Sandstone with some dolomite and shale, undivided; includes Trempealeau, Tunnel City, and Elk Mound Formations
Dolomite with some limestone and shale; includes Galena, Decorah, and Platteville Formations
Dolomite, undivided; includes Cayugan, Niagaran, and Alexandrian series
Dolomite with some sandstone and shale; includes Shakopee and Oneota Formations
Dominantly tholeiitic basalt and basaltic andesite flows and tuffs; associated with sheet dikes, massive and layered metagabbro, and ultramafic rocks. In northeastern Wisconsin, rocks have been named the Quinnesec Formation.
Volcanic rocks in areas of sparse geologic data and nondefinitive magnetic and gravity data
Orthoquartzitic sandstone with minor limestone, shale and conglomerate; includes Glenwood and St. Peter Formations
Gray basalt flows, interflow breccia, tuff, and minor sedimentary rocks
Gray to pinkish-gray, medium-grained, generally equigranular granite to tonalite and granitoid gneiss; locally includes diorite. Intrudes older metavolcanic rocks. Zircon ages range from 1852 +/- 15 to 1862 +/- 5 Ma
Red, coarse-grained rapakivi granite consisting of large (1-3 cm) ovoid alkali feldspar sporadically mantled by plagioclase, interstitial plagioclase, quartz, biotite, hornblende, and ilmenite
Red lithic conglomerate and sandstone; mafic to felsic volcanic flows similar to those of the unnamed formation (unit Yu) are interlayered with the sedimentary rocks.
Shale, dolomitic shale and dolomite; includes overlying Neda Formation (age uncertain) consisting of oolitic iron oxides and shale
Varied gneisses of mostly unknown age in cores of gneiss domes and fault-bounded uplifts (Archean gneiss terranes). Except for the Watersmeet dome (Late to Early Archean), all dated rocks are Late Archean. Includes granite of Late Archean age that transgresses gneisses and amphibolite.
Mainly reddish-brown, medium- to fine-grained lithic arkosic sandstone, siltstone, and micaceous silty shale.
Poorly sorted arkosic sandstone
Quartzofeldspathic gneiss and less abundant amphibolite and migmatite. Includes granitoid rocks. Granitic gneiss at Port Edwards, WI on Wisconsin River has a U-Pb zircon age of 2870 +/- 13 Ma, and gneiss at Jim Falls in Chippewa River valley has a U-Pb zircon age of 2522 +/- 22 Ma (Sims and others, 1989)
Metamorphosed graywacke in poorly exposed areas in northwestern Wisconsin. Sample at one locality (near Blockhouse Lake, 10 km northeast of Park Falls, T. 40 N., R. 1 E.) has a U-Pb zircon age of 1852 +/- 6 Ma (Sims and others, 1985b)
Contains alkali feldspar phenocrysts (0.5-2 cm), subordinate plagioclase and quartz, and rare clusters of biotite with or without hornblende
Thick and stratigraphically varied formation of sedimentary and less abundant volcanic rocks. Metamorphosed graywacke is predominant rock of Michigamme Formation. Calcareous concretions common. Metamorphosed from cholorite to sillimanite grade (Cannon, 1986). Mapped undivided in poorly exposed areas
Interlayered quartzofeldspathic gneiss and amphibolite in Marenisco, Mich., area and northwestern Wisconsin. Protoliths are bimodal intermediate and mafic volcanic rocks (Sims and others, 1984).
Poorly sorted arkosic sandstone
Pink to maroon to light-gray, medium-grained, moderately sorted quartzite. Red argillite (pipestone) interbedded locally. At least 213 m thick
Gray, medium-grained intrusive rocks, including intrusion breccias. Exposed in central Wisconsin. Zircon ages range from 1837 to 1847 Ma
Red to pink, medium-grained leucocratic alkali feldspar granite; contains miarolitic cavities; biotite is altered to opaque oxide minerals; contains fluorite locally. Called red granite previously (Sims, 1990). Exposed in both Pembine-Wausau and Marshfield terranes.
