Abstract
Tributary fan deposits in the Sikkim Himalaya are well preserved in the Teesta Valley in the tectonic zone bounded by the Main Central Thrust and the Main Boundary Thrust. The lithofacies characteristics and assemblage patterns of these deposits bear imprints of the linked effects of tectonic and climatic factors on the sedimentation processes in the river valley. Two tributary streams, namely Turung Khola and Bembung Khola have been studied in detail. Three major fanlobes are preserved at Turung Khola and two fanlobes are preserved at the Bembung Khola. Terraces, floodplains, channel bars, and chute bars are associated with geomorphic features in this part of the Teesta Basin. Longitudinal profiles of the tributaries and that of the trunk river indicate a relationship between the knick points and high SL‐Index values, where fault/thrust intersections are present. Sediment characteristics of these fan deposits indicate that four types of flows viz. debris flows (DF), hyperconcentrated flows (HF), sheet flows (SF), and channel flows (CF) deposited these sequences. The CF deposits are dominant in the fan sequence of the Turung Khola followed by SF deposits, HF deposits, and debris flow deposits respectively. HF deposits are dominant in the F1 fanlobe sequence, whereas the active channel fanlobe is dominated by the CF deposits. The rest of the active channel sequence is composed of SF deposits. On the other hand, the F1 fanlobe of Bembung Khola is made up of DF deposits and F0 fanlobe is composed of channel flow deposits and flood sediment. Interactions between the tributary fans and the main Teesta River are studied using lithofacies and the associated morphological context. From the above analysis, an evolutionary model for depositional and incision processes at the tributary stream fan confluence allows the reconstruction of five phases of incision and aggradation through the last ~45 ka.
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