290-5 Lithofacies changes and geochemical correlation in a rapidly sedimented and deeply subsided deltaic complex from the central to coastal Bengal Basin

Session: Delta Evolution from Rivers to the Shelf: Past, Present and Future Perspectives for Society

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This study primarily aims to examine the Holocene sediments based on their deposition and distribution pattern from the central to the coastal Bengal Basin. We aim to evaluate the changes in lithofacies from fluvial at the central to deltaic coastal Bangladesh, and prepare a refined lithostratigraphy and examine the mechanism of sediment-groundwater interactions. We integrated primary data obtained from core, washed, and exposed Holocene sediment compositional analysis through petrographic study of thin sections, scanning electron microscopy, ICP-MS, 87Sr/86Sr fingerprinting, and carbon-14. We also combined several secondary data sources along with our primary data to determine sediment deposition and distribution patterns, provenance, sediment routing, and subsidence dynamics. The average sedimentation rates found in the southwest coastal region of Bangladesh are 4.1 mm/y, in the south-central coast it is 4.69 mm/yr, and in the east coastal region it is 3.26 mm/yr, respectively. The stratigraphic analysis and basin-wide correlation study reveal that the central Bengal basin is characterized by Pliocene-Holocene clastic sediments, featuring several fining-upward alluvial sequences.  Moreover, the coastal Bengal basin shows Pleistocene-Holocene detrital sediments comprising both coarsening and fining-upward sequences consisting of alternating layers of tidal mud and sand layers of deltaic and estuarine deposits.  Notably, the absence of distinct lithology, such as gravel beds which are observed in the central Bengal Basin, is absent from stratigraphic sequences in the coastal areas. This may suggest that the downcutting of high-energy Himalayan rivers did not continue down to the coastal Bengal basin during the sea-level low, likely resulting from the last glacial maximum. 

Our additional research objective has been to explore whether vegetation indices (NDVI, EVI) can serve as a proxy for monitoring/detecting groundwater salinity in coastal Bangladesh. We employed GIS-based regression models and Remote sensing-based vegetation data to explain the variability observed, considering the response of the dependent variable (salinity) using a set of covariates or independent variables (vegetation indices). We found that salinity is negatively correlated with vegetation, with a correlation coefficient of r = - 0.31. Our visualization maps and analyzed results reveal that groundwater salinity in coastal aquifers is primarily connected with the surface water recharge, storm surge inundation, land-use change, and surface water-groundwater interactions within the shallow aquifers.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10916

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