Morpho-sedimentary records of active tectonics at the Kameng river exit, NE Himalaya

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The Kameng River in the NE India cuts through the rising Siwaliks, the frontal ranges of the Himalaya and forms four levels of alluvial terraces. The geomorphic configuration and overlying alluvial cover of each terrace has been studied to decipher the uplift history of the Himalayan front and varying sedimentation pattern of the river. The ratio (SA ratio) of the thicknesses of alluvial cover and bedrock strath is used as a measure of geomorphic instability (GI) and a GI-Index has been calculated for each incision and aggradation phase. It is suggested that the area was geomorphologically most unstable when the Kameng River was incising from terrace T 2 to T 1. The Luminescence chronology constrained this phase to be at ∼ 7 ka B.P. The older incision phases namely T 4–T 3 and T 3–T 2 occurred ∼ 14 and ∼ 10 ka B.P. respectively. The alluvial cover of the terraces is composed of four major lithofacies viz. (1) Matrix-supported pebble, (2) Clast-supported pebble, (3) Parallel laminated medium sand, and (4) Bioturbated fine sand. The vertical variation of the lithofacies indicates that the river was flowing under a braided pattern. The study suggested that Kameng river incised 95 m from terrace T 4 to T 2 in the span of ∼ 8 ka between 14–6 ka BP, suggested the uplift rate of 11.9 mm/year. However, the total uplift in the Siwaliks, as indicated by the height difference of terraces T 4–T 0 in the area is 105 m which occurred during the last 14 ka at the rate of 7.5 mm/year, indicating a varying rate of Siwalik uplift in the Holocene. The studies corroborate with published incision rates from the central Himalaya in Nepal.