Platteville Limestone

Paleozoic Stratigraphy of the Upper Midwest, USA Dates are approximate, and deposition occurred at slightly different times in different areas

Ordovician :Maquoketa Group (446–440 Ma) ::Neda Formation Brainard Formation Fort Atkinson Formation Scales Formation :Galena Group (454–446 Ma) ::Dubuque Formation (447–446 Ma) Wise Lake Formation (449.4–447 Ma) Dunleth Formation (452–449.4 Ma) Decorah Shale (454–452 Ma) Platteville Limestone / Group (455–454 Ma) Ancell Group (454–455 Ma) Glenwood Shale (~455 Ma) St. Peter Sandstone (~459–~455 Ma) --- Major Erosional Unconformity --- :Knox Supergroup ::--- Major Erosional Unconformity --- Everton Dolomite (only in S. Indiana) --- Major Erosional Unconformity --- Dunleth Formation Prairie du Chien Group * Shakopee Dolomite * New Richmond Sandstone (only in S. Indiana) * Oneota Dolomite * Dunleth Formation * Gunter Sandstone Cambrian :Knox Supergroup (ct'd) ::Jordan Sandstone Potosi Dolomite Potsdam Supergroup/Potsdam Sandstone Megagroup Munising Group Davis Formation Franconia Formation Ironton Sandstone Galesville Sandstone Eau Claire Formation Mount Simon Sandstone

The Platteville Limestone cropping out in Minnehaha Falls Park, Minneapolis, Minnesota. The Platteville Limestone is the less-eroded, layered unit that constitutes the majority of the photo. Below it is a thin, dark layer of Glenwood Shale. Below the shale is a thin, white stripe of St. Peter Sandstone, followed by a slope of eroded St. Peter Sandstone material.

The Platteville Limestone is the Ordivician limestone formation in the sedimentary sequence characteristic of the upper Midwestern United States. It overlies the thin Glenwood Shale, which overlies the thick Saint Peter Sandstone.^[1] It is shot through with dolomitic mottles in an anastomose pattern; this dolotimization occurred after deposition but prior to the development of joints in the rock.^[2]

This difficult-to-erode unit forms the cap of Saint Anthony Falls and the Mississippi River bluffs in most of the Twin Cities area. Sea life was abundant during the Ordivician Period and a large number of marine fossils including corals, bryozoans, brachiopods, clams, snails, cephalopods, and trilobites can be found in the limestone sediments at several areas in the Twin Cities and along the Mississippi River at Minnehaha Falls park and elsewhere.^[1][3][4]

Building material

Many early stone buildings in Minnesota were constructed with this material, which often was quarried at the site.^[5][6] It can often be seen in older neighborhoods as the foundation for a building or a retaining wall. A notable existing example is the Nicollet Island Inn in Minneapolis.

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.
2. Robert H. Griffin (1942). "Dolomitic Mottling in the Platteville Limestone". SEPM Journal of Sedimentary Research 12. doi:10.1306/D4269148-2B26-11D7-8648000102C1865D.
3. ftp://mgssun6.mngs.umn.edu/pub2/mnglance/Mn_Precambrian.pdf
4. http://www.sciencebuzz.org/blog/collecting-fossils-minnesota
5. Millett, Larry (January 1992). Lost Twin Cities. Minnesota Historical Society Press. pp. 43–. ISBN 978-0-87351-273-2.
6. Ancient Tropical Sea Minnesota historical society

• River Bluffs Homepage, University of Minnesota
• Sloan, R.E., ed., 1987, Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley: Minnesota Geological Survey Report of Investigations 35, 232 p., University of Minnesota, Minneapolis.