Paleomagnetism of the upper Keweenawan sediments: the Nonesuch Shale and Freda Sandstone

Abstract

The natural remanent magnetization of the upper Keweenawan Nonesuch Shale and Freda Sandstone has been analyzed with thermal, alternating field, and chemical demagnetization techniques. The results of this study are in good agreement with previously published works by DuBois and by Vincenz and Yaskawa, but place a tighter constraint on the North American apparent polar wander path. Fifty-eight samples, representing nearly 900 m of section, have been collected from the flanks of the Porcupine Mountain uplift. From principally thermal demagnetization analyses, a mean direction of primary magnetization has been calculated for the Nonesuch Shale, with declination 279.8°, inclination +9.8°, yielding a virtual geomagnetic pole position at 176.5° E, 10.3° N, and for the Freda Sandstone, with declination 271.3° inclination + 0.7°, yielding a virtual geomagnetic pole at 179.5° E, 1.2° N. A group of intermediate (secondary) components of magnetization is removed between temperatures of 350 °C and 550 °C, yielding well clustered directions. Its mean direction with declination 280.6°, inclination −9.5°, resulted in a virtual geomagnetic pole at 169.2° E, 3.7° N. This secondary magnetization is assumed to be of chemical origin and is most likely associated with the late Precambrian copper mineralization of the Nonesuch Shale. By thorough sampling of the stratigraphic column it is possible to infer the general direction of motion of a plate as the sediments were deposited. The motion of the North American plate as observed in the upper Keweenawan magnetizations is in agreement with the previously published polar wander paths for the late Precambrian.

The end of Midcontinent Rift magmatism and the paleogeography of Laurentia

“…7; Henry et al, 1977). Both in its age and position, the Michipicoten Island Formation pole bridges the gap between previous poles of the Keweenawan Track and the poles of these initial rift sediments.…”

“…This APWP is buttressed by over half a century of paleomagnetic study in the Midcontinent Rift (Halls and Pesonen, 1982, and references therein) and is foundational to work focused on reconstructing Proterozoic paleogeography (e.g., Li et al, 2008;Evans, 2009). The pioneering work of Rob Van der Voo and his group at the University of Michigan led to the development of poles from Keweenawan sedimentary rocks that extend the Keweenawan Track into the interval of sedimentation that followed rift magmatism (Henry et al, 1977;McCabe and Van der Voo, 1983). This work produced poles that remain in use for paleogeographic reconstructions to this day (e.g., Li et al, 2008).…”

“…Last, the proximity of late-rift volcanic paleomagnetic poles to those of Oronto Group sedimentary formations is consistent with deposition of the Oronto Group shortly after the end of Midcontinent Rift magmatism, although such proximity alone does not require this temporal relationship. It is noteworthy that the Oronto Group poles are relatively insensitive to inclination flattening within sediments, as paleomagnetic data from the Nonesuch Shale and Freda Sandstone exhibit both shallow positive and negative inclinations (Henry et al, 1977), such that inclination correction would have a negligible effect on their mean directions (i.e., both upper and lower hemisphere directions would be steepened during unflattening, leaving a mean direction similar to the original). We propose that age assignments for these Oronto Group poles should be ca.…”

“…The ages of late-stage volcanic rocks also constrain the young end of the Keweenawan APWP before the path transitions into poles from sedimentary rocks, for which it is more difficult to develop firm geochronological constraints. Current compilations of the Keweenawan Track show a significant directional difference between the late-stage pole of the Lake Shore Traps (Diehl and Haig, 1994) and the poles of younger sedimentary rocks (Henry et al, 1977)-even from sedimentary rocks of the Nonesuch and Freda Formations, which were likely deposited soon after or even during eruption of the youngest rift volcanic rocks. Improved paleomagnetic and geochronologic constraints on additional late-stage Midcontinent Rift volcanic rocks could bridge this gap between the cessation of rift volcanism and the deposition of these sediments.…”

Constraints on Conceptual and Quantitative Modeling of Early Diagenetic Sediment-Hosted Stratiform Copper Mineralization

“…Swenson et al [18] suggest that the pore water should be saline, given that the pore waters of most sedimentary basins are saline at depth [35]. In fact, in the case of the White Pine eSSC district, evaporitic salts and/or brines were probably contemporaneous with sedimentation in the Keweenaw rift basin due to a prevailing hot, dry climate at~10 • N latitude [36], even if evaporites as such are not known within the Keweenaw stratigraphy. As noted by White [37] and others, present-day seeps of water in the deep mine workings are invariably highly saline in the Keweenaw district.…”