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Plate 2, Bedrock Geology

Plate 2, Bedrock Geology

Prepared and Published with the Support of COUNTY ATLAS SERIES THE CROW WING COUNTY BOARD OF COMMISSIONERS AND BEDROCK GEOLOGY ATLAS C-16, PART A GEOLOGICAL SURVEY THE MINNESOTA DEPARTMENT OF NATURAL RESOURCES, DIVISION OF WATERS Plate 2—Bedrock Geology Harvey Thorleifson, Director

R. 29 W. By 94°15' R. 28 W. 94°00' R. 26 W. R. 25 W. CASS COUNTY Lizzie R. 27 W. Lake Jail Island 405 Lake Lake 435 5 5 Allen 405 6 Goose 1 6 1 Wolf 1 Y Terrence J. Boerboom and V.W. Chandler 40 6 40 Lake Little Pine  1 Lake Papoose 1405 d 435 Lake 6 Lake 390 Lake 390 Stewart Ku 405 Lake 5 390 390 40 Roosevelt Clough Ku 435 2004 Lake 0 Lake 390 42 405

5 d 405 435 AITKIN COUNT Agr Mitchell 390 Squaw 40 420 0 Lake Lake ei 39  5 d Blue Ku CORRELATION OF MAP UNITS Kego Lake 420 Lake  Anna 6 MAP SYMBOLS mg Lake 390 Ku pq 390 CRETACEOUS MESOZOIC GAIL LAKE TIMOTHY 5 405 Inferred contact—Mainly located from aeromagnetic data, except where locally T. 138 N. FIFTY LAKES Ku LITTLE PINE unconfor mity Penokean orogen 40 EMILY T. 138 N. controlled by information from closely-spaced drill holes. d 405 Duc390k Eagle ei Lake  20 20 Inferred —Mainly interpreted from aeromagnetic data but locally based on d Lake Ruth West 30  33 Lake vt 46°45' drill-hole data. Fold-and-thrust belt 46°45' Ami Fox Butterfield Lake 390 Lake Inferred or structural discontinuity—Sawteeth denote the upper plate. 405 Buchite Lows 390 Interpreted to have mainly north-directed thrust displacement; position is interpreted Ami Lake Ku INTERNAL ZONE 0 Lake mainly from aeromagnetic data. p 39 Lake Mary 405 Brook d  Sand 390 Anticlinal fold axis—Crestline is shown, plunge is unknown. ANIMIKIE gd 390 Lake 390 BASIN hm 31 tl Anticlinal fold axis—Crestline and plunge are shown. Ams 390 36 Trout Lake 36 390 Lake 36 31 70 31 Emily Synclinal fold axis—Troughline is shown, plunge is unknown.  405 Big Trout 36 31 vt Ag Pickerel 36 vs Grass Dahler Bass Lake Daggett Lake 390 70 Lake Lake 390 Lake 390 Synclinal fold axis—Troughline and plunge are shown. Ami 0  390 39 d Ox 5  Ku Ami 375 40 tl Island ei 6 Lake Lake Overturned anticlinal fold axis—Troughline is shown, plunge is unknown. 390  390 1 6 1 1 ei 6 1 6 1 375 Little Overturned synclinal fold axis—Troughline is shown, plunge is unknown. pq 390 405 Pine Ag 375 MEDIAL ZONE 375 Lake tl 405 390 Linear trace of -formation—Inferred from aeromagnetic data. Locally constrained Upper vt 375 by drill-hole data. EXTERNAL ZONE gm Whitefish 420 unconformity Lake Lower Goodrich Lake 5 405 Whitefish Rush 40 Island 390 Linear positive aeromagnetic anomaly—Probably indicative of iron-formation. Lake Lake  Lake rl rv 375 vs 375 Weak, linear, positive aeromagnetic anomaly. 375 375 ROSS LAKE ti  375 420 ei 90 Bedding—Strike and dip are shown: inclined; vertical; ball indicates topping direction 0 80 75 375 375 39 where known. Measured from drill core or taken from Schmidt (1963). T. 137 N. CROSS LAKE Dolney um Lake FAIRFIELD T. 137 N.  375 im Lower Ku Overturned bedding—Strike and dip are shown; this example shows bedding whose Ams 60 JENKINS IDEAL 375 ei  Hay Lake Cross Roger Ross younging direction is north but that dips south. Data taken from Schmidt (1963) lm unconfor mity Agm Lake 375 Lake Lake  Bertha 375 375 d Star Lak375e 390 Inclined metamorphic foliation—Strike and dip are shown.  Lake Velvet Mud 40 unconformity gv Lake f  Lake ? vm 375 0 Test hole—Shown only outside of mined areas; mainly drilled in the early 1900s. 375 375 Ku 39 f  du 375 Clear   fv d 375 Big Bird tl vs sa Lake Kimble Duck Lake Outline of former open-pit iron mine. Lake Lake pq Ku 375 5 375 37 Stark Jenkins 375 390  Upper Greer Bass Upper ei Lake Location of historic underground iron mine shaft—Locations in the Cuyuna North  d Ami 390 375 gs Hay Lake Lake 36 Lake Ku Dean  range were taken from Schmidt (1963); locations in the Cuyuna South range  Eastham Lake vt mg 36 Pine 31 375 36 31 31 Lake Lake 36 were taken from historic maps. 405 31 36 Ossawinnamakee 375 375 Adney 5 Lake 37  Lake ei tl 371  375 mg 375 er 1 1 6 tl 6 Riv Google 1 6 1 94°30' 94°00' 93°30' 93°00' 92°30' ? 