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

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.