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UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL
INSTITUTO DE GEOCIÊNCIAS
PROGRAMA DE PÓS-GRADUAÇÃO EM GEOCIÊNCIAS
ORIGEM VULCÂNICA PARA O TONSTEIN DA JAZIDA DO
FAXINAL (RS):
ESTUDOS MINERALÓGICOS, PETROGRÁFICOS E DE
PALINOFÁCIES.
MARGARETE WAGNER SIMAS
ORIENTADOR – Prof. Dr. Milton Luiz Laquintinie Formoso
CO-ORIENTADORA – Profª. Drª. Margot Guerra-Sommer
BANCA EXAMINADORA
Prof. Dr. Carlos Augusto Sommer – Instituto de Geociências, Universidade Federal
do Rio Grande do Sul
Prof. Dr. João Marcelo Medina Ketzer - Instituto do Meio Ambiente – Pontifícia
Universidade Católica do Rio Grande do Sul
Prof. Dr. Norberto Dani - Instituto de Geociências, Universidade Federal do Rio
Grande do Sul
Dissertação de Mestrado apresentada como requisito
parcial para a obtenção do Título de Mestre em
Geociências.
Porto Alegre – 2008
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Simas, Margarete Wagner
Origem vulcânica para o tonstein da jazida do Faxinal (RS):
estudos mineralógicos, petrográficos e de palinofácies. / Margarete
Wagner Simas. - Porto Alegre : IGEO/UFRGS, 2008.
[117] f. il.
Dissertação (Mestrado). - Universidade Federal do Rio Grande
do Sul. Instituto de Geociências. Programa de Pós-Graduação em
Geociências. Porto Alegre, RS - BR, 2008.
1. Tonstein. 2. Cinzas Vulcânicas. 3. Mineralogia. 4. Petrografia
.
5. Palinofácies. 6. Permiano Inferior. 7. Bacia do Paraná. I. Título.
_____________________________
Catalogação na Publicação
Biblioteca Geociências - UFRGS
Renata Cristina Grun CRB 10/1113
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À minha mãe Paulina Wagner Simas
que sempre esteve ao meu lado
dedico esta Dissertação.
AGRADECIMENTOS
Eu gostaria de expressar meu agradecimento a todos aqueles, pessoas e instituições,
que contribuíram para realização deste trabalho.
Ao Programa de Pós-Graduação em Geociências - PPGGeo e ao Centro de Pesquisas
em Petrologia e Geoquímica - CPGq da Universidade Federal do Rio Grande do Sul – UFRGS que
me propiciaram a realização deste trabalho com o apoio institucional.
Agradeço pelo auxílio financeiro dos Projetos CNPq – FAPERGS (Processo
04/0825.3 – PRONEX) coordenado pelo Prof. Dr. Milton Formoso e pelo Projeto Universal CNPq
(Processo 473301/04-0) coordenado pela Profª. Drª. Margot Guerra-Sommer que foram
imprescindíveis para realização deste trabalho.
Expresso meu agradecimento a todos os membros que compõem a banca
examinadora e que aceitaram julgar este trabalho:
• Prof. Dr. Carlos Augusto Sommer, da Universidade Federal do Rio Grande do Sul,
• Prof. Dr. João Marcelo Medina Ketzer, da Pontifícia Universidade Católica do Rio Grande do
Sul,
• Prof. Dr. Norberto Dani da Universidade Federal do Rio Grande do Sul.
Eu agradeço, particularmente, ao Prof. Dr. Milton L. L. Formoso pela orientação
desta dissertação, pelos imprescindíveis ensinamentos e discussões e pelo inestimável apoio neste
do trabalho.
Meu especial agradecimento à Profª. Drª. Margot Guerra-Sommer, co-orientadora da
dissertação, pelo grande incentivo e apoio, pelas várias discussões que possibilitaram o
desenvolvimento de idéias que foram fundamentais para a concepção desta dissertação, e ainda e
incansável auxílio em cada etapa do trabalho.
Agradeço à estes Grandes Mestres, o Orientador e a Co-Orientadora, pelos
significantes exemplos de vida acadêmica que representam, pela dedicação de ambos neste meu
intento e por acreditarem nas minhas capacidades encorajando-me a retornar a vida acadêmica e a
avançar no conhecimento científico.
Agradeço, ao coordenador do PPGGeo, Prof. Dr. Léo Afraneo Hartmann, pela
oportunidade que me proporcionou de retornar a este programa de pós-graduação e,
particularmente, por conduzir a minha iniciação científica nas geociências.
Eu agradeço, especialmente, ao Prof. Dr. Luiz Fernando De Ros pela valiosa
colaboração em diversas etapas deste trabalho com importantes ensinamentos, observações,
discussões, sugestões e por conceder gentilmente o empréstimo do microscópio no qual foi
realizada a etapa primordial desta dissertação, a petrografia.
Meu especial agradecimento aos co-autores do paper Drª. Miriam Cazzulo-Klepzig
pela relevante contribuição na compreensão e integração de temas desta pesquisa, constante
incentivo e prestimosos conselhos. Ao Prof. Dr. João Graciano Mendonça Filho, Diretor do
Instituto de Geociências da Universidade Federal do Rio de Janeiro (IGEO-UFRJ) e Coordenador
do Lab. de Palinofácies & Fácies Orgânica, por oportunizar-me a inédita aplicação da técnica de
Análises de Palinofácies no estudo de tonsteins, pela análise, discussão da integração de dados e
correções no texto. Também agradeço à MSc. Taíssa Rêgo Menezes do Lab. de Petrografia
Orgânica do CENPES-PETROBRAS que auxiliou na confecção e organização destas análises de
palinofácies
Agradeço à Profª. Drª. Nara Basso do Departamento de Química Orgânica da
PUCRS pela estratégica concessão de horas de análises no Centro Microscopia Eletrônica da
PUCRS, vitais para petrografia e mineralogia deste trabalho, através de convênio entre
pesquisadores do Projeto Nanotecnologia de Bentonitas, por intermédio do Prof. Dr. Milton
Formoso .
Agradeço a Companhia de Pesquisas e Lavras Minerais – COPELMI, na pessoa do
Gerente de Operações, Engº. de Minas Alexandre Grigorieff, pela permissão de visita à Mina do
Faxinal para os trabalhos de campo e coleta de amostras. Agradeço ao Geólogo de Carvão Gustavo
Bastiani, por conduzir-nos ao leito de tonstein da Mina do Faxinal, inteirando-nos de relevantes
informações desta ocorrência. Agradeço ainda ao Supervisor da Mina do Faxinal Eraldo José de
Lima, pelo empenho e prestimosa ajuda durante os trabalhos de campo junto às frentes de lavras,
viabilizando a nossa amostragem e propiciando-nos uma visão ampla das relações do tonstein com
seqüência portadora de carvões aflorantes nos cortes.
Meu especial agradecimento à Secretária do CPGq e do Prof. Formoso, Dona
Carmem Peres por prestimosos esclarecimentos e importante ajuda institucional. Agradeço também
ao Secretário PPGGeo Roberto Pereira e às Auxiliares Administrativas, Elen e Letícia pelos
prestimosos esclarecimentos e ajuda institucional.
Os trabalhos analíticos contaram com a fundamental colaboração de técnicos,
funcionários e bolsistas dos laboratórios do CPGq-UFRGS. Ao Renato Figueira, Jorge, Quelen do
Lab. de Difração de Raios-X. Ao Prof. Dr. Norberto Dani, e aos técnicos e funcionários Júlio, Luiz
e Ronaldo dos Laboratórios de Fluorescência de Raios-X e Geoquímica. Ao Adriano, Sandrinha,
Fabrício e Juliana do Lab. de Preparação de Amostras pelo auxílio nos procedimentos de moagem
de amostras, cortes de seções, confecção de lâminas e polimentos. Às bolsistas de iniciação
científica Isabela e Tatiana pela colaboração desde as etapas de campo até a finalização da
dissertação.
Agradeço ao prestimoso auxílio das bibliotecárias e auxiliares de biblioteca da
Biblioteca de Geociências Renata Grun, Veleida Blank, Ivo e Telmo
Expresso o meu agradecimento aos colegas e amigos, Karen Pires, Liane Calarge,
Renata Schmitt, Flávia Schenato, Heinrich Frank, Vitor Paulo Pereira, Etiene Pires, Maria do
Carmo Cunha, Karin Goldberg e Cris Lenz, com cuja valiosa contribuição fizeram-se presentes para
auxiliar o desenvolvimento deste trabalho.
E, finalmente, expresso meu agradecimento às minhas amadas mãe e irmãs Paulina,
Rosane, Marilene, Cláudia e Elisabete; ao meu amado sobrinho Adriel, ao querido cunhado Martin
Cordovana, por acreditarem na concretização deste trabalho e também ao Maurício, aos pequenos
Ygor, Yasmin, Yuri e Yanko e ao José Ninow por fazerem parte da minha família.
RESUMO
Análises mineralógicas, petrográficas e de palinofácies são registradas em um leito
de tonstein associado a camadas de carvão na Jazida do Faxinal, Rio Grande do Sul, Brasil. A
integração dos dados revestiu-se de grande importância para atribuir uma origem vulcânica para
este argilito caolinítico. O tonstein é uma rocha quase monominerálica, composta
predominantemente por caolinita antigênica. Dispersos na massa caolinítica ocorrem os minerais
piroclásticos: paramorfos de quartzo-ß bipiramidais euédricos, “splinters” de quartzo transparente,
zircão idiomórfico, apatita euédrica, alanita e pseudomorfos de sanidina, os quais são considerados
como uma suíte restrita de minerais vulcânicos de tonsteins distais que preservaram durante a
diagênese. Os minerais primários e suas feições texturais, bem como as relações de campo, indicam
uma origem vulcânica de queda para essa camada. O estudo de palinofácies, inédito para este tipo
de rocha, evidenciou uma composição diferenciada da matéria orgânica estruturada ao longo do
perfil do tonstein. Análises estatísticas do querogênio de diferentes níveis da camada de tonstein
indicaram altas percentagens de fitoclastos (xilema e epiderme) associados à menor
representatividade de palinomorfos. Análises microestratigráficas destes níveis demonstraram que a
saturação e a precipitação dos palinomorfos foram altamente influenciadas pelo intenso processo de
queda de cinzas. O nível basal caracteriza-se por densos aglomerados de esporos e polens, enquanto
o topo é marcado pela preservação de fragmentos de colônias de algas Botryococcus evidenciando
uma deposição subaquosa desta camada. Alguns fragmentos de epiderme (cutículas) evidenciam,
por sua coloração, acentuada alteração termal. Esses dados possibilitaram vincular as peculiaridades
do mecanismo de deposição e preservação da matéria orgânica com o processo de formação do
tonstein relacionado à rápida precipitação de cinzas vulcânicas. O tonstein intercalado em camada
de carvão indica um episódio de sedimentação de tefra durante a deposição da seqüência portadora
de carvão no Permiano Inferior no sul da Bacia do Paraná.
Palavras Chave: Tonstein, Cinzas vulcânicas, Mineralogia, Petrografia, Palinofácies, Permiano
Inferior, Bacia do Paraná.
ABSTRACT
Mineralogical and palynofacies analyses are reported from a tonstein layer
interbedded with coal seams in the Faxinal coalfield, Rio Grande do Sul, Brazil. Integration of data
has far reaching significance for attributing a volcanic origin for this kaolinitic claystone bed. The
tonstein is almost monomineralic rock, composed mainly by authigenic kaolinite. Scattered in the
kaolinitic mass primary pyroclastic minerals occur: euhedral beta-quartz paramorphs and water-
clear quartz splinters, idiomorphic zircons, apatite, allanite and sanidine pseudomorphs; considered
as a restricted suite of silicic volcanic minerals of the distal tonsteins which preserved during
diagenesis. The primary minerals and their textural features, as well as the field relations, indicate a
volcanic air-fall origin. Analyses of the kerogens from different levels of tonstein layer indicate
high percentages of phytoclasts combined with very low palynomorph percentages.
Microstratigraphic analyses of the tonstein profile demonstrated that saturation and precipitation of
palynomorphs were highly influenced by the intense ash-fall process. The preservation of
Botryococcus colonies at the top of the tonstein evidenced the subaqueous deposition of this bed.
The brown color of several cuticle fragments and tracheids was linked to thermal alteration. The
tonstein interbedded in a coal seam indicates an episode of tephra sedimentation during the
deposition of the coal-bearing sequence of the Lower Permian in the southern Paraná Basin.
Keywords: Tonstein, volcanic air-fall origin, mineralogy, petrography, palynofacies, Lower
Permian, Paraná Basin
SUMÁRIO
Texto explicativo da estrutura da dissertação ........................................................... 10
1 INTRODUÇÃO .................................................................................................. 12
1.1 Generalidades ............................................................................................... 12
1.2 Objetivos ....................................................................................................... 16
1.3 Estado da Arte .............................................................................................. 16
1.4 Contexto Estratigráfico ............................................................................... 23
1.5 Metodologia .................................................................................................. 27
1.5.1 Material ................................................................................................... 27
1.5.2 Métodos ................................................................................................... 28
1.5.2.1 Petrografia e Mineralogia ............................................................. 28
1.5.2.2 Palinofácies .................................................................................... 30
1.6 Análise Integradora ..................................................................................... 31
1.7 Referências .................................................................................................... 38
2 ARTIGO SUBMETIDO ……………………………………………..……... 45
“Mineralogy and Palynofacies analyses of the Tonstein of Faxinal coalfield, an altered
volcanic-ash layer from the Lower Permian of Paraná Basin, Brazil”
3 ANEXOS
ANEXO A - Carta de aceitação do manuscrito submetido
ANEXO B - Cópias de resumos e artigos publicados em co-autoria
Resumo
“A Roof Shale Flora da Mina do Faxinal (Sakmariano do Rio Grande do Sul): Uma nova
concepção sobre o processo tafonômico”
Artigo A
“Geochronological data from the Faxinal coal succession, southern Paraná Basin, Brazil: a
preliminary approach combining radiometric U-Pb dating and palynostratigraphy"
Artigo B
“Peat-forming environment of Permian coal seams from the Faxinal coalfield (Paraná Basin) in
southern Brazil, based on Palynology and Palaeobotany”
10
Texto explicativo da estrutura da dissertação
O documento aqui apresentado, obrigatório para a obtenção de título de
Mestre em Geociências junto ao Programa de Pós-Graduação em Geociências da
Universidade Federal do Rio Grande do Sul (PPG-Geo/UFRGS), foi elaborado de acordo
com a Resolução nº. 093/2007, da Câmara de Pós-Graduação, a qual normatiza a
apresentação de dissertações e teses na forma de artigos publicados e/ou submetidos pelo
aluno em periódicos científicos, sendo aqui sintetizados os procedimentos utilizados na
estruturação do Documento Final submetido à avaliação.
Constam desse documento Resumo e Abstract que sintetizam os objetivos e
os resultados obtidos com o desenvolvimento do projeto de pesquisa. O capítulo 1 -
Introdução - corresponde a uma compilação de dados sobre a presença de leitos de tonstein
associados a camadas de carvão no Permiano Inferior da Bacia do Paraná, datações
radiométricas obtidas a partir de análises em zircões procedentes dessas rochas e
correlações estratigráficas com outras seqüências gonduânicas. Considerando a necessidade
de confirmar a origem vulcânica para o tonstein do Faxinal através de estudos petrográficos
e de palinofácies foi delimitado o problema e formulados os objetivos: Generalidades,
Objetivos, Estado da Arte, Contexto Estratigráfico, Metodologia, Análise Integradora,
Bibliografia e Anexos.
No Estado da Arte é apresentado um histórico abrangente dos trabalhos
publicados sobre o tema, abordando as diferentes concepções a respeito da gênese do
tonstein. Na Metodologia são caracterizadas as técnicas utilizadas na preparação e análise
do material, de acordo com as diferentes metodologias de estudo, ou seja, estudos de
Petrografia, Mineralogia e Análises de Palinofácies.
No Contexto Estratigráfico é apresentada uma síntese estratigráfica regional,
usando critérios e estratigrafia de seqüências.
A Análise Integradora é formulada a partir dos objetivos inicialmente
formulados e dos resultados atingidos com o desenvolvimento do projeto de mestrado. A
integração dos resultados de análises petrográfica, mineralógicas, geoquímicas e de
palinofácies possibilitou estabelecer conclusões inéditas a respeito do processo de
deposição do tonstein, ratificando sua origem vulcânica.
11
O capítulo 2 - Corpo Principal da tese é composto pelo capítulo onde é
apresentado o artigo “Mineralogy and palynofácies analyses in the Tonstein of Faxinal
Coalfield, an altered volcanic-ash layer from the Lower Permian of Paraná Basin, Brazil”
submetido durante à realização do mestrado, no qual a mestranda é a primeira autora,
precedido da carta de aceitação da submissão por parte do periódico International Journal
of Coal Geology.
Compõem o capítulo 3 - referente aos Anexos (a carta de Aceitação da
Submissão do artigo, o resumo “A Roof Shale Flora da Mina do Faxinal (Sakmariano do
Rio Grande do Sul): Uma nova concepção sobre o processo tafonômico” e dois artigos
“Geochronological data from the Faxinal coal succession, southern Paraná Basin, Brazil:
a preliminary approach combining radiometric U-Pb dating and palynostratigraphy” e
“Peat-forming environment of Permian coal seams from the Faxinal coalfield (Paraná
Basin) in southern Brazil, based on Palynology and Palaeobotany” publicados durante o
desenvolvimento do mestrado, focados em seqüências portadoras de carvão da Bacia do
Paraná, que forneceram subsídios importantes para a dissertação.
As Referências Bibliográficas estão organizadas por ordem alfabética no
final desse documento e referem-se exclusivamente ao capítulo 1.
12
1 - INTRODUÇÃO
1.1 - Generalidades
A ocorrência de leitos de argilito identificados como tonsteins em jazidas de
carvão no sul da Bacia do Paraná (Formação Rio Bonito, Grupo Guatá) é extensiva
distribuindo-se no Rio Grande do Sul, desde bacias da borda leste (Faxinal, Leão) até a área
da jazida de Candiota. De acordo com modelo geocronológico proposto para a Bacia, com
base em estratigrafia de seqüências de alta resolução (Milani, et al. 1998), o intervalo
portador de carvões ocorre na base da seqüência de segunda ordem Carbonífero-Triássico
Inferior (Fig.1).
Figura 1 - Mapa geológico simplificado da Bacia do Paraná no Brasil com os maiores elementos tectônicos, e
referências geográficas (modificada de Milani, 2003).
13
Leitos de tonstein podem ter ampla distribuição regional, e, por suas
características mineralógicas, freqüentemente podem ser datados através de estudos
isotópicos. A distribuição geográfica dessas camadas, por outro lado, está limitada a
ambientes potencialmente favoráveis à sua deposição como, por exemplo, ambientes
lacustrinos. Sua ocorrência muito freqüente em camadas de carvão está relacionada ao
condicionamento geológico das bacias carboníferas e à energia deposicional do
paleoambiente. A possibilidade de integrar estudos petrográficos, geoquímicos e isotópicos
em tonsteins com dados bioestratigráficos de rochas associadas os caracteriza como
marcadores isócronos no registro sedimentar.
