FLUVIAL DEPOSITION DURING THE EARLY EOCENE IN THE SOUTHERN BIGHORN BASIN (WYOMING, U.S.A.)

River channel morphodynamics are influenced by boundary conditions and internal behaviors of the sediment transport system. These morphodynamics are subsequently recorded in the alluvial stratigraphy. This feature of stratigraphy allows researchers to reconstruct the response of Earth's sediment transport systems to past changes in tectonism, eustasy, and climate. This study examines fluvial sandbodies within the earliest Eocene strata of the Bighorn Basin located in northwest Wyoming, U.S.A. Eocene deposition is represented by the Willwood Formation, which is composed of three lithofacies associations indicative of in-channel fluvial deposition, proximal floodplain crevasse splay deposition, and distal floodplain deposition. We focused on fluvial sandbodies in the southern portions of the Bighorn Basin near Worland, Wyoming. We documented the thickness and lateral extent of fluvial sandbodies as well as internal architecture of the units, and compared these with age equivalent strata in the northern Bighorn Basin from past studies. Fluvial deposition in the southern Bighorn typically produced lenticular sandbodies with 1-3 stories less than 10 meters thick and several tens of meters wide. The sandbodies exhibit fining upwards patterns in grain size and are dominated by trough cross-bedding and ripple cross-lamination with occasional soft sediment deformation near the tops of the unit. Paleocurrent measurements suggest paleodrainage to the northwest, and when combined with recent provenance analyses, indicate erosion of Cretaceous-aged marine shale units within the Bighorn Mountains to the east and southeast. The differences between southern and southeastern fluvial deposition and northern fluvial deposition appears to be the overall size of the sandbodies. In the north, near Cody and Powell Wyoming, fluvial sandbodies can exceed 20 meters in thickness and average ~0.7 km in width. We attribute these differences to a combination of differences in 1) catchment area and 2) provenance. In the south the catchment area is restricted to the Bighorn Mountains whereas the northern rivers' catchment area includes all surrounding Laramide uplifts and potentially distal sources in the Sevier fold-and-thrust belt. Furthermore, the provenance of the southern rivers is dominated by erosion of Cretaceous marine shales, which would yield comparatively little sand-sized sediment. This contrasts with other Laramide uplifts in the basin that had already unroofed their entire sedimentary cover as well as crystalline basement