Flexing of ndian lithosphere due to the continent-continent collision and thrust fold loading produced Gangetic Foreland Basin during Middle Miocene. Synoptic remote sensing studies enabled delineation of major and minor geomorphic features. Major features include: Upland nterfluve Surface (T2); Megafan surfaces; River Valley Terrace (T1); Piedmont fans and Active Flood Plains and minor features comprise Ponds and Alluvial ridges which occur on the upland interfluve surfaces. Major rivers of Gangetic plains are entrenched to more than 20 m. A tectonic and climatic control on the geomorphic evolution via sea-level changes has been hypothesized, however, relative amplitudes and chronology of these processes in shaping the regional geomorphology has not been possible due to paucity of reliable numerical chronology. Limited radiocarbon dates on calcrete and molluscan shells from upland interfluve surfaces and ponds suggest that deposition on upland interfluve continued during 28 - 1 ka BP.
Results of a systematic attempt to provide numerical chronology to the deposition event of sediments in various geomorphic context using Blue-green and nfrared stimulated luminescence techniques are presented here. As discussed below, luminescence ages provide a first order framework to the relationship of major and minor geomorphic features to climate and tectonics.
Samples from the raised Upland interfluve surface, (which reflects an interplay of channel, lacustrine and aeolian processes), were dated across the region. A 20 m thick section at Kalpi on river Yamuna shows (base upward)
- a cross-bedded gravely unit,
- 8 m thick fine sand (with remains of the largest Asiatic elephant tusk, Rhino, Bovines and equus) and,
- top 4 m silt devoid of any fluvial signatures.
Luminescence analysis of this section, bracketed the above mentioned palaeontological and sedimentological evidences of more humid climate to 69 - >32 ka. Sequences on rivers Betwa and Ganges provided similar results and enabled a reasonable inference that deposition of upland interfluve surface began atleast at 80 ka and ended by 10 ka. Local deposition on this surface also took place during Holocene. A northward younging in age of final fluvial activity across the region is also seen. River valley terrace occur in all major rivers, typically ~5 m above the present channel. Present study places activity of meandering rivers with a higher discharge during the Holocene (10-1.5 ka) on these surfaces. Accretion on the narrow, poorly developed youngest Active flood plain began after 1.5 ka. Alluvial ridges occur as geomorphic highs (3-5 m) on upland interfluve surface, comprise grey, cross-bedded fluvial sand at the bottom and reddish brown well sorted aeolian sand at the top. Luminescence ages of 7-6 ka for fluvial sediments (channel activity, humid climate) and 5-4 ka for aeolian sediments (less humid climate) suggest a significant shift in climate during Mid-Holocene, that is also reflected in the aeolian and lacustrine record of Thar desert. Distribution pattern, luminescence dating and sedimentology of Ponds suggest their development due to cessation of fluvial activity on the upland interfluve surface some time during 7-4 ka.
Overall, the geomorphic record of Gangetic basin shows a close correspondence to the Arabian sea record of changes in SW monsoon (water budget). The present 20 m fluvial entrenchment should have been influenced by tectonism and needs further scrutiny. Minor geomorphic features and development of soil in western Gangetic plains bear evidence Late Holocene tectonism.