Microfacies analysis and depositional environments of the Upper Cretaceous Fort Munro Formation in the Rakhi Nala Section, Sulaiman range, Pakistan

Abstract

The Fort Munro Formation is 200-m-thick mixed carbonate–siliciclastic succession of Late Cretaceous (Campanian) age exposed at the Rakhi Nala Section, Sulaiman Range, Lower Indus Basin, Pakistan. The Fort Munro Formation is representing thin- to thick-bedded limestone with interbedded marls, shale and sandstone. Detailed microfacies analysis revealed the recognition of 15 microfacies types (11 carbonate and 4 sandstone). These facies were deposited on both inner and middle ramp platform. The inner ramp sediments are more widespread than the corresponding middle ramp including open-marine, skeletal shoals, semi-restricted, carbonate sand shoals and banks and lagoon depositional environments. The uppermost part of the Fort Munro Formation has received sufficient amount of siliciclasts, thereby producing a sandstone texture. These siliciclasts indicate the tectonic uplift, which thereby increases the source area rejuvenation. Therefore, a mixed carbonate–siliciclastic, moderately storm-dominated homoclinal ramp depositional environment is suggested for the Fort Munro Formation.