Dynamics of Upper Sinemurian Macrobenthic Groups (Bivalves and Brachiopods) Preserved in Organic-Rich Facies of the Lusitanian Basin (Western Iberia)

Abstract

Bivalves and brachiopods are the most abundant macrobenthos of the upper Sinemurian marine assemblages preserved in the Lusitanian Basin (Portugal). The wide variety of life habits of the bivalves and the taxonomic diversity of both groups indicate that these elements should be taken into account for marine assemblages analysis. Although the basin contains many upper Sinemurian outcrops, those in the S. Pedro de Moel and Peniche areas are exceptionally fossil rich. The palaeontological content of the basin’s Água de Madeiros Formation is well preserved and has provided taxonomic and quantitative data regarding the abundance of bivalves and brachiopods. This lithostratigraphic unit shows two distinct sequences of biofacies: (1) shallow-water and soft-bottomed assemblages; and (2) hemipelagic assemblages associated with organic-rich facies. The transition between them corresponds to faunal turnover in the basin, recorded in the Raricostatum Subchronozone. The transition is particularly clear in the S. Pedro de Moel area, but does not seem to be synchronous with that observed in the Peniche area.