shallowing-upward successions

formed by complete and incomplete intermediate rank cycles that vary their distribution

upward in the section, and are bounded by sharp surfaces.

While the lower rank cycles display characteristics that reveal their seasonal signature,

detailed sedimentological characterization and understanding of stratal stacking patterns

related to the intermediate and higher rank cycles supports a genesis linked to syn-sedimentary

tectonic activity. This is particularly suggested by the high facies variability, limited lateral

extension, and frequent and random thickness changes of the intermediate-rank cycles.

Additionally, the four higher rank cycles recognized in the Codó Formation match with

stratigraphic zones having different styles of soft-sediment deformation structures attributed to

seismic activities. Therefore, the several episodes of lake shallowing recorded in the Codó

Formation are linked to seismic pulses that alternated with sediment deposition. This process

would have created significant changes in the lake water level, and resulted in sharply

bounded successions internally displaying deeper to relatively shallower facies associations in

the upward direction.

Key-words: lake system, cyclic sedimentation, evaporites, syn-sedimentary tectonics,

Late Aptian, Brazil.

2.1. INTRODUCTION

Shallowing-upward cycles are one of the most common characteristics of lacustrine

successions, and many authors have related them to orbital forcing (e.g., Olsen, 1986; Glenn

& Kelts, 1991; Olsen & Kent, 1996, 1999; Juhász et al., 1997; Vugt et al., 1998; Steenbrick et

al., 2000; Hofmann et al., 2000; Aziz et al., 2000). Developments in the understanding of

facies relationship and thickness consistency have also allowed lacustrine cycles to be related

to tectonic pulses. However, only a few studies have clearly demonstrated the linkage between

shallowing-upward cycles and tectonics (e.g., Martel & Gibling, 1991; Anadon et al., 1991).

The Codó Formation (Upper Aptian) exposed in several quarries along the eastern

Grajaú Basin, northeastern Brazil (Fig. 2-1), is a saline lacustrine to sabkha complex

characterized by different ranks of shallowing-upward cycles. This unit, particularly well

exposed in the Codó and Grajaú areas, consists chiefly of bituminous shales, evaporites and

limestones, forming a succession up to 25 m thick. Internally, the Codó Formation displays

several depositional cycles formed by episodes of upward shallowing. Although smaller scale

cycles are probably attributed to seasonal fluctuations, intermediate and higher rank cycles

show features that do not match with climate influence. In this work we present a detailed

sedimentological and stratigraphic analysis of the shallowing upward cycles recognized in the

Codó Formation, in order to demonstrate that their genesis are, at least in part, related to syn-

sedimentary tectonic pulses.

2.3. GEOLOGICAL SETTING

The split up of Africa and South America continents led to the establishment of several

rift basins along the equatorial Brazilian margin. The São Luís and Grajaú basins are

expressive examples of this tectonic phase, occupying together more than 150,000 km

. These

basins are interpreted to represent a unique structural feature formed by combination of pure

shear stress and strike-slip deformation associated with an intracontinental rift (Góes &

Rossetti, 2001). Fault displacement (Fig. 2-2a,b) initiated during the Aptian, resulting in a

shallow basin where the Codó Formation was deposited. During the main rifting in the Albian,

fault offsets reached up to 400 m, culminating with the amplification of the rift system.

Vertical to sub-vertical, normal and reverse faults cut through the entire sedimentary package,

with some continuing downward into the crystalline and Palaeozoic basement. Three main

fault trends can be distinguished (Fig. 2-2a): NE/SW, NW/SE, and less commonly E-W

(Rezende & Pamplona, 1970; Azevedo, 1991

The latter is closely associated with the E-W-

oriented Sobradinho Fault, which represents the continuity of the Romanche Transcurrent

Zone (Pindell, 1985). E-W oriented thrust faults with offsets of a few meters are present in

outcrops located in the southern and eastern margins of the basin (Góes & Rossetti, 2001).

The Codó Formation records the Upper Aptian deposition of the São Luís-Grajaú

Basin. Gamma-ray log correlation in this basin shows that the sedimentary record consists,

from bottom to top, of three major, probably second-order, depositional sequences bounded by

regional discontinuity surfaces, interpreted as sequence boundaries (Rossetti, 2001; Fig. 2-3).

The two lowermost sequences (S1 and S2) are Aptian to Middle Albian in age, and show an

internal tripartite organization into lowstand, transgressive and highstand systems tracts

(Rossetti, 2001), as expected in a complete depositional sequence. The lowstand systems tracts

of sequences S1 and S2 comprise continental (i.e., fluvial, deltaic, eolian and lacustrine)

deposits in the southern margin of the Grajaú Basin, which interfinger with shallow

marine/estuarine deposits to the north in the São Luís Basin. In both sequences, the

transgressive systems tracts are characterized by muddy lithologies with marine fossils that

sharply cover the underlying sandier deposits, resulting in wedges that pinch out southward.

Discontinuity surfaces marked by sharp lithological contrast define the base and top of these

successions and are interpreted as transgressive and maximum marine flooding surfaces,

respectively (Rossetti, 2001). The highstand systems tracts are characterized by deposits

displaying aggradational stratal patterns that grade upward into intervals with an overall

prograding character, formed when the relative sea level started to decline at the end of the

highstand stage. In contrast to sequences S1 and S2, sequence S3 does not show a tripartite

internal organization, but consists of several sharply bounded, fining- and then coarsening-

upward successions. At least the two uppermost ones are partly exposed and record estuarine

systems formed during the Late Cretaceous (Rossetti, 2001).

upward successions. At least the two uppermost ones are partly exposed and record estuarine

systems formed during the Late Cretaceous (Rossetti, 2001).

The Codó Formation correlates with the lowstand deposits of sequence S1 described

above (Fig. 2-3). It is interesting to note that, in the southern margin of the basin, the Codó

Formation contains a horizon marked by soft-sediment deformation sandwiched within

entirely undisturbed deposits, which are attributed to syn-sedimentary seismic activity. Four

zones of deformation were recognized (Rossetti & Góes, 2000), which are briefly summarized

here (Fig. 2-4) due to their close relationship with the shallowing-upward cycles discussed in

this paper. They include: (1) zone Z1, with cracks filled by fine-grained calcite crystals, small-

scale faults, fissures and stylolites inclined at a high angle to bedding; (2) zone Z2, represented

by complex convolute folds associated with thrust faults, pseudonodules, and mound-and-sag

structures, the later requiring alternating periods of deposition and sediment deformation; (3)

zone Z3, which is associated with intraformational boulders up to 2.5 m long, consists of

normal faults and fissures that are vertical to near vertical, present ragged morphologies, with

small, delicate edges, and taper both downward and upward after a few centimeters; and (4)

zone Z4, characterized by shales with irregular convolute folds. This vertical succession of

deformation events were attributed to syn-sedimentary shear stresses associated with the early

rifting that gave rise to the São Luís-Grajaú Basin. These syn-sedimentary seismic pulses seem

to have had a strong influence on the evolution of the Codó lake and saline pan/sabkha

complex and on the origin of its shallowing-upward cycles, as proposed in this paper.

2.4. DEPOSITIONAL ENVIRONMENT

A detailed facies analysis of the Codó Formation was presented elsewhere (Paz &

Rossetti, 2001; Rossetti ert al., 2004). However, given its importance to provide a general

overview on the depositional setting and define the shallowing-upward cycles discussed here,

the main descriptions and interpretations will be summarized in this section, together with

some new information that helps to support the proposed paleoenvironmental model. The

Codó Formation can be described in terms of four facies associations in the study areas (Fig.

2-5), which are attributed to central lake, intermediate lake, marginal lake, and saline

pan/sabkha depositional environments. Ostracods, including two genera, Harbinia and

Candona, are abundant throughout these deposits, occurring either dispersed or as thin (up to

10 cm thick) beds of shell mittens. Disarticulated and articulated shells, including both young

and adult individuals, are present, as are freshwater Charophytes algae.

2.4.1. Central lake facies association:

The central lake deposits (Fig. 2-6a,b) occur at the base of the shallowing-upward

cycles, and consist mostly of bituminous black shale and evaporite (mostly gypsum). The

bituminous black shale is the most frequent facies in this association, occurring as packages up

to 3 m thick. It consists of bituminous shales (Fig. 2-6b), with total organic content up to 30%,

which contain plant remains, pyrite crystals that are as large as 0.5 cm, and lenses of native

sulphur. Silt grain sizes include quartz and feldspar grains, while clays are composed mostly

of detrital (irregular, laminated flakes) smectite and, secondarily, kaolinite and illite.

Bioturbation does not occur in this facies association.

The evaporite facies reaches up to 5 m in thickness and includes massive/macronodular

(Fig. 2-6a) and, subordinately, layered gypsum. Packages of gypsum up to 1 m thick showing

well-developed horizontal lamination are locally present. The massive gypsum forms

unstructured bodies intergraded with macronodular gypsum, the latter consisting of

centimeter-sized nodules with or without a shale matrix. The gypsum nodules display an

almond-like form, defined by a web of undulating, horizontal to oblique fractures filled by

bladed crystals up to 0.5 cm thick and with compromise contacts that grow perpendicularly to

the fracture walls. Rosettes of dark gypsum up to 5 cm in diameter are common in this facies.

The massive/macrogranular gypsum is in sharp contact with the layered gypsum, locally

forming diapirs several meters long. The latter comprises laterally continuous, horizontal

alternating dark/light beds that vary from few mm up to 10 cm thick. The dark beds are

formed by either crystals or micronodules of gypsum less than 0.5 cm long distributed within a

matrix of black shale, while the light beds consist of upward-oriented chevron gypsum

crystals.

2.4.2. Transitional lake facies association:

The transitional facies association (Fig. 2-6c) consists of interbedded grey/green shale

and limestones. Bioturbation is locally present, as are symmetrical ripple marks at the top of

some limestone beds. The grey/green shale is the dominant facies in this association, occurring

particularly well developed in the Grajaú area, where it reaches up to 4 m thick. The total

organic content of this facies is much lower than in the black shale, reaching only up to 1 %.

Films of gypsum or calcite as acicular or fibrous, vertically aligned crystals, are locally present

on bed planes.

The limestones can be described in terms of three facies: calcimudstone, peloidal

wackestone-packstone, and sparstone. The calcimudstone is the dominant limestone facies in

this association. It consists of microcrystalline calcite, locally silicified to chalcedony, which

occurs as laterally continuous, either laminated or massive beds up to 15 cm thick. Grains of

quartz and mica (mostly muscovite) occur disperse or parallel to bed planes. The peloidal

wackestone-packstone facies forms beds up to 20 cm thick, characterized by well sorted,

rounded to sub-rounded, spherical to oblate peloids within a micrite matrix. Disarticulated

ostracod shells and microspherules are frequently present, reaching up to 15 % of the

allochemical grains. This facies is commonly parallel laminated and, less commonly, massive.

Films of microbial mats might be present paralleling bed planes. The sparstone facies (cf.

Wright, 1992) consists exclusively of calcite crystals ranging in size from 250-500 µm and

displaying sutured contacts, arranged as a blocky mosaic.

2.4.3. Marginal lake facies association:

The marginal lake facies association (Fig. 2-6d-g) occurs at the top of the shallowing-

upward cycles and comprises a variety of intergrading lithofacies, including: pelite,

intraclastic grainstone, ostracodal wackestone to grainstone, ooidal/pisoidal packstone,

rhythmite (Fig. 2-6D,E), tufa, and sandstone. Wave-ripple marks are commonly observed

within this facies association, and gypsum occurs locally as discontinuous laminae.

Bioturbation is, in general, absent, though a few undetermined tiny traces were locally

observed.

The pelite facies is endurated, massive, and display a blocky texture (Fig. 2-6f). Its

colour varies upward from olive-green to brownish-red. The intraclastic grainstone (Fig. 2-6g)

consists of well sorted and well rounded, fine- to coarse-grained calcite grains. This facies

forms structureless and, less commonly, tabular and sigmoidal cross-stratified beds that are up

to 20 cm thick. Mm-cm long lensoid cavities (i.e., fenestrae), locally filled by mosaics of

sparite, are typical of this facies, as are microkarstic structures, and vadose meniscate calcite

cement. Ostracodal wackestone to grainstone occurs as beds or concretions up to 15 cm thick,

the latter typically displaying articulated shells with a mixture of young and adult individuals.

Ooidal/pisoidal packstone consists of coalescing, well-rounded or elongated ooids and pisoids

(diameter<5 mm) displaying internal botryoidal or fibrous radial fabric. This facies forms

layers up to 5 cm thick, but which are laterally continuous for the entire extension of the

exposures (at least hundreds of meters). Rounded to slightly elongated fenestrae are abundant

in this facies. The tufa facies is a highly porous limestone with sparry calcite filaments

arranged into a dendritic web. Rounded calcite grains less than 200 µm of diameter occur

encrusting the branches. Rhythmite occurs as packages 10-20 cm thick of thinly (millimeter-

scale) laminated, ostracodal wackestone-grainstone, shale, and less commonly, siltstone beds.

Fish are abundant in this facies. Cryptomicrobial mats are abundant in association with

ooidal/pisoidal packstone and rhythmites, being recognized in the field as black, corrugated

films parallel to bed planes.

The sandstone facies is structureless and occurs only locally as lenses up to 0.5 m

thick, being represented by moderately sorted, well rounded, fine-grained, quartz grains.

2.4.4. Saline pan/sabkha facies association

This facies association is up to 6 m thick, being mostly present in the Grajaú area,

where it occurs between lacustrine deposits. This is an evaporite-dominated association

constituted by layered gypsum, gypsarenite, and secondarily tufa, grey/green shale, and

calcimudstone. The three latter facies will not be described here, as they have the same

characteristics described in previous facies associations. Tufa is much more expressive in this

facies association than in the marginal lake deposits, occurring as beds up to 5 cm thick that

disappear laterally within few meters. A typical feature of this facies association is the

preservation of primary horizontal layering.

The layered gypsum (Fig. 2-6h,i) consists of horizontal beds typically formed by

alternating darker and lighter couplets that become progressively thicker upward, varying from

few mm to 30 cm. The darker beds consist of either crystals or micronodules of gypsum, in

general less than 5 mm long, which occur within a matrix of dark mudstone. The lighter

gypsum consist of upward-oriented crystals displaying aligned twin-planes and superimposed

growth faces with acute angles arranged in zig-zag, perpendicularly to the crystal long axe

(i.e., chevron gypsum); the chevron gypsum intergrades with acicular gypsum.

The gypsarenite is interbedded with the layered gypsum, and forms layers few cm thick

of moderate to poorly sorted, rounded to sub-rounded gypsum grains with sizes varying from

very coarse to pebbly. Calcite cement is common. This facies increases in frequency upward

in this association. Shallow (i.e., <2 mm deep) potholes averaging 3 cm in diameter and with

ragged concave up shapes completely filled up by layers of growth-aligned gypsum crystals,

are present at the top of some gypsarenite beds. A film of yellow to light brownish calciferous

clay is settled out on the bottom of these structures.

2.4.5. Interpretation:

Although displaying similar lithologies, a comparison of the sedimentary features and

facies architecture between the Grajaú and Codó areas revealed some basic differences in their

depositional systems. Hence, the Codó area displays deposits formed in comparatively more

stable, well-stratified lakes with significant periods of anoxia and closure, when evaporites

where formed almost exclusively in deeper, central areas. Facies analysis, added to the

abundance of continental ostracods and Charophyte algae, as well as the absence of any

marine fauna, led us to attribute these deposits to a dominantly lacustrine system. The

abundance of shales in the central lake deposits shows a low energy depositional setting.

Pyrite, bitumen, well-preserved fossils (i.e., ostracods and fishes) and the almost complete

lack of bioturbation support anoxic conditions. Under such circumstance, the preservation of

large amounts of organic matter is favoured, being derived from organisms living in upper,

more oxygenated water layers (Trewin, 1986; Martill, 1988).

The transitional lake facies association is characterized by its position between central

and marginal lake facies associations. The abundance of fine-grained deposits, recorded by

calcimudstones and grey/green shales consistent with a low energy environment with

abundant mud deposition below wave base (e.g., Specht & Brenner, 1979; Shinn et al., 1989).

In this sense, this association resembles central lake deposits, but the lighter colour, suggesting

less preservation of organic matter and absence of bitumen, as well as the local bioturbated

horizons, symmetrical ripple marks, and disarticulated ostracod shells, support a shallower

environment relative to the central lake association, with more oxygenated waters and wave

action.

The marginal lake facies association was recognized by its position at the top of the

shallowing-upward cycles and by the abundance of sedimentary features attributed to

subaerial and/or meteoric exposure, such as palaeosoil (recorded by the massive pelite),

microkarstic surface, fenestrae and meteoric cement. The presence of coalescing pisoids with

variable sizes, elongated shapes that are associated with microbial mats suggests stagnant,

shallow water condition and/or even subaerially exposed environments (e.g., Risacher &

Eugster, 1979; Chafetz & Butler, 1980; Schreiber et al., 1981; Tucker & Wright, 1990). Under

such conditions, microbes lead to in situ formation of pisoids (cf. Ferguson et al., 1978), an

interpretation favoured in the study area due to the close association of pisoids and microbial

mats. Fenestrae support formation close to the vadose zone. The concretions of ostracodal

wackestone-packstone associated with these deposits might have resulted from a microbial

influence (Raiswell, 1976) combined with anisotropic permeability, since the concretions

occur within more permeable limestone/shale rhythmite that overlies less permeable shales.

The local occurrence of sandstone facies in this association is probably related to episodic

influx of sands during raining episodes through small deltas.

As opposed to the Codó area, much more ephemeral conditions prevailed in the Grajaú

area, which presented better-oxygenated water pans having evaporite precipitation only in

their margins and along the surrounding mudflats, configuring a saline pan/sabkha complex.

