Abstract: Three-Dimensional Modelling of Alluvial Architecture

J. S. Bridge, S. D. Mackey

A three-dimensional model of alluvial stratigraphy has been developed to simulate the spatial distribution, proportion, connectedness and dimensions of channel-belt deposits in alluvium 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). The location and timing of avulsions are determined by local changes in floodplain slope relative to channel-belt slope (controlled by deposition, compaction and tectonism) and by relative flood magnitude and frequency. Avulsion increases when channel-belt deposition rate increases relative to that on the floodplain and as downvalley slope decreases. The diverted channel follows the locus of maximum loodplain slope.

Experiments in which deposition rate increases (and slope decreases) downstream simulate the effects of rising base level. In this case, avulsion frequency increases in the down-valley part of the floodplain, and channel-deposit proportion and connectedness increase relative to the up-valley part. Down-valley decreases in deposition rate occur in some foreland basins, alluvial fans, and where base-level is falling. In this case, avulsions occur more frequently in up-valley locations, increasing channel-deposit proportion and connectedness here. These trends may be moderated by along-valley or temporal changes in factors such as floodplain width, channel-belt width, or tectonism. An alluvial fan simulation indicates that channel belts can move progressively in a preferred direction ove the fan surface purely by autocyclic mechanisms. Also, avulsion sequences occur where points of avulsion migrate up-valley with a progressive decrease in avulsion period. A coarsening-upward sequence results from increasing channel-deposit proportion and connectedness as avulsion period decreases. Channel belts become concentrated near tectonically down-thrown areas of the floodplain and may offlap away from these areas in response to changes in floodplain elevation due to local deposition and shift in avulsion locations. Apparent dimensions, proportion, and connectedness of channel-belt deposits can vary dramatically as a function of cross-section orientation and location relative to points of avulsion and areas of preferred tectonic subsidence.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994