USING SPATIAL STATISTICS TO IDENTIFY CONTROLS ON RIVER AVULSION AND FLUVIAL SAND-BODY ORGANIZATION IN THE FERRIS FORMATION (CRETACEOUS/PALEOGENE, HANNA BASIN, WYOMING)

The stratigraphic record of alluvial basin-fills provides an important window into the complex interactions of climate, tectonics, and river behavior. Autogenic river processes, particularly channel avulsion, can act to filter and obscure sedimentary signals of climate change and surface deformation. In particular, recent work has shown that channel avulsions may be self-organized on basin-filling time scales, producing stratigraphy which traditionally would be interpreted as resulting from extrinsic forcing. The Ferris Formation (Upper Cretaceous/Paleogene, Hanna Basin, Wyoming) provides a rare opportunity to study basin-scale organization of fluvial deposits, as it has been rotated to a dip of nearly 90 degrees and is exposed in cross-section on the present-day land surface. The unit was deposited in a rapidly-subsiding basin during the Laramide Orogeny, and exhibits stratigraphic organization where clusters of closely-spaced channel deposits are separated from other clusters by large intervals of overbank deposits. It is unknown whether this stratigraphic pattern reflects tectonic or climatic controls on the depositional system, autogenic organization of river avulsions, or some combination thereof. In order to ascertain controls on the stratigraphic distribution of channel deposits in the Ferris Formation we employ statistical methods to model and characterize the spatial distribution of channel bodies. We use field data on auxiliary variables such as maximum grain size, paleoflow depth, paleocurrent direction, provenance, and channel architecture, and examine the distribution of these variables within and across the channel-body clusters. With this approach it is possible to identify which of these variables are spatially correlated to the overall channel-body distribution, and thus indicative of the factors controlling channel avulsion and deposition.