GEOCHRONOLOGY OF LATE PLIOCENE AND EARLY PLEISTOCENE (MEDIAL BLANCAN THROUGH EARLY IRVINGTONIAN) VERTEBRATE FAUNAS FROM THE ALBUQUERQUE BASIN, NEW MEXICO

Five Late Neogene (Blancan and Irvingtonian) vertebrate faunas from the Albuquerque basin contain age-diagnostic mammals and/or associated tephra that permit refinement of their ages. The medial Blancan Pajarito Local Fauna (LF), from the Arroyo Ojito Fm., includes the pocket gopher Geomys paenebursarius, and is associated with a 3.12 Ma fluvially-recycled pumice. The Belen LF, from the Arroyo Ojito Fm., occurs 40 m below the ~2 Ma Llano de Albuquerque surface, and contains the medial/late Blancan Geomys paenebursarius, and three species found in late Blancan or younger faunas: the mole Scalopus (Hesperoscalops) blancoensis, the horse Equus calobatus, and the proboscidean Stegomastodon mirificus. The Santo Domingo LF, from the Sierra Ladrones Fm., contains the horse Nannippus peninsulatus indicating an age older than 2.2 Ma and occurs in deposits interbedded with 2.4-2.7 Ma basalt flows, restricting its age to late Blancan (2.7-2.2 Ma). Although the absence of Neotropical immigrants in the Belen and Santo Domingo LFs is typical of medial Blancan faunas (3.6-2.7 Ma), South American mammals may not have migrated as far north as the Albuquerque basin by the late Blancan. The Mesa del Sol LF, from the Arroyo Ojito Fm. at Tijeras Arroyo, contains the rabbit Hypolagus gidleyi and the horse Equus cumminsii, indicating a medial to late Blancan age (~3.6-2.2 Ma). The Tijeras Arroyo LF, from ancestral Rio Grande deposits of the Sierra Ladrones Fm., overlies the Mesa del Sol LF, and contains an early Irvingtonian (~1.6-1.0 Ma) assemblage including the mammoth Mammuthus meridionalis, the glyptodont Glyptotherium arizonae, and the horse Equus scotti. Strata that produce the Tijeras Arroyo LF contain fluvially-recyled pumice from the 1.61 Ma lower Bandelier tuff. A 1.25 Ma fallout tephra correlated to the Tshirege ash is within 10 m of the top of the Tijeras Arroyo section. A probable disconformity at Tijeras Arroyo between the Mesa de Sol and Tijeras Arroyo LFs may represent a hiatus corresponding to the latest Blancan and earliest Irvingtonian (~2.2-1.6 Ma).