INVESTIGATION OF THE NEOGENE DEPOSITIONAL ENVIRONMENT OF THE EASTERN HIMALAYAN SIWALIK DEPOSITS THROUGH MULTIPROXY ORGANIC ANALYSIS

The depositional environment of the Neogene Siwalik deposits in the Himalayan foreland basin has been inferred as meandering and braided, exclusively fluvial deposits based on sedimentary facies and fossil analysis. These records mostly come from the Western Himalayas. The Eastern Himalayan Siwalik deposits are not as well studied as Western Himalaya and few studies looking at these deposits reported major differences in facies and fossil assemblages. A recent detailed sedimentological analysis of the Siwalik sediment reported major differences in facies with that of the west. Siwaliks of the eastern Himalaya is characterized by the presence of wave and combined flow structures, dark grey mudstones, brackish water tolerant spore-pollens and marine trace fossils. Based on these evidence, recent studies proposed a marginal marine depositional setting characterized by a river-dominated delta. Our study focuses on reconstruction of the vegetation assemblage, to better understand the Neogene depositional environment, as a marginal marine environment is expected to have a different assemblage compared to a terrestrial environment. We analyzed sediment samples collected from the Tista Valley in the Eastern Siwaliks of India. We present new data of lignin, branched/isoprenoid tetraether (BIT) index of glycerol dialkyl glycerol tetraethers (GDGTs), C/N ratio, bulk δ¹³C and n-alkane chain length. Lignin is found in terrestrial vascular plants and analysis of its structure can help to infer abundance, degradation state and type of vegetation present. BIT index values vary from 0 in pure marine to 1 in continent interior soil GDGTs. Thus it can help to indicate if organic material is derived from marine or terrestrial source. Both bulk δ¹³C and Carbon/Nitrogen (C/N) ratio change from terrestrial to marine sources while n-alkane chain lengths vary among different types of vegetation. Comparison of these different proxies can help us to construct a comprehensive record of the Neogene vegetation in the Eastern Siwaliks and ascertain the depositional environment. Terrestrial versus marine affinity of the vegetation has important bearing on the tectonic and paleogeomorphological evolution of the eastern Himalayan Foreland basin.