1 6 46°45' 390 d 390 375 Lake 390 10 miles 375 Nelson An 420 390 Perry imiki 375 River  Lake e Ams Pine Pine Lake ei Crow Wing q Little 390 undivided bs County un ba rocks conformity sin  375 20 T. 136 N. 405 d Pequot Amv Dean 375 Lakes Lake Agu 375  375 Foreland cs  375 vt d d Breezy DEAN LAKE basin elt Sibley Horseshoe 46°30' lt 46°30' Point Lakes MISSION 390 ict) d-thrust be Lake PERRY LAKE  d-thrust b ne fold-an  SIBLEY lm Mille Lacs Medial Zo ownship d T. 136 N. 390 belt (Moose Lake–Glen T 375 ne fold-an Panel) ity 375  er lt ntinu ei e, Cuyuna distr e Structural disco Riv b ural t ruct Pelican  is o st pq vs Deer h lm Ami 1 c Ma Amv Lake External Zo na district) s d 375 Lake c Long (North Rang iti 375 ph Fool Prairie ra G McGrath Gneiss Thrust fault Agu Lake basin (Archean) 375 6 46°15' Medial Zone fold-and-thrust Concealed thrust fault 375 PELICAN y 375  (South range Cuyu it Agr ei i m r 360 375 o PENOKEAN OROGEN NEOARCHEAN  Lake n f d o issipp c Foreland basin Mille n Cullen 375 lm u Miss Lacs Animikie basin (Virginia and Thomson Ams Lakes Bass 31 Indian Jack Rocks of the MidcontinentFormations and equivalents) Lake Lake 375  375 Fold-and-thrust belt 375 36 36 31 Mission im d 93°00' Ag 375 Agm Lougee Internal Zone fold-and-thrust belt External zone (Cuyuna North range) Twin 30 37531 Lake 375  94°30' 94°00' 93°30' Lakes vt 36 ti T. 47 N. Medial zone (Mille Lacs Group) 375 Figure 1. Simplified geologic map of east-central Minnesota showing the major structural 90 Internal zone (granitoid plutons) Amv 50 0 RABBIT LAKE Lake  Island 0 60 elements of Paleoproterozoic rocks of the Penokean orogen, and areas of Archean bedrock Edna d Lakes WOLFORD Lake ei 375 6 60 (white). Modified from Southwick and others (1998, 2001). 6 Agu 1 Rabbit 0 1 Coles 36 360  360 0 rl Perch Lake 57 60 Lake ti Lake ti  50 55 60 ti Carlson 90 vs Carbonaceous sedimentary rocks—Carbonaceous argillite interbedded with 90 55 90 90 Lake INTRODUCTION MILLE LACS GROUP AND RELATED ROCKS Nisswa im im 375 360 thin beds of , hematitic argillite, and fine-grained, thin-bedded, green Johnson 375 80 Cuyuna The bedrock beneath Crow Wing County includes a diverse assemblage of rock types bs Metabasalt and allied rocks—Inferred from meager drill hole data to the east of this 37 Nisswa Clark ti 5 Lake Lake Lake ti carbonate-silicate iron-formation. The unit becomes less carbonaceous and Roy Garden um 70 80 Casey that are mainly Precambrian (Archean to Paleoproterozoic) in age, along with scattered map area to consist primarily of variably amygdaloidal to massive metabasalt. Turner 375   pinches out toward the north end of the Emily district. Mapping is based on Lake Lake 31 90 rv 50 sa Lake Rice Lake  80 Lake 390 outliers of much younger Mesozoic sedimentary rocks of Cretaceous age. The county Metamorphosed under greenschist-facies conditions. Their extension into Crow Black rv 85  7031 drill core data. Lake ti 36 rl Edward Silver 36 Bear 70 375 straddles the northwestern border of the Paleoproterozoic Penokean orogen (Southwick Wing County is based on poorly constrained geophysical evidence. Lake 405 36 ei Iron-formation of the Emily district—Thick- to thin-bedded, granule-bearing cherty Lake 60  Trommald ti and others, 1988) and westward-adjacent Archean rocks. There are no known bedrock q Dam Lake quartzite—Quartzite and sericitic schist. Best known to the east of LAKE EDWARDS 360 375  375 iron-formation interbedded with thin-bedded hematitic iron-formation; locally CENTER 360 65 du outcrops in the county, but concentrated exploratory drilling, coupled with past mining NISSWA underlain by thin-bedded carbonate-facies iron-formation in the southern part of this map area, where it consists of massive to thick-bedded quartzite that typically 46°30' rv Agate  46°30' 375 f 1 activity, provide a good understanding of the bedrock in localized areas. Outside of the 75 35 Lake lacks metamorphic foliation, and micaceous, medium- to thick-bedded quartzose im 1 6 1 6 Cedar the Emily district. In places, black, fine-grained, -poor, carbonaceous 55 ti 70 210 Cuyuna and Emily iron ranges (see Plate 6, Mineral Endowment), the bulk of the county 75  65 390 Black 375 Lake rocks that are typically schistose. Unit includes an unknown proportion of T. 135 N. ti rv du argillite and are interbedded with hematitic iron-formation. Portions of the 371 Bass 65 Crosby Lake has only scattered drill hole control. Consequently, construction of this map relied heavily pelitic schist, and green, quartz-rich schist that contains a substantial component 375 Lake 20 360 390  iron-formation are anomalously rich in manganese (Morey and others, 1991). Lake ti 70 90 65 f on geophysical interpretations and extrapolation of known geology