PALEOMAGNETISM OF THE MOENAVE FORMATION IN SW UTAH: IMPLICATIONS FOR MAGNETOSTRATIGRAPHY ACROSS THE TRIASSIC-JURASSIC BOUNDARY AND INCLINATION SHALLOWING

We present new paleomagnetic data of the Triassic-Jurassic Moenave Formation from two localities in southwestern Utah, Leeds and Warner Valley, exposed in the western transition zone of the Colorado Plateau. Paleomagnetic inclination directions observed in 74 specimens from one sandstone bed are analyzed to quantify the amount of inclination shallowing using the elongation/inclination method described by Kent and Tauxe (2005). Inclination shallowing is measured by a flattening factor that accounts for discrepancies between observed paleomagnetic inclination directions and the inclination directions predicted by the geomagnetic field model TK03.GAD at the same site. Mesozoic strata located in the American Southwest experienced inclination flattening during lithification that may or may not be consistent with the degree of compaction recorded in strata of eastern North American Mesozoic rift basins (f= 0.48-0.64).

Western outcrops of the Moenave Formation consist of fluvial sandstones and mudstone of the basal Dinosaur Canyon Member, which is overlain by predominantly lacustrine sandstones and mudstones of the Whitmore Point Member. Typical thermal demagnetization behavior includes present day viscous overprint that unblocks at about 500°C followed by a characteristic vector component that unblocks by 670-680°C. Chemical demagnetization is most effective after a 30-minute soak in HCl, but specimens mostly disintegrate by 24 hours, suggesting remanence is mostly carried by pigmentary hematite cement.

The Dinosaur Canyon Member sampled to date from at both localities exhibits exclusively normal polarity directions consistent with expected directions for the latest Triassic to earliest Jurassic. Sandstones of the Whitmore Point Member have only been sampled at Leeds and also record normal polarity directions with the exception of one site that exhibits reverse polarity (D: 170.9, I: 13.9, α95: 22.8, n: 4). This reversal may be correlative to E23r recorded in Newark Basin strata and/or SA5r recorded at St. Audrie's Bay. However, these correlations are tentative given the lack of convincing biostratigraphic constraints.