Comparing basin-scale avulsion dynamics in three ancient fluvial systems to understand how floodplain sedimentation influences avulsion patterns

An understanding of the lateral and vertical distribution and geometry of channel-belt sandstones can be inverted to reconstruct the filling history of fluvial sedimentary basins, and provides insight into the relationship between channel avulsion and floodplain aggradation.

Previous studies document relationship between channel and floodplain deposits and the role of allogenic and autogenic controls on basin architecture. Basin-filling models have also been used to evaluate the effect of sediment flux, discharge, floodplain aggradation rate, and subsidence on fluvial stratigraphy. However, reconciling predictions from models with field data is difficult due to scale differences between model results and outcrop exposures.

In this study, we measured the scale and distribution of MSBs using photo panels and digital outcrop models of three ancient fluvial systems within which changes in channel and floodplain deposition have been documented: the Paleocene-Eocene Willwood Formation (Bighorn Basin, Wyoming, USA) and Wasatch Formation (Piceance Basin, Colorado, USA); and the Miocene Huesca Fan (Ebro Basin, Spain). Using a scaled 2D basin-filling model in which avulsion pattern, channel incision and floodplain aggradation rates are changed, we evaluated MSB stratigraphic architecture under a range of conditions that were permissible at the time of deposition of these units. We compared the net-to-gross of scaled sections of modelled stratigraphy to that recorded in the field, to test the statistical likelihood of reproduction of field stratigraphy through solely autogenic forcing.

Model results show that clustered avulsion stacking patterns generally produce high rates of channel reoccupation, and stratigraphy similar to that observed in the Willwood and Huesca outcrops as opposed to Wasatch sections. Our results suggest that avulsion pattern is highly sensitive to floodplain-sedimentation patterns in some systems but not in others. Field observations suggest that differences in floodplain elevation and rates of incision are more likely to cause changes in avulsion frequency and patterns in these systems. These results underscore the importance of constraining controls on floodplain deposition patterns in order to understand long-term controls on avulsion flow-path selection.