Aggradation, incision and interfluve flooding in the Ganga Valley over the past 100,000 years: Testing the influence of monsoonal precipitation

Abstract

In order to investigate fluvial responses to climate forcing, stratigraphic data from seven cores in the Ganga Valley of India and a valley-margin cliff section 1.3km long are compared with proxy records for the intensity of the Southwest Indian Monsoon. Luminescence dates indicate that the strata cover the past ~100ka. Five aggradational periods separated by incisional episodes are apparent in the valley fill, some of which have correlative strata along the valley margin. During Marine Isotope Stages (MIS) 4 and 5, the valley experienced fluvial activity, with thin floodplain successions and probable discontinuities along the interfluve margin. Modest fluvial activity characterised the mid part of MIS 3 at about 37ka. Aggradation of channel sands preceded by incision is documented for late MIS 3 (23 to 30ka), as well as for late MIS 2 to early MIS 1 (11–16ka) with fluvial dates focused around the Younger Dryas period of monsoon reduction. The most recent aggradational period took place in the latest Holocene (<2.5ka). On the proximal part of the interfluve, thin deposits that include lacustrine and aeolian beds continued to accumulate until about 26ka, after which the interfluve was apparently not inundated by the Ganga and underwent degradation through gully erosion. There is little evidence of fluvial activity during the Last Glacial Maximum of MIS 2, when the Ganga appears to have been underfit. The alluvial records fit well with proxy precipitation records, and the main aggradational periods correspond to times of declining monsoonal strength. Although more difficult to constrain, the timing of incisional periods corresponds broadly with periods of monsoonal intensification, and the river appears to have shifted course in its valley mainly during these times.