Earth and Environmental Sciences Dissertations, Theses, and Plan B Project Papers

Persistent link for this collectionhttps://hdl.handle.net/11299/212256

This collection contains some of the final works (dissertations, theses, and Plan B project papers) produced by master's degree and doctoral students in the Department of Earth and Environmental Sciences (or its predecessors) as part of their requirements to graduate. Specific master's degree programs over the years have included Master of Science in Geology, Master of Science in Geological Sciences, Master of Science in Earth Sciences, and Master of Science in Earth and Environmental Sciences (the current iteration as of spring 2024).

To see final works for specific degrees, click the links below. NOTE: Newer dissertations and theses from roughly 2007 on (including all theses from the Master of Science in Earth Sciences program) will not appear in the lists linked below, though some do appear in this collection. (For the most up-to-date new dissertations and theses, see the University of Minnesota Twin Cities Dissertations and Theses collection.) Newer Plan B project papers will appear in the lists.

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    A Mafic Layered Intrusion of Keweenawan Age Near Finland, Lake County, Minnesota
    (1974-04) Stevenson, Robert J
    A gently dipping sequence of layered, differentiated, mafic rocks are exposed in the vicinity of Sonju Lake, Minnesota. These rocks, here named the Sonju Lake intrusion, are in fault contact along their eastern edge with diabasic olivine gabbro and troctolite. To the south, prismatic granodiorite appears to intrude the top of the layered units, and to the north, the base of the intrusion is bounded by diabasic troctolite. All of the rock units in the area are presumed to be Late Precambrian in age. The Sonju Lake intrusion has well-developed igneous lamination and centimeter-scale, gravity stratified, and cryptic layering. The intrusion has a stratigraphic thickness of approximately 3300 feet (1000 meters). The rock units, from the base to the top, are picrite (105 feet), a transition zone (150 feet), troctolite (965 feet), two-pyroxene gabbro (780 feet), augite gabbro (680 feet), apatite-rich ferrodiorite (265 feet), and a granodiorite unit (350 feet). The crystallization sequence based on stratigraphy and textures is (chromian spinel)-olivine-plagioclese-augite-pigeonite(inverted)-ilmenite-magnetite-apatite-alkali feldspar-quartz. Compositions of major minerals vary systematically with stratigraphic height. Electron microprobe and optical studies show that olivine varies from Fo71 in the picrite to Fo12 in the uppermost portion of the apatite- rich ferrodiorite, plagioclase from An83 in the picrite to An46 in the ferrodiorite, and augite from Wo43En46Fs11 in the two-pyroxene gabbro to Wo40En25Fs35 in the uppermost portion of the apatite-rich ferrodiorite. A calculated bulk composition for the Sonju Lake magma is tholeiitic and is between the high-alumina olivine diabase magma of the Pigeon Point sill and the more alkali and titaniferrous tholeiitic magma of the Logan Sills, Canada. In bulk composition, layering, crystallization sequence, and differentiation the Sonju Lake intrusion has many affinities with other Keweenawan intrusions as well as the Tertiary Skaergaerd intrusion of East Greenland. The data presented in this thesis place new chemical and physical restraints on models of Late Precambrian petrogenesis.
