SCOUR IN LARGE BRAIDED RIVERS, AND THE RECOGNITION OF SEQUENCE STRATIGRAPHIC BOUNDARIES

Alluvial scour into shallow marine sediments may be caused by the inci sion of a river adjusting to a new base level(1-4) following a fall in sea level. The identification of such erosion surfaces(1-3) has there fore been pivotal in the reconstruction of past sea-level changes from ancient sedimentary sequences(1-14). Here we report data from a study of the Tamuna river, Bangladesh, one of the world's largest modern br aided rivers(15), which illustrate that bed scour associated with chan nel confluences and bends alone can be substantial-as much as five tim es greater than the mean channel depth. Indeed, the basal erosion surf aces produced by such deep scours have characteristics similar to thos e of boundaries in some ancient sedimentary sequences that have been a ssumed to result from sea-level fall(1-14), potentially leading to rad ically different interpretations of past variation in base level and c limate. We suggest that, to discount unambiguously the influence of fl uvial scour in ancient sediments, the erosive boundary should be great er than five times the mean channel depth and extend for distances gre ater than the floodplain width. Ideally, it should be traceable betwee n different basins.