Abstract

Paleocurrent measurements in the middle Tertiary Brule Formation of northwest Nebraska indicate that ancient streams drained the Laramie Mountains and Hartville Uplift in Wyoming, flowing eastward onto the Great Plains. Uplift in central Wyoming in late Eocene-early Oligocene time provided the impetus to change north-northwest flowing streams in Eocene time to east flowing streams during the Oligocene. Minor structural activity in the Black Hills of South Dakota may have provided sediment for deposition in northwest Nebraska, and also may have diverted northeast flowing streams to the east or southeast. Other factors affecting deposition of the Brule Formation include increasing aridity through the Oligocene, erosion of highlands on the west, and volcanic activity supplying detritus for deposition, especially during deposition of the Whitney Member.

Sedimentary structures in the Brule Formation include cross-stratification, parting lineation, sole marks, ripple marks, grain orientation, disturbed bedding, and graded bedding. They are present principally in the Orella Member of the Brule Formation, and are rare in the overlying Whitney Member. The Orella Member is a fluvial unit with both channel and flood plain beds and features present. The Whitney Member reflects extensive eolian activity during deposition and consists dominantly of fine-grained rocks with very few sedimentary structures.