New Mexico Geological Society Annual Spring Meeting — Abstracts

The Triassic outcrops of south-central New Mexico have received relatively little study, compared to contemporaneous strata in the north-central and eastcentral parts of the state. Triassic sections to the north of Carthage, Socorro County (T05S, R02E), encompass the Middle Triassic Moenkopi Formation and Upper Triassic Chinle Group (Shinarump and San Pedro Arroyo formations). The Moenkopi Formation (Anton Chico Member) overlies the siltstone-dominated Artesia Group and is mostly cross-bedded sandstones and less common beds of mudstone, silstone and intraformational conglomerate. Fragmentary capitosauroid amphibian material near the top of the unit are the only fossils collected from the Moenkopi Formation near Carthage and are consistent with assigning it a Perovkan (Anisian) age.

The Shinarump and San Pedro Arroyo formations unconformably overlie the Moenkopi Formation near Carthage; this is the Tr-3 unconformity, a hiatus of about 10 million years. The San Pedro Arroyo Formation is interbedded mudstone and sandstone except for the Ojo Huelos Member, which is a prominent limestone interval, with occasional conglomeratic lenses that are fossiliferous, and can be used regionally as a marker bed. Lithology within the Ojo Huelos ranges from clean lime mudstone to brecciated and pisolitic limestone. Fossils from the San Pedro Arroyo Formation are characteristic of Late Triassic tetrapod faunas in being metoposaurand phytosaur-dominated. Abundant fossils of large metoposaurid amphibians have been recovered from the lower part of the San Pedro Arroyo Formation and the Ojo Huelos Member. Abundance of large metoposaurids corresponds to the previously established “metoposaurid acme zone” within the Chinle and suggests a pre-Revueltian age for the lower part of the San Pedro Arroyo Formation, including the Ojo Huelos Member.

Isolated phytosaur bones and teeth are found in the San Pedro Arroyo within and above the Ojo Huelos Member, but do not provide genus- or species-level identifications. Thus, the precise age of the upper San Pedro Arroyo Formation is uncertain due to the lack of diagnostic and biostratigraphically useful fossils. The Upper Cretaceous Dakota sandstone and/or a thin section of Upper Jurassic Morrison Formation unconformably overlies the San Pedro Arroyo Formation near Carthage.