New Mexico Geological Society Annual Spring Meeting — Abstracts

Vertebrate fossils collected from the upper part ofthe Gila Group (="Gila Conglomerate") in the Mangas graben in southern Catron County and northern Grant County, southwestern New Mexico, help to constrain the age of this problematic geologic unit. A skull of the rhinoceros
Teleoceras fossiger from a conglomerate on Dry Creek in Catron County is indicative ofan early Hempbillian age (late Miocene, 9-7 Ma). A tuffaceous siltstone exposed along the North Fork of Walnut Canyon southeast of Cliff in Grant County has produced a rich concentration of mammal fossils suggestive of a somewhat younger, late Hempbillian age (latest Miocene or earliest Pliocene, 6-4.5 Ma) based on the co-occurrence of the horses Astrohippus stocki and Dinohippus cf. D. mexicanus and the small fox-like canid Vulpes stenognathus. Other members of this fauna include the rabbit Hypolagus, a small rodent, two species of camels, and anantilocaprid. Similar late Hemphillian faunas are known from the Panhandle of Texas (Coffee Ranch) and Chihuahua, northern Mexico (Yepomera).

Badlands northwest of Buckhorn in Grant County have yielded a fairly diverse vertebrate fauna ofmedial Blancan age (late Pliocene, 3.5-2.5 Ma). The fossils occur in at least three different stratigraphic units within a 25-m-thick interval of basin-fill sediments in the upper part of the Gila Group. An unconsolidated fine sand near the bottom of this interval contains numerous microvertebrates, including small rodents, birds, snakes, and abundant frogs. A greenish-gray clayey sand near the top of the section has produced a partial skeleton of a large wading bird, as well as fragmentary fossils of several other bird taxa. Larger mammals occur with the microvertebrates in the fine sand and in silty and clayey beds just above this unit. A medial Blancan age is indicated by the association of the small horse Nannippus peninsulatus (=N. phlegon), the larger horse Equus (Doliehohippus) simplieidens, and the primitive muskrat Pliopotamys. This fauna also includes the ground squirrel Spermophilus, a rabbit, the badger Taxidea, a medium-sized feild, the peccary Platygonus vetus, the camels Camelops and Hemiauehenia cf. H. blaneoensis, and an unidentified gomphotherlid proboscidean. Correlative medial Blancan faunas occur in the Rio Grande rift valley in south-central New Mexico near Truth or Consequences (Palomas Creek and Cuchillo Negro Creek) and from the San Pedro Valley in southeastern Arizona (Benson).

These preliminary findings indicate that vertebrate fossils will be very useful in placing late Miocene and younger sediments within the upper part of the Gila Group in southwestern New Mexico into a well-documented chronological framework. Future field work will concentrate on increasing the known vertebrate faunas and discovering new localities from both older and younger strata in the Gila Group. Where possible other techniques such as paleomagnetic polarity stratigraphy and radioisotopic dating of interbedded basalts and volcanic ashes will be to used to provide additional geochronological data.