R. 19 W. Prepared and Published with the Support of COUNTY ATLAS SERIES THE CARLTON COUNTY BOARD OF COMMISSIONERS, ATLAS C-19, PART A MINNESOTA GEOLOGICAL SURVEY R. 18 W. THE MINNESOTA DEPARTMENT OF NATURAL RESOURCES, DIVISION OF WATERS, AND Plate 2—Bedrock Geology Harvey Thorleifson, Director St. Louis the Fond du Lac Band of Lake Superior Chippewa CORRELATION OF MAP UNITS 380 420 T. 51 N. Brookston 36 31 35 34 MIDCONTINENT RIFT SYSTEM PENOKEAN OROGEN

400 MAP SYMBOLS 420 ¤2 Keweenawan Supergroup Animikie basin Fold-and-thrust belt 1 6 Inferred geologic contact—Located from aeromagnetic data and limited 2 3 BEDROCK GEOLOGY Hinckley outcrop and drill hole information. 400 mh 440 Sandstone Inferred fault—Interpreted from aeromagnetic data and breaks in Brook River

Martin400

Martin  400  Fond du Lac electromagnetic conductor trends. Lake e dr 400 mf ) Jo 400 Formation ) 440 Inferred thrust fault or structural discontinuity—Sawteeth denote the upper 460 By

mi  Chengwatana plate and indicate dip direction; interpreted from aeromagnetic data.

420 mc Volcanic Group 400 ( MESOPROTEROZOIC ( Carlton dikes Malmo structural discontinuity—An inferred thrust fault; sawteeth denote 400 92°30' 400 420 440 Terrence J. Boerboom Esko oxide Tamarack the upper plate and indicate dip direction; see introduction for details.  400 R. 17 W. gabbro peridotite dn 18 Airborne electromagnetic conductor—Compiled by McSwiggen (1987), STONEY BROOK du Thomson External Internal ARROWHEAD d from unpublished data by Rocky Mountain Energy Company and USX T. 50 N. 2009    Formation zone zone 400 me mt dc (formerly U.S. Steel).

dr 420 unconformity 400 th M Axial surface trace of F2 folds—Inferred from the map pattern of bedrock

Twin to F LOCATION DIAGRAM units; anticline, syncline. Plunge is generally shallow east and west.

er BREVATOR deformed only by second deformation FOND DU LACiv BAND 400 R Lakes unconformity o Strike and dip of inclined bedding—Angle in degrees from horizontal. deformed by first and second deformation

400 r  Lost Simian tt Strike and dip of bedding showing younging direction—Angle in degrees

Lake  from horizontal; inclined, vertical.

pc tm

Lake pg 400 tg Strike and dip of cleavage—Angle in degrees from horizontal; inclined, Ê Brook È 400 vertical. Cleavages dipping less than 60° are generally S , those greater 420 Creek unconformity and possible thrust fault 1 400 36 31 are generally S2. 34 OF LAKE SUPERIOR 420 36  ? 92°45' gs ? S Y 31 mg Trend and plunge of minor fold axis—Angle in degrees from horizontal. T 93°00' Pcscs

N ST. LOUIS COUNTY ST. LOUIS COUNTY R. 20 W. 420 R. 16 W. Y thrust fault R. 21 W. thrust fault Location of recent exploratory boring with short descriptive well log U Hardwood 400 400 O thrust

Lake 400 400 abandonment reports. C Elm PALEOPROTEROZOIC ( 380 360 fault

6 380 N 1 1 S I ( ( Creek ( Location of scientific bore hole—Completed by the Minnesota Geological

t 6 420 . K 1 400 400 6 ( 6 H 6 1 a 1 400 L  T st Simian 6  1 ca I y o me Mille Lacs Survey, with core or cuttings samples available, or water well holes with u

A i Ê s 400 Group cutting sets stored at the Minnesota Geological Survey. CHIPPEWA RESERVATION420 46°45' 400 46°45' Dead 400  400 vg Fish )33 ! Location of exploratory bore hole—Derived mainly by private mineral Lake 400 Creek 400 400 exploration interests. Locations are from Severson and others (2003).

400  (  dr R 380 ga Little 400 400 Cedar iv Samples are stored at the Minnesota Department of Natural Resources, Lake e

400 ag

r Division of Lands and Minerals, Hibbing, Minnesota.

 Malmo structural discontinuity Tamarack 400  River mm 400 dr  Brook 400 g 400 o

)73 Cloquet 380 to Bedrock outcrop—Derived mainly from previous mapping. River ?

