Prepared and Published with the Support of MISCELLANEOUS MAP SERIES THE U.S. GEOLOGICAL SURVEY AS PART OF THE 2007 STATE GEOLOGIC MAPPING PROGRAM ELEMENT (STATEMAP) MAP M-193 MINNESOTA GEOLOGICAL SURVEY Bedrock Geology, Cavity Lake Fire Area Harvey Thorleifson, Director OF THE NATIONAL COOPERATIVE GEOLOGIC MAPPING PROGRAM, AND THE PRECAMBRIAN RESEARCH CENTER, UNIVERSITY OF MINNESOTA DULUTH 91° Some information on this map was formerly published as: R.6 W. R. 5 W. R.5 W. R. 4 W. Jirsa, M.A., and Starns, E.C., 2008, Preliminary bedrock 80

68

80 67 geologic map of the 2006 Cavity Lake fire area, parts of 70 M 84 85 85 55 Ester Lake, Gillis Lake, Munker Island, and Ogishkemuncie 86 80 85 70 Lake 7.5-minute quadrangles, northeastern Minnesota: 85 68 Minnesota Geological Survey Open-File Report 08-5. 85 83 85 Aou 71 58 80 80 80 80 70 Aoc

86 80

Aos 74 F Ast 85 84 74 86 75 81 75 67 BEDROCK Geology of the CAVITY LAKE FIRE AREA, 88 63 Aoc Ast 71 78 72 79 77

78 85 BOUNDARY WATERS CANOE AREA WILDERNESS,

89 85 M 64 88 80 71 86 86 NORTHEASTERN Minnesota 70 80 80 Aos By

70 SAGANAGA TONALITE

81 Mark A. Jirsa Edward C. Starns Mark D. Schmitz T. 66 N. Minnesota Geological Survey ConocoPhillips Alaska Boise State University Aos T. 65 N. T. 66 N. Aos 79 72 72 2017 T. 65 N. Aoc 71 84 83 59 81   d 64 80 58 d 73

78 55 Ast

56 69 53 75 38 87 84 78 70 28 81 72 83 F 85 72 79 81 85 89 89 66 55 72 85 72 INTRODUCTION which lies in fault and unconformable contact with the older rocks (Driese and others, Fault. The unconformity, exposed near Alpine Lake, is marked locally by a 59

85 84 85 80 73 53 67 60 55 2011). Bedding and other structures in the Ogishkemuncie sequence transect the regional thick zone of paleosaprolitic tonalite containing abundant quartz phenocrysts

87 85 89 88 The bedrock geology in this part of the Boundary Waters Canoe Area Wilderness 73 M

52 76 85 east–northeast trend of units that is inferred to have been established during D ; however, in a sericitic matrix (metamorphosed clay-rich saprolite), relict corestones, F 87 86 81 1 84 81 is extremely diverse and unusually well exposed (Fig. 1). Many parts of this area were 55 87 the conglomeratic strata contain low-grade metamorphic minerals and locally a weak rock and clastic dikes composed of the superjacent Ogishkemuncie strata (unit 87 87 mapped to varied levels of detail in the 1930s, and more recently in the 1970s and 1980s 73 63 cleavage assigned to D2, thus constraining deposition to the interval between 2,690 and Aou). See Driese and others (2011) for details. Aoc 87 49 73 (see Index Map), with efforts focused primarily along waterways. A severe windstorm in 78 76 2,680 Ma. The prominent Ogishkemuncie fault is one of a series of northeast-trending, Tonalite—main phase—Light gray to nearly white, massive to trachytoid- 52 68 84 d 1999 blew down trees in much of the region, and a delayed result in 2006 was the Cavity Ast anastomosing fault zones of likely D3 generation that separate discrete structural segments. foliated, medium- to coarse-grained tonalite, having plagioclase in much 59 88 65 Lake forest fire. The fire exposed bedrock and allowed comparatively unencumbered 59 50 A Field relationships imply that some of these faults may represent reactivation of much older 56 86 83 84 G greater abundance than microcline. Large phenocrysts of polycrystalline 62 Aos access to interior parts of the map area, creating a unique and time-sensitive opportunity 87 structures that bounded successor basins into which sediments of the Ogishkemuncie and 54 Aoi quartz are common, and quartz also occurs interstitial to subhedral 80 for mapping. Fieldwork and compilation of prior mapping was conducted in 2007 and Aos 87 81 Jasper Lake sequences were deposited. plagioclase (An ). Small amounts of microcline occur as antiperthitic 55 82 2008, and a preliminary map was produced (Jirsa and Starns, 2008). That map is revised 20-28 80 69 69 84 exsolution in plagioclase, as rims on plagioclase, and as small interstitial Aos 88 D here to incorporate subsequent fieldwork and geochronologic analyses, including one new 68 80 GEOCHRONOLOGY OF NEOARCHEAN ROCKS 87 73 grains. Hornblende is the dominant ferromagnesian mineral, together with Aoc 75 U date acquired by coauthor Schmitz. 74 83 83 minor amounts of augite, biotite, and secondary epidote and chlorite. A 89 The temporal framework for most of the Neoarchean rocks on this map sheet is based 56 78 73 88 The map portrays bedrock that represents crustal evolution spanning the Neoarchean to 80 80 on lithologic and structural similarities with rocks in adjacent areas, with the exception pronounced foliation and mineral lineation—which is inferred to be largely 64 81 53 Mesoproterozoic Eras (Fig. 2), with an emphasis on structural and stratigraphic relationships 82 82 magmatic—is defined by parallel alignment of minerals and rare autoliths. 71 of two high-precision analyses done on samples taken from outcrops (locations A and B 86 75 in the Neoarchean portion. Neoarchean greenstone-granite terrane of the Wawa subprovince 83 on the map). An age of 2,690.83 ± 0.26 Ma for the Saganaga Tonalite (Driese and others, Based on limited geochemical analysis, the tonalite and associated rocks 75 of the Superior Province is represented by a succession of mostly mafic to ultramafic 85 77 89 can be classified as part of the sanukitoid suite (Jirsa and Weiblen, 2007).

