Jurassic polar movement relative to North America

Abstract

Previous paleomagnetic studies of Jurassic rocks have not given concordant results and have led to the conclusion that the Jurassic pole position was possibly close to the present geographic pole. To test that supposition, the Kayenta, Carmel, Entrada, and Summerville formations were sampled from the extensive Jurassic sedimentary sequence in eastern Utah. The Lower and Upper Jurassic Kayenta and Summerville formations gave very similar magnetization directions. However, the Middle to Upper and Upper Jurassic Carmel and Entrada formations proved to be paleomagnetically unstable, and no reliable pole positions could be calculated. Poles calculated from two localities in the Kayenta formation and from the Summerville formation are 98°E, 59°N and 84°E, 59°N (Lower Jurassic) and 86°E, 70°N (Upper Jurassic, Oxfordian); thus they show a lack of major movement of the North American continent before Upper Jurassic (upper Oxfordian) time. Comparison with published Lower Cretaceous pole positions (135–100 m.y. in age) indicates that the 30 degrees of arc movement in the North American polar-wander curve (from the Upper Jurassic to the Lower Cretaceous pole positions) occurred in approximately 30 m.y. Thus the major portion of the spreading in the North Atlantic between Africa and North America did not occur until uppermost Jurassic-Lower Cretaceous times, making this period one of rapid sea-floor spreading compared to that of the Late Cenozoic.