RECORDS OF THE LATE HAUTERIVIAN PLATFORM DROWNING AND MID-BARREMIAN OCEANIC ANOXIC EVENT on THE BILECIK PLATFORM, SAKARYA ZONE, WESTERN TURKEY
In the studied areas, a sudden change in the facies takes place by multiple red hard-ground occurrences with abundant ammonites, belemnites and bivalves at the top of the platform carbonates. Hard-ground surfaces contain macro and micro-scale manganese and iron encrustations, micro- and macro-boring structures, calcite mineralization and glauconite minerals. A red pelagic limestone succession with planktonic foraminifera overlies the last hard-ground surface which took place at the late Hauterivian-early Barremian boundary. This succession is overlain by alternation of grey colored bioturbated limestones with Radiolaria and black shales/marls. 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 an alternation of thicker sandy limestone and shale/marl covering the black shale interval.
Chemical element (major, REE) analysis indicates that Mn encrustation in the hardground is related to hydrothermal and/or hydrogenetic-detrital and associated with continental margin occurrence.
The biostratigraphical frameworks of these sections have been established by using planktonic foraminifera and Radiolaria. Presence of similar microfacies, drowning of platform and occurrence of hard-grounds in the same boundary interval, overlying pelagic succession within the same time interval display very similar characteristic feature correlated with successions in Helvetic Alps and other European localities. This helps to analyze the extension of drowning event and understand the mechanism in behind this event.
The record of “mid-Barremian” black shale interval within the overlying succession including abundant Radiolaria may indicate the record of “mid-Barremian” Oceanic Anoxic Event following the drowning event in the basin. Such an oceanic event on the Sakarya Zone of Pontides has been recorded for the first time in Turkey and can be correlated with “mid-Barremian” Oceanic Anoxic Event recorded in some European basins.