Aoc 65 66 2011) was acquired from samples near the western edge of the intrusion (location A, unit

D 89 76 metavolcanic rocks (circa [ca.] 2,700 Ma), unconformably overlain by calc-alkalic volcanic D 79 A U-Pb date of 2,690.83 Ma was acquired by Driese and others (2011) U M 65 Ast 48° 7' 30" and volcaniclastic rocks (ca. 2,690 Ma), and intruded by the Saganaga Tonalite (also ca. Ast). This map presents a newly acquired age for a trachyandesitic to dacitic intrusion 85 Aou from sample location A at Alpine Lake. U Aou 89 62 2,690 Ma). This succession was uplifted and subaerially eroded to provide detritus to one emplaced into volcaniclastic strata of the Jasper Lake sequence. The intrusion and its host 64 Granodioritic border phase—Gray to pink, moderately to strongly Asd 76 or more successor basins. Some of the Neoarchean terrane can be correlated in a general volcaniclastic rocks have nearly identical textural and mineralogic attributes, implying Asb Amv foliated, locally modally layered, medium- to coarse-grained, quartz- 70 way with that along strike and across the U.S.–Canadian border described by Corfu and synvolcanic emplacement. Analyses by coauthor Schmitz of five single zircon grains 78 Ast bearing hornblende diorite to granodiorite, generally without large quartz 48° 7' 30" Stott (1998). On this basis, the successor basin strata are thought to have been deposited from the trachytic, hornblende- and plagioclase-phyric intrusion (location B, unit Aji; phenocrysts. Autoliths of variably porphyritic, dioritic, and quartz dioritic at about 2,684 to 2,682 Ma—before the primary regional deformation and metamorphic sample UTM location NAD83, 649,327m E., 532,9586m N.) yielded slightly discordant 207 206 phases are common and trend parallel to foliation; xenoliths of metabasalt 83 76 Apm U-Pb ratios, with a weighted mean Pb/ Pb date of 2,690.7 ± 0.6 Ma. Details of these 85 86 -1Aoc event at about 2,680 Ma (Boerboom and Zartman, 1993). All of these rocks were cut by 66 analyses appear in digital files accompanying this map and accessible via the Minnesota and metagabbro increase in abundance near the contact with the Paulson Lake 84 -1 69 mafic dikes inferred to be largely of Paleoproterozoic age. The Neoarchean rocks and Aot 64 88 Geological Survey website. The near synchronicity of this date with that from the Saganaga sequence. Apophosial dikes of aplite, granodiorite, and diorite emplaced into 68 64 45 64 70 65 some dikes are unconformably overlain by Paleoproterozoic metasedimentary strata of 58 F M 60 Aou Tonalite, together with apparent lithologic similarities, implies that magmas related to the the adjacent Paulson Lake volcanic host rocks were moderately to tightly 67 75 the Animikie Group (ca. 1,880 to 1,830 Ma), which includes the Gunflint Iron Formation. 76 76 folded and cataclastically deformed, presumably during D deformation. 70 60 88 latter were the sources for volcanism of the Jasper Lake sequence. 2 68 78 The uppermost layers of iron formation are intensely deformed and overlain east of this 65 75 78 89 75 85 map area by thin lenses of ejecta from a meteorite impact that occurred near Sudbury, Asd Granodioritic, tonalitic, aplitic, and pegmatitic dikes—Trends and distribution U 65 50 64 Api Api Asg Note that nearly all the Neoarchean rocks described here are metamorphosed to at 60 Asd 78 Api Ontario, at ca. 1,850 Ma (Jirsa, 2011; Jirsa and others, 2011). Mesoproterozoic rifting vary, but north–northeast strikes are most common; intrusions are more D 48 76 88 Apm 89 83 87 least low greenschist facies; however, the prefix "meta" is omitted from most rock names -1 67 89 86 75 87 50 is manifest in hypabyssal dikes and sills known collectively as the Logan intrusions (ca. abundant in the Paulson Lake sequence adjacent to the Saganaga Tonalite, 79 85 to emphasize protolith. 57 85 which presumably is the source. Most dikes are plagioclase-phyric. 89 85 1,115 Ma), and several intrusive phases of the Duluth Complex (ca. 1,100 Ma) emplaced 61 54 D Ajx 87 55 81 85 76 55 80 89 KNIFE LAKE GROUP—The Knife Lake Group portrayed here is subdivided 82 75 into both Neoarchean and Proterozoic rocks. One of the most notable geologic features in PAULSON LAKE SEQUENCE—Consists of metamorphosed mafic to ultramafic 84 77 75 U 86 Ajx 75 89 this region is the local preservation of four major unconformities; two within Neoarchean into two informally named sequences: the Jasper Lake sequence that volcanic and hypabyssal intrusive rocks, capped by interbedded felsic tuff 45 71 85 Aji 71 Apm represents an earlier alkalic volcanic, volcaniclastic, and plutonic event; 87 Aoc Ajx 89 rocks, and two in and at the base of Paleoproterozoic strata (Fig. 3). and chert, and overlain locally by graywacke and slate. Metamorphic grade 63 78 and the conglomeratic Ogishkemuncie sequence that is the product of 80 -1 77 increases from greenschist to amphibolite toward the Saganaga Tonalite, 89 Apf Api 85 uplift, development of saprolite, erosion, and deposition. The precise 80 0 Aji 88 76 ACKNOWLEDGEMENTS presumably a contact phenomenon. The sequence has many similarities -1 89 85 Asg subdivision of the two sequences is obfuscated by the fact that both 82 Aoc d Ajc to and may be equivalent with other metabasalt-dominated sequences in 72 The authors thank the many students of the Precambrian Research Center (University 89 89 89 Aps 78 85 Asg sequences contain conglomerate and graded sandstone-mudstone strata, the region, including the Newton Lake Formation (Green, 1970) to the 88 88 80 of Minnesota Duluth) field camps, 2007 through 2011, for assisting in detailed mapping of 85 79 78 78 86 and the region is marked by major anastomosing faults that separate southwest and Greenwater Assemblage to the northeast in Ontario. The 80 Api 85 87 Apm 89 80 70 small areas, and the U.S. Forest Service (Duluth office) for granting permits and providing 85 Aos Aji 87 coherent stratigraphic successions into blocks that likely represent different latter has an age of 2,718 to 2,722 Ma (Corfu and Stott, 1998). Aji preliminary (unprocessed) digital air photo imagery acquired just after the forest fire. We 89 84 crustal levels of exposure. For mapping purposes, the two sequences are M 75 89 Aps Graywacke and slate—Typically dark greenish-gray, locally displays 78 82 Api 88 also acknowledge the guidance of Richard W. Ojakangas (University of Minnesota Duluth), separated by the Ogishkemuncie fault and unexposed contacts inferred to Ajx 87 89 cleavage. Api 85 Apm who enlisted graduate students in the 1970s and 1980s to create a series of Master's be unconformable; however, some portions of the two sequences may be Aji 75 76 Aji Apm 79 89 Apf Felsic tuff and chert—Thinly interbedded layers of white weathering, fine- 45 89 80 89 thesis maps, collectively covering a large portion of the Boundary Waters Canoe Area more or less equivalent. This is based primarily on the shared stratigraphic G Apf 85 Apf grained to aphanitic tuff, and black chert. 89 89 Wilderness—that work was incorporated here. Thanks are extended to John W. Goodge progression from coarse-grained, colluvial, alluvial fan, and fluvial deposits, -1 75 89 Aoc Ajx B 80 Ajv (University of Minnesota Duluth) for his map review. Api Mafic to ultramafic intrusions—Primarily metamorphosed sills and sill Ajx 85 to fine-grained, subaqueous turbiditic strata. Furthermore, lithologically 85 88 89 complexes that vary in relict compositions from peridotite and pyroxenite 79 80 32 Ajx identical peperite (magma emplaced into wet sediment) is present locally Apm (some with polysutured jointing and rare spinifex texture), to gabbro and 89 89 DESCRIPTION OF MAP UNITS in both sequences. The most apparent difference is that conglomerates of 89 d  d 80 the Ogishkemuncie sequence contains clasts of all older units including diorite; fine- to coarse-grained, dark gray to black; locally gray to pale Ajc 89 MESOPROTEROZOIC 68 84 the Paulson Lake sequence, Jasper Lake sequence, and Saganaga Tonalite; brownish-gray where serpentinized and weathered. 89 Aji 85 80 Ajx Asd MIDCONTINENT RIFT INTRUSIVE SUPERSUITE however, the Jasper Lake sequence lacks clasts of tonalite. Apm Mafic to ultramafic volcanic rocks—Steeply dipping flows of tholeiitic 87 -1 88 89 Ajx 89 87 Ajv Duluth Complex Ogishkemuncie sequence—Inferred to be largely a fault-bounded successor basin to komatiitic composition (Vervoort, 1987); dark gray to dark greenish- Aou Ajc 89 Apm 89 88 60 gray, with textures that vary from massive to pillowed to autobrecciated; 83 88 M Tuscarora intrusion of the Layered series deposit containing polymictic (mixed-source) conglomerate, sandstone, 89 commonly variolitic. Dike—Poikilitic troctolite and subophitic augite-troctolite; fine- to medium- and siltstone. Conglomeratic strata contain clasts of all older rock 77 65 Mtd 87 87 types—most prominently of the Saganaga Tonalite, basaltic volcanic and Amv METABASALT AND METAGABBRO—Typically dark greenish-gray, medium- 89 grained and homogeneous. Trachytoid texture and weak modal layering occur 74 78 85 83 near and subparallel to vertical contacts. Unit contains small (6 to 16 inches fine- to coarse-grained mafic intrusive rocks of the Paulson Lake sequence, to coarse-grained, locally variolitic, and variably metamorphosed. Unit Ajs 75 80 80 65 [15 to 40 centimeters]) inclusions of poikilitic troctolitic anorthosite, which and hornblende-bearing volcanic rocks of the Jasper Lake sequence. The is lithologically similar to and likely equivalent with the Paulson Lake 85 82 85 83 88 Ajc sequence (units , ). Ajv 87 presumably were derived from unit Mau. Dike is lithologically similar to sequence contains both sorted, well-bedded conglomerate and sandstone; Apm Api 63 75 50 and unsorted, matrix-supported, lens-shaped deposits. The former may 80 89 and may be an offshoot of unit Mtu. 80 89 Apm 88 89 Aos 66 89 represent deposition in streams that carried detritus washed from adjacent 51 83 66 Ajc Mtu Stratigraphically upper troctolite and augite troctolite—Medium- to coarse- REFERENCES 45 80 75 highlands; the latter may be the product of episodic uplift and alluvial 85 80 89 grained, homogeneous. Modal layering is well developed and generally 72 50M 75 (Numbers in parentheses refer to those shown on the Index to Previous Mapping) 56 52 Ajx 89 50 fan deposition that contributed poorly sorted sediment. Pre-lithification 89 70 55 M 80 U D concordant with unit boundaries that dip gently to the south. 45 73 76 52 80 deformation features, including growth faults and dissected intrafolial Ajv 63 Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, Ajs 84 78 Mld Mtm Melatroctolite—Olivine- and oxide-rich troctolite. A sample in the map

