Relative sea-level fluctuations and ecostratigraphy; applications to Middle/Upper Oxfordian fossil assemblages of east-central Algarve, Portugal

Abstract

The replies of benthic-epibenthic marine macroinvertebrate assemblages to ecospace variations during the Middle/Upper Oxfordian (Plicatilis, Transversarium, Bifurcatus, and Bimammatum Zones) according to fluctuations in the faunal spectra are recorded from six analyzed sections in the east-central Algarve, southern Portugal. The general evolution of the ammonite record during the studied time interval, with a recognizable decrease during the Transversarium-Bifurcatus Zones, can be biased by the better development and better outcrop conditions of the Bimammatum Zone in the area. The other studied chronozones are comparatively worse known and correspond to a comparatively larger development of buildups and/or intervals of tectonic instability in the Algarve, especially during the Transversarium-Bifurcatus Zones. In the recorded ammonite assemblages, the lytoceratids are always accidental elements with less than 2% representation, and their record decreases significantly during the Bimammatum Zone. The general trend of decreasing in the phylloceratids is coherent with the reduction of the ecospace inherent to the sequential evolution proposed by MARQUES & al. (1991). On the contrary, the evolution of the phylloceratids does not offer a clear interpretation according to the sequential scheme recently proposed by PONSOT & VAIL (1991). Significant deviations in the record of some ammonite groups have been recognized in comparatively high-energy deposits and are related to taphonomic effects. In one case it has been interpreted as an evidence of the heterogeneity of ecologic parameters whose recognition is however difficult through facies analysis.