Estimation of the paleoflux of terrestrial-derived solids across ancient basin margins using the stratigraphic record

Abstract

A simple inversion scheme for estimating sediment flux from ancient shelf-margin successions is presented here by treating shelf-margin clinothems as the product of deposition associated with migration of a shelf-edge clinoform with constant shape at a rate equal to the shelf-margin progradation rate. Assuming sediment conservation, deposition can be broken into components of (1) response to subsidence and sea-level changes, and (2) basinward migration of the clinoform profile. Sediment flux can therefore be estimated with knowledge of progradation rate, subsidence/sea-level change rate, and clinoform dimensions. An advantage of this methodology is that it requires only two-dimensional data (i.e., dip-oriented cross sections) rather than three-dimensional volumes, making it ideal for use with sparse data sets as well as with outcrops. This methodology is also useful for analyzing areally limited data sets because it can predict the flux of sediment transported beyond the area of data coverage. The approach is able to accurately reproduce the sediment-flux estimates of previous workers from several margins (the Fox Hills–Lewis, Zambezi, New Jersey, and North Slope margins) using both volumetric and forward-modeling methods. Not only are the predicted distributions for sediment flux across ancient shelf-margins similar to distributions predicted by more data-intensive theoretical models, the estimated magnitudes for paleofluxes favorably compare with measured loads from modern rivers. Growth of continents is achieved in part by accretion of sediment on shelf margins. The rates and patterns of continental expansion are therefore partially dependent on the magnitude and distribution of mass transfer from eroding hinterlands to continental margins, fluxes which also play a critical role in global biogeochemical cycles. Flux estimates cast into a mass-balance framework suggest that approximately two-thirds of continental-margin sediments are exported past the shelf edge into deeper water at long-term geologic time scales. This finding implies that two-thirds of the terrestrial-derived, particulate organic carbon (POC) delivered from rivers to the ocean can be stored in deep water over geologic time scales. The observations presented here indicate that repetitive delivery of sediment to margins by shelf-edge deltas is fundamental to the long-term process of margin accretion.