60 89 75 folds, together with the presence of outsized clasts, are consistent with

Ajs 71 70 area yielded an age of 1,098.81 ± 0.32 Ma (Hoaglund and others, 2010; S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 85 70 71 85 Ajx

F 81 82 76 steep paleoslopes and syn-depositional faulting. The sequence shares many

88 73 58 87 79 73 age date location C near Gillis Lake). Distribution in the western half of 1,850 Ma Sudbury impact event: Geology, v. 33, p. 193-196.

84 M 89 F 88 45 87 Ajs Mth attributes with the ca. 2,682 Ma Auto Road assemblage in adjacent Ontario F 85 81 Ajs 35 89 75 89 the map is speculative. (1) Birkmeier, R., Boldy, T., Brannon, B., Doucette, R., Jirsa, M.A., and Lee, A., 2011, 75 78  (Corfu and Stott, 1998), an age that is consistent with the observation of 85 85 76 65 75 89 85 Ajc 45 gs 78 Ajs 77 Stratigraphically lower augite troctolite—Heterogeneous, with local troctolitic Geologic mapping of Neoarchean rocks near Ogishkemuncie Lake by students of the 75 45 Mtl D (approximately 2,680 Ma) cleavage in the Ogishkemuncie strata. M 85 83 2 61 62 45 rf and olivine gabbroic compositions. Modal layering is well developed and Precambrian Research Center's 2010 field camp [abs.]: Institute on Lake Superior D Ajs 65 89 85 63 Rock types are interbedded on all scales and thus, map units portray 46 80 70 M 73 65 85 77 Aji 68 60 hornfels basalt inclusions are abundant. Geology, 57th Annual Meeting, Ashland, Wis., Proceedings v. 57, pt. 1, p. 5-6. U 74 73 U the predominant lithotype. As a result, the listing of map units below is 52 Mth 76 78 70 82 85 70 82 Basal heterogeneous zone—Troctolite to biotite-rich olivine gabbro Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of 77 40 Ajs 75 65 75 45 D Mth not strictly geochronologic, as unit distribution varies and commonly one 89 65 89 containing abundant inclusions of subjacent Archean country rock and the central Giants Range batholith, northeastern Minnesota: Canadian Journal of Earth Ajc 50 gs unit may lie within another. Nevertheless, within each fault-bounded block, M 78 44 65 75 53 46 basaltic hornfels. Sciences, v. 30, p. 2510-2522. 86F 76 coarse-grained units appear to be more common in stratigraphically lower Ajs 70 50 Mtl 55 52 Corfu, F., and Stott, G.M., 1998, Shebandowan greenstone belt, western Superior Province: 20 Mth 56 Mh Anorthositic series exposures, and finer-grained units in higher ones. 80 84  45 84 80 Mld gs 65 U-Pb ages, tectonic implications, and correlations: Geological Society of America 45 Mau Anorthosite, troctolitic anorthosite, olivine gabbroic anorthosite, and Felsic intrusion—Contains trachytoid-foliated, fresh hornblende and plagioclase Aoi Bulletin, v. 110, p. 1467-1484. 71 79 85 86 U leucotroctolite, undifferentiated—Medium- to coarse-grained. Magmatic phenocrysts in pink, very fine-grained quartzofeldspathic groundmass; it 80 37 OGISHKEMUNCIE FAULT 31 47 foliation defined by plagioclase is variably well developed. Similar rocks (2) Costello, D.E., 2010, Geology and petrogenesis of the Tuscarora Intrusion of the Mth D Mtm forms a northeast-trending unit at least 100 feet (30 meters) wide; rock 85 80 85 southwest of this map area yielded an age of 1,099.0 ± 0.6 Ma (Paces and is identical with portions of the Jasper Lake sequence; contacts are not Duluth Complex, Gillis Lake 7.5' quadrangle, northeastern Minnesota: University of 84 73 41 38 89 30 Miller, 1993). Minnesota Duluth, M.S. thesis, 122 p., 1 pl., scale 1:24,000. 73 82 85 38 Mtl exposed; outcrops are surrounded by sedimentary strata of units Aou and 72 (3) Duex, T., 1979, Unpublished geologic data, Ester Lake area: On file at the Minnesota 46 Mh KEWEENAWAN SUPERGROUP Aos, implying an intrusive origin; however, a tectonic one cannot be ruled Mtd M 15 North Shore Volcanic Group out. Geological Survey. 43 Mh Sandstone, siltstone, and mudstone—Typically forms graded layers, Driese, S.G., Jirsa, M.A., Ren, M., Sheldon, N.D., Brantley, S.L., Parker, D., and Schmitz, 11 Mh Mh Hornfels inclusion—Fine-grained, metamorphosed inclusions of volcanic Aos 70 M., 2011, Neoarchean paleoweathering of tonalite and metabasalt: Implications for 15 (primarily basaltic) and less commonly sedimentary protolith in the Tuscarora locally interbedded with or cut by channels composed of pebble to cobble D U T. 65 N. reconstructions of 2.69 Ga early terrestrial ecosystems and paleoatmospheric chemistry: 77 D intrusion; derivation from the North Shore Volcanic Group is inferred. conglomerate. Sedimentary structures include local cross-bedding, load- 20 Mh 43 casts, and flame structures. Thin, reddish to purplish, magnetite-rich siltstone Precambrian Research, v. 189, p. 1-17. Doi: 10.1016/j.precamres.2011.04.003. Amv U 34 T. 64 N. LOGAN INTRUSIONS 60 20 to mudstone lenses and layers occur locally (shown by map symbol). (4) Fahrenkrog, B., Totenhagen, M., Jaret, S., Watson, T., and Jirsa, M.A., 2010, Geologic Mtl Mld Diabase—Semi-concordant sills and rare dikes of fine- to medium-grained T. 65 N. 12 63 mapping of Archean and Paleoproterozoic rocks near Bingshick Lake, Boundary Waters 80 diabase marked by thin aphanitic margins and irregular zones containing Aoc Polymictic conglomerate—Unit varies from planar and trough cross-bedded Canoe Area Wilderness, by students of the Precambrian Research Center's 2009 field 40 plagioclase phenocrysts. Similar intrusions near Thunder Bay, Ontario conglomerate containing moderately well sorted, rounded to subrounded T. 64 N. camp [abs.]: Institute on Lake Superior Geology, 56th Annual Meeting, International