GEOCHEMICAL CHARACTERISTICS AND DEPOSITIONAL ENVIRONMENT OF LOWER CRETACEOUS SEMBAR FORMATION, WESTERN SULAIMAN FOLDBELT, PAKISTAN
The geochemical and mineralogical studies of the Sembar Formation from four locations (two from the Loralai-Mekhtar areas and two from Quetta area), revealed that on average these shales are composed of 42% SiO2, 16% Al2O3, 0.67% TiO2, 10% CaO, 2% MgO, 3% K2O, 0.46% Na2O, 1.39% FeO, 2% Fe2O3 and 0.13% P2O5. The shale also contain 194±106 ppm Mn, 65±19 ppm Ni, 39±17 ppm Cu, 84±43 ppm Zn, 343±97 ppm Sr, 275±65 ppm Ba, 23±6 ppm Pb, 100±34 ppm Cr, 155±33 ppm Rb, 25±6 ppm Y, 130±45 ppm Zr, 15±3 ppm Nb, and 147±49 ppm V as average amounts of trace elements. The chemical analyses revealed that the average calcite content is 20%, the average quartz content is 16%, average pyrite content is 2.6%, average organic carbon is 3.3%, average organic sulphur 0.15%, and average inorganic sulphur is 1.4%.
The geochemical data indicate that the Sembar Shale is comparatively less siliceous in the Mekhtar and Loralai areas (Eastern part) as compared to Murree Brewery and Gawani Nala areas (Western part), indicating possibly different provenance. The quartz and clay minerals are positively correlated indicating conversion of feldspar and other less stable minerals into clays during moderate to intense paleoweathering conditions. Higher calcite/(Qtz+Clay) ratio in the Murree Brewery and Gawani Nala indicates significant biogenic activities in western part of the depositional basin. The positive and generally linear correlation between CaO and CO2, and between CO2 and CaO+MgO, also indicate biologically active depositional environment in the western part. Based on the values of the degree of pyritization it appears that most of the shales of the Sembar Formation were deposited in an aerobic water conditions in normal marine environment with temporal and special fluctuations in the amount of dissolved H2S in water.