GEOLOGIC MAP OF MINNESOTA

BEDROCK GEOLOGY

Compiled by

G.B. Morey and Joyce Meints

Third Edition2000

The University of Minnesota is an equal opportunity educator and employer

©2000 by the Board of Regents of the University of Minnesota

Base modified from 1990 CENSUS TIGER/Line Files ofU.S. Bureau of Census (source scale 1:100,000) and DigitalChart of the World (ESRI version).

Lambert Conformal Conic ProjectionStandard parallels 33° N and 45° N.

Geology compiled 2000

GIS compilation and cartography by Joyce Meints;digital cartography by Philip Heywood

STATE MAP SERIES S-20, 3rd Edition

Minnesota Bedrock Geology

DIGITAL MAPPING—Numbers refer to areas shown on map above.

1 Jirsa, M.A., Chandler, V.W., and Runkel, A.C., 1999, Bedrock geologic map ofnorthwestern Minnesota: Minnesota Geological Survey Miscellaneous Map Series,Map M-92, scale 1:200,000.

2 Chander, V.W., Jirsa, M.A., and Morey, G.B., 1997, Mineral potential assessmentof northern St. Louis County, southeastern Koochiching County and northeasternItasca County, Minnesota: Minnesota Geological Survey Open-File Report 97-5,9 pls., scale 1:62,000; final report, 26 p.

3 Peterson, D.M., and Jirsa, M.A., compilers, 1999, Bedrock geologic map and mineralexploration data, western Vermilion district, St. Louis and Lake Counties, north-eastern Minnesota: Minnesota Geological Survey Miscellaneous Map Series, MapM-98, scale 1:48,000.

4 Jirsa, M.A., Boerboom, T.J., and Morey, G.B., 1998, Bedrock geologic map of theVirginia Horn, Mesabi Iron Range, St. Louis County, Minnesota: Minnesota Geologi-cal Survey Miscellaneous Map Series, Map M-85, scale 1:48,000.

5 Miller, J.D., Jr., and Chandler, V.W., 1999, Bedrock geologic map of the central DuluthComplex and the western part of the Beaver Bay Complex, Lake and St. LouisCounties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series,Map M-101, scale 1:100,000.

6 Boerboom, T.J., Southwick, D.L., and Severson, M.J., 1999, Bedrock geology of theAitkin 30 x 60 minute quadrangle, east-central Minnesota: Minnesota GeologicalSurvey Miscellaneous Map Series, Map M-99, 2 pls., scale 1:100,000.

7 Boerboom, T.J., Southwick, D.L., and Severson, M.J., 1999, Bedrock geology of theMille Lacs 30 x 60 minute quadrangle, east-central Minnesota: Minnesota GeologicalSurvey Miscellaneous Map Series, Map M-100, 2 pls., scale 1:100,000.

8 Meyer, G.N., project manager, 1995, Geologic atlas of Stearns County, Minnesota:Minnesota Geological Survey County Atlas Series, Atlas C-10, Pt. A, 7 pls., scales1:100,000 and 1:200,000.

9 Jirsa, M.A., Chandler, V.C., Cleland, J.M., and Meints, J.P., 1995, Bedrock geologicmap of east-central Minnesota: Minnesota Geological Survey Open-File Report95-1, 2 pls., scale 1:100,000.

10 Mossler, J.H., and Tipping, R.G., compilers, 2000, Bedrock geology and structureof the seven-county Twin Cities Metropolitan Area, Minnesota: Minnesota GeologicalSurvey Miscellaneous Map Series, Map M-104, scale 1:125,000.

SCALE 1:1,000,0000 50 MILES2525

0 50 75 KILOMETERS2525

SOURCES OF GEOLOGIC MAPPING USED TO COMPILE THIS MAP11 Hobbs, H.C., project manager, 1995, Geologic atlas of Rice County, Minnesota:

Minnesota Geological Survey County Atlas Series, Atlas C-9, Pt. A, 6 pls., scale1:100,000.

12 Setterholm, D.R., project manager, 1998, Geologic atlas of Goodhue County,Minnesota: Minnesota Geological Survey County Atlas Series, Atlas C-12, Pt.A, 6 pls., scale 1:100,000.