yielded an age of 1,115 ± 1 Ma (Heaman and others, 2007). fragments inferred to be largely fluvial; to poorly sorted and matrix-supported conglomerate, with abundant outsized clasts, inferred to be alluvial fan Falls, Minn., Proceedings, v. 56, pt. 1, p. 16-17. PALEOPROTEROZOIC deposits. The fan deposits typically have a prevalence of one clast type over (5) Feirn, W.C., 1977, The geology of the early Precambrian rocks of the Jasper Lake G ANIMIKIE GROUP—A shallowly south-dipping sequence of sedimentary strata others, indicating shifting contributions from various localized sources. Clast area, Cook County, northeastern Minnesota: University of Minnesota Duluth, M.S. types include red jasper and black chert, Saganaga Tonalite, metavolcanic thesis, 146 p. 40 C including iron formation, argillite, slate, and graywacke; metamorphosed to 40 and metagabbroic rocks identical to those in the Paulson Lake sequence Amv 86 Mtm amphibole- and pyroxene-bearing hornfels adjacent to the Duluth Complex (6) Flood, T.P., 1981, Geology of the Cypress, Hanson, and South Arm of Knife Lake area, (Floran and Papike, 1978). (units Apm, Apf, Api), and hornblende- and plagioclase-phyric fragments Boundary Waters Canoe Area: Eastern Vermilion District, northeastern Minnesota: of the Jasper Lake sequence (units , ). No fragments of Jasper Lake < Rove Formation—Slate and hornfels derived from fine- to medium-grained Ajv Aji University of Minnesota Duluth, M.S. thesis, 141 p. rf sequence breccia (unit ) or conglomerate (unit ) have been found, graywacke and mudstone; thinly bedded to laminated. Detrital zircons Ajx Ajc Floran, R.J., and Papike, J.J., 1978, Mineralogy and petrology of the Gunflint Iron Formation, implying that sedimentary units of the Jasper Lake sequence may not have Mtm taken from lower parts of the formation in Ontario yielded ages of 1,827 Minnesota-Ontario: Journal of Petrology, v. 19, p. 215-288. 45 Mtl been lithified at the time of Ogishkemuncie deposition. Locally contains Amv ± 8 Ma and 1,836 ± 5 Ma (Addison and others, 2005). Detrital zircons Fralick, P.W., Davis, D.W., and Kissin, S.A., 2002, The age of the Gunflint Formation, 60 gritty sandstone interbeds and matrix. U D higher in the formation yielded ages as young as 1,777 Ma (Heaman and Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash: Mth Sandy polymictic conglomerate and arkosic sandstone—Basal unit resting Mtl others, 2007). Aou Canadian Journal of Earth Sciences, v. 39, p. 1085-1091. Mtu unconformably on paleosaprolite developed in the Paulson Lake and Jasper Mtm < Gunflint Iron Formation and Sudbury Impact Layer—Siliceous grainstone, Green, J.C., 1970, Lower Precambrian rocks of the Gabbro Lake quadrangle, northeastern gs Lake sequences and the Saganaga Tonalite. It contains abundant angular chert, ferruginous slate, and breccia. Metamorphism to amphibolitic and Minnesota: Minnesota Geological Survey Special Publication SP-13, 96 p. to subrounded quartz grains and sericite inferred to be relict grus derived pyroxenitic hornfels and consumption by the superjacent Duluth Complex Mtu from tonalite, and irregularly shaped, weathered fragments of saprolite (7) Gruner, J.W., 1941, Structural geology of the Knife Lake area of northeastern Minnesota: precludes subdivision of the iron formation exposed in this map into upper that appear to have been semi-ductile (perhaps clayey) during deposition. Geological Society of America Bulletin, v. 52, p. 1577-1642. Mtm and lower sequences as portrayed along strike to the east (Jirsa, 2011). Locally contains outsized fragments of tonalite as large as 7 feet (2 meters) Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, C.A., MacDonald, C.A., and Smyk, M., Iron formation rests unconformably on an eroded surface composed of that appear to have "rolled" into place from inferred fault-valley walls. 2007, Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Neoarchean rocks. The contact is marked by a thin, discontinuous layer region, Ontario: Canadian Journal of Earth Sciences, v. 44, p. 1055-1086. of conglomerate in the easternmost exposures. Reworked volcaniclastic Aot Toe Lake conglomerate and breccia—Oligomictic conglomerate composed of Hoaglund, S.A., Miller, J.D., Jr., Crowley, J.L., and Schmitz, M.D., 2010, U-Pb zircon zircons from the upper parts of the formation exposed in Ontario yielded pale-weathered, angular to subrounded fragments of dacitic composition. geochronology of the Duluth Complex and related hypabyssal intrusions: Investigating a U-Pb age of 1,878 ± 1 Ma (Fralick and others, 2002). The uppermost Poorly bedded to massive. Contains subangular quartz grains similar to the emplacement history of a large multiphase intrusive complex related to the 1.1 Ga layers of iron formation in this map area consist of chaotic breccia inferred unit Aou, and may therefore be sourced in part from Saganaga Tonalite or Midcontinent rift [abs.]: Institute on Lake Superior Geology, 56th Annual Meeting, to be the product of impact-induced seismic deformation during the 1,850 similar intrusions. International Falls, Minn., Proceedings, v. 56, pt. 1, p. 25-26. Ma Sudbury meteorite impact event (Jirsa and others, 2011). Jasper Lake sequence—A complex suite of hornblende- and pyroxene-phyric, (8) Jirsa, M.A., 2011, Bedrock geology of the western Gunflint Trail area, northeast Minnesota: UNDIFFERENTIATED MAFIC DIKES dacitic, trachyandesitic, and locally trachybasaltic volcanic flows, pyroclastic U rocks, breccia, debris flows, and largely oligomictic conglomeratic strata. Minnesota Geological