THE APWP FOR JURASSIC NORTH AMERICA: A CONTRIBUTION FROM THE BLACK HILLS OF WYOMING AND SOUTH DAKOTA

Several interpretations of the North American Apparent Polar Wander Path (APWP) for the middle to upper Jurassic presently exist. An analysis that is able to choose the correct APWP is necessary in order to test hypothesis regarding the accretion and translation of Cordilleran terranes.

Paleopoles identified from samples collected on the Colorado Plateau have pole latitudes of approximately 60N, whereas data collected in New England indicates pole latitudes of approximately 75N. The Colorado Plateau data includes the ~158 Ma Summerville Formation pole of 56.3N, 133.4E and the New England data includes the ~168 Ma Moat Volcanics pole of 78.7N, 90.3E.

In an attempt to resolve this ambiguity, a paleomagnetic study was done on the 164 million year old Sundance Formation of the Black Hills of Wyoming and South Dakota. Part of the North American craton, the Black Hills was subjected to tectonic uplift during the Paleocene to Eocene. 881 specimens from 40 sites were collected from the Sundance Formation, a series of tan and red shallow-marine sandstones and siltstones divided into seven members. 460 specimens from 29 sites were subjected to thermal and AF demagnetization techniques. Data from the Hulett member had the clearest results. This member showed a normal polarity overprint that was removed by 180C and a normal and reverse polarity characteristic direction that unblocked between 200C and 540C. A reversal test has determined that the normal and reverse characteristic directions are antipodal. Paleomagnetic fold tests were inconclusive.

Based on data from twelve sites, the in situ pole position is 83.3N, 144.2E, A95 of 7.5 and in tilt corrected coordinates, 78.0N and 139.8E with an A95 of 8.1. These poles are indistinguishable from the Eocene reference pole of Diehl et al. (1983). We thus interpret the characteristic directions obtained from the Sundance Formation as being the result of Eocene remagnetization, despite the positive reversals test.