Magnetostratigraphy of the Upper Triassic Chinle Group and Implications for the Age and Correlation of Upper Triassic Strata in North America

The Upper Triassic Chinle Group is considered Late Triassic in age, based on palynostratigraphy and vertebrate biostratigraphy. Deposition was thought to have begun during the Carnian and end in late Norian to earliest Rhaetian time. In the Chama Basin, northern New Mexico, strata below the Poleo Formation have been designated Carnian in age. Strata above and including the Poleo Formation are considered Norian in age and uppermost strata, termed Rock Point Formation, were originally assigned a Rhaetian age, then a late Norian age, based primarily on vertebrate biostratigraphy. A new magnetostratigraphy for the Chinle Group in the Chama Basin, and a new detrital zircon (DZ) date from the top of the Bluewater Creek Formation, Zuni Mountains, western New Mexico, change previous age assignments for Chinle Group strata. The DZ date implies that no Carnian-age strata are preserved and Chinle deposition began in early to middle Norian time. Most Chinle strata yield magnetizations with south or north-seeking declinations and shallow inclinations (e.g., Poleo Formation grand mean: D = 183.1°, I = 0.3°, α₉₅ = 5.7°, k = 33.9, N/No = 20/30 sites), which are interpreted as primary, Late Triassic magnetizations. Magnetostratigraphic correlations corroborate the DZ date and indicate that uppermost strata in northern New Mexico are not time equivalent to true Rock Point Formation or the Redonda Formation in eastern New Mexico. Uppermost Chinle Group strata in the Chama Basin are Rhaetian to possibly earliest Hettangian in age. A revision of age assignments of these strata changes the biostratigraphic framework utilized for correlations. For example, the aetosaur Aetosaurus, used as a Norian index fossil, is found in uppermost Chinle Group strata in the Chama Basin, thus extending its age range into the late Rhaetian. The dinosaur Coelophysis, considered Norian in age, is late Rhaetian to earliest Hettangian in age.