PALEOMAGNETISM AND CORRELATION OF TRIASSIC STRATA OF THE COLORADO PLATEAU: NEW RESULTS FROM SE UTAH

Upper Triassic strata on the Colorado Plateau of SW North America have been assigned to the Chinle Formation. In southeast Utah, Chinle strata have been assigned, in stratigraphic order to the Blue Mesa, Moss Back, Upper Petrified Forest, Owl Rock and Rock Point Members. The Carnian-Norian boundary has been placed below the Moss Back Member (Sonsela Sandstone equivalent). At Comb Ridge, the Petrified Forest interval is about 190 meters thick, and it is characterized by brick red mudstones with intercalated medium to fine sandstone beds. We collected a total of 30 paleomagnetic sites from sandstone beds at irregular intervals on the section, averaging about 3m between sites. The samples were subjected to thermal demagnetization, which reveals that the natural remanent magnetization is composed of a north directed overprint stable up to 550°C and a characteristic magnetization (ChRM) of uniform reverse polarity. The ChRM is southeast directed and shallow, and resides in hematite. A preliminary formation mean obtained from 28 pilot demagnetized samples is of Dec=158.1° and Inc=-11.3° (k=16 and alpha95=7.0°). This result is problematic considering that: (1) the magnetostratigraphy reported for the Upper Petrified Forest in Arizona is dominated by normal polarities, and (2) the inferred time equivalent interval in the Newark Supergroup contains alternating intervals of reverse and normal polarity. Because several gaps exist in our sampling, a likely explanation is that normal polarity intervals were systematically “missed”, and that the section represents the polarity intervals E12 to E14 for the Newark sequence.