This depositional setting displays widespread pans and evaporitic flats, the first being

restricted to more depressed local areas of the system. The distinguished depositional setting

recorded in the Grajaú area might have been crucial to control the distribution of the evaporite

deposits. A saline pan/sabkha facies association is indicated by evaporite precipitation in a

very shallow subaqueous environment that was interrupted by periodic exposure. The thin

beds of grey/green shales and calcimudstones are attributed as the record of deeper waters,

probably representing central saline pans. Shallower areas of the pans and surrounding flats

were dominated by evaporite deposition. The dominance of gypsum with well-preserved

horizontal lamination reflects original bedding on flatter-lying environments surrounding the

saline pans. The chevron growth-aligned crystals of the lighter beds attest to primary gypsum

deposition on the floor of brine pools. Precipitation of similar features in many modern and

ancient environments occurs when water depths is less than only 2 m (e.g., Logan, 1987;

Hovorka, 1987; Handford, 1991; Smoot & Lowenstein, 1991). Shallow waters favour a high

degree of supersaturation, as well as a less dense and unstable brines, which are conditions

required to promote the upward growth of crystals. The darker beds in the couplets record

displacive intrasediment growth of crystals beneath the brine from supersaturated pore fluids

in the capillary and/or upper phreatic zone, thus requesting periods of descending ground

waters and eventual exposure (e.g., Kerr & Thomson, 1963, Warren, 1999). These deposits

formed slightly post-depositionally, but still under a strong influence of the depositional

setting as precipitation took place within only a few mm of the depositional surface. The

alternating dark and light gypsum couplets are attributed to pulsating episodes of water table.

The growth-aligned crystals grew up in the interface sediment-brine during times when

saturated brines were flushed into the basin. The formation of such evaporite layers requires a

subaqueous environment with stable phases to allow the aggradation of the upper euhedral

surface of the crystals (Warren, 1999). As the water level falls, precipitation of this type of

crystals is precluded. During these episodes, evaporite precipitation in the study area occurred

only below the sediment interface.

The gypsarenite records moments when evaporite crystals were reworked. Because

these deposits are mostly associated with the layered gypsum, it is possible that its formation

is due to reworking of growth-aligned crystals as the water level slightly decreased, but with

the process occurring still subaqueously. The increased occurrence of this facies upward in the

evaporite section attests to progressive shallowing, an interpretation that is supported by the

presence of numerous potholes attributed to partial evaporite dissolution due to subaerial

exposure. As the evaporite basin was dissected, the deposits were at least momentously kept

above the water level and dissolution took place forming these small depressions. Water

accumulated in these depressions, bringing suspended sediments, that were settle down

forming the clay films. A later submergence of the dissolved planes led to the infill of the

potholes by thin layers of growth-aligned crystals.

2.5. SHALLOWING-UPWARD CYCLES AND DEPOSITIONAL UNITS

2.5.1. Description

Three ranks of shallowing-upward cycles are distinguished in the Codó Formation. The

lower rank cycles display regular thickness ranging from 5 to 10 cm, and consist of facies that

vary according to the position in the lake setting. Hence, the central lake deposits show

interbeddings either of bituminous black shales and evaporites, or bituminous black shales

with streaks of calcimudstone and bituminous black shales with native sulphur. The

intermediate lake deposits display bituminous black shale interbedded with peloidal

wackestone-packstone or grey/green shale interbedded either with calcimudstone or peloidal

wackestone-packstone. The marginal lake deposits show either grey/green shale and

ostracodal wackestone/grainstone, as well as alternations of ostracodal wackestone/grainstone

and/or calcimudstones with cryptomicrobial mats and ooidal/pisoidal packstones. The lower

rank cycles in the saline pan/sabkha deposits consist of alternating dark and lighter gypsum,

consisting of microgranular gypsum grown within shales and vertical aligned gypsum crystals,

respectively. These are locally arranged into packages containing 3 to 7 couplets (Fig. 2-6H); a

succession of 4/7/5/6/6/7/6 couplets was counted at one place.

The intermediate rank cycles vary from a few 0.3 m up to 5.6 m thick (averaging 1.7 m

thick), and contain the lower rank cycles. They can be either complete or incomplete

according to the relative proportion of facies associations representing the several lacustrine

and saline pan/sabkha sub-environments (Fig. 2-7). Hence, complete cycles display facies

associations that record the upward gradation from central to marginal environments of the

lake or saline pan-sabkha system, attesting to the complete preservation of one shallowing

episode. Three different types of complete cycles were recognized (Fig. 2-8): complete cycles

type 1 (C1), where central lacustrine deposits are entirely formed by evaporites and

bituminous black shales; and complete cycle type 2 (C2), where central lake deposits consist

of black and/or grey/green shales and limestones; and complete cycle type 3 (C3), where

grey/green shale and calcimudstone record central saline pan environments, and evaporites

(layered gypsum, gipsarenite) and tufa record shallower saline pan and surrounding evaporitic

flats. Incomplete cycles are defined by shallowing-upward successions where at least one

facies association is lacking. There are also three types of incomplete cycles (Fig. 2-8):

incomplete cycle type 1 (I1), where central and intermediate lake associations dominate;

incomplete cycle type 2 (I2), composed mostly of intermediate and marginal lake facies

associations; incomplete cycle type 3 (I3), consisting of only shallower saline pan and

evaporitic flat deposits.

Fig .2-8: Diagrams illustrating the four types of lower-rank, shallowing-upward cycles of the Codó Formation.

Thickness of individual cycles range from 0.3-5.6 m. See figure 2-7 for legend.

The higher rank cycles define four laterally continuous depositional units, referred as 1

to 4 from bottom to top (Figs. 2-7, 9a,b). Unit 1 is only partly exposed at the base of the

sections and form an interval that reaches up to 3.6 m thick (averaging 2.7 m thick) composed

of thin I1 cycles. Its top is either marked by a horizon of breccia containing clasts of

grey/green shale up to 5 cm long, or by lenses of medium- to coarse-grained sandstones,

which occur along a sharp surface with erosional relief up to 4 m. Unit 2 (Fig. 2-7) reaches up

to 8 m thick (averaging 5.2 m) and contains all types of cycles. Although in number there is a

prevalence of incomplete cycles, this unit displays the highest volume of complete cycles of

the whole Codó Formation exposed in the study area. Up to five successive shallowing-

upward cycles were observed in this unit, with C1 cycles being the thickest and dominant

ones. These occur mainly in profiles 2 and 3 of the Codó area and profiles 7 and 8 of the

Grajaú area (Fig. 2-7). It is noteworthy that the marginal facies of the shallowing-upward

cycles located closer to the top of unit 2 are characterized by the abundance of gypsarenite,

intraclastic grainstone with fenestrae, karstic features and nodular chert (silcrete). The top of

unit 2 is marked by a sharp bounding surface that is either planar or displays an erosional

relief up to 1 m at the outcrop scale.

Unit 3 (Figs. 9b and c), confined to the eastern portion of the study area, is up to 3.8 m

thick (averaging 2.6 m thick) and is comprised mostly (i.e., nearly 80%) of I2 cycles, with the

remaining 20% being represented by C2 cycles. A remarkable and exclusive feature of this

unit is the presence of ooids/pisoids and concretions of ostracodal wackestone-grainstone,

which constitute important stratigraphic markers throughout the study area. The latter occur

invariably at the transition from intermediate to marginal lacustrine deposits, while the

ooids/pisoids are present in marginal lake deposits located close to the top of unit 3. The

concretions bearing articulated ostracod shells include a mixture of juvenile and adult

individuals. These lithologies are interbedded with rhythmites bearing abundant articulated

and disarticulated fish bones, and which also contain a high volume of former microbial mats.

Unit 3 is also bounded at the top by a sharp surface with erosional relief up to 2,5 m.

Fig .2-9: Types of shallowing-upward units present in the Codó Formation, illustrating: a) Unit 2, represented by

one complete cycle type 1 with evaporites and shales. b) Unit 3, presented from base to top by two incomplete

cycles type 1 and one complete cycle type 2. c) The uppermost portion of unit 2 and the base of unit 3, with the

latter showing three incomplete cycles type 1 and an upper incomplete cycle type 2.

The uppermost unit 4 (Fig. 2-9b) is up to 4.6 m thick (averaging 2.2 m thick) and

typically starts at the base with grey/green shales containing only thin (< 1 mm thick) laminae

of gypsite and/or fibrous calcite, both formed by vertically aligned crystals. Upward, the

shales are interbedded with few and thin (mm to few cm) layers of calcimudstones. I1 cycles

are dominant in this interval, but no black shales are present. Secondarily, this unit also shows

I2 cycles. Unit 4 is truncated by a discontinuity surface showing erosional relief up to 5 m,

marked by a red-coloured palaeosol horizon overlain by Albian sandstones and argillites of the

Itapecuru Group (Fig. 2-7).

2.5.2. Interpretation:

The lower rank cycles recognized in the Codó Formation record minor changes in

depositional conditions, attesting to alternations between mud settling and chemical

precipitation of evaporites or limestones. This characteristic, added to the regular thickness

variation, is consistent with seasonal fluctuations, with mud deposition and chemical

precipitation taking place during less and more arid phases, respectively.

The intermediate rank, shallowing-upward cycles reflect periods of progradation of the

lake shoreline resulting from superposition of marginal lake deposits upon intermediate and/or

central lake deposits. The higher rank cycles record several episodes of upward-shallowing

due to lake desiccation, bounded by subsequent floodings. In this sense, they are good

continental analogs for parasequences described in marginal marine settings (Vandervoort,

1997). The maximum shallowing was reached at the top of each unit, and it is marked by

better-developed marginal lacustrine facies associations, evidenced by the features recording

wave reworking and subaerial exposure such as palaeosol, karstic features, fenestrae, and chert

(silcrete) nodules.

Units 1 and 2 reflect the prevalence of anoxic conditions and a water column with

sufficient depth to favour stratification. The abundance of C1 cycles in unit 2, rich in bitumen

and evaporites, is the most representative record of this phase, and is attributed to a phase

when the lake had the maximum relative depth, which is consistent with the fact that in this

unit cycles are thicker than in the other units.

Unit 3 records a time with the greatest development of marginal lake deposits,

suggesting prevalence of shallower water conditions due to lake desiccation. The abundance

of ostracodal wackestone-grainstone displaying a mixture of juvenile and adult, articulated

ostracod shells, as well as both articulated and disarticulated fish bones, attests to episodes of

mass mortality. This event was probably associated to the reduction in the lake area due to

shallowing, which is suggested by the fact that only marginal lake deposits display evidence

for ostracod and fish mortality. Fish mass mortality recorded in lacustrine settings of the

Achanarras Middle Old Red Sandstone of Middle Devonian age from the Orcadian Basin in

Scotlant has also been interpreted in a similar way (Trewin, 1986). The reasons that led to this

rapid drought in the study area will be discussed below.

Unit 4 records a return to dominantly deeper water conditions, but without significant

evaporite precipitation and no black shale formation, as indicated by the prevalence of I1

cycles, characterized by deepest-water deposits with only grey/green shales. This unit is

interpreted as deposited during a time when the lake was oxygenated throughout the water

column and no water stratification was present. The thickness of the shallowing-upward cycles

suggests that the lake depth was again as deep as in unit 2. However, a better understanding of

the conditions leading to the formation of unit 2 is precluded due to the strong erosion

associated with the development of the late Aptian/Albian unconformity (Fig. 2-2).

2.6. ORIGIN OF THE SHALLOWING-UPWARD CYCLES

Analysis of the vertical stratal stacking patterns represented by the shallowing-upward

cycles recognized in the Codó Formation is the key to understand their nature. These cycles

were formed as the lake or saline pan/sabkha base level episodically decreased through time.

In this depositional system, drop in base level, with resulting facies progradation, is caused

chiefly from one or the interaction of the following factors: increase in sediment supply either

from a fluvial drainage or a marine inflow, progressively increased aridity, or increase in

subsidence due to syn-sedimentary tectonics.

In the particular case of the Codó Formation, the influence of sediment supply might

be considered negligible, since siliciclastic sands supplied into the lake system was reduced, as

indicated by their scarcity even in marginal deposits. Deciphering whether climate or

subsidence linked with syn-sedimentary seismic activity, was the main cause for progradation

is not so straightforward, particularly because both factors might be combined in order to

produce shallowing-upward units (e.g., Anadon et al., 1991).

Climate has been claimed to explain many shallowing-upward lacustrine deposits

recorded in ancient and modern environments (e.g., Smoot & Olsen, 1994; Olsen & Kent,

1996; Hofmann et al., 2000; Aziz et al., 2000; Steenbrink et al., 2000). The small, lower rank

cycles, particularly recognized in the saline pan/sabkha deposits of the Codó Formation,

record minor changes in depositional conditions, attesting to alternations between mud settling

and chemical precipitation of either evaporites or limestones. This characteristic, added to the

rhythmic and regular thickness variation, is consistent with seasonal fluctuations, with

alternating mud deposition and chemical precipitation taking place during less and more arid

phases, respectively. This is specially suggested by the succession of evaporite couplets,

which are attributed to pulsating episodes of water table. Although locally observed, packages

displaying regularly distributed bundles varying from 4 to 6 are probably resulting from

seasonal base level changes.

While seasonal fluctuations may be applied to explain the lower rank cycles recognized

in the Codó Formation, the origin of the intermediate and higher rank cycles are probably

related to syn-sedimentary tectonics. A climatic cause seems to be unlikely in these instances

because, if so, then one would expect an increased precipitation of evaporite minerals during

phases of maximum shallowing, when the lake and saline pan level was at the minimum

(Carroll & Bohacs, 1999). This did not happen in the Codó Formation, neither in the

intermediate cycles, nor in the overall higher rank cycles that form the depositional prograding

units described herein.

The intermediate and higher rank cycles recognized in the Codó Formation show

characteristics that are better explained by a tectonic signature. A line of evidence might be

claimed to suggest tectonics as the main cause for the intermeditate rank cycles: 1. the high

variability of facies within individual cycles; 2. the limited lateral distribution, usually on the

order of less than a few tens of meters; and 3. the frequent and random change in cycle

thickness in the upward direction, ranging from few centimetres up to several meters. These

characteristics match better with those typically recorded from tectonically-influenced,

shallowing-upward cycles (Martel & Gibling, 1991; Benvenuti, 2003), as climatically related

cycles are expected to have more regular thicknesses and more monotonous facies distribution

throughout large distances (Hofmann et al., 2000; Harvey, 2003).

The higher rank cycles of the Codó Formation exposed in the Codó area are

particularly suggestive of formation under the influence of syn-sedimentary tectonics. These

units have been correlatable to Grajaú area based on distinctive features of the surfaces

overlying each unit, as formerly discussed in the item 2.5.1. The four higher rank cycles

reflect lower frequency episodes of lake shallowing. The two lowermost units record

progressive episodes of lake shallowing in deep-water, anoxic conditions, when lake

stratification prevailed. Following this phase, progradation of lake shoreline proceeded under

shallower water conditions, as recorded by unit 3. The fossil assemblage preserved in this unit

indicates mass mortality, suggesting that the reduction in the lake area might have been subtle.

Stagnant waters favoured a widespread distribution of microbial mats. A renewed episode of

significant lake deepening took place during deposition of unit 4, which at this time occurred

under dominantly oxidizing conditions.

A tectonic influence is also proposed for the origin of the higher-rank cycles recorded

by deformational structures in the depositional units 1 to 4. This is suggested because these

units have good correspondence with stratigraphic intervals representing different styles of

deformation zones that characterize the Codó Formation in the Codó area. As summarized in a

previous section (see geological setting), these deformation zones (Fig. 2-4) were attributed to

alternating periods of sediment accumulation and shear stress associated with syn-sedimentary

seismic activity linked to the rifting of the São Luís-Grajaú Basin during the Late Aptian

(Rossetti & Góes, 2000). Hence, depositional units 1 and 2 are stratigraphically correlatable

with deformational zones Z1 and Z2 (Fig. 2-10), respectively. In other words, during the two

first episodes of shallowing recorded in the study area, sedimentation in the lake system was

strongly affected first by extensional and, then by compressional forces, as shown by the

upward gradation from small scale cracks and normal faults to complex convolute folds

associated with thrust faults and vertical to sub-vertical stylolites. Deformation would have

initially increased the accommodation space, promoting the development of a thicker higher-

rank shallowing-upward cycle (i.e., unit 1) with increased deeper water facies, as recorded by

a higher volume of C1 and I1 cycles. As the sedimentary succession evolved, the area might

have experienced some compression, with consequent local uplift giving rise to the

development of shallower-water facies at the top of depositional unit 2.

Uplift or more stable tectonic conditions would have produced even shallower-water

environments in the Grajaú area, represented by the saline pan/sabkha deposits that are

represented by complete and incomplete cycles of types C3 and I3, respectively. After this

time, a relative stability seems to have prevailed, with the lake basin becoming progressively

shallower due to low accommodation rate and resulting in deposition of unit 3, which is

characterized by the abundance of I2 cycles. A period of extension led to development of

several normal faults and slumpings at the top of unit 3, which corresponds to deformation

zone Z3. As a result of this extension, new accommodation space was created and the lake

system became relatively deeper, favouring deposition of unit 4, characterized by thicker C2

and I1 cycles. However, anoxic conditions no longer existed during this time, as revealed by

the scarcity of organic matter in the shales of this unit. Tectonic activity seems to have

continued, resulting in convolution of these deposits and producing deformation zone Z4.

Fig .2-10: A representative lithostratigraphic profile of the Codó Formation showing the good match between

higher-rank shallowing-upward cycles, represented by depositional units 2 to 4, and the syn-sedimentary

deformational zones described by Rossetti and Góes (2000). (See Fig. 2-7 for legend).