Survey Miscellaneous Map M-191, scale 1:24,000. Mtu < Diabasic, dioritic, and lamprophyric dikes—Typically fine-grained mafic D d Nearly all of these strata are cut by intrusions of the same hornblende- or (9) Jirsa, M.A., Cowan, H., Kowalik, J., and Niedermiller, J., 2009, Geologic mapping of rocks of uncertain age that may range from Neoarchean to Mesoproterozoic; pyroxene-phyric composition (unit ), and peperite is present locally— Neoarchean rocks near Paulsen [sic] Lake, Boundary Waters Canoe Area Wilderness, Mau some diabase dikes are similar in composition and trend to Paleoproterozoic Aji implying syn-depositional magmatism. The sequence grades stratigraphically by students of the Precambrian Research Center's 2008 field camp [abs.]: Institute dikes exposed elsewhere in the Superior Province. Lamprophyric dikes upward to the south and west from what appear to be largely subaerial on Lake Superior Geology, 55th Annual Meeting, Ely, Minn., Proceedings, v. 55, pt. are similar to the more mafic components of Neoarchean units and . Ajv Aji volcanic rocks (units , ); into variably-bedded, reworked volcanic 1, p. 48-49. Dike widths vary from a couple inches to 66 feet (several centimeters to 20 Ajv Ajx debris (unit ); to graded, presumably water-lain packets containing meters), and most dip steeply. Short segments on the map represent dikes Ajc Jirsa, M.A., Fralick, P.W., Weiblen, P.W., and Anderson, J.L.B., 2011, Sudbury impact varied proportions of interbedded sandstone, siltstone, and conglomerate exposed in outcrop; longer sets of north- and east-trending dikes shown by layer in the western Lake Superior region, in Miller, J.D., Jr., Hudak, G.J., Wittkop, (unit ). The lack of Saganaga Tonalite clasts in this sequence implies the dashed line are inferred from aeromagnetic maps; short dashes delineate Ajs C., and McLaughlin, P.I., eds., Archean to Anthropocene: Field guides to the geology tonalite was unroofed after deposition of conglomeratic strata. where a dike is concealed by surface water. of the Mid-Continent of North America: Geological Society of America Field Guide Interbedded sandstone, tuffaceous siltstone, and pebble conglomerate— 24, p. 147-169. Doi: 10.1130/2011.0024(08). Mau Ajs Mau NEOARCHEAN Typically graded and moderately well sorted. Coarser-grained sandstone (10) Jirsa, M.A., and Miller, J.D., Jr., 2004, Bedrock geology of the Ely and Basswood Lake layers have a "salt and pepper" appearance from mineralogic content of (U.S. portion) 30' x 60' quadrangles, northeastern Minnesota: Minnesota Geological Mau The Neoarchean rocks in the central Boundary Waters Canoe Area Wilderness are inferred here to comprise a Timiskaming-type extensional basin and its apparent wall- and plagioclase, hornblende, and pyroxene grains. The pyroxene grains are Survey Miscellaneous Map M-148, scale 1:100,000. pseudomorphically altered to chlorite locally. Clast content is primarily Mau floor-rocks. The geologic units in the region were parceled by Gruner (1941) into eight Jirsa, M.A., Southwick, D.L., and Boerboom, T.J., 1992, Structural evolution of Archean structural segments separated by anastomosing shear and fault zones, and this map exposes Jasper Lake sequence volcanic and hypabyssal intrusive rocks; lesser rocks in the western Wawa subprovince, Minnesota: Refolding of pre-cleavage nappes parts of the eastern four of those segments. Although rock types are comparatively pristine amounts of metabasalt occur near the Paulson Lake sequence. during D2 transpression: Canadian Journal of Earth Sciences, v. 29, p. 2146-2155. within each segment, correlation of units from one fault-bounded block to another is Aji Intrusion of dacitic to trachybasaltic composition—Irregular shape and R.6 W. R. 5 W. R.5 W. R. 4 W. Jirsa, M.A., and Starns, E.C., 2008, Preliminary bedrock geologic map of the 2006 Cavity Base from U.S. Geological Survey Ester Lake, 1959; Gillis 91° challenging. Each block was uplifted, down-dropped and tilted differently, which resulted trend; typically contains varied proportions of plagioclase, hornblende, and Lake fire area, parts of Ester Lake, Gillis Lake, Munker Island, and Ogishkemuncie GIS compilation by R.S. Lively Lake, 1959; Munker Island, 1959; and Ogishkemuncie 1°45' in different stratigraphic levels of exposure and repetition of strata locally. This map pyroxene phenocrysts. Intrusions cut volcanic and volcaniclastic units Ajv, Lake 7.5-minute quadrangles, northeastern Minnesota: Minnesota Geological Survey Lake, 1959, 1:24,000 quadrangles. Edited by Lori Robinson SCALE 1:24 000 attempts to "unstrain" the rocks within the segments to reveal stratigraphic variations that Ajx, and Ajc, implying emplacement synchronous with deposition. Also Open-File Report 08-5, scale 1:24,000. Universal Transverse Mercator grid, zone 15 1 0.5 0 1 MILE may reflect fluctuations in basin geometry and progressive erosional dissection of basin intrudes the Saganaga Tonalite locally. Zircons from one intrusion cutting 1983 North American Datum (11) Jirsa, M.A., Starns, E., Costello, D.E., Gal, B., Hoaglund, S.A., and Putz, A.J., 2008, wall rocks during deposition. volcaniclastic strata (unit ) yielded a Pb/Pb age of 2,690.7 ± 0.6 Ma. MINN. Ajx Capstone geologic mapping near Gabimichigami Lake, Boundary Waters Canoe 1 0.5 0 1 KILOMETER Three major periods of Neoarchean deformation are recognized in this region (Jirsa Ajc Volcanic conglomerate—Oligomictic; derived primarily from units Ajv, Ajx, Area Wilderness, by students of the Precambrian Research Center's 2007 field camp

