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Estudo paleoecológico e biostratigráfico do Cenomaniano da região de Lisboa (Lousa-Salemas) baseado em ostracodos e microfácies
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Resumo(s)
A sucessão exposta no corte de Lousa-Salemas, maioritariamente do Cenomaniano médio, é caracterizada por alternância entre margas, calcários margosos e calcários compactos, fossilíferos, ricos em ostracodos, moluscos, algas e foraminíferos bentónicos, reflectindo ambiente sobretudo lagunar a marinho pouco profundo. Neste estudo, foram identificados fósseis de vários organismos, como moluscos bivalves (incluindo ostreídeos), gastrópodes, ostracodos (num total de 42 espécies), serpulídeos, equinodermes, algas calcárias (dasicladáceas, carófitas e outras), foraminíferos aglutinados (textularídeos indiferenciados, lituolídeos e outros aglutinados grosseiros), foraminíferos porcelanosos (miliolídeos em geral, incluindo alveolinídeos), foraminíferos hialinos e calpionelídeos. Para além dos fósseis, foram ainda identificados extraclastos de quartzo e raros feldspatos, intraclastos, pelóides e raros oncóides, porosidade e evidências de pedogénese e exposição subaérea, como rizoconcreções, “root-mat”, ferruginização e estruturas microalgais. No estudo dos ostracodos foram identificadas várias espécies típicas de meio lagunar marinho, salobro, mesmo oligohalino. A espécie Fossocytheridea merlensis domina a maior parte dos níveis da sequência de Lousa-Salemas, e a sua grande abundância, muitas vezes aparecendo em monoespecificidade, indica períodos salobros na laguna. O aparecimento das espécies Heterocypris berthoui e Globotalicypridea vulcanosa indicam meio oligohalino. Por outro lado, o aparecimento de espécies marinhas ornamentadas como Paracaudites (Dumontina) grekoffi, Cythereis cacemensis supralmargemensis e Trachyleberidea sp. e de espécies de carapaça lisa e fina como Cytherella aff. medwayensis, Asciocythere aff. burgausensis e Bairdoppilata aff. pseudoseptentrionalis, bem como diminuição ou mesmo desaparecimento de F. merlensis, indicam meio lagunar marinho. Relativamente ao estudo das microfácies, verificou-se que a maioria das lâminas estudadas varia entre biomicrite a bioextramicrite wackestone e packstone, embora também se tenha verificado a existência de grainstones, mudstones e boundstones, cujos principais elementos figurados carbonatados e outros aspectos particulares composicionais presentes nas lâminas indicam o meio em que o nível se formou. Os níveis lagunares salobros caracterizam-se por apresentarem abundantes moluscos (inclusive ostreídeos), ostracodos e alguns foraminíferos aglutinados grosseiros (sobretudo Buccicrenata aff. hedbergi, Pseudocyclammina rugosa, Charentia cuvillieri e Daxia cenomana), juntamente com extraclastos de quartzo e alguma ferruginização incipiente. Verificou-se que quando há aumento de miliolídeos (por exemplo Quinqueloculina robusta e Nummoloculina regularis) face aos aglutinados grosseiros, juntamente com aparecimento de abundantes fragmentos de equinodermes e de dasicladáceas (Neomeris cretacea, Heteroporella lepina e Salpingoporella hasi), estes níveis caracterizam meio lagunar marinho. Por outro lado, a ausência ou fraca representação de bio-elementos, aumento significativo dos extraclastos (sobretudo quartzo) e a pedogénese intensa com exposição subaérea associada (como “root-mat”, rizoconcreções e ferruginização abundante), indicam níveis de meio margino-continental. No topo da sequência ocorrem níveis em que há grande aumento de miliolídeos, incluindo o aparecimento de espécies do género Praealveolina, indicando a passagem para meio marinho pouco profundo de plataforma interna aberta. Com base nos dois tipos de estudos efectuados observou-se então, ao longo de toda a sequência, intercalação entre três ambientes principais, lagunar margino-continental, lagunar salobro, lagunar marinho, passando no topo a um quarto ambiente, marinho aberto, pouco profundo, que indica a passagem do Cenomaniano médio para o Cenomaniano superior. O ambiente lagunar salobro caracteriza-se pela abundância em F. merlensis, e ocasionalmente também por Perissocytheridea estribeirensis ou Gen. Ind. sp. 3, juntamente com bivalves (incluindo ostreídeos) e foraminíferos aglutinados. O ambiente lagunar marinho caracteriza-se pela diminuição ou desaparecimento da espécie F. merlensis e pelo aparecimento de espécies tipicamente marinhas dos géneros Cythereis, Cytherella, Bairdoppilata e Asciocythere, e das espécies P. (Dumontina) grekoffi, Trachyleberidea sp. e Paracypris sp., bem como pela presença abundante de miliolídeos, equinodermes, dasicladáceas e por vezes foraminíferos hialinos. O ambiente lagunar margino-continental é caracterizado por pedogénese e exposição subaérea, podendo chegar a ser oligohalino quando se verifica a presença das espécies H. berthoui e G. vulcanosa, juntamente com F. merlensis e Lycopterocypris? sp.. O aparecimento dos alveolinídeos e abundância em outros macro e microorganismos marinhos marca a passagem para meio marinho pouco profundo, aberto. A sequência de Lousa-Salemas é assim uma boa evidência da transgressão progressiva que terá ocorrido durante o Cenomaniano na região de Lisboa.
