FOX HILLS SANDSTONE SHINGLE STRATIGRAPHY IN THE DENVER BASIN, COLORADO

The Fox Hills Sandstone in the Denver Basin represents the near-shore and beach deposits deposited as the Cretaceous Interior Seaway retreated from the Denver area in Maastrichtian time. Because of the episodic retreat of the seaway, these sandstone beds accumulated in a succession of vertically stacked and off-lapping features that can be mapped in the subsurface across the entire basin area.

Outcrops are modest but serve to confirm the subsurface observations and allow detailed facies relationships to be seen and sampled. Particularly good exposures of stacked shingles occur in the White Rocks area in Boulder County. Our work identifies about ten shingles in the Denver area and closely spaced wells allow our subsurface units to be tied to the surface.

Thickness patterns and stacking architecture suggest that there may have been early subsidence and increased accommodation peripheral to the site of the future Front Range during Fox Hills deposition as far back as 70 MY. Ammonite bio-stratigraphic patterns mapped out by Bill Cobban of the U.S. Geological Survey have been tied into the subsurface allowing the Denver Basin record to be placed in a regional context.

Our observations and depositional patterns compare favorably with other settings in the Interior Seaway, particularly work by Paul Devine on the Point Lookout Sandstone in the Four Corners area and by Cooper Land in the Fox Hills Sandstone of the Greater Green River Basin in Wyoming.