Luminescence chronometry and Late Quaternary geomorphic history of the Ganga Plain, India

Abstract

Flexing of the Indian lithosphere due to the continent-continent collision and thrust fold loading produced the Gangetic Foreland Basin during the Middle Miocene. The basin attained its present configuration during the Late Quaternary. Neotectonics and climate change since the last interglacial influenced the evolution of the present landscape of the Gangetic (Ganga) plains. The regional geomorphic surfaces in these plains are: (1) Upland Interfluve Surface (T₂); (2) Marginal Plain Upland Surface (MP); (3) Megafan Surface (MF); (4) Piedmont Fan Surface (PF); (5) River Valley Terrace Surface (T₁); (6) Active Flood Plain Surface (T₀). Minor geomorphic features comprise ponds, alluvial ridges, and small channels, which occur on the Upland Interfluve Surfaces. The present contribution reviews geomorphic processes that sculpted the basin landscape and, then, using the blue green-stimulated luminescence (BGSL) and infrared-stimulated luminescence (IRSL) ages provides a first order chronological framework for the relationship of major and minor geomorphic features to climate and tectonics. Luminescence dating of the Upland Interfluve Surface (T₂) shows that formation of this surface started before 51 ka and continued at least up to 7 ka. Deposition in the Marginal Plain (MP) began at least at 76 ka and continued after 32 ka bracketing the evidence of a more humid climate during 50-40 ka. Sedimentation on the T₂ and MP surfaces was synchronous. The top of these surfaces gives variable ages ranging from 48 to 7 ka, due to differential erosion of an undulatory topography caused by tectonic activity. Age upper bound for the initiation and termination of the final building phase of Ganga Megafan can be placed at ~26 ka and ~22 ka respectively. Deposition on Piedmont Fan Surface continued between 8 and 3 ka. The River Valley Terrace Surface (T₁) of Ganga River shows the activity of Yazoo type channels during 3-1.1 ka. The last phase of active accretion on the narrow, poorly developed youngest Active Flood Plain Surface (T₀) was after 1.5 ka. The ponds of T₂ surface formed due to cessation of fluvial activity sometime during 8-6 ka. A prominent tectonic activity at 7-5 ka and dry climate at 5 ka produced undulatory topography and high siltation rates in the lakes and ponds. The peripheral bulge, southern Ganga Plain has been tectonically active causing deep incision of rivers and a 40-ka seismic event, and Mid-Late Holocene tectonic activity can be inferred.