Microfacies and Diagenetic Fabric of the Lockhart Limestone, Kotal Pass Section, Northeast of Kohat, Pakistan

Abstract

The present study is intended to describe the carbonate microfacies of the Paleocene Lockhart Limestone inarea northeast of the Kohat City. The analyzed limestone in the area is 65m thick and predominantly medium to thickbedded, nodular to brecciated and richly fossiliferous. Thin interbeds of marl are observed in the main lithological unit.The lower contact of the Lockhart Limestone is faulted with the Samana Suk Formation of the Jurassic age whileconformably overlain by the Patala Formation of Paleocene to Eocene age. The major lithological unit is comprised ofpredominant assemblage of larger benthic and smaller planktonic foraminifers with accretion of ostracods,dasycladacean algae, echinoderms, gastropods and corals. On the basis of field and laboratory observations, twodistinct types of microfacies have been identified as i) Dasyclad-Miliolid Foram Wacke-Packstone microfacies of theinner shelf, sub tidal lagoon, ii) Larger Foram Packstone microfacies of the middle shelf. The Limestone represents acarbonate cyclic sequence marked by three, transgressive, deepening up cycles representing a gradual sea level risecompensated by vertical accumulation of microfacies. The commencement of each cycle is clearly marked by the inputof land-derived siliciclastic sediments and near shore restricted marine faunal/floral assemblage in the inner shelfmicrofacies gradually thinning up section where the microfacies become deeper offshore. The diagenetic modificationsare observed in the shape of compactional framework, dolomitization, aragonite to calcite alteration and spar filledfractures in the main lithological unit of the Lockhart Limestone.