DefinePK

Abstract

In this paper we have used the outcrop data and utilized facies analysis techniques for the dynamic depositional modeling of Chhidru Formation, exposed in the Nammal Gorge and the Chhidru Nala sections in the Salt Range, Upper Indus Basin of Pakistan. Based on the integrated field and petrographic criteria, five different facies types i.e., CHF-1-CHF-5 are recognized. The CHF-1 corresponds to the sandy limestone interbedded with clays facies while CHF-2 is a medium bedded sandy limestone facies. The CHF-3 is represented by the massive sandy limestone facies. The CHF-4 (calcareous sandstone facies) and CHF-5 (thick bedded to massive white sandstone facies) have been identified in the upper part of the two stratigraphic sections. The depositional characteristics of these facies are consistent with the proximal inner shelf to distal middle shelf settings of a siliciclastic-carbonate mixed platform. Based on the integration of the outcrop data, facies information and fusulinid biostratigraphy the dynamic depositional model of the Chhidru Formation is presented. The new data enabled identication of two (02) 3rd order Transgressive-Regressive depositional sequences. The first Transgressive Sequence consists of the Transgressive Systems Tract (TST 1) that further constitutes six parasequences. The second transgressive depositional sequence constitutes the second Transgressive Systems Tract (TST 2) with no significant parasequence. The chronologic calibration fusulnid biostratigraphic age of the Chidru Formation suggest that deposition of the at the base TST 1 started at 252.5 Ma and the topmost beds of TST 2 corresponds to 253.8 Ma, respectively. The global comparison of the relative sea level during 252.5-253.8 Ma shows prominent eustatic signatures, however the flooding surfaces bounding the parasequences are ascribed to the local tectonics.