New Mexico Geological Society Annual Spring Meeting — Abstracts

Triassic strata in the Carthage area (TSS, R2E) are 163 m of redbeds that rest disconformably on Permian strata and are disconformably overlain by a thin sequence of the Upper Jurassic Morrison Formation. Three principal Triassic units can be recognized here: 1) a basal unit that is 15.7 m of dominantly moderate reddish-brown mudstone and sandstone/conglomerate: 2) a middle unit that is 6.0 m of moderate reddish-brown conglomeratic sandstone in which clasts are mostly extraformational chert: and 3) an upper unit of 141 m dominated by moderate reddish-brown and grayish-red mudstone with thin intraformational conglomerates and sandstones in its upper half. The basal unit has been called Santa Rosa Sandstone but is better termed Moenkopi Formation because of its lithologic similarity to Moenkopi outcrops on the Sevilleta Grant northeast of Socorro. Furthermore, typical strata of Santa Rosa in Guadalupe County, New Mexico, are intensively crossbedded quartzarenites and are quite distinct lithologicailly from the basal Triassic unit near Carthage. The Moenkopi outcrops in the Carthage area are the southeasternmost outcrops of this formation.

The middle unit near Carthage arguably is homotaxial with the Shinarump Member of the Chinle Formation on the Colorado Plateau, and the upper unit is equivalent to the lower petrified Forest Member of the Chinle. These stratigraphic assignments are supported by lithologic resemblances, stratigraphic position, regional stratigraphic relationships and fossils. Fossils from the upper unit near Carthage pertain to phytosaurs and metoposaurid labyrinthodonts,indicative of a late Triassic age.