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    The Quaternary Geology of the Wrenshall and Frogner Quadrangles, Northeastern Minnesota
    (1977-06) Zarth, Randee Jo
    Study of late Wisconsin glacial deposits of the Wrenshall and Frogner quadrangles, southwest of Duluth, suggests a revised model for the late- and post-glacial history of the area. Two major sedimentary environments are distinguished: (1) an ice-disintegration environment and (2) a glaciolacustrine environment associated with Glacial Lake Duluth. Sediments produced by ice-disintegration are stratified, moderately to poorly sorted sand and gravel with clasts predominately of Precambrian sandstone, volcanics, granite, and slate; and minor bodies of laminated silt and clay. Topographically, these sediments comprise a wide belt of kettles, kames, and disintegration ridges, that are dissected locally by meltwater channels and tunnel valleys, some of which contain eskers. The lacustrine environment contains the following units: (1) thick, horizontally-bedded sand, (2) cross-bedded sand, (3) laminated silt and clay, (4) massive clay, and (5) massive and stratified dropstone deposits. In the nearshore environment are found moderately sorted and well rounded sand (0.25 mm) with lag boulders and gravel at the shoreline. At 300 to 309 min elevation the sand grades rather abruptly to massive clay. The nearshore facies overlies the offshore facies (silt and clay) indicating progradation of Glacial Lake Duluth. The highest strandline features occur at elevations near 330 m. They are expressed primarily as beach scarps and other well developed shoreline features such as several spits, an offshore bar, and a delta. A prominent linear, northeast trending scarp between 300 and 309 m, previously considered to be a strandline, is here interpreted to be the depositional front of a coarse grained shelf deposited into Glacial Lake Duluth as it stood near its highest stage (330 m). This indicates that what previously had been considered to be two stages, Glacial Lake Nemadji and Glacial Lake Duluth, is actually a single stage, that of Glacial Lake Duluth. The following late- and early-postglacial history is indicated: (1) Ice from the last advance of the Superior Lobe stagnated at the margin of the Lake Superior basin, resulting in the development of an ice-disintegration complex and stratified glacial deposits. (2) Meltwater from the disintegrating ice and the retreating Superior Lobe, along with other sources, was ponded in front of the retreating ice. A lake level rise to 330 m is indicated by a transgressive sequence of sediments. (3) The lake stabilized as Glacial Lake Duluth long enough to develop strong beach features. Sediment supplied at this stage appears to have been derived mainly from the ice-disintegration complex and the Superior Lobe (as ice-rafted sediment). (4) Progradation of the shallow water facies over the deep water facies was the result of sediment laden streams, meltwater and other runoff entering the lake. (5) The lack of a regressive facies indicates a rapid drop in lake level as a lower outlet was uncovered.
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    Petrology and Structural Relations of the Brule Lake Intrusions, Cook County, Minnesota
    (1976-06) Burnell, James Russell, Jr
    The Duluth Complex splits into two east-trending tongues at its eastern extent to form a north ern and a southern prong which expose a strip of older Keweenawan volcanic rocks between them. Brule Lake is located at the westernmost extent of the strip of volcanics just east of the point where the gabbro prongs coalesce. The volcanic rocks are intruded by a series of large sill-like bodies which terminate against rocks of the nearby Duluth Complex. Thus, these intrusions were formed between the time of the eruption of the local Keweenawan volcanic rocks and the subsequent gabbroic intrusions. The intrusive rocks of the Brule Lake area, which occur both as dikes and sills, consists of porphyritic intrusions and subsidiary non-porphyritic bodies. The gabbro porphyries are from 100 to JOO meters thick and extend for 2 to 8 kilometers along strike. The non-porphyritic intrusions are much smaller than the porphyritic units and occur as dikes and sills in the volcanic rocks and in the older porphyry units. The Brule Lake intrusions were derived from magmas of two different sources. Type A magmas, which formed mainly large, porphyritic gabbro units, are older than those smaller dikes and sills formed by the type B magma. Major and trace element concentrations suggest that the type A magmas were developed by a two-stage process. The first stage was one of extensive differentiation of a tholeiitic magma involving principally the fractionation of calcic plagioclase and Mg-rich pyroxene in relatively shallow-crustal magma chambers. The melt was then tapped off, leaving behind a residuum of anorthositic or gabbroic composition. The liquid continued to crystallize plagioclase (now more sodic) in a second chamber, the feldspar this time floating in the liquid. This phenocryst-rich liquid portion was then tapped from the top of the magma chamber and injected along planes of weakness in the overlying volcanic pile. The type B magmas did not undergo the same degree of differentiation as the type A. They were probably derived through a process that involved olivine fractionation to a greater degree than occurred in the type A line of descent. They were then intruded along the same planes exploited by the earlier liquids, incorporating very little of the country rock. The intrusive rocks of the Brule Lake area are chemically similar to other intrusive rocks of northeastern Minnesota, particularly the Logan Intrusions. Although there are chemical differences between the two groups, their overall similarity leads to a conclusion that their origins were probably quite similar and are related to the same phase of Keweenawan igneous activity.