from surrounding areas of mafic volcanic rock fragments. Variable amounts of secondary Hubert rv 45 pq Fawn 375 60 into the county. Pokegama Quartzite—Thin- to thick-bedded, gray, greenish-gray, or reddish-gray, 375 rl 60 375 produce a moderately strong positive aeromagnetic anomaly. In Crow Wing Lake 65 rv 75 quartz-rich to arkosic. Interbedded with a substantial proportion of light gray,  70 Serpent Lake County, this unit is intersected by 6 drill holes that describe it as pink, red, or ti 375  Crow Wing County contains the Cuyuna mining district—a formerly important source Gladstone  70 ti Deerwood variably phyllitic argillite, siltstone, and . The base of the Pokegama Lake ti rl Ironton of iron and manganese ore in which production started by underground mining methods green schist. Gull 40 Quartzite is marked by a moderate positive magnetic anomaly that may be a ti and later changed to open-pit methods. The Cuyuna was discovered in 1904,  Lake du cs Phyllitic schist and carbonaceous garnet-biotite schist—Dominantly gray to red, Reno reflection of heavy mineral concentrations or of minor iron enrichment. Severson 360 rv the first ore was shipped in 1911, and the last open-pit mine closed in 1984, during which fine-grained, and variably ferruginous phyllitic schist; interbedded with gray to 36 Spider Blackhoof Lake  and others (2003) reported disseminated magnetite in quartzite from a drill core 360 375 vm time over 106 million tons of were shipped (Skillings Minnesota Mining Directory, black, carbonaceous, and locally sulfidic garnet-biotite schist (meta-argillite). Lake ti Lake Rice DEERWOOD near the base of the Pokegama Quartzite. 31 ti Lake 2001). The has been informally subdivided into two separate entities—the Thin disrupted chert beds are locally present in argillite. Small garnets are 36 31 IRONDALE390 T. 46 N. Round 390 Cuyuna North range and the Cuyuna South range—based upon contrasting lithological and rv Riverton f NORTH RANGE GROUP variably pseudomorphed by chlorite. May be equivalent to the graphitic schist Lake Portage  du f 390 structural attributes of the iron-formation and host rocks. and slate (unit gs). 375 Lak70e Rabbit Lake Formation ti Olander  pq 6 Lake The portion of this geologic bedrock map that covers the Cuyuna mining district was mg Quartz-rich metasedimentary rocks—Dominantly thick-bedded, gray quartzite and 6  rl Slate, siltstone, and very fine-grained graywacke—Gray to very dark gray, variably 1 390 du constructed with reliance on a series of detailed (scale 1:7,200) geologic maps by Schmidt thin-bedded, red to green mudstone (Severson and others, 2003). Inferred from 70 Hamlet phyllitic; unit includes irregularly distributed tabular beds of carbonaceous slate, 1 Lake (1963). He studied open-pit and underground mines and noted the structural and lithologic aeromagnetic and drilling data to unconformably overlie rocks of Archean age; um as well as thin beds of tuffaceous and carbonate- and sulfide-facies 360 210 attributes of the ore and wallrock. Subsequently, Morey and Morey (1986) compiled and separated from the Pokegama Quartzite on the basis of aeromagnetic data. 360 65 iron-formation, chiefly in the lower part. North 25 375 Lookout 0 390 modified Schmidt's individual maps into one map that covered the entire Cuyuna iron range.  375  Lake 39  tl Trout Lake Formation—Gray to tan, fine-grained to locally granular, cherty dolostone. Long du 85 Consequently, the areas in Crow Wing County where mining or intense exploratory drilling rv Interlayered mafic volcanic and volcaniclastic rocks and hypabyssal intrusions—  Mapped from meager drilling data. Lake rl 390 405 390 Located mainly near the base of the Rabbit Lake Formation. Described by 390 have occurred closely follow the maps by Schmidt (1963) and Morey and Morey (1986). 