Comparison of methods to estimate sediment flux in ancient sediment routing systems

Abstract

The need to predict accurately the volume, timing and location of sediments that are transported from an erosional source region into a basin-depocentre sink is important for many aspects of pure and applied sedimentological research. In this study, the results of three widely used methods to estimate sediment flux in ancient sediment routing systems are compared, using rich input datasets from two systems (Eocene South Pyrenean Foreland Basin, Spain and late-Pleistocene-to-Holocene Gulf of Corinth Rift Basin, Greece) for which mapped, dated sediment volumes provide an independent reference value of sediment accumulation rates. The three methods are: (1) the empirical BQART model, which uses values of drainage basin area, relief, temperature, lithology and water discharge; (2) empirical scaling relationships between characteristic geomorphological parameters of sediment-routing-system segments; and (3) the “fulcrum” model, which uses the palaeohydrological parameters of trunk river channels to estimate downsystem sediment discharge. The BQART model and empirical geomorphological scaling relationships were originally developed using modern sediment routing systems, and have subsequently been applied to ancient systems. In contrast, the “fulcrum” model uses hydrological scaling relationships from modern systems, but was developed principally for application in ancient systems. Our comparative analysis quantifies the sensitivity of the three methods to their input parameters, and identifies the data required to make plausible estimates of sediment flux for ancient sediment routing systems. All three methods can generate estimates of sediment flux that are comparable with each other, and are accurate to at least one order of magnitude relative to independent reference values. The BQART model uses palaeoclimatic and palaeocatchment input data, which are accurate for sub-modern systems but may be highly uncertain in deep-time systems. Corresponding estimates of sediment flux are most sensitive to the accuracy with which the palaeocatchment area is constrained and to palaeoclimatic parameters that reflect temperature and precipitation. The “fulcrum” model uses palaeohydrological input data; its sediment-flux estimates are sensitive to palaeochannel dimensions and, in particular, the duration of bankfull discharge, which is invariably difficult to constrain accurately in deep-time sediment routing systems. This uncertainty can give rise to large potential ranges of sediment-flux estimates. Geomorphological scaling relationships offer comparable, order-of-magnitude accuracy for both sub-modern and deep-time sediment routing systems in which geomorphological segments can be identified, but when used on relatively small sediment routing systems the ranges of sediment volumes deposited can vary greatly, limiting the utility of the technique. We suggest that methods to estimate sediment flux should be chosen for a particular sediment routing system based on the types and uncertainty of available data. Where input parameter values are highly uncertain, such as in deep-time systems, Monte Carlo simulation is an effective tool to calculate probability distributions of estimated sediment flux.