Primitive Mammoth Jaw From Lower Pleistocene Rio Grande Sediments, Socorro, New Mexico

Abstract

A mammoth jaw found as a large clast reworked from ancestral Rio Grande (ARG) deposits in Matanza Arroyo at Socorro, New Mexico, is from one of the geologically oldest and most primitive North American mammoths. Thin, cross-bedded layers of pumice above the jawbearing bed are geochemically identical to Group II pumices in ARG sediments between 1.2 and 1.6 million years in age (Cerro Toledo eruptions of Jemez volcanic field) studied in Hell Canyon. The ARG deposits underlie thick fan deposits of Qvo3, which forms the surface at the Socorro County fairgrounds and airport. This fan surface is traceable down the arroyo to ancestral Rio Grande terrace deposits, 30 m above the modern Rio Grande floodplain and is traceable up Matanza Arroyo to a terrace along Socorro Canyon Arroyo. In 2003, Phillips et al. determined a Cl age of 122 ± 18 ka for this terrace. Although the upper surface and shallow deposits of Qvo3 likely are equivalent to the age of the Qvo3 terrace in Socorro Canyon, the stratigraphy of the alluvium and soils between the jaw and the fan surface suggests a long history of erosion, deposition, and stability.