390 390  Shirt f 390 Lake f In other areas, the bedrock geology has been variably reinterpreted, based largely on the Schmidt (1963) as grayish-green, schistose, chloritized flows and/or sills, and NEOARCHEAN ROCKS 36 fragmental to pelletoidal tuff beds, all largely weathered to gray, brown, or 31 Wolf 36 31 390 availability of high-resolution aeromagnetic geophysical data, which were unavailable to 36 Long f Ami Mafic to intermediate intrusions—Geophysically defined bodies inferred to be small, 375 Lake 390 390 6 Arbor earlier workers. Historical records of drilling were compiled from various sources and red saprolite. He described the larger units south of the Trommald Formation 375 Lake 390 Lake  plug-like gabbroic to dioritic intrusions emplaced into Archean supracrustal 375 used as much as possible in the map compilation; however, the lithologic descriptions in (unit ti) as sill-like bodies that cut across the of the surrounding f Loon    rocks; defined by small, sharp, positive aeromagnetic anomalies. Drilling over Horseshoe Lake f du gv Goose rocks. Many old drill logs record a lithology of green schist, much of which Lake 390 these old drilling reports are typically vague and their use is limited largely to identifying T. 134 N. Lake sa 405 a geophysically similar but larger positive magnetic anomaly some 15 miles 6 1 may be sheared basaltic rock, but is not shown as such on this map because of 6  1 the presence of iron-formation. 1 du ambiguous rock descriptions. (23 kilometers) to the west intersected appinitic hornblende diorite (Southwick Wise du GEOLOGIC HISTORY and others, 1986). 375 ti Trommald Formation—The upper part of the unit is composed of thick-bedded, Lake  du Eagle Agu Granite—Geophysically defined bodies inferred to be felsic plutonic rocks emplaced du 65 Bay Lake Sedimentary rocks of Cretaceous age were deposited roughly 65 to 100 million years ago cherty iron-formation that includes chert beds as thick as 7 feet (2 meters). The 360 Hoppy f Lake 375  into Archean supracrustal rocks. Geophysically characterized by a negative du Lake 80 390 Clearwater over a broad area of Minnesota that extended as far north as the western end of the Mesabi lower part is thin-bedded, unoxidized, oxide-silicate iron-formation; moderately   Ku gravity anomaly and featureless aeromagnetic signature. 360 fv f 390 BAY LAKE Iron Range. Subsequent erosion has removed most of these strata, leaving only scattered deformed and weakly metamorphosed; oxidized zones of porous hematite contain Mud Gilbert Lake variable proportions of chert in beds as thick as 1 foot (30 centimeters). Agr Granitoid rocks, undivided—Inferred from regional aeromagnetic patterns to be Lake Lake OAK LAWN 80 NOKAY LAKE outliers, which in Crow Wing County have been identified locally by drill hole data. The Ku 60 f continuous with well-exposed portions of the Giants Range batholith to the 65 8 390 Cretaceous rocks were deposited unconformably on top of weathered Precambrian bedrock. Mahnomen Formation—The subdivision of the Mahnomen Formation into upper   northeast, which consists dominantly of granite, monzodiorite, and tonalite. sa f du Twin 390 Sloan (1964) suggested that the Cretaceous strata in Crow Wing County are related to the and lower members follows the terminology of Southwick and others (1988). 371 390 375 375 du Island 360 65 Coleraine Formation, which was exposed by mining operations along the western Mesabi  Ams Metasedimentary rocks, undivided—Inferred from scant drill core in and west of Red Sand Rice 85 Mud Lake Heron um Upper member—Gray to light-gray siltstone, argillite, and fine-grained quartz-rich um Lake Lake Lake Nokay this map area to consist of chlorite ± biotite ± garnet-quartz-plagioclase schist, 36 31 Lake Ku Birch T. 45 N. Iron Range, northeast of Crow Wing County. Bolin's (1956) study of Cretaceous strata ; unit includes irregularly distributed beds and lenses of iron-formation  Lake du 390 Lake du 40 390 from a drill core in Crow Wing County described gray , sandy shale, and sand with some of which exhibits relict bedding (Southwick and others, 1986). Typified 360 390 390 and allied anomalously iron-rich sedimentary rocks. The base of the unit is 375 microfauna of Foraminifera and fish teeth, scales, and bone fragments. Based on this fossil by a featureless aeromagnetic pattern and by a variably prominent negative 390 arbitrarily defined as the top of the lowest mapped iron-formation (unit im).  f gravity anomaly. f Maple assemblage, Bolin interpreted the Cretaceous strata to be of Cenomanian age.   