Abstract

Oriented cores, taken at stratigraphic intervals of 9 to 12 inches, have been collected from the basal 500 feet of the Moenkopi Formation along the Dolores River in western Colorado. The NRM and the demagnetized data from these samples indicate that the Lower Triassic magnetic field is characterized by at least eleven reversals. Samples from the normal polarity intervals are initially well clustered and remain so after AC or thermal demagnetization. Samples with reversed polarity are initially scattered and remain so after AC demagnetization. However, after thermal demagnetization to 500° C, they become well clustered and have a direction approximately 180° from that of the normal intervals.

All samples belong to either the normal or reversed groups, and no samples transitional between normal and reverse were observed.

A tentative correlation of the resulting polarity reversal sequence with that from the Chugwater Formation of Wyoming is proposed. A magnetic anomaly pattern similar to this polarity reversal sequence is observed in magnetic profiles at sea, indicating that polarity reversal sequences may be a useful means of both regional correlation and calibration of the sea-floor spreading rates postulated for the ocean basins.

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