Detailed sedimentological investigation of the late cretaceous fort munro formation, western sulaiman range, Pakistan

Abstract

The late Cretaceous Fort Munro Formation in the western Sulaiman Range represents medium to thick-bedded, dark grey limestone displaying rich skeletal components, particularly larger benthic foraminifera (LBF), and bivalves. Three sections from the western Sulaiman Range, namely Spera Ragha, Murree Brewery, and Hanna Lake, were studied for detailed microfacies analysis and diagenetic investigation. A total of nine microfacies have been recognized, including 1 mudstone, 4 wackestone, 2 packstone, and 2 grainstone. Considering the proportionate prevalence of biota, their groupings, and the existence of a micritic matrix in different microfacies, it is deduced that the Fort Munro Formation was deposited inner to middle ramp setting. The inner ramp setting is more widespread than the corresponding middle ramp and is characterized by sub depositional environments including semi-restricted, storm-induced skeletal shoals, lagoons, low energy and protected inner ramp, storm induced skeletal shoals, and open marine. The Fort Munro Formation has undergone significant diagenetic alteration due to various diagenetic events. These events have altered the primary and secondary porosities. The observed diagenetic processes include micritization, cementation, dissolution, neomorphism, mechanical compaction, stylolitization, fractures and veins formation. Paragenetic sequence reveals that the studied carbonates are modified in marine, meteoric, and burial diagenetic environments. Detailed diagenetic analysis of the Fort Munro Formation reveals that most of the primary as well as secondary porosities are deteriorated by the precipitation of different type of cements in a variety of diagenetic environments.