Miller 400 57

Cross 420 o Little Falls  Lake 400 360 gg

400 dr Lake Stoney M Formation 420 F 60

400 CLOQUET 58 Midway T. 49 N. 420 420 w 58 Squa 380 M o

400 o T. 49 N. o 400 420 du First F o BESEMAN RED CLOVER PROGRESS PERCH LAKE Lake 70 THOMSON 420 420 o (  Third 28 MF 360 DESCRIPTION OF MAP UNITS

400 Lake foliated, carbonate-altered; metamorphosed under lower greenschist- metagabbro sill or mafic tuff. 400 400 MF MF M M o ( MESOPROTEROZOIC ROCKS M ¤61 facies conditions. Local undeformed lozenges contain 5 to 10 < Metagabbro and metabasalt—Green, medium-grained, massive hornblende 420 420 FMF Esko g 400 Rice 400 380

420 420 Big ( 55 F Keweenawan Supergroup percent euhedral, white, sericite- and carbonate-altered plagioclase Portage ( Creek (o metagabbro to fine-grained amphibolitic mafic schist. Metagabbro

¦§35 400 400 Lake Perch dn 400 F phenocrysts up to 3 centimeters in size, but in most places the gabbro intersected in drill holes near the center of this unit is deeply weathered

r

360 o grained, well-sorted and well-rounded; composed of about 96 percent is strongly shear-foliated and the phenocrysts are broken and strung at the top, and is composed primarily of millimeter-sized blocky 400 400 Y 400 o

400 400 ( (( T out in the foliation plane. The matrix is composed of chlorite, sericite, hornblende and epidote-chlorite altered plagioclase, in a groundmass F F o quartz (Tryhorn and Ojakangas, 1972). The overall distribution is

Lake M N

31 d F U

400 o carbonate, minor quartz, sphene/leucoxene and opaque granules, and

( MMF 340 60 well constrained by well log data and rare exposures near Holyoke. of epidote, sphene, and fibrous actinolitic amphibole. A single 31 400 36 k 36 O 36 31 400 31 36 31  45 dc ( M  o 300

36 e du

 Woodbury 380 F C minor disseminated pyrite that is typically strung out in the foliation Lake e (

dn to 31M F outcrop at the southeast end of this unit is composed of fine-grained,

Lake r F FMM 36 In Pine County to the south, the Hinckley Sandstone contains karst- S

F o Jaskari C 50 400 M ( I

Lake 340 45 plane; locally overprinted by late rhombohedral carbonate, and rare,

o dark greenish-gray, biotite-bearing amphibolitic schist with minor

U

400 F F F like sinkhole features in areas where the sandstone is thinly covered 400 F o 400 o O

400 400 Wild 360 49 Little coarse-grained, muscovite porphyroblasts. The foliation dip varies

400 M F L by sandy glacial till (Shade and others, 2002). disseminated pyrite. The dominant foliation (S ) in this outcrop 1

o

F o

Gill

)210 .

Rice r ( F M o 40

e 6 M T from 25° east at the north end of the outcrop area, to 60° southeast

Tamarack t M River

6 Cromwell 400 1400 Sawyer Lake t ( 40 5 strikes N. 73° E., but the dip varies from 13° north to 35° south, due

380 4 o 1 S < 1 F n Fond du Lac Formation—Sandstone, mudstone, and conglomerate, light F M F R 6 400 Thomso mf O o 400 F o Creek 6 6 6 o 1 F

Wright i 400 Island M

v 1 FF Reservoir F 50 50 10 at the south; this is at a high angle to the regional structural grain, to a gentle D fold that plunges 10° west.

e 400 F 360 orange to reddish-brown and locally mottled pale green. The base is 2 r River o 1

F 220 9 and may be the result of cleavage refraction through this presumably

Lake 400 400 F F 50 < Dolomitic marble and metagraywacke—Dark gray with a tan-weathered o F

)210 380 ( 8 marked by an approximately 60-foot (18-meter) thick conglomerate mm

400 Torchlight C F 320 200 more competent unit. The main, presumably S metamorphic foliation

reek   1 rind, fine-grained, granoblastic-foliated, locally contains minor, o

Lake d MF o d 260 with clasts of vein quartz cemented by pyrite and marcasite. The Tamarack 70 F F o M o

) Y

Cole M 210o

CarltonF is weakly folded by a later D2 crenulation cleavage. Lake 400 240 thin, deformed veins of recrystallized galena. Lenticular structures, Kettle r F o T conglomerate unconformably overlies the Thomson Formation (unit Lake M M o 7 10 e F Little N M o N tt 26 F 280 River

M M FF I < Graphitic slate and metagraywacke—Gray, rhythmically interbedded, 400 Creek St. LouisF 45 10 tg isolated fold hooks, and boudinaged quartz veins indicate a high

oO F M 85 U < ), and is overlain by fine- to medium-grained, arkosic to subarkosic M to F 40 30 F S

360 360 M FF O variably graphitic, with scattered pyrite cubes up to 2.5 centimeters

( 7 N degree of deformation; pseudo-bedding and foliation dip 20° south.

( 240 C sandstone interbedded with siltstone and mudstone. Trough cross- 400 o 5 4 260 r o 360 260 Mattlia e 260 O

) S

t 23 wide concentrated in carbonaceous/graphitic argillite beds. Bedding Lake t beds in sandstone higher in the stratigraphic section contain clasts Thin argillite beds in marble exhibit strong D2 crenulation cleavage

400 A

21 C O 81 260 Creek Eagle Creek 360 240 L

( Silver S and S1 foliation are typically tightly folded and dislocated, and locally and contain disseminated, oxidized pyrite.

