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| Platteville Formation | |
|---|---|
| Name | Platteville Formation |
| Type | Formation |
| Period | Ordovician |
| Pr\ imary lithology | Limestone, dolomite |
| Other lithology | Shale, chert |
| Named for | Platteville |
| Region | Upper Midwest, United States |
| Country | United States |
| Unit of | Maquoketa Group |
| Underlies | Galena Group |
| Overlies | St. Peter Sandstone |
Platteville Formation
The Platteville Formation is an Ordovician carbonate unit widely recognized across the Upper Midwest of the United States. It is an important stratigraphic marker in the Iowa–Wisconsin–Minnesota region and is notable for its fossiliferous limestones, dolomitization, and role in regional hydrogeology and mineral resources. The unit figures prominently in studies by researchers affiliated with institutions such as the United States Geological Survey, University of Iowa, and University of Wisconsin–Madison.
The Platteville Formation consists predominantly of fossiliferous limestone and locally dolomitized carbonate rock, with interbeds of fine-grained siliciclastic material. Outcrops occur along river bluffs, roadcuts, and quarry faces in counties such as Grant County, Wisconsin, Dubuque County, Iowa, and Winona County, Minnesota. The formation serves as a marker horizon within regional stratigraphic columns used by mapping programs from agencies like the Iowa Geological Survey and Wisconsin Geological and Natural History Survey.
Lithologically, the Platteville comprises micritic to sparry limestone, bedded dolomite, nodular chert, and thin shale horizons; bioclastic grainstones and packstones occur locally. Stratigraphically it commonly overlies the St. Peter Sandstone and is overlain by members of the Galena Group or the Maquoketa Formation depending on locale. Correlative Ordovician units include the Decorah Shale and the Trenton Group in adjacent basins, and sequence stratigraphic analysis aligns it with regional transgressive systems tracts documented by the Indiana Geological Survey and Illinois State Geological Survey.
The Platteville spans the Upper Mississippi Valley region, with mapped extents across eastern Iowa, southwestern Wisconsin, southeastern Minnesota, and parts of Illinois. Key exposures are found along the Mississippi River corridor, in the Driftless Area, and in abandoned quarries near towns such as Platteville, Wisconsin, Dubuque, Iowa, and La Crosse, Wisconsin. Subsurface continuity is traced with well logs from petroleum and water-resource investigations conducted by Chevron Corporation contractors and state surveys.
Fossils are abundant and include brachiopods, trilobites, bryozoans, ostracods, gastropods, and cephalopods, which have been documented by paleontologists at institutions like Field Museum of Natural History and Harvard University. Trace fossils and bioturbation structures are common; some localities preserve articulated faunal assemblages used in biostratigraphic correlations with the Champlainian and Nashville faunas. Fossil collections from Platteville exposures contributed to taxonomic work by researchers such as James Hall and Charles D. Walcott in early North American paleontology.
The Platteville Formation was deposited during the Middle to Late Ordovician, within a shallow epeiric sea associated with the passive continental margin of Laurentia. Depositional models invoke tidal to outer-shelf carbonate ramp settings, episodic siliciclastic influxes, and periodic anoxic events tied to global Ordovician sea-level changes recorded in studies by the Paleontological Society and stratigraphers at Yale University. Chemostratigraphic and conodont biostratigraphy constrain its age relative to global stages such as the Darriwilian and Sandbian.
Platteville carbonates have economic importance as aggregate, dimension stone, and raw material for cement production; quarries historically supplied building stone for municipalities including Dubuque and Galena, Illinois. The formation hosts regional aquifers exploited for municipal and agricultural water supplies, documented by the United States Environmental Protection Agency and state water resources departments. Additionally, karst features and mineral replacement zones have been explored for lead‑zinc mineralization associated with the Upper Mississippi Valley Zinc-Lead District.
The unit was named for exposures near Platteville, with early descriptions by 19th-century geologists working for entities such as the Iowa Geological Survey and the Geological Survey of Wisconsin. Influential monographs and mapping efforts were produced by geologists including Charles A. White and later synthesis by the United States Geological Survey team led by E.O. Ulrich and contemporaries. Continued research by university geoscience departments and state surveys refined stratigraphic boundaries, lithofacies interpretations, and paleontologic inventories through the 20th and 21st centuries.
Category:Ordovician geology of the United States Category:Geologic formations of Iowa Category:Geologic formations of Wisconsin Category:Geologic formations of Minnesota