The seismic interpretation of the shallowing-upward cycles provided here is in

agreement with the structural framework of the São Luís-Grajaú Basin. As previously

presented, the main rift stage of this basin took place during the Albian, but reactivation of

ancient fault systems started earlier in the Aptian, with the establishment of a shallow, but

widespread basin where the Codó Formation was deposited. The presence of deposits with

evidence for soft sediment deformation in this unit is taken as an indication for syn-

sedimentary tectonics (Rossetti & Góes, 2000). At the basin margins, where the study areas

are located, the displacement of faults with small offsets would have favoured the

development of subsiding areas, promoting the establishment of lake systems. The prevalence

of fine- grained and chemical deposits in the depositional setting is not inconsistent with this

model. Tectonically-influenced settings are usually considered to be recorded by a dominance

of coarse-grained siliciclastic deposits. However, modern examples have shown that coarse-

grained deposits will not occur immediately following tectonics. Instead, it has been suggested

that the first response to tectonics in several settings from marine to lacustrine is represented

by fine-grained deposition, as a depositional setting usually takes a time to re-equilibrate and

respond to tectonics (Blair & Bilodeau, 1988). In areas dominated by mild tectonics with fine-

grained and chemical sedimentation, as occur in the Codó Formation, higher rates of

accommodation will give rise to deposition of evaporites and preservation of shales with high

organic content in central lake areas (Carroll & Bohacs, 1999). On the other hand, periods of

quiescence will favour deposition of shallower-water limestones and evaporites as the

accommodation space is reduced. It is possible that the increased sediment reworking,

recorded by the occurrence of gypsarenites and intraclastic grainstones in marginal deposits of

depositional unit 2, might also be related to this tectonic phase. The presence of breccia and

coarse-grained sandstones only at the top of the lowermost higher-rank cycle, represented by

depositional unit 1, is probably an indication for a progressive decrease in the intensity of the

tectonic process through time.

2.7. CONCLUSION

Although more often attributed to climate fluctuations, many shallowing-upward

lacustrine cycles might be resulting from pulsating tectonism taking place contemporaneously

with sediment deposition. The Codó Formation, exposed in the eastern Grajaú Basin, seems to

be an unequivocal example of an ancient lacustrine system displaying two ranks of

shallowing-upward cycles reflecting prograding episodes driven by syn-sedimentary seismic

activity. Despite the seasonal signature recognized in the lower rank cycles, a tectonic cause is

proposed for the intermediate and higher rank cycles described in this unit. In the particular

case of the intermediate rank cycles, a tectonic origin is revealed on the basis of: (1) high

facies variability; (2) limited lateral extension; and (3) frequent and random thickness change.

The higher-rank cycles were also formed as a result of tectonic episodes that alternated with

sediment deposition, a conclusion supported by their matching with stratigraphic zones

characterized by different styles of soft sediment deformation that are attributed to

contemporaneous seismic activity. Based on the observations made in the study area, one can

state that extension affecting lake deposits, with subsequent creation of accommodation space,

promotes the development of prograding successions internally formed by thicker (deeper)

water cycles. Bed shortening by compression and/or stability reduces the water depth and lead

to the development of thinner and shallower-water cycles. Therefore, different styles or/and

intensities of seismic pulses alternating with sediment deposition might cause substantial

changes in lake level, promoting alternating deeper and shallower water phases, and ultimately

resulting in cyclic deposition. Deciphering the genesis of such prograding episodes in ancient

lacustrine successions is a task that requires detailed facies analysis and precise mapping of

the stratal stacking patterns, as well as their association with tectonically-driven structures.

Acknowlegement. The Itapicuru Agroindustrial S/A is acknowledged for the permission to

access the quarries with the exposures of the Codó Formation. This work was supported by the

Conselho Nacional de Desenvolvimento Científico e Tecnológico-CNPq (Grant #460252/01).

The authors want to gratefully thank Paulo Milhomem (Petrobras) for the ostracod

identification.

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3. PETROGRAPHY OF GYPSUM-BEARING FACIES OF THE

LATE APTIAN CODÓ FORMATION

3.1. ABSTRACT

An original and detailed study focusing the petrographic aspects of evaporites from the

Neoaptian Codó Formation exposed in the eastern and southern margins of the São Luís-

Grajaú is presented herein, with the aim to discriminate between evaporites with primary and

secondary origin and reconstruct their post-depositional evolution. Seven phases of evaporite

formation were recognized: 1. chevron (selenite) gypsum; 2. nodular/lensoidal

gypsum/anhydrite; 3. acicular gypsum; 4. mosaic gypsum; 5. brecciated gypsum/gypsarenite;

6. pseudo-nodular anhydrite/gypsum; and 7. rosettes of gypsum. The chevron gypsum, the

nodular/lensoidal gypsum/anhydrite and the brecciated gypsum/gypsarenite display

petrographic characteristics that conform to a primary nature. Their occurrence forming well-

layered, horizontal beds displaying a cyclic arrangement are consistent with this interpretation.

The acicular and mosaic gypsums were formed by replacement of these primary gypsums, but

their formation took place very early in the diagenetic history, being still influenced by the

depositional environment. Noteworthy is that these gypsum morphologies are closely related

to the layered evaporites, serving to demonstrate that their formation were related to

replacements that were mild, not enough to cause any significant change in the primary

Authors: J.D.S. Paz & D.F. Rossetti. Submitted to Anais da Academia Brasileira de Ciências

sedimentary structures. The pseudo-nodular anhydrite/gypsum seems to have originated due to

intrastratal fluids during burial, probably been related to halokinesis. The rosettes of gypsum,

which intercept all the other variety of gypsum, represent the latest phase of evaporite

formation in the study area, resulting from either intrastral waters or surficial waters during

weathering.

Key-words: evaporite, petrography, paleolake, sabkha, late Aptian, São Luís-Grajaú

Basin

3.2. INTRODUCTION.

Ancient evaporites are economically important as sources for salts (e.g., halite and

potash) and metals (e.g., Cu, Zn, Au), and as indicators for structural traps of oil and gas

(Warren, 1999). These deposits are abundant in sedimentary basins located along the Brazilian

continental passive margin, being particularly developed in the Aptian-Albian transition

(Hashimoto et al., 1987; Uesugui, 1987; Silva-Telles, 1996). The evaporites of the Codó

Formation in the São Luís-Grajaú Basin (Fig. 3-1) are the only ones available for surface

studies in the north equatorial Brazilian margin. Detailed facies analysis of fresh exposures in

several open quarries located in the southern and eastern margins of this basin has provided

elements to support a depositional system representing a lacustrine/sabkha complex (Paz &

Rossetti, 2001; Rossetti et al., 2004). In addition, the deposits exposed in these localities

represent an excellent opportunity to better understand the post-depositional processes that

took place after primary evaporite formation. Such approach concerning to the Aptian

Brazilian evaporites from the north equatorial Brazilian margin has not been conducted yet.

A preliminary field investigation showed that the evaporites from the Codó Formation

include several morphologies, suggesting they might not be all primary in origin. Petrographic

studies of evaporites are useful to help deciphering their origin (e.g., Ogniben, 1955; Kerr &

Thomson, 1963; Holliday, 1970; Arakel, 1980; El-Tabakh et al., 1997; Aref, 1998), thus this

type of study applied to the Codó Formation might contribute to better define and/or

reconsider the proposed lacustrine/sabkha system. Furthermore, petrographic studies can serve

as the basis to determine the sequence of events that took place following deposition (e.g.,

Arakel, 1980; Morad et al., 2000; Pérez et al., 2002). Determining the post-depositional

history of these evaporites might help to decide on their potential for future strontium and

sulphur isotopic studies attempting to test the hypothesis of a possible marine influence during

sedimentation, as previously proposed elsewhere (e.g., Batista, 1992; Rodrigues, 1995;

Rossetti et al., 2000).

3.3. GEOLOGICAL SETTING

The Gondwana split up took place through several steps, culminating in the Aptian

with the break up of African and South American continents (Feijó, 1996; Góes & Rossetti,

2001). This process led to final establishment of several rift basins along the equatorial

Brazilian margin, where the São Luís-Grajaú Basin is one of the largest, occupying 150,000

. This basin has been interpreted as a unique structural feature formed by combination of

pure shear stress and strike-slip deformation (Góes & Rossetti, 2001). The main rifting

developed in the Albian, when fault offsets reached up to 400 m and (Fig. 3-2A,B), but fault

displacement initiated in the Aptian, giving rise to a shallow and widespread basin where the

Codó Formation was deposited in the late Aptian. This unit is defined at the top by a

regionally correlatable unconformity marked by erosion and paleosols (Rossetti et al., 2001).

Exposures of the Codó Formation include shallowing-upward deposits attributed to

lacustrine or saline pan/sabkha depositional environments (Paz & Rossetti, 2001; Rossetti et

al., 2004; Fig. 3-3). In the Codó area, where a lacustrine system dominates, the shallowing-

upward cycles consist of bituminous black shales, evaporites and, subordinately,

calcimudstones attributed to central lake depositional environments. These deposits grade

upward into gray/green shale and limestones (i.e., calcimudstone, laminated to massive

peloidal wackestone to grainstone, and sparstone), attributed to intermediate lake

environments. The top of the shallowing-upward cycles comprises massive pelite, shale,

limestone (e.g., intraclastic grainstone, ostracodal wackestone/grainstone, ooidal/pisoidal

packstone, tufa) and rhythmite of limestone and shale. These lithologies display with a variety

of sedimentary features consistent with marginal lake deposition, such as paleosol, karstic

surface, fenestrae, meteoric calcite cement, vadose pisoid. In the Grajáu area, a saline

pan/sabkha depositional environment was proposed (Rossetti et al., 2004), being represented

by evaporite and locally tufa, gray-green shale, and calcimudstone.

Fig. 3-3: Depositional model proposed for the Codó Formation in the Codó and Grajaú areas (cf. Paz & Rossetti,

2001).

The evaporite facies consists of layered gypsum (formed by alternating darker and

lighter couplets that are up to 15 cm thick), gypsarenite, and massive-macronodular gypsum,

which record deposition in flat lying areas, intraformational reworking of previously deposited

evaporites, and remobilization of salts during burial, respectively. Layered gypsum occurs in

both of the study areas, being dominant in the Grajaú area, while gypsarenite was observed

only in the latter. Massive/macronodular gypsum is widespread in the Codó area, occurring

mostly in the nucleus of evaporite lenses that are up to 5 m thick, which enclose chunks of

black bituminous shales. In the Grajaú area, these deposits form diapiric features that are up to

6-8 m high at the outcrop scale. The contact between the massive-macrogranular gypsum and

the layered gypsum is gradational, except locally where the diapirs are well defined by sharp

boundaries.

3.4. PETROGRAPHY OF THE EVAPORITES

The evaporite facies from the Codó Formation were characterized petrographycally

through the analysis of 86 thin sections distributed along four vertical sections (Fig. 3-4).

Based on morphology and crystal relationships, eight phases of evaporite formation were

recognized, which include (Fig. 3-5): 1. chevron (selenite) gypsum; 2. nodular/lensoidal

gypsum/anhydrite; 3..acicular gypsum; 4. mosaic gypsum; 5. brecciated gypsum/gypsarenite;

6. pseudo-nodular anhydrite/gypsum; and 7 rosettes of gypsum. In addition to these evaporite

phases, an episode of carbonate (calcite/dolomite) cementation/replacement took place in

these deposits.

3.4.1. Chevron gypsum (selenite)

This type of gypsum was recorded in both areas, however it is much more widespread

in sections located in the Grajaú area. The chevron gypsum, which represents the lighter

component of the layered gypsum facies, was described in a previous work (e.g., Rossetti et

al., 2004), and it forms horizontal beds that are up to 10 cm thick of vertically aligned crystals.

Under the microscope, the selenite crystals form twin planes and superimpose growth faces

with acute angles arranged as a zig-zag, perpendicularly to the crystal long axis (Fig. 3-6A).

The selenite crystals usually grew up or are mantled by thin discontinuous layers of shales

(mostly smectites). Some thicker layers of chevron gypsum might display packages of crystals

that slumped down into the muds, resulting in a series of segments with superimposed lower

concave-up shapes.

Fig. 3-5: The several phases of evaporites recognized in the Codó Formation, with paragenesis indicated by

lateral position of the boxes.

3.4.2. Nodular/lensoidal gypsum/anhydrite

This evaporite was recorded only in sub-surface, occurring in association with shales.

It consists of either nodules of gypsum or isolated lensoidal gypsum crystals that are parallel

to bedding planes (Fig. 3-6B,C). The nodules are up to 5 mm long, and internally composed of

a dark mixture of anhydrite and gypsum that form a massive cryptocrystalline framework.

3.4.3. Acicular gypsum

The acicular gypsum occurs invariably in association with the chevron gypsum,

likewise forming vertically oriented crystals similar to needles (Fig. 3-6D). Although areas

with dominance of acicular gypsum were observed, in general these types of gypsum are

intergraded, resulting in a closely interlaced framework. Noteworthy it is the frequent presence

of relics and/or ghosts of the chevron gypsum within the acicular crystals.

3.4.4. Mosaic gypsum

This type of evaporite occurs particularly within the dark bundles of the layered

gypsum. It consists of inequidimensional crystals of gypsum averaging 300 µm in length that

are arranged into a mosaic framework. The contact between crystals is usually ragged and

sutured. The mosaic gypsum typically displays an abundance of relics of floating anhydrite,

forming a poikilitic texture (Fig. 3-6E).

3.4.5. Brecciated gypsum/gypsarenite

This type of evaporite occurs most frequently intergrading with the mosaic gypsum

within the darker bundles of the layered gypsum facies. It consists of gypsum that occur as

clasts ranging from µm to 1 cm in diameter (Fig. 3-7A,B). The clasts might be either angular

to subangular, when a fitted texture is recognized, or rounded, forming locally a texture that is

similar to gypsarenite (Fig. 3-7C). The clasts are surrounded by either a continuous or

discontinuous dark cutan formed by a mixture of clay, organic matter and iron oxides. Under

crossed nichols, several clasts might display optical continuity, forming large crystals that can

reach up to 3 mm of diameter (Fig. 3-7D). These contain abundant relics of anhydrite.

3.4.6. Pseudo-nodular anhydrite/gypsum

At the outcrop scale, this evaporite phase consists of either slightly elongated or

spherical fragments up to 5 cm long of evaporites displaying a fitted texture (Fig. 3-8A-C).

Petrographically, the clasts are composed of alabastrine gypsum, fibrous gypsum, and

anhydrite. In general, alabastrine gypsum is the most widespread, occurring as a mosaic of

crystals less than 200 µm in diameter (Fig. 3-8D). The crystals are very limpid, i.e., typically

with no inclusions. The fibrous gypsum occurs secondarily as a series of parallel columns that

can reach up to 1 cm long, which grew within the fractures that define the clasts, being

perpendicular to fracture walls (Fig. 3-8A-C). Unlike to the alabastrine, the fibrous gypsum

might display relics of anhydrite. In addition, ghosts of fibrous crystals may occur within the

alabastrine gypsum (Fig. 3-8D). In places, some elongated clasts are composed of

microcrystalline lath-like anhydrite (Fig. 3-8E). Where these clasts are in contact with mosaics

of large gypsum crystals similar to the ones described above, the latter form concave

embayments toward the anhydrites, leaving a “dust” of anhydrite relics behind (Fig. 3-8E).

Inasmuch, both the anhydrite clasts and the mosaic gypsum might display etched margins as

they grade into fibrous and alabastrine gypsum. Electronic scanning microscopic analysis of

the anhydrite clasts reveal a mixture of anhydrite and limpid gypsum crystals that are similar

to those observed in the nodular/lensoidal gypsum/anhydrite (Fig. 3-9A, compare with Fig. 3-

6C). Noteworthy is that the anhydrite clasts occur as spots that intergrade with alabastrine and

fibrous gypsum.

3.4.7. Rosettes of gypsum

Rosettes of amber-colored gypsum occur disperse within the evaporites of the Codó

area, being restricted to the massive/macronodular gypsum (Fig. 3-8C). The rosettes reach up

to 5 cm of diameter, and consist of fibrous crystals arranged into a radial pattern. The rosettes

unconformably cut into all the other gypsum phases described above.

The carbonates associated with the gypsum/anhydrite consist of calcite cement filling

fractures in the brecciated gypsum/gypsarenite (Fig. 3-9B), even where this facies is

obliterated by pseudo-nodular anhydrite/gypsum. The calcite also occurs as enlarged crystals

that grew upon the gypsum from both sides of the fractures (Fig. 3-9C), in which case it forms

caries in the mosaic gypsum. On the other hand, the alabastrine gypsum enters into relics of

calcite, forming caries toward them (Fig. 3-9D). Rhombs of calcite after dolomite are locally

present (Fig. 3-9E). Celestite might be also present in association with the carbonates,

occurring as pyramidal crystals with etched edges.

Fig. 3-8: Textures of the evaporites from the Codó Formation exposed in the study areas. A) Overlay field

drawing illustrating a spot within the pseudo-nodular anhydrite/gypsum with relics of a complex arrangement

formed by mosaic, acicular and fibrous gypsum, bounded by films of calcimudstone. B) Pseudo-nodular gypsum,

formed by fracturing. C) A spot within the pseudo-nodular anhydrite/gypsum, with nodules of anhydrite bound by

films of fibrous gypsum (arrows). Note the superimposed rosettes of gypsum of variable sizes (rg). D) Alabastrine

(gy) and fibrous (gf) gypsum in gradational contacts. The arrows indicate places where the fibrous gypsum is

partly replaced by the alabastrine gypsum, recorded by numerous crystals of the later over the fibrous gypsum,

which in turn remains as diffuse relics. E) Photomicrography of the nodules shown in C, illustrating their

composition of tiny, equant, lath-like crystals (an). Note that the edges of the nodules were replaced by fibrous

(gf) or mosaic (gy) gypsum.

