Valley and interfluve sediments in the Southern Ganga plains, India: exploring facies and magnetic signatures

Abstract

The western Ganga Plains of India show narrow, incised valleys that contain the major Himalayan-sourced rivers, separated by broad interfluves traversed by small plains-fed rivers. From analysis of four cores and outcrop sections to 50 m sub-surface, this geomorphic setting appears to be of Late Quaternary antiquity. A major river (paleo-Ganga) has been located near its present position since at least 26 ka BP, with indications of southward migration between 11 and 6 ka. Valley fill cores show repeated cycles of valley aggradation in response to Late Quaternary climatic oscillations, in accord with simulation models of drainage-basin response to climate change, and reflect interplay of discharge and sediment supply. Floodplain sediments within the valley fill show a generally weakly magnetic signature and only modest pedogenic activity. The valley margin records a major discontinuity that marks reduction of discharge in the Ganga River during the Last Glacial Maximum, when monsoonal precipitation was greatly reduced and lakes and eolian dunes occupied areas distant from the main channel. The lacustrine facies show marked magnetic enhancement, probably reflecting bacterial action and the availability of organic matter. The interfluve record yields no indication of major-river activity during the past 100 ka. Instead, the near-continuous activity of small plains-fed rivers generated cycles of alluviation and pedogenesis, the latter yielding strong magnetic signatures. Enhanced magnetic signatures are apparent in both valley and interfluve Holocene records, possibly linked to increased monsoonal strength, warmer conditions and enhanced rates of pedogenesis.