Geochemical characteristics of the Silurian shales from the central Taurides, southern Turkey: Organic matter accumulation, preservation and depositional environment modeling

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Abstract The Silurian-age organic-rich and organic-lean shales, located within Aydincik, Gulnar and Silifke districts of Mersin province in central Taurides, southern Turkey, is considered as an unconventional resources. These organic-rich and -lean shales are controlled by the similar geological, tectonic and sedimentary processes, as the passive margin basin has acted as the provenance region throughout the deposition of these shales. Geochemical data of major and trace elements obtained for a total of 39 outcrop samples were interpreted to determine the depositional model, water column productivity, amount of organic matter and preservation conditions. Total organic carbon (TOC) values of these organic-rich and -lean shales range from 0.56 to 4.06 wt. % and 0.01 to 0.38 wt. %. Several redox indicators, such as V systematics (V/Sc, V/Ni) as well as ratios of Ni/Co, Th/U and Mo/Mn, showed that the organic-rich shales were deposited in suboxic to anoxic environments, as opposed to oxic-dysoxic conditions for organic-lean ones. However, organic matter enrichment is not restrained by water column productivity, as indicated by a lack of correlation between TOC and productivity indexes (P/Ti and Ba/Al), but rather probably controlled by dysoxic-anoxic water column environment. In addition, a detrital material input cannot be ignored for its influence on organic matter enrichment in the shale deposition. The increase in the clastic fluxes probably resulted from the deposition in the extended shelf with coastal upwelling. A lack of correlation between K2O, Al2O3, SiO2 and TOC contents among these shales imply that clay minerals may not influence organic matter preservation. According to w(La)N/w(Yb)N and ΣREE values, a high sedimentation rate is determined during the shale deposition, which would result in a dilution of organic matter. Furthermore, slight marine influences with sulphur limitation and low salinity, semi-arid/warm to humid-warm climatic conditions are widespread, according to total sulphur (TS), Sr/Ba, Sr/Cu, chemical index of alteration (CIA), C-value and related discrimination diagrams. In the end, a depositional model of the central Taurides shales is established to display an excellent preservation condition as the major controlling factor for organic matter enrichment in the Silurian shales.