13 Bloomgren, B.A., 1993, Bedrock geology of Waseca County, Minnesota: MinnesotaGeological Survey Miscellaneous Map Series, Map M-73, 3 pls., scale 1:62,500and smaller.

14 Mossler, J.H., project manager, 1998, Geologic atlas of Mower County, Minnesota:Minnesota Geological Survey County Atlas Series, Atlas C-11, Pt. A, 6 pls., scale1:100,000.

15 Mossler, J.H., project manager, 1995, Geologic atlas of Fillmore County, Minnesota:Minnesota Geological Survey County Atlas Series, Atlas C-8, Part A, 5 pls., scale1:100,000.

16 Runkel, A.C., 1996, Bedrock geology of Houston County, Minnesota: MinnesotaGeological Survey Open-File Report 96-4, 3 pls., scale 1:100,000; text, 13 p.

17 Morey, G.B., 1999, unpublished geologic map of the Mesabi iron range, northernMinnesota: Minnesota Geological Survey file map, scale 1:100,000.

NONDIGITAL MAPPINGMorey, G.B., 1996, compiler, Geologic map of Minnesota, bedrock geology: Minnesota

Geological Survey State Map Series, Map S-20 digital version 2, scale 1:1,000,000and references therein.

Every reasonable effort has been made to ensure the accuracy of the factual data onwhich this map interpretation is based; however, the Minnesota Geological Survey doesnot warrant or guarantee that there are no errors. Users may wish to verify criticalinformation; sources include both the references listed here and information on file atthe offices of the Minnesota Geological Survey in St. Paul. In addition, effort has beenmade to ensure that the interpretation conforms to sound geologic and cartographicprinciples. No claim is made that the interpretation shown is rigorously correct, however,and it should not be used to guide engineering-scale decisions without site-specificverification.

Generally excellent to fair outcrop

Generally fair to poor outcrop

Poor to no outcrop

DIAGRAM SHOWING GEOLOGICRELIABILITY BASED ON DENSITY

OF BEDROCK OUTCROP

KITTSON ROSEAULAKE

OF THEWOODS

MARSHALL

POLK

RED LAKE

PENNINGTON

BELTRAMI

CLE

AR

WA

TE

R

NORMAN

KOOCHICHING

CLAY

BECKER

HUBBARD

WADENA

CASS

ITASCA

ST. LOUIS

LAKE

COOK

CARLTON

WILKIN

OTTER TAIL

TRAVERSE

GRANT

STEVENS

DOUGLAS

POPE

TODD

STEARNS

MORRISON

AITKIN

CROWWING

MIL

LE L

AC

S

KA

NA

BE

C

PINE

BIG STONE

SWIFT

CHIPPEWA

LAC QUIPARLE

YELLOW MEDICINE

RENVILLLE

KA

ND

IYO

HI

MEEKER WRIGHT

LIN

CO

LN

LYONREDWOOD

SIBLEY

MCLEOD

NICOLLET

ISANTI CHISAGO

ANOKA

BENTON

SHERBURNE

DAKOTA

CA

RV

ER

WA

SH

ING

TO

N

HENNEPIN

RAMSEY

SCOTT

WINONA

FILLMORE

OLMSTED

WABASHAGOODHUE

DODGE

RICELE SUEUR

BROWN

PIP

ES

TO

NE

MURRAY COTTONWOOD WATONWAN BLUE EARTH WASECA STEELE

ROCK NOBLES JACKSON MARTIN FARIBAULT FREEBORN MOWER HOUSTON

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2

DESCRIPTION OF MAP UNITS

MESOZOIC ROCKSKu Cretaceous rocks, undivided—Dakota, Graneros, Greenhorn, Carlile, Niobrara,

and Pierre formations and their nonmarine equivalents in northwestern,southwestern, and southeastern Minnesota.

Kc Coleraine Formation—Jasper-pebble conglomerate, sandstone, and shale of diverseorigin on the Mesabi range of northern Minnesota, and unnamed sandstoneand shale of nonmarine to marine origin in east-central Minnesota.

