HOLOCENE TECTONO-SEDIMENTARY EVOLUTION OF PARTS OF THE MIDDLE GANGETIC PLAIN

Abstract

Though a fair amount of work on the Quaternary soils/sediments of the Upper Gangetic has been carried out (Srivastava et al., 1994; Kumar et al., 1996; Singh et al., 2006; Bhosle et al., 2008), only limited studies (Mohindra et al., 1992; Gohain et al., 1993) on these aspects of the Middle Gangetic Plains have been undertaken. We have confined our studies to the region between the Ghaghara and Kosi rivers. The region between the Ghaghara and Rapti rivers belongs to the Upper Gangetic plain and the area east of the Rapti forms a major part of the Middle Gangetic plain. Major geomorphic features identified in the study area are: old piedmont, young piedmont, active piedmont, old plains, terminal fans, megafan (Gandak Megafan), terraces and misfit river (Burhi Gandak River). Optically stimulated luminescence ages of C-horizons of soils help to categorize soil-geomorphic units into six groups depending upon the major breaks in plot of ages in the form of bars in decreasing order. Based on these breaks, six morpho-stratigraphic members of a Quaternary Ganga Morphostratigraphic Sequence (QGMS-I to QGMS-VI) are recognized with increasing age i.e.<1.4 Ka, 1.4-4.7 Ka, 4.7-6.9 Ka, 6.9-8.4 Ka, 8.4-10 Ka and > 10 Ka, respectively Using Ground penetrating Radar (GPR), we confirmed the surface expression of two basement faults i.e. West Patna Fault (WPFt) and South Muzaffarpur Fault (SMRFt). The WPFt is a set of normal faults, whereas the SMRFt consist of two major normal faults. Also, GPR studies show that due to in activity of normal faults, terminal fans were deposited on the downthrown sides. Three sandy sequences (each 2-3 m), overlying thick sands of a large river (the River Gandak) on the downthrown block of SMRFt were observed, indicating . repeated activities of the fault. GPR studies show that the older Gandak River underlying some terminal fan deposits consist of braid bar deposits formed due migration of dunes or deposition in upper plane bed phase leading to deposition of sigmoidal to trough cross-bedded/flat-bedded sand in deeper anbranches in the lower parts, whereas in upper parts, mainly lateral accretion and upstream/downstream accretions are very common. GPR investigation of terminal fans indicate that these are mainly lateral accretion sand facies overlain by mud fades, deposited in floodplains and/or in abandoned channels. We have identified three major tectonic features (twenty two faults, two lineaments and three tectonic blocks). The study area is divided into three tectonic blocks namely the Ghaghara-Rapti block, Rapti-Gandak block and Gandak-Kosi block separated by the two major faults i.e. Rapti fault-ll and Gandak fault. The Ghaghara-Rapti Interfluve block is a part of the Upper Gangetic Plains and bounding longitudinal faults like Ghaghara and Rapti Faults are curvilinear with convexity to the SW, like other longitudinal faults in the Upper Gangetic Plains. Here the bounding rivers like Ghaghara and Rapti are incised in nature and the Interfluve is an upland area. The Upland has been a site of deposition since about 10 ka by terminal fans due to the activity of transverse

normal faults and longitudinal Rapti Fault-I. The Gandak megafan block has been marked by northeastward tilting, causing a shift of the Gandak River from west to east over a distance of 80 km during the period 10-8.6 ka. This has led to a higher degree of soil development in the west, which decreases to the east. Since about 4.5 Ka, due to activity of EWtrending faults, four terminal fans have been deposited. The Gandak-Kosi Interfan Block is marked by a number of E-W trending longitudinal faults. Due to activity of the North Sitamarhi fault, at the base of the Old Piedmont, a 44 km wide younger piedmont has developed, which gives a width of 82 km for the whole piedmont, which is the highest in the Gangetic Plains. Activity of a number of longitudinal faults since 1.3 Ka has led to development of terminal fans, which cover major parts of the plains (mainly swampy in nature). The curvilinear Ghaghara and Rapti faults (like other such faults in the Upper Gangetic Plains) were earlier explained due to compression from SW (Parkash et al., 2000). However, the development of E-W trending longitudinal faults in the Middle Gangetic Plains suggest a compression from south. Using Global positioning system Srivedi (2004) found out the Indiaoplate is showing movements towards NE and Nin the Upper and Middle Gangetic Plains, respectively, which are similar to those given by faults inferred by us. However, ages of soils suggest that such deformation patterns have been active at least since -10 Ka in the Upper and since ~4.5ka in the Middle Gangetic Plains.