Foraminiferal assemblages from the Toarcian (Lower Jurassic) ‘Spotted limestone’ of the northern Adriatic Carbonate Platform

Abstract

The present study provides an in-depth study of foraminiferal assemblages from a Tethyan carbonate platform during the late Early Jurassic interval of environmental perturbations. The Toarcian succession from the Adriatic Carbonate Platform was investigated in two sections from Slovenia. In addition to detailed sedimentological and micropalaeontological analysis, carbon and oxygen isotope curves were produced. The succession starts with platform-top facies with abundant and diverse skeletal material, including lithiotid and megalodontid bivalves, dasycladacean algae, corals and sponges. Foraminifera are diverse and abundant, with larger benthic foraminifera (LBF) being the most commonly present. The shallow platform facies is followed by dark grey allodapic limestone, suggesting the formation of an intraplatform basin with restricted circulation of bottom waters. The composition of the transported material highlights the continuation of biotic carbonate production in the shallower parts of the platform. Deposition of the overlying ‘Spotted limestone’, thin-bedded laminated and bioturbated carbonate mudstone coincides with the negative carbon isotope excursion and the onset of the Toarcian Oceanic Anoxic Event (T–OAE). Foraminifera are represented by small lagenids and opportunists. No LBF were detected, and barren intervals are common. With the gradual return of carbon isotope values to less negative values, ooids and bioclasts are once again transported into the basin. The abundance and diversity of foraminifera return to pre-excursion levels, although with fewer species of LBF. The lower diversity of LBF could be related to environmental stress that affected the shallower parts of the platform during T–OAE, such as sea water warming and/or eutrophication.