Lake im Iron-formation member—Thin-bedded, variably green, oxide-silicate-carbonate 1 6 um Whipple Brainerd 405 Most of Crow Wing County is underlain by Precambrian bedrock that varies in age from  Ag Hypabyssal metagabbroic intrusions—Dark green, medium-grained closely Lakes Partridge iron-formation; lithologically similar to the Trommald Formation (unit ti), and du Hanks Lake allied with metavolcanic rocks of unit Agm. Metamorphosed under greenschist White BAXTER f Lake Archean (approximately 2,700 million years of age, or Ma) to Paleoproterozoic (approximately to smaller lenses of iron-formation within units um and lm. Rocks contain 390 Turtle Sand 18 390 Portage Wilson 2,200 to 1,800 Ma). The Archean rocks that underlie the northwest portion of the county to lower amphibolite facies; unit is magnetic and -bearing. Recognized Lake Lake strong metamorphic cleavage. Contains rocks formerly assigned to the Trommald Lake 360 375 Lake in a single drill core at the eastern end of the northern map unit; the extent is f 18 are poorly understood due to lack of drill hole or outcrop data. However, this part of the Formation and the Emily member of the Rabbit Lake Formation (Morey, 1978). du 390 county is thought to be similar to typical Archean "greenstone-granite" terranes that are inferred by moderate, positive aeromagnetic anomalies. er 390 lm Lower member—Predominantly siltstone, fine- to medium-grained quartzose well exposed in northeastern Minnesota and can be extended into the county by the use Agm Metabasaltic and allied hypabyssal metagabbroic intrusions, undivided— Riv 390  graywacke, and argillite; minor quartz arenite and irregularly distributed layers Perch f Grave Rice 390 q Lake f Lake 390 Lake 36 of geophysical imagery. In Crow Wing County, these include greenstone belts made up Characterized by positive linear aeromagnetic anomalies located over a positive 210 Y 36 31 36 31 36 390 and lenses of anomalously iron-rich sedimentary rocks. The top of the unit IT 31 Round predominantly of metamorphosed mafic volcanic rocks, but include metamorphosed gravity anomaly. Recognized in a single drill core at the eastern end of the Baxter U 390 Scott Lake is arbitrarily placed at the lower contact of the lowest mapped iron-formation 360 IN 375 T Lake recognized in drill core and iron-formations inferred from linear aeromagnetic data. The  unit that intersected fine-grained, pale green schist of basaltic protolith. Other N (unit im). O 390 greenstone belts are separated by belts of metamorphosed sedimentary rocks. C linear magnetic anomalies within this unit may be the product of interbedded IS 375 Kenney T. 133 N. D Lake Paleoproterozoic rocks in Crow Wing County are part of the Penokean orogen, a term that MISCELLANEOUS ROCK UNITS iron-formations or discontinuous hypabyssal mafic sills within mafic volcanic L 6 390 A 1 6 gv 1 1 6 1 refers to a belt of deformed rocks that can be traced discontinuously east from central Minnesota rocks. R fv p Mafic to ultramafic intrusions—Small plugs mainly composed of ultramafic to 360 TU f 375 C to the Grenville Front in eastern Canada. Rocks of the Penokean orogen were deformed to Amv Metabasaltic rocks, undivided—Extrapolated from two drill holes located Island U ippi mafic rocks that include serpentinized peridotite, pyroxenite, hornblendite, and R Miller varying degrees between roughly 1,900 and 1,760 Ma, during a protracted episode of deformation approximately 5 miles (8 kilometers) west of the county, which intersect basaltic Lake T  gabbro. Well known from drilling to the south of this map area, recognized by MississS gs Lake Borden gm E f referred to as the (Southwick and others, 1988). In east-central Minnesota, rocks metamorphosed under greenschist-facies conditions (Southwick and others, Bass K Upper South 375 Lake small, sharp, positive aeromagnetic anomalies. Lake LA Russell Long Lake the Penokean orogen is divided into two subterranes—the fold-and-thrust belt, and the younger  1986). Geophysically characterized by linear aeromagnetic anomalies that T 25 390 du Metadiabase—Medium-grained, grayish-green, variably porphyritic; chiefly forms N  Lake bs 18 foreland basin that contains the Animikie Group (Fig. 1). may be the product of small, linear iron-formations, and by a positive gravity E f sills and irregular sheet-like intrusions. Metamorphosed under lower greenschist- 360 P 375 anomaly. ER 390 The geologic evolution of the Penokean orogen fold-and-thrust belt involved deposition of S Garrison facies conditions. Sills lack geophysical definition; unit is mapped only where 360 CROW WING Mille sedimentary and volcanic rocks within a poorly understood basin that formed as a result of extension, known from drilling intercepts and is likely more extensive than shown. Drill  LONG LAKE MAPLE GROVE 405 Lacs sa 375 GARRISON 390 thinning, and sagging of the preexisting Archean crust. Faults that would have developed during hole CW-1 (inclined 45° south), located