Sedimentary processes and facies of Upper Pleistocene alluvial fans in the Purna Valley Basin of central India

Alluvial fans of Upper Pleistocene age occur as transverse depositional systems along the fault-defined Satpura mountain front that marks the northern margin of Purna Valley Basin. Our findings indicate that the fans were built by three main sedimentary processes: debris flow, sheetflood and stream flow. The debris flow fans are the smallest in size and comprise ill-sorted, matrix supported conglomerates showing evidence of deposition on slopes. The sheetflood fans are larger than debris flow fans in size and are composed predominantly of laterally extensive, tabular, planar-bedded couplets of relatively coarse and fine grained sediments. The stream flow fans are the largest in size and represent channelized flows with sheet-like, clast supported, massive gravel beds as the fundamental depositional unit. It emerges from this study that two essential factors controlled the sedimentary processes acting on the alluvial fans. These are : the size of the drainage basins and the tectonic geomorphology of the mountain front. The debris flow and sheetflood fans are restricted to the linear, tectonically active part of the mountain front dissected by small streams only. The stream flow fans developed adjacent to that part of the mountain front which is tectonically less active, erosionally embayed and drained by rivers with large catchments.