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Analysis of the Contact Between the Fox Hills and Laramie Formation Using Subsurface Data
RAYNOLDS, Robert G., Denver Museum of Nature & Science, 2001 Colorado Boulevard, Denver, CO 80205-5798

A series of cross sections have been prepared using electric logs to illustrate the character of the contact between the Fox Hills and Laramie formations in the Denver Basin. The Fox Hills is represented by shallow marine to nearshore and beach facies sandstones and laminated sandstones and shales. It typically has a very distinct top surface and a gradational base into the underlying Pierre Shale. In the Denver Basin, the overlying Laramie Formation is generally shale-rich, contains coals, and isolated fluvial channel sandstone beds. The coals are concentrated in a coal zone commonly within about 100-150 feet from the top of the Fox Hills. The Laramie represents fluvial, overbank, and swamp environments.

As the Fox Hills prograded to the northeast, it rose stratigraphically in a series of shingles. This aggadational pattern has the effect of thinning the overlying Laramie Formation from below, and represents one reason why the Laramie is observed to be thinner on the east side of the basin than on the west. Landward of these aggradatinal step-ups of the Fox Hills there is preliminary evidence suggesting coals may be particularly well-developed.