Stratigraphy and correlation of the Permo-Carboniferous Cutler Group, Chama Basin, New Mexico

Abstract

—Nonmarine siliciclastic red beds at the base of the Phanerozoic section across most of the Chama Basin of northern New Mexico are assigned to the Pennsylvanian-Permian Cutler Group. These strata are here divided into two mappable lithostratigraphic units, the El Cobre Canyon and overlying Arroyo del Agua formations. The El Cobre Canyon Formation is up to 500 m of brown siltstone, sandstone and extraformational conglomerate of an ephemeral braided stream environment that overlies Proterozoic basement in the subsurface and is conformably overlain by the Arroyo del Agua Formation. Siltstone beds of the El Cobre Canyon Formation contain numerous rhizoliths and comprise relatively thin, slope-forming units between multistoried sandstone beds that are arkosic, micaceous, coarse grained and trough crossbedded. The Arroyo del Agua Formation is up to 120 m of orange siltstone, sandstone and minor intraformational and extraformational conglomerate of a braided to anastomosed stream depositional environment. The siltstones are thick, slope-forming units with abundant calcrete nodules between thin sandstone sheets that are arkosic and trough crossbedded. In the Chama Basin, the De Chelly Sandstone (= Meseta Blanca Member of the Yeso Formation) locally overlies the Arroyo del Agua Formation, but at most outcrops the Upper Triassic Chinle Group rests unconformably (some slight angularity is evident) on the Arroyo del Agua Formation. Megafossil plants, palynomorphs and fossil vertebrates indicate the El Cobre Canyon Formation is of Late Pennsylvanian-Early Permian (early Wolfcampian) age. Sparse fossil vertebrates indicate the Arroyo del Agua Formation is of late Wolfcampian age. Correlation of Cutler Group strata southward to Jemez Springs suggests that the Abo Formation is equivalent to the upper part of the El Cobre Canyon Formation and the entire Arroyo del Agua Formation. The lower part of the El Cobre Canyon Formation in the Chama Basin is correlative to mixed marine-nonmarine strata of the upper “Madera Group” at Jemez Springs. FIGURE 1. Index map showing distribution of Cutler Group outcrops in the Chama Basin and location of type sections of El Cobre Canyon and Arroyo del Agua formations. 146 LUCAS AND KRAINER In 1874, E. D. Cope traveled through part of the Chama Basin, observing Cutler Group strata along the Rio Gallinas. Like Newberry, he (Cope, 1875) also considered these red beds to be of Triassic age, primarily because Cope found Late Triassic fossils of unionid bivalves and reptiles in the upper part of the red bed succession (Petrified Forest Formation of Chinle Group of current usage: Lucas et al., 2003). Fossils subsequently collected by David Baldwin from the lower part of the red bed succession convinced Cope (1881; also see Marsh, 1878) of their Permian age. Williston and Case (1912, 1913; also see Huene, 1911) described the Cutler Group red beds in El Cobre Canyon and in the Rio Puerco valley. They applied no lithostratigraphic names to these strata, but did assign them a Pennsylvanian-Permian age. Particularly important was their discovery of a loose brachiopod (Sprifier rockymontanus) in the floor of El Cobre Canyon, which they deemed evidence of a Pennsylvanian age. They based an Early Permian age on the vertebrate fossil assemblages, which they correlated to the lower part of the Wichita Group in Texas. On his geological maps, Darton (1928a, b) assigned the red beds below the “Poleo sandstone,” to the Abo Sandstone of Carboniferous age (Fig. 2). However, on his geologic cross sections, Darton (1928a, fig. 69) simply referred to these strata as “red shale and sandstone” between the Carboniferous Magdalena Group and the Poleo sandstone. He also (p. 21) noted that “bones from the red beds now regarded as representing the Abo sandstone near Coyote, in Rio Arriba County, were classified as Permian by Marsh and Cope and later by Williston and Case.” Wood and Northrop (1946) mapped the geology of the southern flank of the Chama Basin, and called the Pennsylvanian-Permian red beds north of latitude 36oN Cutler Formation, and south of that Abo Formation. Northrop (1950, p. 85) followed up by stating that “the Cutler formation (200’-1100’?)...is the northward equivalent of the Abo and Yeso formations.” Romer (1950, 1960) referred to the strata as “Abo (Cutler) Formation” and reviewed their fossil vertebrates, correlating them to the lower or middle portion of the Wichita Group in Texas. Langston (1953, p. 351) stated that “all Permian red beds in Rio Arriba County are assigned to the Cutler formation.” He described in detail the vertebrate fossil localities near Arroyo del Agua, and documented the fossil amphibians from these localities. Langston correlated the Arroyo del Agua vertebrate fossils to the lower and middle Wichita Group of the Texas section (Langston, 1953, fig. 24). He also discounted the idea that the El Cobre Canyon vertebrate fossils are Pennsylvanian, and assigned them an Early Permian age. Smith et al. (1961) referred to the red bed strata as Cutler Formation and mapped their distribution in the southeastern Chama Basin. They (p. 7) described them as a “seemingly cyclic alternation of cross-bedded, purple, arkosic sandstones which are locally conglomeratic, and of purple and orange mudstones” at least 1500 ft (500 m) thick. Smith et al. (1961, p. 7) also noted that “no lithologic break could be found throughout the section, [so] the entire thickness is mapped as Permian Cutler Formation.” In an appendix, they presented a composite section said to be based on the surface section on the western wall of El Cobre Canyon and on the log of a well drilled in the canyon floor (Fig. 3). Particularly significant was Smith et al.’s (1961) identification of a Pennsylvanian plant locality in the northern end of El Cobre Canyon that they assigned to the Hermosa Formation (we, however, assign this site to the Cutler Group). Baars (1962) well reflected the consensus when he referred the older red beds in the Chama Basin to “Cutler Group undifferentiated” and indicated they are generally equivalent to the Abo Formation and thus of Early Permian age. However, Fracasso (1980) presented megafossil plant (also see Hunt and Lucas, 1992) and fossil vertebrate evidence (also see Vaughn, 1963) that the lower part of the Cutler Group in El Cobre Canyon is of Pennsylvanian age. In the 1980s and 1990s, sedimentological studies of Cutler Group strata in the Chama Basin were published by Eberth and Berman (1983, 1993), Eberth (1987), Fracasso (1987) and Eberth and Miall (1991). Particularly significant was Eberth’s division of the Cutler Group into three depositional cycles he referred to as megasequences (Fig. 2). Berman (1993) summarized the vertebrate paleontology of the Cutler Group strata in the Chama Basin. Our studies of Cutler Group stratigraphy in the Chama Basin began in 2000 (Krainer and Lucas, 2001).