Na jazida de Candiota camadas de tonstein distribuem-se em uma área que
corresponde aproximadamente a 300 Km2 de extensão. Com base em estudos
mineralógicos e químicos, além de relações de campo, Corrêa da Silva (1973), sugeriu que
as camadas argilosas na jazida de Candiota corresponderiam a stratotonstein de acordo com
sistema classificatório de Bouroz, ou dichtertonstein conforme nomenclatura proposta por
Schüller, sendo sua origem detrítica. Dela Fávera et al. (1992) concordam com essa origem,
considerando os tonsteins de Candiota como indicadores de clima seco. Análises
geoquímicas detalhadas nos tonsteins intercalados na Camada Candiota Superior e na
Camada Inferior de carvão em Candiota, realizadas por Formoso et al. (1999) indicaram
para esses leitos, uma origem vulcânica relacionada à dispersão de cinzas por via aérea, de
acordo com os critérios de Bohor e Triplehorn (1993).
O arcabouço palinoestratigráfico do Paleozóico Superior na Bacia do Paraná,
estabelecido detalhado mais recentemente por Souza e Marques-Toigo, (2005), utilizado
em crono-correlações de caráter local, torna-se restritivo no estabelecimento de correlações
em escala regional na Bacia. Estas correlacões ficam comprometias especificamente dadas
as dificuldades de controles faciológicos; desta forma, determinados dados palinológicos
utilizados como pararâmetros bioestratigráficos, poderiam corresponder a ecozonas, sem
significado temporal. No estabelecimento de correlações com escalas de tempo global, por
outro lado, esses parâmetros palinoestratigráficos são também restritivos, levando em
consideração as características endêmicas das floras gonduânicas.
A identificação de rochas vulcânicas, que ocorrem intercaladas à camadas de
carvão na porção sul da Bacia do Paraná, e a aplicação de técnicas de datação radiométrica
14
essas rochas, oportunizaram o início de programas de estudo com a finalidade de
estabelecer o ajuste dos zoneamentos bioestratigáficos a escalas temporais internacionais
com base numérica.
Datações IDITIMS U-Pb efetuadas por Matos et al. (2000, 2001) em zircões
do Tonstein A intercalado na Camada Candiota Inferior indicaram uma idade radiométrica
correspondente a 267.1 ± 3.4 Ma. Cazzulo-Klepzig et al. (2002) analisam o conteúdo
palinológico da camada de tonstein e tentam estabelecer uma calibração entre o
zoneamento palinoestratigráfico proposto por Marques-Toigo (1991) e essa datação
radiométrica, incluindo as palinofloras procedentes do tonstein e carvões associados na
subzona palinoestratigráfica Caheniasaccites ovatus, até então datada como
Artinskiano/Kunguriano, a qual ficou restrita ao intervalo Kunguriano/Roadiano (267 ± 4
Ma.), de acordo com Jin et al. (1997). Esses resultados, porém, mostraram-se
significativamente discordantes em relação a modelos geocronológicos estabelecidos para
toda a sucessão permiana na Bacia do Paraná, utilizando diferentes metodologias (Lavina e
Lopes, 1987; Milani et al., 1998).
Com a finalidade de obterem dados mais consistentes sobre as relações
estratigráficas entre as diferentes jazidas de carvão e os modelos deposicionais já
existentes, foram desenvolvidos diferentes estudos. Os resultados permitiram definir o
intervalo de geração de carvões no Rio Grande do Sul e calibrar dados palinoestratigráficos,
tendo como base, análises radiométricas (IDITIMS U-Pb; SHRIMP U-Pb) realizadas em
tonsteins das jazidas de Candiota e Faxinal. (Guerra-Sommer et al., 2006, in press-a,b,c).
Os dados radiométricos indicaram que a idade média das camadas de tonstein é de 290.6 ±
1.5 Ma. (Guerra-Sommer et al., in press-c) correspondente ao Sakmariano Médio de acordo
com os critérios de Gradstein et al. (2005). O intervalo máximo para a geração dos carvões
na porção sul da Bacia do Paraná restringe-se a ± 2 Ma (Fig. 2), pouco expressivo em
termos de tempo geológico, oportunizando a hipótese de que possa representar um marco
estratigráfico.
15
Figura 2 - Integração dos resultados obtidos entre a datação radiométrica, dados bioestratigráficos e de
estratigrafia de seqüências (modificada de Guerra-Sommer et al (in press-c).
As idades numéricas obtidas nas jazidas de Candiota e Faxinal podem ser
consideradas equivalentes àquelas obtidas em rochas bioestratigráficamente controladas na
Bacia de Paganzo, datadas entre 302 ± 6 Ma e 288 ± 7 Ma., calibrados à zona Pakhapites
fusus - Vittatina subsaccata (FS) registrada também na Formação Tasa Cuna e na parte
superior da Formação El Imperial (Césari, 2007).
Na Bacia do Karoo a equivalência pode ser estabelecida com datações
radiométricas de tufos procedentes de seqüências da porção inferior da Formação Prince
Albert do Grupo Ecca com datação de 289.6 ± 3.8 Ma. até 288 ± 3 Ma. (Bangert et al.,
1999).
Para López-Gamundi (2006) os horizontes de tufos identificados ao longo
das margens continentais nas bacias de San Rafael, Souce Grande, Paraná e Karoo indicam
um intervalo produtor de tufos vulcânicos entre 280 e 260 Ma. relacionado à fase ácida
riodacítica a riolítica do vulcanismo de Choiyoi. Os dados gerados por idades radiométricas
SHRIMP U-Pb para as jazidas de Candiota e Faxinal, indicam idades entre 292 e 288 Ma.,
mais antigas, portanto, para o evento vulcânico, relacionado por Guerra-Sommer et al. (in
press-c) a fase andesítica do referido vulcanismo.
16
A camada de tonstein do Faxinal é enfocada no presente trabalho por suas
peculiaridades: constituí-se em uma camada única, facilmente reconhecível, lateralmente
contínua em toda a jazida; esse leito contém uma densa associação de plantas fósseis
magnificamente preservadas; a presença de um nível carbonático contínuo em sua base
distingue esse leito dos demais já descritos para a Jazida de Candiota.
Análises petrográficas preliminares desenvolvidas no tonstein do Faxinal
(Guerra-Sommer et al., in press-a), com base nas características texturais e mineralógicas
dos minerais primários, permitiram estabelecer uma hipótese de origem vulcânica para esta
rocha. Todavia, o detalhamento analítico desse tonstein, estabelecido através da integração
de análises petrográficas, mineralógicas e de palinofácies permite inferir novos dados a
respeito de sua gênese e sugere uma fonte vulcânica mais próxima como origem das cinzas.
Visando documentar essas inferências, foram propostos para o presente
estudo os objetivos abaixo relacionados.
1.2 - Objetivos
• Caracterizar a petrografia, a composição mineralógica e palinofaciológica do
tonstein intercalado à camada de carvão S da Jazida do Faxinal, visando obter assinaturas
das rochas para confirmar a origem vulcânica e estabelecer comparações com possíveis
fontes piroclásticas.
• Integrar os resultados das diferentes metodologias com a finalidade de confirmar
a origem vulcânica para essa camada argilosa, designada como tonstein (sensu Bohor and
Triplehorn, 1993).
1.3 - Estado da Arte
A história da investigação dos tonsteins inicia-se com o trabalho pioneiro de
Bischof (1863), que assim designa os leitos de argilitos associados a camadas de carvão nas
jazidas da Saxônia, Silésia e Saar, sem, contudo, atribuir conotação genética ao termo
conforme referem às revisões terminológicas de Bouroz, et al. (1983) e Williamson
(1970a). Esta designação foi introduzida no período conhecido como “pré-microscópico”,
17
quando as características mineralógicas diagnósticas eram reconhecidas apenas com
recursos que permitiam observações visuais das rochas. Todavia, mesmo com estas
limitações, muitas dessas características foram posteriormente confirmadas através de
estudos petrográficos, os quais se tornaram efetivos somente com o desenvolvimento da
microscopia.
Termier (1889, 1890) faz as primeiras descrições microscópicas e
petrográficas da composição dos tonsteins considerando elementos de suas texturas
reliquiares e identifica grandes prismas vermiculares de um mineral na matriz
criptocristalina, o qual denominou como “leverrierite”, considerando-o como uma
variedade transicional entre caolinita e mica. Este termo foi usado por um longo tempo em
estudos de tonsteins. No mesmo período Schmitz-Dumont (1894) descobre, em tonsteins,
texturas indicando uma origem vulcânica do material primário e utiliza esses dados para o
estabelecimento da hipótese vulcanogênica para a origem dessas rochas. Esses estudos
foram realizados nas camadas de carvão do Carbonífero Superior da região de Saar. Essa
concepção tornou-se, posteriormente, no conceito genético mais aceito pela comunidade
científica.
No inicio do século XX, a petrografia finalmente é caracterizada como uma
ciência autônoma. A maioria dos estudos petrográficos, porém, dedicaram-se às rochas
ígneas, concentrando-se na investigação de suas microtexturas. As pesquisas de Rosembush
e seus colaboradores neste campo foram destacadamente representativas. Estudos em
rochas sedimentares já haviam sido desenvolvidos por Sorby (1877, 1880) quando é
referida a importância da luz polarizada para análise dessas rochas e descreve texturas
pelíticas, utilizando-as para interpretação genética (Levison-Lessing, 1923).
Ao longo do século XX, a geologia, mineralogia e petrografia das argilas
passam a ser estudadas com detalhe (Ginzburg, 1912, 1915, Zemyatchenskii, 1923) e a
moderna concepção das argilas, com ênfase em seus aspectos mineralógicos, é formulada
inicialmente por Vernadskii (1954).
O intensivo estudo que se fez nesse período no sentido da compreensão da
gênese das argilas ignorou, por outro lado, as rochas caracterizadas como tonsteins, sendo
sua investigação ocasional e irregular, sobretudo quanto aos seus litotipos. Tomando como
referência os estudos realizados em argilas, Ginzburg (1912) propõe três hipóteses para a
18
origem dos tonsteins: pós-vulcânica, relacionado a ambiente paludal e por processo
intempérico. Essa última hipótese consolida seu status de teoria com os trabalhos de
Polynov (1956) e Ginzburg (1963).
A hipótese de uma gênese vulcânica para essas rochas recebeu, todavia,
confirmações adicionais através de diferentes estudos (Rogers, 1914; Termier, 1923;
Stützer, 1931; Hartung, 1942; Petracheck, 1942; Bederke, 1943), mais tarde, substanciada
por Lapparent, 1934 e desenvolvida por Stach (1950). Este conceito é consistente com
aquele estabelecido para as bentonitas (Knight, 1898), “produzidas in situ pela
transformação de cinza vulcânica”.
Hoehne (1953a) apresenta uma visão discordante, caracterizando os
tonsteins, como produtos de intemperismo de sedimentos comuns em um ambiente paludal.
Uma sinopse do desenvolvimento da hipótese sedimentar é apresentada em seu último
artigo, publicado postumamente (Hoehne, 1964).
Nesse período, hipóteses alternativas para a para a origem dos tonsteins são
consolidadas por Diessel (1965), Petrov (1967) e Millo (1968), embasada nos estudos de
Hoehne segundo a qual essa rocha seria produzida pelo transporte de caolinita para a bacia
de sedimentação, desde o local de proveniência. De acordo com ela, tonsteins representam
uma ”laterita argilosa” composta de material transportado em estado coloidal e precipitado
de soluções devido à coagulação (conceito quimiogênico).
Apesar do caráter irregular das investigações, em meados do século XX,
duas propostas alternativas, vulcanogênica ou detrítica, encontram apoio litogenético e
respaldo na comunidade científica. As análises eram, porém, eminentemente descritivas e
sistemas classificatórios não haviam ainda se estabelecido, inexistindo hipóteses a respeito
de correlações regionais e espaciais entre leitos de tonstein. Consequentemente, a
compreensão das possibilidades de aplicação do estudo dessas rochas para a solução de
problemas geológicos não havia ainda sido concebida.
O estabelecimento de sistemas classificatórios para os tonsteins foram
elaborados por Schuller (1951), Bouroz (1962), Masek (1963), Burger (1979, 1985) e Stach
et al. (1982), mas a maior parte destes sistemas é descritiva e geralmente baseada na
textura, morfologia de grãos dos argilominerais, e pseudomorfos; isto é, feições
principalmente relacionadas aos processos pós-deposicionais e não à origem. Dado que
19
essas feições são secundárias e frequentemente variarem lateralmente dentro de uma única
camada, elas pouco contribuem ao entendimento genético ou mineralógico dos tonsteins.
Outros nomes foram propostos para as antigas camadas de cinza vulcânica
não-marinha, tais como “cineritos” (Bouroz, 1962) e “bentonitas caoliníticas” (Spears e
Rice, 1973), mas o termo “tonstein” foi mais aceito e é amplamente utilizado.
O termo “cinerito”, proposto por Bouroz (1962), aplica-se geralmente a
qualquer deposito de cinzas vulcânicas de queda, independente do seu estado de alteração,
ambiente deposicional, ou composição mineralógica presente. Burger (1979) usou o termo
“Kaolin-coal tonstein” para essas camadas, mas esta designação excluía os tonsteins em
camadas de carvão que não são originalmente compostos por caolinita. “Noncoal tonstein”
refere-se de acordo com Burger (1979) a camadas de cinzas vulcânicas alteradas, não-
marinhas, não interacamadadas ou em contato com camadas de carvão.
O termo “tufo” utilizado para definir cinzas vulcânicas consolidadas (Fisher
e Schminke, 1984) deveria, de acordo com Bohor e Triplehorn (1993), ser aplicado para
caracterizar leitos não marinhos de cinzas vulcânicas não associadas com carvões. Fisher e
Schmincke (1984) sugeriram que o termo tonstein fosse eliminado e que o termo bentonita
fosse usado em um senso amplo para representar todas as camadas ricas em argilas de
provável origem vulcânica, delgadas, muito espalhadas, independente de sua composição
de argilomineral ou ambiente deposicional.
O incremento de estudos desenvolvidos na Europa e nos Estados Unidos
comprovou que essas rochas não são restritas somente a bacias paleozóicas, como se
julgava, mas ocorrem também em seqüências portadoras de carvão do Mesozóico e
Cenozóico em diferentes continentes (Addisson et al., 1983; Batchelor, 1995; Cheng et al.,
1996; Diessel, 1985; Burger, 1979; Weiss et al., 1992).
Esses estudos demonstraram que os tonsteins, antes considerados como
efêmeros, constituem-se em rochas típicas de seqüências portadoras de carvões, sendo
ocorrências comuns, embora quantitativamente pouco representadas nos perfis
estratigráficos.
Pesquisadores europeus passaram a aceitar a teoria vulcânica, na segunda
metade do século XX, culminando na monografia detalhando a natureza vulcânica de
tonsteins, apresentada por Dopita e Kralik, (1977). Uma importante contribuição à
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consolidação da teoria vulcânica foi a descrição de tonstein proveniente de seqüências
recentes, contendo inequívocos minerais primários vulcânicos e mesmo vidro vulcânico
(Triplehorn e Bohor, 1986). Essas evidências sobre origem de tonsteins são atualmente
aceitas por quase toda a comunidade cientifica, mesmo pelos mais devotados discípulos de
Hoehne (cf. Burger, 1985b).
A confirmação da hipótese de que tonsteins representam camadas não-
marinhas de cinzas-vulcânicas alteradas, análogas às bentonitas marinhas, foi
extremamente importante para o estabelecimento dos seus usos geológicos. Masek (1963)
demonstrou a relação genética entre estes dois tipos de camadas de cinzas delineando a
transformação lateral de bentonitas para tonsteins em carvões da porção central da Bacia da
Bohemia.
A equivalência entre estes dois tipos de camadas vulcânicas indica que os
tonsteins também podem ser usados do mesmo modo que bentonitas i.e., como isócronas,
marcadores de horizontes, calibradores de biozonas fósseis, e fontes de minerais primários
vulcânicos apropriados à datação radiométrica.
O estudo das texturas do tonstein foi associado à caracterização
mineralógica e petrográfica, constituindo a base da classificação dos tonsteins. A análise
das prototexturas demonstrou que os tonsteins das bacias paleozóicas eram correlacionados
com acumulações eólicas de cinzas vulcânicas. Essas texturas reliquiares são encontradas
em muitos tipos de argila e reforçaram a hipótese vulcanogênica para os tonsteins
(Chrisfidis e Dunham, 1993; Batchelor, 1995).
Todavia Admakin (1995) com base em estudos realizados em jazidas de
carvão da Bacia de Moscou e na Bacia de Irkutsk (Admakin e Portnov, 1987), propõe a
ocorrência de tonstein detrítico, composto de caolinita epiclástica montmorilonítica, além
do tonstein vulcanogênico. Para designar essas rochas Martinec et al. (1989) sugeriu o
termo “paratonstein“. A concepção de Admakin (1995) caracteriza indicadores
litogenéticos para subdividir os tonsteins, de acordo com sua composição inicial,
discriminando alternativamente duas categorias genéticas de tonstein: os “orthotonsteins”
representariam o produto da transformação diagenética de cinza vulcânica transportada pelo
vento, enquanto que os “paratonsteins” seriam produzidos por acumulação de material
caolinítico liberado de crostas intempéricas erodidas. Admakin (1995) afirma que, embora
21
as propriedades genéticas dos diferentes grupos sejam similares, é importante elaborar
critérios objetivos para distingui-los. Os critérios diagnósticos incluem a associação entre
minerais estáveis e protominerais pseudomórficos, texturas reliquiares e características
específicas das camadas de carvão, nódulos de titânio e a paragenêse dos tonsteins com
rochas geneticamente relacionadas na seqüência portadora de carvão.
O conhecimento das características e dos processos genéticos relacionados à
geração de tonsteins propiciou a solução de diferentes problemas geológicos. Dessa forma,
esses leitos passaram a ser utilizados em correlações intrabacinais e interbacinais, em
seqüências portadoras de carvões. Esse método foi usado para reconhecer camadas de
carvão sincrônicas na França, Bélgica e Grã-Bretanha (Bouroz, 1966; Hoehne, 1951, 1953,
1957; Burger, 1979; Spears and Kanaris-Sotiriou, 1979) como também no estabelecimento
de correlações entre seqüências portadoras de carvões nos Estados Unidos e Europa
(Burger, 1985b). Leitos de tonsteins também têm sido utilizados como uma importante
ferramenta para caracterizar a extensão de zonas formadoras de carvão em determinadas
seqüências estratigráficas. Por outro lado, a análise desses leitos tem também propiciado
registro e datação de eventos eruptivos de curta duração e a estimativa de periodicidade em
eventos vulcânicos do passado geológico.
A consolidação da teoria vulcânica para a origem dos tonsteins é apresentada
no artigo “Tonsteins: Altered Volcanic-Ash Layers in Coal-Bearing Sequences” elaborado
por Bohor e Triplehorn (1993). Naquele artigo é apresentada a evolução dos sistemas de
nomenclatura e classificação dessas rochas, detalhada descrição da mineralogia primária e
secundária, bem como a ocorrência geográfica. Tonsteins são reconhecidos como cinza
vulcânica de queda alterada com base nas relações de campo, composição mineralogia e
texturas, geoquímica e idade radiométrica. Essa interpretação amplia seu uso em diferentes
estudos geológicos. Desta forma, os tonsteins têm sido utilizados como: controle de
amostragem geoquímica, estudos de petrografia orgânica e planejamento de mineração.
Correlações regionais e intercontinentais entre estratos não marinhos podem ser
estabelecidas; datações radiométricas de minerais primários vulcânicos permitem a
determinação de idades de camadas de carvão e sua calibração com zonas
palinoestratigráficas. Por outro lado, a presença de múltiplas camadas de tonstein em
espessas camadas de carvão pode ser usada para estudar o estilo e a história do vulcanismo
22
explosivo. A partir desse estudo, que é considerado um marco com relação à evolução
conceitual do estudo de tonsteins, diferentes autores têm estabelecido análises em cinzas
vulcânicas alteradas, em diferentes bacias e em distintas províncias paleogeográficas, em
amplos intervalos de tempo geológico, desde o Devoniano até o Holoceno (López
Gamundí, 1994; Stollhofen et al., 2000; Wüst e Bustin 2001; Creech, 2002; Lopez
Gamundí, 2006).