I

o o G of weathered basalt and reworked sandstone and mudstone at their 400 Lake 400 M 18 70

Merwin o o U transposed, by strong D crenulation cleavage. Similar to unit < , o 45 W tt T. 48 N. Lake s 360 280 bases (Morey, 1967; Kilburg and Morey, 1977). 2 This unit is very poorly constrained and shown somewhat  O Walli e Little Otter tt

400 o D River 78 45 o but characterized by numerous electromagnetic conductors, a higher Lake ( 41 360 < Basaltic lava flow, undifferentiated—Distribution is inferred from positive schematically; the extent is based on a weak, low, aeromagnetic River Bob Horn o o 340 mc Lake T. 48 N. residual Bouguer gravity background anomaly, and better developed Kettle 380 40 gravity and aeromagnetic anomalies that are continuous with areas to anomaly.

o 14 360 260 300 S fabric.

W Kettle Chub the southwest and northeast, where there are also outcrops of basalt 2

e Lake LITTLE FALLS FORMATION

s LAKEVIEW EAGLE CORONA ATKINSON 70 TWIN LAKES SILVER BROOK Drill Hole BM-2 (T. 47 N., R. 20 W., sec. 23) intersects several t 400 400 M o Lake 280 260 assigned to the Chengwatana Volcanic Group (Boerboom, 2001).

Y È 0.5- to 30-foot (0.2- to 9-meter) intervals of conglomerate that

tm ! du 360 Wrenshall

S o Pelitic beds are locally graphitic and commonly contain small, pink,

! Lake 72 360 Venoah contain clasts of perthitic feldspar, granitic to dioritic rock fragments,

B 400 61 the basalt is likely near the surface.

r 4 Lake

an 400 S 24 260 metagraywacke, chert, and marble. This unusual conglomerate may variably chlorite-altered garnet. Flattened carbonate concretions

c 65 5 o ( 280 h È r 400 È < Interflow sandstone—Inferred from a linear topographic depression that 360

F 57 ( o 5 mi r 59 8 mark the unconformity between the base of the Thomson Formation are common in graywacke beds. Topping indicators in drill cores

 o 61o 9 Y mt 74 S 300 corresponds with a linear aeromagnetic low. 400 82 ( 71 o360 Y o and rocks of the underlying fold-and-thrust belt. The conglomerate indicate bedding is predominantly upright and subhorizontal. Early S1

9 S  o F 10 < Carlton diabase dike, dominantly reversely polarized—Dark gray, fine- 4 400 400 dr S ! Lac d P! ! ! o 360 ! Hay and associated graywacke also contain abundant glassy unit quartz metamorphic foliation is parallel to bedding, both are gently folded and 31 !!a ! ! !! La Belle 2 3 rk ! ! 32 Lake 31 grained, granular to ophitic, locally weakly porphyritic dikes, best 31 Heikkila 400 !  31 36 36 31 36 ! ! tg 36 grains possibly derived from an unknown felsic volcanic source. variably crenulated by D2 folds along vertical to steeply south-dipping 36 31 L 360 31  !a S exposed in the St. Louis River valley near Carlton. Dikes exhibit

th dn ke 36 < Graphitic pyritic slate and metagraywacke with minor chert and axial surfaces that plunge shallowly east and west. To the north the unit

pg 360  ! tm 380 ! pc tm 13 chilled margins and horizontal columnar jointing, range from 2 to ( o

400 A 380 o dolomitic marble—Gray, crenulated, tightly folded, rhythmically is fine-grained micaceous and phyllitic; it becomes progressively more 380

C ( 340 59 feet (1 to 18 meters) thick, strike approximately N. 30° E., and

re ! coarse-grained and garnetiferous to south, and to south of this map in

Creek e o interbedded graphitic or carbonaceous slate, meta-argillite, and fine-

k 59 ¤61 Y ! ) dip near vertical (Reichhoff, 1987). All dikes sampled have reversed

6 1 ) 340 Pine County (Boerboom, 2001) the metamorphic grade increases to the

6 1 6  380  1 6 1 6 Clear 1 6 grained metagraywacke metamorphosed under greenschist-facies Y tm 1 tm ) polarity except two: one small dike exposed in the St. Louis River

 6 ! 340 tt  20 Bear 320 Creek staurolite facies. Regional aeromagnetic anomaly patterns, together

( tg ) 260 conditions. Discontinuous layers of chert and brecciated chert are

380 tm Lake valley, and a 7-foot (2-meter) thick dike exposed along the Soo Line with lithological and metamorphic attributes, imply that this unit is 320 ) Flodin generally thin but locally as much as 200 feet (61 meters) thick, and K er trail at Moose Lake (Chandler and Lively, 2003). These are not shown )73 i iv Lake correlative with the Little Falls Formation in east-central Minnesota. n R ( rare beds of dolomitic marble are up to 20 feet (6 meters) thick. g Mahtowa   360 as separate map units. tm tm 340 ! Mud Drill hole MG-3, located at the western end of this unit adjacent

( W ) dn The chert locally contains thin beds of pyrite and pyrrhotite, and e Hizer Dikes are of tholeiitic basalt composition (Fig. 2) and contain 41 Blackhoof

400 s ! the marble contains scattered pyrite cubes, which are also abundant to unit < , intersected a 130-foot (40-meter) thick interval of sheared t Lake Ellstrom vg

