Field test of autogenic control on alluvial stratigraphy (Ferris Formation, Upper Cretaceous-Paleogene, Wyoming)

Internally generated (autogenic) sedimentary processes can obscure signals of tectonic movements, climate conditions, or sea-level fluctuations in alluvial basins. The stratigraphic effects of autogenic dynamics have been considered negligible on time scales of 10 (super 4) -10 (super 6) yr. However, recent physical and numerical models have shown that over basin-filling time scales, self-organization in sedimentary systems can produce stratigraphic patterns similar to those resulting from changing basin boundary conditions. Here, we present the first field-based test for autogenic control on stratigraphy in an ancient alluvial basin. The Ferris Formation (Upper Cretaceous-Paleogene, Hanna Basin, Wyoming) is composed of clusters of closely spaced channel deposits separated by intervals dominated by overbank material--a pattern that has been proposed as an example of a potentially autogenic stratigraphy. In order to evaluate controls on Ferris Formation stratigraphy, spatial patterns of key channel properties were analyzed. These variables (including sand-body dimensions, paleoflow depth, maximum clast size, paleocurrent direction, and sediment provenance) were chosen because they can change in response to climate, tectonic, or sea-level forcing and are commonly measurable in ancient alluvial successions. No clear statistical trends were detected in the measured variables, which suggests that external forces did not likely control stratigraphic organization in the unit. This study demonstrates that autogenic dynamics in natural, field-scale systems can produce organized stratigraphic patterns over much larger spatial and temporal scales than is typically presumed. This finding emphasizes the need for further understanding of autogenic stratigraphy and greater care interpreting climate, tectonic, and eustatic changes from alluvial basins.