New Mexico Geological Society Annual Spring Meeting — Abstracts

Exposures at Pearson Mesa in the Duncan basin along the New Mexico-Arizona border have produced a diverse assemblage of late Pliocene (Blancan) vertebrates. The stratigraphic section at Pearson Mesa consists of more than 60 m of sandstones, mudstones, and sedimentary breccias of the Gila Group. Two distinct vertebrate faunas occur at Pearson Mesa: the early late Blancan Pearson Mesa Local Fauna (LF) is derived from the lower 15 m of the stratigraphic section and the latest Blancan Virden LF occurs in the upper 20 m of the section. The Pearson Mesa fauna consists of 25 species: 3 land tortoises (Gopherus and two species of Hesperotestudo); box turtle (Terrapene); colubrid snake; bird; and 19 mammals. Age-diagnostic mammals from the Pearson Mesa LF include: the ground sloth Paramylodon cf. P. garbanii; the pocket gopher Geomys persimilis; the cotton rat Sigmodon medius; the three-toed horse Nannippus peninsulatus; the one-toed horses Equus cf. E. cumminsii, and E. simplicidens; and the peccary Platygonus bicalcaratus. The association of Paramylodon and Nannippus defines a restricted interval of time in the Blancan between the first appearance of South American immigrants (including Paramylodon) in the southwestern US at ~3.0 Ma and the extinction of Nannippus at ~2.2 Ma. Magnetostratigraphy further constrains the age of the Pearson Mesa LF, with five normally-magnetized samples from the lower part of the section referred to the uppermost Gauss Chron (Chron 2An.1n; 2.58-3.04 Ma). Southwestern early late Blancan faunas (~2.6-3.0 Ma) correlative with Pearson Mesa include: Anapra, NM; Wolf Ranch and 111 Ranch, AZ; and Cita Canyon and Hudspeth, TX. A 10-m-thick sedimentary breccia overlying the Pearson Mesa LF lacks fossils and may represent a hiatus. The Virden LF consists of 22 species: toad; large Hesperotestudo; Terrapene; colubrid snake; lizard; two birds; and 15 mammals. A latest Blancan age (~1.8-2.2 Ma) for the Virden LF is indicated by the presence of the dwarf cotton rat Sigmodon minor and the small camelid Hemiauchenia gracilis, both restricted to latest Blancan faunas, and the association of the glyptodont Glyptotherium arizonae with the coyotelike canid Canis lepophagus. Southwestern latest Blancan faunas correlative with Virden include: La Union, NM and Curtis Ranch and San Simon, AZ.