ABSTRACT: Cretaceous Strata of the Raton Basin, New Mexico and Colorado
C. L. Pillmore, E. G. Kauffman
Cretaceous rocks in the Raton basin range in thickness from a maximum of about 1370 m in the Colorado portion to a minimum of about 1120 m in the eastern part of the New Mexico portion. Composite stratigraphic reference sections were selected in both areas as part of the Western Interior Cretaceous Project of the Global Sedimentary Geology Program.
Cretaceous strata in the basin include, in ascending order: the Aptian(?)-Albian Purgatoire Formation, which lies unconformably on the Upper Jurassic Morrison Formation and consists of the Lytle Sandstone and the Glencairn Shale Members; the upper Albian to lower Cenomanian(?) Dakota Sandstone; the lower to middle Cenomanian Graneros Shale; the upper Cenomanian to middle Turonian Greenhorn Formation, which includes the Lincoln Limestone, Hartland Shale, and Bridge Creek Limestone Members; the middle to late Turonian Carlile Shale, which comprises the Fairport Chalky Shale, Blue Hill Shale, Codell Sandstone, and Juana Lopez Members, and the unnamed upper black shale member; the Niobrara Formation, which spans the uppermost Turonian, the Coniacian, the Santonian, and the lower Campanian and contains the Fort Hays Limestone and Smoky Hill Members; the Campanian and lower Maastrichtian Pierre Shale, which consists of an unnamed lower transition member, the Apache Creek Sandstone and Sharon Springs Members, a middle member, and an upper transition member; and the younger Maastrichtian sequence, which includes the Trinidad Sandstone and the continental beds of the Vermejo Formation and the lower coal zone of the Raton Formation.
The Kiowa-Skull Creek, Greenhorn, Niobrara, Claggett, and Bearpaw transgressive-regressive cycles are represented by the marine units above. Major unconformities are at the base of the Purgatoire, the base of the Dakota, the top of the Codell, the top of the Apache Creek, and the base of the Raton. Evidence of the asteroid impact event that terminated the Cretaceous is preserved at the top of the lower coal zone of the Raton as a thin iridium-rich kaolinitic claystone.
AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990