STRATIGRAPHY, LITHOFACIES, AND BIOFACIES OF THE U-BAR FORMATION (APTIAN-ALBIAN) OF THE BIG HATCHET MOUNTAINS, HIDALGO COUNTY, NEW MEXICO.

The U-Bar Formation crops out in the southern part of the Big Hatchet Mountains, Hidalgo County, New Mexico. This Cretaceous (Aptian-Albian) formation is between 3500 and 4000 ft (1067 and 1219 m) thick, and is divided (in ascending order) into five informal members: brown limestone, oyster limestone, limestone-shale, reef limestone, and suprareef limestone. The brown limestone member is 404 ft (123 m) thick and consists predominantly of thin-bedded, reddish brown and dusky red, oolitic, dolomitic, and gypsiferous limestones. These limestones are dominantly bioclastic wackestones and packstones, and, less commonly, fenestral, peloidal, laminate, and non-laminate micrites. Limestones of this member contain a dwarf and diminutive fauna consisting of thin-shelled bivalves, turritelloid gastropods, and echinoderms. Dasycladacean algae and foraminifera are rare. The oyster limestone member is between 1800 and 2000 ft (552 and 610 m) thick and consists of olive gray to light olive gray, thin- to medium-bedded limestones, and, less commonly, massive limestones, shales, siltstones, and sandstones. Limestones of this member are dominantly wackestones and packstones and typically contain Ostrea-type bivalve biostromes which include Texigryphea, Ostrea, Exogyra, and Gryphea. Turritelloid gastropods, echinoderms, foraminifera, green algae, and the ammonites Sinzowiella cf. spathi, Acanthohoplites cf. berkeyi, and Immunitoceras immunitum are rare to locally common. The limestone-shale member is between 400 and 1041 ft (122 and 317 m) thick and consists predominantly of thin- to medium-bedded, olive gray to bluish gray limestones and interbedded calcareous shales. Limestones of this member are bioclastic mudstones and wackestones and contain a common to locally abundant echinoderm Hemiaster cranium, and the calcareous nannoplankter Colomiella mexicana. The ammonite Douvilleiceras mammillatum and the nautiloid Cymatoceras neohispanicum are rare. Shales of this member contain a diverse assemblage of agglutinate and calcareous performate foraminifera which include Orbitolina texana, Lituola sp., Triplasia sp., Robulus sp., Lenticulina sp., and Globigerina sp. . . . (Author\u27s abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UM