Abstract

An investigation of the magnetic reversal stratigraphy in the Redonda Member and the underlying shale member of the Chinle Formation at Montoya, New Mexico, reveals the presence of at least eight separate polarity intervals in the upper 133 m of the Chinle Formation. At Lucianna Mesa south of Montoya the contact between the Redonda Member and the underlying unnamed shale member occurs 3 m above a transition from normal (below) to reversed polarity (above), the reversal sequence within the Redonda having eight polarity intervals. Measurement of samples collected from the same stratigraphic interval on Redonda Mesa 30 km to the east serves as a check on a portion of this reversal sequence. The Chinle Formation in eastern New Mexico is considered to have been contemporaneous with the Newark group in the eastern United States on the basis of vertebrate paleontology. Published investigations of the Newark group have revealed only normal magnetic polarities. Because the magnetic field reversed for the entire earth the Newark cannot exhibit different polarities from those in the Chinle if the two units acquired their magnetization at the same time. Thus, there are inconsistencies between the paleontologic and paleomagnetic data for the two rock units. These inconsistencies are best answered by assuming the Chinle in New Mexico is partially or completely older than the Newark. The latter explanation is improbable when carried to the extent of assigning the Chinle a Middle Triassic age as proposed by McElhinny and Burek (1971). Until the temporal relation between the Chinle and Newark is clarified, the Newark group should not be used to define a reversal chronology.

A mean paleomagnetic pole position at 79° E. long, 58° N. lat was calculated for approximately the upper half of the Chinle Formation at Montoya. Pole positions for two well-demagnetized polarity intervals (one from each member studied) have been combined with other published Upper Triassic pole positions (demagnetized) to obtain a mean Upper Triassic paleomagnetic pole position at 89° E. long, 62.5° N. lat. The similarity of this mean and of the individual Chinle poles to the published Lower Triassic pole determinations based on demagnetized samples indicates little, if any, latitudinal polar wander-continental drift relative to North America during most it not all of the Triassic.

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