Abstract

The Bearpaw Shale, Fox Hills Sandstone, Lance Formation, and equivalent strata record the last major regression of the western margin of the Cretaceous interior seaway from the craton of North America. In Montana and Wyoming, the Bearpaw (= Pierre) Shale and Fox Hills Sandstone have a gradational boundary over a thick interval of highly bioturbated claystone, siltstone, and fine-grained sandstone. This gradation records a transition from an outer shelf to a foreshore environment. A distinctive succession of sandstone units punctuates this transition and shows a systematic increase in both thickness and complexity of internal stratification. The lower 10 to 20% of this interval contains thin (0.3 to 1 m), laterally-continuous, fine-grained sheet sandstone bodies characterized by erosional bases, internal horizontal bedding, and topped by ripple-drift cross-laminae. The lateral continuity, lack of bioturbation, and stratigraphic position within surrounding lithotypes suggest deposition during episodic storm-surge conditions on the outer shelf. The middle 20 to 30% of the Bearpaw-Fox Hills transition contains sheet- to lenticular-shaped sandstone units characterized by erosional bases, horizontal bedding, and by the dominance of hummocky cross-stratification. These units record deposition under combined-flow, storm-generated current conditions within the inner shelf. The upper 50 to 60% of the transition is a coarsening-upward succession characterized by large-scale sand bodies with a distinctive ridge-and-swale depositional topography. Ridge crests strike NNW-SSE, and are approximately contour parallel to presumed paleoisobaths of the Maestrichtian shelf. Internally, these sandstone bodies have erosional bases, low-angle inclined bedding, hummocky cross-stratified intervals, and trough cross-stratified channel scours near the crests. Ridges are overlain by marine strata and evidence of subaerial exposure has not been found. The ridge system was apparently nucleated during storm-surge conditions, and later modified by contour-parallel currents. The entire transitional interval is overlain by foreshore, strand plain, and continental sediment, marking the final withdrawal of the seaway from the western craton of North America.