Evolution of Pre-Siwalik Tertiary Basin of Himachal Himalaya

Abstract

The Subathu, Dagshai and Kasauli rocks of Himachal sub Himalaya have been classically interpreted as of marine brackish or lagoonal origin. In this study, the pre-Siwalik Tertiary rocks of southern Himachal Pradesh are grouped into five distinct lithofacies, which collectively represent a consanguineous association of sediments deposited in related geomorphic environments: 1. Fossiliferous green shale, locally red, with lenses of limestone interpreted as tidally influenced shelf deposits; 2. Clean white sandstone interpreted as coastal barrier bars; 3. Red mudstone locally green, and lenses of grey sandstone with channel lag interpreted as lagoonal and tidal flat deposits; 4. Grey sandstone with channel lag and red mudstone and siltstone inferred to have accumulated in distributary channel and inter-distributary plains of fluvio-delta complex; 5. Grey, white sandstone and mudstone interpreted as a mixed assemblage of fluvio-delta and coastal barrier facies.

Two systems of currents operated during the deposition of these rocks: one, across the delta plain complex was directed towards south and southwest following the prevailing regional paleoslope; and the other representing longshore currents in adjacent coastal areas operating parallel to shoreline, was directed towards west-northwest.