Evidence for penecontemporaneous magnetization of the Moenkopi Formation

Although paleomagnetic data obtained from red beds of the Moenkopi formation give every indication of being stable, it has been suggested that beds of this type do not acquire magnetizations penecontemporaneously with their deposition and that they could have progressively acquired their magnetizations and thus integrated over long periods of geologic time. Serial correlation tests, dispersion comparisons, and Fourier analyses were carried out on samples possessing stable magnetizations from this formation. The results of these studies indicate that the magnetization was not acquired over a long-term period. These tests also suggest that the time duration for complete magnetization of a specimen 2.2 cm in height was less than one nondipole secular variation period, i.e., less than about 1800 years. Six transitions are recorded in the samples that we have used for this study. These transition records suggest that field changes were abrupt and that the time duration of the field intensity decrease was slightly longer than that of the direction change. A detailed study of the nature of transition patterns yields evidence that the time difference between the completion of magnetizations arising from specularities and pigments was insignificant, on the assumption that both carry parts of the stable magnetization. It is thus strongly suggested that magnetization was penecontemporaneous with deposition.