Report of Investigations

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GEOPHYSICAL AND PETROLOGIC ADVANCES IN UNDERSTANDING THE TAUNTON-BELT WEDGE, AN ARCHEAN TERRANE WITHIN THE MINNESOTA RIVER VALLEY SUBPROVINCE OF THE SUPERIOR CRATON, NORTH-CENTRAL U.S.A.
(2023) Southwick, David L; Chandler, V.W.; Jirsa, Mark A; Boerboom, Terrence J
The Taunton-belt wedge, defined herein, is an unexposed fault-bounded Archean terrane located within the Morton block of the Minnesota River Valley (MRV) subprovince of the Superior Craton. It is bounded on the north by the Yellow Medicine shear zone (YMSZ), a prominent regional structure that extends from the west margin of the Paleoproterozoic Penokean orogen in Minnesota to the east margin of the Paleoproterozoic Trans-Hudson orogen in South Dakota. The south boundary of the wedge is a less distinct curving fault zone that splays west–southwest from the YMSZ in south-central Minnesota and rejoins it in eastern South Dakota. The interior geology of the wedge is poorly known, owing to continuous cover of the Precambrian basement by Phanerozoic strata and Pleistocene glacial deposits. Regional aeromagnetic and ground gravity mapping indicate a broad belt of mafic metavolcanic and related rocks in roughly the northern half of the wedge, and many granitoid intrusions in the southern half. Geophysical signatures characteristic of layered gneiss, such as those observed over known gneiss elsewhere in the Morton block, are minor to absent.
The Influence of Bedrock Topography on the Origin of a Mid-Pleistocene Epoch Glacial Lake in Rock County, Southwest Minnesota
(Minnesota Geological Survey, 2017) Southwick, David L
A circular, closed depression 1.9 miles (3 kilometers) in diameter that was formerly occupied by a shallow lake is located in the glaciated landscape of northern Rock County, southwest Minnesota. The depression is partly framed by Sioux Quartzite and is situated above a bedrock swale on a broad, quartzite-supported upland that is thinly mantled by pre-Wisconsinan glacial deposits. The quartzite-supported upland has been a positive topographic feature since at least the Late Cretaceous Epoch; near the depression, its discontinuous cover of unconsolidated sediment consists of glacial till, outwash deposits, and loess that aggregate to a total preserved thickness between 1 and 82 feet (less than 1 to 25 meters). The depression originated from the melting out of a buried, tabular ice mass that was isolated near the stagnating margin of a mid-Pleistocene Epoch continental glacier. The ice mass was buried in outwash and then further buried by a thin till deposited when the glacier readvanced. The ice mass became isolated and was slow to melt because of its position in a bedrock swale on a topographic high that was near a dynamically fluctuating glacier margin; its relative thickness and protected location in the swale were key factors in its transient preservation. The closed depression that formed upon final melting of the ice mass has survived in the post-glacial landscape because of its location on a geomorphically persistent bedrock upland where the erosive energy of post-glacial and modern streams has been minimal. It has been speculated that this bedrock-framed, geographically unique circular depression may be a deeply eroded meteorite impact structure. No supporting evidence for this speculation has been discovered in the field or laboratory.
Characterization of the Philbrook Intrusion, Central Minnesota
(Minnesota Geological Survey, 2017) Boerboom, Terrence J; Geary, Jesse
The Philbrook intrusion is an informal name for a small, sub-circular mafic pluton that is exposed at the surface along the southeast side of the Long Prairie River near the town of Philbrook, in northeastern Todd County, central Minnesota. An 40Ar/39Ar age of 1,854 ± 4 Ma has been obtained from a sample of what is interpreted to be magmatic hornblende from the intrusion. The pluton is composed primarily of melanocratic to mesocratic diorite, along with substantial proportions of pyroxenite, hornblendite, oxide-apatite rock (nelsonite), and rare anorthosite as inclusions in the mesocratic diorite. The Philbrook intrusion contains abundant hornblende interpreted to be both primary-magmatic and secondary-deuteric, which coupled with local net-veined pegmatitic textures and pervasively saussuritized plagioclase, imply that the magma was rich in hydrous components. Even though primary textures are commonly overprinted by secondary hornblende, relict primary cumulate textures are present in most of the different rock types. Chemically, the entire suite of rocks is high in phosphorous, iron, titanium, and vanadium, and low in silica, magnesium, and potassium. The Philbrook intrusion was emplaced into a suite of interlayered sedimentary, volcanic, and hypabyssal intrusive rocks that have been inferred to be Paleoproterozoic, but have more recently been interpreted to be possibly Archean, in age. The country rocks are schistose and were regionally metamorphosed to the upper greenschist/lower amphibolite facies, either during or before the Penokean Orogeny, or possibly both, depending on their age, which is unknown.
