Jurassic Palaeomagnetism

Johnson, Allen H.; Nairn, A.E.M.

doi:10.1038/240551a0

Cited by 9 publications

(1 citation statement)

References 2 publications

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“…In particular, the Chinle poles of Reeve and Helsley [1972] and Reeve [1975] are excluded in favor of the Shinarump Member, Upper Shale Member, and Owl Rock Member poles. Previous determinations of the Kayenta paleomagnetic pole [i.e., Johnson and Nairn, 1972;Steiner and Helsley, 1974] also are excluded in favor of the palcomagnetic pole from the Kayenta Formation reported here. The Wingate Sandstone pole [Reeve, 1975] is excluded due to large uncertainties associated with its determination (k = 6.1).…”

Section: Late Triassic-early Jurassic Paleomagnetic Poles From the Somentioning

confidence: 99%

Paleomagnetism of the Chinle and Kayenta Formations, New Mexico and Arizona

Bazard¹,

Butler²

1991

J. Geophys. Res.

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Paleomagnetic data were obtained from 22 sites (6–10 samples/site) in the Upper Shale Member of the Chinle Formation, 43 sites in the Owl Rock Member of the Chinle Formation, and 35 sites in the Kayenta Formation. Thermal demagnetization and data analyses indicate that within‐site dispersion is an important criterion for selecting sites which retain a high unblocking temperature characteristic remanent magnetization (ChRM). Site‐mean directions define at least four antipodal polarity zones within each member/formation, suggesting the ChRM was acquired soon after deposition. Fifteen site‐mean virtual geomagnetic poles (VGPs) from the Upper Shale Member of the Chinle Formation yield an early Norian paleomagnetic pole position of 57.4°N, 87.8°E (K = 60, A95 = 5.0°). Eighteen site‐mean VGPs from the Owl Rock Member of the Chinle Formation yield a middle Norian paleomagnetic pole position of 56.5°N, 66.4°E (K = 183, A95 = 2.6°). Twenty‐three site‐mean VGPs from the Kayenta Formation yield a Pliensbachian pole position of 59.0°N, 66.6°E (K = 155, A95 = 2.4°). Combined with paleomagnetic poles from the Moenave Formation and the Shinarump Member of the Chinle Formation, these data record ∼30 m.y. of North American apparent polar wander (APW) within a regional stratigraphic succession. During the Camian and Norian stages of the Late Triassic, Chinle poles progress westward. During the Hettangian through Pliensbachian stages of the Early Jurassic, the pattern of APW changed to an eastward progression. Even after correction for 4° clockwise rotation of the Colorado Plateau, a sharp comer in the APW path (J1 cusp) is resolved near the pole from the Hettangian/Sinemurian (∼200 Ma) Moenave Formation (59.4°N, 59.2°E). Amongst other implications, the sharp change in the APW path at the J1 cusp implies an abrupt change from counterclockwise rotation of Pangea prior to 200 Ma to clockwise rotation thereafter.

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Section: Late Triassic-early Jurassic Paleomagnetic Poles From the Somentioning

confidence: 99%

Paleomagnetism of the Chinle and Kayenta Formations, New Mexico and Arizona

Bazard¹,

Butler²

1991

J. Geophys. Res.

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Apparent polar wander paths carboniferous through cenozoic and the assembly of Gondwana

Irving

1982

Geophysical Surveys

377

193

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Abstract. Directions of magnetization observed from rock-units from a given continental block are conveniently summarized by a path of apparent polar wander (apw) relative to that block. This article documents apw paths for the maj or continental blocks spanning the interval from Late Carboniferous (approximately -300 Ma) to the present. It is shown that by passing moving windowsof 30 or 40 Ma duration over the data an apw path can be obtained with little or no loss of features through smoothing. It also is shown that mean pole positions obtained using Fisher's statistics or by calculating modes do not differ significantly. In the analysis 'poor' results are screened out by applying certain minimum reliability criteria. Unit weight is then given to each of the remaining results. No attempt has been made to weight the results differentially because of the risk of added subjectivity.The paths contain many interesting features. One is an interval of rapid apparent polar motion relative to all major land-masses that occurred sometime in the Late Permian and Triassic. The general form of these motions is roughly established, but their timing and detailed form is poorly known because of inadequacies in the paleomagnetic record. Although it is clear that the motions occurred sometime around the Paleozoic-Mesozoic boundary it is not known whether the motions are or are not contemporaneous from continent to continent.Apw paths are of great importance for many problems in geology and geophysics. As an example, the new paths are applied to the question of how should Gondwana be correctly assembled. The results show that the best available reconstruction is the 1937 assembly of Alex du Toit as quantified by Smith and Hallam, with a minor modification by Barton and Molnar for the fit of Australia and Antarctica. Assemblies, that place Antarctica to the south and wrap the Antarctic Peninsula around the southern Andes, are not supported by the paleomagnetic data. There are, however, several obscurities, which derive, it is argued, from the uncertainties in timing the Late Permian to Triassic transitions alluded to above.

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