PALEOMAGNETISM OF THE MIDDLE JURASSIC SUMMERVILLE FORMATION, EAST CENTRAL UTAH

The paleomagnetism of the late Callovian(?) Summerville Formation was analyzed to obtain a late Middle Jurassic paleomagnetic pole for North America. A total of 281 samples were collected from 35 sedimentary horizons (sites) in a single locality in the San Rafael Swell area of east central Utah. Fifteen site-mean characteristic remanent magnetization (ChRM) directions pass the reversals test and define at least five polarity zones within 52 m of stratigraphic section, suggesting that the ChRM was acquired upon, or soon after, deposition. Magnetizations of some specimens are complex, and several horizons yield anomalous site-mean directions. Data analysis included filtering to provide different combinations of virtual geomagnetic poles for calculation of the paleomagnetic pole. However, editing the data did not change the pole position by more than 5-degrees. The preferred paleomagnetic pole position is 56.3-degrees-N, 133.4-degrees-E (A95 = 7.2-degrees; N = 11 sites). The Summerville Formation paleomagnetic pole is located near the approximately 172 Ma Corral Canyon pole and is statistically indistinguishable from the approximately 151 Ma Glance Conglomerate and approximately 149 Ma Lower Morrison poles. The paleomagnetic pole from the Summerville Formation is located at a much lower latitude and more easterly longitude than the paleomagnetic pole obtained from the approximately 165 Ma Moat Volcanics of New England. We propose that the Jurassic North American apparent polar wander path is an age-progressive band at 55-degrees-N to 65-degrees-N latitude extending from approximately 110-degrees-E longitude at approximately 172 Ma to approximately 150-degrees-E longitude at approximately 149 Ma.