Discovery of Tsunami deposit in the paleo-Kathmandu Lake,central Nepal Himalaya

Journal of Nepal Geological Society, 2010, Vol. 41 (Sp. Issue) Discovery of Tsunami deposit in the paleo-Kathmandu Lake, central Nepal Himalaya *Ananta P. Gajurel1, Tetsuya Sakai2, Ashok Sigdel2, Hideo Tabata3, Nobuo Ooi4, and Bishal N. Upreti1 1 Department of Geology, Tri-Chandra Campus, Tribhuvan University, Ghantaghar, Kathmandu, Nepal 2 Department of Geoscience, Shimane University, Matsue 690-8504, Japan 3 Gifu Academy of Forest Science and Culture, Mino, Gifu 501-3714, Japan 4 ONP Laboratory, Neyagawa, Osaka 572-0021, Japan (*Email: apgajurel@wlink.com.np) Kathmandu Valley is an intermontane basin located on the basement rocks of Kathmandu Nappe in the central Nepal Himalaya. The study is focused on the fluvio-deltaic sediments deposited around the Gothatar area, which is situated to north-east of the Tribhuvan International Airport in the Kathmandu Valley in order to understand the sediment stacking pattern. Stratigraphically, the deposits in the studied area belong to the Gokarna Formation (50-34 ka). The sedimentary deposit of the area can be separated into lower, middle and upper part consisting of very thick, crossstratified, coarse to medium grain sands of dominantly delta front origin; thick, ripple cross-laminated, very fine sand and silt with thick, parallel laminated, dark grey mud of dominantly prodelta and partly interdistributary origin; and alternating layers of thick sands, silt and mud of fluvial and small deltaic origin, respectively. For the first time peculiar sedimentary deposits have been discovered in the middle and upper part of the deposit at three distinct stratigraphic levels corresponding to the Gokarna Formation. Such deposits were observed in five different sites in an area of 500 x 500 m. The peculiar sediments of 0.5 to 1.5 m thick deposit are characterized by: • reverse direction of sediment transportation than the normal regional flow, • syn-sedimentary soft-deformed structures like ball-andpillow, disc and flame structures with small shear zones, • gravels to very fine sand size sediment, • deposits located at delta front zone, and • interfinguring of normal and reverse flow sediments The deposits are interpreted as “Tsunami deposits” in the paleo-Kathmandu Lake that appeared at around 37-38 ka. 151