ESTIMATING SEDIMENT FLUX FROM ANCIENT FLUVIAL SYSTEMS FOR SOURCE-TO-SINK CALIBRATIONS; CHALLENGES AND WAYS FORWARD

Estimating source sediment supply in ancient systems may proceed by reconstruction of landscape yield, sink accumulation, or transport rates. Each approach faces its own challenges. Transport functions are a long considered but still nascent approach. Evaluating total volume of sediment transported over the duration of a stratigraphic unit requires calculation of the rate of sediment delivery from the source to the sink. For fluvial-fed systems, this means reconstruction of the sediment transport rate from the limited information preserved in the units preserving the fluvial feeder system. The Fulcrum approach offers a series of equations that generate sediment transport rates for bankfull conditions from channel-fill dimensions and grainsize. With robust evaluation of field data, these equations have the potential for predicting transport rates to a factor of three. The technique is limited currently to an order of magnitude for longterm sediment flux by additional unknowns. The largest source of error is in translation of the bankfull channel flux rate into more longterm transport rates. This first requires translation of bankfull flux into annual rates. Rating curves support this effort in modern systems, but are not applicable to ancient deposits. Stratigraphic translations have to be extrapolated from the proportion of the bankfull as annual flow and the duration of the bankfull flow. These numbers are not compiled for modern systems and can only be generalized to an order of magnitude. This can be improved if modern rivers are examined more closely and proportions and durations are more tightly defined by river type. From here, assessment of ancient river type (e.g., climate conditions, morphology, etc.) can be used for more direct comparison and improved estimates. In addition, the method only addresses the flow of specific channels. Better knowledge of sediment routing and number and type of rivers is required to determine the proportion of delivery ascribed to each river type observed. Finally, the rivers preserved must be statistically representative of the rivers that once delivered. This requires improved understanding of preservation modes for axial fluvial deposits. Improved estimates of river flux will add independently and with other methods to calibrate stratigraphic source/sink volumes.