Gray to reddish-gray, slightly foliated, porphyritic rhyolite containing aligned phenocrysts of alkali feldspar and lesser amounts of plagioclase in a fine-grained matrix of feldspar, biotite, amphibole, and sparse quartz
Gray, coarse-grained (1-20 cm) plagioclase-rich (An45-53) rock containing interstitial orthopyroxene, clinopyroxene, Fe-Ti oxides, and apatite.
Rhyolite to dacite and, locally, andesite tuff, breccia, and minor sedimentary rocks, including conglomerate. Exposed in central Wisconsin.
Interlayered quartzofeldpathic gneiss and amphibolite between Athens shear zone and Owen fault, central Wisconsin
Succession of poorly exposed interlayered metavolcanic and metasedimentary rocks. Lithic units described in Sims (1990)
Pink, coarse-grained granite to granodiorite containing nearly equal amounts of microcline microperthite, plagioclase, and quartz and 5-10 percent biotite and (or) hornblende. Mafic minerals are interstitial and give a clotty appearance. (1836 +/- 15 Ma)
High-aluminum basalt to low-silica andesite pillowed flows and interlayered dacite to rhyolite tuffs and porphyries in Monico and Mountain areas, northeastern Wisconsin. A rhyolite porphyry at Jennings (Oneida County) has an age of 1869 +/- 6 Ma. Rocks are host to massive sulfide deposits (Crandon and Pelican River)
Mafic to ultramafic bodies defined and delineated principally by positive magnetic and gravity anomalies; includes diorite bodies. Comagmatic with unit Xmi
In northern Wisconsin, granite to granodiorite of varied texture. In southern Wisconsin (not mapped separately), red to pink alkali-feldspar granophyric granite associated with rhyolites of both peraluminous and metaluminous affinities (unit Xr of Marshfield terrane).
Rapakivi granite (wiborgite) containing 70-80 percent coarse (1.5 -5 cm) ovoid alkali feldspars mantled by plagioclase, coarse anhedral quartz, and interstitial hornblende and biotite
Ash-flow tuffs and interbedded volcaniclastic sedimentary rocks and cogenetic granite (unit Xga) exposed as inliers in southern Wisconsin. In central Wisconsin pink, flow-banded rhyolite and chert-cemented breccia inferred to be 1760 Ma.
Dominantly dark-gray, massive, porphyritic tholeiitic basalt. Includes a basal conglomerate and a lean iron-formation in middle of formation
Pink to pinkish-gray, medium- to coarse-grained, equigranular to inequigranular granite to granodiorite in Puritan batholith south of Gogebic Range. Age 2710 +/- 140 Ma (Sims and others, 1977)
Tuff, argillite, graywacke, and minor volcanic rocks of lower greenschist-facies in southern Rusk County. Could be younger in age than adjacent metavolcanic rocks.
gabbro, anorthosite, granite, peridotite
Gray, weakly foliated to massive granite in poorly exposed areas.
Derived from mafic to intermediate pyroclastic rocks and massive to pillowed lava flows. Unit mapped as Ramsey Formation by Prinz (1981) south of Gogebic Range; mapped as Mona Schist and Kitchi Schist in northern complex of Marquette District.
Granite containing plagioclase, microcline, and quartz phenocrysts in a fine-grained granophyric matrix. Probably subvolcanic intrusions related to volcanic rocks of 1835- to 1845-Ma age group
Basalt, andesite and lesser rhyolite flows. Basalt flows near base of the formation contain plagioclase phenocrysts, some in radiating clusters
Well-sorted quartzose sandstone
Traverse Group
Includes Marinette Quartz Diorite of Dunbar Dome (Sims and others, 1985a) and nearby Twelve Foot Falls Quartz Diorite
Red to pink granite consisting of both a coarse-grained and a fine-grained facies. Probably an epizonal intrusion
Massive to layered intrusive gabbroic rocks; weakly metamorphosed and deformed. Exposed in both Pembine-Wausau and Marshfield terranes
Mylonitic rocks of uncertain lithology in Eau Pleine shear zone
Orange to pink, coarse-grained massive granite composed of alkali feldspar sporadically mantled by plagioclase, quartz, and biotite; contains miarolitic cavities
Dark-greenish-gray, pillowed to massive tholeiitic basalt and pyroclastic rocks. Correlated with the Hemlock Formation on basis of geology and similarity in chemical composition.
