Bulletin-20—Stratigraphy of the Colorado Group, Upper Cretaceous, in Northern New Mexico

By C. H. Rankin, 1944, 30 pp., 6 figs., 1 index.

In previous reports on the Cretaceous rocks of NM, writers have followed the nomenclature established by Cross, and have described all the rocks between the Dakota Sandstone and the Mesaverde coal measures as Mancos Shale. Lee and Darton, in describing the rocks of central and northeastern NM, recognize that some subdivisions of the Colorado group can be identified; but in their measured sections these authors don't differentiate formations.

Since 1925 the writer has measured a number of sections of Cretaceous rocks in northern NM while studying the economic geology and oil resources of the region. A study of these sections, which are presented herewith, supplemented by information obtained from the examination of well cuttings, reveals that all divisions of the Colorado Group as described in southern Colorado, except the Fort Hays Limestone and the Apishapa Shale, can be recognized in northern NM. The purpose of this paper is to identify and describe these divisions. The writer has integrated his studies with those of Lee, and also with the excellent descriptions of the rocks of the Colorado Group given by Dane, Pierce, and Reeside.

The recognition and study of the divisions of the Mancos Shale throws some interesting light on a phase of the geologic history of northern NM that has previously received only little attention. The persistence of the Greenhorn Limestone over most of northern NM aids materially in solving the problem of the Dakota Sandstone and contemporaneous strata in this and other areas.