Fluvial Landscape of Brahmaputra Foreland: A Sea Level Controlled Stratigraphy

Indus-Ganga-Brahmaputra foreland is basin formed on a crustal flexure formed continent-continent collision. Large part of the Ganga plain is said to be filled by Himalayan and craton bound sediments where climate variability, tectonics of Himalaya and peripheral bulge are said to be major forcing factor. However, with this study we demonstrate that stratigraphy of Brahmaputra foreland seems to be largely controlled by sea level changes. In this study vertical stratigraphy of a relict alluvial fan is studied in the western Assam plains. The study utilizes geomorphic mapping, lithofacies analysis and geochemical (Sr study) provenance characterization along with optically stimulated luminescence for chronological framework. The fan surface that lies 50-30 m above mean sea level is incised and forms a regional valley terrace T1, composed of meandering channel deposits. Modern rivers are braided and flow on T0 surface. The results suggested that alluvial fan is made up of 3 distinct facies association. The bottommost gravelly-sandy braided channel system; overlain by facies dominant by sheet flows. Towards the top lies deposits of fine grained muddy meandering channels and flood plain facies association. The basal and middle units were deposited during ~22 to ~17 ka that coincides to last glacial maxima (LGM). This indicates that alluvial fan was prograding in response of increased gradient of the transverse Himalayan Rivers due to lowered sea level during LGM and moved much downstream as compared to present day. The Upper most unit was deposited in response of rising sea level as rivers lost gradient and inland sedimentation in muddy channels took place during Early-Mid Holocene. We suggest that mid Holocene sea level drop and increased precipitation allowed river incision and gullying of fan surface. Subsequently, the rivers were meandering on the incised valley. The gravel units found in borehole stratigraphy of Upper Bengal Delta (located ~100 m below modern sea level) relate to prograpdation of alluvial fans in response to lowered sea-level rather than an extreme flood events in axial channel of Brahmaputra as envisaged in earlier study. The Sr isotopic signatures has also pointed towards sediment sources lying in southern front of Himalaya (i.e. Lesser and Higher Himalaya).