Alluvial Valleys of the Ganga Plains, India: Timing and Causes of Incision

Abstract

Analysis of Quaternary successions from the Himalayan Foreland Basin suggests that tectonics, climate, and glacioeustasy have influenced valley and floodplain architecture in different parts of the Ganga Plains. Tectonic and climatic effects are implicated in valley formation near the Himalayan Front, where major faults are active. Climate was an important factor near the craton margin in the western plains, where tectonic activity is minor and subsidence rates are moderate. Age models for this region suggest that discontinuity-bounded sequences formed over timescales of 103 to 104 years in response to variations in monsoonal precipitation and fluid and sediment discharge in the rivers. Modern megafans and interfan rivers of the eastern plains are not incised, probably due to high sediment yields, low unit stream power, and high subsidence rates. Although long-term records are sparse, discontinuities are probably uncommon in this area. Channels on the Ganga–Brahmaputra Delta are not presently incised, but sea-level fluctuations generated thick late Pleistocene valley fills that extend several hundred kilometers inland. Many Himalayan and cratonic rivers across the area experienced Early Holocene incision in response to monsoon intensification following the Last Glacial Maximum, and brought a large sediment load to the delta and the Bengal Fan. Many reaches in axial parts of the Ganga system appear to have migrated progressively southward in the recent past, generating cliffs along their southern valley margins. Such systematic migration reflects collision of India and Asia and resulting uplift at the Himalayan Front. Climatically controlled sequences may be superimposed on this long-term valley migration, which should generate diachronous valley-base surfaces. Valleys on inland alluvial plains may vary from prominent to subtle over short periods and over distances of hundreds of kilometers, as well as avulsing periodically and migrating systematically.