The Lousa-Salemas section, mostly of middle Cenomanian age, is characterized by an alternation of fossiliferous marls, marly limestones and limestones, rich in ostracods, molluscs, calcareous algae and benthic foraminifera, reflecting coastal lagoon to open marine environments. In this study, several organisms were identified, such as bivalves (including oysters), gastropods, ostracods (a total of 42 species), serpulids, echinoderms, calcareous algae (dasycladales, charophytes, and others), agglutinated foraminifers (undetermined textularids, lituolids and other), porcellaneous foraminifers (miliolids in general and alveolinids), hyaline foraminifera and calpionelids. In addition to the fossils, quartz and rare feldspars, intraclasts, pellets and rare oncoids were also identified, as well as porosity and evidence of pedogenesis and subaerial exposure, such as rhizocretions, root-mat, ferruginization and microbial-algal structures. In the study of the ostracods several species were identified, typical of marine, brackish or oligohaline environments. The species Fossocytheridea merlensis dominates most of the levels of the studied sequence, and its great abundance (often appearing in monospecificity), indicates brackish periods in the lagoon. The appearance of the species Heterocypris berthoui and Globotalicypridea vulcanosa indicate oligohaline environment. On the other hand, the appearance of ornamented species such as Paracaudites (Dumontina) grekoffi, Cythereis cacemensis supralmargemensis and Trachyleberidea sp. and of smooth and thin carapace species such as Cytherella aff. medwayensis, Asciocythere aff. burgausensis and Bairdoppilata aff. pseudoseptentrionalis, as well as the decreasing or even disappearance of F. merlensis, indicates a marine lagoon setting. In the study of microfacies, most of the samples studied range from biomicrite to bioextramicrite wackestones and packstones, although there are also grainstones, mudstones and boundstones present in the section. The identification of the carbonate elements and other particular compositional aspects can provide the environment in which the level was formed. Thus, four main environments were established: marginal lagoon with continental exposure, brackish lagoon, marine lagoon and open shallow-marine. The brackish levels are characterized by abundant molluscs (including oysters), ostracods and some coarsely-walled agglutinated foraminifera (mainly Buccicrenata aff. hedbergi, Pseudocyclammina rugosa, Charentia cuvillieri and Daxia cenomana), and also by grains of quartz and some incipient ferruginization. On the other hand, when there is an increase in miliolids (for example, Quinqueloculina robusta and Nummoloculina regularis) and appearance of abundant fragments of echinoderms and dasycladales (Neomeris cretacea, Heteroporella lepina and Salpingoporella hasi), this characterizes as a marine lagoon environment. The absence or weak representation of bio-elements, the significant increase of extraclasts (mainly quartz) and the pedogenesis with associated subaerial exposure (such as root-mat, rhizoconcretions and abundant ferruginization) indicates marginal environment with a strong continental influence in the lagoon. At the top of the sequence there is a large increase of miliolids, including the appearance of species of the genus Praealveolina, indicating the transition into an open shallow-marine environment. Based on the two type of studies, an intercalation of three main environments was recognized throughout the sequence, marginal lagoon with continental exposure, brackish lagoon and marine lagoon, and at the top of the succession a fourth environment was also observed, open shallow-marine, that indicates the transition from middle Cenomanian to upper Cenomanian. The brackish lagoon is characterized by abundance of F. merlensis and occasionally Perissocytheridea estribeirensis and Gen. Ind. Sp. 3, along with bivalves (including oysters) and agglutinated foraminifera. The marine lagoon is characterized by the decrease or disappearance of F. merlensis species and by the appearance of typical marine species belonging to the genus Cythereis, Cytherella, Bairdoppilata and Asciocythere along with other marine species such as P. (Dumontina) grekoffi, Trachyleberidea sp. and Paracypris sp., as well as abundant presence of miliolids, echinoderms, dasycladales and hyaline foraminifera. The marginal with continental exposure lagoon is characterized by pedogenesis and subaerial exposure, sometimes with oligohaline water when in the presence of the ostracods H. berthoui and G. vulcanosa as well as F. merlensis and Lycopterocypris? sp.. The appearance of alveolinids and abundance in other marine macro and microorganisms indicates an open shallow-marine environment. Thus, the sequence of Lousa-Salemas is an evidence of the progressive marine transgression that has occurred during Cenomanian times in the Lisbon region.