RI-68 Quaternary Lithostratigraphic Units of Minnesota
(Minnesota Geological Survey, 2016-03) Johnson, Mark D.; Adams, Roberta S.; Gowan, Angela S.; Harris, Kenneth L.; Hobbs, Howard C.; Jennings, Carrie E.; Knaeble, Alan R.; Lusardi, Barbara A.; Meyer, Gary N.;
Much of Minnesota is covered by sediment of Quaternary age that was deposited during numerous glaciations by ice, wind, and water. In this report, we follow guidelines of the North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature, 2005) to create a framework for establishing formal lithostratigraphic units in Minnesota. We evaluate over 100 lithostratigraphic units that have been identified in Minnesota. Eighty (80) units are considered to be useful lithostratigraphic units of formation and member rank, and these are formally accepted in this report or are recommended to be so in future publications. These 80 units include previously named formal lithostratigraphic units that are recognized and accepted as originally defined, but also formally defined units that we have revised or redefined to better fit into our stratigraphic framework. The remaining lithostratigraphic units have been used informally in earlier reports or are newly named in this report. Additional units that are no longer considered necessary as lithostratigraphic units are abandoned in this report. These units include previously used units of both formal and informal status. Of the 80 lithostratigraphic units recommended to be retained, 47 are formally defined, revised, or redefined in this report. The remaining 33 units are recommended to be formally named in a future Minnesota Geological Survey Report of Investigations.
RI-70, Characterization of the Franklin Peridotite and Other Similar Intrusions in East-Central and Southwestern Minnesota
(2014) Boerboom, Terrence J.
Outcrops of peridotite adjacent to the Minnesota River near the town of Franklin in Renville County were sampled and petrographically characterized as part of a small study funded by the Minnesota Department of Natural Resources in 1997. That study obtained mineral separates with the intent of examining them for kimberlite indicator minerals. The results were not formally published, but rather summarized in an unpublished final report to the Minnesota Department of Natural Resources titled "Mineral Investigations of Franklin Kimberlites." In addition to petrographic and geochemical characterizations, ground magnetic traverses were made across the outcrop area in order to quantify the size and shape of the peridotite body. Based on simple ground magnetic surveys, the peridotite body is approximately 1 square kilometer (0.4 square mile) in area. The peridotite in the outcrops is extensively silicified, most likely by low-temperature alteration associated with lateritic weathering beneath Cretaceous sedimentary strata. Peridotite that is not silicified is composed of olivine (serpentinized), orthopyroxene, hornblende, magnetite, and minor spinel, phlogopite, ilmenite, and sulfide minerals; all the silicate phases are Mg-rich. The silicified peridotite contains abundant secondary quartz and chalcedony, but the silicification did not affect the Fe/Mg ratio, as both silicified and unsilicified peridotite have Mg numbers of 87 to 88. The Franklin peridotite is similar to ultramafic peridotite and pyroxenite bodies in east-central Minnesota, as well as the Cottonwood peridotite body intersected by drilling in northern Lyon County, 55 kilometers (34 miles) west–northwest of the Franklin peridotite and south of the Minnesota River valley. The peridotites in east-central Minnesota are between 1,770 and 1,791 Ma in age, whereas the age of the Franklin and Cottonwood peridotites is unknown.
