Signatures of Palaeofloods in Sandbar-Levee Deposits, Ganga Plain, India

Abstract

Based on architectural element analysis of sandbar-levee deposits an attempt has been made to identify recurring floods in the alluvial rivers. Point bar deposits characterized by low-angled lateral accretion (LA) elements exhibit some channel-ward steeply dipping (18-20°) discordant surfaces. These high-angled surfaces are erosional and show change of facies across them. Such discordant planes present within the bar deposits most probably indicate phases of exceptional floods, when the bar was mostly eroded abd remodelled under high-energy conditions. Similarly, in the braid bar deposits superimposed bar-building events separated either by mud drapes or marked erosional contacts are testimony to high-energy floods. At flood times, a new bar may form and can migrate over pre-existing sand bar, forming huge sandflat. Alternately, a new channel may develop cutting across the existing bar complex producing a channel bar having a different orientation. Multi-storied levee deposits developed at the bank of the channels, and characterized by decimeter to meter scale fining-up sequences, are the best indicators of exceptionally large floods overtopping the channels. The flood events are commonly separated by erosional contacts and mottled horizons containing organic matter, root burrows and faecal-pellet filled earthworm burrow network. © Geol. Soc. India.