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Sequence stratigraphy and depositional setting of the Pliensbachian and Toarcian marly limestones in the Lusitanian Basin, Portugal

Duarte L.

Volumina Jurassica

2006

Vol. 4, no. 1

157-158

The Pliensbachian and Toarcian series in the Lusitanian Basin (Portugal) are generally dominated by hemipelagic deposits, represented by marl/limestone alternations that are very rich in nektonic and benthic fauna. These sediments are included in the following four formations: Vale das Fontes, Lemede, S. Giăo and, partially, Póvoa da Lomba. The weak lateral facies variation, generally observed at the basin scale, suggests that these sediments were deposited in an epicontinental extensional basin on a homoclinal carbonate ramp controlled by eustatic fluctuations and regional tectonics. Considering the Late Triassic – Late Callovian large cycle, the sediments correspond to the maximum transgressive facies which can be widely observed throughout the succession. A detailed studied of several stratigraphic sections in terms of sedimentological, geochemical and paleontological analysis shows that the Pliensbachian-Toarcian series is subdivided into two second-order sequences (SP and ST). The Pliensbachian succession shows a typical second-order transgressive/regressive sequence, with a dominant marly deposition at the base and a calcareous dominant facies at the top. The basal discontinuity of the SP is particularly well observed in the western part of the basin, dating roughly from the Sinemurian/Pliensbachian boundary. The series shows a large transgressive phase, ending in the middle-upper part of the Margaritatus Zone (around Subnodosus/Gibbosus subzones boundary) associated with an organic-rich deposition verified at the basin scale. During the Spinatum Zone the sedimentation returned to a calcareous regime very rich in benthic macrofauna. The upper discontinuity of the SP observed in the whole basin dates from the lowermost Polymorphum Zone (intra-Mirable Subzone). The base of ST (Polymorphum Zone) corresponds to an abrupt flooding event, through a generalised marly accumulation in the whole basin. However, around the Polymorphum-Levisoni interval, an important tectonic activity occurred, responsible for a great sedimentary change with special facies features in some positions of the basin. The dominance of marl observed at the top of the Levisoni Zone marks the maximum peak transgression of the Toarcian second-order sequence, showing some evidence of pelagic deposition, with thin-shelled bivalve-rich (Bositra sp.) horizons. The Upper Toarcian – Lower Aalenian succession shows a regressive trend, ending ST with an upward increase of calcareous and bioclastic content, including ahermatipic corals in the eastern sectors. The upper discontinuity dates from the Opalinum Zone and shows different sedimentary records across the basin. ST it is subdivided into four third-order depositional sequences (St1 to St4), each bounded by regional discontinuities, recognized over most parts of the Lusitanian Basin.