New Mexico Geological Society Annual Spring Meeting — Abstracts

Upper Cretaceous strata exposed as outliers near Roy in Harding County, New Mexico, are assigned to the Graneros Shale and the overlying Greenhorn Formation. The Greenhorn Formation locally consists of three members (in ascending order): Lincoln, Hartland, and Bridge Creek. The Lincoln Member is 8.3 m thick and dominated by yellowish gray and very pale orange calcareous shale and characteristic persistent beds of calcarenite. The Hartland Member is 11.0 m thick and consists mostly of medium and brownish gray bentonitic shale. The Bridge Creek Member here is erosionally truncated by the Neogene Ogallala Formation 4.1 m above its base and consists of interbeds of ledge-forming limestone and calcareous shale. Fossils from the Lincoln Member include numerous specimens of the bivalve Ostrea beloiti Logan and of the selachian Ptychodus anonymus Williston. The Hartland Member lacks biostratigraphically significant fossils. The Bridge Creek Member produces the ammonites Calycoceras sp. and Sciponoceras gracile (Shumard) near its base and Watinoceras coloradoense (Henderson) and Vascoceras birchbyi Cobban and Scott near its top. These fossils can be assigned to the upper Cenomanian Sciponoceras gracile ammonite zone and the lower Turonian Vascoceras birchbyi ammonite zone. Younger ammonite zones normally present in the Bridge Creek Member are not present at Roy, having been stripped off by Neogene erosion.