Cenomanian – Turonian Oceanic Anoxic Event (OAE2) in the Sakarya Zone, northwestern Turkey: Sedimentological, cyclostratigraphic, and geochemical records

Abstract

Abstract The Cenomanian/Turonian oceanic anoxic event as recorded in many parts of the world is characterized by presence of black shales and global carbon isotope peaks. Cyclic variations within the anoxic event and association with pelagic red beds are indicated by many studies. Global tectonic/volcanic controls on the origin and the chronostratigraphic position of the anoxic event within the global sea-level changes are important for understanding the paleoceanographic and paleoclimatic conditions during the deposition of Cenomanian/Turonian sediments. Three measured stratigraphic sections in the Central Pontides and the Central Sakarya region of Turkey on the Sakarya Zone of the Pontides have been studied in detail. Cyclic facies alternations have been analysed and Ficher plot curves have been established for the measured sections. According to the cyclostratigraphic analysis, position of the C/T boundary with black shales/mudstones coincides with the global sea-level rise. This indicates that global sea-level change is one of the controlling parameters of black shale generation at the boundary and the long distance correlation between pelagic/hemipelagic successions of different parts of the Sakarya Zone and with global counterparts displays the effect of a large-scale event rather than local. A positive shift in the δ 13 C curve has been obtained just below the C/T boundary similar to many curves obtained around the world. The depletion of Mn indicates oxygen-depleted bottom conditions. Parallel increase of Cd, P, Ba, and Fe indicates increased productivity. Increase in Zn, Cu, Pb and the presence of volcanoclastic sands just below the boundary indicate the fact that volcanism was present during the deposition of anoxic sediments and affected the productivity. TOC values of black shales reach up to 1, 95% at the boundary but drop suddenly after the boundary in parallel with other proxies. The Milankovitch-scale frequent rise and fall of the sea-level superimposed on larger cycles also triggered the nutrient enrichment and the ventilation of the basin resulting in the repetitive alternation of black shales and red marls or cherts. Therefore, this study indicates that the OAE2 including cyclic black shale and red marl/chert alternations on the Sakarya Zone was developed by the cooperation of volcanism, productivity and small and large-scale sea level changes.