Tectonic impact on the fluvial deposits of Plio-Pleistocene Himalayan foreland basin, India

Abstract

The Himalayan foreland basin comprises a number of subbasins developed in various sedimentary environments during the Plio-Pleistocene phases of Himalayan orogeny. This paper describes the evolutionary processes that led to the development of the Himalayan foreland basin in the Dehra Dun and Subathu subbasins. The Dehra Dun subbasin essentially consists of conglomerate facies whereas Subathu subbasin is primarily composed of sandstone–mudstone intercalated with conglomerate. The two subbasins were sites of braided river systems. Evidence from chronostratigraphy, provenance and tectonic and climatic conditions suggests that differential tectonic movement, sedimentation rate and basin subsidence, coupled with climatic variation, were responsible for the contrasting fluvial facies in these subbasins. Following the merger of the two subbasins at about 5.23 Ma, the conglomerate facies assemblage of the Dehra Dun subbasin passes laterally into a sandstone–mudstone–conglomerate facies assemblage of the Subathu subbasin. Thickly bedded, amalgamated sheet conglomerate of Dehra Dun subbasin has a high sedimentation rate (0.70 mm/year) between 5.23 and 4.8 Ma, with a slow rate of basin subsidence. In contrast, a slow sedimentation rate (0.39 mm/year) in coeval sediments of the Subathu subbasin, with relatively fast subsidence, produced a succession of thick multistorey sandstone body with mature paleosols. Petrographic and clast composition data from Dehra Dun subbasin suggest that the detritus was derived from the pre-Tertiary Lesser Himalayan sequence lying in the north, during an active tectonic phase of the Main Boundary Thrust. Between 4.8 and 1.77 Ma, progradation of posttectonic conglomerate of the Dehra Dun subbasin continued during a quiescent phase of the Main Boundary Thrust. However, as a consequence of partitioning of Subathu subbasin along the Intra-Foreland Thrust, interfingering of buff (piedmont drainage) and grey (trunk drainage) sandstones took place. In contrast, the syntectonic conglomerate of Subathu subbasin originated from the uplifted proximal part of the foreland basin (Tertiary belt) during the active phase of Intra-Foreland Thrust around 1.77 Ma.