Abstract

The volcanic Crowsnest Formation of Albian age (late Early Cretaceous) from the Rocky Mountain fold and thrust belt of Alberta has a stable remanent magnetization with a mean direction of 349°, 59 °(α95 = 5°) and paleopole at 78°N, 108°E(dm = 7°, dp = 5°). The inclination is lower than, and the declination clockwise of, the expected mid-Cretaceous paleogeomagnetic field for cratonic North America. Taken at face value the result indicates that the Crowsnest Formation and the thrust sheet in which it occurs have been transported from the south relative to cratonic North America by 17 ± 6 °(about 1800 km) and rotated 24 ± 10° clockwise. It is also possible that flattening of inclination is caused by magnetic anistropy, but tests show this to be unlikely. A third possibility is that the magnetization is secondary and of latest Cretaceous age, but there are good reasons for believing this is not so. Lastly, it is possible that the unit could have been formed close to its present position relative to the craton but was deposited so quickly that the paleosecular variation was not adequately sampled, and the result is only a "spot" reading of the paleofield. The last is our preferred interpretation of the flattened inclination, but the clockwise deflection of the declination could reflect rotation. Other paleomagnetic data from the fold and thrust belt are generally consistent with the third interpretation.

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