Early Cretaceous benthic associations (foraminifera and calcareous algae) of a shallow tropical-water platform environment (Mljet Island, southern Croatia)

Abstract

The Lower Cretaceous shallow-marine succession of Mljet Island in Croatia records the geological history of the southern part of the Adriatic Platform during its last tectonically quiet period, prior to the Late Cretaceous collision processes between the Adria Microplate and the Eurasian Plate. We studied the Early Cretaceous biostratigraphy of benthic foraminifera and calcareous algae in order to establish a precise, combined benthic biozonation for the Adriatic, which in turn facilitates a better understanding of the global distribution of these microorganisms during the Early Cretaceous. Thirty-four species from 26 genera of benthic foraminifera and 32 species of calcareous algae, distributed among 11 genera, were recovered from the Lower Cretaceous shallow-water carbonate rocks of Mljet Island in southern Croatia. Nine biostratigraphical units, eight of which are based upon the benthic foraminifera and calcareous algae, and one upon the microencruster Bacinella irregularis, have been distinguished within this interval: the Clypeina parasolkani-Humiella catenaeformis Interval Zone (Berriasian–earliest Valanginian), Epimastopora cekici-Pseudoclypeina? neocomiensis Interval Zone (Late Valanginian), Montsalevia salevensis Taxon-range Subzone (Late Valanginian), Clypeina? solkani Abundance Zone (late Early and Late Hauterivian), Salpingoporella melitae-Salpingoporella muehlbergii Interval Zone (Barremian), Bacinella irregularis Assemblage Zone (Early Aptian), Salpingoporella dinarica Abundance Zone (Late Aptian), Orbitolina ( Mesorbitolina) texana-“ Valdanchella” dercourti Interval Zone (Early Albian) and “ Valdanchella” dercourti Taxon-range Zone (Late Albian). The stratigraphical position of the main genera and species of benthic foraminifera and calcareous algae within the Berriasian–Albian interval is discussed, and the established biozones are correlated within the Adriatic Platform domain. The Early Cretaceous diversification of benthic foraminifera in the area investigated can be shown to follow the sea-level curves for that period, and the major foraminiferal turnovers coincide with global sea-level rise and fall. During relative sea-level rises, coupled with reduced oceanic circulation and expansion of nutrient-poor, shallow tropical waters, these organisms were able to diversify into various euphotic habitats, particularly within shallow subtidal environments of the platform interior. Regressive episodes resulted in the reduction of oligotrophic habitats and a decrease in species richness.