Fluvial incision and sequence stratigraphy: alluvial responses to relative sea-level fall and their detection in the geological record

Abstract

Abstract A critical question in sequence stratigraphy is whether or not a falling or lowered sea level will cause a river or delta distributary channel to incise and thus help to initiate an Exxon type-1 unconformable surface. River adjustment to relative sea-level fall was quantitatively modelled by linear diffusion. Although not a new approach to the general problem of river incision, these numerical experiments illustrate the rates and magnitudes of channel incision expected for various combinations of river slope, shelf slope and sediment transport coefficient. The results of the modelling broadly agree with, admittedly uncertain, field observations from the Gulf Coastal Plain of the USA. Incised valleys have limited lateral extent because of the slowness of valley side slope retreat. Successive nodal avulsions upstream of the highest point of channel incision may progressively neutralize the effects of sea-level fall in younger channel courses. It is also possible that incision is reduced or eliminated by rivers with high sediment transfer coefficients: it is shown that rivers may ‘keep up’ with sea-level fall under certain circumstances, constructing a prograding alluvial plain over a previous ramp-like shelf with little slope differential. These conclusions apply to the recognition of geologically ancient incision and thus to the existence or otherwise of local and regional sequence stratigraphical boundaries caused by relative sea-level fall. We briefly discuss examples from the stratigraphical record and conclude that much exciting work remains to be done in the field and on the computer.