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    Glacial History of the Late Wisconsinan Des Moines Lobe in Minnesota: Geomorphic, Lithologic and Stratigraphic Evidence for Two Advances
    (2024-05) Arends, Heather E
    Continental ice sheets play a significant role in the Earth’s climatic system. Reconstructing the growth and decay of the Laurentide Ice Sheet during the Last Glacial Maximum betters our understanding of how ice sheets respond to climate change and contribute to rising global sea levels. The southwestern margin of the Laurentide Ice Sheet formed discrete terrestrial lobes, influenced by bed topography. The Des Moines lobe (DML), channeled by the Red River and Minnesota River valleys, advanced to a terminal position in central Iowa. Four dated ice margins provide chronological constraints for regional correlations: the Bemis at ~17.0 ka cal BP, the Altamont at ~16.2 ka cal BP, Algona at ~14.8 ka cal BP, and the Big Stone moraine at ~14.0 ka cal BP, which marks the transition to what is known as the Red River lobe. The number and timing of DML phases were reconstructed using multiple lines of evidence derived from geomorphic, lithologic, and stratigraphic analyses. In a study area located in southwestern Minnesota and eastern South Dakota, the delineation of subglacial bedforms was used to identify lateral shear margins positioned at the base of topographic highs. Locations of lateral shear margins also correlate to continuous glacial landforms and the greatest compositional variability within the till sheet, observed from modeling 451 DML till sample sites. Spatial relationships indicate that a single, heterogenous till sheet is surfically exposed throughout the study area and bed topography may have influenced ice-flow dynamics to generate faster flow. Correlations of moraines with the stratigraphy of Late-Wisconsinan sediments indicate there is one continuous basal till unit capped by discontinuous sorted, unsorted, and interbedded sediments that extends from the Bemis margin and continues up-ice of the Altamont moraine. A second till sheet overlies this stratigraphy north of the Algona moraine in Minnesota. Results suggest that the DML experienced two phases. The first, associated with the Bemis advance, is followed by a systematic retreat from the study area and reorganization of the ice mass. A second advance is associated with the Algona margin. The onset of global warming, defined by Greenland Interstade 1 (GI-1), occurred soon after the Algona advance at ~ 14.7 ka cal BP. Rapid climate change caused widespread stagnation and ice retreat to the Big Stone margin at a rate of 250 meters/year. The scale of stagnation and resulting surficial landforms is a unique response to unprecedented global warming associated with the GI-1 and probably not representative of earlier DML ice behavior. The ages of proglacial lakes that bound the Big Stone moraine suggest the margin is a recessional feature and does not represent a third advance.