380 ) Lake to 50 percent plagioclase, 15 to 35 percent granular to ophitic augite, ( 340 in metagraywacke and slate below the chert and dolomite horizons. metabasalt flows with rubbly flow tops and minor graphitic argillite Kettle ! 360 340

)  1o ! 360 ( D 4 to 11 percent magnetite, 2 to 10 percent ilmenite, 0 to 6 percent tg Fo 5 340 Creek 240

( rk S 360 ¦§35 e Outcrops located 0.5 mile (0.8 kilometer) north of Mahtowa contain interflow sediment, beneath metagraywacke/argillite and above ) e Roc ( ! k variably altered olivine, minor amounts of felsic mesostasis, and ! rn ! r Creek < River 2 380 o 340 Creek abundant pyrite cubes, as well as rare, retrograde-metamorphosed, graphitic-pyritic argillite; this may be a fault slice of unit vg. ! 380 2 H Munson k  e Mud accessory apatite, biotite, and hornblende (Reichhoff, 1987). tg tm Creek Lake e

380 r T. 47 N. pseudomorphic, rotated garnets like those observed in drill cores in

T. 47 N. 400 ) Y ! ) 380 ! C Several other irregular, 4- to 39-inch (10- to 100-centimeter) ! ! ! ( unit < . The extent of this unit is inferred from linear to discontinuous, )  ! tt REFERENCES ose R tm ! 340 o 380 M iv ! thick dikes too small to show at this map scale were noted in drill

er BLACKHOOF Creek east–west-trending, positive, aeromagnetic anomalies.

AUTOMBA KALEVALA SKELTON!! ! MAHTOWA Blackhoof WRENSHALL280 Allen, D.J., 1994, An integrated geophysical investigation of the Midcontinent Rift

360 Lake Creek cores in south-central, southwestern, and northeastern Carlton County. 360 k < ( Chert—Light gray to black to locally orange-stained (due to oxidized

d ) ea ) Crystal oc 260 pc

D gs ) e  Benfield Sandy Spring R These dikes differ in that they contain abundant fresh to talc-altered System: Western Lake Superior, Minnesota and Wisconsin: Purdue, Ind., Purdue

os dn Lake Lake pyrite), fine-grained, weakly bedded. Composed of very fine-

) o Lake Lake 300

) ) 380 M 360 olivine phenocrysts, and commonly abundant augite phenocrysts, in University, Ph.D. dissertation, 267 p.

È ) ! River 220 grained saccharoidal quartz, rare small pyrite cubes, and minor

) !  ) )

400 mg ( 260 a very fine-grained matrix. Preliminary chemical analyses (Fig. 2, Boerboom, T.J., 2001, Bedrock geologic map and sections, pl. 2 of Boerboom, T.J.,

! 340 muscovite; black chert contains abundant, submicroscopic, dusty,

) ! Nemadji

45 ga

) small ultramafic dikes) show that these dikes are more primitive in project manager, Geologic atlas of Pine County, Minnesota: Minnesota Geological

 Black Creek ) 360 ho opaque inclusions that are not identifiable by optical methods. ) mf of 260 ) ( 360 Fork composition than dikes from the Carlton swarm (unit < ), but not as Survey County Atlas C-13, pt. A, 7 pls., scale 1:100,000.

Lake dr d McSwiggen (1987) identified thin intervals of dark gray chert in drill

M ( 240 primitive as the Tamarack peridotite (unit < ). ) o ( Twentynine S 220 mt Boerboom, T.J., Runkel, A.C., and Chandler, V.W., 2002, Bedrock geology of Pine

 o 360 k core MLCH-11 as being phosphatic. Shown as a separate map unit

Si dn se er ! unk Creek

lve iv North River < Diabase dike, reversely polarized (Cloquet dike)—Gray, fine- to medium- County, Minnesota, chapter 1 of Boerboom, T.J., project manager, Contributions

r ( R 36 dc only where mapped in outcrop.

) ( orn 240 31 ) 36 31 H 31 36 31 o 36

36 31 31 31 36 grained, subophitic to ophitic, pigeonite-, oxide-, and altered olivine- to the geology of Pine County, Minnesota: Minnesota Geological Survey Report

vg o Nemadji MILLE LACS GROUP

cs È of Investigations 60, p. 1-20. 400 Fork bearing, reversely polarized. Informally known as the "Cloquet dike," ) 25

2o 6 360 360 for exposures along the St. Louis River in the town of Cloquet, it is < Graphitic schist, phyllite, and slate—Finely interbedded. Poorly Boerboom, T.J., Severson, M.J., and Southwick, D.L., 1999, Bedrock geology, pl. 1  ( ! dc 15 gs

Barnum 260 C 380 th 46°30' ) r differentiated from the main Carlton swarm because it is unusually constrained, projected from the west where intersected by scattered of Bedrock geology of the Mille Lacs 30 x 60 minute quadrangle, east-central ! ee u 46°30' 6 k 1 6 51 o 360 6 6 1 6 260 1 6 S 1 6 thick (approximately 230 feet [70 meters]; Manzer, 1978) and can drill holes in which rocks display an S crenulation cleavage Kettle 1  1 2 Minnesota: Minnesota Geological Survey Miscellaneous Map M-100, 2 pls., (! ! gg 340 360

! Bear Creek 280 be readily traced for approximately 36 miles (58 kilometers) by a

Lake Hunters Spring superimposed on a layer-parallel S1 schistosity; distribution is based scale 1:100,000. ( River

! C prominent, linear, negative, aeromagnetic anomaly. This dike is also on the extension of the subdued aeromagnetic anomaly pattern. ok 360 ree Chandler, V.W., and Lively, R.S., 2003, Rock properties database: Minnesota 380 Kettle

! ro k

B intersected in the entire length of a 500-foot (152-meter) deep, vertical < Metagabbro—Green, medium-grained, massive. A single outcrop located Geological Survey, .

