Lithostratigraphy and facies analysis of the Siwalik Group, Karnali River section, far-western Nepal Himalaya

Sixth Nepal Geological Congress Lithostratigraphy and facies analysis of the Siwalik Group, Karnali River section, far-western Nepal Himalaya *Ashok Sigdel1, Tetsuya Sakai1 , Prakash D. Ulak2, Ananta P. Gajurel2, and B. N. Upreti2 2 ¹ Department of Geoscience, Shimane University, Matsue 690-8504, Japan Department of Geology, Tri-Chandra Campus, Tribhuvan University, Kathmandu, Nepal (*Email: sigdel.ashok@gmail.com) The Siwalik Group lying in the southern flank of the Himalaya has been considered to be the ancient Gangetic plain deposits. This study analyses the fluvial succession of a large river system at the time of deposition of the Siwalik Group, which is expected to be one of good recorders of the regional changes in climate and tectonics. The 6-km thick Karnali River section, where the Paleo-Karnali River is expected to have been flowing, is the target of the present study regarding stratigraphy and facies of the succession to disclose new findings. The stratigraphy of the Karnali River section is proposed as follows: Chisapani Formation (equivalent to the Lower Siwaliks, 2045 m), Baka Formation (equivalent to the Middle Siwaliks, 2740 m), Kuine and Panikhola Gaun Formations (equivalent to Upper Siwaliks, 1500 m) in an ascending order. The Chisapani Formation is composed of interbedded red mudstones and fine- to medium-grained sandstones. The Baka Formation is composed of medium- to coarse grained- pebbly sandstones interbedded with greenish grey mudstones. The Kuine Formation consists of clast supported, imbricated, pebbles to cobbles conglomerates and Panikhola Gaun Formation consists of thick matrix supported, pebble, cobble to boulder conglomerates. Facies analysis resulted that the Karnali River section recorded the change of fluvial channel style from fine-grained meandering rivers to sandy braided rivers following gravelly braided rivers. Debris flow deposits predominate at the top of the succession. The order of the appearance in fluvial facies is similar to that of other Siwalik successions. The timing of the changes in fluvial style at the Karnali River and at the Tinau Khola sections, where the Paleo-Kaligandaki River is expected to have been present, nearer with each other, implying that the regional changes in climate or tectonics might have affected simultaneously on the large fluvial systems along the southern flank of the Nepal Himalaya. 30