Ju Jurassic rocks, undivided—Unnamed units of green, gray, brown, and red shale,white to tan micritic limestone and dolostone, and white, fine- to coarse-grained sandstone and siltstone; unit contains nodules of chert and gypsum.

PALEOZOIC ROCKSDu Devonian rocks, undivided—Limestone, dolomitic limestone, and dolostone of

the Cedar Valley and Wapsipinicon Groups.

Dl Lithograph City Formation, Coralville Formation and Hinckle and Eagle CityMembers of the Little Cedar Formation (Cedar Valley Group), undivided—Limestone, dolostone, and lesser amounts of shale.

Dc Chickasaw Member of the Little Cedar Formation (Cedar Valley Group)—Light-gray to medium-gray shale and silty shale.

Dw Bassett Member of the Little Cedar Formation (Cedar Valley Group) and PiniconRidge and Spillville Formations (Wapsipinicon Group)—Dolostone and shalydolostone.

Oum Upper and Middle Ordovican rocks, undivided—Limestone and shaly limestoneof the Maquoketa and Dubuque Formations, and limestone and dolostone ofthe Galena Group in the Hollandale embayment of southeastern Minnesota.

Omu Middle and Upper Ordovician rocks, undivided—Sandstone and shaly sandstoneof the Winnipeg Formation and limestone and dolomitic limestone of the RedRiver Formation along the east edge of the Williston Basin in northwesternMinnesota.

Om Middle Ordovician rocks, undivided—Decorah Shale; limestone of the PlattevilleFormation; shaly rocks of the Glenwood Formation; and St. Peter Sandstonein the Hollandale embayment of southeastern Minnesota.

Omd Decorah Shale (Middle Ordovician)—Light-greenish-gray shale and lesser amountsof coquinoid limestone, especially in the upper half of the formation. Mappedas a separate unit where possible.

Ol Lower Ordovician rocks, undivided—Shakopee and Oneota Formations of thePrairie du Chien Group in the Hollandale embayment of southeastern Minne-sota. Unit consists dominantly of dolostone and dolomitic limestone. TheShakopee also contains intervals of quartz arenite, including a pronouncedbasal unit named the New Richmond Member.

�u Upper Cambrian rocks, undivided—Jordan Sandstone; dolomitic, glauconitic,and silty glauconitic rocks of the St. Lawrence and Franconia Formations;Ironton and Galesville Sandstones; sandy and shaly rocks of the Eau ClaireFormation; and the Mt. Simon Sandstone.

MESOPROTEROZOIC ROCKS

�mh Hinckley Sandstone—Buff to tan quartz arenite of lacustrine and eolian origin.

�mf Fond du Lac Formation—Red to dark-brown shale, feldspathic sandstone, andarkose of fluvial origin. Includes the Oldenberg Point Member, a pronouncedbasal unit of quartz-pebble conglomerate in the Duluth area.

�ms Solor Church Formation—Dark-red to dark-brown shale, siltstone, and lithicsandstone of fluvial origin in Scott and Carver Counties; metamorphosed tozeolite facies.

�mc Chengwatana Volcanic Group—Basalt and related volcanogenic and interflowsedimentary rocks in east-central Minnesota.

North Shore Volcanic Group.

�mnp Schroeder-Lutsen basalts—Predominantly ophitic olivine tholeiitic basaltunconformably over older, normally polarized volcanic rocks. Based onits stratigraphic position and geochemical affinities, the unit may be correla-tive with the Lake Shore traps of northern Michigan .

�mnn Normally polarized volcanic rocks, undivided—Basalt, andesitic basalt, rhyolite,and related volcanogenic interflow sedimentary rocks along and inland fromthe North Shore of Lake Superior.

�mnr Reversely polarized volcanic rocks, undivided—Mixed tholeiitic diabasic andporphyritic basalt, trachybasalt, and rhyolite in far northeastern Minnesotaand porphyritic and diabasic basalt near Duluth. Includes units of a basalquartz arenite, Puckwunge Sandstone and Nopeming Formation, innortheastern Minnesota and near Duluth, respectively.

