PALEOMAGNETISM OF THE FRONT RANGE (COLORADO) MORRISON FORMATION AND AN ALTERNATIVE MODEL OF LATE JURASSIC NORTH-AMERICAN APPARENT POLAR WANDER

A paleomagnetic study of the Upper Jurassic Morrison Formation in the Front Range of central Colorado yields high-unblocking-temperature, dual-polarity magnetizations. With respect to known paleohorizontal, the inclinations (absolute mean = 57.3-degrees, 95% confidence interval = 52.3-degrees to 63.1-degrees, N = 8 sites) pass tilt and reversal tests, whereas the dispersion in declinations can be attributed to apparent or real tectonic rotations and sedimentary processes. The site-centered colatitudinal locus of possible Front Range Morrison poles partially overlaps the "upper" pole, but it excludes the "lower" pole from the Morrison Formation on the Colorado Plateau as well as the 151 Ma Glance conglomerate pole from the Basin and Range province of southeastern Arizona. We offer various explanations for these disparities and suggest an alternative model of Late Jurassic North American apparent polar wander through approximately 70-degrees-N which is supported by Late Jurassic European poles (with positive stability tests) transferred to North American coordinates.