Abstract

Lithostratigraphy and vertebrate biochronology allow precise correlation of Upper Triassic strata between West Texas and eastern New Mexico. Upper Triassic strata well exposed in West Texas from Oldham to Scurry Counties are assigned to the Dockum Formation of the Chinle Group consisting of four members: (1) Camp Springs Member, sandstones and extrabasinal conglomerates; (2) Tecovas Member, mostly variegated bentonitic mudstones; (3) Trujillo Member, sandstones and mostly intrabasinal conglomerates; and (4) Cooper Member, mostly reddish brown mudstones. Fossil vertebrates from the Camp Springs and Tecovas Members are of Late Carnian age, whereas those from the Cooper Member are of Early Norian age. Upper Triassic strata in east-central New Mexico, across the Llano Estacado from the West Texas outcrops, correlate as follows: Camp Springs is equivalent to the lower Santa Rosa; Tecovas is equivalent to the upper Santa Rosa/Garita Creek; Trujillo is equivalent to the Trujillo (“Cuervo”); Cooper is equivalent to the lower Bull Canyon. Upper Triassic strata in southeastern New Mexico and in Howard and adjacent counties in Texas are the lower Santa Rosa/Camp Springs overlain by mudstones and sandstones that contain Late Carnian vertebrates and are assigned to the San Pedro Arroyo Formation in New Mexico and an informally termed upper member of Dockum Formation in Texas.

Available data refute several long-held ideas about the Upper Triassic of West Texas. These data demonstrate that: (1) there is a pervasive unconformity at the base of the Dockum Formation that represents much of Triassic time; (2) the Trujillo Member is not correlative with the Santa Rosa of eastern New Mexico: Trujillo is a medial Dockum unit, whereas Santa Rosa is at the base of the Upper Triassic section; (3) very little Dockum mudrock was deposited in lakes; and (4) Dockum rivers flowed almost exclusively to the north, northwest and west; thus there is little evidence of a closed depositional basin in West Texas during the Late Triassic.