New Mexico Geological Society Annual Spring Meeting — Abstracts

Lower Cretaceous strata exposed at Eagle Nest in Luna County (sec. 34, T27S R5W) overlie Permian limestone and are overturned to the southeast. The Lower Cretaceous section here is at least 400 m thick and is assigned to the Hell-to-Finish and U-Bar formations of the Bisbee Group. The Hell-to-Finish Formation disconformably overlies Permian strata (probably Colina Limestone) and is 175 m thick. The lower third of the Hell-to-Finish Formation (about 50 m thick) consists of interbedded cobble conglomerates (clasts are Paleozoic limestone and chert up to 15 cm in diameter) and red-bed mudstones. The upper two-thirds are interbedded red-bed mudstone, calcareous shale, calcarenite, nodular (pedogenic?) limestone, limestone-cobble conglomerate and sandstone.

We place the contact of the Hell-to-Finish and U-Bar formations at the base of a 5-m-thick ledge of bivalve packstone. Above this bed, U-Bar strata are interbedded bivalve and gastropod packstones, calcareous shale and calcarenites. The exposed U-Bar Formation at Eagle Nest is about 230 m thick, if a 75-m-thick covered interval about 25 m above its base is included, but no top of the U-Bar Formation is exposed here. About 100 m below the top of the U-Bar Formation at Eagle Nest, at NMMNH locality 3755 (UTM 3533302N, 279207E, zone 1 NAD 27), fossils of Quadratotrigonia and other bivalves indicate a late Aptian age Lithology and biostratigraphy thus support assignment of the exposed U-Bar section at Eagle Nest to the Carbonate Hill Member. The upper Aptian Carbonate Hill Member of the U-Bar Formation is present at aIl U-Bar sections in southwestern New Mexico and thus provides an important basis for regional correlation of the Lower Cretaceous.