Lucas_hunt_1992_main

Triassic stratigraphy and paleontology, Chama Basin and adjacent areas, North-Central New Mexico

Reviews the Triassic rocks and fossils found in the Chama River Basin, which includes the Ghost Ranch Coelophysis quarry.

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Abstract

Triassic strata in the Chama Basin and along the flanks of the Nacimiento and Jemez Mountains of Rio Arriba and Sandoval Counties, New Mexico, pertain to the Middle Triassic. Moenkopi Formation (Anton Chico Member) and the Upper Triassic Chinle Group (Agua Zarca, Salitral, Poleo, Petrified Forest and Rock Point Formations). The Moenkopi Formation is present only along the flanks of the Nacimiento and Jemez Mountains in Sandoval County. It is as much as 39m thick and consists mostly of grayish red siltstone, mudstone and immature, trough-crossbedded sandstone. The Agua Zarca Formation is as much as 61m thick and consists mostly of trough-crossbedded, quartzose sandstone and siliceous conglomerate. Near Coyote in Rio Arriba County, it overlies "mottled strata" developed in the top of the Pennsylvanian-Permian Cutler Formation. The Salitral Formation is as much as 102m of mostly purplish, smectitic mudstone. The Poleo Formation is up to 41m thick and is mostly grayish yellow, trough-crossbedded litharenites and subarkoses and minor amounts of both intrabasinal and siliceous conglomerate. Above the Poleo Formation, as much as 200m of strata, dominated by reddish brown, smectitic mudstones, constitute the Petrified Forest Formation. South of San Miguel Canyon in Sandoval County, the Poleo Formation is not present, and all of the mudstone section above the Agua Zarca Formation is assigned to the Petrified Forest Formation. Locally, near San Ysidro, the Correo Member is present at the top of the Petrified Forest Formation. In the Chama physiographic basin, the Rock Point Formation, as much as 70m thick and mostly reddish brown and grayish red siltstone and ripple-laminar sandstone, disconformably overlies the Petrified Forest Formation. Three formations of the Chinle Group in north-central New Mexico contain biochronologically important fossils, notably the aetosaur Longosuchus (late Carnian) in the Salitral Formation, the aetosaur Typothorax and the phytosaur Pseudopalatus (early-middle Norian) in the Petrified Forest Formation, and a new genus of phytosaur (late Norian/Rhaetian) in the Rock Point Formation. These fossils and lithostratigraphy allow precise correlation of the Chinle Group strata exposed in north-central New Mexico with other Upper Triassic strata in New Mexico.