Reversal pattern and apparent polar wander for the Late Jurassic

Abstract

A paleomagnetic study of the Upper Jurassic (Kimmeridgian-Tithonian) Morrison Formation near Norwood, Colorado, indicates the existence of thirteen polarity intervals. The seven reversed intervals occupy much more of the section than the six normal intervals, suggesting that during this time the dominant polarity was reversed. Pole positions were computed for each portion of the section where directions were tightly grouped. The pole positions form two separate groups directly related to the stratigraphic position in the section of the samples from which they were computed. The two mean pole positions for the Morrison Formation, 142.2°E, 61.4°N (dp = 4.0°, dm = 6.5°) and 161.8°E, 67.5°N (dp = 3.5°, dm = 5.0°), define a path which includes the Cretaceous pole positions for North America. The data indicate that the Jurassic apparent polar wander curve for North America is approximately a line of latitude (present-day coordinates) connecting published Triassic and Cretaceous pole positions. The data disagree with the commonly held view that the Jurassic portion of the North American apparent polar wander curve includes the present axial dipole.