Early Silurian paleolatitude for central Newfoundland from paleomagnetism of the Wigwam Formation

Abstract

Redbeds and minor volcanic rocks of the Early Silurian Wigwam Formation of the Botwood Group were sampled in the Exploits Subzone of the Dunnage Zone of central Newfoundland. At 18 sedimentary and 3 volcanic sites thermal demagnetization isolated a shallow northward (or southward) magnetization (D = 356.0°, I = -16.8"; k = 12.9; o l , , = 9.2"; n = 21 site means) after correction for the tilt of bedding. A positive fold test demonstrates that this remanence predates regional Siluro-Devonian folding. A primary age for the remanence is indicated by positive conglomerate tests on volcanic clasts, and reversals correlated with sedimentary stratigraphy. Antiparallel remanence directions at sites of opposite polarity indicate that the primary remanence is not partially overprinted by a secondary component. The Exploits Subzone of the Dunnage Zone is interpreted to have been at low paleolatitudes (8.5"s + 5") during the Early Silurian. We reached a similar conclusion for the Notre Dame Subzone of the Dunnage Zone in a previous paleomagnetic study of Early Silurian redbeds and volcanic rocks of the King George IV Lake area. The low paleolatitudes obtained for the Wigwam and King George IV Lake rocks do not differ significantly and, hence, provide no evidence for a large ocean between the two subzones in the Early Silurian. However, the azimuthal orientation of the subzones at that time is still uncertain, and an ocean with its axis oriented in a north-south direction would not be detected by the paleomagnetic method.Les couches rouges et quelques volcanites de la Formation de Wigwam, du Groupe de Botwood, ont Ct C CchantillonnCs dans la Sous-zone dlExploits de la Zone de Dunnage, dans la partie centrale de Terre-Neuve. Les Cchantillons de 18 sites skdimentaires et de 3 sites volcaniques ont Ct C dtsaimantks par chauffage. Aprks correction pour I'inclinaison du litage ils ont fourni une aimantation nord (ou sud), isolCe, peu profonde ( D = 356°C; I = -16,8"; k = 12,9; a9, = 9,2"; moyenne de n = 21 sites). Le test du pli positif dCmontre que la remanence predate I'Cvtnement rCgional de plissement siluro-dtvonien. Un Lge primaire pour la rtmanence est indiquC par les tests positifs de conglomCrat sur les fragments volcaniques, et les inversions sont corrClCes avec la stratigraphie skdimentaire. Les directions antiparalltles de rCmanence aux sites de polarit6 opposCe rkvtlent que la rCmanence primaire n'est aucunement surimprimCe par une composante secondaire. La Sous-zone d'Exploits de la Zone de Dunnage est interprCtCe comme ayant Ctt localiste a de basses palColatitudes (8,5"S f 5") durant le Silurien prtcoce. Une Ctude palComagnCtique anterieure sur les couches rouges et les volcanites, d'Lge silurien prCcoce, dans la rCgion du lac King George IV a conduit 2 la mkme conclusion. Les basses palkolatitudes obtenues pour les roches de Wigwam et du lac King George IV ne diffkrent pas significativement et, par condquent, elles ne fournissent aucune Cvidence de l'existence d'un large octan entre les deux sous-zones au Silurien ...

Early Silurian paleolatitude for central Newfoundland from paleomagnetism of the Wigwam Formation: Discussion

“…Hodych and Buchan have reported, in abstract form, similar shallow inclinations from the red beds of the Sprlngdale Group [27]. The only exception to this general pattern is a shallower inclination (by 5 °) for three felsic flows included in the Wigwam Formation study of Buchan and Hodych [4]. Although the pattern of shallower inclinations in red beds is fairly consistent, it should be noted that the a95's are generally large, so that many of these differences in inclination are within the statistical uncertainty.…”

“…This separation gradually diminishes until there is no distinguishable paleolatitudinal differ- [3], KG = King George IV Lake area of Buchan and Hodych [2], WL = Wigwam Formation of Lapomte [3], WBH = Wigwam Formation of Buchan and Hodych [4], RA = Roberts Arm Group of Van der Voo et al [8], NA = North America [22], SB = eastern Avalon (southern Britain) [24,25], DP = western Avalon (Dunn Point volcanlcs, Nova Scotia) [34]. [3] 28+_ 8° [3] ence in the Silurian.…”

“…Table 2 lists the paleolatitudes of all Silurian studies of central Newfoundland rocks. The Wigwam Formation of the Botwood Group, studied by Lapointe [3] and Buchan and Hodych [4], consists mainly of red beds. In the former study the Wigwam red beds have shallower inclinations than the Lawrenceton volcamcs of the same group (28 ° vs. 47°).…”

“…Shallow inclinations are exemplified by the results from the King George IV Lake area [2] and by the results from the Wigwam Formation of the Botwood Group [3,4]. Intermediate inchnations have been observed from the Lawrenceton Formation of the Botwood Group [3,5].…”

“…The significant dextral offset suggested by the shallow directions is of the order of thousands of kilometers and is thus amenable to paleomagnetic documentation. It is ~mportant to note that steeper inclinations are predominantly associated with volcantcs (Lawrenceton Formation) [3,5], whereas the shallower inclinations are often associated w~th red beds (King George IV Lake area and Wigwam Formation) [2,3,4]. These inclination discrepancies would suggest largely different paleolatitudes for coeval Silurian rocks, which is implausible.…”