( River 340 340 Creek mg

ca Brook ! È ! Creek exploratory borehole located approximately 13 miles (21 kilometers)

! ! 300 in T. 47 N., R. 21 W., sec. 19, contains 2- to 4-millimeter hornblende Clark, R.C., 1985, The structural geology of the Thomson Formation: Cloquet and ! ! (

! (! Hanging River

! ! 360 River southwest of Cloquet. The dike interior contains an estimated 50 clots in saussuritized groundmass; overall unit is composed of ( È 35 Esko quadrangles, east-central Minnesota: Duluth, Minn., University of Minnesota

È! Horn ar le 72 o !! percent fresh, felty, lath-shaped plagioclase; 35 to 40 percent fresh, o plagioclase heavily replaced by clinozoisite and minor chlorite, Lake C t

mm 360 ! ek o Duluth, M.S. Thesis, 114 p.

Nemadji e e

55 ( Cr subophitic to ophitic augite; 5 to 10 percent pigeonite; 3 percent N

F actinolite or hornblende after clinopyroxene, sphene/leucoxene after

20 ( 360 

 Silver mh Heaman, L.M., and Easton, R.M., 2005, Proterozoic history of the Lake Nipigon area,

72 ga Creek olivine that is mostly altered to iddingsite and/or bowlingite; 3 percent

! R Nemadji 15 primary blocky to skeletal ilmenomagnetite, and minor interstitial

 Gillespie ! 320 Ontario: Constraints from U-Pb zircon and baddeleyite dating: Institute on Lake

dn i S ¤61 ! v S y )23

! e È n opaque Fe-Ti oxide minerals; and traces of quartz and apatite.

r to quartz with small apatite needles. Not foliated, and relict primary o T. 46 N.

9 ( 360 S Superior Geology, 51st Annual Meeting, Nipigon, Ontario, pt. 1, Program and È

T. 46 N. g o MOOSE LAKE

23 < Diabase dike inferred from geophysical anomalies—This unit is mostly igneous texture is well preserved despite extensive deuteric and/or

o e

! 20 HOLYOKE n du Abstracts, p. 24-25.

10 360 !  ! Li

( 23 d

SPLIT ROCK SILVER ! Moose BARNUM CLEAR CREEK Creek schematic, drawn to signify the presence of dikes for which the hydrothermal alteration.

400 ( k Holm, D.K., Anderson, R., Boerboom, T.J., Canon, W.F., Chandler, V.W., Jirsa, M.A., Miller,

È r 320

o 15

)27 ( ! e 340 o

l polarity, thickness, and locations are poorly constrained.

( ! È Net 340 t

! ( ! ! F Graham Extent is poorly constrained; the outcrop is located over a

!  ! Lake t

73 i J., Schneider, D.A., Schulz, K.J., and Van Schmus, W.R., 2007a, Reinterpretation of È ( È ! ( 10 ag ) )27 !!

 ! Lake L (

!! ( (

ca !! o < Diabase dike, normally polarized—Inferred from discrete, isolated, negative Bouguer gravity anomaly, implying either that it is part

! R 45 ! È ( 21 dn Paleoproterozoic accretionary boundaries of the north-central United States based on a ( o

35 ck 22 340 340

360( Moosehead 340 linear, positive, aeromagnetic anomalies; dashed where inferred to

È 340 of a mafic sill emplaced into, or a thin skin of mafic rocks thrust-

River ( new aeromagnetic-geologic compilation: Precambrian Research, v. 157, p. 71-79. 40 ( 20 Lake Y

380 te

( 360 r 35 ta T extend beneath the Fond du Lac Formation (unit

o e § ( ¦

S N o v Holm, D.K., Schneider, D.A., Rose, S., Mancuso, C., McKenzie, M., Foland, K.A., and

40 i Anderson ( ! Split ( 340 U

Y R in outcrops or intersected in drill cores. aeromagnetic data indicate that this unit is continuous with similar North

T O

o 

25 o rn mc Hodges, K.V., 2007b, Proterozoic metamorphism and cooling in the southern Lake

N 10 S o C 20 S < rocks to the west that have been mapped on the basis of scattered ( H Moose 380 dr Diabase dike, reversely polarized—Inferred from discrete, isolated,

U ( ! Soper Superior region, North America and its bearing on crustal evolution: Precambrian o 360 ! Coffee 340Lake S O 35 320 340

Echo Lake A linear, negative, aeromagnetic anomalies; dashed where inferred drill hole data.