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    The Sedimentology and Petrology of the Upper Cambrian Mt. Simon, Eau Claire, and Galesville Formations in Southeastern Minnesota
    (1993-06) Churchill, Richard
    The Mt. Simon Sandstone, Eau Claire Formation, and the Galesville Sandstone comprise the Upper Cambrian Dresbachian Stage, and are found as subsurface strata in southeastern Minnesota with scattered small outcrops along the Mississippi and St. Croix Rivers. The formations are relatively flat-lying units located in a structural lowland termed the Hollandale Embayment. The Mt. Simon has an unconformable contact with the underlying Precambrian basement. The Eau Claire is conformable with the underlying Mt. Simon and the overlying Galesville. The Galesville has an unconformable contact with the overlying Ironton Sandstone of the Franconian Stage. The Dresbachian formations thicken from northwest to southeast direction across the state, cross-bedding has a south-southwest direction of dip. The Mt. Simon is primarily a medium- to coarse grained, moderately sorted, thick bedded orthoquartzitic sandstone with some feldspathic sandstone and shale laminae. The Eau Claire contains fine-grained, well-sorted, thin bedded orthoquartzites, arkoses, and quartzose arkoses, and laminated shales. The formation is very fossiliferous and glauconitic. The Galesville consists of medium-grained, well-sorted, thick bedded orthoquartzitic and feldspathic-quartzose sandstones. Petrology of the three formations is simple. The mineralogy is dominated by quartz with secondary feldspars. Mic, clay matrix, carbonate, pyrite, and feldspar cement, glauconite, and fossils are accessory components. Heavy minerals include zircon (dominant in most samples), tourmaline, garnet, rutile, apatite, and opaques, and are well-rounded for the most part. Fossils include trilobites, brachiopods, mollusca, and worm burrows. The Mt. Simon Sandstone, on the basis of lithology, grain size and sorting, bedding and cross-bedding, is a nearshore, highly turbulent, shallow water deposit. The Eau Claire Formation, on the basis of lithology, grain size and sorting, bedding and cross-bedding, and fossils and glauconite, is probably an offshore shelf deposit of quiet, relatively deep water. Characteristics, however, also resemble tidal flat deposits, which the formation may be in part. The Galesville Sandstone, on the basis of lithology, grain size and sorting, bedding and cross-bedding, is a nearshore deposit of turbulent, shallow waters.
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    Characterizing Provenance of the Late Wisconsinan Rainy Lobe Using Fine-Fraction Geochemistry and Detrital Zircon Geochronology
    (2023) Hinkemeyer, Audray
    Till of the Late Wisconsin Rainy lobe, which emanated from the Labradoran sector of the Laurentide ice sheet (LIS), is exposed at the surface from SW Minnesota to the extreme NE part of the State. The Rainy lobe advanced to its maximum limit in southwestern Minnesota well prior to the Last Glacial Maximum (ca. 27-30 ka BP) and retreated into Ontario by 17.9 ka BP. This till exhibits dramatic spatial and temporal changes in provenance from the Hewitt till of SW Minnesota to the Independence till in the NE. Two models have been proposed to explain the lithological differences (particularly carbonate) in Rainy Lobe tills. Goldstein (1989) postulated that the downglacier increase in carbonate in the Hewitt till was the result of progressive incorporation, by regelation or deformation, of older underlying till that was rich in carbonate. Larson (2008) concluded that the changes in sedimentology and landforms record systematic changes in provenance related to changing basal boundary conditions in the interior of the LIS. Early in this phase of glaciation, the sediments reflect long-distance transport from Hudson Bay, and later phases reflect increased proportions of felsic shield lithologies and Duluth Complex rocks with a corresponding decrease in carbonate. These two models of Rainy lobe till sedimentology are evaluated using mixing models, till matrix geochemistry, and detrital zircon geochronology. The multicomponent mixing model is developed to examine sedimentological variability by incorporation of older, underlying tills (e.g. Goldstein, 1989). The mixing model shows that the Hewitt till does not lie on the mixing curve, suggesting that mixing is not a viable model for the origin of the sedimentary variability in the Hewitt till. To evaluate the model of Larson (2008), which implies long vs. short transport distances, twenty-eight samples collected along a transect from SW to NE Minnesota, and eight samples collected from the Hudson Bay lowlands, were processed and sent for geochemical analysis. Fifteen of these samples were processed and analyzed for detrital zircon geochronology using laser-ablation, ICPMS. Results of a 48-element analytical suite were run through a principal component analysis. Factors 1 and 3 distinguished mafic vs felsic igneous rock geochemical signatures and carbonate content, respectively. Results show that Core SLL (Independence) plots positively on factor 1 indicating a short mean transport length. Core CSS (Hewitt, north Wadena drumlin field) in central MN represents an intermediate mean transport length, while core TG (Hewitt, south Wadena drumlin field) in far SW MN has the longest mean transport length. In addition, the samples with the longest transport length plot in high carbonate space with the calcareous Hudson Bay lowland samples, positive on Factor 3. A Kolmogorov-Smirnoff (K-S) and degree of likeness test were used to statistically compare detrital zircon age populations. Results from these statistical tests reveal that high carbonate Hudson Bay lowland ages are statistically similar to samples from central Minnesota (core CSS). Geochemistry and detrital zircon analyses support the model of Larson (2008). Early deposits of the Rainy lobe in SW Minnesota are geochemically similar to the high-carbonate Hudson Bay lowland samples, indicating a distal provenance. This similarity is also observed in the detrital zircon results from statistical analyses. Subsequently younger deposits lose the Hudson Bay lowland signature and start to incorporate more felsic craton and eventually mafic signatures of the Mid-Continent rift system of NE MN.
