Abstract
Detailed examination of the upper part of the Montana group in the vicinity of Golden and Morrison, Jefferson County, Colorado, has revealed the presence of a series of beds more than 1,200 feet thick containing a well developed and varied Fox Hills fauna. These beds have been divided, on the basis of lithology and fauna, into four zones which are continuous and mappable for at least 20 miles along the outcrop. The base of the Fox Hills sandstone as restricted by the United States Geological Survey lies more than 1,000 feet above the first appearance of a typical Fox Hills fauna in the section. The basal members of the overlying Laramie formation rest on different zones of the upper Montana at different localities within the area. This has been interpreted as indicating a disconformity between the two, at least in the Golden area. Evidence is cited to substantiate this interpretation. The Montana shales below the restricted Fox Hills are readily divisible into upper, middle, and lower Pierre, as has been established in northeastern Colorado. The middle Pierre, including several prominent sands, is particularly well developed in this area. Structural and historical implications of the interpretation are discussed.
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