Holocene landscape evolution of the Brahmaputra River valley in the upper Assam Basin (India): Deduced from the soil-geomorphic studies

The Brahmaputra River basin exhibits a variety of landforms, including alluvial fans, old and active floodplains, and piedmont belts. The surface expression of the subsurface basement ridge, known as the Brahmaputra Arc, is characterized by paleosol exposures aligned along its crestal region. Using Optically Stimulated Luminescence (OSL) chronology and paleopedological studies, four stages of landform development have been identified in this region, influenced by neotectonics and climate change. The soils in the study area have been categorized into four members of the Quaternary Brahmaputra Morphostratigraphic Sequence (QBMS). QBMS-I (<1.77 ka) represents the youngest soil found in the active floodplains, QBMS-II (2-3.65 ka) includes the Mikri hills piedmont and adjacent river plains, QBMS-III (4-6.5 ka) covers most parts of the study area and is formed on alluvium. The oldest unit QBMS-IV (>7 ka) is exposed on the crest of the Brahmaputra Arc. These soils provide insights into the geomorphologic history of the region, with QBMS-II and III recording multiple avulsions during the Holocene. Micromorphological studies indicate that these soils have undergone multi-phase development under varying redox conditions. The Mid-Miocene compression in the foreland basin led to the formation of the Brahmaputra Arch, and subsequent sedimentation covered the crestal part. Presently, erosion and removal of sediment from the crestal region have exposed older soils as paleosols. Two alluvial fans occur in the study area, the older one located in the central part and the younger one in the northeast corner. The Naga hill piedmonts also exhibit a similar distribution, with the older segment in the central part and younger segments in the northern and southern parts. These patterns of piedmonts and alluvial fans suggest distinct tectonic episodes responsible for sediment redistribution in the study area. The proximity of the Brahmaputra Arc and the Naga thrust in the central part further supports this observation. The findings of this study are consistent with the episodes of neotectonic activity in the nearby Ganga plain, highlighting their regional relevance. Overall, the study provides valuable insights into the evolution of landforms in the Brahmaputra River basin, highlighting the role of neotectonics and climate change in shaping the landscape.