Fig. 3-9: Textures of the evaporites from the Codó Formation exposed in the study areas. A) SEM view of the

pseudo-nodular anhydrite/gypsum, with lath-like anhydrite (an) interlaced with gypsum (gy). This material comes

from a nodule of anhydrite depicted in figure 8C. B) Calcite (ca) cementing fractures in the pseudo-nodular

anhydrite/gypsum (gy). C) A detail of the pseudo-nodular anhydrite/gypsum showing several fractures (arrows)

filled by a mixture of calcite and muds. Note larger calcite crystals (ca) that grew sidewards from the fractures

trough replacement of gypsum (gy). D) Relics of calcite (ca) within alabastrine gypsum (gy) from the pseudo-

nodular anhydrite/gypsum. E) Rhombs of calcite after dolomite (arrows). (Except for the SEM micrography

shown in A, all the other figures were obtained under petrographic microscope with crossed nicols).

3.5.PARAGENESIS

The combination of facies and petrographic characteristics revealed that the foregoing

described evaporites show features supporting both primary and secondary formations (Fig. 3-

10). In addition, the petrographic study also showed that at least great part of the secondary

gypsum might have been formed under influence of the depositional surface, thus reflecting

Fig. 3-10: Model to explain the formation of primary and early diagenetic evaporite phases recorded in the Codó

Formation.

the original brine characteristics. Evidences for primary evaporite precipitation is particularly

recorded in the layered gypsum. First, this is suggested by the preservation of sedimentary

structures forming laterally continuous, horizontal beds. Second, the primary nature is

confirmed by the presence of chevron gypsum in the lighter bundles of this facies. Such

feature is attributed to the progressive upward precipitation of salts on the floor of shallow

(usually less than 2 m deep), supersatured brine pools (e.g., Logan, 1987; Hovorka, 1987;

Handford, 1991; Smoot & Lowenstein, 1991), as suggested in a previous work (i.e, Rossetti et

al., 2004).

recorded in the layered gypsum. First, this is suggested by the preservation of sedimentary

structures forming laterally continuous, horizontal beds. Second, the primary nature is

confirmed by the presence of chevron gypsum in the lighter bundles of this facies. Such

feature is attributed to the progressive upward precipitation of salts on the floor of shallow

(usually less than 2 m deep), supersatured brine pools (e.g., Logan, 1987; Hovorka, 1987;

Handford, 1991; Smoot & Lowenstein, 1991), as suggested in a previous work (i.e, Rossetti et

al., 2004).

The cyclic alternation of selenite beds with dark gypsum containing nodular/lensoidal

gypsum/anhydrite is also a reflex of changes in the depositional conditions. The formation of

these morphologies is consistent with accumulation taking place a few mm beneath the

depositional surface by displacive intrasediment growth of crystals from supersaturated pore

fluids in the capillary and/or upper phreatic zone (e.g., Kerr and Thomson 1963, Warren

1999). As the brine level decreased, nodular/lensoidal gypsum grew displacively within the

underlying sediments, here represented by shales. This is the most common habit of gypsum

precipitated within sediment, either in mudflats or other environments subjected to palustrine

conditions (Magee, 1991).

The presence of nodules composed of a dark mixture of anhydrite and gypsum forming

a massive cryptocrystalline framework is consistent with this interpretation, being related to

increased evaporation, probably due to climatic changes (Rossetti et al., 2004). Similar masses

of mixing gypsum have been considered as the record of sulphate replacements during

seasonal variations next to the depositional surface (p.e., Arakel, 1980; Mees, 1998; Mees &

Stoops, 2003).

The mosaic gypsum was formed by replacement of anhydrite, as indicated by the fact

the abundant remains of this mineral within it. Mosaics of gypsum crystals with sutured

contacts have been interpreted as a non-equilibrium texture of grain interpenetration at low

temperature (cf. Voll, 1960), probably reflecting formation under early diagenesis (e.g.,

Spencer & Lowenstein, 1990). The occurrence of mosaic gypsum in association with the dark

bundles alternated with selenite/acicular gypsum within the layered gypsum might be taken as

an evidence for replacement soon after deposition.

Extreme low brine level with subaereal exposure resulted in gypsum fracturing due to

desiccation, a process that gave rise to in situ brecciated gypsum and ultimately gypsarenite,

the latter recording local reworking at the surface as recorded in other evaporite deposits (e.g.,

Sanz-Rubio et al., 1999; Schreiber & El Tabakh, 2000). The frequent upward gradation from

the brecciated to the gysarenite is consistent with this interpretation. The presence of mud

cutans surrounding the clasts is attributed either to mechanical infiltration from downward

flows or to the adhesion of residues on clast surfaces during reworking.

Following brecciation, there was a phase of widespread replacement, which gave rise

to mosaics of large gypsum crystals affecting the dark beds of the layered gypsum facies. This

sequence of event is proposed based on the observation that large gypsum crystals encompass

several clasts, attesting to pervasive cementation and/or replacement. The presence of

abundant relics of microcrystalline lath-like anhydrite within the mosaics attests to their

formation following a period of anhydritization. Noteworthy is the fact that the mosaics were

developed only in the darker bundles of the layered gypsum, not affecting the lighter

components. Instead, the chevron gypsum was replaced by acicular gypsum, as indicated by

the presence of ghosts of selenite within the latter. This morphology reflects crystal growth

from very pure, supersaturated fluids, which favors extreme elongation parallel to the c-axis

(Magee, 1991). The close association of the acicular gypsum with the selenites leads to

suggest that formation fluids were driven from remobilization of sulphates from the selenites,

whose formation naturally required high brine saturation. It is important to mention that

neither of these authigenic processes were enough to destroy the primary lamination. This is

taken as evidence to suggest that the formation of mosaic and acicular gypsum took place

cyclically close to the depositional surface just shortly after formation of the individual

bundles.

Fracturing at the surface created a porosity that was cemented by calcite. Calcite also

replaced gypsum near the fracture sides. Under subaerial conditions (i.e., vadose to freshwater

phreatic), sulphate-undersaturated pore fluids dissolves gypsum and/or anhydrite and release

for precipitation of calcite as the CO

has more affinity with calcium than with SO

(cf. Back et al., 1983). Rare dolomite rhombs might have been formed either by replacement

of calcite or simultaneous dolomite precipitation. The dolomite was in turn replaced by calcite.

Dedolomitization is a process that might be closely linked to karstification (Cañaveras et al.,

1996). This process might have contributed to release Sr

, which combined with SO

promoted precipitation of celestite (cf. Olaussen, 1981; Taberner et al., 2002). The celestite

formation in turn also releases Ca

that increases the Ca/Mg ratio, and could dissolve

dolomite and renew precipitation of calcite (p.e., Back et al., 1983; Kushnir, 1985). The close

association of calcite, dolomite and celestite in the study evaporites of the Codó Formation

suggest these processes as the most likely.

The pseudonodular anhydrite/gypsum is interpreted to represent a later phase of

gypsum formation. First, this is suggested by its occurrence restricted to the

massive/macronodular gypsum, where primary sedimentary structures were almost entirely

lost. The presence of massive gypsum grading into the layered gypsum, and the diapiric

geometry enclosing chunks of black bituminous shales is related to salt remobilization during

halokinesis. Second, the pseudonodular anhydrite/gypsum contains limpid alabastrine and

fibrous crystals, not observed in association with the layered gypsum, which is consistent with

the proposed late formation. Fine crystalline gypsum have been considered as secondary in

origin (Holliday, 1970), forming in consequence of hydration, especially of anhydrite, induced

by diapirism or other mechanism that allow percolation of water through evaporite rocks

(Holliday, 1970; Price & Cosgrove, 1990; Warren, 1999). Fracturing seems to have been the

cause of the pseudonodular aspect of this gypsum. Under stress, probably related to salt

remobilization, part of the gypsum might have had a ruptile behavior, ultimately braking apart

to form individual fragments (e.g., Price & Cosgrove, 1990; Marco et al., 2002). Saturated

fluids percolating along the secondary porosity created by this process would have promoted

precipitation of the fibrous gypsum. In the following, there might have had the enlargement of

the fractures due to forcing caused by crystal growth. While this process took place, most of

the primary sedimentary features, as well as the previous phases of evaporites, became

obliterated. As the salt became mobile, there was a pervasive development of alabastrine

gypsum. The later nature of this phase of gypsum is revealed by its limpid aspect free of

anhydrite inclusions and by the fact that it contains ghosts of fibrous gypsum.

The spots with microcrystalline lath-like anhydrite that occur within the macronodular

gypsum is probably a relic of one of the earliest phase of evaporite precipitation in the study

area The paragenesis reconstituted with basis on petrographic relationships shows that these

anhydrites form nodules that were in part replaced by the mosaic gypsum, as revealed by the

fact that the later form concave embayments that enter into the anhrydrite. The margins of this

gypsum is in turn ragged due to reaction with fibrous and alabastrine gypsum. These

relationships support that the anhydrite nodules had an early development relative to all other

gypsum phases present in the pseudonodular gypsum. If so, then it is possible that the

anhydrite is temporally related to the anhydrite formed in the dark bundles of the layered

gypsum, which represents one of the earliest evaporite phases developed in the Codó

Formation.

The final phase of evaporite formation is recorded by the rosettes of gypsum, as

confirmed by the fact that these truncate all the other evaporite morphologies. Aggregates of

large fibrous gypsum crystals forming rosettes similar to the ones of the study area have been

attributed to the action of either intrastratal waters during burial or surficial waters during

weathering (e.g., Shearman, 1966; Holliday, 1970; Warren, 1999).

3.6.CONCLUSIONS

Despite the many varieties of evaporites recorded in the Codó Formation exposed in

the eastern and southern margin of the Grajaú Basin, these deposits display many petrographic

and faciological attributes that are consistent with an early formation either by primary

precipitation or early replacements when the sediments were still under the influence of the

depositional surface. These types of evaporites prevail particularly in the Grajaú area, where

layered gypsum is the dominant facies. In both areas, burial phases of gypsum seem to have

developed only where gypsum was remobilized during halokinesis.

The lack of significant deep diagenetic modification of the Codó Formation is recorded

also by studies focusing the limestones and shales interbedded with the evaporites. The

limestones are dominated by lithologies consisting of calcimudstones and peloidal

wackestones/packstones with only local evidences of cementation or replacement. These

lithologies usually display primary features, as normal grading and horizontal crenulated

lamination. Inasmuch, the shales associated with the evaporites consist almost entirely by

smectite, with only subordinate kaolinite and illite. Among these clay minerals, smectite is far

the dominant one, being represented by detrital flakes, while the kaolinite and illite are mostly

authigenic but related to pedogenetic horizons (Gonçalves, 2004). These data are consistent

with the proposition that burial did not cause significant textural or mineralogical modification

of the Codó Formation.

The petrographic studies presented here strongly motivate to undertake isotopic studies

in the evaporites of the Codó Formation. The analysis should be carried out using samples

from the layered facies only, which preserves evaporites formed both primarily and shortly

after deposition. For these reasons, these deposits should provide information of the original

brine characteristics. On the other hand, the pseudo-nodular anhydrite/gypsum and the rosettes

of gypsum should not be considered in this type of studies, as they represent later stages of

gypsum formation due to deeper salt remobilization. Inasmuch, the spots of anhydrite within

the pseudo-nodular anhydrite/gypsum should be also discarded in these analyses. Despite the

interpretation that these evaporites might have formed contemporaneously to the early-formed

nodular/lensoidal anhydrite, the microscopic studies revealed they were strongly replaced by

limpid, alabastrine gypsum, being inappropriate for isotopic analysis that can be used for

paleoenvironmental purposes.

Acknowledgements. The Itapicuru Agroindustrial S/A is acknowledged for the

permission to access the quarries with the exposures of the Codó Formation. This work was

financed by the Brazilian Council for Research–CNPq (Project #460252/01).

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438p.

4. GENESIS AND PALEOHYDROLOGY OF A SALINE PAN/LAKE

SYSTEM (LATE APTIAN) FROM THE BRAZILIAN EQUATORIAL

MARGIN: INTEGRATION OF FACIES, SR AND S ISOTOPES

4.1. ABSTRACT

Facies analysis was combined with Sr and S isotope data to unravel the brine source of

Late Aptian evaporites from the Codó Formation exposed in the southern and eastern margins

of the São Luís-Grajaú Basin, northern Brazil. Comparisons of facies distribution between the

two investigated areas show: 1. stable, well-stratified and hypersaline lakes with periods of

anoxia and closure prevailing in the eastern margin of the basin, with salt precipitation only in

more saturated, central basin environments; and 2. prevalence of relatively more ephemeral

conditions to the south, where a saline pan complex developed and evaporite precipitation

took place mainly in marginal salinas and surrounding mudflats. In both areas,

expansion/contraction cycles were formed as sedimentation took place, which was followed

by decrease, and then increase, in isotope values. This, combined with the wide dispersion of

Sr and S isotope data within individual depositional cycles, as well as petrographic and

scanning electronic microscopic data, led to the conclusion that diagenesis in some of the

examined facies (i.e., laminated argillite and gypsarenite) was not enough to modify the

primary texture or the geochemical signature. This is because the diagnetic processes took

Authors: J.D.S. Paz, D.F. Rossetti & M. Macambira. Submitted to Sedimentology

place shortly after deposition or even penecontemporaneously, thus the newly formed minerals

still keep the primary signature, thus serving for paleoenvironmental purposes. A non-marine

brine source is suggested by

Sr/

Sr ratios ranging from 0.707824 to 0.709280, which are

higher than those from Late Aptian seawaters (i.e., between 0.70720 and 0.70735). The δ

varies from 16.12 to 17.89 %

V-CDT)

in the eastern margin of the basin, which is in

disagreement with Late Aptian marine values (13 to 16 %

V-CDT)

)

Both geochemical tracers

were influenced by facies characteristics, and thus a model is provided, where expansion of

saline pan/lake systems led to decreasing

Sr/

Sr values due to the release of

Sr from clay

minerals by internal draining of mud flats. During expansion peaks, the

Sr/

Sr values were

lower because this process is cut off due to submergence of mud flats, added to the

introduction of external

Sr-depleted waters resulting from weathering of Permian to

Neocomian marine limestones and evaporites, as well as Triassic to Neocomian basaltic rocks.

Furthermore, the sulphur isotope values decrease in the southern margin of the basin to a range

that varies from 14.79 to 15.60 %

V-CDT)

probably due to increased evaporation in shallower

water settings.

Keywords: Sr and S isotopes; facies; mineralogy; Late Aptian; evaporites; continental

paleoenvironment; Northern Brazil.

4.2. INTRODUCTION

Distinguishing between marine and nonmarine evaporites has been always challenging

(Hardie, 1984; Brookins, 1988; Hovorka et al., 1993; Plàya et al., 2000), especially in places

where the associated deposits lack diagnostic features to help reconstructing the depositional

environments. This problem increases when evaporites evolve from hybrid brines, which is

probably the most likely in many cases (Denison et al., 1998). Lack of sedimentological

parameters has motivated the use of Sr and S isotopes to help investigating the genesis of

many evaporite deposits throughout the world (e.g., Denison et al., 1998; Hovorka et al.,

1993; Playà et al., 2000;

Schreiber & El Tabakh, 2000). Both elements are abundant and

uniformly distributed in seawater, displaying isotope ratios that vary through time in a known

manner (e.g., Burke et al., 1982; Bralower et al., 1997). Once age is well established, these

isotope tracers might provide reliable information on brine sources. Excursions from the

established seawater curve are taken as indicators of continental or hybrid brines (Claypool et

al., 1980; Burke et al., 1982), as long as other influences such as diagenesis can be discarded.

Late Aptian Codó deposits from northern Brazil (Fig. 4-1A) contains evaporites

formed throughout the last stages of the Gondwana break-up, which ultimately led to opening

of the South Atlantic Ocean. Determining whether the evaporitic brine evolved in a marine or

non-marine setting has important implications for the paleogeographic modeling of the initial

Brazilian Equatorial Margin. Hence, facies analysis and isotope geochemistry based on

Sr/

Sr and

S were applied to help ascertain the genesis of evaporites in the Codó

Formation exposed in the eastern and southern margins of the Grajaú Basin, near the towns of

Codó and Grajaú. Based on sequence stratigraphic modelling (Rossetti, 2001), the first

basinward marine transgression in this basin took place during the Albian when rifting fully

developed. However, the occurrence of evaporites up to the basin margins has led to argue a

widespread marine incursion as early as the Late Aptian (Batista, 1992; Rodrigues, 1995). Sr

and S isotope analysis might help to better define the origin of the evaporitic brine source, and

hence determine when the inland areas of the basin experienced the first marine incursion,

narrowing the estimate of the time when the Brazilian Equatorial Margin established an ocean

connection. In addition, previous facies analysis has attributed the deposits studied herein to a

cyclic record formed by episodes of expansion and contraction of saline pan/lake systems (Paz

& Rossetti, 2001). This study might also contribute to analyse the behavior of Sr and S

isotopes relative to facies changes, an approach not extensively focused in the literature yet,

but which is of great significance for reconstructing past hydrologic patterns.

4.3. MATERIAL AND METHODS

Sedimentary facies information available from previous field studies was combined

with isotopic analysis of 36 samples from the Codó Formation collected along fresh quarry

exposures. Isotopic analyses were carried out on whole-rock gypsum (eventually anhydrite)

from selected facies. The samples were subdivided into three aliquots. The first one was

disaggregated and the sulphate minerals picked from dried material. About 10 mg of powered

sample was dissolved in 2.5 ml HCl (2.5 N) and heated at 70

C during 12 hours to allow

complete evaporation. Then, 1 ml HNO

(3.5 N) was added in order to separate the Sr cation.

The

Sr/

Sr isotopic analyses were performed on mass spectrometers from the

Geochronology Laboratory of the Universidade Federal do Para (Brazil). The ratios were

normalized to

Sr/

=0.1194 in order to correct any mass discrimination, with results

indicating mean values of 0.704120+-30x10

-6

for the NBS 987 standard.

The second sample aliquot (circa 1.5 g) was used to quantify Rb by X-ray fluorescence

(XRF), to analyse the contribution of

Rb to the

Sr/

Sr ratio. Rb/Sr ratios vary between

0.01 and 0.02, indicating no need for corrections due to decay contribution (cf. Clauer, 1976).

