Geochemical characterization of late Jurassic-early Cretaceous age limestones from Gumushane (NE-Turkiye): identification of the source of organic matter and paleo-environment conditions

Abstract

Samples from late Jurassic-early Cretaceous age limestones in five different areas in the Gumushane region (NE-Turkiye), were studied using TOC/Rock-Eval, and some trace elements analysis, to evaluate their organic material source, type, thermal maturity, and paleo-environment conditions. The studied carbonate rocks are characterized by 90 m thick in the Baglarbasi section, 420 m in the Gulacar, 584 m in the Unlupinar, 375 m in the Mescitli, and 272 m in the Yuvacik. The carbonate rocks are generally represented by, from bottom to top, dolomite, oolitic, sparitic, sandy, micritic, clayey, and bituminous limestones. Average total organic carbon (TOC) values are 0.14, 0.16, 0.20, 0.12, and 0.20% in the Baglarbasi, Gulacar, Unlupinar, Mescitli, and Yuvacik sections, respectively. Organic material in the limestones consists of Type II-III kerogen, indicating that a mixture of terrigenous higher plants-marine aquatic organisms was the main source. Average T-max values and T-max-based vitrinite reflectance values (Ro %) are calculated as 435 and 0.67, 444 and 0.83, 431 and 0.60, 442 and 0.79, and 437 degrees C and 0.70 in the Baglarbasi, Gulacar, Unlupinar, Mescitli, and Yuvacik sections, respectively, indicating that the samples have generally reached thermal maturity. The limestones in the Gulacar, Mescitli, and Yuvacik areas are probably well-drained rocks that were extracted and originated hydrocarbon efficiently from the limestone fractures. In general, a non-marine-sometimes marine deposition environment for the carbonates is indicated by total sulfur values. V/Ni and Sr/Ba ratios indicate a stratified water column with high-low salinity and anoxic-oxic bottom water circumstances and accordingly a transitional sedimentation environment.