Subvolcanic mafic rocks, undivided.�mbu Beaver Bay Complex and other named and unnamed gabbroic-troctolitic

intrusions—Includes a number of other intrusions in a variety of dikes andsills such as the Endion sill and the Pigeon River Intrusions.

�mbg Selected granophyric and leuco-granitic phases of troctolitic-gabbroic intrusionsin the Beaver Bay Complex.

�evd Metabasalt, metadiabase, and metasedimentary rocks metamorphosed to loweramphibolite facies—Includes fragmental volcanic rocks, mafic hypabyssalintrusions, graphitic argillite, and oxide iron-formation.

�edv Metabasalt, metadiabase, and metasedimentary rocks metamorphosed to lowergreenschist facies—Includes fragmental volcanic rocks, mafic hypabyssalintrusions, graywacke, graphitic argillite and oxide iron formation.

�eps Metagraywacke, metasiltstone and a variety of schistose rocks metamorphosedto the amphibolite facies.

�edq Denham Formation—Quartz arenite and siltstone, oxide iron-formation, marble,mafic hypabyssal intrusions and fragmental volcanic rocks metamorphosedto the staurolite grade of the amphibolite facies.

LATE ARCHEAN ROCKS

Ami Post-tectonic mafic intrusions—Gabbro, peridotite, pyroxenite, and their meta-morphic equivalents. Unit also includes small intrusive complexes of anortho-site, gabbroic anorthosite, and anorthositic gabbro. Generally characterizedby pronounced magnetic signatures.

Agd Multiphase intrusions of hornblende-pyroxene-bearing and biotite-bearing mon-zonite, monzodiorite, diorite, syenite, and granodiorite—Typically postdatesregional metamorphism and deformation associated with the Algoman orogen.

Agr Syntectonic to pretectonic granitoid rocks—Granite and granodiorite of theVermilion Granitic Complex, the Giants Range and Bemidji batholiths, aswell as smaller intrusions of tonalite and monzonite of the Algoman orogenin northern Minnesota. Also includes the Odessa, Sacred Heart, and FortRidgely Granites exposed along the Minnesota River Valley in southwesternMinnesota.

Agm Granite-rich migmatite—Granitic gneiss, paragneiss, schist, and migmatite inthe Vermilion Granitic Complex and other parts of extreme northern Minne-sota. Grades into granitoid rocks.

Ast Saganaga Tonalite of northeastern Minnesota—Emplaced more-or-less contem-poraneously with deposition of metasedimentary and metavolcanic rocks.

Asm Paragneiss and schist-rich migmatite—Grades into undivided metasedimentaryrocks (unit Ams).

Ams Metasedimentary rocks, undivided—Graywacke, slate, local units of conglomerate,arenite, graphitic slate, fine-grained felsic volcanogenic, and volcaniclasticrocks, lean oxide iron-formation (shown in red) and its metamorphic equiva-lents. Includes the Knife Lake Group and the Lake Vermilion Formation innortheastern Minnesota.

Amm Mixed metavolcanic rocks—Mafic to felsic volcanic sequences that have variableamounts of felsic volcanogenic and volcaniclastic rocks and lean iron-forma-tion. Includes parts of the Ely Greenstone and the Soudan Iron Formation(shown in red) in northeastern Minnesota.

Amv Mafic metavolcanic rocks—Dominantly basalt that contains thin sedimentaryunits, including iron-formation (shown in red). Includes parts of the ElyGreenstone and the Newton Lake Formation in northeastern Minnesota. Alsoincludes metabasalt exposed in the Minnesota River Valley.

Agb Anorthosite, gabbroic anorthosite, and anorthositic gabbro of the Mentor maficintrusive complex in Polk County.

Adg Gabbro, diorite, peridotite, and associated komatiitic flows of the Deer Lakesequence in Itasca County and the upper part of the Newton Lake Formationin Lake and St. Louis Counties.

Aps Paragneiss, schist, and amphibolite—Amphibolite-facies equivalent of units Amvand Ams; locally includes abundant intrusions of unit Agr.

