Insights Into Diagenetic Evolution and Paleoenvironment of Eocene Jatta Gypsum in the Kohat Basin, Pakistan

Abstract

This study examines the depositional and diagenetic evolution of the Eocene Jatta Gypsum in the Kohat Basin, Pakistan, using sedimentological, petrographic, mineralogical, geochemical, isotopic, and Raman spectroscopic data. Lithofacies analysis identified selenitic, radial, discoidal, massive, laminated, and nodular gypsum, along with gypsum arenite and anhydrite nodules, reflecting deposition in restricted sabkha-lagoonal settings. Petrographic and SEM observations revealed pseudomorphic anhydrite laths, secondary calcite and celestite replacements, Fe-oxidation zones, alabastrine and granoblastic textures, and satin spar veins, documenting a complex multi-stage diagenetic history. Geochemical data indicate that high Sr and Sr/Mn ratios in selenitic and massive facies reflect pure evaporitic precipitation, whereas nodular and satin spar facies, enriched in Si-Al, Mn-Ag-Cu, and Rb, record detrital influx, microbial sulfate reduction, and hydrothermal fluid circulation. Isotopic values (delta S-3(4): 16.3-17.6 parts per thousand CDT; delta O-1(8): 19.2-22.2 parts per thousand SMOW) confirm microbial activity, strong evaporation, and episodic freshwater influx. Raman spectra provide further confirmation: sharp SO42- stretching bands (1008-1012 cm(-)(1)) indicate primary gypsum, while broadened signals and weakened hydration bands reflect dehydration to anhydrite and fluid-driven transformations. Overall, the Jatta Gypsum represents a multi-regime evaporitic system, where pure evaporitic facies formed under strongly evaporitic arid conditions, while detrital-rich facies reflect semi-arid restricted settings modified by diagenesis, microbial processes, and hydrothermal influence. These results offer new insights into the paleoenvironmental and tectonic evolution of the Kohat Basin during the late stages of the India-Eurasia collision.