Conglomerate, sandstone, mudstone, shale, marlstone, siltstone, and minor lignite, deposited in marine and non-marine settings; likely Cenomanian to Campanian age. Unit outline is the product of contouring the stratigraphic top and base, from which an isopach grid was created. Because the distribution is patchy, unit boundaries were drawn from the gridded data to represent locations where more than 25 feet (8 meters) of thickness occurs. As a result, many areas outside of the unit boundaries may be overlain by thin Cretaceous strata and the unit is depicted without a contact line.
Mafic metavolcanic rocks. Includes minor volcaniclastic and hypabyssal intrusive rocks metamorphosed to lower greenschist to lower amphibolite facies; includes the Ely Greenstone (~2,722 Ma).
Slate and graywacke. Includes the Rove (~1,836 to 1,777 Ma), Virginia (~1,832 Ma near base), and Thompson Formations (youngest zircons in the Thompson Formation ~1,790 Ma).
Granitic intrusion. Includes the Sacred Heart (~2592, 2,603 Ma) and Ortonville granites, the Shannon Lake Granite (~2,674), and other intrusions having low gravity and magnetic signatures.
Biotite schist of graywacke protolith, and schist-rich migmatite.
Foliated to gneissic tonalite, granodiorite, and diorite. Includes the Lookout Mountain tonalite (~2,718 Ma) of the Giants Range batholith and other intrusions within batholithic complexes defined by a low-to-moderate gravity signature with magnetic foliation apparent from aeromagnetic maps.
Limestone, shaley limestone, and dolostone; includes the Maquoketa Formation, and the Stewartville, Prosser, and Cummingsville Formations of the Galena Group. Also includes limestone and dolostone of the Red River Formation, and sandstone and shale of the Winnipeg Formation in northwest Minnesota.
Dolostone, sandy to silty dolostone, and sandstone; includes the Shakopee Formation and Oneota Dolomite of the Prairie du Chien Group.
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.
Volcaniclastic rocks of felsic to intermediate composition.
Sandstone, siltstone, and shale; includes the Wonewoc Sandstone, Eau Claire Formation, and Mt. Simon Sandstone.
Granite to granodiorite. Variably magnetic.
Granitic orthogneiss and migmatite. Geophysical map patterns imply this unit intruded other gneisses.
Shale, dolomitic limestone, and sandstone; includes the Decorah Shale of the Galena Group, the Platteville and Glenwood Formations, and the St. Peter Sandstone.
Unconformably overlies units described below (Mns, Mnu, Mnr, and Mnb).
Schist and slate of graywacke-mudstone protolith.
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.
Schist of sedimentary protolith. Metamorphosed to upper greenschist to amphibolite facies.
Granitoid intrusion. Constrained solely by low gravity and magnetic signatures.
Quartzite, mudstone, and local conglomerate of fluvial and marine origin (~1,760 to 1,630 Ma; 1,902 ± 55 Ma rhyolite pebble in basal conglomerate).
Granite-rich migmatite. Contains neosome of variably magnetic biotite granite similar to the Lac La Croix Granite, and paleosome of tonalite to granodiorite gneiss and schist.
Granodiorite to hornblende diorite, locally magmatically foliated. Includes the Britt Granodiorite (~2,685 Ma) and other intrusions having moderate gravity and magnetic signatures.
Schist and tonalitic to granodioritic paragneiss. Typically adjacent to large batholithic complexes.
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.
Granitoid gneiss with dioritic to amphibolitic enclaves. Produces moderately high and varied gravity and magnetic signatures.
Anorthositic intrusions and inclusions, undifferentiated (~1,099 Ma).
Includes the Mahnomen and Rabbit Lake Formations and Trout Lake marble. Unit descriptions from GEOLEX - Mahnomen Formation: Upper member described as gray to light-gray siltstone, argillite, and fine-grained quartzose sandstone containing irregularly distributed beds and lenses of iron-formation and allied anomalously iron-rich sedimentary rocks; base of unit defined arbitrarily as the top of the lowermost mappable iron-formation layer of the middle, iron-formation member. Middle, iron-formation member--interlayered cherty and slaty iron-formation that is similar lithologically to the smaller iron-formation lenses within the upper and lower members; contains rocks formerly assigned to the Trommald Formation and the Emily Member of the Rabbit Lake Formation (Morey, 1978). Lower member--predominantly siltstone, fine- to medium-grained quartzose graywacke, and argillite; also includes minor quartz arenite and irregularly distributed layers and lenses of anomalously iron-rich sedimentary rocks. Rabbit Lake Formaiton: gray to very dark-gray slate, siltstone, and very fine-grained graywacke; includes irregularly distributed layers and lenses of carbonaceous slate and also carbonate- and sulfide-facies iron-formation, chiefly in the lower part of the unit.
