New Mexico Geological Society Annual Spring Meeting — Abstracts

The only Upper Cretaceous strata exposed in Quay County, NM are outliers along the Llano Estacado escarpment along the northwestern side of the Bonita fault. The most extensive of these is in the SE1/4 sec. 23 and SW1/4 sec. 24, T9N, R33E, just east of the outliers originally mapped by Dobrovolny et al. (1946, USGS Oil &Gas Inv. Prelim. Map 62). The Upper Cretaceous strata here are assigned to the Graneros Shaie and overlying Greenhorn Limestone.

The Graneros disconformably overlies the Pajarito Formation (Dakota Group), and the Greenhorn is disconformably overlain by the Neogene Ogallala Formation. The Graneros Shale is 32 m thick and mostly consists of gray calcareous shale and a few thin, fossiliferous grayish orange calcarenite beds. The basal limestone bed of the overlying Lincoln Member of the Greenhorn Limestone produces an extensive fossil assemblage of the oyster Ostrea beloiti Logan and the selachian taxa Cretodus semiplicatus, Cretoxyrhina mantelli, Ptychodus anonymus, Ptychodus occidentalis ,Squalicorax falcatus and Squalicorax curvatus.

The Greenhorn Limestone is about 50 m thick and consists mostly of thin interbeds of pinkish gray chalky limestone, calcareous shale and bentonitic ash beds. The Lincoln, Hartland and Bridge Creek Members can be recognized at this outcrop. The Lincoln Member is about 34 m thick and consists of yellowish brown shale and thin ledges of calcarenite. The Hartland Member is about 9 m thick and consists of gray shale and white bentonitic ash beds. The Bridge Creek Member is at least 8 m thick and consists of alternating beds of chalky limestone and calcareous shale. A limestone bed in the Bridge Creek Member produces numerous inoceramid bivalves (especially Mytiloides mytiloides) and ammonoids (especially Watinoceras reesidei).

The Quay County outlier indicates that Upper Cretaceous strata at least as young as earliest Turonian were deposited throughout east-central New Mexico and into West Texas. Cenozoic erosion removed these strata from most of this region.