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2 2006

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Recent data on the ostracod fauna from the Lower and Middle Toarcian of Peniche, Portugal

Pinto S., Cabral M., Duarte L.

Volumina Jurassica

2006

Vol. 4, no. 1

204-205

At Peniche region, western Portugal, a rather continuous marly limestone succession ranging in age from the Sinemurian to the earliest Aalenian crops out. The ostracod fauna from a 74 m thick packet within this succession has been studied, encompassing deposits that range from the topmost Pliensbachian (topmost Spinatum Biozone) to the Middle Toarcian (Polymorphum, Levisoni, Bifrons and basal Gradata biozones). The studied levels belong to Lemede (2 m) and Cabo Carvoeiro Formations. From the 47 samples collected in the marly layers, a single one is azoic. Sample numbers and bed subdivision are according to biozonation in Duarte (1995), which considers 5 packets: topmost Spinatum, Polymorphum, Levisoni, Levisoni+Bifrons, Bifrons+basal Gradata. The systematic study of the ostracods has been undertaken, and some palaeoecological aspects have also been addressed, namely hydrodynamics (carapace/valve ratio), bathymetry and oxygenation, based on marker genera/groups. Around 8000 individuals were obtained (1 individual =1 carapace or 1 valve), which are commonly badly preserved. At least 58 species belonging to 22 genera have been identified until now. The top of Spinatum Biozone displays both high diversity (8-15 species/sample) and abundance (222-402 individuals/sample) of ostracods. The dominant species are from the genus Ogmoconcha, Ogmoconchella and Liasina, associated with Polycope, Paracypris, Ledahia, among others. The Polymorphum Biozone shows high diversity (3-22 species/sample) and variable abundance (1-150 individuals/sample). The genera Ogmoconcha, Ogmoconchella and Liasina still dominate and, from the first levels of the biozone, heavily ornamented species of Kinkelinella are present; the genus Isobythocypris occurs for the first time. In Levisoni Biozone, the ostracod diversity (1-4 species/sample) and abundance (1-150 individuals/ sample) are low, in contrast with previous biozones. Ogmoconcha and Ogmoconchella disappear, and the genus Cytherella appears for the first time, becoming dominant together with Liasina in the first levels of the biozone. In the upper part, Kinkelinella dominates and at the top Bairdiacypris and Cytheroptheron occur for the first time. Levisoni+Bifrons shows that ostracod diversity is moderate (5-11 species/sample) and abundance is high (60-310 individuals/sample). Species from the genera Bairdiacypris and Kinkelinella dominate, Cytherella and Patellacythere are common. Bifrons+basal Gradata shows higher diversity (3-18 species/sample) than previous unit and variable abundance (16-468 individuals/sample). Bairdiacypris, Cytherella and Kinkelinella dominate, and Macrocypris and Trachycythere(?) occur for the first time. All of the recognized species have been found in formations of a similar age in Western Europe, and are benthic marine species, except the genus Polycope (pelagic). Most of the species indicates a deep marine setting, with variable oxygenation levels, from normal to low (predominance of Metacopina and Cytherella), for the all section. The water energy was also variable, being stronger at the topmost Spinatum and Polymorphum biozones, within which ostracod valves are dominant, and lower at the Levisoni and Levisoni+Bifrons biozones, within which ostracod carapaces clearly dominate.

Microbial mounds from the restricted-marine Sinemurian succession of S. Pedro de Moel (Lusitanian Basin, Portugal): preliminary palaeoecological interpretation

Azeredo A., Duarte L., Cabral M.

Volumina Jurassica

2006

Vol. 4, no. 1

77-77

In the Lusitanian Basin the Sinemurian corresponds to a marginal- to restricted-marine succession, representing the early stage of sea flooding in the recently formed basin, following a Late Triassic rifting event. Except for its topmost part, the Sinemurian succession belongs to the Coimbra Fm., composed of dolostones, dolomitic limestones and limestones. This unit is dominated by peritidal facies towards the eastern, more landward zone of the basin, whereas in the W (as at S. Pedro de Moel) more distal (though not quite deep) facies occur. At S. Pedro de Moel the Coimbra Fm. is well exposed and the section displays a succession mostly composed of argillaceous and/or dolomitic limestones and fossiliferous/skeletal limestones (bivalves, gastropods, ostracods), interbedded with a few marly levels. Although fossil remains are commonly present in variable amounts, some beds seem to be azoic, others exhibit rare, deformed burrows and very rare ostracods. Undulating, irregularly bounded, laminar levels are common and, locally, centimetre to decimetre-thick concentrated skeletal/fossil layers occur. However, towards the middle part of the section, well-preserved, dome-shaped stromatolites occur, in clear contrast with the under- and overlying bedded deposits. Towards the top of the succession, the calcareous/clay ratio increases. This section is still under study, so we only make a preliminary palaeoenvironmental approach here. The microbial mounds have an average maximal thickness of 0.75 m and show different fabrics, sometimes within the same mound: laminated, stromatolitic crusts; clotted, peloidal micrite; micritic and sparitic threads; degraded, tuft-like filamentous structures; enhanced fenestral-like porosity; dense, slightly darker micrite. These features suggest that they were formed through hardening of calcified cyanobacterial and other microbial films, whose early disintegration also would have contributed with autochthonous mud for the mounds. However, a more detailed study is clearly needed. It is known from the literature that microbialites, as a whole, may form in a wide range of environmental conditions, though some associations or particular morphologies may give us more accurate ecological information. However, a crucial basic condition is a very low to low background sedimentation. In the present case, it is suggested that the stromatolites grew under low-energy, restricted water-circulation and low rate of mixed terrigenous and calcareous mud deposition. A likely nutrient-poor substrate, the existence of terrigenous material and, maybe, slight (?)hypersalinity would have inhibited a more significant development of epibenthic and heterotrophic organisms, favouring the microbial community. The low energy prevented physical erosion which, coupled with the absence of predators, allowed the development of the well-defined dome morphologies. In contrast, towards the top of the succession a more open setting prevailed, with better water-circulation, probably better oxygenation and somewhat higher sedimentation rate with dramatic decrease of clay material. Though most of the upper deposits are still low-energy ones (biomicrite mudstones and wackestones, with a few ostracods, gastropods, bivalves, rare hyaline forams), skeletal/intraclastic/peloidal packstones and rarer grainstones occur more frequently, attesting for the less protected environment. These conditions did not allow the continuation of microbial growth.