The physical and geochemical character of the post‐depositional alterations in the Early‐Middle Jurassic carbonates on the western margin of the Indian Plate: Implications for cement morphologies and dolomitization

Abstract

The physical and geochemical character of the post‐depositional changes present in the Early‐Middle Jurassic carbonate‐dominated units (Samana Suk, Shinawari, Chiltan and Loralai formations) of the Indus Basin, Pakistan have been addressed. The study is based on 11 sections along a N‐S transect on the western margin of the Indian Plate to provide an insight into the nature of cementation and dolomitization behavior by using an integrated petrographic and geochemical analysis. The carbonates located in the northern proximity are dominated by the eogenetic signatures of hardgrounds, biogenic micritization, and iron‐containing burrows. The pattern and yellowish‐brown coloration of the burrows is distinct, and shows a positive relationship with the Fe, Si, Al, K, Mn concentrations, and meteoric phase. The transition from the Early to Middle Jurassic is marked by a correspondent increase in the dolomitization and bioturbation but its intensity decreases from north to the south. The packing index and marine phreatic environments generally increase towards the south. The planar‐s to planar‐e protodolomite and dolomite proper have particularly targeted the micritic facies and allochems, courtesy of retaining high‐magnesian calcite waters, and the geochemical analysis also reveals some dedolomitization, associated with sequence boundaries. The pore occluding cements of the Samana Suk and Chiltan formations are Fe deficient and have low Sr and Mn levels, as compared to the Shinawari Formation. The latter has an elevated concentration of Fe, Mg, Si, Al, Sr, Mn, and the Fe + Mn versus Sr relationship and Mn* values suggest that cementation dominantly occurred in the oxic‐dysoxic zones of sub‐tropical climate.