No Brasil, o estudo pioneiro de Correa da Silva (1973), desenvolvido com
base em relações de campo, e algumas análises mineralógicas e químicas, classificou os
tonsteins de Candiota como “Stratotonstein” de acordo com a classificação de Bouroz,
(1962) que atribui a essa rocha uma origem detrítica. Della Favera et al. (1992) concordam
com essa interpretação considerando esses tonsteins como indicadores de clima seco.
Todavia, Formoso et al, (1999), através de análises geoquímicas detalhadas
indicam para esses leitos, origem vulcânica relacionada a dispersão de cinzas por via aérea,
de acordo com os critérios de Bohor e Triplehorn (1993).
Datações radiométricas U/Pb em zircões provenientes de tonsteins da jazida
de Candiota (Matos et al., 2000, 2001) indicaram idade de 267 ± 3,4 Ma. Cazzulo-Klepzig
et al. (2002) tenta estabelecer uma calibração entre o zoneamento palinoestratigráfico
estabelecido por Marques-Toigo (1991) e as idades indicadas; todavia a idade proposta
mostrou-se incompatível com os esquemas estratigráficos vigentes para a Bacia do Paraná
(Milani, et al. 1998).
Com a finalidade de obter dados mais consistentes que possibilitassem uma
melhor correlação com os modelos vigentes, Guerra Sommer et al. (2006, in press-a,b,c)
estabelecem datações com base em análises radiométricas IDTMS U-Pb em tonsteins das
jazidas de Candiota (296 ± 4,2 Ma.) e Faxinal (285,4 ± 8,6 Ma.). Os resultados obtidos em
análises de zircões procedentes de tonsteins das jazidas de Candiota e Faxinal indicaram
para os mesmos uma idade média de 290,6 ± 1,5 Ma.
Interpretações de Coutinho et al. (1988) e Coutinho e Hachiro (2005) através
de estudos de distribuição, mineralogia, petrografia, e proveniência e significado dos
depósitos de cinzas na bacia do Paraná, inferem a proveniência destas cinzas atravessando a
patagônia alcançando a Austrália.
23
Guerra-Sommer et al. (in press-c) com base em dados de Sato e Lhambias
(1994), consideram que o vulcanismo do grupo Choiyoi inferior poderia representar a fonte
mais provável para a precipitação distal de cinzas no sul da Bacia do Paraná durante o
Sackmariano.
1.4 - Contexto Estratigráfico
Eventos tectônicos e eustáticos no sul da Bacia do Paraná no sul do Brasil
induziram vários ciclos de terceira ordem de elevação do nível do mar durante a longa
duração englobada pela Superseqüência I do Gonduana. Esta superseqüência abrange
quatro seqüências denominadas de terceira-ordem, da base para o topo, S1, S2, S3, e S4
(Holz, 1998; Holz et al., 2000). O intervalo estudado focalize-se na Formação Rio Bonito
que inclui o estrato portador de carvão e compreende a seqüência S2 e a parte inferior da
S3. A seqüência S2 corresponde a um trato de sistema de mar baixo (LST), em sua base,
seguido por um trato de sistema transgressivo (TST) e no topo, trato de sistema de mar alto
(HST). A parte inferior de S3 representa novamente um LST. O intervalo de topo do S3
pertence à Formação Palermo e representa um TST e um HST. O LST de S2 é composto de
duas paraseqüências (Holz et al., 2000) que contém camadas fluvio deltáicas progradantes e
umas poucas camadas de carvão. O LST é recoberto por um TST de quatro paraseqüências
(pântano associado à barreira/laguna). As camadas de carvão nas três paraseqüências basais
são espessas e contínuas, enquanto na paraseqüência superior elas são finas e descontinuas.
O HST compreende uma única paraseqüência de fácies marinha (Fig. 2).
A jazida de carvão do Faxinal (UTM N6651.5 / E432.7), minerada pela
Companhia de Pesquisas e Lavras Minerais (COPELMI), está localizada perto do Município
de Arroio dos Ratos, cerca de 120 km a sudoeste de Porto Alegre e próxima às jazidas de
carvão de Água Boa e Sul do Leão (Fig. 3).
24
Figura 3 - Mapa de localização da Mina do Faxinal.
Estas jazidas estão situadas em um graben, referido previamente como o
paleovale Leão/Mariana Pimentel (Ribeiro et al., 1987). Esta é uma estrutura alongada
tendendo a SE-NW em sua porção oriental e E-W para o oeste. O graben, inserido no
embasamento, tem 60 km de extensão e até 5 km de largura. As três jazidas de carvão são
blocos estruturais abatidos, principalmente controlados por um sistema de falhas N40ºE, e
sua extensão é limitada pela erosão subseqüente. A jazida do Faxinal (reserva de 18 x 10
6
toneladas de carvão) está situada na parte leste do graben. A sucessão do Faxinal inclui
cinco camadas de carvão, designadas, da base para o topo: I, IM, M, MS, e S. As camadas são
intercaladas com siltitos, lamitos, arenitos e paleosolos (Fig. 4).
25
Figura 3 – Perfil litológico, fácies, paraseqüência, geocronologia e dados radiométricos da Jazida do Faxinal
(modificado de: Guerra-Sommer, in press-a,c).
O presente estudo focaliza-se em uma camada argilosa de coloração cinza
clara, de aproximadamente 10 cm de espessura, fossilífera, intercalada na camada de carvão
(S). A camada está exposta ao longo dos cortes da mina a céu aberto e mostra,
principalmente, limites superior e inferior abruptos (Fig. 5).
26
Figura 5 – Em a) vista aérea da Mina do Faxinal (imagem de satélite – Google Earth 2007); b) vista de perfil
do corte 8, onde as setas indicam a fina camada de tonstein; c) detalhe da camada de carvão S, com o tonstein
intercalado.
27
1.5 - Metodologia
1.5.1 - Material
Sete amostras integrais da camada de tonsteins foram coletadas
aleatoriamente de diferentes pontos da frente de lavra da Mina do Faxinal, no Corte 8
(UTM N6652,3/E431,3) em uma extensão de 100 m. As amostras são denominadas: FX-0,
FX-1, FX-2, FX-3, FX-4, FX-5, FX-7, conforme constam nas figuras 6, 7 e 8. Em cada
amostra tonstein, de cerca de 10 cm de espessura, foram identificados três níveis distintos,
mesoscópicamente observáveis e confirmados por análises petrográficas e de difração de
raios X. Estes níveis, aqui denominados níveis microestratigráficos, constituem-se de: nível
basal (b) correspondente a um carbonato calcítico com caolinita; o nível intermediário (m)
composto por um argilito caolinitico com siderita, pirita e calcita; e o nível de topo (t)
corresponde a um argilito caolinítico (Fig. 6)
Figura 6 – Perfis das amostras do tonstein da Jazida do Faxinal, mostrando os três níveis microestratigráficos
de base, intermediário e topo; laminação primária, matéria orgânica (folhas e ramos fósseis). Na amostra FX-
1 (a,b,c) verifica-se uma concreção calcítica lenticular na base; FX-0 (d), FX-2 (e), FX-3 (f), FX-4 (g)
apresentam coloração rosada no nível intermediário devido à presença de siderita; FX-5 (h) e FX-7 (i); (c)
estrutura de escape de fluídos na concreção calcítica (canto inferior direito); (f) mostra lentes piríticas (c.a.
1cm de espessura) entre os níveis calcítico e siderítico; (g) falhas de deslizamento de pequena escala; em
(b,c,e,h,i) perturbação da laminação pela queda de ramos.
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1.5.2 – Métodos
1.5.2.1 – Petrografia e Mineralogia
Análises mineralógicas e petrográficas foram desenvolvidas a partir de sete
amostras de argilitos selecionadas. A elaboração de quinze lâminas petrográficas foi
efetuada no Spectrum Petrographics Inc. (Vancouver, WA, USA) tendo em vista a
qualidade necessária para a realização dos estudos propostos.
A microscopia convencional foi a ferramenta fundamental para identificar e
caracterizar a mineralogia primária (piroclástica) e secundária (autigênica), bem como as
texturas vulcânicas piroclásticas, aspectos deposicionais (microlaminação) e diagenéticos.
Para tal análise foi utilizado o microscópio petrográfico (Leitz – Laborlux 12 POL S).
As análises de Microscopia Eletrônica de Varredura (MEV) foram obtidos
nos Centros de Microscopia Eletrônica: MEV-PUCRS e MEV-UFRGS. Foram obtidas
imagens eletrônicas pelos detectores de Elétrons Retroespalhados e de Elétrons Secundários
e análises químicas qualitativas pelo EDS (Espectrômetro de dispersão em energia de raios-
X) em algumas amostras de tonsteins, em lâminas delgadas e fragmentos de rocha,
confirmando os dados já observados ao microscópio óptico petrográfico convencional.
O detalhamento mineralógico realizado a partir de difração de raios-X
(DRX), com análises qualitativas e semi-quantitativas de rocha total, foi obtido pelos
métodos do pó para sete amostras, cada qual dividida nos três níveis distintos, observáveis
a vista desarmada (Fig.7). Para observação dos argilominerais (caolinita) presentes nos
argilitos do topo da camada de tonstein, foram preparadas lâminas de sete amostras com
material orientado e submetidas à técnica de amostra natural (Fig. 8). Estas análises foram
realizadas no Laboratório de Difração de Raios-X do IG-UFRGS, com o difratômetro
modelo SIEMENS Bruker AXS D 5000, com radiação Kα em tubo de cobre (Cu) (nas
condições 40 kV e 25mA) e filtro de níquel (Ni).
29
Figura 7 – Padrões de difração de raios-X de rocha total da camada de tonstein mostrando diferentes
proporções entre minerais caolinita (Kln), calcita (Cal), siderita (Sd) e Quartzo(Qtz) primário, evidenciando a
distinção mineralógica que marca os 3 níveis microestratigráficos.
30
Figura 8 – Padrões de difração de raios-X de amostra orientada natural do topo da camada de tonstein
mostrando um único argilomineral presente, a caolinita muito bem cristalizada.
1.5.2.2 – Palinofácies
As análises de palinofácies foram efetuadas nos diferentes níveis (b, m, t) de
cinco amostras integrais (cada uma dividido em b, m, t) do leito de tonstein. O material foi
primeiramente tratado com ácidos clorídrico HCL e fluorídrico HF, seguindo pelo líquido
denso cloreto de zinco (ZnCl
2
), a fim de concentrar a matéria orgânica. A matéria orgânica
isolada foi então montada de forma dispersa na lâmina. A técnica de preparação empregada
foi o procedimento palinológico padrão, não-oxidativo.
A caracterização palinofaciológica foi baseada em análises quantitativas da
matéria orgânica particulada de acordo com procedimentos descritos por Tyson (1995) e
foram contadas um total de 300 partículas para cada amostra, observada em microscópio
óptico de luz transmitida em luz branca e ultravioleta (Mendonça Filho, 1999).
31
1.6 - Análise Integradora
O tonstein da jazida do Faxinal (RS) constitui-se em um conspícuo marco
estratigráfico correlacionado ao Permiano Inferior da Formação Rio Bonito, sul da Bacia do
Paraná. Esta unidade litológica com aproximadamente 10 cm de espessura, estende-se por
dezenas de quilômetros e possui limites abruptos (Ribeiro et al., 1987) (Fig. 5b,c). Análises
petrográficas, mineralógicas e de palinofácies na camada de argilito intercalada à camada
de carvão S na jazida do Faxinal, forneceram argumentos importantes para confirmar uma
origem vulcânica para essa rocha.
A caracterização petrográfica do tonstein evidencia uma rocha texturalmente
fina, síltico-argilosa, com laminação irregular e com orientação sub-paralela de minerais
piroclásticos alongados ou suas pseudomorfoses (Huff e Spears, 1989) (Fig, 9a,b,c). O
tonstein é uma rocha constituída predominantemente por caolinita autigênica (Fig. 7, 8,
10a,b,c). Dispersos na massa caolinítica ocorrem os minerais primários piroclásticos:
paramorfos de quartzo-beta bipiramidal euédrico e splinters de quartzo transparente, zircão
idiomórfico, apatita euédrica, alanita e pseudomorfos de sanidina (Fig. 9d,e,f,g,h,i,j,k,l).
Estes minerais piroclásticos são considerados como uma suíte restrita de minerais
vulcânicos de tonsteins distais, cujas características foram preservadas durante os processos
diagenéticos (Bohor e Triplehorn, 1993).
A presença dos minerais piroclásticos e suas texturas reliquiares, assim como
as características de campo desta camada, constituem-se nos principais diagnósticos para
atribuir-se a proveniência vulcânica ao tonstein. Tais minerais primários são típicos de
magmas silicosos, o que confirmam estudos geoquímicos a partir de elementos menores e
traços menos móveis (razões Zr/Ti versus Nb/Y), conferindo ao tonstein uma fonte
vulcânica de cinzas riodacíticas a riolíticas (em preparação).
Nesta camada foram observados três níveis textural e mineralogicamente
distintos, caracterizados como níveis microestratigráficos, (medindo c.a. 3 cm cada)
divididos em: base (b), intermediário (m) e topo (t) (Fig. 6). Nos níveis intermediário e de
base, houve variações nas condições de Eh e pH, as quais produziram sequencialmente as
seguintes fases eodiagenéticas: caolinita, pirita, siderita e calcita (Fig. 7). Na porção média
32
do tonstein, a caolinita é substituída por siderita associada à pirita (Fig. 10g,h,i,j)e, no nível
de base, a caolinita sofre uma pronunciada substituição por calcita (Fig. 10b,c,k).
Figura 9 - Fotomicrografias de lâminas delgadas mostrando feições texturais e minerais piroclásticos do
tonstein do Faxinal: a, b) laminação irregular devido à variação granulométrica; b, c) orientação sub-paralela
marcada por partículas alongados de “splinters” de quartzo e pseudomorfos de feldspatos; d) paramorfo de
quartz-ß bipiramidal euédrico; e) “splinters” de quartzo transparente; f) “splinters” de quartzo mostrando
feições de embaimento (superior esquerdo), em forma curva (superior direito); g) zircão idiomórfico zonado e
com inclusão em matriz caolinítica; h) apatita euédrica, face basal em matriz caolinítica e prismática com
substituição parcial por siderita (inferior esquerdo); i) alanita em forma elíptica, imagem de MEV e análise
química por EDS; j, k, l) pseudomorfos de feldspatos substituídos por caolinita, formas rômbicas; l) seção
primática de feldspato pseudomorfo. Legenda: Ap - apatite, ß-qtz - quartzo-ß, Kln – kaolinite, Sd - siderite.
33
Figure 10 - Fotomicrografias de lâminas delgadas mostrando minerais autigênicos do tonstein do Faxinal: a,
b, c) caolinita em vermículos; c) caolinita engolfada por calcita (nível de base); d) caolinita fina em agregados
granulares constituindo a matriz microcristalina; e,f) acordeons de caolinita; a, d, e, f) são imagens de MEV;
g, h) siderita marrom amarelada como agregados granulares dispersos em matriz caolinítica; g) siderita
engolfando pirita; h) pirita entre lamelas de caolinita (centro); i) feição em detalhe mostra pirita substituindo
parcialmente a caolinita; j) calcita substitui alguns minerais prismáticos euédricos preservando formas
reliquiares; k) caolinita vermicular e splinter de quartzo engolfados e substituídos por grandes cristais de
calcita poiquilotópica (nível de base); l) calcita com textura “cone-em-cone” engolfando zircão primário
(centro), deslocando e substituindo toda caolinita na concreção calcítica lenticular da base do tonstein.
A composição mineralógica do nível de topo constituí-se de até 95 % de
caolinita autigênica muito bem cristalizada (Fig. 8, 10a,b,e,f) formada a partir de intensa
lixiviação e alterações produzidas pelos ácidos orgânicos presentes no ambiente paludal,
34
bem como por transformações diagenéticas dos depósitos de cinzas vitroclásticas de queda.
No nível de topo são encontrados todos os minerais piroclásticos reliquiares observados na
camada de tonstein, que juntos perfazem cerca de 2%.
O estudo de palinofácies, inédito para este tipo de rocha, evidenciou uma
composição diferenciada da matéria orgânica estruturada, ao longo do perfil. Análises
estatísticas indicaram altas percentagens de fitoclastos (xilema e epiderme) associados à
baixa representatividade de palinomorfos (Tabela 1). Alguns fragmentos de epiderme
(cutículas) evidenciam, por sua coloração, acentuada alteração termal. O nível basal
caracteriza-se por densos aglomerados de esporos e polens, enquanto o topo é marcado pela
ocorrência de fragmentos de colônias de algas Botryococcus. Esses dados possibilitaram
vincular as peculiaridades do mecanismo de deposição e preservação da matéria orgânica
com o processo de formação do tonstein relacionado à rápida precipitação das cinzas
vulcânicas.
Peculiaridades do mecanismo de formação do tonstein relacionadas a uma
rápida deposição de cinzas vulcânicas de queda foram consideradas para interpretação de
dados de palinofácies.
A presença de maciças associações amalgamadas de esporomorfos (pólens
monossacados e bissacados) (Fig. 11a,b,c) no nível basal do tonstein, atestado pela
utilização de luz ultravioleta, pode ser explicada através da atuação do processo que gerou a
precipitação e deposição de forma muito rápida dos esporomorfos desde as estruturas
produtoras de esporomorfos na planta mãe. Em ambiente detrítico, após a dispersão aérea
aleatória, as fases de saturação, precipitação e sedimentação dos esporomorfos ocorrem em
processos mais lentos (Traverse, 1994). As condições particulares encontradas neste
material podem ser explicadas pela rápida incorporação dos grãos de pólen concomitantes à
precipitação das cinzas.
A excelente preservação das colônias de Botryococcus, que correspondem a
algas coloniais microscópicas do fitoplancton (Tyson, 1995) (Fig.11d), evidenciada no topo
da camada de tonstein, em diferentes amostragens, usando luz ultravioleta, enfatiza sua
deposição subaquática. Essa interpretação foi apresentada também por Creech (2002) para
tonsteins do Newcastle Coal Measures, Permiano Superior do norte da Bacia de Sydney,
35
Austrália. A presença de lâmina d’água protegeria o depósito de cinza de redistribuição
subseqüente por ação da chuva e escoamento superficial (Creech, 2002).
No grupo dos fitoclastos as cutículas estão bem representadas ao longo de
todo o perfil microestratigráfico (Fig. 11f,g,h,i,j). Alguns exemplares apresentam estruturas
epidérmicas compatíveis com o grupo das glossopteridales (Fig. 11f,h,i). Por outro lado, a
coloração marrom de alguns fragmentos cuticulares é atribuída à alteração térmica,
considerando que a cinza ainda estava quente quando tomou contato com a superfície da
folha.
Os elementos do xilema (traqueídeos) são representados por fragmentos
opacos grandes, alongados, equidimensionais, angulosos, com pontoações preservadas,
indicando a ausência de transporte extensivo, de outra forma, fitoclastos bioestruturados,
não opacos, listrados, estriados, bandeados ou perfurados representam séries de traqueídeos
gimnospérmicos com espessamento helicoidal, escalariforme e com pontoações areoladas
(Fig. 11e). A presença de membranas que envolvem a pontoação (torus) em alguns
fitoclastos, constitui evidência muito forte de sepultamento rápido (Tyson, 1995). Alguns
fitoclastos de lenho opacos, de coloração marrom (Fig. 11g,h,i), sugerem um grau de
alteração termal diferenciado, provavelmente relacionado à temperatura da cinza.
