Characterizing and interpreting sand-body clustering in the Ferris Formation, Hanna Basin, Wyoming

A primary goal of sedimentary geologists is to interpret past tectonic, climatic, and eustatic conditions from the stratigraphic record. It is typically assumed that deposits accumulated over timescales of 104-106 years dominantly record basin boundary conditions such as subsidence or sediment and water supply rates. Consequently stratigraphic changes in alluvial-basin fills are routinely interpreted as the result of past tectonic movements or changes in climate or sea level. Recent physical and numerical models have shown that sedimentary systems can exhibit self-organization on basin-filling time scales, suggesting that structured stratigraphic patterns can form spontaneously rather than as the result of changing boundary conditions. The Ferris Formation (Upper Cretaceous/Paleogene, Hanna Basin, Wyoming) exhibits stratigraphic organization where clusters of closely-spaced channel deposits are separated from other clusters by intervals dominated by overbank material. In order to determine the potential for autogenic self-organization in this ancient deposit, spatial-point-process statistics are used to quantitatively compare the stratigraphic pattern of the Ferris Formation to stratigraphy from a physical experiment designed to highlight intrinsic variability in alluvial depositional systems. The spatial patterns of auxiliary variables such as maximum grain size, paleoflow depth, and paleocurrent direction are examined throughout the Ferris Formation in order to determine the factors that controlled channel avulsion and deposition within the basin.;The Ferris Formation is shown to have similar stratigraphic characteristics to the experimental deposit suggesting that autogenic organization of channel avulsions over basin-filling timescales may have played a role in constructing the clustered stratigraphic pattern observed in the unit. Additionally the study area lacks strong trends sand-body properties through the stratigraphic succession and between cluster groups. Consequently there is no indication that the stratigraphic pattern observed in the Ferris Formation was driven by systematic changes in climate, tectonics, or sea level.