Paleomagnetism and magnetic fabrics from the Springdale and Wigwam Redbeds of Newfoundland and their implications for the Silurian paleolatitude controversy

Abstract

In many mobile belts, paleomagnetic directions from clastic sedimentary rocks are typically shallower than directions from associated igneous rocks. This discordance raises questions about the reliability of sedimentary rocks and can lead to controversial paleogeographic reconstructions. In Newfoundland, the controversy over Silurian paleogeography arises from a difference between results from clastic redbeds (with anomalously shallow directions that place Newfoundland at the paleoequator in the Silurian) and coeval epicontinental volcanics (with steeper directions that place Newfoundland at a more southerly paleolatitude). This study tests the possibility that the anomalously shallow redbed directions are the result of internal strain or inclination error related to deposition. We specifically compare coarse- and fine-grained lithologies under the assumption that such remanence-altering effects will be different in rocks with differing mechanical competence. We show that inclinations correlate with lithologic variations, but in a manner opposite to that predicted for the accumulation of internal strain. Based upon a strict set of demagnetization criteria, clearly defined single-component characteristic directions are only observed in the finer grained sandstones. These well-determined directions yield a pre-folding and pre-rotational dual polarity magnetization that places Newfoundland at a paleolatitude of 23° (S) ± 9° in the Silurian. This revised paleolatitude is consistent with the volcanic results and with the paleolatitude predicted for Newfoundland from the North American reference path. In contrast, coarse-grained sandstones yield shallower directions that appear to be composites of opposite polarity magnetizations or weakly defined directions with strongly contaminating overprints. Because the shallow magnetizations in the coarser grained samples may have originated as a depositional remanence, we also suspect inclination error related to deposition. The degree of inclination error is consistent with values predicted for redbeds from previously published experimental results. These observations suggest that grain-size considerations are important in assessing the reliability of redbed magnetizations. Recognition of the relationship between grain size and remanence acquisition allows us to resolve the Silurian paleolatitude controversy for Newfoundland.