The third sample aliquot (0.3 mg) was used for δ

S determination carried out in the

Iso-Analytical Ltd Laboratory, England. Vanadium pentoxide catalyst was added to the

sample in a tin capsule, placed in an automatic sampler and combusted at up to 1700° C. The

combusted gases were then swept in a helium stream over combustion catalysts (tungstic

oxide/zirconium oxide) and through a reduction stage of high purity copper wires to produce

, N

, CO

, and water. Water was removed using a Nafion™ membrane. Sulphur dioxide

was resolved from N

and CO

on a packed GC column at a temperature of 30 °C. NBS-127

(barium sulphate, δ

V-CDT

= +20.3 ‰) distributed by the IAEA, Vienna. NBS-127, IAEA-S-

1 (silver sulfide, δ

V-CDT

= -0.3 ‰) and Iso-Analytical IA-BaSO

(barium sulphate, δ

V-

CDT

= +11.0 ‰) were used for calibration and correction.

Petrographic analysis of all evaporite samples and, whenever necessary, scanning

electron microscopy were performed to provide the best constraints in the evaluation of

possible diagenetic changes of the Sr and S values during burial, and thus decide on their

potential use as paleohydrological indicators.

4.4. GEOLOGICAL FRAMEWORK AND PALEOENVIRONMENTAL CONTEXT

The sedimentary fill of the São Luís-Grajaú Basin consists of three 2

-order

sedimentary sequences (Fig. 4-1B) deposited from the Late Aptian to Quanternary (Rossetti,

2001). The Codó Formation developed during the lowstand stage of the oldest sequence. The

prevalence of the polen Sergipea variverrucata in the deposits of the study area confirms their

Late Aptian age (Batista, 1992; Antonioli, 2001; Rossetti et al., 2001). This unit records the

first deposits accumulated in a broad and shallow depression formed by mild tectonic

stretching before the main Albian rifting stage. These Late Aptian deposits are represented

throughout the basin by deltaic, eolian, lacustrine, as well as transitional to shallow marine

deposits. A sharp discontinuity surface with a paleosol horizon occurs between the Late

Aptian and the Albian deposits (Rossetti et al., 2001; Fig. 4-2A).

Fig . 4-2: A) A view of the unconformity between the Codó Formation and the overlying Albian deposits of the

Itapecuru Group. (Gy= gypsum; Lsh=interbedded limestone and shale). B) Lithostratigraphic profile

representative of the Codó Formation in the study area, with indication of the shallowing upward cycles formed

by central lake (1), intermediate lake (2) and marginal lake (3) deposits. The letters to the left locate figures A-F.

C) Bituminous shale (black areas) interbedded with evaporite (white areas) from central lake deposits. D)

Interbedded limestones (lm) and shales (darker areas from intermediate lake deposits. E) Interbedded lime-

mudstone (Ml) and pisoidal limestones (Pl) and F) Rhythmite of lime mudstone (white beds) and microbial mats

(black beds) from marginal lake deposits.

In the study area, the Late Aptian Codó Formation consist ofs a 25 m-thick lacustrine

prograding succession typically formed by shallowing-upward cycles (Fig. 4-2B). The base of

these cycles includes evaporites and bituminous black shales attributed to central lake deposits

(Fig. 4-2C). Pyrite and native sulphur are locally abundant in the shales. The central lake

deposits progressively grade upward into laminated argillite interbedded with limestones

(calcimudstone, laminated to massive peloidal wackestone to packstone and sparstone),

attributed to intermediate lake environments (Fig. 4-2D). Bioturbation is locally present, as are

symmetrical ripple marks. Ostracods represent circa 15-25% of the grains in the peloidal

limestones, which may contain microbial mats. The top of the shoaling-upward cycles

comprises a variety of intergrading lithofacies, including: 1) massive blocky pelite that varies

upward from olive-green to brownish-red colors; 2) fenestral calcarenite formed by calcite

grains; 3) ostracodal limestone (wackestone to grainstone); 4) pisoidal packstone (Fig. 4-2E)

with elongated and agglutinating pisoids of various sizes interlaminated with microbial mats;

5) tufa; and 6) ostracodal limestone/shale rhythmite with abundant microbial mats (Fig. 4-2F).

These deposits show an abundance of sedimentary features (i.e., paleosols, karstic surfaces,

fenestrae, meteoric cements, vadose pisoids) typical of subaerial and/or meteoric exposure,

consistent with their deposition in marginal lake environments. Most of the described

sedimentary facies are also observed in the Grajaú area. However, in this locality evaporites

dominate over the other facies, being represented by layered gypsum and gypsarenite.

The shallowing-upward cycles of the Codó Formation are arranged into a higher

category of lower frequency cycles that are bounded by sharp discontinuity surfaces (Fig. 4-

2B). At the base of these lower frequency cycles the superimposed shallowing-upward cycles

are characterized by a higher volume of central lake deposits, while in its top shallowing-

upward cycles with increased volume of marginal lake deposits dominate. It is noteworthy the

presence of lags of intraformational calcimudstone and shales, clastic gypsum, as well as

rhythmites with abundant microbial mats, at the base of the lower frequency cycles (Fig. 4-

2B). The origin of these cycles is attributed to expansion and contraction of the lake system,

probably related to syn-sedimentary seismic activity (Paz, 2000).

4.5. CHARACTERIZATION OF THE EVAPORITES

4.5.1. Description

The evaporites of the Codó Formation, represented by gypsum and locally anhydrite,

occur as discontinuous lenses that are up to 4.5 m thick and circa 300 m long at the outcrop

scale. In the Codó area, these deposits are interbedded with black shales, and exclusively

occur in deeper central lake deposits, while in the Grajaú area they are linked to facies

associations typical of shallower waters with evidence for exposure to meteoric and/or vadose

conditions and pedogenesis.

The evaporites consist of three facies: layered gypsum, gypsarenite, and

massive/macronodular gypsum. Layered gypsum dominates, and is comprised by laterally

continuous, alternating dark/light horizontal beds that vary from few mm up to 10 cm thick

(Fig. 4-3A-C). The dark beds are formed by either crystals or micronodules of gypsum less

than 0.5 cm long distributed within a matrix of black shale. The light beds consist of upward-

oriented fibrous gypsum crystals. The gypsarenite (Fig. 4-3C) is interbedded with the layered

gypsum, forming layers few cm thick of moderate to poorly sorted, rounded to sub-rounded

very coarse to pebbly gypsum grains. The massive/macronodular gypsum forms unstructured

bodies (Fig. 4-3D) that are closely intergraded with macronodular gypsum consisting of

centimeter-sized gypsum nodules, with or without a shale matrix. The gypsum nodules display

an almond-like form defined by a web of undulating, horizontal to oblique fractures. Gypsum

crystals up to 0.5 cm thick that grew perpendicularly to the fracture walls fill these fractures.

Where fractures are several cm long, gypsum nodules replaced by anhydrite up to 3 cm in

length are present (Fig. 4-3E). Rosettes of dark gypsum up to 5 cm in diameter are common in

this facies. The massive/macronodular gypsum is in sharp contact with the layered gypsum,

locally forming diapirs several meters long.

Petrographic analysis of the evaporite facies showed that the darker beds are formed by

nodular gypsum displaying laths up to 20 µm long of gypsum, and secondarily, by anhydrite,

as well as by relics of calcite (Fig. 4-4A and B). The nodules were pervasively recrystallized

to gypsum crystals averaging 200 µm long, internally displaying laths of gypsum and/or

anhydrite, as well as relics of calcite. The volume of matrix is low in these nodules. The

authigenic gypsum is in optical continuity beyond the nodule boundaries (Fig. 4-4C). The

crystals from the lighter beds of the layered gypsum occur as aligned twin planes and

superimposed growth faces with acute angles zig-zagging perpendicularly to the crystal long

axes, and characterizes the chevron gypsum (Fig. 4-4D). These crystals are intergrown by

acicular gypsum containing relics and/or ghosts of the chevron gypsum. In the gypsarenite

facies (Fig. 4-4E), clasts were replaced by a mosaic of gypsum crystals varying from 100 µm

to 2 mm in length. These display optical continuity beyond their boundaries and present

abundant inclusions, similarly to the gypsum in the darker beds. The massive/macronodular

gypsum is pervasively replaced by mosaics of either alabastrine gypsym (i.e., crystals up to

200 µm in diameter), or coarser crystals reaching up to 2 mm in diameter (Fig. 4-4F). As

opposed to the previously described evaporite facies, the gypsum crystals in this facies lack

inclusions, resulting in limpid mosaics. The anhydrite nodules are fine-grained to

cryptocrystalline and internally display few tabular anhydrite crystals up to 250 µm. The

nodules replace large (circa 8 mm) crystals of gypsum with well-preserved cleavage; they are,

in turn, locally replaced by fine-grained gypsum.

4.5.2. Depositional setting

As previously stated, detailed facies analysis of the Codó Formation led to attribute it

to a dominantly continental setting. This interpretation will be further supported by the isotope

data presented in this paper. However, before going into that issue, it is important to detail the

depositional setting of the evaporites as extracted from facies data, because both vertical and

lateral variations in paleoenvironmental conditions seem to have affected the distribution of

the isotope values in this instance.

Stable and well-stratified hypersaline lakes with significant periods of anoxia and

closure prevailed in the Codó area, which led to the preferential precipitation of salts in more

saturated deeper central areas (Paz & Rossetti, 2001). Much more ephemeral conditions seem

to have occurred in the Grajaú area, where more oxygenated water pans with evaporite

precipitation in their margins and along surrounding mudflats prevailed (Fig. 4-5). This is

demonstrated by the dominance of layered gypsum and gypsarenite in association with

marginal and intermediate lake environments. In these facies, the chevron gypsum crystals of

the light-layered gypsum attest to primary deposition on the floor of brine pools. Precipitation

of similar features in many modern and ancient environments occurs when water depth is less

than 2 m (e.g., Hovorka, 1987; Logan 1987; Handford, 1991; Smoot & Loweinstein, 1991).

The darker layered gypsum is attributed to displacive intrasediment growth of crystals beneath

the brine from groundwater brines in the capillary and/or upper phreatic zone, thus implying

periods of eventual exposure (e.g., Kerr & Thomson, 1963; Southgate, 1982; Warren, 1999).

The gypsarenite records periods when evaporite crystals were reworked. Therefore, the

evaporites in the Grajaú area were dominantly formed in subaqueous, but shallower

environments than in the Codó area, with better defined alternating periods of stable and

ephemeral waters, a condition that led to envisage formation in a saline pan complex with

widespread evaporitic mudflats.

Beside facies characteristics, a continental setting is also suggested by the abundance

of ostracods mostly including the genus Harbinia and Candona, presence of charophytes, and

absence of any marine tolerant fauna.

4.6. SR AND S ISOTOPES FROM THE EVAPORITES

Thirty-six samples were collected for Sr and S isotope analysis. However, only the

results obtained from the layered gypsum and gypsarenite facies were used for

paleoenvironmental purposes, because these lithologies showed the least degree of textural

and/or mineralogical modification, while the massive/macronodular gypsum was strongly

affected during burial, as discussed in the following section.

The results (Table 4-I; Fig. 4-6) for the layered gypsum and gypsarenite indicate

variable

Sr/

Sr ratios ranging from 0.708481 to 0.709280 in the Codó area and 0.708272 to

0.708935 in the Grajaú area, with only one sample displaying 0.708104 in the later. The

massive/macronodular gypsum displayed the lowest values, ranging from 0.707824 to

0.708190. Discarding these values from our results, it is noteworthy that the

Sr/

Sr ratios

vary from higher to lower and then higher again upward within low frequency cycles. The Sr

content ranges from 212 to 2051 ppm (mean value of 435 ppm). These values vary inversely

to the Sr isotopes within individual cycles.

Table 4-I: Results of the strontium and sulphur isotope analyses from the evaporites of the Codó Formation in the

study areas.

Sample #

Sr/

2σ δ

S (v-CDT)

Evaporite Facies

1 0.709280 18 16.59

2 0.708901 15 17.09

3 0.708860 9 17.89

4 0.708558 15 17.47

5 0.708652 12 16.99

6 0.708481 14 -2.23

7 0.708675 53 16.12

8 0.708992 48 16.50

Layered gypsum

9 0.708116 17 15.85

10 0.708153 14 15.66

11 0.708134 14 15.29

12 0.707824 46 15.21

13 0.707989 30 -

14 0.707960 34 -

Massive/

macronodular

gypsum

15 - -5.57

16 0.708821 25 15.04

17 0.708831 24 15.15

18 0.708822 19 14.79

19 0.708754 29 15.34

20 0.708416 20 15.10

21 0.708561 15 15.06

22 0.708441 27 15.60

23 0.708413 59 15.35

24 0.708272 49 15.12

25 0.708442 27 15.30

26 0.708555 23 15.54

27 0.708935 28 15.29

28 0.708842 22 14.56

29 0.708758 26 14.60

30 0.708477 27 14.69

31 0.708443 25 14.75

Layered gypsum

32 0.708190 32 15.52

33 0.708090 30 15.14

34 0.708138 55 15.24

35 0.708104 17 15.07

36 0.708104 11 14.60

Massive/

macronodular

gypsum

100

The

S ratios from layered gypsum and gypsarenite in the Codó area varied from

16.12 to 17.89 %

o (

V-CDT)

, with only one sample having an anomalous value of –2.23 %

o (

V-CDT)

The Grajaú area, in turn, showed distinctively lower values ranging from 14.79 to 15.60 %

o (

V-

CDT)

, with one sample displaying a negative value of –5.57 %

o (

V-CDT)

. The distribution of S

isotope within individual low frequency cycles changes following the same pattern shown by

the Sr isotope data, except for local inverse covariance where marginal lake deposits

dominate. The massive/macronodular gypsum in both areas showed S isotope values between

15.07 and 15.85 %

o (

V-CDT)

4.7. BURIAL OVERPRINT

An important step to consider, before analysing the significance of the Sr and S isotope

data as potential paleoenvironmental indicators, is to eliminate diagenetic influences. A line of

evidence suggests that, if the deposits were modified after deposition, diagenesis was not

enough to cause any significant changes in the isotope values, which seems to rather reflect

the primary water composition.

First, absence of significant diagenetic imprint is suggested by the overall wide

dispersion of Sr and S isotope data within individual depositional cycles. Second, there is a

consistent change in isotope values along the deposits formed during the expansion and

contraction phases of the saline pan/lake systems, revealing a depositional, rather than a

diagenetic signature. Third, petrographic and scanning electronic microscopic studies helped

to select samples suitable for isotope investigation. In fact, most of the analyzed samples

revealed at least a certain degree of replacement, recrystallization, cementation, neomorfism or

dissolution. However, this study also led to conclude that the mineralogical modifications

observed in the layered gypsum and gypsarenite, as explained in the following, were not

enough to cause any significant change in the isotope signal, as opposed to the

massive/macronodular gypsum that was pervasively modified during burial.

As previously discussed, the crevron crystals in the lighter portion of the layered

gypsum record primary deposition on the floor of brine pools. The horizontal alternations of

lighter and darker gypsum in this facies represent a product of sedimentary processes. The

darker layered gypsum formed slightly after deposition, but still under strong influence of the

101

depositional setting as precipitation took place within only a few milimeters below the

depositional surface (cf. Warren, 1999). The chevron gypsum was in part replaced by acicular

gypsum, as indicated by relic and ghost features of chevron gypsum. But even in this case the

effects of diagenesis were rather mild, not disturbing bedding planes. Although ultimate

evidence is lacking, the absence of other types of secondary gypsum in these beds suggests

that such replacement took place also shortly after deposition, thus still under a strong

influence of the primary brine. A second phase of gypsum would have affected this facies, as

well as the gypsarenite, which locally resulted in gypsum crystals that grew beyond nodule or

clast boundaries. However, this process was not extensive enough to replace the internal laths

of anhydrite/gypsum and/or relics of calcite. This fact, added to isotope values similar to those

from the layered gypsum, as will be shown in the following sections, led us to attribute an

eodiagenetic origin for this second phase of gypsum formation. Gypsum crystals formed

shortly after deposition will keep a primary isotope signature, because the precipitating fluids

derive from dissolution of evaporites formed in the same depositional site (e.g., Denison et al.,

1998).

On the other hand, the massive/macronodular gypsum records salt replacement and

recrystallization during deeper burial in association with halokinesis. As this process took

place, the original structures were greatly obliterated, forming a dominantly massive deposit.

The pervasive mosaics of limpid crystals typical of this facies might have required several

phases of replacement, during which any primary sedimentary feature was lost. Although

some of the macronodular texture might reflect relics of a primary nodulation, the majority of

the macronodules originated by salt displacement, giving rise to fracturing and precipitation of

bladed gypsum by introduction of sulphate-saturated intrastratal waters. The close association

of anhydrite nodules with fractures points to their formation by local dewatering due to

overpressure. In fact, these samples revealed anomalously the lowest isotope values, which is

consistent with its distinct post-depositional evolution. Furthermore, samples with clay

minerals, as revealed under the microscope, were not applied for analysis of geochemical

tracers in order to avoid Sr contamination.

102

4.8. DISCUSSION

Considering that a significant diagenetic imprint was discarded in samples of layered

gypsum and gypsarenite, the Sr and S isotopes could be used for paleoenviromental purposes.

On the other hand, as expected, the values obtained for the massive/macronodular gypsum

differed from the average strontium ratios. This is attributed to later growth of limpid gypsum

crystals and secondary nodules of anhydrite, as characterized by petrographic studies. As a

result, the isotope data from the massive/macronodular facies were discarded from our

analysis. In addition, the sample from the Grajaú area that also showed an anomalously low Sr

isotope value was also excluded, as this sample comes from a red-stained gypsum horizon

only 0.2 m below a discontinuity surface probably related to prolonged subaerial exposure at

the top of a shallowing-upward succession.

