New Mexico Geological Society Annual Spring Meeting — Abstracts

Lower Cretaceous strata exposed in southwestern New Mexico (Hidalgo, Luna, Grant, Sierra and Dona Ana counties) are assigned to the Hell-to-Finish, U-Bar and Mojado formations of the Bisbee Group. A lack of regional stratigraphic studies (most have focused on a single mountain range), the relative rarity of biostratigraphically useful fossils and the apparently rapid lateral facies changes in parts of the Lower Cretaceous section have hindered the development of convincing regional correlations. However, a lithologically and biostratigraphically distinctive interval of the U-Bar Formation, the Carbonate Hill Member, is present in all outcrop areas of the U-Bar Formation in southwestern New Mexico. It thus provides a sound basis for the correlation of U-Bar sections throughout the region.

Zeller (1965, NMBMMR Memoir 16) introduced the term "oyster-limestone member" of the U-Bar Formation in the Big Hatchet Mountains for the same lithostratigraphic unit that Gillerman (1958, NMBMMR Bulletin 57) had already named the Carbonate Hill Limestone. The Carbonate Hill Member of the U-Bar Formation is 60 to 610 m thick and consists mostly of dark gray to black, bioclastic limestone (packstone) characterized by numerous bivalve or gastropod shells and interbedded with calcarous shale, calcarenite and minor limestone-pebble conglomerate. It yields late Aptian ammonites (e.g., Kazanskyella spathi, Acanthoplites berkeyi, Hypacanthoplites immunitis) in the Peloncillo and Big Hatchet Mountains. At all outcrops, a diverse assemblage of bivalves from the Carbonate Hill Member includes species of Quadratotrigonia that indicate a late Aptian age. At some outcrops, the large, late Aptian-early Albian foraminiferan Orbitolina texana is present in the Carbonate Hill Member.

The Carbonate Hill Member of the U-Bar Formation crops out in the Big Hatchet, Little Hatchet, Animas, Peloncillo, Victorio and East Potrillo Mountains, and at Eagle Nest in the West Potrillo Mountains. It thus provides the basis for a first order correlation of U-Bar Formation outcrops throughout southwestern New Mexico.