RI-69 Reexamination of the Minnesota River Valley Subprovince with Emphasis on Neoarchean and Paleoproterozoic Events
(2014-03-06) Southwick, David
The Minnesota River valley subprovince (MRV) is a fragment of Mesoarchean continental crust that was sutured to the southern margin of the Superior craton about 2,600 m.y. ago. The suturing event induced widespread regional metamorphism and local anatexis in a dominantly orthogneissic crust and ended with the emplacement of numerous granite plutons. In the Paleoproterozoic era the MRV was a tectonically rigid part of the cratonic foreland with respect to Penokean (geon 18), Yavapai (geon 17), and Mazatzal (geon 16) accretionary events. As such, it was affected by crustal extension and the emplacement of mafic dikes associated with the ca. 2,070 Ma opening of the pre-Penokean ocean. Subsequently, internal shear zones that had formed during Neoarchean docking of the MRV crustal block were reactivated in response to stresses applied during cycles of Paleoproterozoic stretching and subsequent compression from the south and southeast. Most of this reactivation is inferred to have taken place between 2,000 and 1,750 Ma. The Minnesota segment of the Great Lakes tectonic zone, the Neoarchean suture, was not significantly reactivated, whereas the Appleton shear zone and the Yellow Medicine shear zone both were. Six sets of mafic dikes were emplaced in the interval between 2,070 and ca. 1,750 Ma. Two sets that were emplaced early in the interval are the southwesternmost members of the pre-Penokean Kenora–Kabetogama/Fort Frances dike swarm. Two and perhaps four younger dike sets were emplaced during a period of vigorous crustal heating and magmatic activity that affected much of the MRV in early- to mid-geon 17. Numerous plugs and small plutons also were emplaced in early- to mid-geon 17. These intrusions range in composition from peridotite to granite and are comparable to rock types within and satellitic to the East-Central Minnesota batholith; they are most abundant in the eastern and southern parts of the MRV, relatively near the inferred Penokean and Yavapai tectonic fronts. Transtensional stress during the extensional stage of the Mazatzal orogenic cycle generated differential subsidence of crust south of the Yellow Medicine shear zone and produced en echelon fault-bounded depressions that became depocenters filled by supermature clastic sediment ancestral to the Sioux Quartzite. The Sioux Quartzite was deposited, lithified, and hydrothermally altered over a prolonged time interval that may have begun as early as ca. 1,730 and ended as recently as ca. 1,280 Ma.
RI-67 Lunisolar Tidal Signatures in the ~1.9 GA Pokegama Formation, Northeastern Minnesota: Implications for the Lunar Orbit
(2010) Ojakangas, Gregory W.; Ojakangas, Richard W.
Specimens of siltstone beds from a roadcut in the lower member of the ~1.9 Ga Pokegama Formation at the village of Midway near Virginia, Minnesota contain laminae that record possible neap-spring cycles and a strong diurnal inequality. These laminae provide support for the tidal origin of the formation proposed by R. Ojakangas (1983) on the basis of other sedimentary structures. In order to enhance observed cyclicities in the presence of significant diagenetic alteration, a new algorithm was developed that transforms digital images of lamina sequences to horizontal uniformity, and then averages them horizontally to reduce noise and enhance lamina boundaries. Although too short and noisy to provide results of appreciable confidence, spectral analysis of a lamina sequence from the Pokegama Formation suggests minimum values for the lunar orbit radius (greater than 30 earth radii) and length of day (greater than 10 hours) at the time of deposition, consistent with theoretical expectations and extrapolation of other cyclic tidal data. The analysis presented here illustrates the potential value of identifying well-preserved tidal sequences in Proterozoic sedimentary rocks of the Lake Superior region.
RI-52 Mineral Development in Minnesota: Past History, Present Trends, and Future Possibilities
(Minnesota Geological Survey, 1998) Morey, G.B.
Mineral development, like all development in Minnesota in the 21st century, will occur within the principles of sustainable development. However those principles are not entirely applicable to the extractive-minerals industry inasmuch as mining depends on the utilization of nonrenewable resources. Because mineral deposits once mined are gone forever, sustainable development in mining can be achieved and through continuous replacement of the commodity consumed. The rate at which many mineral resources must be replaced can be reduced by careful conservation measure and by the use of substitutes. Recycling contributes to our current supplies or most metals, but recycling alone cannot meet all of our societal demands for metal products. Thus for mineral resources, "replacement" to a large extent means discovering new mineral deposits. Minnesota has had a long history of metal mining extending back to 1884 when iron ore was first shipped from the Vermilion range. Since then, Minnesota has produced nearly 3.5 billion tons of iron ore for the United States steel industry. Iron mining will continue to be of considerable importance in the foreseeable future. The Mesabi range contains more than 170 billion tons of crude or 36 billion tons of iron-ore concentrate that will be recoverable for more than 200 years using current open-pit mining methods. Geologic studies also show that a vast, but low-grade copper-nickel resource consisting of 4.1 billion tons of material that has an average copper value of 0.7 percent and a copper to nickel ratio of 3.33 to 1. Other commodities, including low-value, laree-volume industrial materials also are important to the state's economy. Industrial materials mined and used include construction aggregate (sand and gravel or crushed stone) dimension stone, clay, silica sand, and peat. Geologic studies started in the mid-sixties have defined a modern conceptual framework that has established that the state has a large economic potential for a variety of commodities including gold, zinc, copper, nickel, and uranium. Many technical problems stand in the way of a discovery, but Minnesota has considerable potential for future mineral development. Therefore given enough effort, the probability of finding a new mineral deposit is fairly large. The discovery of a mineral deposit of sufficient size and grade is the first step toward developing a mine, but a discovery will not in itself lead to mining. For a mineral deposit to be mined requires that the commodity can be extracted profitably with existing technology and under current economic conditions. Thus in the end, the decision to develop a deposit turns on economic and public policy factors. Economic decisions depend on specific factors such as taxes, royalties, environmental costs, and interest rates, as well as on less specific questions regarding potential impacts on the quality of life. In the end, a program leading from discovery to mine depends on a strong working relationship between the public and private sectors. The partnership like all partnerships require confidence based on mutual respect and understanding. Each sector has a role to play and each must freely contribute to the process. Mineral development like sustainable develop in general will occur only when we take a long-term view. Mineral development from discovery to mine typically takes a long time and involves large sums of money. It is the public sector's responsibility to provide geologic maps at regional and intermediate-scales and other kinds of geologic information that will support the exploration process. It is the private sector's responsibility to explore and develop diligently. In the final analysis, both sectors must recognize that a mining company profits today must provide the capital for tomorrow's exploration ventures.