Light- to dark-gray, feldspathic, fine-grained sandstone, argillaceous siltstone, and argillite. Near base, ferruginous argillite contains beds of cherty sideritic and pyritic iron-formation
Surrounds gneiss domes in Gogebic County, Mich. and Vilas County, Wisc., and occurs in north-central Dickinson County. Probably correlative with part of Blair Creek Formation (unit Xbc). Previously correlated with uppermost unit of Blair Creek Formation (Sims and others, 1984).
Gray to pink, equigranular to porphyritic granite to granodiorite containing microcline microperthite, concentrically zoned plagioclase, quartz, biotite, and hornblende
Contains flow-oriented xenoliths of biotite schist and sillimanite-bearing quartzite
Pink, coarse-grained granite consisting of microcline microperthite and oligoclase phenocrysts in a fine-grained matrix of quartz, potassium feldspar, and oligoclase. Hornblende and (or) biotite occur in clots
Oronto Group; Freda Sandstone; Conglomerate member
Maroon but locally white, gray, and red quartzite (quartz arenite) with a basal quartzose conglomerate. At Flambeau Ridge (Chippewa County) consists of conglomerate. Distinguished from other quartzite units in being strongly deformed and metamorphosed. Includes Flambeau, Rib Mountain, McCaslin, and Thunder Mountain Quartzites of local usage
Includes andesite tuff. Occurs in northeastern Wisconsin. Dacite has age of 1866 +/- 39 Ma.
Medium-gray, mottled, medium-grained, equigranular tonalite and granodiorite. Intrudes rocks of the Milladore Volcanic Complex.
Granophyre
Lava flows, mostly basalt, andesite and felsite flows and subordinate interflow sedimentary rocks.
Gray to pinkish-gray, fine- to medium-grained, massive granite containing scattered phenocrysts of potassium feldspar. Exposed in northeastern Wisconsin
Mafic to felsic flows, pyroclastic rocks, impure quartzite, and conglomerate in Eau Claire River, Eau Claire and northern Clark Counties. Rhyolite has zircon ages of 1,858 +/- 5 Ma. Possibly correlative with Milladore Volcanic Complex.
Pink to red, medium- to fine-grained granite containing miarolitic cavities
Dominantly Fortune Lakes Slate, but includes other rocks in poorly exposed areas. Correlated with upper part of Baraga Group.
Pyroxene syenite, amphibole syenite, tabular syenite, and nepheline syenite. Age approximately 1520 Ma (Sood and others, 1980).
Flow-banded rhyolite, welded tuff, volcanic conglomerate, and volcanogenic sedimentary rocks. Exposed in both Pembine-Wausau and Marshfield terranes.
Gray, green, and brown lithic siltstone, shale, and sandstone. Copper sulfides and native copper occur locally near base
Baraga Group; Michigamme Formation; Banded cherty and slaty iron-formation
Powder Mill Group; Kallander Creek Volcanics; Upper rhyolite member
Dark-gray basalt and minor porphyritic andesite. Generally strongly magnetic with reversed remanent magnetism. Underlain by a thin unit of quartzose sandstone (Bessemer Quartzite).
Pale-orange to pink, fine-grained porphyritic aplite
Pink and gray, layered quartzofeldspathic schist of intermediate volcanic composition and associated thin iron-formation. Felsic gneiss at Arbutus Dam on Black River at Hatfield has U-Pb zircon age of about 2800 Ma (Sims and others, 1989)
Mellen Intrusive Complex; Granite (about 1000 Ma)
Brown to dark-gray, coarse-grained (0.3-1.5 cm) inequigranular monzonite containing subhedral, zoned plagioclase, alkali feldspar, mafic silicates (fayalite, hypersthene, ferroaugite-hedenbergite, hornblende, and biotite), and interstitial quartz
Interlayered gabbro, mafic accumulates, gabbroic anorthosite and anorthosite (Cummings, 1984) in Eau Claire River (T. 27 N., R. 8 W.). Age uncertain, but interpreted as Archean (Sims, 1990).
