Bulletin 115—Stratigraphy of the Upper Cretaceous Niobrara Formation in the Raton Basin, New Mexico

By G. R. Scott, W. A. Cobban, and E. A. Merewether, 1986, 34 pp., 3 tables, 16 figs.

Contributes new detailed descriptions of the stratigraphy, fossils, and organic matter in the Niobara Formation in northeastern NM. The discovery of natural gas in the Denver Basin focused much attention on the Niobara. Correlating the formation in the Raton Basin with the Niobara near Pueblo, Colorado, the authors found very similar faunal sequences but significant lithologic differences. Additionally, the authors collected and analyzed samples from eight localities as part of an evaluation on oil and gas resources in the Raton Basin.

The purpose of this report is to present new information about the stratigraphy, fossils, and organic matter in the Upper Cretaceous Niobara Formation in the Raton Basin, NM. This study contributes stratigraphic sections and descriptions of all parts of the Niobara, descriptions and illustrations of the marine molluscan fossils present, and a tabulation and appraisal of some constituent organic matter. The rock units and fossils of the Niobara Formation in the Raton Basin are correlated with those near Pueblo, Colorado. The present work provides a biostratigraphic framework for future, more detailed investigations of the petrography, mineralogy, sedimentary structures, depositional environments, and other aspects of the Niobara Formation.

The index fossils of the Niobara are time-equivalents of fossils in other parts of the world and permit correlation of the Niobara rocks with rocks elsewhere. Many of the fossil species are the same as those listed for the Pueblo area. The remaining species, which typify some of the faunal zones of the Coniacian, Santonain, and Campanian Stages, have been found in other regions. The discovery of gas in the upper chalk unit of the Smoky Hill Shale Member of the Niobara Formation in the eastern part of the Denver Basin in 1972 focused much attention on the formation. The upper chalk unit in the Denver Basin is 20–50 ft thick and is regarded as a primary reservoir that has high porosity and low permeability; however, the permeability is enhanced by natural and induced fractures. Unfortunately, in northeastern NM the upper chalk is almost entirely replaced by calcareous shale that contains orange-weathering limestone. In this region, the upper calcareous shale probably has little potential for the production of gas or oil, although it probably is a source rock for oil and gas in some areas.