Fragmentos de carvão, não opacos, não bioestruturados, são comuns no nível
basal do tonstein (Fig. 11k,l). Sua presença pode ser explicada pelas estruturas de escape de
fluidos, formados pela expulsão vertical de água ou pelo escapamento de gás produzido
pela decomposição da matéria orgânica na camada de turfa subjacente (Reineck e Singh,
1986) (Fig. 6b,c). No nível basal do tonstein também são observadas estruturas de
deformação como falhas de pequena escala e distorções da laminação, resultantes da
movimentação e deslizamento de camadas, penecontemporâneas, associadas à rápida
sedimentação, e que ocorrem principalmente sob a ação da gravidade, e neste caso estão
relacionadas à queda de troncos, ramos ou tufos foliares no sedimento fino subaquoso
(Reineck e Singh, 1986) (Fig. 6e,g,h).
36
Figura 11 - a, b, c) Esporomorfos amalgamados; d) Colonias de Botryococcus; e) Traqueídeo de
gimnosperma com pontoações e toros preservados; f) Esculturações epidérmicas de cutícula superior de
Glossopteris; g, h, i) cutícula inferior de epidermes de Glossopteris h, i) Cutícula termicamente alterada; j)
Cutícula fina; k, l) Fragmentos de carvão. a, c, d, f, g, h, i, l) Obtidas por fluorescência; b, e, j, k) Obtidas por
microscopia por luz transmitida (luz branca).
37
Tabela 1 - Análises de Palinofácies do Tonstein da Jazida do Faxinal - de cinco amostras, cada qual dividida
em três níveis de topo, meio e base. Legenda: Along.: Alongado; Equid.: Equidimensional; Corr.: Corroído;
Listr.: Listrado; Estr.: Estriado; Band.: Bandado; Perf.: Perfurado; Cut.: Cutícula; N.Degr.: Não degradado;
Degr.: Degradado; Memb.: membrana; Spor.: Esporos; Indet.: Indeterminado; Botry.: Botryococcus; MOA:
Matéria Orgânica Amorfa.
38
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45
2 ARTIGO SUBMETIDO
“Mineralogy and Palynofacies analyses of the Tonstein of Faxinal coalfield, an altered
volcanic-ash layer from the Lower Permian of Paraná Basin, Brazil”
Journal of International Coal Geology
46
Mineralogy and Palynofacies analyses of the Tonstein of Faxinal
Coalfield, an altered volcanic-ash layer from the
Lower Permian of Paraná Basin, Brazil.
Margarete Wagner Simas
1*
, Milton Luiz Laquintinie Formoso
1
, Margot Guerra Sommer
1
,
João Graciano Mendonça Filho
2
, Miriam Cazzulo Klepzig
1
1
Instituto de Geociências, UFRGS, Av. Bento Gonçalves, 9500 -91501.970, Porto Alegre, RS, Brazil;
2
Instituto de Geociências – UFRJ – Ilha do Fundão - 21949-900 - Rio de Janeiro, RJ, Brazil
*Contact author.
E-mail address: [email protected]
Abstract
Mineralogical and palynofacies analyses are reported from a tonstein layer interbedded with
coal seams in the Faxinal coalfield, Rio Grande do Sul, Brazil. Integration of data has far reaching
significance for attributing a volcanic origin for this kaolinitic claystone bed. The tonstein is almost
monomineralic rock, composed mainly by authigenic kaolinite. Scattered in the kaolinitic mass
primary pyroclastic minerals occur: euhedral beta-quartz paramorphs and water-clear quartz
splinters, idiomorphic zircons, apatite, allanite and sanidine pseudomorphs; considered as a
restricted suite of silicic volcanic minerals of the distal tonsteins which preserved during diagenesis.
The primary minerals and their textural features, as well as the field relations, indicate a volcanic
air-fall origin. Analyses of the kerogens from different levels of tonstein layer indicate high
percentages of phytoclasts combined with very low palynomorph percentages. Microstratigraphic
analyses of the tonstein profile demonstrated that saturation and precipitation of palynomorphs were
highly influenced by the intense ash-fall process. The preservation of Botryococcus colonies at the
top of the tonstein evidenced the subaqueous deposition of this bed. The brown color of several
cuticle fragments and tracheids was linked to thermal alteration. The tonstein interbedded in a coal
seam indicates an episode of tephra sedimentation during the deposition of the coal-bearing
sequence of the Lower Permian in the southern Paraná Basin.
Keywords: Tonstein, volcanic air-fall origin, mineralogy, palynofacies, Lower Permian, Paraná
Basin
1. Introduction
47
Tonsteins are claystone beds, synchronous with the sedimentary rocks in which they
are interbedded. Tonstein beds extend over large distances and usually contain minerals
that can be dated by isotopic analyses. Their geographical distribution is limited to sites
with favorable preservation potential, such as lacustrine environments. Their limitation to
coal measure sequences reflect the higher depositional energy that operates outside of the
topographic peat-accumulating depression.
Evidence of volcanic activity is widespread in different coal successions in the
southern Brazil, which are historically assigned to the Rio Bonito Formation (Fig. 1), a
fluvial-marine sandstone and shale prone lithostratigraphic unit. These rocks are recorded
by discrete and continuous horizons of clay beds identified as tonsteins interbedded within
coal seams in Candiota and Faxinal coalfields (Formoso et al., 1999, Guerra-Sommer et al.,
2006).
Corrêa da Silva (1973) suggested, on the basis of field relationships and some
mineralogical and chemical analyses, that the clay beds in the Candiota Coalfields can be
classified as stratotonstein (according to Bouroz’s scheme) or dichtertonstein (apropos the
Schüller nomenclature). Furthermore Della Favera et al. (1992) agrees with a detrital origin
for those tonsteins.
Formoso et al. (1999) studied three of these clay beds, two in the upper Candiota
coal seam (8-10 and 1-3 cm thick) and one in the lower coal seam (1-2 cm thick). The
samples, both oriented and unoriented, were analyzed by XRD (Siemens D500-CoK, 40
mA – 30 kV). Advanced kaolinization suggests an in situ diagenetic alteration to nearly
pure kaolinite (ca. 90%): preservation of primary minerals is poor. The presence of
euhedral beta-quartz is a reliable indicator of volcanic origin. The higher content of U and
Th, negative Eu anomalies, and high La/Yb signal that source rock was acidic.
Geochemical diagrams support the view that the tonsteins are of acidic volcanic origin, in
agreement with a volcanic ash origin accepted for these beds (Bohor and Triplehorn, 1993).
On the other hand, field relationships of these tonsteins, particularly their extensive
distribution, relative thinness, and continuity, indicate not to be a detrital, but also an ash-
fall volcanic origin (Formoso et al., 1999).
48
Radiometric data in clay beds characterized as tonsteins in different coalfields in the
southern Brazilian coal succession were made by several authors (Matos et al., 2000, 2001;
Guerra-Sommer et al., in press-a,b) using IDTIMS U-Pb method to date zircons.
Ion microprobe (SHRIMP II) dating of zircons from tonsteins interbedded with coal
seams from the Candiota and Faxinal coalfields (Early Permian, Rio Bonito Formation,
Paraná Basin, Brazil) are presented by Guerra-Sommer et al. (in press-c). The mean ages
obtained (290.6 ± 1.5 Ma) are more precise than previously published intervals.
Calibrations of chronostratigraphic data with radiometric ages show that the main coal
succession from the southern Basin is constrained to the Middle Sakmarian. The ± 2 Ma
time interval of deposition supports the hypothesis that the coal-generating process was
quite rapid in terms of geological time. The new results have far-reaching significance for
correlations of the Basin with sequences of the Argentinian Paganzo Basin (302 ± 6 Ma and
288 ± 7 Ma) and also with the Karoo Basin, with the lowermost Ecca Group (288 ± 3 Ma
and 289.6 ± 3.8 Ma). This new evidence supports the presence of an active and widespread
Lower Permian explosive volcanic event in western Gondwana, which is interpreted as the
same volcanism which produced the Choiyoi Group in western Argentina.
Previous petrographic studies developed by Guerra-Sommer et al. (in press-a) from
the tonstein layer interbedded on the Upper Coal Seam (S) in the Faxinal coal field
confirmed that the claybed is composed of authigenic and probably pyroclastic, ash-derived
minerals.
The Faxinal tonstein bed, in particular, is the focus of this work for several reasons.
Firstly, it is a unique, highly recognizable, laterally persistent claybed on the coal field
profile. Secondly, SHRIMP zircon dating of the tonstein bed allowed for absolute ages for
its deposition (290.6 ± 1.5 Ma). Thirdly, this kaolinitic claybed contains a high proportion
of fossil plants, represented mainly by leaves, in excellent stage of preservation. Fourthly,
the presence of a continuous carbonatic band (microstratigraphic level) at the base of this
altered ash-fall deposit represents a conspicuous feature, absent in other tonstein layers of
the basin.
Taking into account the available geological data and its effect on the tonstein
formation process for the southern Paraná Basin, in Brazil, the aim of the present
contribution is to make accurate previous petrographic studies with the purpose of: to
49
document petrographic analyses in different levels of this tonstein layer; calibrate the
petrographic and mineralogical results with palynofacies analyses; integrate results from
different methodologies in order to confirm the volcanic origin for this claystone bed,
designed as tonstein sensu Bohor and Triplehorn (1993).
2. Geological and stratigraphic setting
The Paraná Basins is a large (1,400,000 km2) intracratonic sag basin covering part
of southern Brazil, Paraguay, Uruguay and Argentina. Basin floor subsidence, in addition to
Paleozoic sea-level changes, created six second-order sequences deposited from the
Ordovician to Late Cretaceous, separated by regional unconformities (Milani, 2003). The
coal-bearing Rio Bonito Formation, which is the object of this study, occurs at the base of
the second-order Carboniferous–Early Triassic sequence. The study area comprises the
Candiota and Faxinal coal fields, which is part of the southeastern outcrop belt of the Rio
Bonito Formation of the Paraná Basin, southern Brazil (Fig. 1).
Detailed sedimentological and stratigraphic work demonstrated that coal deposits of
the Rio Bonito Formation in the Candiota and Faxinal coal fields occur adjacent to paralic,
i.e., estuarine, deltaic, backshore, foreshore and shoreface siliciclastic deposits, and that
coal was deposited in back-barrier environments (Holz, 1998). Tissue preservation and
gelification index corroborate with deposition in a coastal swamp environment (Alves and
Ade, 1996). Deposition occurred at the cool temperate climatic belt (Scotese, 2002) or,
according to the criteria of Rees et al. (1999), in the cool temperate bioma, at a
palaeolatitude of 50° approximately (Scotese, 2002). The Permian in the Paraná Basin was
a period of overall transgression of the shoreline, punctuated by short regressive episodes
(Zalán et al., 1990). The Rio Bonito Formation integrates part of two third-order
depositional sequences of the Carboniferous Early Triassic second-order sequence, named
S2 and S3 (Holz et al., 2002).
The Faxinal coalfield, mined by the Companhia de Pesquisas e Lavras Minerais
(UTMN6651.5/E432.7) (COPELMI), is located near the town of Arroio dos Ratos, about
120 km southwest of Porto Alegre and close to Água Boa and Sul do Leão coalfields (Fig.
2). These coalfields are situated in a graben, referred to previously as the Leão/Mariana
50
Pimentel paleovalley (Ribeiro et al, 1987). It is an elongate structure trending SE-NW in its
eastern portion and E-W to the west. The graben, inserted in the basement, is 60 km long
and up to 5 km wide. The three coalfields are downthrown structural blocks, mainly
controlled by a N40°E fault system, and their extent is limited by subsequent erosion. The
Faxinal coalfield (producing 18 × 106 ton coal/tons of rock by open pit) is situated in the
eastern part of the graben.
The Faxinal succession includes five coal seams, named, from base to top: I, IM,
M, MS, and S. the seams are interbedded with siltstones, mudstones, sandstones, and
paleosols (Fig. 3). The present study focuses on a light gray clay-bed, approximately 10 cm
thick that is laminated to massive and fossiliferous and interbedded with the upper coal
seam (S). The bed is exposed along the cutbanks of the open pit and displays mostly sharp
lower and upper boundaries (Fig. 3 and 4a,b).
The bed present on its base, a continuous carbonatic level for all crop out section,
with 3 to 6 cm of thickness, and a claystone upper level, from which it was described a
palaeoflora mostly composed by Glossopteris leaves, also occurring Cordaites and
filicopsids leaves, conifers and fronds of undetermined botanical affinity (Guerra-Sommer,
1988).
3. Petrography and Mineralogy
Methodology
• Sampling techniques
Tonstein samples were collected from the different cutbanks of the open pit areas
(Fig. 4a,b). In each sample three different levels were analyzed independently, taking into
account their secondary mineralogical characteristics. The base level (b) is calcitic
carbonate composition with kaolinite; the intermediate level (m) besides kaolinite contains
often siderite and secondarily pyrite and calcite; at the top level (t) the kaolinite is always
dominant (Fig. 4c,d,e,f).
• Analytical techniques
The samples were analyzed using the following methodologies: 1) conventional
optical microscopy analyses on 13 thin sections (Leitz - Laborlux 12 POL S).; 2) Scattered
51
electronic microscopy (SEM) analyses of semi-quantitative chemical analyses by Energy-
dispersive X-ray spectrometer (EDS) and images acquisitions by Backscattered electrons
(BSE) and Secondary electron images (SE) in thin section and rock mount in stub, at the
Electronic Microscopy Centers of the Pontifícia Universidade Católica do Rio Grande do
Sul and Universidade Federal do Rio Grande do Sul; 3) X-ray diffraction (XRD) analyses
(Siemens-Bruker AXS D5000 - CuKα - 25 mA - 40 kV) using powder diffraction
technique to whole rock to identify the mineralogical composition qualitatively and semi-
quantitative of 21 samples (seven of each levels) (Fig.5a,b,c), and natural oriented
preparation to identify the clay minerals (7 samples of the top level) (Fig.5d), the XRD
analyses were made in the laboratory of IG-UFRGS).
Petrography
The tonstein at the Faxinal Coalfield is a continuous bed with constant thickness
(about 10 cm), showing sharply bounded upper and lower contacts, widespread for
kilometers (Ribeiro et al., 1987).
Texturally is a claystone to mudstone, silty clay with medium sand particles,
light gray colors. Irregular lamination is observed that was characterized by granulometric
variation (Fig. 6a,b) and marked by thin black lenses of organic matter (coalfield leaves)
(Fig. 4c,d,e,f). Some Tuffaceous character are observed in thin sections of the original ash
preserved as sub-parallel orientation of the elongate quartz splinters and pseudomorphs
feldspars (Fig. 6b,c) (Huff and Spears, 1989).
The tonstein is composed by primary minerals derived from volcanic ash-fall
and dust, and by secondary minerals were formed during the diagenetic processes. This
rock is constituted mainly by authigenic kaolinite, make up around 90% of its mineralogical
composition.
Relict pyroclastic minerals occur into the kaolinitic mass and correspond to:
euhedral bipyramidal beta-quartz paramorph and water-clear quartz splinters, idiomorphic
zircon, apatite, allanite and sanidine pseudomorph, which characterize distal tonsteins. The
presence of other diagenetic minerals that formed after the kaolinization of pyroclastic ash
fall was also observed. These early diagenetic mineral phases were established in the
following sequence: kaolinitepyritesideritecalcite.
52
Small differences in the colors of the hand samples are observed, that due to the
variations in the mineralogical composition (kaolinite, siderite, calcite), which
characterizing microstratigraphic levels (Fig. 4c,d,e,f). Thus the tonstein bed is here divided
in three levels that denominate: base level (b), intermediate level (m) and top level (t). The
microstratigraphic levels are observed in the XRD patterns of the sequential analysis of
samples to the each level (Fig. 5a,b,c).
In the base microstratigraphic level calcite predominates, replace almost all
kaolinite and the primary minerals such as quartz, zircon, apatite and others (Fig. 7c,j,k,l).
The base level display a continuous carbonatic band (3 cm thick), as well as some
carbonate concretion lenses (6 cm thick in its center) (Fig. 4c,d,f). The secondary phases
such as pyrite and siderite are formed mainly into the intermediate level (Fig. 5a,d).
Kaolinite is the mainly secondary phase present on the top level, making up 95 % of this
level and all the pyroclastic mineral suite of this claystone, showing an irregular
laminations by variation of grain size (clay, silt up to fine sand particles), that could suggest
a wind variations during an eruption or, possibly, pyroclastic pulses, by alterning ash and
dust, with more or less glass particles or crystals. Therefore, the characteristics of the top
level develop the more typical tonstein composition, structures and textures.
Mineralogy
The minerals were divided in two groups: pyroclastic minerals, which preserved
their characteristics during the action of weathering and diagenetic processes over the
tephra, and the authigenic minerals, that were post-depositional formed in the peat swamp
environment.
• Pyroclastic minerals:
Quartz is the most abundant volcanic mineral. There are many euhedral grains
of beta-quartz paramorph, with bipyramidal habit, without prism development (Fig. 6d).
Quartz, generally is the most common and abundant primary mineral present in
tonsteins. It crystallizes as phenocrysts from silicic magmas in the high-temperature phase
(beta) form, inverting to the low-temperature phase (alpha paramorph) when temperatures
fall bellow 573ºC (Bohor and Triplehorn, 1993).
The characteristically clear-water volcanic quartz occurs as splinters, showing
elongated and almost always deformed, wedged, but delicate forms (Fig. 6c,e,f). Also
53
subhedral crystals or fragments were identified with sharply angular, triangular to cuspate
forms, probably representing broken bipyramids (Bohor and Triplehorn, 1993). Few
anhedral crystals of elongated, with embayment or curved forms are observed (Fig. 6f). The
elongate quartz particle grain size varies between 38 µm to 230 µm, with average around of
123 µm. The bipyramidal crystals displaying an average grain size of 68 µm. In the powder
X-ray diffraction (XRD) analyses of the whole rock, the quartz was detected in low
quantities, presenting intensities (counts) relatively lower in the base level when compared
with intermediate and top levels. (Fig. 5a,b,c,d)
Zircon occurs as prismatic, idiomorphic elongate crystals with grain size
average of 80 µm (Fig. 6g). Most crystals display bipyramidal termination, straight walls.
This zircon morphology is characteristic of volcanic rocks, and therefore, may be useful for
correlation of pyroclastic rocks (Kowallis and Christiansen, 1989; Guerra-Sommer et al,
2007b,c). A second zircon population is equant, rounded, with grain size ranging from 10
to 100 µm, but those detrital grains are rare.
Apatite show clear crystals with euhedral hexagonal and prismatic forms. Grain
sizes vary from 25 to 70 µm, mostly around 50 µm (Fig. 6h). Apatite is found in most fresh
volcanic ashes of silicic composition; however, this mineral is not present in most tonsteins
because of its susceptibility to acid dissolution (Bohor and Triplehorn, 1993). The apatite of
the Faxinal tonstein seems to be pyroclastic, and its dimensions are similar to the volcanic
zircon dimensions. An accurate study in the apatite is necessary to verify this question.
Zircon and apatite represent less than 0.1% and with quartz make up around 2 % of the
rock.
Feldspar pseudomorphs replaced by kaolinite show euhedral equant habit
(almost square to elongated pseudohexagonal or rhombic shape) and rare prismatic forms
(Fig. 6j,k,l). The pseudohexagonal forms vary between 140 and 400 µm whereas prismatic
are from to 80 µm on average. Relict feldspar, with irregular form and 50 µm in diameter,
display intense dissolution along the cleavage planes and present K-feldspar composition
by EDS-SEM analyses Baveno-like gemination is observed in euhedral rhombohedral grain
of pseudomorph potassium feldspar (see Fig. 5f in Guerra-Sommer, in press-a).
