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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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The Neda iron formation is a hematite and goethite-rich oolitic ore which occurs in lens-shaped deposits on top of the Maquoketa Shale at only a few locations in the Wisconsin area. Its origin has been a puzzle for over a hundred years, but there have been suggestions that it is the westernmost extension of the Queenston Shale. Paleomagnetic studies were undertaken to see if paleopole directions from the two formations could aid in determining the origin of the Neda.
Thermal demagnetization of the Neda samples indicates the remanence is carried by hematite. Chemical demagnetization suggests the remanence is produced by the interstitial material rather than the oolites. The paleopole from 25 samples is at S 45.4°, W 48° (^agr 95 = 16°). This pole position is similar to Late Mississippian to Early Permian of North America rather than Late Ordovician. This suggests that the hematite in the ore was produced from dehydration of goethite during Late Mississippian time.
Queenston Shale samples from western New York were similarly measured. Thermal demagnetization indicates the remanence is carried by hematite and the pole position from 8 samples is at S 45°, W 38° (^agr 95 = 10°). This pole position is very similar to that of the Neda. This indicates that both formations were presumably subjected to the same post-depositional chemical changes in the late Paleozoic, but it does not conclusively show that the Neda is in fact the western extension of the Queenston. This late Paleozoic pole position has been found in almost all red sediments of Ordovician age in North America, both folded and nonfolded, which suggests that the remanence is not simply due to deformation produced by the Appalachian orogeny.
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