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    Design and application of a proxy system model for the quantitative reconstruction of hydroclimate variability recorded by oxygen isotopes in lacustrine carbonate sediment
    (2021-01) Fernandez, Alejandro
    Oxygen isotope analyses of lacustrine sediment, which are widely used as proxies of past climatic variability, have become increasingly reliant on computational modeling approaches that allow for quantitative interpretations of past hydroclimate, constraining of water resources’ sensitivities to changes in climate, and direct comparisons of proxy data with climate models. In this study, we present the development, structure and application of a Proxy System Model (PSM) designed for Castor Lake (Washington, U.S.A): a well-understood, highly monitored small lake system. The principal goal is to improve upon the understanding of the relationships between climate and the stable oxygen isotope (18O) proxy system in the context of lake sediments, by addressing the impacts that a variety of climate variables, as well as non-climate relate factors such as basin morphology, vegetation, hydrologic setting and lake mixing, have on the isotopic signatures of resulting sediments in the lake, as well as to provide a quantitative basis from which well-informed reconstructions of past climate can be made. Following a calibration process based on over a century of compiled daily weather data as well as approximately 14 years of in-situ continuous measurements of lake level, temperature and water oxygen isotope samples, the PSM was shown to accurately reproduce seasonal, interannual and century-scale trends of sediment oxygen isotope values and water balance, with varying degrees of accuracy for different timescales. Model-based reconstructions of hydroclimate variables for an early Holocene (~10000 years B.P.) δ18O maximum in the Castor Lake sediment record show that cold-season (i.e. winter) precipitation and relative humidity must have been lower by 21% ± 5% and 14% ± 7%,respectively, in order for the observed sediment δ18O signature to be produced and recorded. Furthermore, air temperature and warm-season precipitations seem to be negligible controls on sediment δ18O signatures, opposite to what was expected following the temperature dependence of carbonate sediment formation and isotopic fractionation. These results showcase the advantages of the application of PSMs to the analysis of paleoclimate proxy records as a way to make well-informed quantitative interpretations of past climate change through the constraining of physical, chemical and biological processes that impact the formation of the sedimentary archive.
