GEOMORPHOLOGICAL AND PEDOLOGICAL EVOLUTION OF THE DEOHA/GANGA-GHAGHARA INTERFLUVE

Abstract

Regional mapping using remote sensing techniques and extensive fieldwork helped to identify and map 24 soil-geomorphic units in the study area. On the basis of Luminescence ages soil-geomorphic units are grouped into five members of a Morphostratigraphic Sequence (QGMS) with ages of <1.7 ka B.P., 1.8-3.6 ka B.P, 4.6-6.4 ka B.P., 6.8-10 ka B.P and >10 ka B.P. Except for two units with sandy parent material, all the other members with loamy parent materials show systematic increase in the degree of soil development from Member QGMS-I to V. Modern hydrological set-up suggests that most of area is dry sub-humid and form wetland due to impervious layers at shallow depths. Major pedological processes are salinisation, alkalinisation, illuviation, calcrete development and gleying. Regional mapping of soil-geomorphic units, tectonic features and dating of soils have helped to decipher that the Interfluve between Deoha/Ganga- Ghaghara Rivers is bound by longitudinal faults, along the bounding rivers. The major longitudinal faults trend N-S or NNE-SSW in the northern region; they turn N-S in the central region and take easterly to SEE direction in the south, giving the Interfluve a curvilinear shape. Due to curvilinear nature of the longitudinal faults, strike of the set of six transverse normal faults in the plains change from approximately E-W direction in the northern regions to NE-SW in the middle central region and run NEE-SWW in the southernmost region of the Interfluve. Downthrown sides are to the south and to east in northern region and western Hi region, respectively. The transverse faults represent an extensional regime normal to general maximum compressive regime acting in the NE-SW direction. Due to activity of different segments of various transverse faults during a probably arid to dry subhumid climatic period of 10-5 ka for the region, terminal fans were deposited on the downthrown blocks. Also, tilting of smaller blocks leading to shifting away of large rivers seems to be additional control on distribution of soils and sedimentation in the Interfluve. Repeated activity of normal transverse faults followed by quiescence periods leading to formation of soils may give rise to a sequence of terminal fan deposits topped by soils, very similar to palaeosol assemblages observed from the Upper Siwalik Group (Thomas et al., 2002).