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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.