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| Maquoketa Group | |
|---|---|
| Name | Maquoketa Group |
| Type | Geological group |
| Age | Late Ordovician |
| Period | Ordovician |
| Primary lithology | Shale, Limestone |
| Other lithology | Dolomite, Siltstone, Sandstone |
| Named for | Maquoketa, Iowa |
| Region | Midwestern United States |
| Country | United States |
| Unit of | Upper Ordovician strata |
| Subunits | Decorah Shale; Dubuque Formation; Tomah Formation; Guttenberg Formation; Brainard Shale |
| Underlies | Silurian |
| Overlies | Cincinnatian Series |
Maquoketa Group is a Late Ordovician sequence of sedimentary rocks widespread across the Midwestern United States, notable for its shale-dominated facies, fossil assemblages, and stratigraphic importance in regional correlations. It records a major transgressive-regressive cycle and is integral to studies linking North American cratonic sequences with global events such as the Hirnantian glaciation, the Taconic orogeny, and biotic turnovers recognized in Paleobiology Database, International Commission on Stratigraphy, and regional geological surveys.
The unit crops out across Iowa, Illinois, Wisconsin, Minnesota, Indiana, Missouri, and Michigan and is correlated with coeval strata in Ontario and the Cincinnati Arch region, informing interpretations by researchers affiliated with United States Geological Survey, Illinois State Geological Survey, and university programs at University of Iowa, University of Wisconsin–Madison, Iowa Geological Survey, and Indiana Geological and Water Survey. Stratigraphic frameworks developed during syntheses at institutions such as Lamont–Doherty Earth Observatory, Royal Ontario Museum, and Field Museum of Natural History have placed this group within broader frameworks discussed in publications of the Geological Society of America and the American Geophysical Union.
The Maquoketa Group comprises multiple formations including shale-dominated units like the Decorah Shale and carbonate intervals such as the Dubuque Formation and Guttenberg Formation, with local equivalents termed Tomah Formation and Brainard Shale. Lithologies include organic-rich black shale, argillaceous limestone, dolomite, calcareous siltstone, and storm-influenced sandstone, described in stratigraphic columns used by Kansas Geological Survey and Ohio Geological Survey. Chemostratigraphic and biostratigraphic correlation employs data from conodont zonations established by researchers at Smithsonian Institution, Harvard University, and Yale University, and integrates isotope studies promulgated in journals of Elsevier and Springer Nature.
Outcrops and subsurface equivalents span the Interior Plains, the Michigan Basin, and the Illinois Basin, extending from Missouri River margins to the Great Lakes rim. Mapping efforts by USGS and state surveys show thickness variations from tens to several hundred meters, with onlap patterns recorded along structural elements such as the Cincinnati Arch, Kankakee Arch, and Wisconsin Arch. Correlation to units in Ontario and the Appalachian Basin involves work by teams at Queen's University, Carleton University, and University of Toronto.
Fossil assemblages include brachiopods, trilobites, bryozoans, graptolites, conodonts, cephalopods, ostracods, corals, and rare vertebrate microremains, documented in collections at Smithsonian Institution, Field Museum of Natural History, University of Iowa Paleontology Repository, University of Michigan Museum of Paleontology, and Royal Ontario Museum. Notable taxa include elements comparable to faunas described from Cincinnatian Series localities, and comparisons are drawn with faunal lists curated by Paleobiology Database, International Paleontological Association, and authors publishing in Journal of Paleontology and Palaeogeography, Palaeoclimatology, Palaeoecology. Studies by paleontologists affiliated with Ohio University, Kent State University, and Indiana University have used Maquoketa fossils for biostratigraphic zonation and paleoecological reconstruction.
Sediments reflect a shallow epicontinental sea affected by eustatic rises and falls tied to the Late Ordovician glacio-eustatic events recognized by investigators at University of Cambridge, University of Oxford, and Columbia University. Interpreted environments include offshore anoxic basins, storm-dominated outer shelf, and restricted carbonate platforms; these interpretations are supported by sedimentological analogs from studies at Woods Hole Oceanographic Institution and isotope work from teams at Scripps Institution of Oceanography. Connections to the Taconic orogeny and siliciclastic influx from eastern sources are discussed in syntheses by authors associated with Pennsylvania State University and Rutgers University.
Shales within the group host organic-rich horizons investigated for petroleum source potential in reports by US Energy Information Administration and American Association of Petroleum Geologists, while carbonates have been quarried for aggregate and cement by companies registered with American Concrete Institute and regional producers in Iowa and Illinois. Trace-metal enrichments and phosphate occurrences have been evaluated in environmental and mining studies conducted by Environmental Protection Agency and state mineral resource programs. Groundwater aquifer interactions are considered in hydrogeologic assessments by USGS Water Resources Division and state water resource departments.
The unit was named for exposures near Maquoketa, Iowa, during 19th-century surveys by geologists associated with the Iowa Geological Survey and was subsequently refined through work by stratigraphers at University of Iowa, Iowa State University, Geological Survey of Canada, and USGS staff. Key publications by figures connected to Charles D. Walcott-era paleontology, later stratigraphic syntheses published in Geological Society of America Bulletin, and regional monographs by state surveys have shaped current usage. Ongoing revisions integrate biostratigraphy from conodont specialists at University of Copenhagen and chemostratigraphy from isotope laboratories at Caltech.
Category:Ordovician geology of the United States