Triassic stratigraphy of the southeastern Colorado Plateau, west-central New Mexico
— Spencer G. Lucas


Triassic strata are well exposed along the southeastern Colorado Plateau and in adjacent areas in west-central New Mexico (McKinley, Catron, Cibola and parts of Valencia and Socorro counties). The oldest Triassic strata are assigned to the Anton Chico Member of the Moenkopi Formation, which disconformably overlies Permian strata and is disconformably overlain by the Upper Triassic Chinle Group. The Moenkopi Formation in west-central New Mexico is as much as 68 m thick, and consists of mostly grayish-red and reddish-brown sandstone, mudstone siltstone and conglomerate. Overlying Triassic strata in west-central New Mexico are assigned to seven formations of the Chinle Group (in ascending order) – the Shinarump /Zuni Mountains formations, the Bluewater Creek/San Pedro Arroyo formations, the Petrified Forest Formation (including Blue Mesa, Sonsela and Painted Desert members), the Owl Rock Formation and the Rock Point Formation – and to the Wingate Sandstone. As much as 24 m of silica-pebble conglomerate/sandstone and pedogenically-modified and color-mottled siltstone, mudstone and sandstone of the Shinarump and Zuni Mountains formations, which are lateral equivalents, are at the base of the Chinle Group. The 50-60 m of Bluewater Creek Formation are dominated by red sandstones and mudstones and are laterally equivalent to the southeast to the color-mottled San Pedro Arroyo Formation. The Petrified Forest Formation is up to 441 m thick and is divided into the (ascending) Blue Mesa, Sonsela and Painted Desert members. The Blue Mesa Member is 21-45 m thick and mostly purplish and greenish bentonitic mudstone with calcrete nodules and a white tuffaceous sandstone locally present at the base. The Sonsela Member is 15-61 m thick and mostly yellowish-gray, cross-bedded sandstone and siliceous conglomerate with fossil logs. The Painted Desert Member is up to 325 m thick and is mostly reddish brown mudstone and siltstone. It includes some relatively thin, but laterally persistent sandstone units, the Perea and Correo beds. The Owl Rock Formation is up to 35 m thick and characterized by reddish siltstone and beds of white limestone. The Rock Point Formation is up to 70 m thick and consists of repetitively-bedded, reddish-brown sandstone and siltstone. The Wingate Sandstone is the stratigraphically-highest Triassic (?) unit in west-central New Mexico. The Moenkopi Formation is Anisian in age, whereas the Chinle Group ranges from middle Carnian to Rhaetian in age. The base of the Moenkopi Formation is a regional unconformity (Tr-0/Tr-2 unconformities), and the base of the Chinle Group is the regional Tr-3 unconformity. There are two within-Chinle regional unconformities, Tr-4 at the Sonsela base and Tr-5 (=J-0) at the Rock Point base. The Middle Jurassic Entrada Sandstone overlies Triassic strata across west-central New Mexico at the J-2 unconformity

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Recommended Citation:

  1. Lucas, Spencer G., 2021, Triassic stratigraphy of the southeastern Colorado Plateau, west-central New Mexico, in: New Mexico Geological Society, 71st Annual Fall Field Conference, September 2021, Geology of Mount Taylor, Frey, Bonnie A.; Kelley, Shari A.; Zeigler, Kate E.; McLemore, Virginia T.; Goff, Fraser; Ulmer-Scholle, Dana S., New Mexico Geological Society, Guidebook, pp. 229-240.

[see guidebook]