High-resolution chronostratigraphy and isopach maps of the Niobrara Formation and Sharon Springs Member of the Pierre Shale (Cretaceous, Western Interior seaway) were constructed using data from >700 well logs in E Colorado, NW Kansas, W Nebraska, SW South Dakota, and SE Wyoming. The Niobrara Fm. was divided into 14 chronostratigraphic units and the Sharon Springs into 2 units. The results were used to determine spatial and temporal variation in sedimentation and in organic-matter content in these two units. The variations in sedimentation, in turn, were interpreted as variations in biologic productivity, clastic influx, or paleobathymetry and related bottom currents. Variations in organic-matter content were interpreted as variations in biologic productivity or dilution by clastic influx. The relationship of the sediment-water interface to storm wave base was interpreted from the distribution and evolution of unconformities.

The conclusions resulting from this work were: (1) spatial and temporal variation in sedimentation were controlled principally by variations in carbonate productivity and paleobathymetry; (2) the Niobrara Formation was never far below storm wave base and sea level did not change during Niobrara deposition; (3) areas of high organic-matter content shifted throughout the interval except for persistent elevated organic-matter content at the southern end of the study area, and there was a long-term secular trend of increasing organic-matter content through both units. The lack of a consistent geographic pattern in the distribution of organic carbon may be an artifact of the limitation of the study area to the deepest part of the basin. Studying the margins in a similar fashion, however, may be difficult because the sedimentation patterns there are strongly influenced by factors extrinsic to the basin and the signal may be obscured.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah