A Lower Ordovician geomagnetic pole from Pennsylvania

Abstract

Basalt, diabase, and arkose were sampled from the Middle Ordovician Martinsburg Formation in Lebanon County, Pennsylvania, for paleomagnetic study. The sampled units, situated within blocks emplaced in the Middle Ordovician sedimentary basin during the Taconic orogeny, are believed to be Lower Ordovician. In the course of orogenic deformation, the sampled units and host rock were extensively folded, a factor which had to be accounted for in the derivation of the paleomagnetic pole. The Lower Ordovician virtual geomagnetic pole, calculated after alternating field and thermal cleaning of 26 samples from four sites, lies at 159.8°E longitude and 35.8°N latitude. The agreement of this pole with other Lower Paleozoic poles for the North American plate demonstrates that intense regional deformation need not preclude the recovery of a valid paleomagnetic pole. There does exist a possibility of uncorrected rotation of the block in which the rocks were emplaced.