New Mexico Geological Society Annual Spring Meeting — Abstracts

Thirty vertebrate fossil assemblages from New Mexico date from the early Pliocene (~4 Ma) to the early Pleistocene (~1 Ma), corresponding to the Blancan (4.5-1.8 Ma) and early Irvingtonian (1.8-1.0 Ma) North American land mammal "ages" (NALMA). Correlation of faunas to one of these two NALMAs is based on biostratigraphic comparisons with similar faunas elsewhere in western North America. Other geochronological data, including magnetostratigraphy and radioisotopic dates, provide important corroboration for the age of these sites. We summarize the available radioisotopic dates and magnetostratigraphy associated with Blancan and Irvingtonian vertebrate faunas in New Mexico.

The Truth or Consequences Local Fauna (LF) from the Palomas Formation (Fm.) is one of the few early Blancan (4.5-3.6 Ma) vertebrate faunas from New Mexico. The Tor C LF occurs in normally magnetized sediments that probably correlate with the Cochiti Subchron (4.29-4.18 Ma). A date of 4.5 Ma on a basalt flow at the base of the Palomas Fm. provides a maximum age for the T or C LF. There are numerous medial Blancan (3.6-2.7 Ma) faunas from New Mexico. The Pajarito LF from the Arroyo Ojito Fm. is associated with a fluvially-recycled pumice dated at 3.12 Ma. The medial Blancan Elephant Butte Lake and Cuchillo Negro Creek LFs, from the axial facies of the Palomas Fm., are correlated with a basalt flow dated at 2.9 Ma. The medial Blancan Hatch LF from the Camp Rice Fm. correlates with a magnetostratigraphic section at Hatch Siphon that spans much of the Gauss Chron (3.58-2.58 Ma), and includes a pumice-clast conglomerate dated at 3.12 Ma. Magnetostratigraphy of the Camp Rice Fm. at Cedar Hill demonstrates that the medial Blancan Tonuco Mountain LF is younger than the beginning of the Gauss Chron (3.58 Ma) and older than the top of the Kaena Subchron (3.04 Ma).

Late Blancan (2.7-1.8 Ma) faunas are characterized by the first appearance of South American immigrant mammals in the southwestern US at about 2.7 Ma, in the uppermost Gauss Chron just prior to the Gauss/Matuyama boundary (2.58 Ma). The Mesilla A Fauna from the Camp Rice Fm. occurs in normally magnetized strata correlated with the upper Gauss Chron (3.04-2.58 Ma). The presence of the South American immigrant Glyptotherium further constrains the age of this fauna to younger than 2.7 Ma. The occurrence of the South American immigrant
Glossotherium in the Pearson Mesa LF from Gila Group strata that correlate with the upper Gauss Chron suggests a similar age to Mesilla A. A combination of biostratigraphy and magnetostratigraphy apparently constrains the ages of the Pearson Mesa and Mesilla A faunas to between 2.7 Ma (earliest appearance of South American immigrants) and 2.6 Ma (GausslMatuyama boundary), although the magnetostratigraphic data do not rule out the possibility that these two faunas could be as old as 3 Ma.

The early Irvingtonian Tijeras Arroyo LF occurs in Sierra Ladrones Fm. strata containing fluvially-recyled pumice from the lower Bandelier tuff dated at 1.61 Ma. These same strata have produced one of the earliest and most primitive specimens of the mammoth Mammuthus known from North America. The early Irvingtonian Mesilla C Fauna from the Camp Rice Fm. also contains primitive Mammuthus, and occurs in reversely magnetized strata of the Matuyama Chron between the top of Olduvai Subchron (1.77 Ma) and the bottom of the Jaramillo Subchron (1.07 Ma). A 1.59 Ma pumice date from near the base of this same stratigraphic interval provides a maximum age for the Mesilla C fauna.