Quaternary Geology and Geomorphology of a Part of Ghaghara-Rapti-Gandak Sub-Basins of Indogangetic Plain, Uttar Pradesh

The Quaternary sediments of the Indogangetic plain in eastern part of Uttar Pradesh, can broadly be subdivided into two groups lithostratigraphically, viz. the Older Alluvium and the Newer Alluvium. The former is subdivided into basal Banda Older Alluvium (BOA) comprising of variegated clays and clastic sediments of Chitrakoot Formation encountered in the boreholes in the southern part and its equivalent Upper Siwalik in the northern part and an upper Varanasi Older Alluvium (VOA) and Palaeo Bhur deposits. The Newer Alluvium includes Bhat Alluvium, GhagharalRapti/GandakiChhoti GandaklChhoti Sarju Terrace Alluvium and their Recent Alluvia. The lithounits correspond to two broad geomorphic units, viz . an upland - the Bhangar surface and a low lying area demarcating the flood plains - the Khadar surface. The former consists of a piedmont zone of alluvial fans and an alluvial plain. while the later consists of terraced valley. The evolutionary history of Quaternary sedimentation commenced with a change in the configuration of Middle Siwalik basin converting it into much wider one by transgressing southward further over Bundelkhand gneissic complex and Vindhyan basement probably, during Upper PliocenelLower Pleistocene with sediment supply in the southern part from the Peninsular provenance constituting the Banda Older Alluvium and with sediments derived from Himalaya in northern part of the basin to constitute the Upper Siwalik. This was followed by a break in sedimentation caused due to upliftment during Middle Pleistocene resulting in a new basin formation superimposed over previous one, with sediment influx from the north, to the deposition of Varanasi Older Alluvium. The Upper Pleistocene glaciation brought an end to this sedimentation phase. The gradual warming up and retreat of glaciers accompanied with a large amount of sediment supply led to the formation of Bhat Alluvium in piedmont zone. A number of fluvial regimes, namely Ghaghara, Rapti, Gandak etc. carved out their new, narrow basins with warming up and increased precipitation during Holocene represented by their terraces. The ongoing phase of sedimentation is restricted to within the present active flood plain in response to reduced discharge and sediment supply.