Paleomagnetism of Late Triassic and Jurassic sediments of the southwestern United States.

Abstract

Paleomagnetic poles were obtained from the Chinle, Kayenta, Summerville, and Morrison Formations. Combined with paleomagnetic poles from the Moenave Formation, poles from the Chinle and Kayenta formations record ∼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° clockwise rotation of the Colorado Plateau, a sharp corner in the APW path (J1 cusp) is resolved near the pole from the Hettangian/Sinemurian (∼200 Ma) Moenave Formation (59.4°N; 59.2°E). The J1 cusp implies an abrupt change from counterclockwise rotation of Pangea prior to 200 Ma to clockwise rotation thereafter. Paleomagnetic poles obtained from the Summerville and Morrison formations are consistent with the Middle and Late Jurassic APW path described by the Corral Canyon and Glance Conglomerate paleomagnetic poles as well as a Late Jurassic Cusp (J2 cusp) in the APW path. The APW path described by the J2 cusp, a single Morrison Formation pole, and mid-Cretaceous paleomagnetic poles suggest from ∼150-126 Ma the North American plate experienced a minimum rate of motion of 0.93°/m.y. which is similar to rates calculated for the Late Triassic (0.73°/m.y.) and Jurassic (0.66°/m.y.). This rate is much lower than rates based on previous APW paths. Thermal demagnetization and data analysis indicate that within-site dispersion is an important criterion for selecting sites which retain a high unblocking-temperature, characteristic remanent magnetization (ChRM). This criterion was used to define at least three stratigraphically-distinct, antipodal polarity-zones within each member/formation, suggesting the ChRM was acquired soon after deposition. ChRMs from 15 to 22 sites in the Upper Shale Member of the Chinle Formation define an early Norian paleomagnetic pole position of 57.4°N, 87.8°E (K = 60, A₉₅ = 5.0°). ChRMs from 18 to 43 sites in the Owl Rock Member of the Chinle Formation define a middle Norian paleomagnetic pole position of 56.5°N, 66.4°E (K = 183, A₉₅ = 2.6°). ChRMs from 23 of 35 sites in the Kayenta Formation define a Pliensbachian pole position of 59.0°N, 66.6°E (K = 155, A₉₅ = 2.4°). ChRMs from 15 to 35 sites in the Summerville Formation define a late Callovian pole position of 53.8°N, 133.6°E (K = 25, A₉₅ = 7.5°). ChRMs from 15 sites in the Morrison Formation (9 from the study of Steiner and Helsley [1975]) define a single, ∼147 Ma, paleomagnetic pole position of 64.1°N, 152.4°E (K = 113, A₉₅ = 3.6°).