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Paleozoic Stratigraphy of the Upper Midwest, USA
Dates are approximate, and deposition occurred at slightly different times in different areas
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Ordovician
Platteville Limestone / Group (455–454 Ma)
Ancell Group (454–455 Ma)
Glenwood Shale (~455 Ma)
St. Peter Sandstone (~459–~455 Ma)
--- Major Erosional Unconformity ---

Cambrian

Potsdam Supergroup/Potsdam Sandstone Megagroup
Munising Group
Davis Formation
Franconia Formation
Ironton Sandstone
Galesville Sandstone
Eau Claire Formation
Mount Simon Sandstone

The Decorah Shale is a fossiliferous shale that lies above the Platteville Limestone and below the Galena Limestone in the sedimentary sequence that formed from the shallow sea that covered central North America during Ordovician Time. Where it crops out in the Upper Midwest, especially in the Twin Cities, it is a popular place for amateur fossil collecting. It contains trilobites, brachiopods, horn corals, gastropods, crinoids, and large numbers of bryozoans.[1] Cephalopods may also be found in the lower layers of the Decorah Shale.

References

  1. ^ Mossler, J. and Benson, S., 1995, 1999, 2006, Fossil Collecting in the Twin Cities Area. Minnesota at a Glance: Minnesota Geological Survey: University of Minnesota.







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