Abstract

Strata between the Glenwood and Maquoketa formations in Fillmore County, Minnesota, and adjacent areas of Minnesota and Iowa, are representative of these same beds cropping out farther to the north and west in the Mississippi Valley. They have received little modern study, and have heretofore not been subdivided on lithologic grounds alone.

The Platteville Formation is a dominantly carbonate sequence between the Glenwood Formation and the shale of the Decorah Formation; it consists from the base upward of the sandy dolomitic limestone of the Pecatonica Member, with multiple corrosion zones at its top; the limestone of the McGregor Member, with a corrosion zone near its base; and the Carimona Member, of interbedded limestone and minor shale with a conspicuous bentonite at or near its base in Fillmore County. The Decorah shale includes the Spechts Ferry bentonite near its base, contains hematite oölites at the top toward the west, and is not divisible into the members recognized to the southeast. The Galena Formation consists of three members: the Cummingsville cyclical limestone and shaly limestone at the base; the Prosser limestone, which has a group of corrosion zones at its top; and the Stewartville dolomitic limestone, which has a feldspathized shale near the base. The Dubuque Formation, of interbedded limestone and shale, contains two feldspathized shale beds and is thought to be the highest Middle Ordovician rock in the area.

Large collections of macrofossils were made, and their stratigraphic distribution recorded in detail and summarized in a faunal list. These data show clearly that the terms Black River and Trenton cannot be applied meaningfully to Minnesota Ordovician rocks on the basis of macrofossils.

The geologic history recorded by these rocks and fossils is interpreted as marine transgression continued from the time of deposition of the St. Peter-Glenwood, stillstand and then regression during Decorah deposition, continued regression and stillstand during Galena deposition, and renewed transgression of greater magnitude from Dubuque through Maquoketa deposition. The historical implications of corrosion zones are discussed.

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