A sequence stratigraphic model of benthic foraminiferal facies trends with Triassic and Jurassic examples

Abstract

Abstract Foraminiferal facies trends are outlined in a new sequence stratigraphic model embracing depositional conditions from alluvial plain to offshore shelf settings. The biofacies proxies include distribution of benthic species, faunal diversities and frequency of normal marine (contra restricted) taxa. In the transgressive–regressive framework, biofacies trends are portrayed as assemblage expansion and reduction separated by trend changes. Assemblage expansion means upward increase in diversity and frequency of normal marine taxa, while assemblage reduction means upward decrease of the same proxies. The model is based on two modern bathymetric profiles and five Mesozoic sections, the latter documenting three settings in a transgressive–regressive sequence: 1) A deeper basinal setting, with hypoxia at maximum flooding, shows a trend change from assemblage expansion to reduction in the transgressive systems tract, marking a shift from the restricting effect of shallow water to that of hypoxia. The maximum flooding reveals a change from assemblage reduction to expansion. 2) An intermediate basinal setting with normal shelf conditions at maximum flooding is typified by continuous assemblage expansion in the transgressive systems tract, while the maximum flooding shows a change from expansion to reduction. 3) At a basin marginal position the maximum transgression is marked by extremely low diversities and relatively homogeneous assemblages. The study demonstrates that changing foraminiferal facies trends signify depositional conditions reflecting arrangement of sequence components such as flooding events, systems tracts and boundaries. Assemblage trend changes are discrete stratigraphic horizons signalling shifts in the effect of environmental factors (water depth, salinity, oxygenation) in the sequence architecture.