Late Quaternary History of the Ganga Plain

Abstract: Flexural subsidence of the Indian lithosphere created the Ganga Plain foreland basin in front of the Himalayan orogen, which attained its present configuration during the late Quaternary Climate change, neotectonics and base level changes influenced the evolution of the present landforms of the Ganga Plain The regional geomorphic surfaces are (1) Upland Intelfluve Surface (T2), (2) Marginal Plain Upland Surface (MP), (3) Megafan Surface (MF), (4) Piedmont Fan Surface (PF), (5) River Valley Terrace Surface (T1) and (6) Active Flood plain Surface (T0) The Upland Interfluve Surface (T2) is widespread reference surface, other surfaces are either incised or overlie this surface Formation of these surfaces is essentially related to the climate cycles of late Pleistocene-Holocene (128 ka BP) These surfaces are depositional and have a succession of overlying sediments, younger than the time of formation of the respective surfaces The sediments coming from the Himalaya undergo significant chemical weathering in the alluvial plain where much Na, Ca, Sr, K, Mg, is removed, while As, Cr, Ni and Th are enriched In suitable conditions smectite and kaolinite are formed These sediments are partly eroded and transported to the delta region and beyond The Ganga Plain consists of a large number of river systems, originating in different parts of the Himalaya, Peninsular Craton, and alluvial plain formed at different times during the late Pleistocene-Holocene Surface runoff and groundwater are the main sources of water It is difficult to make meaningful scheme of classification of channels Based on the sediment-water transfer patterns, the channels are classified into three broad categories The river systems are in different stages of evolution The active perennial rivers are incised within the alluvium, and show avulsion on a scale of 104 -105years, some channels, part of the ancient anastomosing network, act as flood channels and show avulsion on a scale of 101-102 years The Upland Interfluve Surface (T2) exhibits a number of microgeomorphic featuies which have helped in the reconstruction of the history of the Ganga Plain duing the last -20 ka BP 20-13 ka BP - new drainage of low density, 13-8 ka BP - extension of drainage system, high rainfall, 8-6 ka BP - channel disruption due to base-level rise and tectonics and formation of large lakes which retained most water-sediment of T2-Surface, 6-4 ka BP - dry climate, increased terrigenous clastic sediments in lakes, 4-2 ka BP - increased si Ration of the lakes, moderate rainfall, 2-0 ka BP - fragmentation and drying of lakes, strong anthropogenic influence The basin exhibits active tectonism, which is mostly in the form of contractional system in the piedmont zone, and prominent extensional system in the southern part of the basin One of the most significant events in Ganga Plain history is tectonics of 8-5 ka BP, which produced areas of centripetal drainages and changed the fluvial landscape into a landscape of ponds and lakes with few high grounds It was helped by the climate change from humid to dry around 5 ka BP