3-DIMENSIONAL MODEL OF ALLUVIAL STRATIGRAPHY - THEORY AND APPLICATION

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, chan nel-belt and overbank sedimentation rates, avulsion location and perio d, compaction, and tectonism (tilting and faulting). In this model, a floodplain surface of variable width and length is occupied by a singl e channel belt. Changes in floodplain topography are produced by spati al and temporal variation of channel-belt and floodplain deposition ra tes and by compaction and local or regional tectonism. The location an d timing of avulsions are determined by local changes in floodplain sl ope relative to channel-belt slope and by flood magnitude and frequenc y. The diverted channel belt follows the locus of maximum floodplain s lope. At the end of each simulation, architectural parameters are calc ulated, including channel-deposit proportion and connectedness and the dimensions of channel-belt sandstone bodies. Three-dimensional perspe ctive diagrams, mesh surfaces, and two-dimensional stratigraphic secti ons can be plotted to illustrate depositional surfaces (time planes) a nd the location and geometry of coarse-grained channel-belt deposits w ithin 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 r atios. Avulsion sequences develop where points of avulsion shift up va lley with a progressive decrease in avulsion period. Such sequences ma y produce successions in which channel-belt proportion and connectedne ss vary vertically with a cyclic period of 10(3) to 10(5) years. Down- valley increases in aggradation rate or down-valley decreases in flood plain slope (for example, associated with a rise in base level) may re sult in an increase in channel-belt proportion and connectedness becau se of high avulsion frequencies in down-valley regions of the floodpla in. Down-valley decreases in aggradation rate (as in alluvial fans, fo reland basins, and during base-level fall) may result in high avulsion frequencies in up valley parts of the floodplain. Tectonic tilting an d faulting locally increase avulsion probabilities, and channel belts generally shift toward areas of maximum subsidence. Under certain cond itions, however, depositional topography may cause channels to shift a way from areas of maximum subsidence. Channel-deposit proportion and c onnectedness are generally high near downthrown areas of the floodplai n, but distribution (clustering) of channel belts may not be a reliabl e indicator of fault geometry or displacement. Models of alluvial arch itecture that consider only sediment accumulation rate as the main con trolling factor are oversimplified The three-dimensional model present ed here predicts many of the features of channel behavior observed in modern rivers, but there is a pressing need for better models and adeq uate natural data to test them.