Abstract

A vector analysis of chemical leaching and thermal demagnetization results of 179 specimens of the Hopewell Group shows that three magnetizations were acquired in a nearly parallel direction but at different times as indicated by polarity. Field evidence indicates that magnetization occurred within 35 m.y. of deposition. It is suggested that these red beds were magnetized in three stages during a magnetization process which began at the time of deposition; hence producing one detrital (DRM) and two chemical (CRMA and CRMB) remanent magnetizations in that order. The two CRMs can be successively removed chemically to uncover the DRM. The CRMB is demagnetized by thermal treatment at 600–674 °C, but, the DRM and the CRMA cannot be separated thermally. This accounts for apparently aberrant directions obtained after thermal demagnetization at 674 °C, because the resultant vector of the remaining (and sometimes oppositely directed) DRM and CRMA is not directed along the direction of the magnetizing field. However, by vector analysis of the combined chemical and thermal results, the directions of the three magnetizations can be determined and an accurate field direction (174°, + 15°; α95: 3°; pole: 36 °N, 123 °E) is thus obtained. More of the information contained in the rocks can also be retrieved from the within-specimen magnetization observed by cutting specimens at some stage during chemical or thermal treatment. For example, the results indicate the following about field reversals: Several of them occurred at the Pennsylvanian–Mississippian boundary; the intensity of the field may remain constant at the beginning of a reversal; reversals may be rapid, and, in some instances, of short duration, leading to the suggestion that, at reversal time, the field may be subjected to an oscillatory motion before stabilizing in the same or opposite polarity.

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