Cenozoic Himalayan foreland basin: An overview and regional perspective of the evolving sedimentary succession

Abstract

The collision between the Indian and Asian plates during Cenozoic resulted in the creation of a peripheral foreland basin occurring all along the southern flank of the Himalaya. In this chapter, the broad tectonic framework, chronostratigraphy, climate, dispersal pattern, depositional systems and large-scale paleogeography of the foreland basin are reviewed with emphasis on the eastern Himalaya. The earliest foreland deposits, in the western and central Himalaya, are of the Eocene Age and comprise marine sediments. The contemporaneous sediments in the eastern Himalaya are sparingly exposed. The overlying Neogene deposits, all over the foreland basin, are referred to as the Siwalik Group and are divided into three broad subdivisions: the mudstone-dominated Lower, the sandstone-dominated Middle and the conglomeratic Upper Siwalik Subgroups. The coarsening upward of the entire Siwalik succession is inferred to reflect the southward progradation of the Himalayan thrust front. Significant differences are observed in the foreland deposits from west to east in the lithology, fossil content, palynoflora and sedimentary facies. In the west-central sector, many of the studies infer deposition of the Siwalik Group in the Kosi-like megafans, fed by the transverse Himalayan drainages, in a warm subtropical seasonal paleoclimate. Recent works on the eastern part of the foreland reveal a deltaic to a shallow marine deposition, particularly for the lower part of the Siwalik Group, as inferred from the occurrence of grey mudstone, trace fossils of marine affinity, brackish water-tolerant spore pollens, a few marine invertebrate fossils and abundance of wave-generated structures that are in sharp contrast to the abundant land vertebrate fossils, red mudstones and fluvial deposits of the western Himalaya. In Nepal, drastic reduction in the number of vertebrate fossil taxa and occurrence of mostly aquatic species that flourished in swampy environments imply a gradual transition in the niche towards more aquatic and marine environment. From Potwar to Nepal, the Lower and Middle Siwalik rocks reveal a major component of the paleoflow to the southeast suggesting the probable existence of major axial drainage (paleo-Yamuna?). The S to SW paleoflow recorded from the Middle Siwalik of the Arunachal Himalaya hints at the existence of an axial drainage (paleo-Brahmaputra) flowing to the west. The paleomagnetic age data from the west-central Himalaya indicate deposition of the Siwaliks during 19 to 2 Ma and show upward increase in the sedimentation rate consistent with the hinterland-ward thickening of the foreland succession. Paleomagnetic profiles in the eastern Himalaya, on the other hand, bracket Siwalik deposition between 13 and 2 Ma and denote a slight upward decrease in the sedimentation rate probably related to the uplift of the Shillong Plateau. It is imperative from the available data that the sea receded from the western part of the foreland basin by Early Miocene, while a shallow marine/deltaic environment persisted till Pliocene in the eastern part of the foreland.