The

Sr/

Sr and

S analyses have been successfully used to distinguish marine

and non-marine brines throughout the world (Table 4-II). These methods have been

increasingly applied in evaporites because: 1. evaporites are rich in strontium and sulphur; 2.

evaporites are widespread in both marine and non-marine settings, and their distinction is very

problematic where the associated deposits have no other diagnostic paleoenvironmental

features, requiring more sophisticated geochemical procedures; 3. other geochemical tracers

commonly used in paleoenvironmental reconstructions show many conflicting results (e.g.,

Hovorka et al., 1993); 4. distribution of Sr and S ratios in seawater is well known for the

Phanerozoic; 5. low permeability in evaporites difficults the introduction of external water

carrying Sr and/or S that may modify their isotopic composition; and 6. Sr isotope is very

sensible to detect the introduction of marine waters in continental settings.

Previous studies focusing on Sr isotopic composition of carbonates and evaporites,

originated from marine-based brines, have established values between 0.70720 and 0.70735

for the Aptian (Bralower et al., 1997; Jones & Jenkins, 2001; Fig. 4-7). The data generated

here are much higher than expected for Late Aptian marine deposits, suggesting a continental

brine source. The difference of the Sr isotope values from the Codó Formation relative to the

Aptian seawater is at least of 0.00075. Similar variations of strontium isotope data relative to

the seawater have also been measured from many other continental deposits recorded in the

literature (e.g., Faure et al., 1963; Jones & Faure, 1967, 1972; Faure & Barret, 1973; Denison

103

et al., 1998; Vonhof et al., 1998; Playà et al., 2000). Modern settings record even lower

differences (i.e, 0, 00021) when continental and marine brines are compared, as measured for

instance in the Abu Dhabi Sabhka (Müller et al., 1990). Therefore, the high Sr isotope data

from the layered gypsum and gypsarenite are related to a continental brine source, which is

consistent with the proposed depositional model interpreted with basis on facies analysis.

Table 4-II:

Sr/

Sr values of non-marine source brines and rocks.

Age (

Sr/

seawater)

Origin Material

Sr/

Sr N

E. Great Salt Lake

Brine 0.7167 to 0.7179 9

W. Great Salt Lake

Brine 0.7125 to 0.7139 4

Lakes and rivers from

Canadian Shield

Brine 0.712 to 0.726

Lake Vanda (Antartida)

Brine 0.7149

Holocene

(0.709173)

Lake George

Brine 0.7184

Pebas Formation

Carbonate 0.708160 to

0.709051

10 Miocene

(0.7081 to 0.7089)

Eastern Betics

Evaporite 0.70804 to 0.70888

Jurassic

(~0.7069)

(~0.7073)

Todilto Formation

Carbonate

Gypsum

0.707273 to 0.708673

0.706873 to 0.708073

Beacon Supergroup

Carbonate 0.7150 to 0.7280 12 Permian

Salado-Tansill formations

Evaporite 0.7069 to 0.7076 5

Devonian

(~0.7077 to 0.7086)

Beacon Supergroup

Carbonate 0.7260 to 0.7291 3

N*= number of samples.

Jones and Faure (1972);

Faure et al. (1963);

Jones and Faure (1967);

Vonhof et al. (1998);

Playà et al. (2000);

Denison

et al. (1998);

Faure and Barret (1973).

104

In addition to the high

Sr/

Sr values, their wide distribution in a same section and

between sections, with a standard deviation of 0.000262, conforms to the proposition of

continental-derived brines (Playà et al., 2000). Oceans favor geochemical homogenization,

thus marine deposits should provide more consistent Sr isotope data during a given time

interval (Denison et al., 1998). For instance, studies in a modern sabkha from the Persian Gulf

indicate a standard deviation of

Sr/

Sr much higher (i.e., 0.000177) in continental brines

than in marine brines (i.e., 0,000042) (Müller et al., 1990). The standard deviation of the Sr

105

isotope values in the Codó Formation is much higher than measured in this modern analog,

reinforcing a continental origin for the brine.

The upward decrease, and then increase, in

Sr/

Sr values within the low frequency

depositional cycles in both of the study areas seem to be better explained by facies changes

related to expansion and contraction of saline pan/lake systems (Fig. 4-8). Analysis of facies

distribution within these cycles suggests either rapid or progressive expansion of the

depositional system. Rapid expansion is recorded where central lake black shales and

laminated argillites overlie directly the basal discontinuity surfaces at the base of the cycles.

On the other hand, progressive expansion is suggested where this surface is overlain by clastic

evaporites, rhythmites, and lags of intraformational calcimudstone/shales that grade upward

into laminated argillites and black shales. Contraction of the depositional system is recorded

by the increased dominance of marginal facies in the uppermost portions of the low frequency

cycles.

The distribution of

Sr/

Sr values according to the expansion and contraction phases

of the saline pan/lake systems reinforces their primary nature. Hence, the progressive

expansion of the depositional system gave rise to evaporitic precipitation. While evaporites

precipitated in central environments, low lying marginal areas dominated by mud flats were

exposed to substantial leaching due to weathering (Fig. 4-8). During this process, the draining

of clay minerals might have released

Sr, increasing

Sr/

Sr values of the evaporitic

precipitating brines (cf. Hovorka et al., 1993; Playà et al., 2000). Similar facies control on the

distribution of

Sr/

Sr ratios has been documented in contraction/expansion cycles in the

Eocene Green River Formation (cf. Rhodes et al., 2002).

Following initial expansion, increased contribution of external drainage resulted in a

period of maximum expansion of the saline pan/lake systems. Flooding promoted water

dilution, ending the evaporite precipitation. Increased inflow also resulted in the introduction

of large volumes of muds in suspension, which were eventually settled down, forming the

laminated argillites and black shales. Expansion caused flooding of widespread marginal mud

flats and, as a consequence, the release of

Sr from clay minerals was cut off, resulting in

relatively lower

Sr/

Sr values.

106

Additionally, the Sr isotope ratios might have decreased due to the contribution of Sr

released from basaltic source rocks. This is suggested due to the fact that the study areas are

located near Permian to Neocomian marine-derived limestones and evaporites from the

intracratonic Parnaíba Basin, as well as Triassic to Neocomian basaltic rocks associated with

the Xambioá-Teresina Arch (Fig. 4-1A). Weathering of these rocks might have contributed

with significant amounts of

Sr-depleted waters into the evaporitic basin, resulting in

relatively lower

Sr/

Sr values. However, it is noteworthy that, even considering the

influence of a basaltic sources, the

Sr/

Sr values are still much higher than expected to Late

107

Aptian seawaters. Mixing of waters from both basaltic and sedimentary rocks might have

provided continental brines with

Sr/

Sr ratios comparable to the ones obtained for the study

areas.

During a following stage, the inflow was interrupted and the saline pan/lake systems

contracted. As the introduction of mud progressively reduced and evaporation increased,

carbonates were formed and marginal facies became more widespread. As a consequence, the

Sr/

Sr data increased.

Likewise the strontium, sulphur is also abundant and uniformly distributed in

seawaters, and its isotope ratio changed through time, opening the possibility for its use to

determine brine sources (Claypool et al., 1980). During the Late Aptian, the δ

S values

ranged from 13 to 16 %

V-CDT)

(Claypool et al., 1980; Fig. 4-7). Similarly to strontium, the

sulphur isotope values obtained in the Codó Formation are highly variable, as expected in

continental-derived brines. In addition, the values from the Codó area are higher than 16 %

V-

CDT)

, thus also conforming to continental-derived brines. However, values from the Grajaú

area are lower, situating in the upper part of the range expected for marine waters. A marine

contribution to the saline pan complex could explain these values, but this is not supported by

the strontium nor by the facies data, as previously discussed. Therefore, if hybrid brine is to be

considered in this instance, then the marine contribution was not enough to cause a significant

geochemical imprint other than in the S isotope composition. Taking into account previous

studies discussing this method, this interpretation is disregarded here because in general

sulphur isotope is considered much less diagnostic of brine sources than strontium isotope

(Chivas et al., 1991; Denison et al., 1998; Playà et al., 2000). Considering the difference in

facies patterns between the study areas, an alternative hypothesis is that the higher rates of

evaporation in the Grajaú area, favored by the prevalence of shallower evaporitic

environments, might have contributed to the depletion in

S. In seawaters, this process might

account for about 2%o of

S depletion (Hoefs, 1980), though there is no information available

from lake systems, and the effect in the gypsum precipitate-brine relationship is an issue open

for further research.

Considering a non-marine brine interpretation, then the changes in S isotope values

between the Codó and Grajaú areas might have had a facies control. The increase in δ

108

values in the Codó area (Fig. 4-7) is attributed to a more restricted environment, resulting from

the prevalence of a hydrologically closed lake system, as revealed by facies analysis (Paz and

Rossetti, 2001). Under such conditions, the δ

S tends to be increased due to the activity of

sulphate-reducing bacteria, resulting in SO

depletion in the water and increasing the δ

(Hoefs, 1980). On the other hand, more oxygenated conditions would have prevailed in the

Grajaú area, which led to a relative enrichment of sulphates and a consequent decrease in δ

The two anomalous negative sulphur isotope values obtained in the study area were derived

from evaporite sampled from central lake black shales recording the beginning of flooding

stages, and they may be related to recycling of underlying sulphides.

4.9. CONCLUSIONS

This study led to several conclusions: 1. when combined with facies analysis and

optical (petrography and scanning electron microscopy), Sr isotope is a powerful tool for

distinguishing evaporites derived from marine and non-marine brines. This effort led to the

confirmation that the deposition of the Codó Formation in the southern and eastern margins of

the Grajaú Basin, northern Brazil, took place dominantly in saline pan/lake depositional

systems; 2. Sr isotopes revealed to be very sensible to detect hydrological changes in this type

of environments; 3. however, fluctuations on the distribution of Sr values due to changes in

sedimentary patterns are not enough to preclude distinguishing between marine and non-

marine evaporites; 4. during initial expansion phases, the Sr isotopes in saline pan/lake

environments tend to decrease due to leaching of

Sr from clay minerals of mud flats exposed

to internal drainage; 5. during maximum expansion of this type of system, these values

progressively decrease due to the cut off of this process, as the mud flats become flooded; 6, in

the particular case of the study area, the lowering in Sr isotope values might have additionally

caused the introduction of

Sr-depleted waters derived from weathering of older marine

limestone and evaporites, as well as basaltic rocks; and 7. interruption of external drainage and

contraction of the system caused increased carbonate precipitation, with consequent slight

increase in Sr isotope values. The data from the study area allowed also to conclude that the

application of sulphur isotope as a parameter to decipher brine sources is limited. This is

because, even where strontium values and geochemical tracers indicate a non-marine brine

109

source, the sulphur values might coincide with those from seawater for a given time interval,

as observed in the Grajaú area. Hydrologically closed systems tend to develop anoxia, when

the activity of sulphate-reducing bacteria results in SO

depletion in the water, with the

consequent increase of δ

S. On the other hand, more oxygenated conditions lead to a relative

enrichment of sulphates and a consequent decrease in δ

S. Therefore, significant fluctuations

in sulphur isotope composition might take place due to changes in hydrological regime, and

thus the use of this parameter alone is not recommended to decipher between marine/non-

marine brines.

Acknowledgements. The Itapicuru Agroindustrial S/A is acknowledged for the permission to

access the quarries with the exposures of the Codó Formation. This work was financed by the

Brazilian Council for Research–CNPq (Project #460252/01).

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pp.1-66. Elsevier, Amsterdam.

Hardie, L.A. (1984) Evaporites: Marine or non-marine?: Am. J. Sci, 87, 193-240.

Hoefs, J. (1980) Stable isotope geochemistry. Springer-Verlag, Berlim, 208 pp.

Hovorka, S.D. (1987) Depositional environment of marine-dominated bedded halite, Permian

Sant Andreas Formation, Texas. Sedimentology, 34, 1029-1054.

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transition in Permian evaporites of the Palo Duro basin, Texas: geochemical response.

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Jones, C.E. and Jenkins, H.C. (2001) Seawater strontium isotopes, oceanic anoxic events,

and seafloor hydrothermal activity in the Jurassic and Cretaceous. Am. J. Sci. 301, 112-

149.

Jones, L.M. and Faure, G. (1967) Origin of the salts in Lake Vanda, Wright Valley, southern

Victoria Land, Antarctica. Earth Planet. Sci. Lett., 3, 101-106.

Jones, L.M. and Faure, G. (1972) Strontium isotope geochemistry of Great Salt Lake, Utah.

Geol. Soc. Am. Bull., 83, 1875-1880.

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Kerr, S.D. and Thompson, A. (1963) Origin of nodular and bedded anhydrite in Permian

shelf sdiments, Texas and New Mexico. AAPG Bull., 47, 1726-1732.

Logan, B.W. (1987) The Lake MacLeod Evaporite Basin, Western Australia–Holocene

Environments, Sediments, and Geological Evolution. AAPG Mem., 44, 140 pp.

Müller, D.W., McKenzie, J.A. and Mueller, P.A. (1990) Abu Dhabi Sabkha, Persian Gulf,

revisited: application of strontium isotopes to test an early dolomitization model. Geology,

18, 618-621.

Paz, J.D.S. (2000) Análise Faciológica da Formação Codó (Aptiano Superior), na Região de

Codó (MA), Borda Leste da Bacia do Grajaú. M.Sc. Thesis, Universidade Federal do Pará,

Belém, PA, 117 pp.

Paz, J.D.S. and Rossetti, D.F. (2001) Reconstrução paleoambiental da Formação Codó

(Aptiano), borda leste da Bacia do Grajaú, MA. In: O Cretáceo na Bacia de São Luís-

Grajaú (Eds. D.F. Rossetti., A.M. Góes and W. Truckenbrodt), pp. 77-101. Goeldi Press,

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Miocene evaporites of the Eastern Betics, SE Spain: sedimentological and geochemical

evidence. Sed. Geol., 133, 135-166.

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438 pp.

113

5. PALEOHYDROLOGY OF A LATE APTIAN LACUSTRINE

SYSTEM FROM NORTHEASTERN BRAZIL WITH BASIS ON THE

INTEGRATION OF FACIES AND ISOTOPIC GEOCHEMISTRY

5.1. ABSTRACT

The Codó Formation records the initial evolutionary stages of an intracontinental rift

system formed along the Brazilian equatorial margin in the Late Aptian. Deposits of this unit

exposed in the eastern margin of the Grajaú Basin include evaporites, bituminous black shales

and limestones. These lithologies were formed in a low energy, well stratified, predominantly

anoxic and hypersaline lake system developed in a dominantly arid/semi-arid climate. This

lacustrine succession is internally organized into three categories of shallowing-upward

cycles, with the first- and second-order cycles being related to seismic activity associated with

fault reactivations, and the third-order cycles recording seasonal fluctuations. Studies

emphasizing petrography and analysis of the minor and trace elements Fe, Mg, Sr, Mn, and Na

led to the evaluation that the Codó Formation was appropriate for isotopic investigations

having paleoenvironmental and paleohydrologic purposes. The results of this study revealed a

wide distribution of dominantly low δ

C and δ

O values, ranging from –5.69 ‰ to –13.02

‰

(PDB)

and from –2.71‰ to –10.80‰

(PDB)

, respectively. This paper demonstrates that the

Authors: J.D.S. Paz & D.F. Rossetti. Submitted to Palaeogeography, Palaeoclimatology,

Palaeoecology

114

isotope ratios vary according to seismically-induced shallowing-upward cycles, in general

becoming lighter in their bases, where central lake deposits dominate, and progressively

heavier upward, where marginal lake deposits are more widespread. In addition, to confirm a

depositional signature for the analysed samples, this behavior led to introduce a seismic-

induced isotope model. Hence, lighter isotope ratios appear to be related with flooding events

promoted by subsidence, which resulted in the development of a perennial lake system, while

heavier isotope values are related to ephemeral lake phases favored through uplift and/or

increased stability. Furthermore, the results show that a closed lake system dominated, as

indicated by the overall good positive covariance (i.e., +0.42 to +0.43) between the carbon and

oxygen isotopes, though open phases are also recorded by negative covariance values of –

0.36. During closed phases, the δ

O displayed the highest interval of variation (i.e., –3.63‰

to –4.89‰) due to the increased residence time, while this variation was low (i.e., –0.09‰ to -

1.87 ‰) during open lake phases, when there was a balance in the water isotope composition

caused by the continuous basin inflow.

5.2. INTRODUCTION

C and δ

O records have been successfully applied for reconstructing the evolution

and circulation patterns of oceanic basins throughout the geological time (e.g., Abell and

Williams, 1986; Charisi & Schmitz, 1995; Hendry & Kalin, 1997). These studies are mostly

based on the principle that the organic matter in marine sediments is characterized by

extremely uniform isotopic compositions, which vary according to climate, as well as oceanic

hydrology. The interpretation of these geochemical indicators in lacustrine settings is more

complex, mostly because lake environments are more diversified, as evidenced by a wider

distribution of carbon isotope ratios ranging from –25.9 to –10.5‰ (Bird et al., 1991). Since

the pioneer work of Stuiver (1970), several documentations on modern and Quaternary lake

systems have provided the basis for an increased discussion on the many parameters that

might influence the isotopic composition of the total inorganic carbon dissolved in lake waters

(e.g., Fritz et al., 1975; Katz et al., 1977; Anderson & Arthur, 1983; MacKenzie, 1985;

Hillaire-Marcel & Casanova, 1987; Bellanca et al., 1989; Gasse et al., 1989; Talbot & Kelts,

1990; Rosenmeier et al., 2002; Herczeg et al., 2003; Russell et al., 2003). Despite these

115

efforts, differentiation among the mechanisms that lead to variations in isotope composition of

lake waters is not straightforward, as local causes might be mistaken by externally forced

environmental changes (Talbot, 1990). In addition, in contrast to marine and modern

lacustrine systems, the record of chemical changes in ancient lake deposits is yet very limited

(Bird et al., 1991; Lister et al., 1991; Camoin et al., 1997; In Sung & Kim, 2003), which have

precluded a wider use of these geochemical tracers for paleoenvironmental purposes. Thereby,

geochemical analysis of carbonate lacustrine deposits from a larger volume of analogs, where

local causes can be distinguished from those of regional significance, are still needed in order

to provide a full understanding of the mechanisms controlling lacustrine carbonate

sedimentation.