Dolostone, sandy dolostone, limestone, and shale; includes the Chickasaw Shale and Bassett Member of the Little Cedar Formation, and Pinicon Ridge and Spillville Formations.
Granite, reddish, variably porphyritic, massive; includes the St. Cloud Red (~1,779 Ma), Foley (~1,774, 1,779 Ma), Pierz (~1,779 Ma), and Pease granites, and Richmond charnockitic granite (~1,772 Ma). An unnamed granite in south-central Minnesota is slightly older at ~1,792 Ma.
Calc-alkalic volcanic and volcaniclastic rocks.
Granitic to granodioritic orthogneiss. Includes the McGrath Gneiss (~2,752 ± 15, 2,557 Ma) and unnamed units.
Foliated to gneissic tonalite and granodiorite.
Leucogranite. Occurs along the Yellow Medicine Shear Zone and elsewhere, primarily in batholithic settings.
Gabbro, peridotite, pyroxenite, lamprophyre, and metamorphic equivalents. Includes the Oaks intrusion (~2,671 Ma) in the Wabigoon subprovince, and a ~2,639 Ma lamprophyre in the western Wawa subprovince; locally defined by variably high gravity and magnetic signatures.
Gray granodiorite to granite intrusions. Includes the Reformatory (~1,783 Ma), Freedhem (~1,775, 1,776 Ma), Isle (~1,779 Ma), and Warman (~1,787 Ma) intrusions.
Claystone, siltstone, and sandstone. Albian age is tentatively inferred from a single analysis of pollen.
Conglomerate, lignite, sandstone, and shale; shallow marine and non-marine. Late Cenomanian age is inferred. Despite exposures in mines on the Mesabi Iron Range, the position of much of the unit contact is highly speculative.
Troctolite. Includes the Boulder Lake, Tuscarora (~1,099 Ma), Partridge River (~1,098 Ma), South Kawishiwi, and part of the Western Margin intrusions.
Primarily mafic flows.
Gabbro. Includes the Western Margin, Greenwood Lake, Bald Eagle (~1,098), Lake One, Lake Three, Wilder Lake, and Osier Lake intrusions.
Pink biotite granite that is variably magnetic and locally pegmatitic (~2,665 to 2,667 Ma).
Volcanogenic graywacke and mudstone. Includes the Lake Vermilion Formation.
Gabbroic intrusions. Including the Sawmill, London, Brule River, Hovland, and Pigeon Point intrusions.
Interlayered volcanic, volcaniclastic, sedimentary, and hypabyssal intrusive rocks. Includes parts of the Mille Lacs Group, North and South Range Groups, and Glen Township Formation.
Dolostone, limestone, and shale; Includes the Lithograph City Formation, Coralville Formation, and Hinkle and Eagle Center Members of the Little Cedar Formation.
Interlayered volcanic and sedimentary rocks.
Tonalite to leucodiorite (~1,792, 1,793, 1,800 Ma).
Lower sequence, primarily basalt. Includes the Ely's Peak and Grand Portage basalts and Hovland lavas (~1,108 Ma).
Undifferentiated mafic to felsic lava flows. Includes the Lakeside, Lakewood, and Cross River lavas.
Syenite, monzodiorite, granodiorite, and diorite. Commonly hornblende- or pyroxene-bearing; Includes the Coon Lake, Side Lake, and Linden (~2,666 Ma) plutons of north-central Minnesota.
Gabbro, ferrogabbro, and gabbronorite. Includes the Lax Lake and Cloquet Lake gabbros, Sonju Lake (~1,096 Ma), Dam Five, Wilson Lake (~1,096 Ma), Fourmile Lake, and Upper Manitou intrusions (~1,096 Ma).
Amphibolitic to dioritic gneiss.
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.
Tonalite-leucodiorite plutons. Includes the Grygla, Red Lake Falls, and other plutons having low magnetic and gravity signatures.
Rockville Granite and rocks inferred to be related. Coarse-grained and pink to white (~1,780 Ma).
Granophyre, ferromonzodiorite, and leucogabbro. Includes the Mt. Weber (~1,106 Ma), Whitefish Lake (~1,109 Ma), Beth Lake, Misquah Hills (~1,106 Ma), and Cucumber Lake (~1,106 Ma) granophyres, and Wine Lake monzodiorite.
Rhyolite and icelandite. Includes the Cross River, Palisade Head (~1,096 Ma), Devil Track, Maple Hill, Grand Marais, and Kimball Creek lavas.
Magnetic intrusions, undifferentiated. Typically too small to ascertain gravity expression.
Shale, limestone, dolomite, siltstone, and sandstone.
