New Mexico Geological Society Annual Spring Meeting — Abstracts

Upper Triassic strata are exposed across most of New Mexico (except the southwestern portion of the state) and are nonmarine sedimentary rocks of fluvial, lacustrine and eolian origin assigned to the Chinle Group. The Chinle Group in New Mexico is of late Carnian (Tuvalian)-Rhaetian age based primarily on tetrapod biochronology supported by less precise biochronology provided by palynomorphs, megafossil plants, fishes, ostracodes and charophytes. Five stratigraphic terranes can be recognized in Chinle Group exposures in New Mexico: Colorado Plateau edge (McKinley, Cibola, Valencia Counties), Chama-Hagan basins (Rio Arriba, Sandoval and Bernalillo Counties), Dry Cimarron (Union County), Tucumcari basin (Santa Fe, Mora, Colfax, San Miguel, Guadalupe, Quay, Harding and De Baca Counties) and Carthage-southeastern plains (Socorro, Lincoln, Chaves, Eddy, Lea Counties). In the Colorado Plateau-edge terrane, the Chinle Group is as much as 500 m thick and consists, in ascending order, of the Shinarump, Bluewater Creek, Petrified Forest, Owl Rock and Rock Point Formations. In the Chama-Hagan-basins terrane, Chinle Group strata are as much as 412 m thick and consist, in ascending order, of the Agua Zarca, Salitral, Poleo, Petrified Forest and Rock Point Formations. The Dry Cimarron terrane exposes (in ascending order) the Baldy Hill, Travesser, Sloan Canyon and Sheep Pen Formations; total Chinle Group thickness is only as much as 282 m. Exposures of the Chinle Group in the Tucumcari basin terrane are the most extensive in the state. Maximum thickness is 593 m, and the following formations, in ascending order, are represented: Santa Rosa, Garita Creek, Trujillo, Bull Canyon and Redonda. The least extensive and thinnest (as much as 130 m) Chinle Group exposures in New Mexico are in the Carthage-southeastern plains terrane. Two formations-Santa Rosa overlain by San Pedro Arroyo-are represented. Subsurface data from the Lovington oil pool in Lea County, however, preserve a Chinle Group section as much as 458 m thick similar to that exposed in the Tucumcari basin terrane.

The base of the Chinle Group is a profound unconformity (Tr-3 unconformity) that separates it from underlying Middle Triassic (Moenkopi Formation) or Upper Permian (Guadalupian Artesia, San Andres or Glorieta Formations) strata. A pervasive unconformity (Tr-4 unconformity) is present in the middle of the Chinle Group, at approximately the Carnian-Norian boundary, at the base of a sandstone-conglomerate complex variously termed Sonsela, Poleo, Cobert Canyon and Trujillo. Another extensive unconformity (Tr-5 unconformity) separates the Rock Point, Travesser and Redonda Formations from underlying Chinle Group strata. Middle Jurassic (Entrada Sandstone) or Lower Cretaceous (Dakota Group) strata unconformably overlie the Chinle Group in New Mexico.