Mesozoic and Cenozoic Structures of Pocatello Region: Transition from Thrust Belt to Hinterland, Southeast Idaho

William D. Burgel, David W. Rodgers, Paul Karl Link

Remapping of upper Proterozoic and Cambrian rocks in the northern Bannock Range south and east of Pocatello, Idaho, indicates a polyphase deformation history characterized by both shortening and extension. Map-scale folds and faults, related to shortening in the transition zone between the Idaho-Wyoming thrust belt and the hinterland to the west, are present, but have been offset and rotated along superimposed normal faults.

The oldest structure evident is the north-trending, east-vergent Rapid Creek fold. This fold involves upper Proterozoic and Cambrian strata in the hanging wall of the Putnam thrust fault. The fold is truncated by east-striking subvertical faults that are inferred to be coeval tear faults. Fold and fault geometries suggest the Rapid Creek fold formed over a footwall ramp, whose location may have been controlled by basement normal faults related to Proterozoic rifting. Further evidence of Mesozoic uplift lies in the sub-Neogene unconformity on Cambrian rocks in the Pocatello area, compared to areas of the thrust belt 15 km (9.3 mi) to the northeast, where similar tuffaceous strata rest on Triassic limestones, indicating 7 km (4.3 mi) of pre-Neogene erosion near Pocatello.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.