Microfacies and Diagenetic Analysis of Lockhart Limestone, Shah Alla Ditta Area, Islamabad, Pakistan
Zeeshan Khattak, SA-8435
shaneekhattak@gmail.com
Bahira Univeristy Islamabad, Pakistan Abstract
The Larger Foraminifera made a major contribution to Paleocene and Eocene biota. The microfacies and diagenetic setting of the Paleocene Lockhart Formation were analyzed in Shah Alla Ditta area, Islamabad. The Lockhart Formation is highly fossiliferous, comprising of thin nodular limestone with interbedded shale/marl. The lower contact is unconformable with Jurassic Samansuk formation while the upper contact is conformable with Margalla Hill Limestone.
Rock samples were collected from the field and thin sections were prepared. Measured thin sections were analyzed under the microscope and on the basis of detailed petrographic study three types of microfacies were identified and interpreted based on Dunham’s textural classification, allochem type, fossil content and sedimentary structures. The microfacies include: LH-MF 1: Foraminiferal Wackestone/Biomicrite Microfacies LH-MF 2: Algal Foraminiferal Wackestone Microfacies, LH-MF 3: Mixed Bioclastic Mudstone Microfacies. A number of larger benthic foraminifera, and the lack of Planktonic foraminifera in the Lockhart Limestone indicate shallow to deep water of restricted inner to middle shelf environment of deposition. Larger foraminifera and dasycladacean algae are the two
major fossil contents present in the Lockhart Formation. Larger foraminiferal species like Lockhartia hameii and Lockhartia conditi present in the formation confirm the age of Lockhart Limestone as Paleocene.
The main diagenetic fabric recognized in the limestone reveals mechanical and chemical compaction, deep burial water pressure, pressure solution and tectonics-related frac- turing. The nodularity in the limestone is credited to pressure solution.
Introduction
The Shah Alla Ditta area is located in the foothills of the Margalla hill ranges which are part of the NW Himalayan foreland fold and thrust belts of Pakistan and are related to the main zone of Himalayan convergence (Jadoon, 1992). A major thrust fault, the Panjal-Khairabad fault, divides the NW Himalayan sequence into a deformed southern zone often referred to as the external or Foreland zone, and a deformed and metamorphosed zone also known as the Hinterland zone (DiPietro et al., 1996; Pivinik et al., 1996) (Figure 1). The Paleocene stratigraphic succession is well exposed throughout the area.
STUDENT ARTICLE
Figure 1. Accessibility map of the study area, field section is shown by black square (Google generated map)
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