MORPHOLOGY AND SIGNIFICANCE OF DIFFERENT FLUVIAL SANDSTONES BODIES IN THE EARLY EOCENE WILLWOOD FORMATION, BIGHORN BASIN, WYOMING

Detailed examination of the alluvial architecture in a stratigraphic interval of the early Eocene Willwood Formation (Bighorn Basin, Wyoming) shows the variability of fluvial sandstone bodies. Large-scale multisory sandstones are up to 10 m thick and 1.5 km wide and consist of medium to coarse sand. Although few in number, they represent most of the sand in the study interval. Small-scale multistory sandstones are 6-7 m thick, consist of fine to medium sand, and form laterally traceable sheets over distances of hundreds of meters. Single-story ribbon sandstones with width/thickness ratios of ~5 are isolated in mudrocks. Associated with the simple ribbons are thin (<2m) sheet sandstones that can be traced laterally for up to 2 km.

Analysis of the relationship among these different sandstone types shows the complexity of the fluvial system. The large-scale sheets are interpreted as the trunk channels. The smaller multistory sandstones formed by the amalgamation of channel deposits from a single sinuous channel. The solitary ribbons are commonly grouped at particular stratigraphic levels that formed channel complexes with subparallel or what appear to be convergent paleoflows. Together with the ribbons they are interpreted as crevasse splay complexes and probable avulsion belt deposits. The stratigraphic interval also shows fine-grained channel fill deposits of two kinds. They are interpreted as abandoned channel fills and muddy channel belt deposits.

Mapping in the study area shows evidence of topographic control exerted by earlier channel sands on later channel position and sandstone deposition. This study also points to the significance of anastomosed channel belts as temporary, unstable elements of an otherwise meandering fluvial system.