Diagenetic evolution of Early Eocene Chorgali Formation in the Upper Indus Basin of Pakistan: outcrop, petrographic and geochemical signatures

The study investigates the Early Eocene Chorgali Formation in the Khair-e-Murat Range of the Upper Indus Basin of Pakistan. The Chorgali Formation has previously been vastly studied for depositional settings and biostratigraphy but have not been evaluated for detailed diagenetic evaluation. Therefore, the present study is carried out to understand the detailed paragenetic sequence of the Chorgali Formation at its type locality in the Khair-e-Murat Range. Outcrop investigations were performed, and samples were collected for laboratory analyses. The collected samples were further studied for detailed petrography and stable isotope geochemistry (delta 13C and delta 18O). Field, petrographic and geochemical observations were integrated and a paragenetic sequence of diagenesis was developed. The paragenetic sequence of the Chorgali Formation shows the multiple stages i.e., cementation, micritization, compaction, dissolution, dolomitization, pyritization and dedolomitization. The dolomitization in the study section is from mimetic (preserved original depositional fabrics) to non-mimetic fabric (replaced the original depositional fabrics). The stable delta 13C and delta 18O isotope values of the dolomite matrix vary from delta 13C (-38.87 to 0.04 V-PDB parts per thousand) and delta 18O (-10.12 to -3.95 V-PDB parts per thousand) respectively. The isotope values of different calcite types of cement range delta 13C (-35.88 to -4.76 V-PDB parts per thousand) and delta 18O (-13.66 to -3.42 V-PDB parts per thousand), respectively. The isotopic values of studied samples of delta 13C and delta 18O range from (delta 13C -9.98 to delta 18O-7.84 V-PDB parts per thousand). The values are highly depleted, suggesting the meteoric origin of dolomitization. The diagenetic events range from shallow burial eogenetic to deep burial telogenetic diagenesis. The paragenetic sequence of these diagenetic events with help of filed, petrographic and isotopic signature offers insights into the pre- and post-depositional history of the studied sequence and the regional basin evaluation. Additionally, it enhances the understanding of how various diagenetic processes have altered porosity and permeability.