Some evidences suggest that the pseudomorph feldspars were sanidine, as the
potassium feldspar composition, abundant pseudo-hexagonal to rhombic or elongate
squares forms, probably (010) faces, and these minerals are replaced by fine kaolinite
54
which preserves the forms of the crystals and rare features similar to twins. There are
indications of very subordinate presence of plagioclase pseudomorphs, presenting prismatic
forms replaced by kaolinite and with features similar to albite twinning (Fig. 6l). The
relative amounts of plagioclase and sanidine pseudomorphs are possibly modified in
relation to the parent ash, because plagioclase is commonly found altered or dissolved
during weathering, rarely as sanidine (Bandfield and Eggleton, 1990).
Allanite was observed by optical microscopy and also detected and confirmed
by electron microscopy (SEM) at the same sample, FX-0 on the base and the top levels. It
presented elongated forms with rounded edges and apex, as an ellipse, with dark brown
color, high birefringence, but it seems in metamict state, and measuring from 55 µm to 18
µm (Fig. 6i – right upper). The SEM-EDS analyses show the Ce, La and Y (REE) and Fe,
Ca, Si and Al as elemental composition (Fig. 6i – left lower).
Allanite is a component of the restrict suite of primary volcanic minerals of the
most distal tonsteins (Bohor and Triplehorn, 1993). It was reported in tonsteins from
Eoceno coal beds in Wyoming (Bohor et al., 1979). Nevertheless, it is easily altered, and
only rarely found in tonsteins in trace amounts, characteristically as corroded crystals
(Bohor and Triplehorn, 1993). Taking into account these allanite characteristics and the
considerations pointed out in this paper, this mineral is has probably a pyroclastic nature,
but further investigations will be able to verify its origin.
The presence of bipyramidal beta-quartz paramorphs, elongate idiomorphic
zircons, as well as, sanidine pseudomorphs, allanite and apatite with compatible forms and
dimensions, suggest that the tonstein was derived from an silicic volcanic tuff. Based on
geochemical studies of less mobile element (in preparation), the ratios Nb/Y against
Zr/TiO
2
which discriminate between volcanic magma series and rock types (Winchester
and Floyd, 1977) are confirming the mineralogical observations and suggest a rhyodacitic
to rhyolitic for the volcanic source to the altered ashes.
• Authigenic minerals
Kaolinite is the most common and abundant mineral in the coal-tonstein bed. X-
ray diffraction analyses of the tonstein showed very well-crystallized kaolinite as the sole
component of the clay fraction (Fig. 5a,b,c,d). It constitutes around 90% of the tonstein bed,
55
and is formed by the alterations, eodiagenetic transformations by replacement of many
primary pyroclastic minerals and particles, such as glass, biotite, feldspars, and possibly
other minerals.
The kaolinite comprise since very fine flakes and granular aggregates that is the
tonstein matrix (Fig. 7d), until large aggregates such as vermicular forms and booklets with
size up to 1000 µm (Fig. 7a,b,c,e,f). Exceptionally kaolinite vermicular aggregates with up
to 700 µm are found from the base until the top of the tonstein bed (Fig. 7b). In thin section
and X-ray diffraction patterns, kaolinite constitutes more than 90% of the top level of the
tonstein layer (Fig. 5a and 7a). On the intermediate (m) and base (b) levels, there is a
reduction in the amount of kaolinite due to the replacement by siderite and calcite (Fig. 5b,c
and 7c,g,k).
Pyrite is the most common sulphide in the tonstein, appearing notably on the
intermediate level, especially as nucleus within siderite masses (Fig. 7g). It displays tiny
euhedral crystals with cubic habit, small spherical framboids, infilling kaolinite {001}
cleavages and some other cavities displaying replacement of organic materials (Fig. 7h,i).
In the tonstein sample FX-3, the pyrite shows lenses (0,7 x 6 cm) on the
boundary of intermediate and base level, that marks the separation of calcitic rich level
(base) to the siderite rich level (intermediate). Some rounded pyritic concretions (like balls)
are observed near the lower limit of the tonstein (Fig. 4f). The pyrite was formed in the early
diagenesis, on the bacterial sulphate reduction zone, where there was Fe
2+
availability in
reducing conditions.
Siderite occur mainly on the intermediate level, in some samples (FX-0, FX-2,
FX-3 and FX-4) comprise 10 to 20% of the rocks. Siderite is also present at the top level of
the FX-0 sample ranging from 1% on the upper part, up to 20% on the lower part of this
level, displaying the bedding distribution (Fig. 4c,f). The siderite is yellowish brown in thin
section, forming irregular massive aggregates up to 200 µm, as well as disseminated
crystals smaller than 5 µm (Fig. 7g,h,j,k and 6k). In the most of siderite aggregates there is
a pyritic nucleus, i.e. the siderite engulfing pyrite (Fig. 7g). Siderite precipitates when a
solution containing abundant Fe2+ and HCO3- evaporates or the pH increases. Under unusual
conditions, where either the supply of ferrous iron is large or a reducing environment is
maintained by abundant organic matter, siderite can precipitate in large quantities (Krauskopf,
1967).
56
Calcite is common mineral in the basal level of the tonstein and small amounts
are observed in the lower part of the intermediate level (mainly in the FX-1). The calcite
was the latter diagenetic mineral phase, that occurs as irregular aggregates and like large
poikilotopic crystals, and therefore, it includes and replaces other minerals of the tonstein
(Fig. 7c,j,k).
At the bottom of the tonstein layer (sample FX-1), a white lenticular concretion
occurs with up to 6 cm thickness to almost 1 m length. This concretion is formed
predominately by calcite, that in some places make up to 98%, and present cone-in-cone
texture, with displacive precipitation that dislocated and replaced all other minerals (Fig. 4d
and 7l).
Organic matter is black, brown to yellow colored in thin section, occurring as
small fragments, visible as thin laminated lenses, mainly produced by coalified leaf
fragmentation process, making up to 1 to 5 % of the rock (Fig. 4c,d,e,f).
The reflectivity of the vitrinite – Ro (%) of the coalified tracheids in the tonstein
(0,45) corresponds to around 60ºC to the Permian temperature curves, in other words, these
temperature, as well as, the authigenic minerals formed in the tonstein are compatible with
eodiagenetic conditions.
4. Palynofacies
Methodology
• Kerogen classification
The organic matter (POM: Palynological Organic Matter) was divided according
Lorente and Ran (1991) into for major groups: palynomorphs, structured debris, amorphous
matter and indeterminate matter. At the present paper the scheme use to classify dispersed
organic matter in transmitted light microscopy is derived and simplified from the
classification of Tyson (1995). The palynofacies characterization was based on quantitative
analyses of the particulate organic matter and a total of 300 particles were counted for each
sample according to the criteria of Mendonça Filho (1999).
• Sampling techniques
They followed the same criteria used for petrographic analyses. Tonstein
samples were collected from the different cutbanks of the open pit areas aiming to compare
57
the distribution of the POM along the ash fall deposition profile. In each sample three
different levels were analyzed independently. The base level (b) is calcitic carbonate
composition with kaolinite; the intermediate level (m) besides claystone contains siderite
and calcite; at the top level (t) the kaolinitic claystone is dominant (Fig. 4).
• Analytical techniques
For palynofacies studies the samples were first treated with HCl and HF acids
followed by heavy liquid (ZnCl
2
), in order to concentrate the organic matter. The isolated
organic matter was then mounted on strewn slides. The preparation technique employed
was the standard non-oxidative palynological procedure.
Results
Statistical analyses of the kerogens for the three different levels of five tonsteins
samples indicated high percentages of phytoclasts combined low palynomorph percentages
(Table 1).
Qualitative observation of phytoclasts in sedimentary basins is determined by
several factors (Cole, 1987); rapidly deposited proximal sediments (e.g. flood deposits)
commonly have poorly sorted phytoclast assemblage. On the other hand, proximal energy
sediments (e.g. delta tops swamps, lagoons) are also generally poorly sorted. Generally,
large amounts of phytoclasts particles are related to proximal depositional conditions.
Otherwise, distal low energy facies have better sorted assemblage; small percentages of
palynomorph would indicate distance of terrestrial source (Tyson, 1995).
Nevertheless, at the present case study, peculiarities of the mechanism of the
tonstein formation, related to rapid volcanic ash-fall deposition, must be taking into account
for palynofacies analyses interpretations. This process is considered as geologically
instantaneous events, according Prothero (1990). During eruptions, the pyroclastic material
was ejected to high altitudes and transported by tropospheric air flows over significant
distances and deposited over a peat-surface.
Palynofacies analyses established in the tonstein layer challenges palynological
results obtained by Guerra-Sommer et al. (2007a) for the Faxinal tonstein. Those analyses
characterized well-preserved cuticles showing affinity with Glossopteridophyta and
Cordaitophyta and very rare spores and pollen grains, frequently poorly preserved and
undetermined. Palynofacies analyses, using blue light excitation or ultraviolet irradiation,
58
identified massive associations of hundreds well preserved amalgamated sporomorphs
(bissacate and monossacate pollen grains) in the basal level (b) composed of calcitic
claystone (Fig. 8a,b,c). Ash fall might have played an important role in the anomalous kind
of preservation of the continuous band of those clusters at the base of the tonstein. These
aggregates, judging by their excellent preservation, seem to be deposited directly from the
atmosphere, with a short transport from their place of origin.
The random dispersion of pollen in the atmosphere using wind currents, typical
from Permian gymnosperms, discharged a great quantity of pollen grains, which were
dispersed the bulk of it is lost in lakes and bogs (Traverse, 1994). Thus, the presence of
amalgamated clusters of sporomorphs is a feature that would be explained by special
environmental conditions. Such conditions would be supplied if large amounts of mature
pollen grains, tapped directly from male inflorescences, were buried rapidly and readily
incorporated in ash-fall deposits. Consequently the saturation and precipitation of those
grains in clusters were intensely influenced by the intense ash-fall process.
The dominance of pollen grains over spores reflects the composition of the
megaflora preserved within the tonstein layer, characterized by the dominance of
Glossopteris and Cordaites leaves (Guerra-Sommer, 1988).
The magnific preservation of Botryococcus colonies (Chlorococcales), evidenced
at the top level (t) of the tonstein layer in different samples by blue light excitation or
ultraviolet irradiation, emphasize the subaqueous deposition of the tonstein layer (Fig. 8d).
These forms encompass microscopic phytoplankton colonial algae, from freshwater
lacustrine, fluvial, lagoonal and deltaic facies (Traverse, 1955; Pocock, 1972; Claret et al.,
1981; Batten and Lister, 1988; Cole, 1987; Riding et al., 1991; Williams, 1992), but also
occur in unstable salinity regimes (Naggapa, 1957; Hunt, 1987). A subaqueous intraseam
tonstein deposition was also inferred by Creech (2002) for Late Permian Newcastle Coal
Measures of the northern Sydney Basin (Australia), based in organic coal petrography,
sedimentology and paleobotany. In that study the presence of Botryococcus was considered
as an important marker of subaqueous deposition. Thus, the palynological composition of
the sporomorph group is consistent with a protection of volcanic ash deposit from
subsequent redistribution by rain fall and surface runoff.
59
At the phytoclast group, cuticles are well represented in the three
mycrostratigraphic levels (b, m, t). However, as it was emphasized by Tyson (1995), size
data from sediments rich in macroscopic plant fragments, such as in the present case study
(Fig. 9a) should be interpreted with great care, as the breakdown of large leaf fragments
during maceration may completely distort the nature and size characteristics of the
palynofacies assemblage.
The segregation of a cuticle of a leaf may occur either by biochemical activity,
physical disintegration, or a combination of process. Complete degradation of
parenchymatous tissues within a plant organ can result in the isolation of the protective
cuticle (Traverse, 1994). The well preserved leaf fragments with upper and lower cuticles
stuck together represented in figure 8f,g,h,i,j and figure 9b,c,d are rare phytoclasts in the
palynofacies assemblage.
These thick fragments of cuticles correspond, actually, to fragmented leaf
laminae, still composed by parenchyma, epidermis and cuticle layers, not yet “softened” by
bacterial attack (Garden & Davies, 1988) (Fig. 8f,g,h.i). This kind of preservation can be
explained by the fast ash-fall deposition process. Large canopy leaves, sometimes arranged
in whorls, are particularly sensitive to ash coating, which predisposes them to abscission
(Fig. 9a). Subsequent falls of the ash and dust dislodge such leaves, which become
incorporated in the air-fall deposits, remaining not degraded (Spicer, 1991). Otherwise, the
brown color of several cuticle fragments could be the results of the thermal alteration (Fig.
8h,i). Nevertheless, the excellent preservation of some isolated cuticles permits the
identification of diagnostic epidermic sculptures typical of Glossopteris specie (Fig. 8f and
9d) described from the same tonstein level (Guerra Sommer, 1988).
The most conspicuous lignified phytoclast are fragments of “xylem elements”
comprising tracheids; large, opaque elongate (lath shape), equidimensional (equant shape)
and corroded wood phytoclasts with internal mycrostructural grain suggest absence of
extensive transportation. On the other hand, translucent biostructured phytoclasts striped,
striated, banded and pitted represents series of gymnosperm tracheids with helical,
scalariform thickening and with bordered pits. The preservation of the pit-closing
membrane in some phytoclasts constitutes a strong evidence of rapid burial process;
otherwise, this organic structure would be easily degraded by bacteria (Eaton and Hale,
60
1993) (Fig 8e). The xylem phytoclasts might be originated by branches and shoots from the
falling tree canopy, commonly observed with the naked eye in the tonstein layer (Fig. 4d,e).
Some opaque biostructurated brown color wood phytoclast suggest a different thermal
degree (Figs 8h,i).
Translucent not biostructurated coal fragments are frequent, mainly close to the
lower boundary of the base (b) level (Fig. 8k,l). Their occurrence into de tonstein layer can
be explained by sedimentary structures locally observed, similar to pit and mound structure,
and slump structures, such as distorted bedding and small-scale faulting. The pit and mound
or mud volcanoes are structures related to vertical water expulsion, or possibly the escaping
gas formed by decomposition of organic matter in underlying sediments (Reineck and
Singh, 1973; p. 48). The structures similar to slump structure, such as distorted bedding and
small-scale faulting, are penecontemporaneous deformation structure resulting from
movement and displacement of already deposited sediment layer, generally associated with
rapid sedimentation (Reineck and Singh, 1973, p. 79). In the present case study, this
process probably occurs due to fall of branches, trunks and foliar tufts, that is a common
feature on the base level of the tonstein (Fig. 4d,e,f).
5- Final Remarks
The tonstein bed of Faxinal Coalfield corresponds to a conspicuous stratigraphic
marker for the Rio Bonito Formation to the Paraná Basin. Integration of petrographic,
mineralogical and palynofacies analyses confirm a volcanic air-fall origin for this claystone
bed.
Taking into account the presence of a restricted suite of primary volcanic
minerals such as euhedral bipyramidal beta-quartz paramorph and water-clear quartz
splinters, idiomorphic zircon, euhedral apatite, allanite and euhedral sanidine pseudomorph
phenocrystals, this claystone bed can be characterized as a distal tonstein. On the other
hand, the constitution of this rock composed mainly by in situ kaolinite, exclude a detrital
origin hypothesis and points out to a typical volcanic-ash alteration.
The texture of the primary minerals, the irregular laminations evidenced by
granulometric variation and sub-parallel orientation of primary minerals reflects pyroclastic
61
ash and dust deposition characteristics. Small color differences are consequence of
weathering alterations and diagenetic transformation in the mineralogical composition
(kaolinite, siderite, calcite), characterizing three different microstratigraphic levels.
The authigenic kaolinite was formed by intense leaching of the organic acids
generated from the abundant organic matter in the paludal environment over the pyroclastic
glass particles, feldspars, micas and other minerals. The sulphide and carbonate authigenic
phases such as pyrite, siderite and calcite are formed respectively after kaolinite, due to
changes in the pH conditions, and distributing preferentially in the three distinct
microstratigraphic levels.
Palynofacies analyses of the kerogens from different levels indicated high
percentages of phytoclasts, combined with low palynomorph content. Microstratigraphic
analyses reflected the presence of hundreds of amalgamated sporomorphs at the base level
(b) and the presence of algal Botryococcus colonies at the top level (t). Special
environmental conditions can be inferred by this kind of preservation, linked to a rapid
saturation and precipitation of palynomorphs associated to a subaqueous deposition of the
bed, supported by the presence of algal colonies. These evidences challenge hypotheses of
a detrital input for the structured organic matter and points to a rapid burial process. Thus, a
volcanic air-fall deposition hypothesis fits well with palynofacies analyses interpretations.
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Figure Captions
Figure 1 - Simplified geological map of the Paraná Basin in Brazil with major tectonic
elements and geographic references (after Milani, 2003).
Figure 2 - Location map of the Faxinal Coalfield.
Figure 3 - Lithological log, facies, parasequence, geochronology and radiometric data of the
Faxinal Coalfield (after Guerra-Sommer in press-a,c).
Figure 4 - Profiles from the Faxinal tonstein in the outcrop a) showing field relations with coal
seams and b) interbedded in the coal seam S. Hand samples profile: c) FX-0 sample, d) FX-1 (with
a calcitic concretion lense into the base level), e) FX-5, f) FX-4. Arrows in a) and b) points to the
Faxinal tonstein bed; in c) aim the boundaries of the sideritic intermediate level; in d) the left arrow
aim to the organic matter (fallen branches) disturbing bedding, and the right arrow points to the
fluid escaping structure; in e) aim to the branches showing disturbance in the lamination; f) arrows
point to small-scale faulting; in pictures c), d), e) and f) the general aspects showing lamination
marked organic matter (black lines) and microstratigraphic levels: base (b), intermediate (m) and
top level (t).
Figure 5 – X-ray diffraction patterns of whole rock (by the powder method - 2º to 72º 2-Theta)
from the Faxinal tonstein samples separated in a) top level, b) intermediate level, c) base level,
displaying diagenetic phase minerals as: kaolinite, calcite, siderite, pyrite and one pyroclastic
mineral: quartz; d) XRD pattern of oriented preparations (natural – air dry state - 2º to 32º 2-Theta)
of the top level samples displaying only the very well crystallized kaolinite (001) and (002) basal
planes. Legend: Kln – kaolinite, Cal – calcite, Qtz – quartz, Sd – siderite.
Figure 6 - Photomicrographs of thin sections showing pyroclastic features and minerals from the
Faxinal tonstein. a, b) irregular lamination by granulometric variation; b, c) sub-parallel orientation
66
the elongated pseudomorphs feldspar and quartz splinters; d) euhedral bipyramidal crystals of beta-
quartz paramorph; e) delicate elongated shapes of water-clear quartz splinters; f) water-clear quartz
splinters showing particles with embayment (upper left side), and curved shape (upper right); g)
idiomorphic zircon with bipyramidal terminations, within kaolinite matrix, displaying straight
walls, is zoning crystal with rounded inclusion at the center h) apatite showing euhedral hexagonal
basal face within kaolinite matrix and prismatic face with replacement by siderite on the left end; i)
allanite elliptical forms with rounded edges, are presenting SEM image and EDS chemical analyses;
j, k, l) feldspar pseudomorphs replaced by kaolinite, with rhombic forms (pseudohexagonal to
square elongated), in l) a prismatic section of feldspar pseudomorph with relict features similar to
albite twins (center of the picture). Legend: ap – apatite, ß-qtz – beta-quartz, kln – kaolinite, sd -
siderite.
