Journal - Small-amplitude lake-level fluctuations recorded in aggrading deltaic deposits of the Upper Pleistocene Thimi and Gokarna formations, Kathmandu Valley, Nepal

Small-amplitude lake-level fluctuations recorded in aggrading deltaic deposits of the Upper Pleistocene Thimi and Gokarna formations, Kathmandu Valley, Nepal

Tetsuya Sakai

Department of Geology and Mineralogy, Division of Earth and Planetary Sciences,
Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan

Ananta Prasad Gajurel and Bishal Nath Upreti

Department of Geology, Tri-Chandra Campus, Tribhuvan University,
Ghantaghar, Kathmandu, Nepal

Hideo Tabata

Gifu Academy of Forest Science and Culture, Mino, Gifu 501-3714, Japan

Jour. Nepal Geol. Soc., 2001, Vol. 25 (Sp. Issue), 43–51


Abstract

Small-amplitude lake-level fluctuations have been recognized from the aggrading delta-plain deposits in the lower parts of the Thimi and Gokarna formations, Kathmandu Valley, Nepal. The delta-plain deposits consist of gravelly sand beds of fluvial channel origin (coarse-sediment interval) and alternation of fine to very fine sand and sandy silt beds (fine-sediment interval). Wave-generated structures occur in the sand beds of the fine-sediment intervals.

 

The vertical and lateral facies changes suggest that the deposition of a set of coarse- and fine-sediment intervals associated with prograding delta front deposits was controlled by a lake-level rise and fall sequence superimposed on a long-term lake-level rise trend. The aggradation of fluvial sediments occurred during a lake-level rise period with sufficient sediment supply to fill a newly created accommodation space on the delta plain. The observation of wave-generated structures in an overlying fine-sediment interval suggests that the delta plain was subsequently inundated due to further lake-level rise, exceeding the sedimentation rate. Subsequent delta progradation occurred during a lake-level stabilized phase after a lake-level fall. The small-amplitude lake-level changes are thought to be attributable to seasonal wet and dry cycles, as inferred based on the presence of peculiar aggrading delta successions, implying that lake-level fluctuations may have occurred over short time scales, and on the results of a previous palaeopalynological study in which a moist palaeoclimate was inferred in the lower part of the Gokarna Formation in particular.

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