Litostratigrafia do Jurássico Inferior da região de S. Pedro de Moel (Bacia Lusitânica, Portugal)

O Jurássico da região de S. Pedro de Moel constitui uma importante referência no estudo da Bacia Lusitânica. Neste trabalho apresenta-se uma caracterização litostratigráfica das unidades carbonatadas do Jurássico Inferior aflorantes neste sector da bacia, apoiada num novo esboço cartográfico. Em S. Pedro de Moel definem-se as Formações de Coimbra, Água de Madeiros, Vale das Fontes, Lemede e S. Gião, totalizando o intervalo compreendido entre o Sinemuriano Inferior e o Toarciano Médio. As três primeiras unidades mostram um registo estratigráfico muito completo, enquanto as duas mais recentes afloram de forma limitada, em consequência da intensa fracturação existente na região. [ABSTRACT]: The Jurassic in S. Pedro de Moel area is an important reference for the study of the Lusitanian Basin. In this work a lithostratigraphic characterization of the Lower Jurassic carbonate units that crop out in this sector of the basin is presented. This is supported by a new cartographic framework. In S. Pedro de Moel, the Coimbra, Água de Madeiros, Vale das Fontes, Lemede and S. Gião Formations are defined, comprising the Lower Sinemurian to Middle Toarcian interval. Whereas the first three formations show a very complete stratigraphical record, the latter two units are very poorly exposed, as a consequence of the intense tectonic activity in this area

The Project PTDC/CTE-GIX/098968/2008 - High resolution stratigraphy of the Lower Jurassic organic-rich marine series in the Lusitanian Basin (Portugal): Initial expectations and results

Apresenta-se neste trabalho uma caracterização e os principais resultados do Projecto “Estratigrafia de alta resolução das séries marinhas ricas em matéria orgânica do Jurássico Inferior da Bacia Lusitânica”, financiado pela Fundação para a Ciência e Tecnologia (Portugal). Este projecto centrou-se numa análise estratigráfica multidisciplinar baseada num vasto conjunto de parâmetros bióticos e abióticos da sucessão margo-calcária rica em matéria orgânica do Jurássico Inferior da bacia, envolvendo sete objetivos principais: 1. Análise sedimentológica (mineralógica, textural e composicional) e sequencial da sucessão hemipelágica do Jurássico Inferior (Sinemuriano superior–Toarciano); 2. Precisões biostratigráficas a partir do registo de amonóides do intervalo compreendido entre o Sinemuriano superior e o Pliensbaquiano superior; 3. Análise paleontológica e paleoecológica da macrofauna bentónica (fundamentalmente braquiópodes e bivalves); 4. Análise biostratigráfica, taxonómica e paleoecológica da fauna de ostracodos (Sinemuriano–Toarciano); 5. Análise quimiostratigráfica baseada principalmente nos isótopos estáveis de carbono (Sinemuriano–Pliensbaquiano); 6. Análise geoquímica e de petrografia orgânica (carbono orgânico total, pirólise Rock-Eval e biomarcadores) do Sinemuriano–Pliensbaquiano; 7. Análise dos padrões de variação dos raios gama (Sinemuriano–Toarciano.This work presents the main results of the Project ”High resolution stratigraphy of the Lower Jurassic organic-rich marine series in the Lusitanian Basin (Portugal)”, supported by the Portuguese Foundation for Science and Technology. This project was focused on a multidisciplinary stratigraphic analysis based on the study of a range of biotic and abiotic parameters of the Lower Jurassic marl–limestone succession rich in organic matter, embracing seven main objectives: 1. Sedimentological (mineralogical, textural and compositional) and sequential analysis of the whole Lower Jurassic hemipelagic succession (Upper Sinemurian–Toarcian); 2. Ammonite biostratigraphic precisions of the Upper Sinemurian and Upper Pliensbachian; 3. Palaeontological and palaeoecological analysis of the benthic macrofauna (especially brachiopods and bivalves); 4. Biostratigraphical, taxonomic and palaeoecological analysis of the ostracod fauna (Sinemurian–Toarcian); 5. Chemostratigraphic analysis based mainly on carbon stable isotopes (Sinemurian–Pliensbachian); 6. Organic geochemical and petrographic analysis (total organic carbon, palynofacies, Rock-Eval pyrolysis and biomarkers) of the Sinemurian–Pliensbachian; 7. Vertical variation patterns of gamma-ray analysis (Sinemurian–Toarcian). Besides the new results in each specific scientific domain (for example, biostratigraphy, palaeoecology, sedimentology, sequence stratigraphy and cyclicity, chemostratigraphy and organic geochemistry), the integration of all data, in several cases at highresolution level, allows emphasizing the achievement of two main transverse goals: the improvement of the stratigraphy and sedimentary knowledge of the Lower Jurassic series of western Iberia margin, with implications in the palaoenvironmental and palaeoceanographic interpretations of the Tethyan pre-Atlantic basins dynamics and global correlation; taking into consideration the huge number of organic geochemical parameters analyzed, the results of this research contributed significantly to understand the role of the different Sinemurian and Pliensbachian organic-rich intervals as potential hydrocarbon generation sources (quality of organic matter and maturation).Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEFondo Europeo de Desarrollo Regional (COMPETE- FEDER)Fundação para a Ciência e a Tecnologia (FCT)pu

BIO-SEDIMENTARY SIGNATURES OF HIGH-FREQUENCY SALINITY/SUBAERIAL EXPOSURE CHANGES: EXAMPLES FROM THE OXFORDIAN OF PORTUGAL (CABAÇOS FORMATION)

The detailed sedimentary and micropalaeontological analysis of a complex association of continental to marginal-marine deposits from the Oxfordian of Portugal (Cabaços Formation) has allowed the recognition of high-frequency, subtle changes in the environmental conditions. The main factors controlling the palaeobiological responses to such minor-scale fluctuations were also identified. Two factors have shown to be especially significant: subaerial exposure duration and frequency (estimated by assigning type of features to the exposure index) and salinity trends, as suggested by the sedimentary and fossil records. In the west of the basin, salinity fluctuations were much stronger and more frequent (fresh- brackish-restricted marine-hypersaline), and subaerial exposure more marked for longer periods, than in the east of the basin. The microfossil assemblages, as a whole, but in particular the ostracod faunas, show differences in abundance, diversity, dominant species, degree of intrageneric and intraspecific variations, both along the successions and between west and east. The western populations seem to have been much less stable, which suggests that high-frequency changes in salinity (more than its absolute values) and degree of exposure were the most important controls on the palaeobiota. PD