Sea Level-Influenced Sedimentation in the Lower Brahmaputra Foreland Basin: Unraveling Dynamics and Provenance

The climate-tectonic-sea level relationship in an active mountain belt, like the Himalayas and its foreland, can be better understood by delving into the sedimentary stratigraphy of the alluvial tracts developed in the foreland basins. Alluvial fans, in particular, operate as a natural depository of sediments that can be utilized to assess the role of geological processes working between the source and sink. The physiography of these fans, including channel patterns, aggradation, and incision, is influenced by tectonics, sea level variations, and local factors like precipitation and slope, impacting the availability of eroded materials. This study based on geomorphic mapping, detailed sedimentological analysis, lithofacies analysis, and geochemical (Sr-Nd analysis) provenance characterization, as well as optically stimulated luminescence (OSL) ages of a relict alluvial fan, provides a dated sedimentation framework for the western Assam lowland areas. The fan surface lies ~40 m above mean sea level, is incised, and forms a regional valley terrace T₁ composed of meandering channel deposits. Modern braided rivers flow on the T₀ surface. The findings suggest that the alluvial fan is composed of three distinct lithofacies associations and aggraded during 27 to 3 ka. The bottom-most cross-bedded gravelly-sandy facies (Phase-1) indicate progradation of the fan during LGM, owing to the increased gradient of the Himalaya-bound rivers. Sheet flood deposits (Phase-2) in the middle facies formed during the Late Pleistocene-early Holocene with rising sea levels and increased precipitation. In the Mid-Late Holocene, the uppermost facies (Phase-3) deposited as rivers, responding to elevated sea levels, lost their gradient, leading to inland sedimentation within muddy meandering channels. Modern-day Gravel Sand transition zone lies much upstream of the transverse rivers in comparison to the gravel sand transition zone of the paleofan. This indicates that alluvial fan was prograding in response of increased gradient of the transverse Himalayan Rivers due to lowered sea level during LGM.  Our analysis found that falling sea level during the late Holocene was associated with greater precipitation and allowed the river to incise, to form gullies over the fan surface and form the valley terrace T₁. The gravel units found in borehole stratigraphy of Upper Bengal Delta (located ~100 m below msl) relate to progradation 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 provenance fingerprinting using Sr-Nd isotopic composition suggest that the deposits of phase-1 and phase-2 were equally sourced from the Higher and lesser Himalaya while the phase-3 deposit along with the sediments of the meander scroll deposit mimicked the composition of the modern-day bed-load which are dominated by Higher Himalayan sediments.