Deltaic coastline of the Siwalik (Neogene) foreland basin: evidences from the Gish River section, Darjeeling Himalaya SUCHANA TARAL and TAPAN CHAKRABORTY * Geological Studies Unit, Indian Statistical Institute, Kolkata, India 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. Received 23 April 2016; accepted 27 September 2016 KEY WORDS Siwalik Group; eastern Himalayan foreland basin; facies analysis; deltaic sedimentation; palaeogeography; tectonics and sedimentation 1. INTRODUCTION The Neogene Siwalik Group was deposited in the peripheral foreland basin that formed on the southern flank of the rising Himalayan mountain belt (Molnar, 1984; Burbank et al., 1996). The structure, sedimentology, vertebrate palaeontology, palaeomagnetism and provenance of the Siwalik succession and tectonic evolution of the basin have been intensely studied in different parts of Western Himalaya and Nepal (Tandon, 1976; Johnson et al., 1985; Willis, 1993; DeCelles et al., 1998; Kumar et al., 1999; Brozovic and Burbank, 2000; Barry et al., 2002; Szulc et al., 2006; Goswami and Deopa, 2015). However, in comparison to such extensive studies in Western Himalaya and Nepal, the Siwalik rocks in the Eastern Himalaya have remained less studied. The sedimentological studies of the Siwalik succession in the Western Himalaya and Nepal have identified it as deposits from large meandering to braided streams and their associated floodplains (Willis, 1993; Willis and Behrensmeyer, 1994; Khan et al., 1997; Kumar et al., 1999, 2004; Nakayama and Ulak, 1999, Friend et al., 2001; Thomas et al., 2002). The channel-fill sandstones in these successions interlayer with red mudstones and pedogenized mudstone with calcareous nodules that formed in their floodplains. These channel systems were inferred to be part of large transverse megafans analogous to the Kosi megafan of the Gangetic alluvial plain (Khan et al., 1997; Zaleha, 1997; Thomas et al., 2002; Kumar et al., 2004; Sinha et al., 2007; Goswami and Deopa, 2015). However, a preliminary examination of the Siwalik rocks in the Eastern Himalayan sector indicates significant differences in the characters of these rocks from that observed in its type section in the Western Himalaya. The major differences include absence of freshwater vertebrate fossils and red mudstones with calcium carbonate nodule-bearing palaeosols. In the Siwalik succession of Eastern Himalaya, the presence of thick dark grey mudstone (Karunakaran and Ranga Rao, 1976), palynoassemblages and trace fossils *Correspondence to: T. Chakraborty, Geological Studies Unit, Indian Statistical Institute, 203 B. T. Road, Kolkata 700108, India. E-mail: tapan@isical.ac.in Copyright © 2017 John Wiley & Sons, Ltd. GEOLOGICAL JOURNAL Geol. J. (2017) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/gj.2886