Interpretation of 250 mi (400 km) of reflection seismic data in conjunction with surface maps and well data along the east flank of the Denver basin, reveals two distinct types of Late Cretaceous faulting. An early Laramide, basement-controlled fault system is the dominant structural style in the zone of flank deformation. Basinward from the fault system is an associated new tectonic style which has now been recognized in the Cretaceous foreland basin. Deltaic sedimentation and overpressured shale masses initiated a shallow-depth growth fault system similar to the tectonic style of many Cenozoic sequences along continental margins.

The shallow growth fault system is approximately 10 mi (16 km) wide and 30 mi (48 km) long, and affects the uppermost Cretaceous strata. Seismic data indicate three or four major trends of listric normal faults that do not appear to extend below a depth of 5,000 ft (1,524 m) in the Pierre Shale. Antithetic horst-graben fault blocks are found on the basinward side of each major fault. Near-surface growth fault movement is indicated by a five-fold thickening of the Fox Hills Sandstone from a normal 75 ft (22.9 m) to 400 ft (121.8 m), and the presence of thicker mineable coal beds in the Laramie Formation in downthrown blocks.

Recognition of growth fault systems will play an important role in future exploration for petroleum and coal in the Rocky Mountain region.