Paleomagnetism and mangetostratigraphy of the Upper Triassic Petrified Forest and Poleo formations, north-central New Mexico, and the Bluewater Creek and Lower Petrified Forest Formations, central New Mexico
Kate E. Zeigler, John W. Geissman, and Spencer G. Lucas
As demonstrated by vertebrate biostratigraphy and palynostratigraphy, the Upper Triassic Chinle Group spans most of the Late Triassic. The Chinle Group was deposited by a fluvial system and consists predominantly of red to purple mudstones with some red to orange siltstones and red to buff sandstones. In the Chama basin, north-central New Mexico, both lower and upper Chinle Group strata are well-exposed and are being used to develop a more complete magnetic reversal chronology for the Late Triassic for the American Southwest. Our sampling methods concentrate on hematitic mudrocks, using an intricate block sampling technique. These materials typically carry a well-defined, well-grouped magnetization dominated by pigmentary hematite that is unblocked below about 660ºC. For example, a single horizon (level 39) in the Painted Desert Member of the Petrified Forest Formation yields an estimated site mean of D=172.9º, I=7.0º, α95=5.5º and k=102.5 (N = 8 independent samples). Indurated, coarser-grained deposits are sampled by more conventional methods. Sandstones and siltstones of the Poleo Formation contain both detrital and authigenic hematite and some detrital magnetite as the magnetization carriers and typically yield a well-defined magnetization at the site (single bed) level (e.g., site 11, eight independent samples, yields an estimated site mean of D=187.4º, I=0.8º, α95=3.4º and k=268.1). A very preliminary polarity reversal stratigraphy has been developed for the Poleo and Petrified Forest formations in the Chama basin. The Petrified Forest Formation includes multiple polarity intervals. All accepted sites in the Poleo Formation are of reverse polarity. The upper part of the Bluewater Creek Formation and lower Blue Mesa Member (Petrified Forest Formation) in the Zuni Mountains, central New Mexico, appear to be entirely of reverse polarity.
- Zeigler, Kate E.; Geissman, John W.; Lucas, Spencer G., 2005, Paleomagnetism and mangetostratigraphy of the Upper Triassic Petrified Forest and Poleo formations, north-central New Mexico, and the Bluewater Creek and Lower Petrified Forest Formations, central New Mexico, in: Geology of the Chama Basin, Lucas, Spencer G.; Zeigler, Kate E.; Lueth, Virgil W.; Owen, Donald E., New Mexico Geological Society, Guidebook, 56th Field Conference, pp. 115-128.