Figure 7 - Photomicrographs of thin sections showing authigenic minerals: a, b, c) kaolinite in
elongate vermicular stacks, with well-developed hexagonal outline (a); c) kaolinite are engulfing by
calcite (base level); in d) fine kaolinite like granular aggregates as a microcrystalline matrix; e,f)
showing “booklets” of kaolinite; a, d, e, f) are SEM images; g, h) yellowish brown siderite as
granular aggregates dispersed in a kaolinitic matrix; in g) siderite engulfing pyrite aggregates; h)
pyrite infilling kaolinite cleavages (center top) and partially replacing kaolinite, detailed feature in
i); j) calcite replace some euhedral prismatic minerals preserving relict forms into the kaolinitic
matrix and with siderite aggregates in right side of the picture; k) kaolinite vermículos and quartz
splinter are engulfing and replaced by large poikilotopic calcite (base level); l) calcite with “cone-
in-cone” texture engulfing zircon remnant (center), dislocate and replace all kaolinite on the bottom
calcitic concretion lense.
Figure 8 - a,b,c) are amalgamated sporomorphs; d) is Botryococcus colonies; e) is a tracheid of
gimnosperm with bordered pits and toros preserved.; f) is epidermis sculpture of upper cuticle of
Glossopteris; g,h,i) are lower epidermis cuticles, and h,i) are thermally altered; j) is a thiny cuticle;
k,l) are coal fragments. The a, c, d, f, g, h, i and l) obtained by fluorescent light; b, e, j and k) are
obtained by transmitted light microscopy (white light).
Figure 9 - a) foliar tuft of Glossopteris papillosa sp.; b and c) epidermic tissue of lower laminar
face; d) epidermic sculpturations in upper laminar face of Glossopteris papillosa sp.
Table 1 - Palynofacies analyses of the tonstein of Faxinal Mine - five samples with three levels
each (top, middle and base). Lath.: elongate (lath shape); Eq.: equidimensional (equant shape);
Corr.: Corroded; Listr.: striped; Estr.: striated; Band.: banded; Perf.: pitted; Cut.: Cuticle; N.Degr.:
not degrated; Degr.: degrated; Memb.: membrane; Spor.: spors; Indet.: Indeterminate; Botry.:
Botryococcus; AOM: amorph organic matter.
67
Fig. 1
68
Fig. 2
69
Fig. 3
70
Fig. 4
71
Fig. 5
72
Fig.6
73
Fig. 7
74
Fig. 8
75
Fig. 9
76
Table 1
77
3 ANEXOS
78
ANEXO A - Carta de aceitação do manuscrito submetido
79
80
ANEXO B - Cópias de resumos e artigos publicados em coautoria
81
Resumo
“A Roof Shale Flora da Mina do Faxinal (Sakmariano do Rio Grande do Sul): Uma nova
concepção sobre o processo tafonômico”
XX Congresso Brasileiro de Paleontologia
82
A Roof Shale Flora da Mina do Faxinal (Sakmariano do Rio Grande
do Sul): Uma nova concepção sobre o processo tafonômico.
A Roof Shale Flora of Faxinal Mine (Sakmarian of Rio Grande do
Sul State): A new conception about the taphonomic process.
Margarete Wagner Simas
a
, Margot Guerra-Sommer
b
& Miriam Cazzulo-Klepzig
b
a
Programa de Pós-Graduação em Geociências-UFRGS
b
Instituto de Geociências-UFRGS
Uma origem hipoautóctone associada à lenta sedimentação em ambiente límnico,
tem sido proposta para a deposição de Roof Shale Flora intercalada a uma camada de
carvão na Mina do Faxinal, RS (Sakmariano da Bacia do Paraná).
Essa paleoflora procedente de um nível de argilito, até então caracterizado como
tonstein de origem detrítica, é composta majoritariamente por folhas de glossopterídeas
identificadas como as espécies Glossopteris brasiliensis sp. n., Glossopteris similis-
intermittens n. sp., Glossopteris papillosa n. sp., Glossopteris rio-grandensis sp. n.;
estruturas reprodutivas correspondentes à Plumsteadia sennes Rigby e sementes,
caracterizadas como Platicardia sp.; folhas de Cordaitanthales, relacionadas a uma só
espécie, Rufloria gondwanensis sp. n.; fragmentos de frondes estéreis, caracterizadas como
Pteridophylla (sensu Boureau & Doubinger, 1975) correspondentes exclusivamente a
sphenopterídeas (Sphenopteris cf. S. ischanovensis, Sphenopteris sp.) ocorrem em
baixíssima representatividade.
Estudos recentes comprovaram uma origem vulcânica para esse tonstein,
modificando radicalmente a concepção sobre o processo tafonômico responsável pela
preservação da paleoflora. Dessa forma, a excelente preservação da paleoflora, seu espectro
composicional e a paleossucessão passam a ser explicadas por processo autóctone ocorrido
em um estreito (diminuto) intervalo de tempo em ambiente relacionado a deposição de
cinza vulcânica.
83
Artigo A
“Geochronological data from the Faxinal coal succession, southern Paraná Basin, Brazil: a
preliminary approach combining radiometric U-Pb dating and palynostratigraphy.”
Journal of South American Earth Sciences
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Geochronological data from the Faxinal coal succession, southern Paraná Basin, Brazil: a
preliminary approach combining radiometric U-Pb dating and palynostratigraphy
Margot Guerra-Sommer
1
*, Miriam Cazzulo-Klepzig
1
, Rualdo Menegat
1
, Milton Luiz Laquintinie
Formoso
1
, Miguel Ângelo Stipp Basei
2
, Eduardo Guimarães Barboza
1
, and Margarete Wagner Simas
1
1
Instituto de Geociências, UFRGS, Av. Bento Gonçalves, 9500 -91501.970, Porto Alegre, RS, Brazil;
2
Centro de Pesquisas Geocronológicas, USP, Rua da Reitoria, 109 - 05508-9000 Butantã, São Paulo,
SP, Brazil
*Contact author.
E-mail address: margot.somm[email protected] (M. Guerra-Sommer)
Abstract
A radiometric zircon age of 285.4 ± 8.6 Ma (IDTIMS U-Pb) is reported from a tonstein layer
interbedded with coal seams in the Faxinal coalfield, Rio Grande do Sul, Brazil. Calibration of
palynostratigraphic data with the absolute age show that the coal depositional interval in the southern
Paraná Basin is constrained to the Sakmarian. Consequently, the basal Gondwana sequence in the
southern part of the basin should lie at the Carboniferous–Permian boundary, not within the Sakmarian
as previously considered. The new results are significant for correlations between the Paraná Basin and
the Argentinian Paganzo Basin (302 +
6 Ma and 288 ± 7 Ma) and with the Karoo Basin, specifically
with the top of the Dwyka Tillite (302 ± 3 Ma and 299.2 ± 3.2 Ma) and the lowermost Ecca Group (288
± 3 Ma and 289.6 ± 3.8 Ma). The evidence signifies widespread latest Carboniferous volcanic activity
in western Gondwana.
Keywords: U-Pb zircon dating, Coal succession, Paraná Basin, Palynostratigraphy, Sakmarian
1. Introduction
The Paraná Basin occupies some 1,700,000 km
2
in southeastern South America and comprises six
depositional supersequences (cf. Milani, 2003) that resulted from second-order eustatic and tectonic
events. These supersequences are, from base to top, (1) Rio Ivaí (Ordovician/Silurian), (2) Paraná
(Devonian), (3) Gondwana I (Carboniferous–Early Triassic), (4) Gondwana II (Late Triassic), (5)
Gondwana III (Jurassic–Early Cretaceous), and (6) Bauru (Late Cretaceous) (Fig. 1).
The Gondwana I Supersequence is a second-order transgressive–regressive cycle consisting
of a basal transgressive package and an overlying regressive package. The transgressive interval
corresponds to lithostratigraphic units known as the Itararé Group and the Rio Bonito and Palermo
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formations. The regressive package comprises the Irati, Serra Alta, Teresina, Rio do Rasto, and Sanga
do Cabral formations.
Palynology has been used to establish formal biostratigraphic zonations (Daemon and
Quadros, 1970; Marques-Toigo, 1991; Souza and Marques-Toigo, 2003) that facilitate regional and
interregional correlations. However, some correlation problems arise from the extension of the basin,
the endemism of the Gondwana flora, and the poor resolution of facies correlations in contiguous areas
of the same basin. These issues emphasize the need for reliable chronostratigraphic markers.
Evidence of volcanic activity is recorded in the coal succession within the Rio Bonito
Formation of the southern Paraná Basin as tonsteins and discrete and continuous horizons of clay beds,
coeval with the interbedded coal seams of the Faxinal, Sul do Leão, and Água Boa coalfields (Ribeiro
et al., 1987) (Fig. 2).
Tonsteins are excellent time markers for stratigraphic and basin analysis, because they
were evidently deposited during a limited time interval (Huddle and Englund, 1996). Combining
biostratigraphic data from the coal seams with biostratigraphic and radiometric data from the tonsteins
provides a useful tool for stratigraphic correlation and dating.
The purposes of this article are to (1) document the petrographic components of the clay bed
interbedded with the upper coal seam of the Faxinal, confirming its identity as a tonstein; (2) obtain a
radiometric age determination through conventional U-Pb analysis of zircons isolated from that rock;
(3) calibrate the palynostratigraphic zonation through radiometric dating, thus offering additional
criteria for the construction of a geochronologic depositional model for the coal succession in the
southern Paraná Basin, and (4) propose a preliminary correlation with the International Stratigraphic
Chart of the International Commission on Stratigraphy [ICS], (2004).
2. Geologic and stratigraphic setting
Tectonic and eustatic events in the southern Paraná Basin of southern Brazil induced several third-
order sea level cycles during the long duration embodied by the Gondwana I Supersequence. This
supersequence comprises four third-order sequences termed, from base to top, S1, S2, S3, and S4
(Holz, 1998; Holz et al., 2000). The studied interval focuses on the Rio Bonito Formation that includes
the coal-bearing strata and comprises sequence S2 and the lower part of S3. The S2 sequence
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corresponds to a Lowstand System Tract (LST), at its base, followed by a Transgressive System Tract
(TST) and, at the top, to a Highstand System Tract (HST). The lower part of S3 represents again a
Lowstand System Tract. The top interval of S3 belongs to the Palermo Formation and represents a
Transgressive and a Highstand System Tract
The Lowstand System Tract of S2 is composed of two parasequences (Holz et al., 2000) that
contain prograding fluvio-deltaic beds and a few coal beds. The LST is overlain by a Transgressive
System Tract (TST) of four parasequences (swamp associated to barrier/lagoon). The coal beds in the
three basal parasequences are thick and continuous while in the uppermost parasequence they are thin
and discontinuous. The Highstand System Tract (HST) comprises a single parasequence of marine
facies (Fig.8 a).
The Faxinal coalfield, mined by the Companhia de Pesquisas e Lavras Minerais (UTM-
N6651.5/E432.7) (COPELMI), is located near the town of Arroio dos Ratos, about 120 km southwest
of Porto Alegre and close to Água Boa and Sul do Leão coalfields (Fig. 2). These coalfields are
situated in a graben, referred to previously as the Leão/Mariana Pimentel paleovalley (Ribeiro et al,
1987). It is an elongate structure trending SE-NW in its eastern portion and E-W to the west. The
graben, inserted in the basement, is 60 km long and up to 5 km wide. The three coalfields are
downthrown structural blocks, mainly controlled by a N40°E fault system, and their extent is limited by
subsequent erosion. The Faxinal coalfield (producing 18 × 10
6
ton coal/tons of rock by open cut) is
situated in the eastern part of the graben.
The Faxinal succession includes five coal seams, named, from base to top: I, IM, M, MS,
and S. The seams are interbedded with siltstones, mudstones, sandstones, and paleosols (Fig. 8b). The
present study focuses on a light gray clay bed, approximately 7 cm thick that is massive and
fossiliferous and interbedded with the upper coal seam (S). The bed is exposed along the cutbanks of
the open pit and displays mostly sharp lower and upper boundaries.
3. Paleobotanical and palynological data
The rich compression taphoflora hosted by the Faxinal tonstein is predominantly gymnospermous
(Guerra-Sommer, 1992). Fragments of glossopterids (leaves, branches, and reproductive organs)
constitute 70% of the entire association; taxa identified include Glossopteris brasiliensis, G. papillosa,
G. similis-intermittens, Plumsteadia sennes, and Platycardia sp. Cordaitean leaves (Rufloria
gondwanensis) are subordinate; very delicate filicoid fronds (Sphenopteris cf. ischanovensis), often
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cross-cutting the lamination, represent understory herbaceous forms. The integration of paleobotanical
and coal petrographic studies of the tonstein indicate a forest swamp flora. An Artinskian–Kungurian
age was inferred mainly from the evolutionary characteristics of the glossopterid foliar morphology
represented mainly by the Glossopteris-type of venation (Fig. 3).
Ash fall should have played an important role in the plant preservation. Spicer (1991),
studying the effects of ash fall from recent volcanism, pointed out that delicate groundcover plants tend
to be buried rapidly and are thus readily preservable. Moreover, large amounts of canopy leaves may be
incorporated in ash-fall deposits because they are particularly sensitive to ash coating, which
predisposes them to abscission.
Palynological studies of the Faxinal coalfield (Guerra-Sommer et al, 1984; Dias and
Guerra-Sommer, 1994; Cazzulo-Klepzig, 2001) have shown some differences in relation to other
Brazilian Gondwana coals, such as quantitative dominance of gymnosperm pollen grains over
pteridophytic spores.
The basal portion of the coal seam (S), subjacent to the tonstein, carries a palynoflora
dominated by well-preserved fragments of gymnosperm cuticles and woody material. Monosaccate
pollen grains, produced by gymnosperm vegetation (including Cordaitophyta), comprise
Cannanoropollis diffusus (Tiwari) Dias-Fabrício 1981, Plicatipollenites malabarensis (Potonié and
Sah) Foster 1975, and P. gondwanensis (Balme and Hennelly) Lele 1964. Glossopteridophyta is
represented by such bisaccate pollen grains as Limitisporites rectus Leschik 1956, L. delasauccei
(Potonié and Klaus) Schaarschmidt 1963, Scheuringipollenites medius (Burjack) Dias-Fabrício 1981,
and Vesicaspora wilsonii Schemel emend. Wilson and Venkatachala 1963. This palynoflora thus
reflects the important contribution of gymnosperms to the peat-forming plant community. However,
spores produced by lycopsids and ferns are unrepresented. The presence of algae-like elements such as
Maculatasporites minimus, M. gondwanenssi, and Portalites gondwanensis Nauhys, Alpern & Ybert
1969 was confirmed by the present study. Similar palyno-assemblages were identified in the tonstein,
with abundant well-preserved cuticles showing affinity with Glossopteridophyta and Cordaitophyta.
Spores are poorly preserved and undetermined.
Palynofloras at the top of the coal seam (S), overlying the tonstein, are different. They are
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strongly dominated by pteridophytic spores derived mainly from lycopsids and ferns (Lundbladispora
braziliensis (Pant & Srivastava) Marques-Toigo & Picarelli 1984, Punctatisporites gretensis forma
minor Hart 1965 and Kraeuselisporites apiculatu sJansonius 1962), associated with presumed algal
cysts such as Maculatasporites gondwanensi Tiwari 1964, M. minimus Segroves 1967 and Portalites
gondwanensis Nahuys,Alpern & Ybert 1969.
The composition similarity between palynofloras in the basal portion of the upper coal
seam and those in the tonstein indicates that the paleoecological conditions in which coal formed
persisted during tonstein accumulation. These data support the hypothesis of rapid deposition, probably
spanning days or weeks, as in recent ash falls (Huddle and Englund, 1996). Accordingly, the change in
palynofloras of the coal beds that overlie the tonstein horizon in Faxinal’s upper coal seam may be
attributed to the chemical effects of ash-fall deposition.
In a regional context, the Faxinal palynoflora has been assigned to Interval J of Daemon
and Quadros (1970) or the uppermost portion of the Caheniasaccites ovatus/base of the
Hamiapollenittes karooensis subzones of Marques-Toigo (1991). This assignment is based mainly on
the presence of Maculatasporites minimus Segroves 1967 and M. gondwanensi Tiwari 1964s according
to Dias and Guerra-Sommer, 1994.
In the present study, the Faxinal coal-bearing strata are assigned to the Hamiapollenittes
karooensis subzone, in accordance with the framework proposed by Souza and Marques Toigo (2003)
for the Paraná Basin.
4. Petrography of tonsteins
The tonstein beds of the Faxinal coalfield are light gray, very fine-grained rocks with thin, black lenses
of coal or organic matter (Fig. 8b). They are composed of authigenic and probably pyroclastic, ash-
derived minerals.
In the authigenic minerals, the bulk of the matrix consists of very fine (1–10 µm) or prismatic
kaolinite, the latter packed in “booklets” (approximately 100 µm). The prismatic kaolinite is probably
the alteration (authigenetic) product of preexisting feldspars (Fig. 4a, b, d). Kaolinite constitutes nearly
85–90% of the rock, which verifies an ash-fall derivation.
Siderite and calcite comprise 8–12% of the rock. Siderite is yellowish brown in thin section,
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forming irregular, 100–150 µm aggregates, as well as individual crystals (<5 µm) (Fig. 4d, e). Calcite
also forms irregular aggregates up to 100 µm (Fig. 4c).
Organic matter is brown to yellow in thin section. It constitutes 2–4% of the rock, occurring as
small fragments and lenses or as fracture infillings (Fig. 4a, b).
Some sulfide grains were identified by SEM. Pyrite is the most common sulfide, 1–50 µm in
grain size. It displays euhedral to anhedral cubic habit or skeletal texture, especially within siderite
masses (Fig. 4d, e). Galena also occurs as cubic, subhedral crystals about 2 µm in size (Fig. 4f).
Of the pyroclastic minerals, zircon occurs as prismatic, euhedral to subhedral crystals (50 µm
by 20–30 µm). Most crystals display pinacoidal or bipyramidal terminations and straight walls, though
a few may show corrosion features (Fig. 5a, b). A second zircon population consists of
equidimensional, rounded, clearly detrital grains with a size range of 30 to <10 µm.
Apatite forms euhedral to subhedral, hexagonal to subhexagonal (rarely prismatic) crystals.
Grain sizes are 5–90 µm, mostly around 50 µm. Some crystals display corroded rims (Fig. 5c, d).
Quartz is anhedral to subhedral, commonly fragmented or splintery, but in many grains, the
bipyramidal habit is still recognizable. Grain size varies between 3 and 55 µm, and the bipyramidal
crystals display an average grain size of 20–30 µm (Fig. 5e). Zircon and apatite represent less than
0.3%. Added to quartz, they make up approximately 2% of the rock.
Feldspar pseudomorphs and relics are also present. Kaolinite-replaced feldspar pseudomorphs
show subhedral, prismatic, and sometimes rhombohedral shapes, with Baveno-like gemination (Fig.
5f). Prismatic shapes average 100 µm, whereas rhombs vary between 100 and 360 µm. Relics of K-
feldspar are 50 µm in diameter; their irregular shapes indicate intense corrosion along cleavage planes
(Fig. 5f). Some of these features suggest the presence of sanidine among the feldspars from the original
volcanic rock. The presence of quartz with pyroclastic characteristics suggests that the tonsteins
derived from an acidic volcanic tuff.
5. Radiometric dating
Radiometric analyses were conducted at the Geochronological Research Center (CPGeo-Igc) of the
University of São Paulo (USP). Zircon dating was achieved with conventional Isotope Dilution
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Thermal Ionization Mass Spectrometry (IDTIMS) and plotting on the Tera-Wassenburg diagram
(Ludwig, 1993, 2001) (Fig. 7). The methodology is detailed in Basei et al. (1995).