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    Sedimentation in the Duluth-Superior Harbor, Lake Superior
    (1983-04) Barlaz, Dora Beth
    The sedimentary environment of the Duluth-Superior Harbor was investigated by examination of surface sediment samples, borehole stratigraphy, seismic reflection profiles, and aerial photographs. Harbor sediments are predominantly silt. Sand is found in a narrow band along the shores of the embayment, on the lower reaches of the St. Louis river bed, and at the base of constrictions on the floor of the dredged channels in Superior and St. Louis Bays. Lag is found upstream in the St. Louis River and in the ship entryways. Clay lies in isolated depressions on the harbor floor and in tributary embayments lining St. Louis Bay. Total organic carbon concentrations correlate well with grain size variation. Percentages range from 0.06 to 5 percent dry weight, with the highest concentrations occurring in the fine-grained sediments of the inner bays. Mineralogy of the harbor sediments is relatively uniform. The medium sand fraction is dominated by lithic fragments. Opaque minerals, amphiboles, and pyroxenes constitute the majority of heavy minerals in the fine fraction. In the clay-sized fraction, relative percentages of smectite, illite, kaolinite, and chlorite vary systematically with bulk-sediment textures. Kaolinite and chlorite are concentrated in the finer sediments, whereas illite is more abundant in coarser sediments. The distribution of surface sediment texture reflects exposure to currents produced by seiches, river currents, ship traffic and wind generated waves. Engineering borehole data and 3.5 kHz seismic reflection profiles were used to reconstruct stratigraphy and Holocene history of the area. The variation of sediment types records changing water levels and environments in western Lake Superior. Boreholes 10 to 61 m deep contain 7 lithologic units: till, glacial outwash sand, glaciolacustrine clay, postglacial peat, silt, clay, and nearshore sand. Seismic profiles contain 3 main reflectors which roughly correspond to the lithologic units observed in borehole logs. The two sets of spits outlining the harbor are Holocene features, possibly formed by emergence of offshore bars, submergence of coastline ridges, or spit progradation by longshore currents or "self generation". Post-formational modifications of the spits include breaching of inlets and inland sediment transfer by wind and water. Numerous comparisons can be drawn between the St. Louis River estuary and coastal estuaries. The St. Louis River estuary is small in comparison to marine embayments, having boundaries determined by the geology of the region. Marine estuaries predate the St. Louis River embayment by a few thousand years, inundated in response to glacial melting and isostatic rebound, respectively. Circulation in a marine estuary, driven by tidal currents and wind, is based on density stratification caused by the mixing of saline and fresh water. In the freshwater St. Louis River estuary, there is no density stratification, and periodic currents are driven more by seiches than tides. Shoreline changes over the past 120 years were described from historic maps and aerial photographs. Development of the shoreline and alteration of the environment through dredging have been accompanied by a rising water level on the southwestern shore of Lake Superior.
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    Surficial Geology of the Isabella Quadrangle, Northeastern Minnesota
    (1981-03) Friedman, Albert Louis
    A surficial-geologic map was constructed from field work, laboratory analysis, airphotos, and topographic maps. The study area is located in Lake County in northeastern Minnesota. Parts of the Isabella Quadrangle lie within the Toimi Drumlin Area, Border Lakes, Area, and the North Shore Highland. Bedrock in the surrounding areas is composed of Duluth Complex plutonic rocks, the North Shore Volcanic Group, Superior syncline elastics, and Vermilion District intrusive and metamorphic rocks. Surficial materials were distinguished as to genesis and provenance. Two distinct provenance groups were identified: Brown-colored drift incorportating rock fragments from the Superior syncline, North Shore Volcanic Group, and part of the Duluth Complex; Gray-colored drift incorporating rock fragments almost entirely from the Duluth Complex with minor Vermilion District input. Surficial materials were divided into genetic units a.s follows: Till, Ice-Marginal Gravels, Other Ice-Contact Deposits, Proglacial Outwash, Lag Acumulations, Loess, and Peat Deposits. The most prominent landforms are the merging Highland and Vermilion Moraines composed primarily of brown and gray ice-marginal gravels. Additional minor brown-till end moraines are situated in front of the Highland Moraine, and are superposed on a gray-till ground moraine that represents the northeast corner of the Toimi Drumlin Area. These minor moraines are in turn partially buried by the Vermilion Moraine. The area behind the Vermilion Moraine is characterized by extensive outwash complexes, a long esker system, and a series of parallel ridges composed of gray till. Lakes have formed in outwash-plain depressions that resulted from wastage or stagnantice blocks. The non-organic surficial material.s in the area were deposited during the St. Croix (ca.>20,500 yr B.P.) and Automba (ca.>16,000 yr B. P.) pha.se of the late Wisconsin. Conterminous Rainy and Superior Lobes emanating from the Rainy Lake area and Superior Lowland deposited the materials composing the Toimi Drumlin Area during the St. Croix phase. During the Automba phase the Superior Lobe advanced laterally out of the Superior Lowland in the southeast while the Rainy Lobe flowed southwestward into the study area. The ice masses met near the town of Isabella, producing interlobate terminal moraines. A sub lobe of the Superior Lobe extended beyond the Highland Moraine, producing the minor brown-till end moraines. As this sublobe receded, part of the Rainy Lobe extended into the vacated area, forming a sharp bend in the Vermilion Moraine. This extension is described as an ephemeral, thinned, frozen-base ice mass that sheared off sections of the substrate into thrust-block ridges. The extension stagnated and disintegrated rapidly as the Rainy Lobe receded, forming extensive ice-contact and outwash deposits. Loess was winnowed from exposed brown-drift outwash plains and deposited as a thin, discontinuous blanket over the entire region. Extensive wetlands formed in poorly-drained depressions and other lowlying areas.
