# Maquoketa Shale Canonical page: https://lithomap.app/resources/geologic-formations/united-states/maquoketa-shale Last reviewed: September 17, 2026 Page purpose: Comprehensive United States geologic formation reference for human readers, search systems, and language models. Major region: Midwest / Great Lakes Regional setting: Upper Mississippi Valley and Great Lakes basins Age: Late Ordovician Age detail: Late Ordovician; subdivisions and rank vary across the Upper Midwest Depositional setting: Shallow to deeper marine shelf during changing sea level Rock types: Shale, Mudstone, Limestone, Dolostone, Siltstone States in recognized usage: Illinois, Indiana, Iowa, Minnesota, Missouri, Wisconsin ## Summary The Maquoketa Shale is a widespread Upper Midwest marine succession of shale and carbonate beds, known for diverse fossils and for major changes in facies and stratigraphic rank across state lines. ## Distribution Maquoketa rocks crop out around the margins of the Illinois and Michigan basins and beneath glacial deposits across much of the region. The name may denote a formation or group containing several formations and members. Bluish or greenish shale, fossiliferous carbonate beds, and locally sandy or iron-rich intervals occur at different levels. Glacial cover and weathering mean the unit is far more extensive underground than surface exposure suggests. ## Geological history Marine mud accumulated across a broad shelf while storms and shifts in water depth introduced carbonate debris and silt. Oxygen conditions and sediment supply varied, producing fossil-rich limestone, quiet-water shale, and locally pyritic or phosphatic beds. Later burial compacted the shale and cemented carbonate layers. Erosion before younger Silurian deposition created important regional breaks, and Pleistocene glaciers subsequently concealed or re-exposed the bedrock. ## Fossil record Brachiopods, bryozoans, mollusks, trilobites, graptolites, crinoid fragments, and cephalopods occur, with distinctive assemblages in particular members. Some beds contain abundant small pyritized or phosphatic shells. Fossil abundance is highly uneven. Calcareous layers may be crowded with shells while nearby shale is sparsely fossiliferous, so local member identification matters. ## Minerals and geological resources Clay minerals, quartz, calcite, and dolomite are common; pyrite and phosphate occur locally. Iron oxides develop during weathering, and the Neda interval at the top is locally rich in goethite and hematite ooids. The shale acts as a confining layer in regional groundwater systems. Pyrite-bearing material can acidify drainage when excavated or exposed to air and water. ## Field context - Glacial sediment commonly hides the bedrock contact even where maps show the unit beneath the surface. - Wet shale slopes are weak and can slump beneath resistant carbonate beds. - Confirm whether the local survey uses Maquoketa as a formation or a group. ## Connected field guides - Shale (rock; Named rock type and mineral context) — https://lithomap.app/resources/rock-identification/shale - Dolostone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/dolostone - Limestone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/limestone - Mudstone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/mudstone - Siltstone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/siltstone - Calcite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/calcite Mapped location and formation context support an identification but cannot establish one. Compare several physical properties and use laboratory analysis when the material remains ambiguous. ## Connected state geology guides - No directly overlapping state guide is currently published. ## Authoritative sources - Geolex Maquoketa references — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/UnitRefs/MaquoketaRefs_2628.html - Hydrogeology of the northern Midwest — U.S. Geological Survey: https://www.usgs.gov/publications/hydrogeology-cambrian-ordovician-aquifer-system-northern-midwest-united-states-a ## Formation photograph - Image: https://lithomap.app/_delivery/formations/maquoketa-shale.webp - Description: Layered Maquoketa Group exposure at Kankakee River State Park, Illinois - Creator: Steven Baumann - License: Public domain — https://creativecommons.org/publicdomain/mark/1.0/ - Source record: https://commons.wikimedia.org/wiki/File:Maquoketa_Group.jpg Formation-level information does not prove that a fossil or mineral occurs at a specific coordinate. It also does not establish ownership, access, collecting permission, present conditions, or specimen identity.