Stratigraphy, paleomagnetism and magnetostratigraphy of the Upper Triassic Chinle Group, north-central New Mexico and preliminary magnetostratigraphy of the Lower Cretaceous Cedar Mountain Formation, eastern Utah.

摘要

The Upper Triassic Chinle Group is prominent in Mesozoic stratigraphy of the American Southwest. Revisions to Chinle Group nomenclature in the Chama Basin, northern New Mexico, include abandoning the term Zuni Mountains Formation, incorporating the lower member of the Salitral Formation into the Shinarump Formation, and the lower member of the Petrified Forest Formation into the Poleo Formation.;New paleomagnetic and magnetostratigraphic data have been compiled from the Chama Basin. Sampling at all sections focused primarily on hematitic mudrocks, using a block sampling technique. These rocks typically carry a well-defined magnetization residing primarily in pigmentary hematite, with laboratory unblocking temperatures below 660°C. All Chinle strata, except Shinarump Formation, typically yield magnetizations with either south or north-seeking declinations and shallow inclinations (e.g., Poleo Formation grand mean: D = 183.10°, I = 0.3°, alpha95 = 5.7°, k= 33.9, N/No = 20/30 sites), and are interpreted as primary, Late Triassic magnetizations.;Paleomagnetic poles for each unit seem indicate little or no apparent polar wander during deposition of most Chinle strata, followed by an abrupt shift of poles to the northeast from inferred Rock Point strata. These paleomagnetic poles could provide further evidence for a Late Triassic "stillstand", but more likely corroborate new age data suggesting there is no Carnian strata preserved. Pole positions also imply that strata termed Rock Point Formation in the Chama Basin are not correlative to either Rock Point strata in Arizona and Utah or to the Redonda Formation in eastern New Mexico. They are most likely time-correlative to the Triassic-Jurassic Moenave-Wingate Formations. A topographic high was present during Rock Point deposition in the late Norian, and no upper Norian strata were deposited in the modern Chama Basin region.;The Cedar Mountain Formation is well exposed in central Utah, and is one of few documented Lower Cretaceous sedimentary sequences in the western United States. Samples from 14 sites in the lower Cedar Mountain Formation provide preliminary paleomagnetic data to assess the possibility of constructing a robust magnetostratigraphy. A reverse polarity interval demonstrates that lower Cedar Mountain strata were deposited prior to the Cretaceous long normal and are probably Barremian in age.