Includes Stambaugh Formation, Hiawatha Graywacke, Riverton Iron-formation, and Dunn Creek Slate (James and others, 1968). Correlated with lower part of Baraga Group
Massive to flow-banded rocks of greenschist metamorphic grade in northeastern Wisconsin; virtually undeformed.
Medium-grained granodiorite gneiss in northwestern Marathon County.
Interbedded cherty and slaty iron-formation. Locally, secondary enrichment has formed "soft ore" bodies. In eastern part of Gogebic Range, Ironwood Iron-formation is intercalated with pyroclastic rocks of Emperor Volcanic Complex, and in eastern half of T. 47 N., R. 43 W. (where mapped within unit Xip) consists entirely of black slate
Mapped where units are too thin to show separately on map (Gogebic Range). Palms Formation includes Bad River Dolomite and Sunday Quartzite of Chocolay Group
Pale-red, medium- to fine-grained, leucocratic, mylonite gneiss exposed near Neillsville, Wisc.
Mellen Intrusive complex; Olivine gabbro
In Dickinson County, Mich., includes Randville Dolomite, Sturgeon Quartzite, and Fern Creek Formation. In Marquette range, between Marquette Bay and Champion, includes Wewe Slate, Kona Dolomite, Mesnard Quartzite, and Enchantment Lake Formation
Diabase, gabbro, and ferromonzodiorite. Primarily sills including the Endion, Lester River, Silver Cliff, Stony Point, Split Rock, Lafayette Bluff, Lake Clara–Lichen Lake, Pigeon River, and Reservation River intrusions.
Contains 30-45 percent resorbed quartz phenocrysts (3-4 mm) and less abundant alkali feldspar and plagioclase phenocrysts (4-5 mm) in a fine-grained matrix of quartz, two feldspars, biotite, and hornblende
Exposed in northeastern Wisconsin
Basalt and basaltic andesite. Includes the Leif Erickson, Lakeside (~1,098 Ma), Sucker River, Larsmont, Two Harbors, Crow Creek, Gooseberry River, Gustafson Hill, Baptism River, Good Harbor Bay, Cascade River, Croftville, Red Cliff, and Marr Island lavas.
Unconformably overlies units described below (Mns, Mnu, Mnr, and Mnb).
Sandstone, siltstone, and shale; includes the Wonewoc Sandstone, Eau Claire Formation, and Mt. Simon Sandstone.
Variably metamorphosed dikes, sills and subconcordant sheets of diabasic rocks. Probably coeval (in part) with volcanic rocks of Hemlock Formation (unit Xh).
Poorly exposed. Includes Clarksburg Volcanics member which is mostly mafic to intermediate pyroclastic rocks in Marquette trough, between Marquette Bay and Champion
Magnetite-quartz iron-formation and associated talc schist in aluminous quartzofeldspathic schist (unit Wt)
Generally dark-gray basalt, andesite, and felsite flows and subordinate interflow sedimentary rocks
Surface exposures limited to northeastern Allamakee County; subcrops beneath Mississippi River alluvium in Allamakee County. Includes Ironton and Galesville members. Primary lithology: sandstone, fine to coarse grained, upper part fossiliferous (Ironton). Thicknesses: 125-155 ft (38-47 m).
Surface exposures limited to Allamakee and northeast Clayton counties; subcrops beneath Mississippi River alluvium in southern Allamakee and Clayton counties. Primary lithologies: sandstone, very fine to coarse grained, quartzose to feldspathic (Jordan); silty dolomite to dolomitic siltstone (St. Lawrence); sandstone, very fine to fine grained, glauconitic to very glauconitic, common greensands (Lone Rock). Secondary lithologies: siltstone, part argillaceous (St. Lawrence, Lone Rock). Minor: silty shale partings (St. Lawrence, Lone Rock); dolomite, part sandy; intraclastic dolomite, sandstone, and siltstone. Maximum thicknesses of total interval: 260-310 ft (79-95 m). Formation thicknesses: Lone Rock Fm, 110-140 ft (34-43 m); St. Lawrence Fm, 60-80 ft (18-24 m); Jordan Sandstone, 80-110 ft (244-34 m).