Mafic to ultramafic hypabyssal intrusive complexes composed of gabbro, pyroxenite, diorite, and anorthosite. Includes the Mentor and Deer Lake complexes, intrusions of the Newton Lake Formation, all within the Wawa subprovince; and the UBD intrusion (~2,685 to 2,695 Ma) in the Wabigoon subprovince.
Granitic intrusion of unknown age. Low gravity and magnetic expression.
Granitoid intrusions, variably magnetic. Includes the Ann Lake granite (~1,784 Ma) and Glendorado pluton (~1,788 Ma).
Locally includes thin basal sandstone and conglomerate. Capped by an irregular layer of brecciated iron-formation and ejecta derived from the 1,850 Ma Sudbury meteorite impact.
Granitic rocks, undifferentiated. Largely inferred from geophysical maps.
Quartzofeldspathic orthogneiss and schist; local metaconglomerate; age unknown.
Includes conglomerate and siliceous mudstone.
Mylonitic, gneissic, and schistose rocks of plutonic and volcanic protolith.
Minong volcanic sequence (~1,095 Ma).
Dam Lake quartzite.
Felsic intrusive rocks, granophyre. Includes parts of the Eagle Mountain (~1,098 Ma) and Pine Mountain (~1,095 to 1,098 Ma) intrusions.
Mafic intrusions including pyroxenite, peridotite, gabbro, and lamprophyre. Defined largely by magnetic signature. One intrusion in Morrison County is ~1,791 Ma, and lithologically similar intrusions cut the Foley Granite (~1,774 Ma).
Diabase and ferrodiorite. Includes the Beaver River diabase, Silver Bay (~1,096 Ma), Milepost 7, Leveaux, Shoepack Lake, Cabin Creek, and Monker Lake intrusions.
Graywacke and slate
Gabbro, pyroxenite, diorite, and lamprophyre. Includes the Watab quartz diorite (~1,780 Ma), St. Wendell quartz gabbro, and an unnamed diorite (~1,786 Ma).
Gabbroic cumulates. Includes the Poplar Lake (formerly Nathan's layered series) and Crocodile Lake intrusions (~1,107 Ma).
Monzonite, quartz monzonite, and granodiorite. Includes the Farm Lake phase of the Giants Range batholith.
Migmatite dominated by tonalitic to granodioritic neosome.
Gabbroic to dioritic intrusion. High to moderate gravity and magnetic signature. Includes Providence and Cottonwood intrusions in southwestern Minnesota.
Volcanogenic lithic sandstone, conglomerate, siltstone, graywacke, and slate.
(~2,690 Ma) - Tonalite to granodiorite; typically contains large quartz phenocrysts and granodioritic to dioritic autoliths.
Diabase and gabbro sills and dikes; locally porphyritic (~1,108, 1,115 Ma).
Graywacke and slate with graphitic and sulfidic zones.
Undifferentiated volcanic rocks and volcanic hornfels.
Dioritic to granodioritic intrusion of uncertain age. Moderate gravity and magnetic expression.
Amphibolitic schist and gneiss. Both extrusive and intrusive protoliths are likely.
Troctolite, augite troctolite, and troctolitic gabbro of the Houghtaling Creek troctolite (~1,099 Ma).
Slate and graywacke with thin carbonate layers.
Mafic volcanic and hypabyssal intrusive rocks of uncertain age. Adjacent to and within the Yellow Medicine shear zone.
Granophyre and granite. Includes the Blesner Lake, Finland (~1,098 Ma), and Cloquet Lake granophyres (~1,095 Ma).
Mafic to ultramafic (komatiitic) volcanic rocks. Includes the Newton Lake Formation and Deer Lake sequence.
Hornblende-phyric volcanic flows, breccia, and tuff.
Granodiorite, variably foliated. Includes the Bradbury Creek (~1,877 to 1,857 Ma) and Philbrook (~1,854 Ma) intrusions.
Mille Lacs granite (~2,009 Ma).
Iron-formation in the Cuyuna North and South Ranges and Mille Lacs Group. Includes the Trommald Formation. Unit description from GEOLEX - Trommald Formation: interlayered cherty and slaty iron-formation, moderately deformed and weakly metamorphosed.
Lithic sandstone (reworked saprolite), marble, and mica schist (youngest detrital zircons greater than 2,000 Ma).
Conglomerate, lithic sandstone, and graywacke, undifferentiated. Includes the Midway sequence, Seine Group, and a part of the Knife Lake Group known as the Ogishkemuncie conglomerate that contains clasts of ~2,690 Ma Saganaga Tonalite (unit Ast). Deposition of alluvial fan and fluvial sediments occurred in fault-bounded basins.
Gabbro to troctolite (~1,099 Ma).
Tonalite. Locally migmatitic, variably magnetic; contains abundant biotite schist paleosome of graywacke protolith (presumably derived from the Little Falls Formation, unit Pls).
