Autogenic and allogenic controls on the temporal palaeographic evolution of the Himalayan foreland basin: Insights from facies analysis of the lower Siwalik succession, Kumaun Himalaya, India

The lower Siwalik succession in the south-central Kumaun Himalaya records Middle Miocene fluvial sedimentation in the Himalayan foreland basin, the largest foreland basin of the world. Detailed facies analysis reveals three distinct facies associations, one of which is sand dominated channel deposits, and the other two are mudstone-sandstone, and mudstone dominated overbank deposits. The initial sedimentation in the region was in channels and frequently/extensively flooded overbank areas of a meandering/anastomosing river system. Activities along basement structures sometimes caused upheaval of the basin so that the streams got incised, and overbank areas rose up beyond the reach of flood waters. As a result, the fluvial sedimentation in these upland areas ceased, the sediments that had already deposited there were subjected to extensive pedogenesis, and occasionally reworked and redistributed by sheet flows and shallow channels. The channel pattern in the region gradually changed to braided type due to channel adjustments in response to rejuvenated tectonic activities and monsoon intensification in the hinterland. These factors caused increased influx of coarser sediments in the channels, which led to gradual steepening of the channel, and once the steepening crossed the threshold, the channel changed from meandering to braided type. Interpretation of our results is contrary to the general belief that Siwalik fluvial system changed from meandering streams to braided streams during the Middle Siwalik times, and the fluvial system in the studied part of the Siwalik basin underwent this change much earlier, during the sedimentation of Lower Siwalik.