Thick and stratigraphically varied formation of sedimentary and less abundant volcanic rocks. Metamorphosed graywacke is predominant rock of Michigamme Formation. Calcareous concretions common. Metamorphosed from cholorite to sillimanite grade (Cannon, 1986). Mapped undivided in poorly exposed areas
Red, brown, and white quartzose sandstone, and minor siltstone, shale and conglomerate
Prairie du Chien Group includes Oneota Fm (Coon Valley, Hager City members) and Shakopee Fm (New Richmond Sandstone, Willow River members); St. Peter Sandstone includes Readstown and Tonti members. Sub-St. Peter erosional surface bevels and locally truncates Prairie du Chien strata. Primary lithologies: dolomite, part cherty, part sandy, mostly unfossiliferous, common voids and vugs (Prairie du Chien); sandstone, quartzose, very fine to medium grained (St. Peter, Shakopee). Secondary lithologies: dolomite, laminated to stromatolitic, part intraclastic; dolomite, fossil molds (primarily mollusks); interbedded very fine to fine sandstone and dolomite (Coon Valley, Shakopee); conglomerate, dolomite and chert clasts in sandy matrix (Readstown). Minor: oolitic dolomite and oolitic chert (Shakopee); shale, green, part sandy (Shakopee); shale, green-brown to gray, part laminated, silty to sandy, part fossiliferous (locally in St. Peter of Winneshiek Co.); coarse sand grains (St. Peter, Shakopee); breccia, dolomite-chert clasts; chalcedony/quartz nodules. Maximum thicknesses total interval: generally 300-400 ft (90-120 m), may reach thicknesses to 700 ft (210 m) (locally thick St. Peter in Winneshiek Co.). Unit thicknesses: Prairie du Chien Group, 225-335 ft (69-102 m), locally thinner where overlain by thick St. Peter; Oneota Fm, 130-225 ft (40-69 m), locally thinned beneath thick St. Peter; Shakopee Fm, 55-100 ft (17-30 m), locally thinned to absent beneath thick St. Peter; St. Peter Sandstone, generally 40-75 ft (12-23 m), varies 25-225 ft (8-69 m), may locally reach thicknesses to 700 ft (210 m) in Winneshiek Co.
Varied gneisses of mostly unknown age in cores of gneiss domes and fault-bounded uplifts (Archean gneiss terranes). Except for the Watersmeet dome (Late to Early Archean), all dated rocks are Late Archean. Includes granite of Late Archean age that transgresses gneisses and amphibolite.
Dominantly tholeiitic basalt and basaltic andesite flows and tuffs; associated with sheet dikes, massive and layered metagabbro, and ultramafic rocks. In northeastern Wisconsin, rocks have been named the Quinnesec Formation.
Dominantly Fortune Lakes Slate, but includes other rocks in poorly exposed areas. Correlated with upper part of Baraga Group.
Dark-greenish-gray, pillowed to massive tholeiitic basalt and pyroclastic rocks. Correlated with the Hemlock Formation on basis of geology and similarity in chemical composition.
Sandstone, siltstone, and local conglomerate. Includes the Hinckley Sandstone and Fond du Lac (youngest detrital zircons ~1,000 Ma) and Solar Church Formations; deposition in eolian, fluvial, and lacustrine environments.
In Dickinson County, Mich., includes Randville Dolomite, Sturgeon Quartzite, and Fern Creek Formation. In Marquette range, between Marquette Bay and Champion, includes Wewe Slate, Kona Dolomite, Mesnard Quartzite, and Enchantment Lake Formation
Anorthositic intrusions and inclusions, undifferentiated (~1,099 Ma).
Trempealeau Formation
Munising Formation
Generally dark-gray basalt, andesite, and felsite flows and subordinate interflow sedimentary rocks
Subcrops beneath Mississippi River alluvium in Allamakee County; surface exposures absent. Primary lithologies: sandstone, very fine to fine grained, part argillaceous; shale, silty, gray to green-gray, part glauconitic. Secondary lithology: siltstone, part argillaceous, part dolomitic. Minor: dolomite. Thicknesses: 120-145 ft (37-44 m).
Sandstone, siltstone, shale, and dolostone; includes the Jordan Sandstone and St. Lawrence Formation, and the Mazomanie, Lone Rock, and Davis Formations of the Tunnel City Group.
