Triassic Stratigraphy in the Zuni Mountains, West-central New Mexico

—The Triassic System in the Zuni Mountains consists of one Middle Triassic unit, the Moenkopi Formation, the Upper Triassic Chinle Group, and the Upper Triassic-Lower Jurassic(?) Wingate Sandstone. The Chinle Group in the Zuni Mountains consists of six formations, in ascending order: Zuni Mountains (named here; = “mottled strata” of previous workers), Shinarump, Bluewater Creek, Petrified Forest, Owl Rock, and Wingate formations. The Moenkopi Formation rests on an unconformable surface that represents the compound Tr-0 through Tr-2 unconformities, and Moenkopi strata locally rest on either the San Andres or Glorieta formations, both of Early Permian age. Moenkopi strata in the Zuni Mountains are relatively thin (< 25 m thick) sequences of thinly bedded sandstone, siltstone, and conglomerate, overlain by the Zuni Mountains and/or Shinarump formations, which lie on a surface corresponding to the Tr-3 unconformity. The Moenkopi Formation in the Zuni Mountains is lithologically identical to the Anton Chico Member of the Moenkopi Formation in eastern New Mexico, and is presumably correlative to both that unit and the Holbrook Member in western Arizona, and is thus of Perovkan (early Anisian) age. We coin the term Zuni Mountains Formation to replace the long-used informal term “mottled strata.” Zuni Mountains Formation strata are color-mottled siliciclastics that record pedogenic modification of strata associated with development of the Tr-3 unconformity. Shinarump Formation strata are principally conglomerates and sandstone that represent channel deposits of a fluvial system that aggraded in Tr-3 paleotopography during early Late Triassic time. Bluewater Creek Formation strata are present across the Zuni Mountains and are typically 50-60 m thick. These strata primarily record floodplain deposition, including both proximal (coarse-grained, crevasse-splay) and more distal (finegrained, floodplain) deposits. In the western two-thirds of the Zuni Mountains, the contact between the Bluewater Creek Formation and the overlying Petrified Forest Formation is defined by a bright white, tuffaceous sandstone that appears to represent a reworked ashfall from a discrete volcanic event. This horizon is not evident in the eastern third of the unit’s outcrop area, where the contact between the two units is a more arbitrary mudstone-on-mudstone contact. Strata of the Blue Mesa Member of the Petrified Forest Formation are primarily highly bentonitic mudstone with well-developed calcrete (siderite) nodules that record floodplain deposition and paleosol development. The Blue Mesa Member is overlain disconformably by the Sonsela Member on a surface corresponding to the Tr-4 unconformity. Sonsela strata are broadly similar to Shinarump Formation strata but are considerably more extensive in outcrop area and form the prominent cuesta south of Interstate 40 between Gallup and Bluewater. Erosion associated with the development of the Tr-4 unconformity generated at least 20 m of paleotopography across the study area that was subsequently filled by Sonsela Member fluvial deposits. The Sonsela grades upward into predominantly fine-grained strata of the Painted Desert Member, which represents a somewhat drier floodplain environment than the Blue Mesa Member. Across the Zuni Mountains, the Perea Bed is a prominent bench-forming, sandstone-dominated unit stratigraphically low in the Painted Desert Member that represents the deposits of a low-sinuosity fluvial system. The Painted Desert Member is overlain conformably by the Owl Rock Formation, as much as 25 m of siltstone, sandstone, and largely pedogenic calcrete that forms persistent benches across the northern edge of the Triassic outcrop belt in the Zuni Mountains. The Owl Rock Formation is disconformably overlain by the Wingate Sandstone, which some workers formerly (and incorrectly) included in the Entrada Sandstone. The surface separating the Owl Rock Formation from the Wingate Sandstone is the Tr-5 unconformity. These outcrops of the Wingate Sandstone in the Zuni Mountains are relatively thin (~35 m) and represent a mix of eolian and water-lain or waterreworked strata and are the only strata assigned to the Wingate Sandstone in the type area. 246 247 HECKERT AND LUCAS usage), he considered lower Chinle beds to be part of the Moenkopi Formation. By 1928, he had adopted Gregory’s (1917) term “Chinle” for all units above the Sonsela and below the Entrada. However, he still considered the lower Chinle to be Permian in age and therefore assigned strata of the lower Chinle Group (below the Sonsela Member of modern usage) to his concept of Moenkopi Formation. Between Darton’s studies, however, Mehl et al. (1916) had reported the first tetrapod fossils from the Fort Wingate area, including a pelvis of an aetosaur and the snout of a phytosaur, both from below the Sonsela Member and indicating a Late Triassic age for these strata. Mehl et al. (1916) thus were the first to establish a Late Triassic age for most of the well-exposed red-bed badlands of the lower Chinle Group in the Zuni Mountains. This stratigraphy remained relatively unstudied and unchanged until the 1950s, when uranium exploration spurred additional research, principally that of Smith (1954, 1957, 1958a,b), Cooley (1957, 1959a,b), and much of the USGS work published by Stewart et al. (1972a,b). Smith (1954) was the first to subdivide the Chinle of the Zuni Mountains into lower, middle, and upper members, with his informal “lower member” including all Triassic strata below the Sonsela Member, a lithostratigraphy he followed in a summary article on the Zuni Mountains (1957) and in later published mapping (1958a,b). He was also the first to recognize the distinct stratigraphic positions of the Sonsela (higher) �� �� ������������