Late Quaternary stratigraphy and infilling of the Meghna River valley along the tectonically active eastern margin of the Ganges-Brahmaputra-Meghna Delta

Abstract

Here sediment provenance from 48 drill cores across a fluvial valley is used to highlight fluvial channel dominance, anticlinal Pleistocene structures, Holocene channel occupations, and Holocene tectonic channel steering in the Meghna valley, which is alongside a convergent margin in the Ganges Brahmaputra Meghna Delta. Sediment provenance shifted in the late Pleistocene and early Holocene from being supplied by more local rivers that were part of the Barak, Surma, and Kushiara rivers that drain the Indo-Burman ranges to being mainly supplied by the Brahmaputra River, which accounts for the majority of sedimentation in the Holocene. Places in the Meghna valley that do have Pleistocene Brahmaputra sediment, appear to be rare, older than the latest Pleistocene channel-levee sand deposits, and are linked to anticlinal folds. The center of Lalmai anticline, which outrcrops in the Meghna valley, is comprised of ~50 m of Brahmaputra braidbelt sands, while the western side of the valley has several cores with Pleistocene Brahmaputra sediments that are likely buried anticlinal structures with deformation tilt rates allowing for the Brahmaputra to be excluded from the Meghna valley in the late Pleistocene. The Holocene occupation history of the Meghna valley by the Brahmaputra River of several occupations of varying time from this study agrees with previous work. Based on Brahmaputra Holocene deposits and cross valley gradient that slight tilting of the land surface due to deformation alone managed to steer the main channel at least 50 km westwards during the Holocene during the 7.5 to 5 kyr occupation.