COASTLINES OF THE CRETACEOUS FOX HILLS SANDSTONE IN THE DENVER BASIN, COLORADO

The paleo-coastlines of the Cretaceous Interior Seaway in the Denver Basin area can be reconstructed by mapping the beach and shoreface deposits of the Maastrichtian Fox Hills Sandstone using subsurface data and outcrops. The Fox Hills Sandstone is considered to be a regressive package of sandstone beds deposited during the retreat of the Interior Seaway in a typical off lapping or shingling pattern, however episodic landward stepping events can be identified as well. Subsurface information from a dataset of over 1350 geophysical well logs that penetrate the Fox Hills Sandstone and from outcrops along the basin edge between Colorado Springs and the Wyoming border were used to set up a stratigraphic framework. Twelve linear beach ridges are identified, with a maximum mapped strike length of over 250 kilometers. The beach and upper shoreface ridges average approximately 8-10 kilometers in width. The beach ridges step up and are progressively younger in an eastward direction, spanning about 100 m of overall stratigraphic section. In part as a result of this stepping up pattern, isopach maps of the overlying Laramie Formation show a systematic thinning to the east from a maximum of about 200 meters in the west to less than 60 meters to the east. Coastlines were generally linear with occasional embayments. Landward stepping units appear in the western and deepest parts of the Denver Basin, suggesting early subsidence and increased accommodation probably due to the onset of the Laramide Orogeny. In these cases, Laramie Formation facies (including coal beds) occur within the package of shoreface sandstone beds typically ascribed to the Fox Hills Sandstone by field mappers. Changes in the stacking pattern of the Fox Hills Sandstone are systematic and well defined. These patterns control the net thickness of sandstone at any given locality and our mapping has been useful to water resource planners who consider the potential of the Fox Hills Sandstone as a bedrock aquifer in the Denver Basin.