Mylonite, varied protolith. Age of shearing unknown and likely protracted.
Clam Falls volcanic sequence (~1,102 Ma).
Mafic intrusions (gabbro, diabase) of unknown age. Includes the LL, BKV, and Lake Washington intrusions in Meeker County.
Sandstone with some dolomite and shale, undivided; includes Trempealeau, Tunnel City, and Elk Mound Formations
Ferromonzodiorite forming the upper contact zone.
Knife Lake Group conglomerate. Clasts consist largely of hornblende-bearing volcanic rocks.
Gabbroic, noritic, and anorthositic intrusions.
Interlayered metavolcanic, volcaniclastic, and hypabyssal intrusive rocks. Metamorphosed to amphibolite facies.
Iron-formation. Includes the Soudan Iron Formation and many unnamed units.
Gray basalt flows, interflow breccia, tuff, and minor sedimentary rocks
Interflow sandstone, siltstone, and conglomerate.
Porphyritic quartzofeldspathic dikes. Typically hypabyssal intrusions associated with felsic volcanic sequences.
North Branch mafic volcanic sequence.
Diabase, diorite, pyroxenite, and gabbro; reversed magnetic polarity; includes the Esko and Tamarack intrusions emplaced into unit Pas (Slate and graywacke of Animikie Group).
Belle Fourche Formation (as described in Kb). Mowry Formation: medium- to dark-gray shale; marine offshore sediment; as thick as 55 metres (180 feet). Newcastle Formation: light-gray, fine- to medium-grained sandstone; marine shoreline and offshore sediment; as thick as 45 metres (150 feet). Skull Creek Formation: medium-gray to dark-gray shale; marine offshore sediment; as thick as 40 metres (140 feet).
Greenschists and amphibolites; banded iron formation; stretched pebble conglomerates; metabasalt; serpentinite; felsic tuff; mylonite; intermediate felsic plutonic rocks; migmatite; layered gneiss.
Yellowish- to brownish-gray limestone and dolomite; offshore marine sediment; as thick as 215 metres (700 feet).
Light-gray, fine to coarse sandstone and gray shale; river, lake, and nearshore marine sediment; as thick as 135 metres (450 feet).
Puckwunge and Nopeming Sandstones.
Dark-gray to black, silty to sandy shale with several zones of septarian, fossiliferous, carbonate concretions. Contains up to three sandstone units in the upper portion of the formation and sandy calcareous marl at the base. Thickness up to 330 ft (100 m).
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.
Dolomite with some sandstone and shale; includes Shakopee and Oneota Formations
Poorly sorted arkosic sandstone
Blue-gray to dark-gray, fissile to blocky shale with persistent beds of bentonite, black organic shale, and light-brown chalky shale. Contains minor sandstone, conglomerate, and abundant carbonate and ferruginous concretions. Thickness up to 1,000 ft (305 m).
Pink and reddish to tan, siliceous, fine- to coarse-grained, iron-stained orthoquartzite with minor metamorphosed conglomerate and mudstone layers. Estimated thickness greater than 1,000 ft (305 m).
Light-gray to greenish-gray shale and light-gray sandstone; marine nearshore and offshore sediment; as thick as 125 metres (400 feet).
Well-sorted quartzose sandstone
White to dark-gray argillaceous chalk, marl, and shale. Weathers yellow to orange. Contains thin, laterally continuous bentonite beds, chalky carbonaceous shale, minor sand, and small concretions. Thickness up to 150 ft (46 m).
Dark-gray, greenish, and varicolored shale with local limestone (Swift and Rierdon Formation equivalents); mainly nearshore marine sediments; as thick as 180 metres (600 feet).
Dark-gray, noncalcareous, pyritic, poorly fossiliferous shale, with numerous sandstone layers at the base. Thickness up to 110 ft (36 m).
Black opaline spiculite, gray to black shale, yellow-brown to gray chalk, gray silty clay, and pink quartz-rich sandstone. Includes the Split Rock Creek Formation and other near-shore facies of the Dakota Formation, Graneros Shale, Greenhorn Formation, Carlile Shale, Niobrara Formation, and Pierre Shale. Thickness up to 400 ft (122 m).
Pink to dark-red, coarse-grained granite composed of orthoclase, quartz, and biotite.
White to buff, brown, or gray limestone, gypsum, and anhydrite; offshore marine sediments; as thick as 190 metres (625 feet).
Gray shale, mudstone, marl, calcarenite, and shaly limestone grading upward into light-gray to tan, alternating marl and thin-bedded, fossiliferous limestone. Thickness up to 40 ft (12 m).
Black, intermediate- to coarse-grained diabase composed of plagioclase, pyroxene, and olivine.