In spite of the complex response, the available information concerning to carbon and

oxygen isotope variations has arrived to some important generalizations. The most significant

one for paleoenvironmental interpretation is the covariance of these geochemical tracers as an

indicator of hydrologically closed lake systems, as opposed to the non-covariance in inlet

lakes (e.g., Eicher & Siegnethaler, 1976; Gasse et al., 1987, 1989; Talbot, 1990; Talbot &

Kelts, 1990). Additionally, these isotopes have been applied for climate reconstructions of

lake systems (e.g., Talbot & Keltz, 1990; Lister et al., 1991; Valero-Garcés et al., 1995).

These applications are, however, highly dependent on the full understanding of facies

distribution and of the possible modifications occurred during burial.

The goal of this paper is to contribute for the documentation of δ

C and δ

O values in

ancient lacustrine systems, and discuss the causes of these variations analysing their

relationship with shallowing-upward cycles within a Late Aptian succession formed during the

early stages of a passive marginal rift. This unit, represented by the Codó Formation, is well

exposed along several quarries in the eastern margin of the Grajaú Basin, where detailed

studies focusing facies and facies architecture, stratigraphy, petrography, as well as Sr and S

isotopes, have provided the basis to support deposition in a dominantly lacustrine setting (e.g.,

Rossetti et al., 2000; Paz and Rossetti, 2001). An integrated approach combining facies and

isotope geochemistry provided the basis to analyze the distribution of carbon and oxygen

isotopes in this ancient lake system, as well as to investigate the main parameters controlling

its hydrology.

116

5.3. GEOLOGICAL SETTING

The Codó Formation records the first deposits accumulated within a broad and shallow

depression formed by mild tectonic streching before the main rifting stage that culminated

with the origin of the Equatorial South Atlantic Ocean during the Albian. These deposits are

well represented in the Grajaú Basin (Fig. 5-1A), a semi-graben formed by the

combination of

pure shear stress and strike-slip deformation (Azevedo, 1991; Góes & Rossetti, 2001).

This rift,

which is connected with the São Luís Basin to the north, became an aborted intracontinental

structure as the continental break up migrated northward.

The sedimentary fill of the São Luís-Grajaú Basin (Fig. 5-1B) reaches up to 4,000 m

thick in the depocenters, and consists chiefly of Cretaceous deposits organized into three

depositional sequences, i.e., S1, S2 and S3, formed during the Late Aptian/Early Albian,

Early-Middle Albian and Middle Albian/Late Cretaceous, respectively (Rossetti 2001). The

lowermost sequence S1 contains the Codó Formation, objective of this paper, and represents a

succession up to 450 m thick of sandstones, evaporites, shales and limestones. This sequence

displays a tripartite subdivision into systems tracts (Rossetti 2001), with the lowstand deposits

grading progressively southward from shallow marine to continental (i.e., fluvial, deltaic, and

lacustrine) in nature. The lowstand deposits are overlain by strata formed in the transgressive

systems tract, and consist of a wedge of richly fossiliferous (mostly bryozoans, equinoderms,

foraminifera and dinoflagellates) shales that pinches out to the basin margins. The highstand

systems tract consists of shallow marine to continental deposits typically displaying stratal

patterns varying upward from aggradational to progradational.

The maximum thickness of the Codó Formation in the Grajaú Basin is 150 m (Rezende

& Pamplona, 1970). Its paleontological content mostly includes pollens, continental ostracods,

insects, and fishes, which are all in agreement with a dominantly lacustrine interpretation for

the depositional system. Additionally, pollen has been recovered from these deposits and

allowed the establishment of a precise late Aptian age with basis on the presence of Sergipea

variverrucata (Batista, 1992; Lima, 1982; Rossetti et al., 2001). The Codó Formation either

grades downward into fluvial and deltaic deposits of the Grajaú Formation (e.g., Mesner &

Wooldridge, 1964), or sharply overlies an unconformity over older Paleozoic and Triassic

basement rocks. Its upper contact is an unconformity with Albian shallow marine, green to

117

brownish-red mudstones interbedded with fine- to very fine-grained, cross-stratified

sandstones of the Itapecuru Group (e.g., Rossetti & Truckenbrodt, 1997; Rossetti et al., 2001).

Fig . 5-1: A) Location of the study area in the Codó region, eastern margin of the Grajaú Basin. B) Stratigraphy

and main tectonic stages of the São Luís-Grajaú Basin.

118

5.4. FACIES ARCHITECTURE AND DEPOSITIONAL SYSTEM

5.4.1. Description

A detailed facies analysis of the Codó Formation exposed in the eastern margin of the

Grajaú Basin was previously reported elsewhere (e.g., Paz and Rossetti, 2001). There, the

Codó Formation consists of a lacustrine succession up to 25 m thick, attributed to three main

sub-environments (Fig. 5-2): 1. central lake deposits, consisting of evaporites and bituminous

black shales, locally with pyrite and native sulphur; 2. intermediate lake deposits, represented

by laminated argillites and limestones; and 3. marginal lake deposits, including massive

blocky pelites, fenestral calcarenites, ostracodal and pisoidal limestones, rhythmites of

limestones and microbial mats, as well tufas. Paleosol, karstic feature, meteoric cement and

vadose pisoid, typical of subaerial and/or meteoric exposures, are frequent features of the

marginal lake deposits.

Three categories of cycles were recognized in the Codó Formation (Fig. 5-3). A

detailed description of these cycles is presented in a separate paper (Paz and Rossetti,

submitted), but a summary is included here as they are critical to help understanding the

meaning of the isotope signals. Third-order cycles display regular thickness ranging from 5 to

10 cm, and consist of facies that vary according to the position in the lake setting. Hence, the

central lake deposits show interbeddings either of bituminous black shales and evaporites, or

bituminous black shales with streaks of calcimudstone and bituminous back shales with native

sulphur. The intermediate lake deposits display bituminous black shale interbedded with

peloidal limestone or green to gray laminated argillites interbedded either with calcimudstone

or peloidal wackestone-packstone. The marginal lake deposits show either green to gray

laminated argillites and ostracodal wackestone to grainstone, as well as alternations of

ostracodal and/or lime mudstones, microbial mats and vadose pisoidal packstones.

The second-order cycles consist of either complete or incomplete successions with

upward transitions from central to marginal lake deposits, with the latter displaying high

internal facies variability when comparing one cycle to another. These cycles are characterized

by limited lateral extension, as well as frequent and random thickness changes, which vary

from few cm up to 5 m.

119

120

Fig . 5-3: Shallowing-upward cycles of the Codó Formation. A) Examples of first- and second-order cycles. B-D)

Third-order cycles formed by alternations of bituminous black shale with streaks of lime-mudstone (Bsl) and

bituminous black shales with native sulphur (Bss) (B), ostracodal grainstone (Gro) and vadose pisoidal packstone

(Pp) with microbial mats (M) (C), ostracodal grainstone (Go) and wackestone (Wo) (D).

The first-order cycles define four episodes of shallowing (Fig. 5-4), organized from

bottom to top as units 1 to 4. Unit 1 is only partly exposed at the base of the sections,

consisting of bituminous black shales interbedded with calcimudstones, and are attributed to

central and intermediate lake settings. Unit 2 reaches up to 8 m thick and contains, at the base,

black bituminous shales interbedded with evaporites, which grade upward into limestones,

laminated argillites and massive block pelites displaying a variety of features related to

intermediate and marginal lake settings. Evaporites are, in general, absent or occur only as

milimetric lenses or isolated crystals of gypsum. Unit 3 reaches up to 4 m thick and is

constituted by intermediate and marginal lake deposits similarly to the underlying unit, but

constituted by intermediate and marginal lake deposits similarly to the underlying unit, bu

121

122

with an increased frequency relative to the latter. A remarkable and exclusive feature of this

unit 3 is the presence of oolites and calcareous (i.e., ostracodal packstone) concretions in its

upper portion, which constitute important stratigraphic markers. The uppermost unit 4 is up to

5 m thick, being represented by laminated argillites containing only thin (<1 mm thick)

laminae of gypsum or calcimudstone.

The first-order cycles closely match with stratigraphic horizons displaying syn-

sedimentary soft sediment deformation that occur between undeformed deposits (Fig. 5-4), an

observation that was crucial to reveal their genesis. Hence, units 1 and 2 correspond

respectively to undeformed strata, and the deformation zones 1 and 2 described in Rossetti and

Góes (2000). Deformation zone 1 consists of spar-filled cracks interconnected with small-

scale faults, fissures and stylolites inclined at a high angle to bedding. Deformation zone 2

consists of strata with complex convolute folds associated with thrust faults, pseudonodules,

and mound-and-sag structures, the latter correspond to synclines and anticlines mantled by

sigmoidal laminations inclined toward the sag centres. Unit 3 corresponds to a deformed

interval consisting of normal faults and fissures that are vertical to near vertical, present

ragged morphologies with small, delicate edges, and taper both downward and upward after a

few centimetres, being associated with intraformational boulders up to 2.5 m long. The

uppermost unit 4 consists of shales with irregular convolute folds.

5.4.2. Interpretation

The facies present in the Codó Formation exposed in the eastern margin of the Grajaú

Basin supports a low energy, well stratified, anoxic and hypersaline lake system developed

under a dominantly arid/semi-arid climate (Paz and Rossetti, 2001).

The third-order cycles record minor changes in depositional conditions, attesting to

alternations between mud settling and chemical precipitation of evaporites or limestones. This

characteristic, added to the regular thickness variation, is consistent with seasonal fluctuations,

with mud deposition and chemical precipitation taking place during less and more arid phases,

respectively.

The higher-order cycles seem to have a different origin. The second-order cycles

record successive episodes of upward gradation from deeper to relatively shallower lake

environments, which resulted in the superposition of marginal lake deposits upon intermediate

123

and/or central lake deposits. The high facies variability, when comparing one cycle to another,

the limited lateral extension and the frequent and random thickness variations are attributes

that match better with tectonically driven (e.g., Martel & Gibling, 1991; Benvenuti, 2003),

rather than more symmetrical climatic cycles (e.g., Olsen, 1986; Goldhammer et al., 1990;

Smoot & Olsen, 1994; Steenbrinck et al., 2000).

The first-order cycles also appear to have resulted from syn-sedimentary tectonics (Fig.

5-4), as suggested by their good correlation with the deformation zones attributed to

contemporaneous seismic activity related to fault reactivation (Rossetti and Góes, 2000).

Based on this fact, it has been proposed that the Codó lake system was affected by alternating

periods of distension and stability and even compression (Paz and Rossetti, chapter 2). The

prevalence of central lake deposits at the base of the first-order cycles would have formed

during higher subsidence, promoted by extension. On the other hand, the more widespread

distribution of marginal lake deposits in the top of these cycles would record periods of higher

stability or uplift. In addition to affect the development of the shallowing-upward cycles, these

processes appear to have had a strong control on the isotope evolution of this lake system, as

discussed in this paper.

5.5. GEOCHEMICAL TREATMENT

C and δ

O data were analyzed from fresh samples in recently exposed quarries, to

guarantee they were free from the influence of modern weathering. In this study, the analyses

were performed using whole-rock limestone samples due to the fact that only ostracods are

present in the studied deposits, and their distribution is not uniform to guarantee a good record

of the individual cycles throughout the succession. Stable isotopic analysis has been

successfully performed in whole-rock carbonates with the advantage of minimizing possible

deviations related to vital effects, and even diagenesis (e.g., Urey, 1947; Camoin et al., 1997).

Twenty milligrams of powered sample reacted in vacuum with 100% of orthophosphoric acid

at 25

C during 12 hours. The released CO

and H

O were captured with liquid N

. The CO

was separated from the water with a solution of alcohol and liquid N

in an off-line gas

extraction line, and thereafter taken to the VG Isotech SIRA II mass spectrometer in the Stable

Isotope Laboratory at the Universidade Federal de Pernambuco (LABISE/UFPE). The results

124

are reported in δ notation, which is defined as the per mil deviation from a standard. The

Peedee Belemnite Standard (‰ PDB) were used to the notation of both isotopes. Replicate

analysis gave a standard deviation (2σ) lower than 0.02 %o for δ

C and 0.03 %o for δ

The isotopic analyses were run using samples selected after their study under the

microscope to assure a primary signature. In addition, a double check on the diagenetic

influence was made through the analysis of Fe, Sr, Mg, Mn and Ca. The procedure consisted

in drying 1.5 g of sample at 1000

C for 2 hours. Samples were then fused using lithium

tetraborate and lithium fluorite, and analysed in the X-Ray Fluorescence Spectrometer of the

Stable Isotope Laboratory at the Universidade Federal de Pernambuco (LABISE/UFPE).

5.6. EVALUATION OF DIAGENETIC IMPRINT

The petrographic analysis of 83 limestone samples from the Codó Formation allowed

to evaluate its diagenetic signature, observing the amount of lime mud, recrystallization,

replacement, cementation, and fracturing. Several authigenic processes were observed, the

most important ones including recrystallization of calcite, cementation and filling of fractures

and secondary porosity by mosaics of calcite, replacement of micrite and ostracod shells by

chert and chalcedony, and pyrite formation within ostracod shells and dispersed in

calcimudstones. Despite these modifications, it was possible to select 53 samples consisting of

microfacies either not affected or only mildly affected by diagenesis, which enhanced their

potential to preserve the carbon and oxygen composition as a reflex of the depositional record.

The samples used in this study included mostly calcimudstone (36%) and ostracodal

wackestone to grainstone (45%; Fig. 5-5), and subordinately fenestral calcarenite (8%),

pisoidal packstone (6%), and peloidal packstone to grainstone (6%).

125

126

After the petrographic studies, samples were additionally investigated using

geochemical analysis of Fe, Sr, Mn, and Mg to help detecting diagenetic modification at a

higher degree of reliability. These elements are the principal trace consituents in the calcite

structure of both marine and non-marine limestones. Considering that their values remained

constant through time, which seems to have been the case at least for most of the Phanerozoic

(Holland, 1978), a comparison among values commonly expected from stratal waters provides

information to detect potentially significant diagenetic influences. The results (Fig. 5-6) show

that, in general, all the samples that appeared to be petrographically suitable for isotope

analysis contain minor and trace elements as expected for continental deposits not affected by

diagenesis. Exceptions are samples with high Fe content, which is due either to high volume

of organic matter or pyritization. However, these samples were also included in the isotope

analysis presented here, considering that: 1. the other geochemical tracers are within the range

expected for diagenetically non-affected rocks; 2. the isotope values do not show any

divergence with respect to the other samples; and 3. they derive from facies with pedogenetic

influence associated with marginal lake deposits, a situation that favours an increased Fe and

Mn circulation.

127

128

5.7. STABLE ISOTOPES

The δ

C isotope curves show an overall similar pattern in all the studied profiles, with

ratios ranging from –5.69 ‰ to –13.02 ‰ (Table 5-I). In general, two short intervals with

higher values at the base and top of the profiles are separated by a large interval with depleted

values (Fig. 5-7). The behavior of the isotope curves reflects the position of the first-order. For

instance, unit 2 displays increasing values at the bottom, followed by a short negative

excursion to ultimately oscillate toward heavier and, in more rarely, lighter values at the top.

The values in unit 3 are more regularly distributed, varying upward from lighter to heavier.

Table 5-I: δ

(

‰

PDB)

and δ

(

‰

PDB)

values observed in the study area. A, B and C mean the vertical profiles

showed in the Fig. 5-7.

Prof

Sample δ

O δ

Prof

Sample δ

O δ

01 pisoidal packstone -5,49 -10,10 31 ostracodal packstone -6,87 -10,96

ost

acoda

pac

sto

-7

24 -11

,80

ost

acoda

pac

sto

5,3

2 -

9,30

udsto

8,9

1 -1

0,03

ost

acoda

pac

sto

-7

,69

-1

0,95

udsto

17 -

pac

sto

-7

2 -11

udsto

8,96

ost

acoda

pac

sto

8,38

-11

,00

est

8,36

6,65

ost

acoda

pac

sto

-7

,83

-11

ost

acoda

pac

sto

9,9

udsto

8,69

-11

ost

acoda

pac

sto

8,36

-1

0,9

udsto

41 -11

,03

udsto

-7

22 -

pac

sto

9,36

-1

0,89

udsto

8,9

2 -

pac

sto

-12

,38

udsto

6,09

41 f

est

9,83

-1

udsto

8,6

2 -

6,3

1 42 f

est

8,00

-1

0,00

ost

acoda

pac

sto

8,50

ost

acoda

pac

sto

8,6

4 -12

44 f

est

9,9

4 -11

,80

udsto

8,83

-11

ost

acoda

pac

sto

6,05

-11

,99

ost

acoda

pac

sto

-11

ost

acoda

pac

sto

-7

2 -12

7 47

udsto

8,99

-12

ost

acoda

pac

sto

3,39

-7

,98

udsto

24 -1

3,0

ost

acoda

pac

sto

-4

9,6

udsto

8,6

2 -11

pac

sto

-4

-1

0,8

udsto

-11

ost

acoda

pac

sto

6,6

2 -1

pac

sto

8,06

ost

acoda

pac

sto

6,5

1 -1

ost

acoda

pac

sto

6,65

-1

ost

acoda

pac

sto

27 -12

,39

ost

acoda

pac

sto

-7

42 -

5,69

udsto

-1

0,8

-11

udsto

-1

0,36

-11

udsto

-1

44 -11

udsto

-7

,58

-11

,89

ost

acoda

pac

sto

-7

-12

,56

ost

acoda

pac

sto

-4

77 -12

ost

acoda

pac

sto

3,0

1 -

6,8

ost

acoda

pac

sto

-2

71 -7

,55

udsto

-4

,80

9,0

129

The fluctuation in carbon isotope ratios is fairly comparable within individual second-

order cycles, particularly in the intermediate and upper portions of the successions, where

values, in general, increase, or less commonly, remain constant. Downward in the cycles, the

carbon ratios oscillate slightly between lighter and heavier. A change in pattern occurs in the

lower portions of the profiles (Fig. 5-7), where there is an upward trend to lower values.

