PALEOGEOGRAPHY OF THE COLORADO PLATEAU REGION, PENNSYLVANIAN THROUGH CRETACEOUS, AND MID-MORRISON PLATEAU ROTATION

The Colorado Plateau resided in equatorial latitudes during mid-Pennsylvanian (Supai Gp) through Early Jurassic (Kayenta Fm) time. The center of the Plateau (e.g., Four Corners area) lay at paleolatitudes of 4-7^o N. No paleomagnetic data exist for most of the Middle Jurassic, but by latest Middle Jurassic (late Callovian, Summerville Fm), the Plateau center lay at 21^o N. An apparent still-stand of the region prevailed during the late Callovian and Oxfordian, shown by statistically identical paleopoles from the Summerville and overlying lower Morrison Formations; this is observed at locations both on the Colorado Plateau and on the craton. The consistency of paleopole location suggests that the J-5 unconformity may have encompassed little time. Thereafter, the Plateau and North America moved rapidly northward.

Paleomagnetic data suggest that after deposition of the lower sand-dominated (Salt Wash) member of the Morrison Formation, and prior to the deposition of the variegated claystones of the Brushy Basin Mbr, the Colorado Plateau was rotated substantially clockwise (4-5^o) relative to cratonic North America. The Plateau and craton paleopoles from the lower sand-dominated Morrison beds are appreciably displaced from one another, whereas the upper claystone paleopoles are statistically identical at the 95% confidence level. Although a physical hiatus is difficult to establish within this fluvial depositional package, two changes in sedimentologic characteristics coincide with both the difference in poles and relationship between Plateau and craton poles. First, sedimentation changes from sand-dominated to clay-dominated; moreover, the onset of volcanism also coincides with the paleopole changes. The change in grain size suggests a change in base level or eustasy, compatible with an intervening tectonic event, and the onset of volcanism is further compatible with preceding tectonism. The age of the postulated hiatus and rotation can be established both from a comparison of Morrison magnetostratigraphy to the sea-floor pattern and by the Ar/Ar ages from ash beds in the formation; this event appears to have taken place at ~152 Ma. The most recent evaluation of Colorado Plateau rotation suggests that the Plateau has been rotated a total of 9+/-3⁰ clockwise relative to cratonic NA since mid-Morrison times.