PRELIMINARY PALEOMAGNETIC DATA FROM THE YELLOW CAT AND POISON STRIP MEMBERS OF THE LOWER CRETACEOUS CEDAR MOUNTAIN FORMATION (BARREMIAN – APTIAN), GREEN RIVER, CENTRAL UTAH

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. At Crystal Geyser, south of Green River, the Cedar Mountain Formation is disconformable on the Jurassic Morrison Formation and is overlain by the Dakota Formation. Samples from a total of 19 sites provide preliminary paleomagnetic data to assess the possibility of a robust magnetostratigraphy. Five sites were collected in isolated sandstone bodies in the upper Morrison Formation, 11 sites were in sandstone, carbonate and travertine beds in the Yellow Cat Member of the Cedar Mountain Formation and 2 sites were collected from the Poison Strip Member of the Cedar Mountain Formation. Specimens were treated with both thermal and alternating field demagnetization. The Morrison Formation sites yielded either poorly defined to incoherent magnetizations, or well-behaved magnetizations that are inconsistent with expected Late Jurassic magnetizations (e.g., site 1: D = 346.3°, I = 42.6°). Specimens from the Yellow Cat Member yield magnetizations that ranged from incoherent to well-behaved, though most specimens yield poor to moderate quality demagnetization results (normal polarity grand mean: D = 004.2°, I = 61.0°, α₉₅ = 5.0°, k =70.0, N = 4 sites; reverse polarity grand mean: D = 164.4°, I = -65.6°, α₉₅ = 10.9°, k = 38.7, N = 1 site). Specimens from the Poison Strip Member also yield poor demagnetization responses. Rock magnetic experiments indicate that magnetic remanence in these strata is carried by a low coercivity phase, probably magnetite. Although the overall quality of the paleomagnetic data is not good, the presence of a well-defined interval of reverse polarity during deposition of the lower Cedar Mountain Formation is consistent with a pre-C34N, possibly M3r, age of deposition during the Barremian. Further field tests are also needed to better ascertain that these data truly represent Early Cretaceous magnetizations, and not a signature of considerably younger age.