The Lousa-Salemas section, mostly of middle Cenomanian age, is characterized by an alternation of fossiliferous marls, marly limestones and limestones, rich in ostracods, molluscs, calcareous algae and benthic foraminifera, reflecting coastal lagoon to open marine environments. In this study, several organisms were identified, such as bivalves (including oysters), gastropods, ostracods (a total of 42 species), serpulids, echinoderms, calcareous algae (dasycladales, charophytes, and others), agglutinated foraminifers (undetermined textularids, lituolids and other), porcellaneous foraminifers (miliolids in general and alveolinids), hyaline foraminifera and calpionelids. In addition to the fossils, quartz and rare feldspars, intraclasts, pellets and rare oncoids were also identified, as well as porosity and evidence of pedogenesis and subaerial exposure, such as rhizocretions, root-mat, ferruginization and microbial-algal structures. In the study of the ostracods several species were identified, typical of marine, brackish or oligohaline environments. The species Fossocytheridea merlensis dominates most of the levels of the studied sequence, and its great abundance (often appearing in monospecificity), indicates brackish periods in the lagoon. The appearance of the species Heterocypris berthoui and Globotalicypridea vulcanosa indicate oligohaline environment. On the other hand, the appearance of ornamented species such as Paracaudites (Dumontina) grekoffi, Cythereis cacemensis supralmargemensis and Trachyleberidea sp. and of smooth and thin carapace species such as Cytherella aff. medwayensis, Asciocythere aff. burgausensis and Bairdoppilata aff. pseudoseptentrionalis, as well as the decreasing or even disappearance of F. merlensis, indicates a marine lagoon setting. In the study of microfacies, most of the samples studied range from biomicrite to bioextramicrite wackestones and packstones, although there are also grainstones, mudstones and boundstones present in the section. The identification of the carbonate elements and other particular compositional aspects can provide the environment in which the level was formed. Thus, four main environments were established: marginal lagoon with continental exposure, brackish lagoon, marine lagoon and open shallow-marine. The brackish levels are characterized by abundant molluscs (including oysters), ostracods and some coarsely-walled agglutinated foraminifera (mainly Buccicrenata aff. hedbergi, Pseudocyclammina rugosa, Charentia cuvillieri and Daxia cenomana), and also by grains of quartz and some incipient ferruginization. On the other hand, when there is an increase in miliolids (for example, Quinqueloculina robusta and Nummoloculina regularis) and appearance of abundant fragments of echinoderms and dasycladales (Neomeris cretacea, Heteroporella lepina and Salpingoporella hasi), this characterizes as a marine lagoon environment. The absence or weak representation of bio-elements, the significant increase of extraclasts (mainly quartz) and the pedogenesis with associated subaerial exposure (such as root-mat, rhizoconcretions and abundant ferruginization) indicates marginal environment with a strong continental influence in the lagoon. At the top of the sequence there is a large increase of miliolids, including the appearance of species of the genus Praealveolina, indicating the transition into an open shallow-marine environment. Based on the two type of studies, an intercalation of three main environments was recognized throughout the sequence, marginal lagoon with continental exposure, brackish lagoon and marine lagoon, and at the top of the succession a fourth environment was also observed, open shallow-marine, that indicates the transition from middle Cenomanian to upper Cenomanian. The brackish lagoon is characterized by abundance of F. merlensis and occasionally Perissocytheridea estribeirensis and Gen. Ind. Sp. 3, along with bivalves (including oysters) and agglutinated foraminifera. The marine lagoon is characterized by the decrease or disappearance of F. merlensis species and by the appearance of typical marine species belonging to the genus Cythereis, Cytherella, Bairdoppilata and Asciocythere along with other marine species such as P. (Dumontina) grekoffi, Trachyleberidea sp. and Paracypris sp., as well as abundant presence of miliolids, echinoderms, dasycladales and hyaline foraminifera. The marginal with continental exposure lagoon is characterized by pedogenesis and subaerial exposure, sometimes with oligohaline water when in the presence of the ostracods H. berthoui and G. vulcanosa as well as F. merlensis and Lycopterocypris? sp.. The appearance of alveolinids and abundance in other marine macro and microorganisms indicates an open shallow-marine environment. Thus, the sequence of Lousa-Salemas is an evidence of the progressive marine transgression that has occurred during Cenomanian times in the Lisbon region.
Descrição
Tese de mestrado em Geologia, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, em 2018
Palavras-chave
Ostracodos Microfácies Cenomaniano médio Paleoambientes Lousa-Salemas Teses de mestrado - 2018