Deltaic coastline of the Siwalik (Neogene) foreland basin: evidences from the Gish River section, Darjeeling Himalaya

Abstract

This paper presents the sedimentological analysis of about 1.3 km succession of Neogene Siwalik Group exposed in the Gish River section, Darjeeling foothills. Contrary to the existing alluvial fan‐braided stream depositional model, the facies analysis shows that the succession accumulated in a shallow marine deltaic setting. Nine facies identified represent deposition from various processes related to unidirectional and oscillatory current, suspension settlement in quiet water and also to processes related to soil formation. Abundant trace fossils of marine affinity occur throughout the succession and include Cylindrichnus, Rosselia, Teichichnus, Rhizocorallium, Chondrites and Zoophycos. The studied succession has been subdivided into seven facies associations that can be interpreted in terms of the different sub‐environments of a river‐dominated delta. The lower ~550 m of the succession comprises sand–mud alternation of delta front, delta mouth and delta plain deposits (FA3, 4, 5 and 6), organized in stacked, 10–30 m coarsening‐upward units, and is inferred to represent progradation of the delta lobes. The overlying 300 m mud‐dominated interval is dominated by prodelta to open marine bay‐fill succession (FA1 and 2); the pebbly sandstones and conglomerates of the uppermost ~500 m represent a braidplain delta environment. A kilometre‐scale coarsening‐upward trend of the Siwalik deposits of this section is attributable to southward propagation of the thrust front related to the activity of the Main Boundary Thrust (MBT). The sedimentological analysis shows that a marine embayment existed in the Eastern Himalaya during the Siwalik time and the deltaic succession reported here provides the link between the upland transverse drainages recorded from Western Himalaya and the deep marine Bengal Fan succession. Copyright © 2017 John Wiley & Sons, Ltd.