Paleomagnetism of the Wilberns Formation (Texas) and the Late Cambrian paleomagnetic field for North America

Abstract

The natural remanent magnetizations of Late Cambrian siltstones of the Wilberns Formation have been analyzed with alternating field, thermal, and Chemical demagnetization techniques. From mainly thermal demagnetization analyses a mean direction of magnetization has been calculated: declination D of 101.5 ø, inclination I of + 13.5 ø, k = 171, and ag• = 4.6 ø for seven sites (55 samples), yielding a virtual pole position at 6.4øN, 158.8øE, dp= 2.4 ø, and dm= 4.7 ø. This polar error (dp, din) represents the oval of 95% confidence about the pole position. Comparisons with previously published Late Cambrian data reveal good agreement between the thermally isolated directions of magnetization. The difference in the published results obtained through alternating field demagnetization can be explained by a late Paleozoic remagnetization or by a previously unexpected Cambrian shift or loop of the apparent polar wander path.

Paleomagnetism of the Cambro-Ordovician McClure Mountain alkalic complex, Colorado

“…has not yet been presented. Furthermore, Larson et al [6] report a pole near the top of the loop for the Black Canyon diabases in southwestern Colorado, which are dated at 497 + 16 Ma [6], clearly younger than the Point Peak pole of Van der Voo et al [3] at the base of the loop and thus discordant with the loop. However, the Black Canyon pole has not been conclusively shown to represent a primary TRM and is far removed from the Lower Ordovician poles of Jackson et al [7] and Dankers and Lapointe [8], which are also near the base of the loop.…”

“…Still, many segments of the APWPs are poorly defined or questionable, even for well-studied continents; one such segment is the Cambrian section of the North American APWP. Published poles define a loop extending from 5°N, 158°E (the Lower Cambrian Tapeats formation [2]) northwest to 59°N, 89°E (the upper Middle Cambrian Abrigo formation [2]) and returning to 6°N, 159°E (the Upper Cambrian Point Peak member of the Wilberns formation [3]). There are two reasons why the loops is suspect: (1) it requires an unusual se-quence of rotations and counterrotations for the North American continent during the Cambrian, and (2) the upper part of the loop overlaps the upper Paleozoic segment of the North American APWP.…”

Paleozoic paleomagnetism and northward drift of the Alexander Terrane, southeastern Alaska

“…The presence of antipodal reversals in these rocks lends additional support to an older, presumably Late Devonian, age of magnetization.. The available Late Cambrian, Ordovician, and Silurian poles were discussed by Van der Voo and French [1977] and Watts and Van der Voo [1979] and for each period these poles form rather well defined clusters (Figure 13 and Table 3).…”

Neotype and Redescription of the Upper Cambrian Anthaspidellid Sponge, Wilbernicyathus Donegani Wilson, 1950

“…The Wilberns Formation was deposited on a wide, shallow plat form on the southern edge of Laurentia, which straddled the equa tor at that time (Lochman-Balk, 1970;Van der Voo et al, 1976). Outcrops of the Wilberns Formation are restricted to the Llano Uplift region of central Texas (Barnes and Bell, 1977), although there is an extensive subcrop in Texas.…”