Sediment Flux and Load Estimation for Ancient Shelf-Margin Successions
Sediment supply for ancient siliciclastic successions is generally expressed in a qualitative fashion (i.e. high, moderate, low). More recent work has utilized shelf-margin accretion rates as proxies for sediment supply. This method is useful for comparing relative sediment supply between different shelf margins but does not give actual sediment fluxes which would allow comparison with modern analogs. Here we calculate sediment flux by treating shelf-margin clinoforms as equilibrium profiles created by advection at a rate (i.e. the celerity) equal to the shelf-margin progradation rate. Assuming sediment conservation, deposition is a consequence of a) subsidence, and b) basinward migration of the equilibrium clinoform profile. Sediment flux is therefore a function of subsidence rate, progradation rate, and elevation of the clinoform profile. Width-averaged sediment flux was estimated for 15 clinothems from the Cretaceous Lewis Shale of the Washakie Basin using this methodology and converted to sediment load using the basin width. Average sediment load was estimated at 9.2 mt/yr using five cross-sections oriented in the direction of shelf-margin accretion. This result is very close to estimates of sediment load for this succession from total rock volumes (9 mt/yr). An obvious advantage of this methodology is that it only requires two-dimensional data (i.e. dip-oriented cross-sections) rather than three-dimensional volumes. Furthermore, sediment flux can be estimated with minimal age data since time is expressed entirely within rates of subsidence and progradation. An inverse application of this technique arises when sediment flux is already known but the basinward end of the clinoform succession is not observed. An estimate of the fraction of the sediment budget transported beyond the observed profile can be made by integrating the deposited sediment to that point.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009