RI-51 Geologic Setting and Descriptive Geochemistry of Archean Supracrustal and Hypabyssal Rocks, Soudan-Bigfork Area, Northern Minnesota: Implications for Metallic Mineral Exploration
(Minnesota Geological Survey, 1998) Southwick, D.L.; Boerboom, Terrence J.; Jirsa, M.A.
Geochemical data are presented for a suite of samples chosen to represent as nearly as possible the compositions of unaltered rocks within the Archean greenstone belt of the Soudan-Bigfork area in northern Minnesota. These data are interpreted within the stratigraphic and structural framework provided by geologic mapping, and are used to deduce probable depositional and tectonic environments for the volcanic and sedimentary sequences described. In addition, the data indicate regional differences in intrinsic mineral potential, as judged from the presence or absence of geochemically favorable rock types associated with economic deposits of metallic minerals elsewhere in the Superior Province. The greenstone belt as a whole has attributes consistent with development in an oceanic volcanic-arc setting that is broadly analogous to modern arcs of the western Pacific basin. Supracrustal rocks in the Soudan belt (southern portion of the Soudan-Bigfork area) include a lower volcanic cycle that consists of the upper parts of a calc-alkaline volcanic edifice which evolved with time from a submarine basaltic pile into an emergent or nearly emergent dacitic eruptive center, and an upper cycle that represents submarine fan deposits mingled in time and space with tholeiitic basalt of possible back-arc origin. The Newton belt (northern portion of the Soudan-Bigfork area) consists of several fault slices made up chiefly of tholeiitic basalt and pyroclastic and epiclastic dacitic rocks. Because of structural shuffling and imbrication, the rock sequences within the Newton belt cannot be reliably correlated from slice to slice, which handicaps paleotectonic interpretation. Geochemically they are consistent with a back-arc origin. Basaltic sequences in the Soudan belt are evolved to iron-enriched compositions and exhibit very low background values for gold. Basaltic sequences in the Newton belt are moderately more magnesian and auriferous, and therefore may be somewhat better candidates for harboring economic gold deposits. The dacitic rocks of the Soudan belt fall into the least favorable geochemical category for hosting massive- sulfide deposits, based on empirical associations throughout the Superior Province, whereas the geochemical attributes of dacitic rocks in the Newton belt remain inadequately characterized in this respect. Differentiated gabbro-pyroxenite-peridotite sills are absent from the Soudan belt but are characteristic of the Newton belt. Peridotite sills within the Deer Lake Complex contain subeconomic amounts of magmatic Cu-Ni sulfide minerals and are petrologically reasonable, but untested, candidates for hosting platinum-group minerals. On the basis of these broad lithogeochemical criteria, the mineral potential of the Newton belt would appear to warrant serious further investigation.
RI-49 Contributions to Quaternary Studies in Minnesota.
(Minnesota Geological Survey, 1998) Patterson, Carrie J.; Wright, H.E. Jr.
RI-48 Pre-Late Wisconsinan Till Stratigraphy of North-Central Minnesota
(Minnesota Geological Survey, 1997) Meyer, Gary N.
