Subtidal stromatolites from the Sinemurian of the Lusitanian Basin (Portugal) |
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Authors: | Ana C Azerêdo Ricardo L Silva Luís V Duarte M Cristina Cabral |
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Institution: | 1.Departamento de Geologia and Centro de Geologia, Faculdade de Ciências,Universidade de Lisboa,Lisbon,Portugal;2.Departamento de Ciências da Terra, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, Coimbra and Centro de Geologia,Universidade de Lisboa,Lisboa,Portugal;3.Departamento de Ciências da Terra, Faculdade de Ciências e Tecnologia, IMAR-CMA,Universidade de Coimbra,Coimbra,Portugal |
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Abstract: | Well-preserved dome-shaped carbonate stromatolites occur in the lowermost part of the Sinemurian of the Lusitanian Basin (Portugal),
at S. Pedro de Moel region (W of the basin). Deposition in the region took place on a westward-dipping carbonate ramp. The
stromatolitic mounds are not found anywhere else in the Sinemurian of the basin and therefore are regarded as specific bioevents.
In contrast to marginal-marine stromatolitic crusts, subtidal carbonate mounds other than sponge-mounds have been seldom reported
in the Lower Jurassic, in particular in the Sinemurian, either from Europe or North-Africa. Therefore, the case documented
here contributes to enhance the knowledge on stromatolites of this age in the Peri-Tethyan and Proto-Atlantic regions. The
depositional setting of the studied succession is interpreted as a mainly low-energy, restricted marine one, punctuated by
higher-energy episodes and, locally, subjected to more open marine influence. The existence of a topographic high and detached
shoals at a more distal location of the ramp is likely, considering regional seismic evidence, the record in offshore (to
the W) wells of peloidal/ooid wacke-packstones with detrital quartz and occurrence of a few ooid grainstones in the studied
section. The inferred positive relief would act as a physical constraint that, coupled with the low-gradient of the ramp,
defined an embayment-like environment in which the prevailing ecological conditions must have been, for the part of the succession
bearing the stromatolites, unfavorable for many benthic organisms, favoring the microbial community. The upper part of the
succession suggests stepwise environmental openness to more marine influence alternating with frequent environmental restriction. |
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