The Platteville Formation, a thin Middle Ordovician unit, is
subdi vided into several members in Minnesota. The basal Pecatonica and
upper Carimona Members are present throughout all the Minnesota/western
Wisconsin outcrop belt. The medial McGregor, which is recognized in the
southern part of the Minnesota outcrop, is replaced by the Mifflin, Hidden
Falls, and Magnolia Members in the Twin Cities area. Distribution of the
Mifflin is restricted to the Twin Cities structural basin; the other two
units extend slightly beyond the Twin Cities basin. Their distribution may
have been influenced by proximity to a paleoshoreline north of the present
The Platteville Formation was deposited in a transgressing sea as part
of the Middle Ordovician-Silurian Tippecanoe sequence. It is interpreted
to have been deposited on a shallow marine shelf in deeper water than the
shoreward facies represented by the underlying St. Peter Sandstone and
Glenwood Formation. Fossils of stenohaline organisms are present
throughout the formation. The formation has no exposure indicators, stromatolites,
or intraclasts. Features indicative of shallower water, such as
ferruginous ooids and coquina layers, are present in the Twin
Cities/western Wisconsin area.
Hardgrounds--surfaces of submarine nondeposition indicative of low
rates of sedimentation--characterize the Pecatonica and lower McGregor
Members, but are less common in the upper part of the formation. The
Carimona Member is characterized by limestone beds separated by shale beds
and partings that thicken toward the west and north. This increased detrital
sedimentation during deposition of the Carimona Member and the succeeding
Decorah Shale may indicate uplift along the Transcontinental Arch.
The proportion of dolomi te to calci te in the carbonate rocks of the
Platteville increases toward the north and northeast. In southern
Minnesota the formation is principally limestone; in the Twin Cities
Metropolitan Area and western Wisconsin, it has a high proportion of dolomite.
The formation does not have any diagenetic fabrics indicative of
subaerial exposure, and deposition of the overlying Decorah Shale appears
to have followed deposition of the Platteville without interruption. The
pervasive dolomitization is probably a result of burial diagenesis.
Mossler, John H..
RI-33 Sedimentology of the Middle Ordovician Platteville Formation Southeastern Minnesota.
Minnesota Geological Survey.
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