Geochemistry of the Miocene oil shale (Hançili Formation) in the Çankırı-Çorum Basin, Central Turkey: Implications for Paleoclimate conditions, source–area weathering, provenance and tectonic setting

Abstract

The geochemistry of oil shale units of Hançili Formation in the Çankırı-Çorum Basin (Central Turkey) was studied using various chemical analyses. The mineralogical composition of the samples were preliminarily investigated using Pearson's correlation coefficients of selected elements, indicating that the samples contain abundant clay minerals as well as K-feldspar and carbonate. The K2O/Al2O3 and SiO2/Al2O3 ratios indicate that the major proportion of clay minerals is composed of smectite. Si, Al, Ti, K, Na, and Fe reside in clay minerals, while Ca, Mg, and Mn are mostly associated with carbonates (e.g., calcite and dolomite) and phosphorous (P) is present as apatite. The total REE content of the oil shale range from 19 to 113ppm. The chondrite-normalized patterns of the oil shale show LREE enrichments, HREE deficits, negative Eu anomalies and negligible Ce anomalies. In general, major, trace and rare earth element abundances suggest that the studied oil shale in the Çankırı-Çorum Basin are mainly from the intermediate rocks, mixed with small amounts of basic rocks, and that their source rocks are mostly deposited in the continental collision setting. The REE geochemistry of the oil shale suggests that these samples were derived from a consistent terrigenous source and the Eu anomaly was inherited from the source rocks. The paleoclimate index (C-value), varies between 0.07 and 1.22 reflecting a generally semi-arid to humid conditions. In addition, Rb/Sr (~0.22) and Sr/Cu (~9.09) ratios support the idea that warm and humid conditions prevailed during deposition of the Hançili Formation. Sr/Ba ratios (0.54–3.7) of the studied samples suggest a paleoenvironment with variable salinity. The co-variation among this factor and paleoclimate indicators suggest that variations in climatic conditions exerted a primary control on salinity. The substantially low C-value and Rb/Sr ratio and significantly high ratios of Sr/Cu and Sr/Ba and also elevated carbonate content of four samples indicate that these samples were deposited during hot and arid periods and that the paleoclimate controlled the terrigenous sediment flux to the lake basin. The chemical index of alteration (CIA), A-CN-K ternary diagram, and high Th/U ratios indicate that the parent rocks of the oil shale experienced moderate chemical weathering (steady state).