Aqz Felsic to intermediate volcanic and volcaniclastic rocks, mica schist, phyllite,and granitoid rocks—Variably and cataclastically deformed. Unit forms aero-magnetic “quiet zone” and probably contains some rocks of Paleoproterozoicage.

MIDDLE ARCHEAN AND OLDER ROCKS

Amg Migmatitic gneiss, amphibolite, and granite—Montevideo and Morton Gneisses(3,600–3,000 m.y.) in the Minnesota River Valley, southwestern Minnesota;McGrath Gneiss (2,750 m.y.) east of Mille Lacs Lake; components of HillmanMigmatite southwest of Mille Lacs Lake; and Sartell Gneiss in Stearns County.Inferred to include various younger rocks, including granitoid intrusions inthe Hillman Migmatite and pillowed basalt in poorly exposed areas ofsouthwestern Minnesota.

Geologic contact—Located principally by inference or by airborne magneticsurveys except in the very few places where the contact is exposed.

Inferred trace of a steeply dipping fault.

Inferred trace of a thrust fault or structural discontinuity that is interpretedto have involved thrust displacement—Teeth are on the upper plate.

Iron-formation.

Duluth Complex.

�mdg Felsic series—Granophyric granite and related felsic rocks.

�mt Troctolitic and gabbroic cumulate rocks—Constitute at least nine named andseveral unnamed intrusions.

�ma Anorthositic series—Plagioclase-rich gabbroic cumulates and related rocks.

�mg Early gabbros—Gabbro and related rocks in northeastern Minnesota that havepetrologic affinities to the Logan Intrusions.

�ml Logan Intrusions—Diabase, porphyritic diabase, gabbro, and related felsic sillsand dikes.

PALEOPROTEROZOIC ROCKS

�es Sioux Quartzite—Red quartzite of fluvial to possibly marginally marine origin.Includes quartz-pebble conglomerate, claystone (catlinite, also calledpipestone), a basal (rhyolite) pebble conglomerate in Pipestone County, anda basal (granite, quartz, chert, iron-formation) conglomerate in Nicollet Countyon the Minnesota River.

�eop Post-tectonic intrusions of the Penokean orogen—Small stocks of olivinepyroxenite in Morrison County; small plutons of hornblende-rich diorite andgabbro that contain layers and lenses of nelsonite, pyroxenite, and anorthositein Todd County.

�egr Late-tectonic intrusions of the Penokean orogen—Includes the St. Cloud andRockville Granites and Reformatory granodiorite of east-central Minnesota,the Section 28 granite, the Cedar Mountain Complex, and other unnamedintrusions exposed along the Minnesota River Valley in southwesternMinnesota.

�egd Syntectonic intrusions of the Penokean orogen—Includes the Pierz Granite, theFreedhem and Bradbury Creek Granodiorites, and several unnamed intrusionsof granite, granodiorite, tonalite, and gabbro in east-central Minnesota.

�emt Unnamed schistose, volcanic, and hypabyssal rocks of mafic composition andvolcanic, volcaniclastic, and intrusive rocks of felsic composition—May becorrelative with rocks of the Wisconsin magmatic terranes.

Animikie Group.

�eg Shale, siltstone, feldspathic graywacke, and associated volcaniclastic rocks—Includes the Rove Formation in Cook County, the Virginia Formation inSt. Louis, Itasca, and Lake Counties, and the Thomson Formation in CarltonCounty.

�eif Iron-formation—Includes the Gunflint Iron Formation in Cook County andthe Biwabik Iron Formation and subjacent units of arenite and conglomerateassigned to the Pokegama Quartzite in Itasca, St. Louis, and Lake Counties.Also includes thin lenses of iron-formation (Remer Member) in the VirginiaFormation in Itasca County.

�eq Pokegama Quartzite—Quartz arenite, siltstone, and shale. Shown only in CrowWing County.

Little Falls Formation.

�elf Quartz-rich slate, argillite, and schist in the northwestward extent of the unitand coarse-grained megacrystic garnet-staurolite-schist in the southeastwardextent—Unit has an uncertain stratigraphic position relative to otherPaleoproterozoic stratified units but is apparently younger than the MilleLacs and North Range Groups.

North Range Group.

