Micropaleontological assemblages, paleoenvironments and sedimentary evolution of Cretaceous deposits in the Algarve (Southern Portugal)

Abstract

The Cretaceous of the Algarve comprises some lithostratigraphic units (carbonatic in shallow-water environments, siliciclastic in estuarine or fluviatile areas) which are essentially dated by their micropaleontological assemblages which contain calpionellids, lituolids, orbitolinids, involutinids, dasycladacean algae, charophytes. The formations are interpreted in the following terms: one regressive-transgressive cycle (late Oxfordian-basal Valanginian), two regressive sequences (Hauterivian-Barremian, Bedoulian) and one transgressive sequence (Gargasian-middle Cretaceous), separated by three major discontinuities at the base of the Valanginian, lower Bedoulian and upper Bedoulian. The sedimentary evolution and the geometry of deposits are controlled by various geodynamic factors the influence of which is analyzed: the relative changes of sea level are the main factor which determines the repartition of the formations in the different isotopic zones; the rate of subsidence on this passive continental margin (well marked during the Aptian and Albian) and sedimentary influxes increasing during the Hauterivian-Barremian contribute to the vertical regressive or transgressive evolutions. Orogenic movements occur only in the Neocomian, near a diapiric area. These factors record, on the south portuguese continental shelf, different stages of the history of the Northern Atlantic: the Neocimmerian and the Bedoulian riftings induces major discontinuities and sedimentary gaps. The beginning of the expansion of the Northern Atlantic determines the increasing of subsidence during the Aptian and Albian.