Lake 340 360 C L Research, v. 157, nos. 1-4, p. 106-126.

! Lake 32 È Moose 31 G < N 31 36 to extend beneath the Fond du Lac Formation (unit mf). The first < Graphitic schist, silicate-facies iron-formation, chert, and metagabbro—A

I 36 36 31 o 36 cs o 31 31 320 36  U Holst, T.B., 1982, Evidence for multiple deformations during the Penokean Orogeny K 36 19 340 340 È A' 31 31 mi 360 340 Net Lake O dike northwest of and parallel to the Cloquet dike (unit < ) has an single drill core in T. 46 N., R. 21 W., sec. 6., intersected variably T Sand dc

I 30 360 14 Lake 400 D in the Middle Precambrian Thomson Formation, Minnesota: Canadian Journal A 25 aeromagnetic signature identical to the Cloquet dike, and may be of foliated metagabbroic rock, silicate-facies iron-formation, and of Earth Sciences, v. 19, p. 2043-2047. PINE COUNTY R. 21 W. R. 17 W. PINE COUNTY similar size and composition. graphitic-sulfidic argillite capped by a 6-inch (15-centimeter) thick R. 20 W. R. 19 W. R. 18 W. R. 16 W. ———1984, Evidence for nappe development during the Proterozoic Penokean 93°00' 92°45' 92°30' GIS compilation by R.S. Lively < Esko oxide gabbro—Dark gray to grayish-black, dominantly medium- bed of white pyritic chert. Metagabbroic rocks, now biotite-carbonate- me orogeny, Minnesota: Geology, v. 12, p. 135-138. Digital base modified from the Minnesota Department of SCALE 1:100 000 Edited by Lori Robinson grained, felty-prismatic oxide gabbro delineated by a strong, negative, chlorite-quartzofeldspathic schist, contain relict feldspar phenocrysts Transportation BaseMap data; digital base annotation by 1 0 1 2 3 4 5 MILES circular, aeromagnetic anomaly. Based on a ground magnetic survey, where least deformed; abundant biotite indicates possible potassic Ikola, R.J., 1967, A geophysical investigation of the geologic structure of Carlton the Minnesota Geological Survey. County, Minnesota: Minneapolis, University of Minnesota, M.S. thesis, 80 p. 1 0 1 2 3 4 5 6 7 8 KILOMETERS 95° 94° 93° Ikola (1967) inferred that this unit forms a gently southeast-plunging alteration. Iron-formation is composed dominantly of coarse-grained Elevation contours were derived from the U.S. Geological Kilburg, J.A., and Morey, G.B., 1977, Reconnaissance geologic map of Esko quadrangle, Survey 30-meter Digital Elevation Model (DEM) by the body approximately 4,265 feet (1,300 meters) long and 1,969 feet granoblastic quartz, with lesser garnet, grunerite, stilpnomelane, CARLTON Minnesota Geological Survey. biotite, chlorite, magnetite, and apatite. St. Louis and Carlton Counties, Minnesota: Minnesota Geological Survey COUNTY (600 meters) wide. contour interval 20 METERS Animikie basin Miscellaneous Map M-25, scale 1:24,000. Universal Transverse Mercator Projection, grid zone 15 Samples of well cuttings from different locations have a uniform Unit was mapped on the basis of continuity with moderate- 1983 North American Datum Animikie texture and mineralogy, implying that the intrusion is neither zoned amplitude, positive, linear, magnetic anomalies that extend from Manzer, G.K., 1978, Petrology and geochemistry of Precambrian mafic dikes, basin Minnesota, and their bearing on the secular chemical variations in Precambrian One cleavage nor has cumulate layering. Generally contains approximately 20 to 30 Mille Lacs Lake to western Carlton County. A drill core obtained outlier Northern domain basaltic magmas: Houston, Rice University, Ph.D. dissertation, 230 p. INTRODUCTION Two conglomeratic horizons may be present: one near the base of the southern domain and the other near the base 47° Two cleavages percent fresh, commonly skeletal plagioclase, 20 to 30 percent fresh, from the western end of this anomaly belt outside of Carlton County of the northern domain. A conglomerate intersected by a deep drill hole near the base of the twice-deformed southern Southern domain prismatic, purplish-green augite commonly cored by plagioclase, 15 (Boerboom and others, 1999) intersected rock types generally similar McSwiggen, P.L., 1987, Geology and geophysics of the Denham-Mahtowa area, east- Carlton County is located near the southern edge of the Canadian shield, and is underlain entirely by Precambrian domain may signify an unconformity between the older Mille Lacs Group and the base of the Thomson Formation. Archean bedrock to 20 percent total skeletal magnetite and platy ilmenite, and 10 to to those described above. central Minnesota: Minnesota Geological Survey Miscellaneous Map M-63, bedrock that ranges from Paleoproterozoic (approximately 2,200 to 1,800 million years ago, or Ma) to Mesoproterozoic 45 percent fine-grained groundmass of devitrified glass filled with < Porphyritic metabasalt, metadiabase, and metasedimentary rock— scale 1:48,000. A second conglomerate, located near the base of the once-deformed Thomson Formation, is based on a report by nuity vg (approximately 1,100 Ma) in age. External zone fold-and-thrustconti belt (includes Mille l dis fibrous-matted plagioclase and augite, magnetite, and trace amounts of Massive to pillowed, grayish-green, variably porphyritic metabasalt Mooney, H.M., Craddock, C., Farnham, P.R., Johnson, S.H., and Volz, G., 1970, Winchell (1899), which described a conglomerate with pebbles of older graywacke, slate, and black chert, in the tura Most of the bedrock is covered by glacial sediment, but bedrock outcrops are abundant in the St. Louis River valley Lacs Group) truc vicinity of what was then called Otter Creek station. Attempts to find this outcrop have been futile, but it is proposed o s