Evolution of fluvial style in the Siwalik Group in the foothills of the Nepal Himalaya

Abstract

Abstract Middle Miocene to Pleistocene fluvial sediments of the Siwalik (Churia) Group are widely distributed in the southern frontal area of the Himalaya. The succession is about 6 km thick, and was derived from denudation of the Himalayan orogen. The Siwalik Group in Nepal is well exposed in the Surai Khola area (western Nepal), and the Hetauda–Bakiya Khola area (central Nepal). The group is separated into northern and southern belts by the Central Churia Thrust (CCT). Eight facies associations (FA1 to FA8) are recognized in the Siwalik Group in these areas. They are interpreted as the deposits of fine-grained meandering, flood-flow-dominated fine-grained meandering, sandy meandering, deep sandy braided, comparatively shallow sandy braided, anastomosed, gravelly braided, and debris-flow-dominated braided systems, respectively. FA6, FA7 and FA8 occur only in the southern belt. In each area, the sedimentary succession generally coarsens upwards. The accumulation of these facies associations, related to the paleomagnetic time frame, indicates that flooding increased dramatically from about 10.5 to 9.5 Ma, and fluvial style changed from meandering to braided between 9.0 and 6.5 Ma. A gravelly fluvial system prevailed after 3.0 to 2.5 Ma. The evolution of these fluvial styles is intimately related to the uplift of the Himalaya and associated thrust movements, and consequent effects on atmospheric circulation and precipitation. By comparison with the characteristics of the submarine Bengal Fan deposits, it is apparent that an increase in flood flow is strongly influenced by increased precipitation due to onset and intensification of monsoon climate. The sandy braided system was mainly induced by regional tectonic uplift, and the gravelly fluvial system may have been formed due to regional thrust movements along the Himalaya frontal area.