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GEOCHEMICAL CHARACTERISTICS FOR DETERMINING THE DEPOSITIONAL CONDITIONS OF OIL SHALES IN THE ALASEHIR GRABEN IN WESTERN TURKEY
Abstract
The Alasehir Graben hosts a thick sedimentary sequence of terrestrial clastics deposited from the Early Miocene to the present. Oil shales occur with thin- to medium-bedded sandstones, siltstones, and shales in the upper part of Alasehir Formation. This study evaluates depositional settings and paleoenvironmental conditions via inorganic geochemistry of oil shales. X-Ray Diffractometer results show the presence of quartz, clay, mica, feldspar, calcite, chlorite, pyrite, and ilmenite. Al2O3 shows strong positive correlations with TiO2, Na2O, Cr, Sn, Ga, Rb, Cs, and Zn; moderate negative correlations with CaO, Sr, and MgO; weak positive correlations with REE+Y; and weak negative correlations with Fe2O3, MnO, Th, and U. Element ratios (e.g., Ni/Co, Cr/V, La/Sc, Eu/Eu*) indicate derivation from shale, schist, and gneiss. REE contents (125.9-267.8 ppm; avg. 186.8 ppm) are close to PAAS (184.77 ppm), with flat patterns and slight LREE depletion relative to HREE. Weathering indices indicate moderate weathering and initial stages of lateritization. The oil shales likely accumulated rapidly in deep lacustrine or nearshore settings under predominantly reducing conditions, periodically oxygenated, with limited sediment recycling. Depositional environments ranged from freshwater to hypersaline, occurring under predominantly hot, arid climates with occasional humid intervals. The findings clarify the occurrence of oil shale and the environmental evolution in the Alasehir Graben and similar basins.
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