AUTOCYCLIC AVULSION CLUSTERS IN ALLUVIAL BASINS: EXAMPLE FROM THE MAASTRICHTIAN/PALEOCENE OF WYOMING

Avulsion is the dominant process controlling the stacking pattern of channel-belt sand bodies. River avulsions typically involve abrupt abandonment of channels and recur on time scales of millennia or less. Local avulsion involves short distance displacement of channels such that a river may rejoin its former channel downstream. Regional avulsions are displaced farther and may not rejoin the previous channels. In addition, a larger-scale avulsion-controlled architecture can be seen in some alluvial basins. Avulsion clusters are groups of individual avulsive sand bodies separated from other clusters by muddier basin fill that only contains sporadic channel-belt bodies. Where observed in the Maastrichtian of Wyoming (Ferris and Lance formations), clusters record up to a few tens of individual sand bodies dispersed over a cross-valley width of a kilometer or less. Vertically, individual clusters are separated by muddy lanes of dominantly flood plain deposits.

Where described elsewhere these features have been interpreted as controlled by changes in relative sea level. In this view, the cluster-lane succession is tied to varying rates of aggradation controlled directly by rates of relative sea-level rise. However, where seen in long cross-strike sections in Wyoming, muddy lanes appear to run laterally into avulsion clusters, suggesting there is always an avulsion cluster somewhere in the basin. Lateral spacing of clusters is on the scale of kilometers and, thus, may be missed in outcrops or in subsurface observations of more local extent.

In contrast, we interpret clusters to be composed of local avulsions in one part of the alluvial basin that build topography to the point that regional avulsion occurs moving the river channel to a lower part of the basin. In this view avulsion clusters are autocyclic events that occur less frequently and over larger areas than the local avulsions that compose the cluster. As such, these events are not necessarily tied to sea level.