Similarly to the carbon, the oxygen isotope ratios obtained for the Codó Formation are

dominantly low, ranging from –2.71‰ to –10.80‰. The behavior of the curves along the

profiles, as well as within the first- and second-order cycles, also shows patterns that are very

similar to the ones described for the carbon, but with oscillations toward lighter values in the

lower and intermediate portions of the cycles. Exception is an increased value in the second-

order cycles located at the lower portions of profiles A and C, where the oxygen isotope

values increase.

Comparisons of the carbon and oxygen curves reveal a positive covariance in profiles

B and C, with values ranging from +0.42 to +0.43, while profile A shows a negative

covariance of –0.36 (Fig. 5-8). This behavior is reflected in both the first- and second-order

cycles. The only exception is the second-order cycle at the base of profile C, which displays an

inverse covariance.

5.8. ISOTOPIC CHARACTERIZATION OF THE LACUSTRINE SYSTEM

The carbon and oxygen data presented here have valuable application for further

support the lacustrine signature of the Codó Formation, as well as reconstruct its

paleohydrology and evolution through time. This procedure was made possible only

considering the primary signature of these data, as confirmed by the petrographic and minor

and trace elements discussed earlier in this paper. In addition, the wide distribution of the

carbon and oxygen values throughout the analysed profiles, their overall good covariance, and

pulsating oscillations according to the first- and second-order shallowing-upward cycles, as

described above, led to exclude a significant diagenetic influence.

130

131

132

The results of the isotope analysis are in agreement with a lacustrine interpretation for

the Codó Formation exposed in the eastern margin of the Grajaú Basin, as proposed in

previous publications (e.g., Campbell et al., 1949; Aranha et al., 1991; Rossetti et al., 2000;

Paz & Rossetti, 2001). Several observations related to the carbon and oxygen isotope data

support this interpretation. First, a non-marine setting is indicated by the exclusive occurrence

of values lighter than –5.15 ‰ for the carbon, which is well below the range of about –2‰

and +5‰ expected for marine limestones (e.g., Deines, 1980; Hoefs, 1980). Second, the

carbon values obtained in the study area are consistent with Late Aptian continental deposits,

since marine limestones of this age display values ranging from +2%o to +4%o (Jones &

Jenkins, 2001; Fig. 5-9). Third, the ratios –2.71‰ to -10.80 ‰ for the oxygen isotopes are also

consistent with a continental setting (Talbot, 1990; Bird et al., 1991). A marine-influenced

continental environment might show lighter values of up to –5‰ (e.g., Ingram et al., 1996;

Hendry & Kalin, 1997; Fig. 5-10), but this hypothesis is very unlikely in this instance because

91% of the analysed samples are below this value. Fourth, the overall wide range of both δ

and δ

O values, is typical of continental-derived waters, as considered in a number of works

(e.g., Talbot & Kelts, 1990; Casanova & Hillaire-Marcel, 1993; Camoin et al., 1997; Fig. 5-

10). Fifth, the good covariance between the oxygen and carbon isotope data (Fig. 5-8),

although not exclusive to, is more consistent with a non-marine setting (Turner et al., 1983;

Gasse et al., 1987; Talbot, 1990; Talbot & Kelts, 1990; Charisi & Schmitz, 1995).

133

Fig . 5-10: Plots of carbon and oxygen stable isotope data from several marine and lacustrine deposits throughout

the world (Modified from Hendry and Kalin, 1997), and their comparison with data obtained in the Codó

Formation. This diagram shows that the isotopic composition of the Codó Formation is in conformity with

isotope data from lacustrine limestones.

In addition to support a non-marine deposition, the carbon and oxygen isotope data

revealed to be valuable for reconstructing lake paleohydrology. Both of these isotopes have

been used directly or indirectly to interpret climate (Fig. 5-11). In fact, temperature and

hydrological balance are the main controllers of isotopic composition in lake systems (Kelts &

Talbot, 1990; Lister et al., 1991). It is well known from studies of modern settings that

temperature causes fractionation of the oxygen in a constant ratio of 0.26 ‰/

C in the

bicarbonate-water-carbonate system (cf. Craig, 1965; Friedman & O’Neill, 1977). However,

the wide range of δ

O values observed in the Codó Formation would require a temperature

gradient equivalent to about 40

C, not expected considering a low paleolatitudinal location

(>10

) of the study area during the Late Aptian (Scotese et al., 1989). On the other hand, the

balance between influx and evaporation causes drastic changes in lake isotopic composition

134

(Talbot, 1990; Lister et al., 1991). Evaporation leads to enriched

O, as lighter

O escape to

the atmosphere. Conversely, high water inflow results in the return of

O from the

atmosphere, causing depletion in δ

O values. Thus, low δ

O values have been related to

higher lake levels, while high δ

O are attributed to lower lake levels, parameters that have

been indirectly related to climate (e.g., Talbot, 1990; Caimon et al., 1997; Lister et al., 1991).

Carbon isotope ratios also have a direct relation to climate. Hence, higher δ

C values

have been associated with aridity, while lower δ

C values indicate relatively more humid

climates (e.g., Talbot & Keltz, 1990; Valero-Garcés et al., 1995). This interpretation is based

on the fact that dry climates favour evaporation, increased influence of C4-path vegetation

type, lower influx, and lake stratification, which ultimately lead to organic matter preservation

with the consequent output of

C from the lake water.

A close relationship between the carbon and oxygen isotope ratios and the first- and

second-order shallowing-upward cycles is recorded in the study area. These changes are

135

analysed in the following in terms of facies development, which is not necessarily related to

climate changes, as widely applied in the literature (e.g., Olsen, 1986; Smoot & Olsen, 1994;

Goldhammer et al., 1990; Steenbrick et al., 2000; Hofmann et al., 2000; Aziz et al., 2000). In

this instance, the variation in both isotopes from lighter to heavier values along individual

second-order cycles follows upward gradations from central to marginal facies deposits, which

are related to changes in subsidence rates, as previously discussed (Fig. 5-12). Such facies

stacking requires alternating episodes of lake deepening and shallowing, which is associated

with increased subsidence and the return to relative stability or even uplift, respectively.

Extreme shallowing might have culminated with periods of desiccation, as indicated by

deposits with paleosols. Heavier isotope values recorded during these situations could be

attributed to a significant enhancement of the isotopic exchange with the atmosphere. This is

because as the water became extremely shallow, evaporation increased significantly due to

heating (Fig. 5-12). Differences in carbon values according to location in the lake system, with

marginal areas displaying higher values, have been also noted by other authors (e.g., Camoin

et al., 1997; Casanova & Hillaire-Marcel, 1993). The loss of

C to the atmosphere as a cause

to enrich the

C in the dissolved inorganic carbonate appears to be an active process in the

epilimnion of lakes with low water inflow (Stiller et al., 1985; Talbot, 1990; Talbot & Kelts,

1990).

The upward lighter trends of carbon and oxygen isotope values observed in the lower

portions of the studied successions is related to lake stratification and bottom anoxia, as

suggested by the prevalence of central lake deposits consisting of evaporites and bituminous

black shales. Such situation leads to

C burial, increasing the amount of

C dissolved in the

central lake waters (Herczeg, 1988; Talbot & Kelts, 1990). The subtle upward decrease in

isotope values would record the transition to intermediate lake environments, where this effect

was less significant. In addition, as opposed to the shallowing-upward cycles located upward

in the studied succession, marginal lake deposits that could contribute to increase the isotope

values through atmosphere exchange, as proposed above, are in general lacking in the upper

portions of these cycles.

136

137

The overall upward high-low-high trend of the isotope values coincides with the

position of the first-order cycles. As presented earlier, these cycles record main episodes of

lake desiccation superposed upon second-order cycles. The lightest values in the lower

portions of the first-order cycles is related to periods of maximum flooding, when the lake was

established and deeper water levels prevailed, as reflected by the better development of central

lake deposits. As the lake evolved, increased evaporation led to a lower water level,

progressively enhancing the isotope values and ultimately culminating with lake desiccation,

when the isotope values reached the heaviest ratios. This trend, well documented in unit 3, is a

little modified in the lower portions of unit 2, where the heaviest ratios were recorded as a

result of lake stratification and anoxia, as this time coincides with the maximum development

of evaporites and/or bituminous black shales in this lacustrine basin. The abundance of these

lithologies in unit 2, where relatively deeper water facies prevail, lead to propose that, rather

than evaporation, lake stratification and anoxia were crucial to form the evaporites (e.g.,

Warren, 1999). As the lake evolved through time, shallower water conditions became

progressively better developed and evaporation increased, but with restrict or no precipitation

of evaporative minerals.

The overall moderate positive covariance between the carbon and oxygen isotope

curves shown in profiles B and C is attributed to the prevalence of a closed lake environment,

a pattern that have been noticed in many other ancient and modern lake systems throughout

the world (e.g., Eicher & Siegenthaler, 1976; Gat, 1981; McKenzie, 1985; Gasse et al., 1987;

Janaway & Parnell, 1989; Talbot, 1990; Talbot & Kelts, 1990; Lister et al., 1991). It is

interesting to note that unit 2 of profile C coincides with a period of high covariance. This is

consistent with the presence of evaporites in these beds, typical of closed basins (Warren,

1999). In spite the evidences for lake closure, a comparison between the isotope curves shows

alternating phases of higher and relatively lower covariance, attributed to episodes of lake

opening. This can be particularly seen in profile A (Fig. 5-7), where the overall covariance

between carbon and oxygen isotopes is negative.

It is interesting to observe that the δ

O displayed the highest interval of variation

during closed phases, which ranged from -3.63‰ to –4.89‰, against the interval of variation

of –0.09‰ to -1.87 ‰ that characterizes open phases. This is because closed lakes have better

138

chances to show oscillations in water levels as the residence time increases, which lead to

enhance the δ

O. Conversely, the isotopic composition of open lakes is more stable due to the

balance caused by the continuous basin inflow, as recorded in several lake systems, such as

Lake Henderson (Stuiver, 1970), Lake Huleh (Stiller & Hutchinson, 1980) and Greifensee

(McKenzie, 1985).

5.9. FINAL REMARKS

The carbon and oxygen isotopic composition of the Codó Formation in the eastern

Grajaú Basin can be directly related to facies changes, as revealed by the good correspondence

between the isotope values and the shallowing-upward cycles. Deciphering the causes of these

changes through time, whether related to climate fluctuation or to any other variation in the

basin, such as subsidence or sediment inflow, is not so straightforward. There has been an

agreement among the authors in relating carbon and oxygen isotopes with lake hydrology,

which is often used directly or indirectly to make inferences about climate (e.g., Smoot &

Olsen, 1994; Steenbrick et al., 2000; Hofmann et al., 2000; Aziz et al., 2000). However, this

study shows that significant changes in carbon and oxygen isotope composition of lake waters,

resembling climatic cycles, can be also related to fluctuations in subsidence rates caused by

syn-sedimentary seismic activity. In this case, combination of isotope and sedimentological

data provided the key for distinguishing which of these factors left the most significant imprint

in the sedimentary record.

We have shown in this paper the close relationship of both carbon and oxygen isotopes

with the first- and second-order shallowing-upward cycles that characterize the studied unit.

The dominant asymmetric nature of these cycles, inferred with basis on the high facies

variability when comparing one cycle to another, the limited lateral extension, as well as the

frequent and random thickness changes, led to propose that climate was not their primordial

controlling factor. On the other hand, sedimentological data favour to attribute these cycles to

syn-sedimentary seismic activity associated with the early tectonic evolution of the São Luís-

Grajaú Basin during the Late Aptian. The Codó Formation was deposited just prior to the main

rifting that culminated with the process of opening of the South Atlantic Equatorial Ocean.

During this initial time, there was the development of a shallow, but extensive subsiding

139

intracontinental basin (Azevedo, 1991; Góes & Rossetti, 2001). This characteristic matches

well with the presence of shallow lakes in marginal areas of the basin, where faults with

reduced offsets are expected to have prevailed. Subsidence gave rise to local water

accumulation, forming closed and perennial lake systems, but as compression took place, and

the area was uplifted, the development of ephemeral lakes appears to have been favoured. This

situation is recorded in the studied profiles by a change from shallowing-upward cycles with

dominance of central and intermediate lake deposits, to cycles with well-developed marginal

lake deposits, as occurs in the lower and upper portions of the first-order cycles, respectively.

Such facies arrangement records the upward transition from periods of maximum flooding to

periods when the lake level fell to lower levels. The carbon and oxygen isotopic composition

of such lake basins is expected to be characterized initially by light values, but as the residence

time increases due to shallowing, heavier values are reached due to the high evaporation-

inflow budget provided by the progressive decrease in water level.

Because arid climate prevailed along the Brazilian equatorial margin during the Late

Aptian (Lima et al., 1980; Lima, 1982; Batista, 1992; Rodrigues, 1995; Rossetti et al., 2001),

evaporite precipitation took place in central lake areas, induced by water stratification and

bottom anoxia (Kirkland & Evans, 1981). The highest value of carbon isotope coinciding with

the moment of maximum formation of evaporites and bituminous black shales is consistent

with this interpretation.

Therefore, different styles and/or intensities of seismic pulses alternating with sediment

deposition might cause changes in the lake level, promoting alternating periods of flooding

and fall in the lake level, and resulting in well-developed asymmetric shallowing-upward

cycles. Such scenario will ultimately affect the overall isotope composition of lake waters.

Acknowledgments. The Itapicuru Agroindustrial S/A is acknowledged for the

permission to access the quarries with the exposures of the Codó Formation. This work was

financed by the Brazilian Council for Research–CNPq (Project #460252/01). The authors are

grateful to Dr. Alcides Sial e Dr. Valderez Ferreira for helping with the geochemical analyses.

140

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147

6. CONCLUSÕES GERAIS

1. A Formação Codó consiste em depósitos dominantemente continentais nas áreas

estudadas, onde se distinguem dois contextos deposicionais: a) lagos estáveis, bem

estratificados e salinos, com períodos de fechamento e conseqüente anoxia do sistema

prevalecendo na margem leste da Bacia de São Luís-Grajaú, com evaporitos restritos à porção

central nas partes mais profundas da bacia lacustre; e b) complexos de sabkha/saline pan na

porção sul da Bacia de São Luís-Grajaú, onde as condições climáticas e/ou tectônicas

favoreceram o desenvolvimento de lagos efêmeros, com a formação de extensas planícies de

lama contendo abundante precipitação de evaporitos centrada nas partes marginais do sistema;

2. A Formação Codó na área de estudo depositou-se de forma cíclica, formando ciclos

de arrasamento ascendente, interpretados como registro de períodos de expansão e contração

do sistema lacustre-sabkha-saline pan, gerados por flutuações climáticas e tectônicas;

3. Ciclos climáticos foram revelados por diversos pares milimétricos de fácies

deposicionais, arranjados em estilo semelhante a varves;

4. Ciclos tectônicos foram revelados por: a) alta variabilidade de fácies; b) limitada

extensão lateral dos ciclos; c) mudanças aleatórias na espessura e freqüência dos ciclos; e d)

correspondência com zonas de deformação sísmica sindeposicional;

5. O Sr mostrou-se como um parâmetro paleoambiental sensível, registrando valores

muito distintos daqueles esperados para águas marinhas neoaptianas;

148

6. Os valores de Sr também tiveram boa correlação com os ciclos deposicionais de

menor freqüência (i.e., ciclos de ordem superior), sendo sua variação atribuída a períodos de

contração e expansão da bacia lacustre;

7. O S mostrou-se menos sensível como indicador paleoambiental. Embora os valores

sejam condizentes com salmoura de origem continental na área de Codó, isto não se repetiu na

área de Grajaú, onde os valores diferem em uma faixa mais ampla do que aquela esperada em

ambientes com influência continental. É possível que os valores de S distintos entre as duas

áreas estudadas estejam refletindo variações nas condições de oxigenação do sistema

deposicional, com valores maiores na área de Codó, onde condições anóxicas prevaleceram,

incentivando a proliferação de bactérias sulfato-redutoras que influenciaram fortemente o

fracionamento do S e deixaram o sulfato residual da água do lago enriquecido

preferencialmente em

8. Este trabalho demonstrou que flutuações nas taxas de subsidência, causadas por

atividade sísmica sin-sedimentar pode responder por variações nos valores dos isótopos de C e

O muito semelhantes àquelas observadas em sucessões resultantes formadas por flutuações de

ordem climática;

9. Diferentes estilos e/ou intensidades de pulsos sísmicos, alternados com deposição

sedimentar, causam mudanças no nível do lago e promovem períodos de inundação e

dessecação do sistema deposicional durante episódios de estiramento e compressão,

respectivamente. A deposição de carbonatos com valores de δ

C e δ

O preferencialmente

mais leves, associada com a instalação preferencial de lagos perenes na fase de inundação, e

de valores de δ

C e δ

O mais pesados, associados com a instalação de lagos efêmeros

durante dessecação, foram bem marcantes no ciclos de ordens intermediária e superior.

10. A análise de fácies detalhada e o mapeamento de padrões de empilhamento estratal

são a chave para a interpretação de sucessões progradantes como no caso da Formação Codó.

A combinação deste tipo de informações com dados isotópicos de C, O, Sr e S propicia

interpretações mais refinadas de sistemas deposicionais continentais onde ocorrem depósitos

de evaporitos e calcários. Além disto, este tipo de integração possibilita a disponibilização de

informações visando-se reconstituir condições paleohidrológicas do sistema deposicional e

dos fatores controladores de sua evolução.

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