Description, classification, and geologic context of a Lower Pleistocene primitive mammoth jaw from Matanza Arroyo near Socorro, New Mexico

A mammoth jaw found as a large clast in the channel of Matanza Arroyo near Socorro, New Mexico, is here termed the Matanza mammoth. Cemented pebbly sandstone attached to the jaw shows that it was reworked from partially cemented ancestral Rio Grande sediments exposed in the channel 50–90 m upstream. This jaw is from one of the geologically oldest and most primitive North American mammoths. The mammoth fossil is a left dentary fragment with incomplete m2-3. The small size of the molars, widely spaced plates, very thick enamel and shape of the dentary identify it as a very early mammoth, Mammuthus meridionalis. The Matanza mammoth is comparable in morphology and geologic age to other primitive Irvingtonian mammoths from New Mexico that document the presence of Mammuthus in the state between 1.6 and 1.25 Ma. Ancestral Rio Grande (ARG) sediments associated with the jaw are exposed discontinuously along the channel floor and lower walls of Matanza Arroyo from 220 m upstream to the arroyo’s mouth, 1,440 m downstream. At the same elevation as the jaw-bearing bed within ARG deposits are small pebbles of Rabbit Mountain obsidian (1.44 ± 0.01 Ma) and thin, cross-bedded layers of pumice. These clasts are geochemically identical to Group II pumices of Cerro Toledo eruptions of the Jemez volcanic field found in ARG sediments between 1.25 and 1.61 Ma in age and studied farther north. The relatively uniform geochemistry of pumices (all Group II) and lack of later pumices from the Jemez Mountains (such as Tshirege tephra of the 1.25-Ma Valles Caldera eruption) suggest that the mammoth jaw is older than 1.25 Ma. At Matanza Arroyo, pebbly sand beds of the ARG (Sierra Ladrones Formation of upper Santa Fe Group) interfinger with channel and inter-channel deposits of the Socorro Canyon fan and are unconformably overlain by thick boulder-gravel deposits of the Socorro Canyon fan that contain at least two buried soils with stage III–IV pedogenic carbonate horizons and a stage III soil along the east-sloping geomorphic surface at the Socorro County fairgrounds and airport. These deposits all reinforce the early Pleistocene age for the fossil.