Continuous cores of complete Quaternary sections at more than 60 sites in north-central Minnesota yield evidence for at least nine pre-late Wisconsinan tills. Found only in the subsurface, they are of both northwest and northeast provenance. The oldest till-the Mulligan Lake of northeast provenance-does not have a recognized counterpart to the south. Tills of northwest provenance-Wirt Lake, Bigfork, Eagle Bend, Funkley, and Browerville-are correlated with tills recognized earlier in central Minnesota. The younger northeastprovenance tills-Shooks, First Red, and Saum-also are correlated with till units recognized in central Minnesota. The subsurface record indicates alternating northeastern and northwestern advances during each major glaciation in north-central Minnesota. The distinct couplets of northeast- and northwest-source tills imply shifting locations for the Labrador and Keewatin ice centers during the Pleistocene. The Pleistocene stratigraphic sequence of Minnesota is divided into five informal "event" units based on the subsurface record. Evidence for the earliest of these--event V-is limited, but its tills are similar to tills of event X. During event W, ice carrying detritus of Rainy provenance extended from north-central Minnesota into central Minnesota. During this interval, ice carrying detritus of Winnipeg provenance moved south across and probably beyond Minnesota. During event X, ice carrying material of Superior provenance dominated flow from the Labrador center into north-central Minnesota at a time when ice carrying Rainy-provenance detritus apparently did not enter the state. Throughout event X, ice carrying Winnipeg-provenance detritus flowed southeastward into Minnesota, and probably again well south and east of the state. Event Y was marked by a resurgence into central Minnesota of ice carrying Rainy-provenance debris, with ice carrying Winnipeg-provenance debris again flowing into the state in a more southward direction. The final event Z comprises the late Wisconsinan glaciation when Keewatin ice expanded from a direction more to the west of Minnesota (incorporating material of Riding Mountain provenance) than earlier advances of ice carrying Winnipeg-provenance material. Sediments of event Y are probably illinoian or older, whereas those of events X, W, and V are probably pre-illinoian in age. The sedimentary record is evidence that the Keewatin and Labrador ice centers were both phenomena of the Laurentide ice sheet throughout the Pleistocene, although their locations probably shifted considerably through time.
RI-47 Contributions to the Quaternary Geology of Southwestern Minnesota
(Minnesota Geological Survey, 1997) Patterson, Carrie J.
RI-46 Hydrothermal Systems in Manganese-Rich Iron-Formation Ofthe Cuyuna North Range, Minnesota: Geochemical and Mineralogical Study of the Gloria Drill Core
(Minnesota Geological Survey, 1996) Melcher, Frank; Morey, G.B.; McSwiggen, Peter L.; Cleland, Jane M.; Brink, S.E.
The iron-rich Trommald Formation of Early Proterozoic age on the Cuyuna North range in east-central Minnesota is the largest resource of manganese in the United States. To better elucidate the complex history of the manganese oxides and to investigate their compatibility with in situ leaching techniques, the U.S. Bureau of Mines drilled a core near the Gloria mine (sec. 28, T. 47 N., R. 29 W.) to an inclined depth of about 1200 feet. It intersects a complete section of Trommald Formation 553 feet thick, as well as short intervals of the overlying Rabbit Lake and underlying Mahnomen Formations. At the Gloria site the lowermost part of the Trommald Formation consists of chlorite-bearing hematite iron-formation, and contains features indicative of syndepositional reworking-possibly under shallow-water conditions that include granule-rich layers and pebble-size conglomerate. Much of the overlying thin-bedded facies consists of carbonate-silicate iron-formation broken in places by beds of breccia. In the breccia, sulfides--especially pyrite filled interstitial voids or form discordant composite veins along with quartz, manganese oxides, carbonates, and stilpnomelane. Pyrite contains included pyrrhotite, chalcopyrite, and arsenopyrite, and is replaced by magnetite, in turn replaced by martite. The silicate-carbonate iron-formation contains little manganese <2 wt. percent MnO), but the breccias are marked by elevated values of Mn, as well as Cu, As, Sb, S, Sr, Y, Ca, and P. Within the thin-bedded facies, a transitional interval of about 15 feet separates silicate-carbonate iron-formation below from oxide iron-formation above. Isocon analyses show that oxidized strata can be derived from unoxidized strata by a variety of decomposition reactions, all essentially removing Mg, K, Ca, Na, P, and resulting in a mass loss of 40 to 50 percent. Manganese oxides appear in oxidized strata as discordant veins and as concordant, massive layers and lenses, where they are admixed with goethite and hematite. Massive layers are partly enveloped by brecciated oxides of several kinds, have maximum MnO values of 12 wt. percent, and are enriched in Ba, Sr, Ag, and U. Manganese values increase markedly just above the contact between the thin-bedded facies and the overlying thick-bedded facies. Manganese oxide-rich layers just above the contact have brecciated or "gnarled ore" textures and are conformably interlayered with intervals of decomposed oxide iron-formation containing abundant secondary limonite. The manganese oxide-rich layers are intercalated with thick beds of admixed chert and hematite; they have a relict micronodular texture and discordant textural features, involving manganese minerals such as manganite, cryptomelane, and pyrolusite. Whole rock assays show that they can contain as much as 50 wt. percent MnO. They also have elevated Ca, Mg (attributed to secondary carbonates), and P values and are enriched in Ba (as much as 1.8 percent), Pb, Sr, Ag, As, and the LREE. The uppermost part of the thick-bedded facies consists of oxide iron-formation where primary hematite is abundant but where manganese oxides are lacking. The overlying Rabbit Lake Formation is an epiclastic unit that contains thick beds of carbonaceous shale, thin layers of tuffaceous material, and intercalated beds of sulfide (pyrite) and oxide (hematite) iron-formation. The sulfide iron-formation (35.7 wt. percent Fe203, 26 percent S) contains elevated values of Au (31 ppb), As, Cu, Co, Ni, Pb, Sr, V, Mo, and Se (230 ppm). Many of the textural and mineralogical features observed in the Gloria core are consistent with hydrothermal fumerolic processes that started in late Mahnomen time and persisted into early Rabbit Lake time.