Lava flows, mostly basalt, andesite and felsite flows and subordinate interflow sedimentary rocks.
Includes andesite tuff. Occurs in northeastern Wisconsin. Dacite has age of 1866 +/- 39 Ma.
Red, brown, and white quartzose sandstone, and minor siltstone, shale and conglomerate
Surrounds gneiss domes in Gogebic County, Mich. and Vilas County, Wisc., and occurs in north-central Dickinson County. Probably correlative with part of Blair Creek Formation (unit Xbc). Previously correlated with uppermost unit of Blair Creek Formation (Sims and others, 1984).
Ferromonzodiorite forming the upper contact zone.
Pink to pinkish-gray, medium- to coarse-grained, equigranular to inequigranular granite to granodiorite in Puritan batholith south of Gogebic Range. Age 2710 +/- 140 Ma (Sims and others, 1977)
Derived from mafic to intermediate pyroclastic rocks and massive to pillowed lava flows. Unit mapped as Ramsey Formation by Prinz (1981) south of Gogebic Range; mapped as Mona Schist and Kitchi Schist in northern complex of Marquette District.
Trenton Limestone
Light- to dark-gray, feldspathic, fine-grained sandstone, argillaceous siltstone, and argillite. Near base, ferruginous argillite contains beds of cherty sideritic and pyritic iron-formation
Dominantly dark-gray, massive, porphyritic tholeiitic basalt. Includes a basal conglomerate and a lean iron-formation in middle of formation
Interlayered quartzofeldspathic gneiss and amphibolite in Marenisco, Mich., area and northwestern Wisconsin. Protoliths are bimodal intermediate and mafic volcanic rocks (Sims and others, 1984).
Basalt, andesite and lesser rhyolite flows. Basalt flows near base of the formation contain plagioclase phenocrysts, some in radiating clusters
Red lithic conglomerate and sandstone; mafic to felsic volcanic flows similar to those of the unnamed formation (unit Yu) are interlayered with the sedimentary rocks.
Dark-gray basalt and minor porphyritic andesite. Generally strongly magnetic with reversed remanent magnetism. Underlain by a thin unit of quartzose sandstone (Bessemer Quartzite).
Powder Mill Group; Kallander Creek Volcanics; Upper rhyolite member
Poorly exposed. Includes Clarksburg Volcanics member which is mostly mafic to intermediate pyroclastic rocks in Marquette trough, between Marquette Bay and Champion
Clam Falls volcanic sequence (~1,102 Ma).
Mainly reddish-brown, medium- to fine-grained lithic arkosic sandstone, siltstone, and micaceous silty shale.
Mapped where units are too thin to show separately on map
Gabbro. Includes the Western Margin, Greenwood Lake, Bald Eagle (~1,098), Lake One, Lake Three, Wilder Lake, and Osier Lake intrusions.
Minong volcanic sequence (~1,095 Ma).
Includes Stambaugh Formation, Hiawatha Graywacke, Riverton Iron-formation, and Dunn Creek Slate (James and others, 1968). Correlated with lower part of Baraga Group
Gray, green, and brown lithic siltstone, shale, and sandstone. Copper sulfides and native copper occur locally near base
Exposed in northeastern Wisconsin
Interbedded cherty and slaty iron-formation. Locally, secondary enrichment has formed "soft ore" bodies. In eastern part of Gogebic Range, Ironwood Iron-formation is intercalated with pyroclastic rocks of Emperor Volcanic Complex, and in eastern half of T. 47 N., R. 43 W. (where mapped within unit Xip) consists entirely of black slate
Subcrops beneath Mississippi River alluvium in northeastern Allamakee County; surface exposures absent. Overlies erosional surface on Precambrian granitic basement rocks. Primary lithology: sandstone, very fine to very coarse grained, part iron-stained. Minor: shale, silty to sandy, gray, red, green; quartz granules; granitic pebbles. Thicknesses: 365-405 ft (111-123 m).
Oronto Group; Freda Sandstone; Conglomerate member
Mafic to ultramafic bodies defined and delineated principally by positive magnetic and gravity anomalies; includes diorite bodies. Comagmatic with unit Xmi
Primarily mafic flows.