Named as the upper formation of the Chinle Group for San Pedro Arroyo, a dry tributary of the Rio Grande north of NM 380 in the northern fourth of T 5 S, R 2 E, Socorro Co., NM. The type section is the broad outcrop belt in the San Pedro Arroyo and beyond north of Carthage in Socorro Co. This formation is also present in Lincoln and Valencia Cos. from the eastern edge of the Lucero uplift at Carrizo Spring to the Capitan Mountains, areas that lie in the Orogrande and San Juan basins. This new name replaces use of Chinle Formation, the term used by previous workers in south-central NM. Term "Chinle Formation" considered unsatisfactory because in this area it did not include "Shinarump Formation" which is part [or member of Chinle Formation of some authors] of the Chinle on the Colorado plateau. San Pedro Arroyo is 123 m thick at its type section where it consists of mudstone, sandstone, conglomerate, siltstone, and a little limestone. Sequence of lithologies shown on cross sections. The mudstones are bentonitic and variegated. The sandstones are purple and red, cross bedded, laminar, and ripple laminated, micaceous, and they are interbedded with conglomerates of mudstone and calcrete clasts. A prominent persistent pisolitic limestone bed 1.6 m thick lies 1.4 m above base; the limestone is newly named Ojo Huelos Member of Chinle. Overlies Santa Rosa Formation, basal formation of Chinle Group in this area. Underlies Morrison Formation locally, or where Morrison is missing, the Mesa Rica Formation of the Dakota Group. Is of late Carnian age in the lower part; age of upper part not determined beyond Late Triassic. Nomenclature charts. One measured section (partial) at Carrizo Arroyo in N1/2 NE1/4 NE1/4 sec. 7, T 6 N, R 2 W, Valencia Co. Fragments of amphibians and phytosaurs found at type. Divided on correlation chart into a lower member, Ojo Huelos Member, and an upper member.

Source: GNU records (USGS DDS-6; Denver GNULEX).