Minnesota Geology > Soudan Banded Iron Formation by ahochstaedter

The late Archean (older than 2.5 billion years old) Soudan Banded Iron Formation is part of the Ely Greenstone Belt in northern Minnesota. This glacially scoured outcrop occurs in Soudan State Park, site of the oldest iron mine in Minnesota, which operated from 1884 to 1962.

The banding reflects differing concentrations of hematite (dark) and chert (light), which results from the precipitation of ferric Iron and Silica, respectively, from an ancient sea. These banded iron formations occur only in the Precambrian. Similar sediments are not being deposited today. Since the Iron is oxidized, these deposits are telling us something about the introduction of Oxygen into the Earth's atmosphere.

This outcrop is deformed by several genereations of folding as well as brittle demormation. In this image, the F2 folds are the most prominent with axial planes that trend from lower left to upper right and steeply plunging axes.

The main source for the information presented here is Jirsa and Green (2011, GSA field Guide 24, pp. 25-45)