in SW1/2SE1/4NW1/4 sec. 10, T. REFERENCES CASS COUNTY 371 p 390 405  Lake this extensional phase may have provided conduits for magmas to work their way to the surface 390 q T. 44 N. 46 N., R. 28 W., intersected 511 feet (156 meters) of moderately sheared to Boerboom, T.J., Severson, M.J., and Southwick, D.L., 1999, Bedrock geology, pl. 1 of Bedrock geology  South to be extruded as volcanic rocks or emplaced as hypabyssal (shallow-level) mafic intrusions. um f 405 390 phyllonitic, fine- to medium-grained metagabbro that contains skeletal ilmenite of the Mille Lacs Lake 30 x 60 minute quadrangle, east-central Minnesota: Minnesota Geological 375 Long These faults also would likely have contributed to a complex basin topography, producing local Lake 169 grains replaced by leucoxene. Local highly sheared intervals within the gabbro Survey Miscellaneous Map M-100, 2 pls., scale 1:100,000. f sub-basins within the larger basin, and may have provided pathways for hydrothermal fluid 405 contain textures indicative of fragmental mafic volcanic rocks, and beneath the Jack Pine Bolin, E.J., 1956, Upper Cretaceous Foraminifera, Ostracoda, and Radiolaria from Minnesota: Journal 390 (heated ground water) circulation. gabbro the drill hole intersected interbedded mafic fragmental volcanic rocks, Lake 390 of Paleontology, v. 30, no. 2, p. 278-298. T. 44 N. 45 After extensional basin development, the rocks underwent a compressional phase of siliceous graywacke, chert, thin to thick intervals of tightly folded iron-formation, 390 Carlson, K.E., 1985, A combined analysis of gravity and magnetic anomalies in east-central Minnesota: 405  deformation, the Penokean orogeny, which can also be thought of as basin closure. This orogenic argillite, and graphitic argillite. q Minneapolis, Minn., University of Minnesota, M.S. thesis, 138 p. p 405 phase induced folding, thrust faulting, and of the basin-filling rocks, leading to  f cs Smith Lake gv Metasedimentary and metavolcanic rocks—Graphitic schist and slate, and lean 390 that part of the orogen now referred to as the fold-and-thrust belt (Fig. 1). A younger, secondary Holm, D.K., Van Schmus, W.R., Boerboom, T.J., Schweitzer, D., and Schneider, D., in press, U-Pb f Holt iron-formation; includes an unknown proportion of mafic to intermediate igneous AITKIN COUNTY zircon geochronology of Paleoproterozoic plutons from the northern mid-continent, U.S.A.: Long 405 basin (the foreland basin, Fig. 1) formed north of and on top of the partially eroded fold-and- rocks similar to unit vm. 40 Lake Lake thrust belt. Sedimentary rocks of the Animikie Group that were deposited into this foreland basin Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis: 5 390 fv Felsic volcanic rocks—Fine-grained, dark-colored, amygdaloidal. Described by Van 360 31 31 36 31 36 31 46°15' unconformably overlie Archean rocks to the north and Paleoproterozoic rocks of the fold-and- Geological Society of America Bulletin. 46°15' 36 36 390 390 Hise and Leith (1911) as horizontally layered, 15 to 25 foot (5 to 8 meter) thick  thrust belt to the south. The Animikie Group includes the Biwabik Iron Formation on the Mesabi Holst, T.B., 1982, Evidence for multiple deformation during the Penokean orogeny in the Middle f 36 layers of amygdaloidal extrusive rocks that unconformably overlie steeply dipping Iron Range and iron-formation of the Emily district in the Cuyuna range. Precambrian Thomson Formation, Minnesota: Canadian Journal of Earth Sciences, v. 19, p. 30 slate and iron-formation; occurs in sec. 6, T. 44 N., R. 30 W. and sec. 7, T. 45 N., R. 405 2043-2047. Crow Rocks of the fold-and-thrust belt contain early recumbent (overturned) folds, refolded 29 W. Age is uncertain, possibly as young as Mesoproterozoic (Keweenawan). Wing 1 390 6 1 6 1 6 1 6 Camp 1 by west-trending, upright folds. In contrast, only the later upright folds affected strata Morey, G.B., 1978, Lower and Middle Precambrian stratigraphic nomenclature for east-central Lake Lake f Iron-formation—Oxide-, silicate-, and carbonate-facies iron-formation interbedded with  375 along the southern margin of the overlying Animikie Group. Thus, rocks of the fold- Minnesota: Minnesota Geological Survey Report of Investigations 21, 52 p. f 360 375    375 metavolcanic and graphitic rocks of units vm, gv, and sa. Shown only where 375 and-thrust belt have undergone two periods of deformation (Holst, 1982), whereas rocks COUNTY p 5 known from exploratory drilling and inferred from magnetic anomaly patterns. Morey, G.B., and Morey, D.D., 1986, Distribution of iron-formations in the main Cuyuna range, east- River 40 of the younger, overlying Animikie basin were only affected by the second deformation.   