GEOMORPHOLOGY, SEDIMENTOLOGY AND PEDOLOGY OF THE INTERMONTANE DEHRADUN VALLEY, NORTHWEST HIMALAYA

Abstract

The Dehradun Valley, a synclinal intermontane valley in the Siwalik Formations, is separated from the Lesser Himalayan formations in the north by a major intraplate thrust, the Main Boundary Fault (MBT). The Siwalik Formations underlying the valley and Southern Siwalik Range (Mohand anticline) are thrust over the Indogangetic Plain sediments along the active Himalayan Frontal Fault. Major parts of the Dehradun Valley are covered by three fans from the west to east - Donga, Dehradun and Bhogpur fans, deposited by stream^ debouching from the Lesser Himalaya into the valley. A comprehensive study of geomorphology of the valley, and sedimentology and stratigraphy of the Quaternary gravels (commonly referred to as the Doon Gravels) filling major parts of the Dehradun Valley is lacking; the present study is an attempt to fill this gap. C Digital Terrain Models (DEM, slope map, aspect map and profiles) were prepared for identification of geomorphological and tectonic features of the Dehradun Valley. Along with the DTMs, field checks, examination of topographic maps and OSL dates of terrace sediments were carried out. For working out the pedology, sedimentology and stratigraphy of the Dehradun Valley 30 selected river-cut exposures were studied in detail. In addition 7 pedons were studied for surface soil characteristics. Each section was measured and logged. Various properties of conglomerate and fine sediments were noted in the field. Thin-section of soils were prepared and studied for micromorphological studies. Ages of selected samples were determined by the OSL method. Major faults in the area are the MBT and Santaurgarh and Bhauwala Faults, which are subparallel and trend approximately E-W. The later two faults are considered to have been formed by the same compressional forces, which were responsible for the MBT. These faults define the boundaries of major geomorphic units from north to south: Lesser Himalaya, Dissected Siwalik Hills, Remnant Siwalik Hills (Pediments) and Piedmont Plain,. Thus major geomorphic units were strongly affected by activityof the Main Boundary Thrust System. Various fans in Dehradun Valley have formed due to major relief being provided by activity of the MBT. Afew ages of fan suggest that the deposition of Dehradun fans mainly started at about 50 ka. Pediment surfaces (first report from the Himalaya) formed in the Remnant Siwalik Hills during the period 30-10 ka and have been inferred to have formed due to dynamic equilibrium 1 type, so that streams could cause lateral erosion and lead to formation ofwide surfaces (pediment) (Bull, 1990). Segmentation of various stream profiles and hill profiles suggest that the MBT, Santaurgarh and Bhauwala faults are presently active and the Dehradun Valley is undergoing differential uplift. Deeper entrenchment upto 50 mcaused by various rivers in northern parts as compared to central part and convergence ofthe Yamuna terraces in the Remnant Siwalik Hills/Piedmont Plain in the northern part of the Dehradun Valley along with comparison of the longitudinal profiles of terraces of the Yamuna and the Ganga rivers with those of respective rivers indicate higher uplift in the north as compared to the central portion of the valley. This is further supported by the rate of uplift calculated by the OSL dates and the height of river terraces form respective river beds. Lithostratigraphically, the Doon Gravels are divided into five Units in the Donga and Dehradun fans and three Units in the Bhogpur fan on the basis of lithofacies and the degree of development of soils and OSL dates of selected samples. The Donga and Dehradun fans are characterized by sediment gravity flow deposits (colluvium, debris flow and mudflow deposits), in the proximal zone, mostly mudflow deposits and minor braided stream deposits in the mid zone and mainly overbank material with minor mudflow deposits in the distal zone during the period 50-10 ka. Also the soils were very weakly to weakly developed, moderately to very strongly developed and weakly developed in the proximal, mid and distal zones respectively. The degree of development of soils was a function of mainly rate of sedimentation and to some extent entrenchment of the streams into the fan surface. The Bhogpur fan has been marked by deposition from braided stream since 50 ka .