RI-45 Iron-Formation Protolith and Genesis, Cuyuna Range, Minnesota
(Minneota Geological Survey, 1995) McSwiggen, Peter L.; Morey, G.B.; Cleland, Jane M.
The Cuyuna iron range in east-central Minnesota is unique in the Lake Superior region because of its large manganiferous iron ore resource. The protolith to the ore has traditionally been considered a Lake Superior-type iron-formation similar to other Early Proterozoic iron-formations in the region. However, recent stratigraphic, mineralogical, and geochemical studies show that the Trommald Formation-the principal iron-formation of the Cuyuna North range-is not the product of a simple sedimentological regime. The presence of the minerals aegirine, barite, Ba-feldspar, and tourmaline within or associated with the iron-formation shows that hydrothermal and exhalative processes were very important during deposition of the iron-rich strata. This new interpretation has significant implications for mineral exploration in this part of Minnesota. It implies that parts of the Cuyuna range may be likely areas for exploration for sediment-hosted, submarine exhalative, PbZn- Ag deposits.
RI-44 Gravity Investigation for Potential Ground-Water Resources in Rock County, Minnesota
(Minnesota Geological Survey, 1994) Chandler, V.W.
The gravity method was used to investigate the thickness and potential ground-water resources of Cretaceous and Quaternary sediments in Rock County, Minnesota. This fill, which partially covers the Early Proterozoic Sioux Quartzite, is a major source of ground water for the county, but its deeper parts are very poorly known because of poor drill-hole control. Local gravity signatures reflecting the low-density fill were isolated from a smooth, regional field reflecting deep, intrabasement sources by a graphical cross-proftle procedure, which incorporated data from outcrops, drill holes, and seismic soundings, to define the regional field. At control points where fill thickness was known, the regional field value was determined by using a Bouguer slab approximation with a density contrast of 0.60 g/cm3 to strip out the local effect of the fIll. Additional control on the regional field was provided by iterative analysis of cross-profIles. Because it is assumed to be smooth, the regional field can be defmed by relatively few control points, and subtraction of this field from the observed gravity data produces a residual map of the fill signatures. The residual field was transformed into estimates of fIll thickness by using the same Bouguer slab approximation and density contrast of -0.60 g/cm3, and the elevation of the Precambrian bedrock was estimated by subtracting the fill thickness from the surface elevation. In southwestern, southeastern, and northeastern Rock County, the combined thickness of the Cretaceous and Quaternary deposits is interpreted to exceed 200 m (600 feet). The thick fill in southwestern Minnesota connects with a buried channel in South Dakota that contains several known aquifers. Potential ground-water resources may also be associated with several buried channels cut into the edges of a plateau of Sioux Quartzite in the northwestern and central parts of the county. Along the southern margin of this plateau, a buried and somewhat dissected escarpment is interpreted to be associated locally with at least 215 m (650 feet) of unconsolidated fill. Additional resources may lie within the fractured rock and thickened fIll in a northwest-striking fracture zone in the Sioux Quartzite, which may extend in the subsurface across the county. The results of this study indicate that the gravity method is an effective reconnaissance-scale tool for ground-water exploration in the Sioux Quartzite areas of southwestern Minnesota.
RI-43 Short Contributions to the Geology of Minnesota, 1994
(Minnesota Geological Survey, 1994) Southwick, D.L.
RI-42 Industrial Minerals-Today and Tomorrow: the Raw Materials to Build the Upper Midwestworkshop Proceedings
(Minnesota Geological Survey, 1993) Sidder, Gary B.; Sims, P.K.
