Sedimentary facies analysis of the Miocene Siwalik Group, Karnali River section,

Sedimentological Society of Japan Sedimentary facies analysis of the Miocene Siwalik Group, Karnali River section, Nepal Himalaya: Implications for paleoenvironment and paleoclimate reconstruction *Sigdel, A., Sakai, T. (Shimane Univ.), Ulak, P.D., Gajurel A.P., and Upreti, B.N. (Tribhuvan Univ.) *(E-mail: sigdel.ashok@gmail.com) Fluvial deposits of the Miocene Siwalik Group (4000–6500 m thick) accumulated in the Himalaya foreland basin. The Siwalik Group thus constitutes an important archive of Himalayan uplift and related climate changes. This study analyzes the fluvial succession of a large river system (Paleo-Karnali River) in which local climatic effects should be minimal. The Karnali section is expected to contain a good record of regional changes in climate and tectonics. Based on facies analysis, eleven depositional facies and six facies associations (FA1-FA6) were identified, in ascending order: red mudstones and fine to medium-grained sandstones (FA1); coarse-grained sandstones with flood-flow deposits, ephemeral stream deposits and grey mudstones (FA2); medium to very coarse-grained and pebbly sandstones (FA3-FA4); followed by thick pebble, cobble to boulder conglomerates (FA5-FA6). The individual facies associations represent fine-grained meandering river systems (FA1), flood flow-dominated meandering river system (FA2), deep (FA3) and shallow (FA4) sandy braided river systems, followed by gravelly braided river systems (FA5) and a debris flow-dominated braided river system (FA6), respectively. The individual fluvial channel deposits tend to coarsen and thicken upward. These features indicate the depositional environment shifted from a distal to more proximal part of the Paleo-Gangatic plain, as reported from other Siwalik successions. The Karnali succession also records climate changes. The change from fine-grained meandering river deposits with red soils (15.8-13.5 Ma) to the flood flow-dominated meandering river deposits with greenish grey mudstones (13.5-9.6 Ma) indicates increased water discharge after 13.5 Ma, which resulted from increased precipitation. An increase in precipitation is also reflected by the appearance of ephemeral streams and playa lake facies in FA2. The order of the appearance of fluvial depositional styles in the Karnali River section is similar to those of the Tinau Khola and Surai Khola sections, but the timing of fluvial facies changes differ. The earlier appearance (3-4 Myrs) of a flood flow-dominated meandering river system in the Karnali River section may have been due to the larger catchment size of the Paleo-Karnali River than those in the other areas. The newly discovered climate shift at about 13.5 Ma in the Siwalik Group is interpreted to be related to intensification of the ‘Indian Summer Monsoon’. Further analysis is required to determine whether this event was related to global climate change, or to uplift of the Himalaya.