The Geology of the Upper Dunleith Formation (Prosser Member Galena Formation) of Middle Ordovician Age in Southeastern Minnesota

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The Geology of the Upper Dunleith Formation (Prosser Member Galena Formation) of Middle Ordovician Age in Southeastern Minnesota

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The upper Dunleith Formation of the Galena Group was deposited in the Hollandale Embayment of the Ordovician sea. The four members of the upper Dunleith Formation: the Rivoli, Sherwood, Wall and Wyota are composed mostly of fine-grained calcium carbonate (micrite) with lesser amounts of shale and coarse grained calcite (sparite). T'he area studied is located in the upper Mississippi River Valley, 30 miles south of Minneapolis-St. Paul southward to northern Iowa. Isopach maps made from outcrop information and data from insoluble residues show that the terrigenous material found in the upper Dunleith strata was likely supplied from the Transcontinental Arch from the west and northwest. Fossil types found within upper Dunleith strata indicate the sea in which upper Dunleith strata was deposited was clear, warm, calm and of normal marine salinity. However, the strata of the upper Dunleith Formation at Mantorville, Minnesota, suggest a different environment, one of which was likely hypersaline, as evidence from the lack of stenohaline organisms and the presence of euryhaline organisms and dolomitic strata. The members of the upper Dunleith Formation can be precisely correlated by the use of argillaceous horizons, fossil zones, chert nodule horizons, corrosion zones, bentonite beds, stylolites, mottled zones and other sedimentary features. The differentiation of the members in the upper Dunleith Formation is usually determined by the presence of distinct argillaceous horizons that occur at the top of each member. Where the argillaceous horizons are less prominant, other features of the member are useful in locating an approximate contact. Corrosion surfaces or zones are features of both local and large lateral extent within the carbonate strata. These surfaces represent a period of nondeposition and/or errosion. Corrosion zones probably indicate a sedimentary environment when the chemistry of the sediment-water interface favored solution rather than deposition of carbonate, associated with mechanical erosion. Four bentonite horizons are present within the upper Dunleith Formation and are useful time stratigraphic features. These horizons occur 3 to 7 feet below the Rivoli Member top, at the Rivoli-Sherwood contact, 3 to 6 feet into the Sherwood Member and approximately in the middle of the Wall Member. The mineralogy of the bentonites is different than those of the shales and clays. Bentonites are usually composed of euhedral (or subhedral) zircon, apatite and hornblende grains. Other heavy minerals of biotite, garnet and tourmaline also occur in some samples in minor amounts. The source of the bentonites as determined from paleomagnetic data (Dott and Batten, 1976) was probably the northeastern portion of the Appalachian mobile belt.


A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by David J. Stone in partial fulfillment of the requirements for the degree of Master of Science, August 1980.

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Stone, David J. (1980). The Geology of the Upper Dunleith Formation (Prosser Member Galena Formation) of Middle Ordovician Age in Southeastern Minnesota. Retrieved from the University Digital Conservancy,

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