bornite and chalcopyrite. Geochemical analyses (Fig. 2) show that the and interbedded fragmental volcanic rocks and mafic tuff composed Refraction seismic investigation of the northern midcontinent gravity high: Journal Malm ARCHEAN east of Cloquet, and are locally abundant from Atkinson to Moose Lake, and in the southwestern part of the county. that the southern domain represents an earlier proto-Animikie depositional basin marked by a basal conglomerate, McGrath Gneiss Esko oxide gabbro is similar in composition to the Carlton dikes. of actinolite, chlorite, epidote, sphene/leucoxene, carbonate, and rare of Geophysical Research, v. 75, no. 26, p. 5056-5086. The electromagnetic anomalies shown on this map are from McSwiggen (1987), who compiled them from data Lake < biotite; plagioclase phenocrysts are strongly altered to chlorite and which was deformed and uplifted in D1 to provide sediment to the main, northern domain of the Animikie basin, the mt Tamarack peridotite—Lithology is based on Minnesota Geological Morey, G.B., 1967, Stratigraphy and petrology of the type Fond du Lac Formation, Duluth, collected by mineral exploration companies. Most drill cores and cuttings from past drilling are stored at the Minnesota Mille base of which is marked by the second conglomerate. Figure 1. Simplified geologic map of Lacs DOUGLAS FAULT Survey test drilling just west of Carlton County, which intersected epidote. Mafic hypabyssal intrusions are medium-grained, variably Minnesota: Minnesota Geological Survey Report of Investigations 7, 35 p. Department of Natural Resources, Division of Lands and Minerals, Hibbing, Minnesota. About 80 of these cores and schistose; they locally contain possible pyroxene phenocrysts replaced Fold-and-thrust belt—Mille Lacs Group and Little Falls Formation east-central Minnesota showing the major Animikie an ultramafic intrusive rock (wherlite; Southwick and others, 1986), Morey, G.B., and Ojakangas, R.W., 1970, Sedimentology of the Middle Precambrian cuttings were examined for this project; information from the others was gleaned from Severson and others (2003). structural elements of the Mesoproterozoic 46°30' basin fold-and-thrust belt and on abbreviated descriptive well logs from mineral exploration by actinolite. Other rock types intersected at depth in deep drill holes Thomson Formation, east-central Minnesota: Minnesota Geological Survey In areas between drill holes and bedrock outcrops, the bedrock geology is inferred from geophysical data. The bedrock in southwestern Carlton County is composed dominantly of metamorphosed mafic volcanic and Midcontinent Rift System, Paleoproterozoic outlier Mesoproterozoic activity focused mainly to the west of Carlton County, where this include variably graphitic metagraywacke and argillite, silicate-facies Report of Investigations 13, 32 p. Prior to construction of this map, the most recent bedrock geologic map that included Carlton County was a sedimentary rocks, including dolomitic marble and possible carbonate-silicate facies iron-formation, all interlayered on rocks of the Penokean orogen, and Archean Sedimentary rocks body is demarcated by an irregular, linear, aeromagnetic anomaly iron-formation, and dolomitic marble; the latter ranges up to 1,500 Volcanic rocks Morey, G.B., and Van Schmus, W.R., 1988, Correlation of Precambrian rocks of the regional compilation by Southwick and others (1988). The western edge of Carlton County was included on regional various scales, that contain the same structural attributes as those in the southern domain of the Thomson Formation. that shows both normal and reverse polarity. In Carlton County the feet (457 meters) in thickness (Sellner and others, 1985; Severson Superior Province. Modified from Paleoproterozoic Lake Superior region, United States: U.S. Geological Survey Professional Paper compilation maps by Boerboom and others (1999), and McSwiggen (1987) included the southwestern portion of the Bedding and S foliation in the fold-and-thrust belt are generally near horizontal, which hinders correlation of Internal(includes zone Little Falls Formation) and others, 2003; Southwick and others, 2005). The metasedimentary 1 Southwick and others (1988) and Boerboom Animikie basin associated aeromagnetic anomaly broadens out into a more subdued, 1241-F, p. 31 p. county as part of a larger-scale regional compilation map. rock types using geophysical anomalies. Interpretation of bedrock composition based on aeromagnetic data is also units are restricted to deeper levels beneath the mafic rocks. All rock and others (1999). Animikie Group circular, positive anomaly with a rim of higher magnetic expression. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and complicated by numerous linear anomalies inferred to be created by Mesoproterozoic diabase dikes, which mask (Thomson Formation and equivalents) types are metamorphosed under lower greenschist facies conditions. Test drilling in this outer rim has intersected 100 to 300 feet (30 to related mafic intrusions, northeastern Minnesota: Geochronological insights to GEOLOGIC SETTING AND HISTORY subtle features in the surrounding bedrock. Fold-and-thrust belt Overall this unit is similar to metagabbro in drill cores within unit Major 91 meters) of variably thermally metamorphosed Thomson Formation physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated External zone thrust fault < . PALEOPROTEROZOIC BEDROCK The Malmo structural discontinuity is thought to be a thrust fault that trends eastward through Carlton County (unit