TRUE NORTH and others, 1992). The first (D ) may be equated with the Shebandowanian orogeny in and Aji; some metabasaltic clasts occur near the Paulson Lake sequence, contour interval 20 feet Every reasonable effort has been made to ensure the accuracy of the factual data on which this map 1 [abs.]: Institute on Lake Superior Geology, 54th Annual Meeting, Marquette, Mich., MAGNETIC NORTH MAGNETIC adjacent Ontario (Percival and others, 2006) to the north, which occurred at about 2,695 which presumably is the source; varies from poorly sorted to moderately National geodetic vertical datum of 1929 interpretation is based; however, the Minnesota Geological Survey does not warrant or guarantee that Proceedings, v. 54, pt. 1, p. 40-41. well sorted; fragments typically are angular to subrounded, and outsized there are no errors. Users may wish to verify critical information; sources include both the references Ma. It produced large-scale folds and block-thrust structures, but no penetrative fabrics in APPROXIMATE MEAN Jirsa, M.A., and Weiblen, P.W., 2007, Field trip 6: Geology along the Gunflint Trail: listed here and information on file at the offices of the Minnesota Geological Survey in St. Paul. In addition, early-formed, largely submarine volcanogenic rocks. The second deformation event (D ) at clasts as large as 3 feet (1 meter) are common; contains thin layers and DECLINATION, 2017 2 Institute on Lake Superior Geology, 53rd Annual Meeting, Lutsen, Minn., Proceedings, lenses of pebbly sandstone similar to that of unit . The apparent absence QUADRANGLE effort has been made to ensure that the interpretation conforms to sound geologic and cartographic about 2,680 Ma (Boerboom and Zartman, 1993), occurred during the Minnesotan orogeny Ajs principles. No claim is made that the interpretation shown is rigorously correct, however, and it should v. 53, pt. 2, p. 143-168. LOCATION and produced regional penetrative fabrics, folds, and prograde metamorphism from low of tonalite clasts is notable. not be used to guide engineering-scale decisions without site-specific verification. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related greenschist to low amphibolite facies. A D event produced faults and localized shear zones, Ajx Volcanic breccia—Typically massive and poorly to moderately well sorted; The views and conclusions contained in this document are those of the authors and should not be interpreted 3 mafic intrusions, northeastern Minnesota: Geochronological insights to physical, but no regionally penetrative fabric in this area. It did, however, reorient axes of some D composed of angular to subangular, hornblende-, pyroxene-, and plagioclase- MAP SYMBOLS as necessarily representing the official policies, either expressed or implied, of the U.S. Government. 2 petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga CORRELATION OF MAP UNITS folds (F ) to strikes that nearly parallel the major fault and shear zones. The Neoarchean phyric, dacitic to trachybasaltic fragments; lenses and discontinuous layers This map is submitted for publication with the understanding that the U.S. Government is authorized 2 Midcontinent rift system: Journal of Geophysical Research, v. 98, p. 13997-14013. Geologic contact—Approximately located. Dashed where concealed by surface water. to reproduce and distribute reprints for governmental use. Supported by the U.S. Geological Survey, supracrustal strata and hypabyssal intrusions are divided here into informal sequences of reworked and tuffaceous sediment are present locally; pyroclastic and Intrusive rocks Supracrustal and hypabyssal intrusive rocks Unconformity—Approximately located; typically marked by the metamorphic products of National Cooperative Geologic Mapping Program, under assistance Award No. 07HQAG0126. based on the manifestation of these deformation events, local correlation of units within debris flow origins are inferred. Percival, J.A., Sandborn-Barrie, M., Skulski, T., Stott, G.M., Helmstaedt, H., and White, D.J., 2006, Tectonic evolution of the western Superior Province from NATMAP and MIDCONTINENT RIFT KEWEENAWAN weathering, and fragments of the underlying rock in overlying strata; unconformities each sequence, and the presence of unconformities or angular breaks between sequences. Volcanic flow, flow breccia, and tuff—Dacitic to trachybasaltic; contain Ajv Lithoprobe studies: Canadian Journal of Earth Sciences, v. 43, p. 1085-1117. INTRUSIVE SUPERSUITE SUPERGROUP commonly truncate internal fabrics of the subjacent rock. In apparent chronologic order, these sequences are here named the Paulson Lake (units prominent hornblende, pyroxene, and/or plagioclase phenocrysts in a fine- North Shore Volcanic Group Fault—Approximately located. Faults in Paleoproterozoic and Mesoproterozoic rocks typically prefixed Ap), Jasper Lake (units prefixed Aj), and Ogishkemuncie (units prefixed Ao). The (12) Severson, M.J., 1978, Petrology and sedimentation of Early Precambrian graywackes Duluth Complex grained ground mass that varies in shades of deep green, pink, purple, gray, are dip-slip; letters indicate relative vertical displacement; up (U), down (D). Faults Jasper Lake and Ogishkemuncie sequences are informal subdivisions of the Knife Lake or white; locally autobrecciated. in the eastern Vermilion District, northeastern Minnesota: University of Minnesota Group. Stratigraphic facing in rocks of the Paulson Lake sequence and an unnamed but Duluth, M.S. thesis, 145 p. Mtd shown in the Neoarchean rocks are largely inferred from linear topographic depressions, SAGANAGA TONALITE—Formal name for an intrusion that consists of a large Anorthositic series Mtu locally verified by offsets or shear fabrics in outcrop. They have both dip-slip and strike- lithologically identical metabasalt to the south delineate a major west-plunging syncline, central area of distinctive, quartz-porphyritic hornblende tonalite, and a local (13) Vervoort, J.D., 1987, Petrology and geochemistry of the Archean of the Jap Lake area, likely formed during D deformation. The sequence is unconformably overlain in the core MESOPROTEROZOIC slip displacements, and it is likely that some have been reactivated several times with 1 granodioritic border phase. The intrusive contact between the granodioritic northeastern Minnesota: University of Minnesota Duluth, M.S. thesis, 193 p. Note: Mtm Tuscarora intrusion different senses of offset. The Ogishkemuncie fault is a complex zone of anastomosing of the syncline by strata of the Jasper Lake sequence, which was folded during late D or Jap Lake was renamed post-1987 to Paulson Lake. of the Layered Series 1 border phase and the Paulson Lake sequence is characterized by abundant shear, with pervasive iron-carbonate alteration present locally. Dashed where concealed early phases of D2, and intruded by the Saganaga Tonalite and related intrusions (units (14) Vinje, S.P., 1978, Archean geology of an area between Knife Lake and Kekekabic Mtl country rock inclusions in the granodiorite, and pervasive intrusions of

