Stratigraphic and structural evolution of the Kumaon Lesser Himalaya

Abstract

Six years of geological investigations comprising studies of sedimentology, geomorphology, photo-interpretation and field checks carried out by a team of 6–8 geologists have led to a reappraisal of Lesser Himalayan geology. The Algonkian and Early Palaeozoic basin derived its sediments from the south. The Caledonian orogeny had a limited effect, the Himalayan basin was sliced into two parts by a central NW-SE trending arch which became a source of sediments during the later Palaeozoic and Mesozoic evolution. The Damta group of turbidites are of Algonkian-Cambrian age; they serve as a “supra-basement” over the whole region and may have partly been taken up in the later metamorphism of the central zone of the Himalaya. Facies changes between the Deoban-Shali carbonate sequence and the Chandpur-Basantpur black shales and shales are worked out. During the Mesozoic and Tertiary, facies changes are controlled by old arches and fundamental lines of weakness which can be recognized even in the present seismic and geomorphological evolution. The Blaini boulder beds, extensively quoted in literature as tillites and used as “fool-proof” marker beds are shown to be a slumped series at the base of the Krol foredeep sequence. The Krol belt rocks are autochthonous and find an extension across the Tehri-Meenus high in the Kophara black shale series that wedge out towards the north. The occurrence of the Eocene nummulitics is two-fold: 1. (a) in concordance with the Jurassic rocks, this position occurs in quiet synclinal areas; 2. (b) in discordance over all the earlier deposited series, this position occurs over the arches witnessing late Mesozoic erosion. Reversal of metamorphism and production of gravity slides is a logical result in the evolution of the basin.