Palaeohydrological Reconstructions Based on Analysis of a Palaeochannel and Toba-Ash Associated Alluvial Sediments in the Deccan Trap Region, India

The Pleistocene-Holocene palaeohydrological and palaeoenvironmental conditions of the Deccan Trap rivers are not Well-Established. This is mainly on account of the limited extent of alluvial deposits and scarcity of palaeochannels to estimate former river discharges. This paper reports the results of recent investigations it a palaeochannel and the Volcanic-Ash associated alluvial materials at two sites located in the Semi-Arid parts of the Deccan Trap region. The palaeochannel is preserved in, and defined, by the volcanic ash. The ash is believed to be associated with the 74 ka Toba Mega-Eruption.

The geometry of the palaeo and modern channels were determined by undertaking EDM surveys. The mean, maximum and bankfull discharges of the palaeo and modern channels were estimated on the basis of published relationships between discharge and channel dimensions. The estimates indicate a bankfull discharge of about 15-40 m³s¹ for the palaeochannel and between 600 and 900 m³s¹for the modern channel. The estimates suggest that modern discharges are higher by about one order of magnitude.Stratigraphical studies at the sites show that the ash layer is often associated with thick black clays, referred to as fissured clays, which belong to marine isotope stage 3 andior 4 On the basis of modern analogs it was inferred that the Ash-Associated fissured clays represent sediments of seasonal, Low-Energy wetlands. The sedimentological characteristics indicate that during and after the Toba event the monsoonal climate was drier and river discharges were depleted. This inference is also supported by estimated mean and maximum discharges passing through the Kukadi Palaeochannel. We infer that at the time of the Toba event, which occurred during marine isotope stage 5-4 transition, the monsoon discharge and rainfall were much lower than the present in the Deccan Trap region.