Abstract

As a part of studies of low-permeability gas reservoirs in the northern Great Plains, the regional geologic setting of the Niobrara has been investigated in North Dakota, South Dakota, and Nebraska. In North Dakota and South Dakota, 2 Niobrara chalk tongues grade into calcareous shales, which intertongue northward and westward with thick noncalcareous shales of the Pierre Shale. In Nebraska, 3 Niobrara chalk tongues and intervening calcareous shales thicken southward as a unit. An isopach map of the interval from the Niobrara base to the Ardmore Bentonite Bed in the overlying Pierre Shale shows an oblate area of thin sediments extending from southwest to northeast out of the Nebraska Panhandle and into central South Dakota. Erosion along the basal Niobrara unconformity is greatest in the area of thin strata and an unconformity is locally developed on the top of the Niobrara in that same area. Structural contours of the Ardmore Bentonite Bed suggest that the area of thin Niobrara strata presently approximates the south flank of the Williston Basin and north flank of the Denver and Kennedy basins. Chalk tongues are interpreted as low-angle shelf surfaces, known as carbonate ramps, which sloped gently to the northwest and southeast off a paleotectonic high. The paleotectonic high cut obliquely across the seaway and was close to the position of the Transcontinental Arch that influenced Paleozoic sedimentation. The present-day stratigraphy and structural setting of the Niobrara are different north and south of the arch crest.--Modified journal abstract.

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