Carbonate Niobrara Group in the Piceance Basin
This research helps resolve an ongoing debate about the extent of carbonate rocks in Niobrara Group into the western part of the Cretaceous Western Interior Basin. It was commonly accepted that these sediments are present in the Denver Basin where they consist of chalk and interbedded shale and that the chalk facies did not extend further to the west into the Piceance Basin. In the western part of Colorado intervals below the Late Cretaceous Castlegate Sandstone and above the Early Cretaceous Dakota Sandstone are known as Lower Mancos Shale and Mancos B shale/siltstone. Identification of interbedded carbonate and shale section thicker than 300 ft, corresponding to Lower Mancos Shale interval, in the New Castle section road cut located along highway I-70 (N39o34’352”, W107o28’820”) allows for the carbonate lithologies in the Niobrara Group to be extended to the Piceance Basin. The New Castle section also contains Early Cretaceous very fossiliferous Juana Lopez Limestone and very fine grained Early Cretaceous Frontier Sandstone which can be easily identified and were deposited below the Niobrara Group. Gamma ray-spectrometer logging of the formations was conducted in the outcrop and the gamma ray log was correlated to geophysical logs from the Douglas Creek Arch in the western Piceance Basin. The results suggest that carbonates in the Niobrara Group and Frontier Sandstone extend further west to the Douglas Creek Arch. However, Juana Lopez Limestone is difficult to identify on the geophysical logs from the Arch due to its very thin nature. Further petrologic and stratigraphic analysis will determine the extent and establish the thickness of the carbonate interval in this area. Scanning Electron Microscope techniques are incorporated to characterize lithology and faunal content of the carbonates and interbedded shales. The Niobrara terminology used in the Denver Basin should be adopted in the Piceance Basin and separated from the Mancos Shale terminology.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009