by surface water. prefixed As) at about 2,690 Ma. Both supracrustal sequences and associated intrusions were Lake, eastern Vermilion district, northeastern Minnesota: University of Minnesota

Mth granodiorite into the Paulson Lake sequence. The western edge of the M Axial surface trace of F fold—Plunge is shown locally; most axes are steeply inclined; uplifted, eroded, and contributed detritus to the conglomeratic Ogishkemuncie sequence, Duluth, M.S. thesis, 176 p. F 1 intrusion is truncated by an irregular unconformity and the Ogishkemuncie Mau Logan intrusions Mh Hornfels anticline, syncline. Axial surface trace of F fold—Inferred plunge is shown by arrow; most axes are steeply Mld 2 F inclined. Inferred from stratigraphic facing and cleavage/bedding relationships; anticline, ANIMIKIE GROUP M syncline. MESOPROTEROZOIC  Rove Formation Strike and dip of bedding—Stratigraphic facing is indicated by bar and ball where determined; rf 92° CAVITY LAKE 91° Duluth Complex inclined, vertical, overturned; stratigraphic facing in areas lacking clear bedding are MAP AREA 91° W. unconformity shown by arrow and "T". 90° Sudbury Impact Layer (not mapped separately) MUNKER ISLAND PALEOPROTEROZOIC Strike and dip of magmatic or metamorphically modified igneous foliation—Established SAGANAGA TONALITE PALEOPROTEROZOIC ESTER LAKE gs Gunflint Iron Formation primarily from magmatic segregations of plagioclase phenocrysts and modal or textural Rove Formation layering in Mesoproterozoic intrusions, and from modal layering and orientation of Grand 4 48° Portage Gunflint Iron Formation unconformity autoliths in Neoarchean intrusions; inclined, vertical. 2  Mafic dikes d Strike and dip of cleavage and schistosity—Inferred to be related to D deformation and Ely 3 WAWA SUBPROVINCE OF THE SUPERIOR PROVINCE 2 regional metamorphism; inclined, vertical. 48° 7' 30" NEOARCHEAN Knife Lake Group D 3 Strike and dip of outcrop-scale shear or fault zone—Offset sense is unknown; locally MESOPROTEROZOIC Ogishkemuncie sequence 2 Knife Lake Group D contains sericite, chlorite, quartz, and iron carbonate; inclined, vertical. Duluth Complex 3 Aoi 2 and other intrusions Ogishkemuncie sequence Strike of outcrop-scale strike-slip fault—Nearly vertically dipping; offset sense: dextral, 6 Aos Aoc Aou sinistral. North Shore Mudstone + sandstone Aot Volcanic Group Bearing and plunge of mineral or clast lineation SUPERIOR Conglomerate Ajs D1 unconformity 1 48° 7' 30 N. Bearing and plunge of minor fold axis—Includes anticline, syncline, and folds of "Z" and PALEOPROTEROZOIC Jasper Lake sequence 2 12 "M" morphology. LAKE 13 8 Ajc Virginia + Rove Formations Jasper Lake sequence 1 5 9 Saganaga Tonalite Aji Ajx NEOARCHEAN Strike and dip of mafic to intermediate dike, undifferentiated—Includes lamprophyre and Ajv Biwabik + Gunflint Iron Formations pyroxenite dikes of probably Neoarchean age, diabase and diorite of likely Paleoproterozoic Intrusions 2 3 Asb age, and diabase of likely Mesoproterozoic age; inclined, vertical. 4 4 Ast Asd Volcaniclastic rocks 14 Strike and dip of felsic dike—Inclined, vertical. NEOARCHEAN 11 1 unconformity Surface trend of magnetite-rich mudstone and chert—Typically occur as steeply dipping, Western Superior Province Knife Lake Group—Ogishkemuncie sequence Paulson Lake sequence Saganaga Tonalite (2,690 Ma) multiple thin lenses of reddish to purplish-red mudstone within finer-grained strata of 47° Wabigoon Quetico Knife Lake Group—Jasper Lake sequence unit . Batholith and dikes Aps Aos Lake 7 Apf Superior Granitoid batholiths and plutons Api Apm Amv Outcrop—Bedrock exposure; typically represents areas of multiple, scattered outcrops. 2 Wawa 10 Consists of new mapping by the authors and outcrops digitized from previous work. Volcanic and volcanogenic rocks Paulson Lake sequence DULUTH COMPLEX and similar rocks D , D , D refers to approximate relative timing OGISHKEMUNCIE 1 2 3 Border of the Cavity Lake forest fire area Kilometers GILLIS LAKE of regional Neorchean deformation events 0 10 20 30 40 50 Hypabyssal intrusions LAKE G Sample location—Location from which geologic samples were acquired for geochronologic Figure 1. Bedrock contact and fault lines (magenta) superimposed on 1-meter lidar imagery demonstrating the influence of bedrock structure 48° N. C 1 kilometer analyses; A—Near Alpine Lake; Driese and others (2011); B—Southeast of Jasper Lake and lithologic composition on land-surface topography. It should be noted that the map was created largely before availability of high-resolution Duluth 0 10 20 30 Mafic-ultramafic flows Miles (UTM location 649,327m E., 532,9586m N., NAD 83, field station G260); new data lidar data. Streamlined forms in the northwest part of the map area are Quaternary glacial sediments. 91° 1 mile INDEX MAP described here; C—West of Gillis Lake; Hoaglund and others (2010). Figure 2. Generalized bedrock geology of northeastern Minnesota showing the location of the Cavity Lake Map area (solid magenta line), extent of the Cavity Lake fire (dashed magenta line), outlines map area (black square). Inset map shows the location of Neoarchean rocks within the Wawa subprovince of Figure 3. Simplified bedrock geologic map highlighting major lithologic subdivisions, generalized stratigraphic facing of 7.5-minute quadrangles, major contacts and faults within published map sheet (gray lines), The University of Minnesota is an equal opportunity educator and employer the Superior Province. (black arrows), and unconformities (dashed lines, numbered). and areas of incorporated archived mapping (numbered; see references).