The first record of the “Mid-Barremian” Oceanic Anoxic Event and the Late Hauterivian platform drowning of the Bilecik platform, Sakarya Zone, western Turkey

Abstract

The late Hauterivian platform drowning has been recorded in the Hauterivian/Barremian boundary beds in western Turkey. This drowning event recorded by presence of hardgrounds, glauconite- and phosphate-bearing facies over the platform carbonates is followed up by pelagic carbonates.“Ammonitico Rosso” like reddish colored, thick-bedded limestones include multiple hardgrounds, abundant ammonites, belemnites and bivalves just below the Hauterivian/Barremian boundary. The hardground surfaces contain macro and micro-scale manganese and iron encrustations, micro- and macro-boring structures, calcite mineralization and glauconite minerals. Geochemical analysis indicates that Mn encrustation in the hardground is related to hydrothermal and/or hydrogenetic-detrital and associated with continental margin occurrence.A red pelagic limestone succession with planktonic foraminifera overlies the last hardground surface at the Hauterivian–Barremian boundary and is followed up by the alternation of gray colored bioturbated sandy/silty limestones with radiolaria and black shales or sandy/silty mudstones. A black shale level including ammonite, manganese, iron, pyrite and glauconite minerals takes place around the early Barremian–late Barremian boundary. At the top of the sections, there is a succession composed of alternation of thicker sandy limestone and shale/marl covering the black shale interval.The record of “mid-Barremian” black shale interval over the platform carbonates and the overlying pelagic succession with radiolaria indicate that the “mid-Barremian” Oceanic Anoxic Event took place after the drowning of the platform. Stable δ13C and δ18O isotope excursions have been detected within black shale zone and display parallel pattern with European sections. This study presents the first record of the mid-Barremian anoxic oceanic event on the Sakarya Zone of Pontides in Turkey.