A Lower Ordovician paleomagnetic pole from the Oneota dolomite, Upper Mississippi River Valley

Publisher Website
Google Scholar
Abstract
Of 149 oriented specimens of the lower Ordovician Oneota dolomite subjected to stepwise thermal or alternating‐field demagnetization, 50 contained a characteristic shallow east‐southeasterly or west‐northwesterly component of magnetization identifiable on orthogonal vector projections. For 48 other specimens the demagnetization trajectories defined greats‐circle paths which intersect close to the characteristic direction. The site‐mean directions for the characteristic component were determined by using both the characteristic directions and great‐circle data, and these yield a formation‐mean direction obtained from 10 sites with a declination of 105°, an inclination of +2°, and a precision parameter k of 17.6 (α95 = 11.9°). The mean of the site virtual poles falls at 10.4° N, 166.4° E (K = 48.0, A95 = 7.0°), which is close to previously published late Cambrian paleopoles. Both polarity groups have predominantly positive inclinations, suggesting the presence of an uncleaned downward component whose unblocking‐temperature and coercivity spectra overlap very strongly with those of the characteristic magnetization. We interpret this downward component as a drilling‐induced remanent magnetization, and we hope to have removed its contribution by averaging the dual‐polarity results. Multicomponent magnetizations were also present in samples collected from blocks of Oneota dolomite incorporated into the overlying New Richmond sandstone (lower Ordovician), but only six out of eight samples yielded well‐defined greats‐circle paths. The best fit intersection of these defines a northerly and steep recent component, while the directions of the higher‐stability components are different for each block. Due to the small number of samples and imperfect resolution of magnetic directions, the conglomerate test is not conclusive; nevertheless it suggests that the stable remanence in the Oneota dolomite was acquired prior to the deposition of the New Richmond sandstone.