Biostratigraphic Setup And Depositional Model Of Margalla Hill Limestone, Hazara Basin Northwest Himalayas, Pakistan

Abstract

This study investigates the biofacies identification, stratigraphic distribution, and depositional model of the Eocene Margalla Hill Limestone in the Ayubia and Nathiagali sections, located in the southeastern Hazara sub-basin of the NW Himalayas, Pakistan. Fieldwork involved collecting rock samples at regular intervals, where diagenetic features were observed, and log charts were created using CorelDraw. A total of 76 samples, 40 from Ayubia and 36 from Nathiagali, were collected, and after preparing thin sections, the samples were analyzed under a petrographic microscope for microfacies, fossils, porosity, and permeability. The limestone, which is approximately 60 meters thick at Ayubia and 54 meters at Nathiagali, consists of grey to pale grey limestone interbedded with shale and argillaceous limestone. At the Ayubia section, eight microfacies were identified, including Mudstone, Bioclastic Mudstone, Bioclastic Wackestone, Algal Wackestone, Assilinid Nummulitic Wackestone, Nummulitidae Alveolina Wackestone, Assilinid Wackestone, and Alveolina Wackestone. In contrast, the Nathiagali section revealed nine microfacies, such as Mudstone, Bioclastic Wackestone, Assilinid Wackestone, Nummulitidae-Lockhartian Wackestone, Coralline-Algal Wackestone, Assilinid-Discocyclinal Wackestone, Discocyclinal-Assilinid Wackestone, Discocyclinal Mudstone, and Assilinid Packstone. The study also highlights larger foraminifera fauna and explores the depositional environment. Diagenetic processes such as compaction, cementation, micritization, bioturbation, dissolution, and dolomitization affected the reservoir's porosity, with dissolution, dolomitization, and fracturing enhancing porosity, while compaction and cementation reduced it. The formation's development is linked to a single 3rd-order cycle with several 2nd-order system tracts (TST and HST), driven by sea-level fluctuations, with local tectonic effects playing a significant role, leading to deviations from global sea-level models.