Zircon crystals were split into four fractions for radiometric dating. The M(-3)1 fraction
was not considered for age computation because of the probable recent loss of Pb, which would explain
the younger radiometric age in relation to the other three fractions.
Two different populations were analyzed. Zircon population type 1 (M(-3) fraction) are
prismatic, with well-defined bipyramidal terminations. They are highly crystalline, transparent,
colorless, commonly with inclusions and micro-fractures. Pyramidal faces are well developed and not
abraded. The length-to-width ratio is 2:1 (Fig. 6a).
Zircon population type 2 (M(-4) fraction) are prismatic with well-defined bipyramidal
terminations. They are highly crystalline, transparent, and colorless, sometimes with inclusions and
micro-fractures. Pyramidal faces are not well developed. The length-to-width ratio is 5:1 (Fig. 6b).
Despite morphological differences observed in zircon populations 1 and 2, they are
considered to be coeval. Zircon crystallization is interpreted as having occurred at 285.4 ± 8.6 Ma. This
age represents the average of U
238
/Pb
206
ages of the five fractions analyzed (Table 1). The typology of
zircon crystals supports an origin from ash falls produced by explosive volcanic activity.
6. Stratigraphic implications of the zircon dating
The zircon age of 285.4 ± 8.6 Ma, obtained from the tonstein layer in the Faxinal coalfield, is clearly
significant for the integration of absolute time and stratigraphy. As summarized in Fig. 8a, the isotopic
dating facilitates geochronological calibration of the coal succession in the southern Paraná Basin.
Correlation with the International Stratigraphic Chart (Gradstein and Ogg, 2004) enhances
constraints on the age of the coal-bearing strata. It is assumed that the Faxinal coal seams were
deposited during the Sakmarian (285.4 ± 8.6 Ma), which predates the Artinskian–Kungurian age
proposed for the coal deposits (Milani, 2003). The present study reveals that, according to the
stratigraphic framework of the coal succession in the basin, based on high-resolution sequence
stratigraphy (Holz, 1998; Holz et al., 2000; Holz and Kalkreuth, 2004), the Faxinal coal seams occur
within the TST of the S3 third-order Sequence 2 (Fig. 8a).
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Palynostratigraphic data assign the main Faxinal coal succession to the Hamiapollenites
karooensis subzone, now dated as Sakmarian. Considering that coal-forming floras in the southern
Paraná Basin are significantly different (Cazzulo-Klepzig, 2001), the particular paleoecological
character of the coal-forming flora at Faxinal, dominated by gymnosperms, may represent not only
local paleoenvironmental conditions but also a stratigraphic signature.
In the western Gondwana realm, similar radiometric ages obtained from ash-fall rocks in
Argentina and South Africa provide additional significance to the new data reported herein.
The Fusacolpites fusus-Vittatina subsaccata Interval Zone was defined by Césari and
Gutiérrez (2001) in the Bajo de Veliz Formation (Paganzo Basin, Argentina). Geochronologic data
from basaltic intrusions intercalated with the lower part of the La Colina Formation (equivalent to the
Bajo de Veliz Formation) yield ages of 302 ± 6 Ma and 288 ± 7 Ma (Thompson and Mitchell, 1972)
for the palynofloras. This zone is distinct from the overlying Lueckisporites virkkiae/Weylandites
lucifer Assemblage Zone, which was correlated by Souza and Marques-Toigo (2003) with the
Lueckisporites virkkiae Interval Zone of the Paraná Basin. The latter zone is typical of the Palermo and
Irati formations, succeeding the coal-bearing Rio Bonito Formation. Santos et al. (2006) obtained
SHRIMP zircon age data from bentonitic ash-fall layers intercalated with the Irati sedimentary rocks,
assigned to the Lueckisporites virkkiae zone, and reveal an age of approximately 278.4 ± 2.2 Ma. This
new dating confirms the biostratigraphic data from Souza and Marques-Toigo (2005).
In the Karoo Basin, South Africa, Bangert et al. (1999) report U-Pb ages of 288.1 ± 3 Ma
in tuff beds of the lowermost Prince Albert Formation (Ecca Group), just above the Dwyka boundary in
the southernmost part of the main Karoo Basin (Sakmarian; cf. Gradstein and Ogg, 2004). These tuff
beds are interbedded with shales genetically related to the upper part of the deep-water deglaciation
sequence IV (Visser, 1997). Bangert et al. (1999) do not calibrate biostratigraphic zonal schemes with
radiometric data, which hinders stratigraphic correlations between the Paraná and Karoo basins.
Because glacial deposits in the Paraná Basin differ lithostratigraphically from those in the
Karoo Basin, radiometric data could be important for establishing the timing of depositional processes
during glacial and the subsequent coal-generating systems in different basins of western Gondwana.
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The source of the ash falls in the Karoo Basin, as well as in Argentina and southern Brazil,
has been envisaged as located in Patagonia and West Antarctica, forming an extensive volcanic arc
situated to the south and west in the pre-breakup Gondwana configuration (López Gamundi, 1994,
Limarino et al., 1996, Stollhofen et al., 2000).
Although determination of the ash source was not the goal of this study, petrographic
evidence indicates that the volcanic ash of the Faxinal Coalfield was generated nearby.
7. Conclusions
The integrated studies in the Faxinal Coalfield of southern Paraná Basin (Brazil) yield the following
conclusions:
• Petrographic study of the clay layer interbedded with the upper coal seam (S) reveals a volcanic
ash-fall origin for this sediment, thus identifying it as a tonstein.
• The Faxinal coal occurs within the TST of the S3 third-order Sequence 3 in the sequence
stratigraphy framework proposed for the coal succession in the southern Paraná Basin.
• The coal-bearing strata are assigned to the Hamiapollenites karooensis subzone according to the
palynostratigraphic framework for the southern Paraná Basin.
• The radiometric age of 285.4 ± 8.6 Ma, obtained through U-Pb zircon dating of the tonstein and the
calibration of biostratigraphic data, preliminarily places the regional stratigraphy of Brazilian
Gondwana sequences in an international context. The southern Brazilian coal succession is dated
as the base of the Sakmarian (i.e., older than the Artinskian–Kungurian assignment proposed
previously).
• An indirect conclusion is that the oldest rocks of the basal Gondwana sequence in the southern
Paraná Basin, presently considered Sakmarian, are constrained to the Carboniferous–Permian
boundary.
• Correspondence between radiometric data from the Faxinal tonstein and those from the lowermost
Ecca Group (288.1 ± 3 Ma and 289.6 ± 3.8 Ma) in the Karoo Basin implies widespread, end-
Carboniferous volcanism in Western Gondwana.
Acknowledgments
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The study was supported by the Brazilian agency CNPq (personal research and study grants). The
manuscript benefited from helpful comments and English corrections by the reviewers Dr. O.R. López-
Gamundi and Prof. G.J. Playford.
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and Permian Geology and Stratigraphy, Abstracts. Buenos Aires, Argentina, pp. 58.
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., São Paulo, Proceedings, pp. 503-515.
Milani, E.J. (1997) Evolução tectonoestratigráfica da Bacia do Paraná e seu relacionamento com a
geodinâmica fanerozóica do Gondwana Sul ocidental. Porto Alegre, unpublished PHD thesis.
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Encontro sobre a Estratigrafia do Rio Grande do Sul: Escudo e Bacias, Porto Alegre, 2003, 18-22.
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do paleovale Leão/Mariana Pimentel. III Simpósio Sul Brasileiro de Geologia, Curitiba, Paraná,
Brasil. Actas pp. 335-350.
Santos, R.V., Souza, P.A., Alvarenga, C.J.S., Dantas, E.L., Pimentel, M.M., Oliveira, C.G., Araújo,
L.M. (2006) Shrimp U-Pb zircon dating and Palynology of bentonitic layers from the Permian Irati
Formation, Paraná Basin, Brazil. Gondwana Research, v.9, pp. 456-463.
Souza, P.A., Marques-Toigo, M. (2003) An overview on the palynostratigraphy of the Upper Paleozoic
strata of the Brazilian Paraná Basin. Revista del Museo Argentino de Ciencias Naturales, Nueva
serie, v.5 (2), pp. 205-214.
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Taphonomy: realizing the data locked in the fossil record, v.9, 71-113.
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Visser, J.N.J. (1997) Deglaciation sequences in the Permo-Carboniferous Karoo and Kalahari basins of
southern Africa: a tool in the analysis of cyclic glaciomarine basin fills. Sedimentology, v.44, pp.
507-521.
Figure captions
Fig. 1. Simplified geological map of the Paraná Basin in Brazil with major tectonic elements and
geographic references (after Milani, 1997).
Fig. 2. Location map of the Faxinal coalfield.
Fig. 3. Glossopteris from the tonstein: (a, d) details of the venation pattern; (b) epidermal pattern; (c)
foliar tuft.
Fig. 4. Petrography of the Faxinal tonstein, showing authigenic minerals. (a, b) Large kaolinite
“booklets,” lenses of siderite, and organic matter within fine kaolinite crystals; (c) large calcite
aggregate with siderite and kaolinite; (d, e) thin section and SEM images, respectively, of pyrite within
siderite aggregates, surrounded by kaolinite; (f) small galena crystals under SEM. Legend: kaolinite,
kaol; organic matter, o.m.; calcite, ca; siderite, si; pyrite, py; galena, ga.
Fig. 5. Photomicrographs and SEM images, including features of possible volcanic minerals from the
Faxinal tonstein. (a, b) Euhedral zircon, with straight walls and one corroded end, within kaolinite
matrix; (c, d) euhedral and subhedral apatite, hexagonal to subhexagonal, within kaolinite matrix; (e)
quartz splinters and fragmented, corroded bipyramidal crystals; (f) K-feldspar relic displaying
corrosion features along cleavage plane (upper left); feldspar pseudomorph replaced by kaolinite
(center); arrow points at feature resembling Baveno twinning. Legend: zircon, zir; apatite, ap; quartz,
qz; K-feldspar, Kfelds; kaolinite, kaol; siderite, si.
Fig. 6. (a) Sample MS-1, population type 1 magnetic fraction M (-3); (b) Sample MS-1 population type
2, magnetic fraction M(-4).
Fig. 7. Tera-Wasserburg diagram for zircon crystals from the Faxinal tonstein.
Fig. 8. (a) Geochronology, lithostratigraphy, biostratigraphy, sequence stratigraphy, and radiometric
data of Carboniferous-Permian succession of the southern Paraná Basin. Biostratigraphy sensu Souza
and Marques-Toigo (2003); sequence stratigraphy sensu Milani (2003); third-order sequence sensu
Holz et al. (2000). The position of the lithological log of Faxinal coalfield is identified by the rectangle
in the right-hand column. The black star indicates the radiometric data from the present study (285.4 ±
8.6 Ma); the white star indicates the radiometric data (278.4 ± 2.2 Ma) from Santos et al (2006). (b)
Lithological log, facies, parasequence, geochronology and radiometric data of the Faxinal Coalfield.
Table 1. Statistical data from zircons of the tonstein in the upper coal seam of the Faxinal coalfield.
ZIRCON TYPOLOGY–SHAPE: P(x/y)–prismatic crystal (length/width); Dt–prismatic crystal with
double termination well developed; Py–prismatic crystal with pyramidal faces well developed;
COLOR/TRANSPARENCY: T, transparency crystal; C, colorless crystal; INTERNAL PATTERNS: I,
crystal with common inclusions; F, crystal with common fractures.
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Table 1
106
Artigo B
“Peat-forming environment of Permian coal seams from the Faxinal coalfield (Paraná
Basin) in southern Brazil, based on Palynology and Palaeobotany.”
Revista Brasileira de Palentologia
Rev. bras. paleontol. 10(2):PROVA, Maio/Junho 2007
© 2007 by the Sociedade Brasileira de Paleontologia
PROVAS
5
PEAT-FORMING ENVIRONMENT OF PERMIAN COAL SEAMS FROM THE
FAXINAL COALFIELD (PARANÁ BASIN) IN SOUTHERN BRAZIL, BASED ON
PALYNOLOGY AND PALAEOBOTANY
MIRIAM CAZZULO-KLEPZIG, MARGOT GUERRA-SOMMER, RUALDO MENEGAT
Instituto de Geociências, UFRGS, Av. Bento Gonçalves, 9500, 91501-970, Porto Alegre, RS, Brazil.
miriam.klepzig@ufrgs.br, mar[email protected], [email protected]
MARGARETE WAGNER SIMAS
Programa de Pós Graduação em Geociências, UFRGS, Av.Bento Gonçalves, 9500, 91501-970, Porto Alegre, RS, Brazil.
JOÃO GRACIANO MENDONÇA FILHO
Instituto de Geociências, UFRJ, Prédio de Ciências e Matemática, Bloco F, Ilha do Fundão, 21941-590, RJ, Brazil.
ABSTRACT – Coal seams from the Faxinal coalfield (Rio Bonito Formation, Lower Permian of Paraná Basin) are
compositionally distinct from other South Brazilian coal palynofloras. The dominance of bisaccate and striate pollen
grains such as Alisporites, Scheuringipollenites, Limitisporites, Vesicaspora and Protohaploxypinus in the palynofloras
reflect a peat-forming plant community composed namely of glossopterids, cordaites and conifers. Subordinate trilete
spores derived from lycopsids, sphenopsids and filicopsids (e.g. Lundbladispora, Punctatisporites, Granulatisporites,
Leiotriletes, Deltoidospora, Calamospora) are less abundant, occurring in variable proportions. Algae-like elements,
commonly found in south Brazilian coal palynofloras (Portalites, Tetraporina, Brazilea and Quadrisporites), are scarce
and Botryococcus was not recorded. This palynological feature as well as the record of abundant woody tissues, cuticles
and other phyterals is quite different from palynoassemblages identified in other southernmost Brazilian coal seams.
The dominance of woody seed plants in the Faxinal peat-forming vegetation, low proportion of pteridophytic plants
and scarcity of algal elements suggest a different peat-forming environment for the Faxinal coal seams compared to other
coals from Brazil that have been studied. By integration of palynological and macrofossil data with available sequence
stratigraphical studies on the coal-bearing strata in south Brazilian Paraná Basin, a landscape unit has been outlined, that
is different from that proposed for the Candiota coalfield. Faxinal coals were interpreted to have accumulated in inland
coastal plain mires, linked to delta fluvial settings, at relatively high sea-level, but under low marine influence.
Key words: Coal palynofloras, peat-forming flora, palaeoecology, Faxinal coalfield, landscape unit, Paraná Basin,
Brazil.
RESUMO – A palinoflora dos carvões da Jazida do Faxinal (Formação Rio Bonito, Permiano Inferior, Bacia do Paraná)
caracteriza-se por uma composição palinológica distinta daquelas reconhecidas para outros carvões do sul do Brasil. O
predomínio de grãos de pólen bissacados e estriados, como Alisporites, Limitisporites, Scheuringipollenites, Vesicaspora
e Protohaploxypinus reflete a presença de uma vegetação formadora das turfeiras constituída principalmente por
glossopterídeas, cordaites e coníferas. Esporos triletes derivados de licófitas, esfenófitas e filicófitas, abundantes na
maioria das palinofloras dos carvões sul-brasileiros, como Lundbladispora, Punctatisporites, Granulatisporites,
Leiotriletes, Calamospora, Deltoidospora, Cristatisporites e Vallatisporites, ocorrem em baixa proporção. Representantes
do grupo das algas (Botryococcus) e elementos incertae sedis ou acritarcas, comumente identificados nos carvões da
Bacia do Paraná (Portalites, Tetraporina, Brazilea e Quadrisporites), ocorrem em baixíssima frequência. Estas características
palinológicas, associadas à identificação de abundantes fragmentos de lenho, cutículas e outros fiterais relacionados a
glossopterídeas e cordaites, evidenciam significativa diferença na composição da vegetação formadora das turfeiras,
provavelmente originada por mudanças nas condições do paleoambiente. Através da integração dos dados microflorísticos,
megaflorísticos e paleoecológicos com modelos deposicionais já definidos para sucessões de carvão no sul da Bacia do
Paraná com base em estratigrafia de seqüências, foi reconstruída, de modo tentativo, a unidade de paisagem condicionadora
da acumulação da turfa geradora dos carvões de Faxinal. A unidade de paisagem proposta, diferente daquela indicada para
a jazida de Candiota, é relacionada a áreas mais internas de planície costeira, vinculada à sistema fluvio-deltaico, em nível
relativo de mar alto, porém com influência marinha menos marcada.
Palavras-chave: Palinologia de carvões, megafloras, Jazida de Faxinal, paleoecologia, unidade de paisagem, bacia do
Paraná, Brazil.
REVISTA BRASILEIRA DE PALEONTOLOGIA,10(2), 2007
6
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INTRODUCTION
Many depositional models of coal formation have been
described in the literature (Stach et al., 1982; McCabe, 1987;
Diessel, 1992; DiMichele & Phillips, 1994; Nowak & Górecka-
Nowak, 1999) often using different kinds of data for the
reconstruction of the original environment. Coal petrography,
palynology, palaeobotany, organic geochemistry, and
sequence stratigraphy are usually the main methods used to
define the features of coal depositional environments. The
palynological composition of coals has been widely applied
to characterize the peat-forming environment, considering
that pollen and spores overwhelmingly derive from the peat-
forming flora and thus reflect the compositional changes that
occurred during peat accumulation.
Considering that vegetation is very sensitive to slight
environmental changes, reflected in palaeofloristic
composition, the association of palynological studies with
palaeofloristic data can provide important information on the
ecological preferences of different plant groups, original
edaphic conditions, and mire hydrology, as well as changes
in these variables during the interval of peat formation (Smith,
1962). On the other hand, the interpretation of miospore
succession within coals is based on the assumption that the
miospore-forming plants are autocthonous or
hypoautocthonous and represent a local population of parent
plants.
The most important studies on the peat-forming
vegetation of the South Brazilian coals (Figure 1) were those
of Marques-Toigo & Corrêa da Silva (1984), Guerra-Sommer
et al. (1991) and Corrêa da Silva (1991) which integrated
palynology, palaeobotany and coal petrography in an attempt
to define the main organic constituents of the coals. Their
results demonstrated that these coals have characteristics
that are indicative of an origin in limno-telmatic moors, where
pteridophytic herbaceous and arborescent plant material
accumulated after some transport, promoting
hypoautochthonous coal seams.
On the other hand, sequence stratigraphical studies
applied to the main coal-bearing strata in southernmost Brazil
(Paraná Basin) demonstrated that whereas some coal seams
were predominantly originated in fluvial settings, the most
important coal beds derived from a lagoon-barrier system
linked with a third-order transgressive systems tract,
subdivided into four parasequences sets, each set
representing a major transgressive pulse in the sedimentary
history of the basin (Holz, 1998; Holz & Vieira, 2001; Holz &
Kalkreuth, 2004).
Palynological studies conducted on the most important
coal-bearing strata in the southern Paraná Basin, mainly those
Figure 1. Location map of the Faxinal coalfield and stratigraphic chart of the Paraná Basin.
7
CAZZULO-KLEPZIG ET AL. – PEAT-FORMING ENVIRONMENT OF COAL SEAMS IN BRAZIL
PROVAS
of Ybert (1975), Burjack (1978), Dias-Fabrício (1981), Bortoluzzi
et al. (1980), Cazzulo-Klepzig et al. (1982), Corrêa da Silva et
al. (1984), Marques-Toigo et al. (1984), Picarelli et al. (1987),
Araújo et al. (1985) and Meyer (1999), demonstrated, in a
general way, the presence of rich and diversified palynofloras,
dominated by spores derived from herbaceous and
arborescent lycopsids. Gymnosperm pollen grains appeared
as subordinate components in the palynofloras. In addition,
for the majority of the palynoassemblages, organic-walled
palynomorphs of doubtful botanical affinity related to algae-
like elements or acritarchs were commonly recorded together
with fragments of Botryococcus braunii.
