Triassic stratigraphy in the Zuni Mountains, west-central New Mexico
Andrew B. Heckert and Spencer G. Lucas

Abstract:

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, Petrifi ed 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 modifi cation of strata associated with development of the Tr-3 unconformity. Shinarump Formation strata are principally conglomerates and sandstone that represent channel deposits of a fl uvial 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 fl oodplain deposition, including both proximal (coarse-grained, crevasse-splay) and more distal (fi negrained, fl oodplain) deposits. In the western two-thirds of the Zuni Mountains, the contact between the Bluewater Creek Formation and the overlying Petrifi ed Forest Formation is defi ned 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 Petrifi ed Forest Formation are primarily highly bentonitic mudstone with well-developed calcrete (siderite) nodules that record fl oodplain 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 fi lled by Sonsela Member fl uvial deposits. The Sonsela grades upward into predominantly fi ne-grained strata of the Painted Desert Member, which represents a somewhat drier fl oodplain 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 fl uvial 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.


Citation:

  1. Heckert, Andrew B.; Lucas, Spencer G., 2003, Triassic stratigraphy in the Zuni Mountains, west-central New Mexico, in: Geology of the Zuni Plateau, Lucas, Spencer G.; Semken, Steven C.; Berglof, William R.; Ulmer-Scholle, Dana S., New Mexico Geological Society, Guidebook, 54th Field Conference, pp. 245-262.

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