New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

A recently discovered assemblage of vertebrate fossils 53 km south-southwest of Socorro, here named the Simon Canyon Local Fauna (LF), is one of the most precisely dated early Pleistocene (late Blancan North American Land Mammal Age – NALMA) faunas in New Mexico. The site is located near the lower (eastern) end of Simon Canyon in the central San Marcial Basin, Socorro County, in central New Mexico. The fossils were collected primarily from two axial-fluvial sand tongues of the Palomas Formation. The lower 4 m of the 12 m-thick, upper sand tongue yielded most of the fossils and also contained pebbles of obsidian. Trace-element analysis of three obsidian clasts, using XRF methods, indicate a close match with the 2.23 ± 0.15 Ma El Rechuelos Rhyolite. A 0.8 m-thick, coarsening-upward, pebble-boulder pumice bed is present 4 m above the top of the upper sand tongue. Its geochemistry supports a correlation with the Otowi Member of the Bandelier Tuff (OMBT), assuming this bed is the same as the Fort Craig pumice. We interpret that this pumice was transported here via a dam-burst flood within thousands of years after the associated Toledo caldera eruption in northern New Mexico at 1.62 Ma. Thus, the age of the upper axial-fluvial sand tongue is bracketed between 2.4 and 1.6 Ma. The Simon Canyon LF consists of 15 species of vertebrates, including seven mammals known from Blancan NALMA vertebrate faunas in New Mexico and elsewhere in the southwestern U.S.: Glyptotherium texanum (glyptodont), Canis cf. edwardii (wolf), Equusscotti (large horse), and four camels, Camelops sp., Gigantocamelus spatulus, Hemiauchenia cf. blancoensis, and Hemiauchenia gracilis. Except for Equus scotti and Camelops sp., these Blancan vertebrates were collected in the lower third of the upper axial-fluvial sand tongue or within 1 m below it. The most age-diagnostic of these species are Glyptotherium texanum, a mammalof South American origin and a participant in the Great American Biotic Interchange that first arrived in New Mexico at the beginning of the late Blancan (~2.7 Ma) and became extinct at the end of the early Irvingtonian NALMA (~1.0 Ma), and Hemiauchenia gracilis, a small camel known only from the late Blancan (~1.6–2.5 Ma). Conspicuously absent are Blancan mammals that became extinct at ~2.2 Ma (e.g., Borophagus, Nannippus) as well as Mammuthus, a Eurasian immigrant whose first appearance in North America defines the beginning of the early Irvingtonian at ~1.6 Ma. Thus, the mammalian biochronology of the Simon Canyon LF indicates a latest Blancan age (~1.6-2.2 Ma) for the fauna collected in the upper axial-fluvial sand. This biochronologic age is remarkably consistent with the independent 1.6-2.4 Ma age provided by the overlying OMBT pumice bed and the El Rechuelos Rhyolite obsidian clasts found in the lower part of the upper axial-fluvial sand tongue.