PALEOMAGNETISM OF THE CHINLE AND KAYENTA FORMATIONS, NEW-MEXICO AND ARIZONA

Paleomagnetic data were obtained from 22 sites (6-10 samples/site) in the Upper Shale Member of the Chinle Formation, 43 sites in the Owl Rock Member of the Chinle Formation, and 35 sites in the Kayenta Formation. Thermal demagnetization and data analyses indicate that within-site dispersion is an important criterion for selecting sites which retain a high unblocking temperature characteristic remanent magnetization (ChRM). Site-mean directions define at least four antipodal polarity zones within each member/formation, suggesting the ChRM was acquired soon after deposition. Fifteen site-mean virtual geomagnetic poles (VGPs) from the Upper Shale Member of the Chinle Formation yield an early Norian paleomagnetic pole position of 57.4-degrees-N, 87.8-degrees-E (K = 60, A95 = 5.0-degrees). Eighteen site-mean VGPs from the Owl Rock Member of the Chinle Formation yield a middle Norian paleomagnetic pole position of 56.5-degrees-N, 66.4-degrees-E (K = 183, A95 = 2.6-degrees). Twenty-three site-mean VGPs from the Kayenta Formation yield a Pliensbachian pole position of 59.0-degrees-N, 66.6-degrees-E (K = 155, A95 = 2.4-degrees). Combined with paleomagnetic poles from the Moenave Formation and the Shinarump Member of the Chinle Formation, these data record approximately 30 m.y. of North American apparent polar wander (APW) within a regional stratigraphic succession. During the Carnian and Norian stages of the Late Triassic, Chinle poles progress westward. During the Hettangian through Pliensbachian stages of the Early Jurassic, the pattern of APW changed to an eastward progression. Even after correction for 4-degrees clockwise rotation of the Colorado Plateau, a sharp corner in the APW path (J1 cusp) is resolved near the pole from the Hettangian/Sinemurian (approximately 200 Ma) Moenave Formation (59.4-degrees-N, 59.2-degrees-E). Amongst other implications, the sharp change in the APW path at the J1 cusp implies an abrupt change from counterclockwise rotation of Pangea prior to 200 Ma to clockwise rotation thereafter.