Three-dimensional model of alluvial stratigraphy; theory and applications

Abstract

Abstract A three-dimensional model of alluvial stratigraphy has been developed to simulate the spatial distribution, proportion, and connectedness of coarse-grained channel-belt deposits in alluvial strata as a function of channel-belt width, floodplain width, bankfull channel depth, channel-belt and overbank sedimentation rates, avulsion location and period, compaction, and tectonism (tilting and faulting). In this model, a floodplain surface of variable width and length is occupied by a single channel belt. Changes in floodplain topography are produced by spatial and temporal variation of channel-belt and floodplain deposition rates and by compaction and local or regional tectonism. The location and timing of avulsions are determined by local changes in floodplain slope relative to channel-belt slope and by flood magnitude and frequency. The diverted channel belt follows the locus of maximum floodplain slope. At the end of each simulation, architectural parameters are calculated, including channel-deposit proportion and connectedness and the dimensions of channel-belt sandstone bodies. Three-dimensional perspective diagrams, mesh surfaces, and two-dimensional stratigraphic sections can be plotted to illustrate depositional surfaces (time planes) and the location and geometry of coarse-grained channel-belt deposits within finer-grained overbank deposits. The model predicts that channel-belt proportion and connectedness and dimensions of sandstone bodies vary as a function of distance from avulsion points and cross-section orientation. Upstream from avulsion points, sandstone bodies have low width/thickness ratios because of aggradation in a fixed channel belt. Immediately downstream from avulsion points, channel belts tend to be connected, resulting in sandstone bodies with high width/thickness ratios. Avulsion sequences develop where points of avulsion shift up valley with a progressive decrease in avulsion period. Such sequences may produce successions in which channel-belt proportion and connectedness vary vertically with a cyclic period of 10 3 to 10 5 years. Down-valley increases in aggradation rate or down-valley decreases in floodplain slope (for example, associated with a rise in base level) may result in an increase in channel-belt proportion and connectedness because of high avulsion frequencies in down-valley regions of the floodplain. Down-valley decreases in aggradation rate (as in alluvial fans, foreland basins, and during base-level fall) may result in high avulsion frequencies in up-valley parts of the floodplain. Tectonic tilting and faulting locally increase avulsion probabilities, and channel belts generally shift toward areas of maximum subsidence. Under certain conditions, however, depositional topography may cause channels to shift away from areas of maximum subsidence. Channel-deposit proportion and connectedness are generally high near downthrown areas of the floodplain, but distribution (clustering) of channel belts may not be a reliable indicator of fault geometry or displacement. Models of alluvial architecture that consider only sediment accumulation rate as the main controlling factor are oversimplified. The three-dimensional model presented here predicts many of the features of channel behavior observed in modern rivers, but there is a pressing need for better models and adequate natural data to test them.