central Minnesota: Minnesota Geological Survey Miscellaneous Map M-60, scale 1:48,000. du 390 vm Metavolcanic rocks—Mafic to intermediate flows and volcaniclastic rocks; Mud 375 Aeromagnetic maps and a study of aeromagnetic and gravity data (Carlson, 1985) show ISON   du Lake f du metamorphosed under greenschist-facies conditions. These rocks are shown only Morey, G.B., and Southwick, D.L., 1995, Allostratigraphic relationships of Early iron- 390 that the anomaly patterns characteristic of the Cuyuna North range continue to the northeast Nokasippi formations in the region: Economic Geology, v. 90, no. 7, p. 1983-1993. Round beneath rocks of the Animikie Group (Fig. 1). where intersected by drilling and are probably more abundant than portrayed on MORR Lake 375  the map. A drill hole located approximately 5 miles (8 kilometers) southwest of Morey, G.B., Southwick, D.L., and Schottler, S.P., 1991, Manganiferous zones in Early Proterozoic FORT RIPLEY 360 hm For a complete synopsis of the geologic evolution of east-central Minnesota, refer to  ROOSEVELT the southwest corner of Crow Wing County, within the projection of map unit gv, iron-formation in the Emily district, Cuyuna iron range, east-central Minnesota: Minnesota du ST MATHIAS DAGGETT BROOK PLATTE LAKE 390 0 Erskine390 Whitefish Southwick and others (1988), Southwick and Morey (1991), Morey and Southwick, (1995),  39 Twentytwo Lake also intersected metamorphosed under greenschist-facies conditions. Geological Survey Report of Investigations 39, 42 p. f 375 gs Lake and Southwick and others (2001). p Lake sa Metasedimentary rocks—Slate, argillite, and metasiltstone; includes a substantial Schmidt, R.G., 1963, Geology and ore deposits of the Cuyuna North range, Minnesota: U.S. Geological T. 43 N. Brook T. 43 N. but unknown quantity of mafic hypabyssal intrusions (unit du) and fragmental Survey Professional Paper 407, 96 p., 11 pls. f ett DESCRIPTION OF MAP UNITS NTINUITY mafic volcanic rocks; may correlate in part with the Mahnomen Formation. Severson, M.J., Zanko, L.M., Hauck, S.A., and Oreskovich, J.A., 2003, Geology and sedex potential of 360 Dagg Mud gv Bass 390 MESOZOIC ROCKS Metamorphosed under lower greenschist-facies conditions. hm Lake Early Proterozoic rocks, east-central Minnesota: Natural Resources Research Institute Technical Lake  390 405 gs Graphitic schist and slate—Graphite-bearing schist, phyllite, and slate interbedded Report NRRI/TR-2003/35, 160 p. RAL DISCO Ku Cretaceous rocks, undivided—Includes poorly lithified, commonly pyritic on a fine scale. Rocks typically display a crenulation cleavage superimposed Skillings Minnesota Mining Directory, 2001, Skillings Minnesota mining directory: Duluth, Minn., Mud 390 390 , white sandstone, black lignite, gray to brown shale, and white 360 Lake UCTU on a layer-parallel schistosity. 360 390 Bulldog kaolinitic shale. The unit was recognized from drill hole logs and is probably Westmorelandflint Publishing, 184 p. Lake  O STR Rock more widespread than shown. The thickness of strata varies from 20 to greater gd Granodiorite—Inferred from geophysical data to form the core of a north-trending Sloan, R.E., 1964, The Cretaceous system in Minnesota: Minnesota Geological Survey Report of 390 Lake structural dome; rimmed by gneissic rocks of the Hillman tonalite (unit hm). Sebie MALM 390 than 200 feet (6 to 61 meters). Investigations 5, 64 p. Platte Lake 360 p hm Hillman tonalite—Medium- to coarse-grained, irregularly layered, light- to dark-gray Lake 405 COUNTY LACS MILLE Southwick, D.L., Boerboom, T.J., and McSwiggen, P.L., 2001, Geochemical characterization of 375 PALEOPROTEROZOIC ROCKS  tonalite with local gneissic banding; known from outcrops located 6 miles (10 Paleoproterozoic volcanic and hypabyssal igneous rocks, east-central Minnesota: Minnesota 36 31 36 31 gd 36 375 36 31 375 390 31 kilometers) south of eastern Crow Wing County and extrapolated into the county 345 375 36 Long Hannah ANIMIKIE GROUP Geological Survey Report of Investigations 57, 33 p. Lake Lake on the basis of aeromagnetic data. Where exposed, the unit contains enclaves  Southwick, D.L., and Day, W.C., 1983, Geology and petrology of Proterozoic