O + K 1 cleavage. Graywacke beds locally contain small pyrite cubes, and dark gray sulfidic-graphitic argillite and fine-grained metagraywacke of Investigations 60, p. 55-72. A common method of referring to successive generations of deformation and metamorphism in rocks is to graben-bounding normal faults were reactivated into reverse faults that uplifted deeply-buried lava flows in the central 2

Na zoned carbonate concretions that are generally 6 to 10 inches (15 with minor, thin, chert beds and local, thin, pyrrhotite beds that Southwick, D.L., Meyer, G.N., and Mills, S.J., 1986, Scientific core drilling in assign numbers to each successive event. For example, the first generation of deformation is referred to as D , the axis of the rift approximately 2.5 miles (4 kilometers) relative to the crust on either side (Allen, 1994), juxtaposing 1 0 0 to 25 centimeters) but as much as 3 feet (1 meter) in length and are central Minnesota: Summary of lithological and geochemical results: Minnesota 20 30 40 50 60 70 80 commonly contain muscovite porphyroblasts. Thin intervals of associated metamorphic fabric (schistosity) as S , and associated folds as F ; and the second generation as D , S , them against the rift-flanking sandstone units, which are estimated to be up to 1.9 miles (3 kilometers) thick (Mooney 30 40 50 60 Na2O + K2O MgO 1 1 2 2 SiO2 flattened in the S cleavage plane. quartz-carbonate-stilpnomelane schist or dark green garnet-bearing Geological Survey Information Circular 23, 186 p. and F , respectively. Using this terminology, D refers to deformation associated with the Penokean Orogeny, and D and others, 1970; Allen, 1994). The uplifted central block of volcanic rocks is known as the St. Croix horst, and the Mg# (atomic) 2 2 1 2 For a more thorough lithologic description of the Thomson schist indicate local iron-formation components, and beds of marble, Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map (scale refers to deformation during the Yavapai Orogeny. main reverse fault that bounds the west edge of the horst is known as the Douglas Fault (Fig. 1). Refer to Boerboom Figure 2. Comparative geochemical plots of Mesoproterozoic mafic intrusive rocks. Carlton dikes are from exposures in the St. Louis River valley (Reichhoff, 1987); Esko oxide Formation refer to Morey and Ojakangas (1970) and Wright and up to 91 feet (28 meters) thick, may project to the bedrock surface as and others (2002) for a more complete explanation of the Midcontinent Rift System development. gabbro results are from unpublished analyses provided by the Minerals Processing Corporation; small ultramafic dikes are from preliminary analyses on file at the Minnesota Geological 1:250,000) of the Penokean orogen, central and eastern Minnesota, and accompanying Thomson Formation—Animikie Group others (1970), and for a more thorough description of the structural map unit < . Both bedding and parallel S foliation are crenulated text: Minnesota Geological Survey Report of Investigations 37, 25 p. Diabase dikes adjacent to the rift are both normally and reversely polarized. The reversely polarized dikes Survey (provided by Jeff Thole, Macalester College); Tamarack peridotite/wherlite analyses were published by the Minnesota Geological Survey (Southwick and others, 1986). mm 1 aspects refer to Holst (1982, 1984) and Clark (1985). by D deformation. The Thomson Formation, composed dominantly of slate and graywacke, is here extended farther south than were likely emplaced prior to a magnetic reversal that occurred at about 1,098 Ma (Paces and Miller, 1993), and the 2 Southwick, D.L., Morey, G.B., Christopher, J.M., McSwiggen, P.L., and Boerboom, < Hornfels—Fine-grained, spotted biotite hornfels produced by thermal The eastern segment of this unit is composed of fine-grained, depicted on earlier maps (for example Southwick and others, 1988; Boerboom and others, 1999), and includes an emplacement of normally polarized dikes likely post-dates these. Some of these dikes can be mapped confidently by th T.J., 2005, Preliminary description and interpretation of the "Hattenberger" deep Malmo structural discontinuity metamorphism of the Thomson Formation (units < and < ) due to crenulated, variably graphitic phyllite and metagraywacke that is east–west belt marked by linear to discontinuous aeromagnetic anomalies that reflect stratigraphic intervals of elevated linear aeromagnetic anomalies, but other anomalies normally associated with diabase dikes, such as those that cross to tt test well, Carlton County, Minnesota: Minnesota Geological Survey Report levels of pyrite and pyrrhotite. These may be analogous to thin iron-formations in the lower part of the Michigamme emplacement of the nearby Tamarack peridotite (unit