Sandstone body character, river styles, and geomorphology of the lower Eocene Willwood Formation, Bighorn Basin, Wyoming, USA

Youwei WANG, Timothy Baars, Hiranya Sahoo, Joep Storms, Allard Martinius, Philip Gingerich, Hemmo Abels

The lower Eocene Willwood Formation of the Bighorn Basin, Wyoming, USA, is an alluvial succession with a sand content varying around 25%. It has been studied intensively for palaeontology, palaeoenvironments and palaeoclimates, as well as sedimentological and stratigraphic analysis. Channel dynamics were studied at a relatively low resolution throughout the basin over the geological time from late Palaeocene to early Eocene. Here, a high-resolution study is reported to complement previous research at the basin scale. Efforts are made to document the characteristics and river planform styles of most sandstone bodies encountered through ca 300 m of alluvial stratigraphy in a 10 km2 area of the Deer Creek part of the McCullough Peaks area situated in the basin axis of northern Bighorn Basin. Four channel facies associations are recognized and ascribed to four river planform styles: crevasse channel, trunk channel, braided-like channel and sinuous-like channel, with the latter two types dominant. Braided-like and sinuous-like channel sandstone bodies differ significantly in thicknesses, being on average 6.1 m versus 9.0 m, but they have similar palaeoflow–perpendicular widths of on average 231 m and palaeoflow directions of on average N 003°. Braided-like and sinuous-like river planform styles show no spatial dependency in the 10 km2 study area. Results of this study are in line with existing basin-scale depositional models that are composed of a single axial system fed by several transverse systems dominantly from the west. The feeding of these systems could be influenced by palaeoclimate changes possibly controlling their contribution over time, thereby impacting river planform styles. At the same time, changing water discharge hydrograph, sediment load, and overbank cohesiveness may have equally driven the observed river planform style changes within the basin without a major role of catchments.