Gandak Fan - A Macro Quaternary Feature of Middle Ganga Plain, Uttar Pradesh and Bihar

Gandak fan, is a large lobate feature trending in NW-SE direction, restricted between the fluvial regimes of Little Gandak in the west and Burhi Gandak in the east, in parts of eastern Uttar Pradesh and north Bihar. It is characterised by distinct geomorphology, manifested by clusters of tals, palaeochannels, oxbow lakes and spill over channels left out by a past history of river migration and flash floods of Gandak and its predecessors. The terrain is characterised by wide spread drapes of newer altuvium sediments locally known as Bhat Alluvium forming a calcareous, fine sandy to silty soil. Granulometric properties of the sand sequence associated with ths unit indicate dominance of very fine grained sand with median value of 3.0 phi. The clay mineral study of this soil unit indicates preponderance of calcite mineral (30%) besides illite, chlorite and muscovite.

Based on the analysis and synthesis of data, an attempt has been made to interpret the morphology of Gandak fan and the genesis of widespread occurrences of BhatAlluvium of this region. The model envisaged for the formation of the fan dwells on possible cataclasmic neotectonic events related to Gandak Tear and other structures in the vicinity. The neotectonic event triggered widespread river migration/avulsion and cascades of mud flows released from breach and spill over of some large lacustrine bodies formed earlier by damming of the river course in the Jhawani Valley in upstream of Gandak catchment area in Nepal. In addition, the geological and geomorphological set up of Gandak Fan has also relevance to environmental scenario of the area especially to the perpetual floods in large tracts of north Bihar.

It is also surmised that the cascades of mudflows related in the formation of fan and rampant floods in the area might be the causative factors for burial of many important temples and monasteries of Buddhist period, such as Kushinagar, Vaishali, Lauria, etc.