RI-41 Bottom Sediments and Organic Geochemical Residues of Some Minnesota Lakes
(Minnesota Geological Survey, 1992) Swain, F.M.
Holocene lake sediments of Minnesota are represented by six facies: I) northeastern-allogenic littoral sand and gravel, and profundal clay and copropel, in Precambrian crystalline rocks, with low to high total carbohydrates, high and variable amino acids, low to moderate hydrocarbons, and low to high pigments; II) northwestern-mixed allogenic clastics, and authigenic copropel and marl, in calcareous glacial drift and Pleistocene lake beds, with moderate carbohydrates, low amino acids, and stratigraphically variable pigments; III) north-central-authigenic marl, copropel, and allogenic sediments in thick calcareous glacial drift, with moderate carbohydrates, low amino acids, and stratigraphically variable pigments; N) east-central-authigenic copropel, marl, and allogenic clastics in calcareous and noncalcareous glacial drift, with high carbohydrates and amino acids, high hydrocarbons and polar lipids, high pigments, all stratigraphically variable; V) west-central and southwestern-allogenic silt, marl, and sapropel in calcareous glacial drift and gypsiferous Cretaceous shale, with high carbohydrates and amino acids, stratigraphically variable, high aromatic hydrocarbons; and VI) southeastern-allogenic fine clastics and copropel in Paleozoic clastic and carbonate rock and pre-Wisconsin drift, with known organic residues similar to those in Facies IV.
RI-40 Sedimentary Rocks of Dresbachian Age (Late Cambrian), Hollandale Embayment, Southeastern Minnesota
(Minnesota Geological Survey, 1992) Mossler, John H.
The Dresbachian Mt. Simon Sandstone, Eau Claire Formation, and Galesville Sandstone of the Hollandale embayment of southern Minnesota are divisible into eleven major lithofacies and several subfacies. Because the formations are almost exclusively in the subsurface in Minnesota, lithofacies descriptions are based on cores, well cuttings, and geophysical logs. Along the eastern side of the Hollandale embayment, the lower Mt. Simon Sandstone consists of a thin basal conglomerate lithofacies overlain by medium- and large-scale, crossstratified and planar-stratified sandstone. Middle Mt. Simon is principally interbedded, coarsely interlayered sandstone, siltstone, and shale; and thin- to medium-bedded, structureless or crossstratified sandstone. The upper Mt. Simon is structureless sandstone with Skolithos and shelly (coquinoid) sandstone. These lithofacies resemble those from outcrops in western Wisconsin described by Driese and others (1981). Toward the west and south in south-central Minnesota, at the embayment center, mediumand large-scale, cross-stratified sandstone dominates in the Mt. Simon. Along the western side of the embayment, structureless sandstone dominates. There are fewer thin shale and siltstone beds in the Mt. Simon near the embayment center than along the eastern side of the embayment. Near the embayment center, the uppermost Mt. Simon Sandstone and basal Eau Claire Formation contain ferroan oolites and coated grains that are scattered in some beds and are the principal sand-sized particles in others. Ferroan oolites and coated grains are not observed in outcrop. Along the eastern side of the Hollandale embayment, the Eau Claire is composed principally of mixed sandstone and shale lithofacies and greensand lithofacies resembling Eau Claire lithofacies that crop out in western Wisconsin (Huber, 1975), especially in cores along structural strike with the Wisconsin outcrops. Red sandstone and shale lithofacies and dolostone lithofacies are at the base of the Eau Claire in south-central Minnesota. These are overlain by a ripple-cross-stratified or troughcross- stratified subfacies of the greensand lithofacies that is much thicker than laterally equivalent beds of greensand lithofacies to the north and east. The Galesville Sandstone, mostly structureless, planar-stratified, or trough-cross-stratified sandstone, appears to be conformable and interbedded with the Eau Claire Formation. The upper part of the Eau Claire and Galesville appear to be part of an upward coarsening sequence. There is evidence of slight disconformity between the Galesville and overlying Ironton Sandstone. The basal conglomerate of the Mt. Simon is interpreted as a braided fluvial deposit. Medium- to coarse-grained sandstone lithofacies of the lower Mt. Simon are interpreted as braid plain, braid delta, and littoral deposits. Fine- to medium-grained sandstone beds and shale beds in the middle Mt. Simon are interpreted as distal braid delta deposits. Sandstone beds in the upper Mt Simon are interpreted as sand shoals and tidal flat deposits. Beds of interbedded, finegrained sandstone and shale in the basal Eau Claire that are tidal flat deposits culminate this initial prograding sequence. Toward the end of the sequence deposition, ferroan oolites formed nearshore, where some were reworked by shifting tidal channels. Red sedimentary rocks were deposited in high tidal flat, channel, and deltaic environments. Carbonate rock was deposited in the southwestern Hollandale embayment as detrital sedimentation ended. The greensand lithofacies of the medial Eau Claire Formation, which records marine transgression at the base of the next prograding sediment sequence, is succeeded by shaly lagoonal deposits, sandy or shaly tidal flat deposits (upper Eau Claire Formation), and sandy foreshore or shoreface deposits (Galesville Sandstone). Variation of sandstone composition reflects selective mechanical reduction of contained potassium feldspar grains as observed by Odom (1975). Medium- to coarse-grained sandstone is quartzose, and very fine to fine-grained sandstone is highly feldspathic. This variation, reflected on gamma logs, helps to distinguish lithofacies. . Sandstone in core from southwestern Minnesota contains accessory minerals, including diaspore, that indicate the contribution of sediment from the Proterozoic Sioux Quartzite to the Mt. Simon Sandstone.
