The Siwalik Group, known as the molassic sediments exposed in association with Himalayan uplift, was deposited during the middle Miocene to the early Pleistocene as a 4 – 6 km thick succession of fluvial sediments at the southern front of the entire Himalayan belt. The Siwalik Group has been traditionally subdivided into three units which are the Lower, Middle and Upper Siwaliks (Auden, 1935; Hagen, 1969; Yoshida and Arita, 1982; Quade et al., 1995; DeCelles et al., 1998; Gautam and Fujiwara, 2000; Ojha et al., 2000; Robinson et al., 2006; Sharma et al., 2007) and locally modified subdivisions into up to five subunits have been applied (Glennie and Ziegler, 1964; Sharma, 1973; Tokuoka et al., 1986, 1988; Sah et al., 1994; Corvinus and Nanda, 1994; Dhital et al., 1995; Ulak and Nakayama, 1998; Sigdel et al., 2011). The common three-fold classification starts in general with the mudstone-dominated Lower Siwalik, grading upward into the sandstone-dominated Middle Siwalik and the Upper Siwalik with conglomerate-dominated sediments.
This study focuses on the fluvial facies, geochemistry and petrography of sedimentary rocks of the Siwalik Group in the Khutia Khola section, Far Western Nepal. Previous studies indicated the presence of a much more extensive succession of finegrained mudsized sediments in the Khutia Khola section (Quade et al., 1995; Ojha et al., 2000; Sharma et al., 2007) than in other Siwalik successions (cf. Tokuoka et al., 1986, 1988; Sah et al., 1994; Corvinus and Nanda, 1994; Dhital et al., 1995; Ulak and Nakayama, 1998; Sigdel et al., 2011). This muddy succession provides a much better archive than the sand-dominated successions because the loss of sediment by stream erosion will be smaller than in the sand-dominated units, and the sediments may provide a good example of interfluve environments of the Siwalik phase. The results from the Khutia Khola are compared to those from equivalent sediments in the adjacent Karnali River section, which are known to have been deposited by the large paleo-Karnali River system. The Siwalik deposits in these two sections are important records of Himalayan Uplift and the related climatic and environmental changes in the western part of the Nepal Himalaya. A detailed comperative study of these sediment records will improve our understanding of these key processes during Neogene period and their effects on a regional and global scale.
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