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

Samples from late Jurassic–early Cretaceous age limestones in five different areas in the Gümüşhane region (NE-Türkiye), 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 Bağlarbaşı section, 420 m in the Gülaçar, 584 m in the Ünlüpınar, 375 m in the Mescitli, and 272 m in the Yuvacık. 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 Bağlarbaşı, Gülaçar, Ünlüpınar, Mescitli, and Yuvacık 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 Tmax values and Tmax-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 °C and 0.70 in the Bağlarbaşı, Gülaçar, Ünlüpınar, Mescitli, and Yuvacık sections, respectively, indicating that the samples have generally reached thermal maturity. The limestones in the Gülaçar, Mescitli, and Yuvacık 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.