Triassic stratigraphy, biostratigraphy and correlation in east-central New Mexico
— Spencer G. Lucas, Andrew B. Heckert, and Adrian P. Hunt
Triassic strata in east-central New Mexico are siliciclastic red beds of Middle and Late Triassic age. As much as –500 m thick, the Triassic section is assigned to the Middle Triassic Moenkopi Formation (Anton Chico Member) and overlying Upper Triassic Chinle Group (Santa Rosa, Garita Creek, Trujillo, Bull Canyon and Redonda Formations). The Anton Chico Member of the Moenkopi Formation is as much as 46 m thick and is mostly trough crossbedded micaceous litharenite. It disconformably overlies Middle Permian red beds of the Artesia Group, and is disconformably overlain by the Upper Triassic Santa Rosa Formation. The Santa Rosa Formation consists of three members: (1) basal, Tecolotito Member, as much as 34 m thick and mostly grayish-orange, trough crossbedded and ripple laminated quartzarenite; (2) medial, Los Esteros Member, as much as 44 m thick and mostly red-bed mudstone; and (3) upper, Tres Lagunas Member, as much as 46 m thick and mostly orange and yellowish brown, crossbedded quartzarenite. The Garita Creek Formation conformably overlies the Tres Lagunas Member and is as much as 76 ID thick and mostly red-bed mudstone. It is disconformably overlain by the Trujillo Formation, a medial sandstone complex of the Chinle Group as much as 68 m thick. The Bull Canyon Formation conformably overlies the Trujillo and is as much as 110 in thick and mostly red-bed mudstone. We name a new unit in the Bull Canyon Formation, the Saladito Point Bed, for a discrete, laterally persistent sandstone 12.2 m thick and approximately 30 m above the base of the formation. The Bull Canyon Formation is disconformably overlain by the Redonda Formation, which is up to 92 m thick and mostly laterally continuous, repetitive beds of red-bed sandstone and siltatone. The Middle Jurassic Entrada Sandstone disconformably overlies the Redonda Formation, although the Triassic section is also erosionally truncated locally by younger units throughout east-central New Mexico.
The Anton Chico Member of the Moenkopi Formation correlates with the uppermost Holbrook Member of the Moenkopi Formation in eastern Arizona. The occurrence of the capitosaurid amphibian Eocyclotosaurus in the Anton Chico Member indicates a Perovkan (Anisian) age. The erosional surface at the base of the Moenkopi Formation in east-central New Mexico is a compound unconformity corresponding to the Tr-0, Tr-1 and Tr-2 unconformities of earlier authors.
Chinle Group strata in east-central New Mexico correlate readily to the Chinle lithosome in West Texas, Oklahoma, Colorado and across the Colorado Plateau. These strata define three major sequences, in ascending order: the Shinarump-Blue Mesa (Santa Rosa-Garita Creek), Moss Back-Owl Rock (Trujillo-Bull Canyon) and Rock Point (Redonda) sequences. Unconformities at the base of the Santa Rosa, Trujillo and Redonda Formations correspond to the Tr-3, Tr-4, and Tr-5 unconformities, respectively. A lower-order sequence boundary may be present in the Chinle, expressed as a regionally persistent, planar-crossbedded litharenite exposed in east-central New Mexico (Saladito Point Bed), western New Mexico (Peres Bed) and in east-central Arizona at the Petrified Forest National Park (Flattops 2 and Painted Desert 3 sandstones).
Numerous lines of biostratigraphic evidence, principally tetrapod fossils, but also including megafossil plants, ostracodes and invertebrates, constrain the ages of Upper Triassic strata in east-central New Mexico and support the correlations advanced here. These indicate that the Tecolotito Member of the Santa Rosa Formation is of Otischalkian (early-late Carnian) age, and the Los Esteros and Tres Lagunas members and the Garita Creek Formation are of Adamanian (latest Carnian) age. The Tr-4 unconformity approximates the Carnian-Norian boundary, and the Trujillo and Bull Canyon Formations are of Revueltian (early-mid Norian) age. The Redonda Formation is of Apachean (late Norian- Rhaetian) age. The type Revueltian fauna is known from the Bull Canyon Formation in Revuelto Creek, and the type Apachean fauna is the vertebrate fauna of the Redonda Formation in Apache Canyon, both in Quay County.
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- Lucas, Spencer G.; Heckert, Andrew B.; Hunt, Adrian P., 2001, Triassic stratigraphy, biostratigraphy and correlation in east-central New Mexico, in: Geology of the Llano Estacado, Lucas, Spencer G.; Ulmer-Scholle, Dana S., New Mexico Geological Society, Guidebook, 52nd Field Conference, pp. 85-102.