Triassic stratigraphy and paleontology, Chama Basin and adjacent areas, north-central New Mexico

Abstract

~Triass ic strala 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 Formati on (Anton Chico Member) and the Upper Triassic Chinle Group (Agua Zarca, Sali tral , Poleo, Petrified Forest and Roc k Point Fomiations) . The Mocnko pi Formation is present only along the flanks of the Nacimiento and Jemez Mountains in Sandoval County. It is as much as 39 m thick and consists most ly of grayish red siltstone , mudstone and immature , trough-crossbedded sandstone. The Agua Zarca Formation is as much as 61 m thick and consists mostly of trough-crossbedded. q uartzose sandstone and siliceous conglomerate. Near Coyote in Rio Arriba County, it overlies " mott led strata" developed in the top of the Pennsy lvanian-Permian Cutler Form ation. The Salitral Formation is as much as 102 m of mostly purpl ish, smcct iti c mudstone. The Poleo Formation is up to 41 m thick and is mostly grayish ye llow, trough-crossbedded litharenites and subarkoses and minor amounts of both intrabasinal and siliceous conglomerate . Above the Poleo Formation , as much as 200 m of strata , dominated by reddi sh brown, smectitic mud stones, constitute the Petrifi ed Forest Formation . South of San Miguel Canyon in Sandoval County, the Poleo Format ion is not present. and all of the mudstonc sec tion above the Agua Zarca Formation is ass igned lo the Petrifi ed Forest Formation . Locally, near San Ysidro , the Correo Member is present at the top of the Pet rified Forest Formation. In the Chama physiographic bas in, the Rock Point Fom1a1ion, as much as 70 m thick and mostly reddish brown and gray ish red siltstone and ripplelaminar 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 Lonxosuch1u (late Camian) in the Salitral Formati on , the aetosaur Tvpothorax and the phytosaur Pseudopalatu.1· (early -middle Norian) in the Petrified Forest Formation, and a new genus of phytosaur (late Norian/Rhaet ian) in the Rock Point Formation. These foss ils and li thostratigraphy allow prec ise correlation of the Chinle Group strata exposed in north-central New Mexico with other Upper Triassic strata in New Mexico.