Date of this Version
DeGraw, H.M. The Pierre-Niobrara Unconformity In Western Nebraska. The Geological Association of Canada Special Paper Number 13
The Pierre-Niobrara unconformity, one of several significant unconformities in the Cretaceous System of the Western Interior region, has not generally been recognized. Detailed correlation of electric logs of a large number of wells in western Nebraska provides evidence of its existence. Isopach maps of the beds occupying three identifiable stratigraphic intervals - a redefined Niobrara Formation, an unnamed uppermost Niobrara unit, and an unnamed basal Pierre unit, which includes the Ardmore Bentonite - provide recognizable geologic patterns that permit reconstruction of the stages of development of the unconformity. The upper part of the Niobrara Formation has been truncated in several areas of western Nebraska. Truncation is most pronounced in northwestern Nebraska, where the unnamed uppermost Niobrara unit, elsewhere more than 100 feet thick, is entirely absent. Throughout much of western Nebraska, however, such truncation is subtle and not easily recognized. The unnamed basal unit of the Pierre Shale consists of silty shale and bentonite. Although discontinuous, it attains a thickness of more than 60 feet in at least one locality. Early Pierre topography appears to have been structurally controlled, uplands tending to coincide with uplifts. The basal Pierre unit is especially well developed in two south-southwest-trending troughs which appear to coincide with synclines. The silty shale well may be of fluvial origin, having been derived from the Niobrara strata of adjacent uplands. Detailed mapping and analysis of unconformities, such as the one between the Niobrara and Pierre Formations, enables the geologic history of the Cretaceous Western Interior region to be interpreted more fully.