THE FIRST CUT IS THE DEEPEST: HUGE, INCISED CHANNELS AT THE BASE OF THE TERMINAL CRETACEOUS HELL CREEK FORMATION

During the Late Cretaceous, tectonic movement in western North America, related to thrusting and foreland deformation, contributed to regional uplift in the area of what is now south-central Montana on the western edge of the Western Interior Seaway. One result of the rapid drop in base level can be seen near Jordan, Montana. A 30 meter thick channel sand at the base of the Hell Creek Formation is incised into the underlying aeolian and subaqueous sands of the Fox Hills Formation. Several channels of this thickness and approximately 50 meters wide incise and meander through the Fox Hills Formation. The cliff-forming, cross-bedded, medium-grained beach sands of the Fox Hills Formation remained as islands until the basal channels of the Hell Creek coalesced to the northeast and created a channel approximately 5 kilometers wide. After the convergence of the basal channels, the aeolian sands of the Fox Hills Formation are completely obliterated and the thickness of the Fox Hills Formation is reduced to approximately 6 meters. Although intervals of channel deposits occur throughout the Hell Creek section, the basal channel, marking the change from the underlying Fox Hills Formation, is the only channel that shows incision. The thickness and width of the basal channel suggests a high energy depositional environment. Fossils have not been found within this basal channel sequence of the terminal Cretaceous Hell Creek Formation.