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Abstract

A preliminary reversal stratigraphy has been resolved for three Lower to Middle Ordovician units in northern Arkansas. At least three magnetic components are present in these units. The first component removed (component A) with both thermal and AF demagnetization is steep and northerly. It reflects a modern viscous remanent magnetization (VRM), with possible minor contribution by recent goethite. The second component (component B), which yields the reversal stratigraphy, is shallow and easterly or westerly. The third component (component C) is shallow and south-southeasterly. Isothermal remanent magnetization (IRM) acquisition and thermal demagnetization reveals magnetite as the dominant magnetic phase in all samples. Many component B samples and some component C samples contain hematite as well.

Component B directions are similar to interpreted Ordovician directions from previous studies. A conglomerate test on mineralized breccias from the Rush MVT district demonstrates that component B was acquired prior to brecciation. Resolvable component B directions are much more common in sandy dolomite and sandstone of the Everton Formation and St. Peter Sandstone than in samples with no terrigenous sand. This suggests a link between the influx of cratonic material (including detrital magnetite) into the carbonate depositional basin and acquisition of component B magnetization. Component C yields a paleopole at 119.2°E and 49.5°N, which falls very near the middle portion of the Permian segment of the apparent polar wander path for stable North America. Component C is likely a chemical remanent magnetization (CRM) acquired during Permian time.

Polarity changes in component B can be correlated stratigraphically between sampled sections within the study area. Most of these correlations are consistent with correlations in lithology and magnetic susceptibility. The presence of a correlatable magnetostratigraphy is further evidence that component B was acquired during or soon after deposition. The preliminary polarity reversal pattern from this study crudely matches that of Piper (1987) but contains considerably more reversals than a compilation by Torsvik and Trench (1991).

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