New Mexico Geological Society Annual Spring Meeting — Abstracts

The U-Bar Formation is divided into five members which are, in ascending order: Brown Limestone, Oyster Limestone, Limestone-Shale, Reef Limestone, and Suprareef Limestone. The formation consists primarily of medium to thin-bedded bioclastic limestones alternating with thin beds of gray shales. The lower part of the formation contains lenses and thin beds of sandstone. Near the top of the formation is a massive carbonate containing rudist bioherms that are· from 20 feet (6 meters) to 500 feet (152 meters) thick.

Ammonoids in the Oyster Limestone Member of the U-Bar Formation permit a relatively precise age designation. Sinzowiella spathi and Kazanskyella arizonica recovered from the Oyster Limestone Member have also been observed in the Pacheta Member of the Lowell Formation of southeastern Arizona. The age of the Pacheta Member has been·determined to be equivalent to the Middle Gargasian beds of France (Upper Aptian).

Cephalopods of the overlying Limestone-Shale Member of the U-Bar Formation contain both Douvilleiceras mammilatum (d'Orbigny) and the nautiloid Cymatoceras neohispanicum. D. mammilatum is now considered to be basal Middle Albian or medial Glen Rose (Trinity) equivalent. The basal Brown Limestone Member contains silicified, unrecognizable fragments of ammonoids. No ammonoid control has been established for the Reef Limestone arid Suprareef Limestone Members. They are undoubtedly Late Albian, but no evidence exists to extend the age of the U-Bar Formation to the Cenomanian, but it reflects aspects of Fredricksburg and Washita units of West Texas.