�enr Rabbit Lake Formation—Mudstone, graywacke, iron-rich strata, and associatedmafic metavolcanic rocks metamorphosed to the greenschist facies. Includesthin beds of carbonate-silicate iron-formation.

�ent Trommald Formation—Carbonate-silicate iron-formation overlain by hematiteiron-formation and associated manganese oxide deposits. Also containssubstantial quantities of volcanic and hypabyssal rocks of generally maficcomposition. Metamorphosed to the greenschist facies.

�enm Mahnomen Formation—Claystone, shale, siltstone, and graywacke meta-morphosed to the greenschist facies.

Mille Lacs Group and related rocks of the Penokean fold-and-thrust belt.

�emq Quartz arenite, siltstone, and chert-rich dolostone of the Trout Lake Formationin Crow Wing County.

�esa Slate, argillite, and metasiltstone metamorphosed to the lower greenschistfacies—Includes lesser amounts of mafic hypabyssal intrusions, and frag-mental mafic volcanic rocks.

�egv Graphitic schist, slate, and silicate iron-formation metamorphosed to the lowergreenschist facies and related conditions—Includes substantial quantitiesof mafic to intermediate igneous rocks.

�egs Graphitic schist, phyllite and slate interbedded on a fine scale.

�eqd Quartzite at Dam Lake—Quartz arenite and sericitic quartz schist; includesa substantial component of mafic volcanic rock fragments.

�em Metadiabase, undivided.

MINNESOTA GEOLOGICAL SURVEYD.L. Southwick, Director

UNIVERSITY OF MINNESOTA

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KITTSON ROSEAU

LAKE OF THE WOODS

KOOCHICHING

KANABEC

MILLELACS

ISANTI

CHISAGO

ANOKA

SHERBURNE

BENTON

RAMSEY

DAKOTASCOTT

HENNEPIN

WRIGHT

CARVER

GOODHUE

WABASHA

WINONA

HOUSTON

STEELE

DODGE

MOWER

FREEBORN

LE SUEUR

NICOLLET

SIBLEY

MARTIN

JACKSON

BROWN

COTTONWOOD

NOBLES

MURRAY

LYON

ROCK

PIPESTONE

LINCOLN

YELLOW MEDICINE

LAC QUI PARLE

CHIPPEWA

RENVILLE

KANDIYOHIMEEKER

SWIFT

MC LEOD

STEARNS

POPE

STEVENSBIG STONE

GRANT

DOUGLAS

TODD

WILKIN

OTTER TAIL

WADENA

MORRISON

CASS

HUBBARD

CLAY

NORMAN

MAHNOMEN

CLEAR-WATER

RED LAKE

POLK

PENNINGTON

MARSHALL

BELTRAMI

WASH-INGTON

ST LOUIS

CROW WING

CARLTON

PINE

RICE

OLMSTED

FILLMORE

FARIBAULT

WASECABLUE

EARTH

REDWOOD

TRAVERSE

BECKER

ITASCA

LAKE

COOK

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North RangeGroup

Little FallsFormation

Algoman intrusive rocks

Mille Lacs Group and rocks of thePenokean fold-and-thrust belt

LATEARCHEAN

MIDDLEARCHEAN

ARCHEANSaganagaTonalite

PALEOPROTEROZOIC

PROTEROZOIC

MESOZOIC

PALEOZOIC

MESOPROTEROZOIC

PHANEROZOIC

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Metamorphic and Igneous Rocks

Stratified Rocks

Upper Cretaceous

Upper andMiddleDevonain

Upper andMiddleOrdovician

Lower Ordovician

Upper Cambrian

CRETACEOUS

CAMBRIAN

ORDOVICIAN

DEVONIAN

JURASSIC

�mg �ml

�mbu

�ma �mt

DuluthComplex

LoganIntrusions

Earlygabbros

KeweenawanSupergroup

Middle Ordovician

�eq

�mbg

Kc

Omd

�emt

�eps

Agb

�egv�esa�emq

�enm

�ent

Amg

Jurassic

�ms

�edq

Adg

Subvolcanicmafic intrusions

�mnr

�mdg

CORRELATION OF MAP UNITS