Although this type of palynoassemblage has been
identified for the majority of South Brazilian coals, some
compositional differences, however, have been noted for a
few coal seams, mainly in terms of the relative abundances of
the most important taxa.
Cazzulo-Klepzig (2002) outlined a hypothetical
reconstruction for the landscapes of peat-forming
environments related to the most important coal seams in the
southern Paraná Basin. This study was based on the
composition of the palynofloras, botanical affinity of spores
and pollen grains, habit of the parent plants, environmental
conditions favorable for the growth of the vegetation,
quantitative changes in the palynological content among
distinct coal beds, and the range of tolerance to changes in
salinity shown by algae and algae-like elements. The author
emphasized, however, that although palynology and
palaeobotany are very useful tools for environmental
reconstruction, the limitations of these methods should be
taken into account (e.g.: Di Michele & Phillips, 1994).
For the Candiota coal seams, which represent the most
economically valuable coals in Rio Grande do Sul State (Figure
1), Cazzulo-Klepzig et al. (2005) outlined a tentative palaeo-
reconstruction of the peat-forming environment of the two
most important coal seams (Lower and Upper Candiota Coal
Seams). For a paleoenvironmental interpretation, sequence
stratigraphical information (Holz ; 1988; Holz & Vieira, 2001
and Holz & Kalkreuth,2004) was integrated with the
palynological and paleobotanical data.
Two distinct scenarios were delineated, related
respectively to high sea-level conditions and low sea-level
conditions. The peats accumulated in paralic conditions,
specifically a lagoon-barrier system, and were influenced by
episodic sea-water floods (Figure 2). Although the lowland
coastal mires could be sporadically flooded, they were
isolated from direct marine influence by a barrier island,
precluding major marine ingressions. Meyer (1999) recorded
the presence of Navifusa and Cymatiosphaera in the Lower
Candiota coal palynofloras and expanded the
paleoenvironmental interpretation for the coal formation.
Focusing on the Charqueadas coalfield, Cazzulo-Klepzig
et al. (1993) noted the strong compositional similarity between
the palynofloras identified and the Candiota coal
palynofloras. The detected changes in the plant-communities
among the distinct coal seams, reflecting palynofloristic
content, were caused by changes in the peat-forming
environment. A palaeoreconstruction of the changing
environment formation was tentatively outlined, characterized
by lateral and vertical organic facies.
For the Faxinal coalfield, the focus of this paper (Figure
1), Guerra-Sommer et al. (1983) provide the only
palynological studies on the coal seams and document,
from preliminary analyses, the palynological content of
the coals. These authors mentioned the presence of spores
Figure 2. Lithostratigraphic profile of the Faxinal coal succession
indicating the beds were the studied samples were collected (P1,
P2, and P3).
REVISTA BRASILEIRA DE PALEONTOLOGIA,10(2), 2007
8
PROVAS
derived from arborescent lycopsids (Lundbladispora)
together with rare gymnospermous pollen grains
(Potonieisporites), although illustrations of the diagnostic
forms were not coincident with the list of the palynotaxa
identified. In addition to miospores, abundant and well-
preserved woody fragments and epidermal remains
(cuticles) were identified as belonging to gymnosperms.
In a tonstein layer interbedded in the uppermost coal seam,
a rich compressed megaflora represented by leaves,
branches and reproductive structures of glossopterids as
well as leaves of cordaites, sometimes preserved as foliar
tuffs, was also recorded (Guerra-Sommer, 1983, 1988).
Results from analyses of the glossopterid epidermal
remains indicate xeromorphic patterns. This author
emphasized that remains of cordaites, despite being
abundantly represented in the Faxinal megaflora,
essentially in the tonstein, are poorly represented in other
South Brazilian antracophylic floras.
Together with the pteridophytic spores, Dias & Guerra-
Sommer (1994) mentioned for these palynofloras a
significant amount of Maculatasporites minimus and
Maculatasporites gondwanensis, considered by some
authors as related to fungi, and abundant pollen grains with
botanical affinity with gymnosperms, mainly cordaites,
conifers and glossopterids. This palynological composition,
very similar to that found in the coal seams, is not a common
characteristic of the South Brazilian coal palynofloras.
Results from petrographic and geochemical analyses
(Guerra-Sommer et al., 1983) have indicated a significant
amount of woody plant- material, suggesting growth of the
peat-forming vegetation in a telmatic environment (forest
moor).
Based on petrological analyses of the Faxinal coals, Henz
(1986) defined the organic matter as derived from woody
material, mainly represented by vitrinite, with subordinate
exinite and inertinite constituents, which were accumulated
in a telmatic environment (forest moor or peat-forming forest
vegetation sensu Hacquebard & Donaldson, 1969). The
maceral composition of coals indicated reducing conditions
and a shallow water table.
Data from sequence stratigraphical studies of the Faxinal
coal-bearing sequenceare very important for a
paleoecological interpretation, but to date have not been
published.
The present study takes into account the peculiar
composition of the palynofloras, reflecting a different type
of peat-forming vegetation than that of other well recognized
coal palynofloras from the Paraná Basin. This study was
also modulated in order to provide an accurate palynological
revision aimed at reviewing and characterizing qualitatively
and quantitatively the palynofloristic content as well as
defining the peat-forming plant-communities related to the
Faxinal coal seams. Additional objectives were to interpret
the environmental conditions of peat-accumulation and to
reconstruct tentatively the landscape under which Faxinal
peats accumulated and differed from the other known Brazilian
coals.
GEOLOGICAL SETTING
The Paraná Basin is a large intracratonic basin, located in
the central-east part of the South American Platform, covering
a surface area of about 1,700,000 km
2
. According to Milani et
al. (1998) the basin comprises six stratigraphic
megasequences bounded by interregional unconformities.
The Carboniferous-Early Triassic megasequence includes the
major coal-bearing strata related to isolated coalfields
cropping out from the southernmost part of Rio Grande do
Sul State through Santa Catarina State, to the northern portion
of Paraná State (Figure 1). The overall transgressive trend at
the top of Itararé Group, the basal sedimentary unit in the
study area, is essentially represented by marine deposits and
points to a relative rise in sea level rise that was later
interrupted during the Rio Bonito deposition. Coal
occurrences are historically assigned to the Rio Bonito
Formation, a fluvial to marine sandstone and shale- rich
lithostratigraphic unit of Early Permian age.
The Faxinal coalfield is located in Guaíba city (UTM N665,5;
E432,7). Prior stratigraphical and sedimentological studies (Paim
et al., 1983; Piccoli et al., 1983) linked the depositional conditions
to the presence of alluvial fans that blocked the fluvial
sedimentation creating favorable areas for organic matter
accumulation. Applying sequence stratigraphy methods, Holz
et al. (2000) and Holz & Kalkreuth (2004) developed a model for
palaeoenvironmental evolution of the coal-bearing strata of the
South Brazilian coalfields. In this model, the most economically
important coal seams occur within the transgressive tract of
Sequence 2 as part of a barrier-lagoon system.
MATERIALS AND METHODS
For the present study, samples were collected from three
coal beds in the Faxinal coal-bearing strata, as indicated in
the columnar profile (Figure 2 - P1, P2 and P3). Extraction of
the palynomorphs from samples was carried out using a
routine process developed at the Geological Survey of Canada
(GSC). Samples were processed in Schulze’s solution (65%
nitric acid, HNO3) saturated with potassium chloride (KClO3),
and neutralized in potassium hydroxide (KOH), enabling the
rapid maceration of palynomorphs. Only samples from coal
seams 1 and 2 were taken for the study. Palynomorphs from
coal seam 3 were poorly preserved.
A visual count of a minimum of 200 miospores was
undertaken to determine the relative proportions of miospore
taxa. Palynological slides are stored at the Department of
Geology of the Geological Survey of Canada.
Assignments of dispersed spores and pollen grains to their
respective parent-plant groups, such as lycopsids, ferns,
sphenopsids, glossopterids, cordaites and conifers, were based
on compilations of Balme (1995) and Quadros et al. (1995).
Paleoecological interpretation for the other organic-walled
palynomorphs followed mainly concepts adopted by Tiwari et
al. (1994), Batten & Grenfell (1996), Guy-Ohlson (1992),
Cazzulo-Klepzig (2001), and accurate criteria of other authors
to infer the paleoecology of Botryococcus mentioned below.
9
CAZZULO-KLEPZIG ET AL. – PEAT-FORMING ENVIRONMENT OF COAL SEAMS IN BRAZIL
PROVAS
PALYNOLOGY
There is strong similarity in the palynological results from
the main coal seams analysed, with a dominance of bisaccate
pollen grains produced by arborescent vegetation, with the
following species: Alisporites australis,
Scheuringipollenites medius, S. minimus, Limitisporites
rectus and Vesicaspora wilsonii. Within this group,
Scheuringipollenites and Alisporites are the most abundant
bisaccate pollen found (more than 55%). These pollen grains
reflect the remarkable presence of glossopterids in the ancient
vegetation (Gould & Delevoryas, 1977).
Monosaccate forms are common (around 12%) and show
a low species diversity being represented by only three
species: Cannanoropollis korbaensis, Potoniesporites
braziliensis, and Plicatipollenites malabarensis. Among
these, Cannanoropollis is proportionally the most abundant
in the assemblage. The number of monosaccate genera
identified in the palynoflora indicates that the cordaites also
constituted an important component in the palaeoplant-
communities.
Striate pollen grains, reflecting the presence of conifers
(more than 12%) are indicated by the record of distinct species
of Protohaploxypinus, mainly Protohaploxypinus limpidus,
Protohaploxypinus hartii, and Protohaploxypinus spp.
The number of trilete spores is relatively low and
represents only a small proportion of the miospore fraction
(around 17%), which is the opposite of the palynological
content in the majority of other South Brazilian coals. The
most common spores are derived from sphenopsids and
filicopsids (around 10%): Calamospora sahariana,
Calamospora plicata, Convolutispora candiotensis,
Deltoidospora directa, Granulatisporites, Granulatisporites
micronodosus, Leiotriletes virkki, Punctatisporites gretensis
and Murospora torifera. The genus Lundbladispora, the
most abundant spore in South Brazilian coals, is rare (3%).
Palynological studies of the tonstein interlayered in the
uppermost coal seam indicated a strong similarity between
the two palynofloras.
Organic-walled palynomorphs of uncertain botanical
affinity (incertae sedis or acritarchs), commonly found in
the coal palynofloras of the southern Paraná Basin, such as
Portalites, Tetraporina, Brazilea, Quadrisporites and
Maculatasporites, are extremely rare in the Faxinal coals
(less than 2%). Cazzulo-Klepzig (2001) emphasized the
palaeoecological significance of these palynomorphs for
peat-environment reconstruction, particularly their relation
to the gradient from fresh, to brackish, to marine conditions.
The coccal algae Botryococcus, considered as a good
palaeoecological marker, is commonly found in other coal
seams but it was not recorded in the Faxinal coals. This
microalga has been supposed to be more widespread in
lagoonal and lacustrine environments with low salinity
(Guy-Ohlson, 1992), although Botryococcus has been
recognized as having a wider tolerance to salinity compared
to other microfossils (Grice et al., 1998; Zippi, 1998; Vincent
& Tyson, 1999; Versteegh et al. 2004).
Figure 3. Histograms showing the relative abundance of major
group of spores, pollen and other palynomorphs in the Faxinal coal
seams (A), in the Lower Candiota coal seam (B); and in the Upper
Candiota coal seam (C).
REVISTA BRASILEIRA DE PALEONTOLOGIA,10(2), 2007
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The palynological content, dominated by pollen derived
from arborescent vegetation, and characterized by a scarcity
of algal forms, demonstrates significant compositional
differences in relation to the coal palynofloras identified in
other South Brazilian coals (Cazzulo-Klepzig, 2002). This
paleofloristic feature, showing a dominance of woody tissues,
tracheids and cuticular fragments was also previously noted
in the petrological analyses of Guerra-Sommer et al. (1983)
and Henz (1986).
These differences could be caused by topological
features or different stages in paleoecological succession
(Cairncross & Caddle, 1998; Glasspool, 2003). In relation to
Figure 4. A, Deltoidospora directa (Balme & Hennely) Hart, 1965 and Calamospora sahariana Bharadwaj, 1953; B, Granulatisporites
angularis Staplin,1960; C, Scheuringipollenites medius (Burjack) Dias-Fabricio, 1981; D, Scheuringipollenites ovatus (Balme & Hennely)
Foster, 1975; E, Alisporites splendens (Leschik) Foster, 1979; F-G, Vesicaspora wilsonii (Schemel) Wilson & Venkatachala, 1963; H,
Protohaploxypinus hartii Foster, 1975; I, Protohaploxypinus amplus (Balme & Hennely) Hart, 1964; J, Protohaploxypinus limpidus (Balme
& Hennelly) Balme & Playford, 1967; K, Vittatina sp.
11
CAZZULO-KLEPZIG ET AL. – PEAT-FORMING ENVIRONMENT OF COAL SEAMS IN BRAZIL
PROVAS
the parent plants, although it is possible to outline the
probable peat-forming vegetation on the basis of the
palynological composition, it is important to take into
account that whenever the problem of dispersed spores
and pollen is considered, a fossil plant community tends to
be underestimated (Traverse, 1988; Nichols, 1995).
Cairncross & Caddle (1998) and Glasspool (2003) describe
the complex relationship between the proportion of spores
and pollen grains relative to the abundance of their parent
plants, considering that the palynological record may not
provide a true representation of the types and relative
abundance of plants within peat-forming vegetation.
Distinct factors such as differential spore and pollen
production, introduction into the mires of extraneous,
regional pollen grains, and oxidation of the palynological
material, can lead to a misrepresentation of the original mire
vegetation. On the other hand, organic-walled palynomorphs
commonly found in South Brazilian coals, such as algae-
like forms and acritarchs, can be often estimated due to
different preparation methods (a large-mesh size allows
small microfossils to be lost), transport, and potential for
preservation. Figure 3 shows a comparison between the
relative abundance of the most important plant groups
reflected in coal palynofloras of Faxinal and Upper and Lower
Candiota.
PALEOENVIRONMENTAL INTERPRETATION
Different conditions for the peat-formation can be inferred
for the Candiota and Charqueadas coalfields, compared to
those of Faxinal (Cazzulo-Klepzig et al., 1993, 2005).
Considering the peculiar palynological and palaeobotanical
features shown by the coals of Faxinal, the palynofloras,
which are dominated by glossopterids, small cordaites and
conifers, have a low representation of pteridophytic spores,
a small fraction of algae-like forms and absence of
Botryococcus. On the other hand, the abundance of woody
plant material and cuticular fragments belonging to
arborescent plants (mainly cordaites and glossopterids),
found together with palynological assemblages that reflect
the same vegetation (Guerra-Sommer et al., 1983), emphasize
the presence of different peat-forming plant communities in
relation to those recorded for other South Brazilian coal seams.
Although the cordaites, glossopterids and some conifers
have been linked to mesophytic-xerophytic environments,
for the palaeoenvironment interpretation of Faxinal coal-
bearing strata, it is important to recognize some of their
palaeoecological features , in order to explain their presence
in areas adjacent to the mire. According to Guerra-Sommer
(1988), the epidermal morphology of the cordaites identified
in the Faxinal coals could represent a mechanism to prevent
dessication or a response to changes in compositional
features of the soil, such as a deficit in N or in chemical
characteristics of the available water in the mires. Such
physiological and morphological features could have
permitted these plants to partition the ecological resources
of the mire.
Cridland & Morris (1963) and Falcon-Lang (2005)
mentioned the very large ecological amplitude of cordaites
and the possibility of these plants representing mangrove
trees. These papers discussed the possibility that the
cordaites occasionally experienced brackish incursions and
adapted to elevated salinity by modification of the rooting
system and leaves and demonstrated that some species grew
in coastal plains. Small cordaites, flourishing in marine-
influenced coastal habitat, in close proximity to the brackish
sea coast, adapted to periodically submerged conditions.
In the same way, arborescent lycopsids could also tolerate
these palaeoecological features (Habib & Growth, 1967).
According to Raymond (1988), some cordaites could live in
lowlands, co-occurring with calamiteans, tree-ferns and
lycopsids, typical of hygrophylous-mesophylous
environments.
According to DiMichele & Aronson (1992), hydraulic
and energetic competition of phothosynthetic and
reproductive activities may have made Paleozoic
glossopterids, more vulnerable to fluctuations in water
supply, influencing their occupation of drier areas adjacent
to the mire. Knoll & Niklas (1987) concluded that
xeromorphic structures shown by some glossopterids, as
identified in the Faxinal coal seam (Guerra-Sommer et al.,
1992), can be indicative that they originally grew in higher
lands. In the Permian, they could have migrated to peat-
forming environments in lowlands, co-occurring with
lycopsids, ferns and sphenopsids. The high representation
of arborescent glossopterids and cordaites with deciduous
leaves and the relatively low abundance of herbaceous
plants with perennial leaves in the megaflora of Faxinal are
indicative of an hypoauthochtonous taphocenose (Birks &
Birks, 1980). On the other hand, the low compositional
variation of the Faxinal megaflora also reflected in the
palynoflora, could indicate that the habitat in which these
plants grew were intrinsically stressful conditions
(DiMichele et al., 1985).
The Faxinal mire landscape was likely located in a more
inland area than that of Candiota. In Faxinal, coal was formed
in wet forest swamp environments, situated in a marine-
influenced lower delta plain setting, presenting favorable
conditions to the development of arborescent vegetation
(Figure 5). This is in opposition to the conditions under which
Candiota and Charqueadas coals formed. The dominance of
arborescent vegetation, low proportion of arborescent
lycopsids and scarce record of algal elements define this
scenario (Figure 6).
ACKNOWLEDGMENTS
The authors especially appreciate the important
assistance of J. Utting (GSC) in the laboratory
procedures. The authors would like to thank three
anonimous reviewers for their constructive and helpful
criticism of the manuscript. This research was supported
by Fundação de Amparo à Pesquisa do Rio Grande do
Sul (FAPERGS).
REVISTA BRASILEIRA DE PALEONTOLOGIA,10(2), 2007
12
PROVAS
Figure 5. Reconstruction of the landscape unit and plant-communities related to the peat-forming coal in high relative sea level (Faxinal coal seam and Lower Candiota coal seam, plant
communities are detailed in Figure 6); 1, hydrophylous plant-community; 2, hygrophilous plant-community; 3, hygrophylous/mesophylous plant-community; 4, mesophylous plant-community;
5, xerophylous plant community.
13
CAZZULO-KLEPZIG ET AL. – PEAT-FORMING ENVIRONMENT OF COAL SEAMS IN BRAZIL
PROVAS
Figure 6. Dominant plant-communities in the landscape unit related to the Faxinal coal seams: 1, algae and algae-like elements (hydrophylous plant-community); 2, herbaceous and/or shrub
like plants (lycopsids, ferns and sphenopsids (hygrophylous plant-community ); 3, arborescent lycopsids( hygrophylous/mesophylous plant-community ); 4, cordaites and glossopterids
(mesophylous plant-community); 5, conifers (xerophylous plant-community).
REVISTA BRASILEIRA DE PALEONTOLOGIA,10(2), 2007
14
PROVAS
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Received in February, 2007; accepted in June, 2007.
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