Depositional processes and sedimentation pattern in an intermontane basin: Insights from the Imphal Basin, Indo‐Myanmar Range, NE India

Abstract

Intermontane basins make a distinct category of sedimentary basins, but nevertheless, the depositional processes and sedimentation patterns therein are not well understood. Quaternary sediment‐filled Imphal Basin in the Indo‐Myanmar Range provides us with an excellent opportunity of lending insight into these sedimentological aspects of an intermontane basin. Detailed facies analysis brings out that five distinct facies associations (“FAA”–“FAE”) have developed in different parts of the basin, each one of which indicates a specific subenvironment of deposition. Facies characters and spatial distribution of facies associations reveal that the basin fill records interlinked lacustrine, fluvial, and alluvial fan processes. Comprising mud and peat, the facies association “FAA” represents deposits of a lake and lakeshore swamp. The facies association “FAB” comprises mud interfingered with crudely bedded gravel and represents suspension fallout in a lake with intervening gravel‐rich, hyperconcentrated flows that diffusively plunged into the lakebed. The sand‐dominated facies association “FAC” and gravel‐dominated facies association “FAD” were deposited in fluvial domains of meandering and braided streams, respectively. Rimming the basin margin, the gravel‐dominated facies association “FAE” was deposited in the alluvial fan–interfan areas. Widespread development of the lacustrine facies (“FAA” and “FAB”) around the Loktak Lake along with extensive peat development over the lacustrine mud suggests lakeshore regression and development of vast swamps under humid conditions. The regression under humid conditions could be a result of fast and enhanced influx of finer sediments into the lake that transcended the creation of accommodation space and ultimately led to faster infilling of the lake margins favouring development of vast swamps in due course of time.