Foraminiferal evidence for paleogeographic and paleoenvironmental changes across the Coniacian–Santonian boundary in western Ukraine.

Abstract: Planktonic and benthic foraminiferal assemblages, oxygen stable isotopes, calcium carbonate content and total sulfur and magnetic susceptibility have been studied from the middle Coniacian to middle Santonian interval of the Dubivtsi succession (western Ukraine). At the Coniacian–Santonian boundary significant changes took place in foraminiferal assemblages. Keeled, deep-water forms, which are a common to dominant group in planktonic foraminiferal assemblages in the late Coniacian decline in abundance in the Santonian, where the assemblages are dominated by heterohelicids and Hedbergella. Late Coniacian benthic foraminiferal assemblages dominated by large, calcareous epifaunal (oxic) forms in the early Santonian became dominated by small, thin-walled, infaunal (dysoxic) species with significant increase of agglutinated foraminifers within assemblages. Changes in foraminiferal assemblages correspond with lithological changes from upper Coniacian limestones to lower Santonian marls. An increase in total sulfur in the lower Santonian and increase in magnetic successibility values up the succession are recorded. Changes in foraminiferal assemblages along with decreasing calcium carbonate content and increasing magnetic susceptibility indicate that during the early and middle Santonian the Tethyan connection with the studied part of the south-central European epicontinental sea was partially limited by an area uplifted as a consequence of Subhercynian tectonic movements; thus it constituted a partially restricted basin with sedimentation in an oxygen depleted environment. [Copyright &y& Elsevier]