Upland interfluve (Doab) deposition: Alternative model to muddy overbank deposits

Abstract

Major alluvial plains contain large tracts of fine-grained muddy sediments, deposited away from the main river channels, which are mostly classed as overbank or floodplain deposits. Systematic study of the Ganga plain shows that such large tracts of deposition of muddy sediments are located several metres above the major channels, and are not flooded by overtopping of the major river channels. These surfaces are here designated as upland interfluve areas (Doab) where deposition of fine-grained sediments takes place independent of the processes operating in the main channels. The surfaces show distinct depositional domains with characteristic deposits. These include higher sloping surfaces (mottled silt), lower flat surfaces (variegated clayey silt), gulleys (sandy silt), small channels (mottled silty sand), ponds (shelly sandy clayey silt), lakes (shelly clayey silt). These deposits are prone to diagenetic changes, especially the development of calcrete horizons. Redistribution of these domains through time produces characteristic mud-dominant alluvial stratigraphy as observed in the Late Quaternary deposits of the Ganga plain. This succession shows similarity to mud-dominant deposits of the Siwalik succession. These Doab deposits are distinct from the overbank deposits formed close to the river channels affected by channel processes. It is argued that many of the thick mud-dominant fluvial deposits of the ancient fluvial record are products of deposition in upland interfluve areas.