Preliminary report on the Pierre Shale in North Park, Colorado, and its correlation to Boulder, Middle Park, and Northwest Colorado

Abstract

The Pierre Shale consists of interbedded marine shale and sandstone representing nearly continuous marine sedimentation during Late Cretaceous time. The Pierre Shale and time-equivalent marine units in the western interior United States contain 28 ammonite range zones. The ammonites show such rapid evolution and such wide geographic extent that their range zones are useful for time-stratigraphic correlation of marine rocks. The Pierre Shale is exposed around the margins of the North Park basin in Colorado where its maximum thickness is about 5,500 feet. The formation overlies the Upper Cretaceous Niobrara Formation and is unconformably overlain by the Paleocene and Eocene Coalmont Formation. The Pierre Shale, which consists of an upper sandy member and a lower shaly member, is primarily marine in origin; however, the sandy member includes minor amounts of coal. Twelve ammonite range zones can be recognized in North Park, including ammonites as old as the early Campanian Baculites obtusus and as young as the early Maestrichtian Baculites eliasi. Zonal distribution indicates that the contact between members becomes older to the northwest. Comparison of the biostratigraphy of the Pierre Shale in North Park with that of the Pierre Shale near Boulder, Colorado, and in Middle Park, and of time-equivalent units in northwestern Colorado, indicates the following: (1) the Pierre Shale in North Park correlates to the upper Mancos Shale, lies Formation, and the lower Williams Fork Formation in northwestern Colorado; (2) two of the three strandlines, represented by nonmarine deposits in the sandy member, correlate to regionally recognized regressions of the Pierre sea; (3) parts of the North and Middle Park structural basin were eroded 1,900-2,300 feet deeper than the Boulder area during the Laramide orogeny; and (4) northwestern North Park was eroded 2,000 feet deeper than northeastern North Park and the area of the stratigraphic section in Middle Park.