mafic dikes, north-central 94°15' R. 28 W. MORRISON COUNTY vt Virginia and Thomson Formations—Interbedded, medium- to dark-gray argillite, of biotite-garnet-cordierite schist and hornblende schist, which are angular in R. 32 W. R. 31 W. R. 30 W. 94°00' R. 29 W. Minnesota and western Ontario: Canadian Journal of Earth Sciences, v. 20, no. 4, p. 622-638. argillaceous siltstone, and feldspathic to lithic graywacke. Includes thin lenses of shape and contain an internal foliation that is commonly discordant to layering  Digital base modified from the Minnesota Department of iron-formation that grade into the carbonaceous sedimentary rocks (unit vs) near in the surrounding tonalite; both the tonalite and enclaves are commonly cut by Southwick, D.L., Meyer, G.N., and Mills, S.J., 1986, Scientific core drilling in central Minnesota: Transportation BaseMap data; digital base annotation by the unit base in the vicinity of the Emily district (see Plate 6). Metamorphosed thin dikes of weakly deformed granite pegmatite. Geochronologic data (U-Pb Summary of lithologic and geochemical results: Minnesota Geological Survey Information the Minnesota Geological Survey. GIS compilation by R.S. Lively Every reasonable effort has been made to ensure the accuracy of the factual data under lower to middle greenschist-facies conditions. Slaty cleavage typically zircon) yield an age of 1,797 ± 4 Ma (Holm and others, in press). Circular 23, 186 p. Universal Transverse Mercator Projection, grid zone 15 on which this map interpretation is based; however, the Minnesota Geological Edited by Lori Robinson is at a high angle to bedding, implying that the strata are folded, and the fold Southwick, D.L., and Morey, G.B., 1991, Tectonic imbrication and foredeep development in the Penokean 1983 North American Datum gm Granite at Mille Lacs Lake—Chlorite- and biotite-bearing, medium- to coarse- SCALE 1:100 000 Survey does not warrant or guarantee that there are no errors. Users may axes are probably oriented mostly northeast, similar to the map-scale folds in orogen, east-central Minnesota—An interpretation based on regional geophysics and the results wish to verify critical information; sources include both the references listed grained, grayish-maroon granite. Recrystallized, cut by narrow shear bands, 1 0 1 2 3 4 5 MILES the iron-formation of the Emily district. The folds are probably similar in style here and information on file at the offices of the Minnesota Geological Survey and affected by retrograde metamorphism; these features collectively indicate of test-drilling, in Sims, P.K., and Carter, L.M., eds., Contributions to the Precambrian geology in St. Paul. In addition, effort has been made to ensure that the interpretation to those seen in outcrops near Carlton, about 60 miles (97 kilometers) east of of the Lake Superior region: U.S. Geological Survey Bulletin 1904-C, p. 1-7. 1 0 1 2 3 4 5 6 7 8 KILOMETERS that the intrusion was emplaced prior to the end of Penokean deformation. conforms to sound geologic and cartographic principles. No claim is made that this map area, where the strata are folded into upright, moderately tight folds the interpretation shown is rigorously correct, however, and it should not be used Granite yields a U-Pb zircon age of 2,009 ± 7 Ma (Holm and others, in press). Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map (scale 1:250,000) of the accompanied by an axial-planar, subvertical cleavage. to guide engineering-scale decisions without site-specific verification. Boerboom and others (1999) contains a detailed description. Penokean orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Investigations 37, 25 p. d Diabase dikes—Variably magnetic dikes of the Kenora–Kabetogama swarm ©2004 by the State of Minnesota, Department of Natural (Southwick and Day, 1983). The width of the dikes is unknown, but exposed Van Hise, C.R., and Leith, C.K., 1911, The geology of the Lake Superior region: U.S. Geological Resources, and the Regents of the University of Minnesota dikes in northeastern Minnesota that are part of this same swarm range from Survey Monograph 52, 641 p., 49 pls. LOCATION DIAGRAM The University of Minnesota is an equal opportunity educator GEOLOGIC ATLAS OF CROW WING COUNTY, MINNESOTA 150 to 400 feet (50 to 120 meters) in width. Emplaced prior to the deposition and employer of rocks of the Animikie and Mille Lacs Groups.

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