Abstract
The Neda Iron Formation (latest Ordovician) is a hematite and goethite rich oolitic ore which occurs in lens shaped deposits on top of the Maquoketa shale (Late Ordovician), at only a few locations in Wisconsin, Iowa and Illinois. Thermal demagnetization of samples from these three states, indicates the remanence is carried by hematite. Chemical demagnetization suggests the remanence is produced by the interstitial material rather than the oolites.The paleopole averaged for all sites from 25 samples of the Neda ore is at S45.4°, W48° (α95 = 16°). This pole position is similar to late Mississippian to early Permian of North America rather than latest Ordovician. The difference between the known age of the Neda and the time of magnetization suggests that the hematite in the ore was produced from dehydration of goethite probably as a result of tectonic uplift in Wisconsin during late Mississippian time. In addition, these results add evidence to the theory that the Neda ore was formed in two stages. The oolites were produced in a near shore environment during the shoaling of the Ordovician seas. The oolites were then incorporated in iron rich muds. Burial and later uplift dehydrated the iron hydroxide to hematite.
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