RI-39 Manganiferous Zones in Early Proterozoic Iron-Formation in the Emily District, Cuyuna Range, East-Central Minnesota
(Minnesota Geological Survey, 1991) Morey, G.B.; Southwick, D.L.; Schottler, Shawn P.
Early Proterozoic strata in the Emily district of the Cuyuna iron range of east-central Minnesota unconformably overlie older folded rocks of the North range (North range group). They are correlative with strata of the Animikie Group on the Mesabi iron range, which consist of a lower quartz arenitic sequence (Pokegama Quartzite), an intermediate iron-rich sequence (Biwabik Iron Formation), and an upper feldspar-rich, graywacke-shale sequence (Virginia Formation). In the Emily district, however, the stratigraphic position of the Biwabik Iron Formation is occupied by three units of iron-formation separated by intervening sequences of black shale. Manganese occurs in the lowest iron-rich unit (informally termed Unit A of the Ruth Lake area). Unit A can be divided into six lithotopes. They are: (1) an epicIastic lithotope of quartz-rich siltstone and shale; (2) a mixed epiclastic jaspery chert litho tope; (3) an oolitic and pisolitic lithotope; (4) a thick-bedded lithotope of cherty or granular iron-formation; (5) a mixed thick- and thin-bedded lithotope characterized by thick intervals of slaty or nongranular iron-formation; and (6) a ferruginous chert lithotope. In general, lithotopes 1, 2, and 3 have shallowwater attributes, whereas lithotope 6 was deposited in quieter, presumably deeper water. Lithotopes 4 and 5 interfinger, and thus were deposited in generally similar sedimentological regimes in water of intermediate depth that was variably affected by currents. Unit A was deposited during two trangressive-regressive cycles in a basin that deepened to the north. Well-rounded grains of terrigenous quartz, which persist throughout lithotopes 1-5, imply that much of the sedimentation occurred relatively close to strandline. Although Unit A in the Ruth Lake area has many mineralogical and chemical attributes typical of "ordinary" iron-formation, it contains manganese at levels that are one or two orders of magnitude larger than the norm. Manganese oxides are distributed throughout lithotopes 1-5 as disseminated grains, as thin pods or lenses, and as layers as thick as 1.5 meters that typically contain about 10 percent Mn, but some contain as much as 20-30 percent. In addition, Unit A contains two major, laterally persistent zones about 15 to 18 meters thick, in which the manganese tenor has been enriched to the 10-50 percent range by secondary processes. Both enriched zones more or less coincide with stratigraphic positions occupied by the ooliticpisolitic lithotope. They contain various proportions of psilomelane, cryptomelane, hematite, and quartz. Goethite and manganite may be locally abundant, and where they occur they are secondary phases that formed during a period of intense chemical weathering in Late Jurassic or Early Cretaceous time. Primary stratigraphic, textural, and mineralogic attributes of the Ruth Lake strata correspond to those used by James (1955) to define the oxide (hematite) facies of ironformation. The hematite and chert are syngenetic, but the manganese oxides are more likely epigenetic. It is inferred that the manganese oxides were deposited in porous and permeable parts of the Ruth Lake sequence by a reflux process involving reducing solutions that leached manganese from older rocks of the North range group. The principal mechanisms for manganese concentration are inferred to have been early diagenetic, and therefore to have operated in Early Proterozoic time. Mesozoic weathering phenomena have been imposed on the rocks and have caused some redistribution of manganese. The abundance of manganese makes the Ruth Lake area in the Emily district a potential target for in situ mining techniques currently being developed by the U.S. Bureau of Mines and the Mineral Resources Research Center of the University of Minnesota.

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