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    Development of Archean Lode-Gold and Massive Sulfide Deposit Exploration Models using Geographic Information System Applications: Targeting Mineral Exploration in Northeastern Minnesota from Analysis of Analog Canadian Mining Camps (Volumes I-III)
    (2001-12) Peterson, Dean M
    Detailed lode-gold (LG) and volcanogenic massive sulfide (VMS) deposit mineral potential maps have been developed from new mapping and compilation of a 2270 mi2 area of the Late Archean Wawa Subprovince of the Superior Province in northern Minnesota. The mineral potential maps have been developed by the integration of ore deposit models for lode-gold and volcanogenic massive sulfide deposits into an exhaustive geological, geochemical, and geophysical Geographical Information System (GIS) data compilation of the study area. In addition, detailed GIS geological compilations from the three largest lodegold mining camps of the Superior Province of Canada (the Hemlo, Timmins, and Kirkland Lake mining camps) have been completed, and are incorporated in the lode-gold mineral potential model. Methods used to predict mineral potential include both knowledge-driven (LG and VMS models) and data-driven (LG only) analysis of information derived from the new GIS geologic map compilations, and from databases of geochemical and geophysical data for the Minnesota study area. The mineral potential analysis includes the use of fuzzy logic techniques in ranking the importance of specific types of information derived from the ore deposit models. In addition, fuzzy logic techniques have been used in the digital overlay of hundreds of maps portraying specific geologic data, into the final LG and VMS maps showing base- and precious-metal mineral potential of the Minnesota study area.
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    Geology and Sulfide Mineralization of the Duluth Complex - Virginia Formation Contact Minnamax Deposit, St. Louis County, Minnesota
    (1980-08) Matlack, William Fuller
    The Minnamax deposit, near Babbitt, Minnesota, is a large, low-grade magmatic iron-copper-nickel sulfide deposit at the contact of the Duluth Complex and the Virginia Formation. The Virginia Formation consists of pelitic hornfels with locally abundant calc-silicate pods and contains diabase intrusions - all have been deformed and metamorphosed to the pyroxene hornfels facies by the Duluth Complex. The Duluth Complex, emplaced as a crystal mush, consists of sulfide-bearing troctolitic rocks, commonly noritic near the contact, which contain barren gabbro to peridotite xenoliths. The Duluth Complex - Virginia Formation contact is highly irregular, characterized by numerous Duluth Complex apophyses and Virginia Formation xenoliths. Granite to diorite dikes cut all lithologies. Sulfide mineralization, consisting of pyrrhotite, pentlandite, chalcopyrite, and cubanite, is primarily disseminated in the troctolitic rocks within 1000 feet of the contact. Sulfide veins fill fractures and breccia zones in both the Duluth Complex and the Virginia Formation. Pelitic hornfels adjacent to the veins and the sulfide-bearing troctolitic rocks commonly is replaced by sulfides. In one area the veins are sufficiently concentrated to form a high-grade sulfide body, the Local Boy deposit. The veins probably formed by filter-pressing of sulfide liquid from the troctolitic rocks into fractures. Some distinctly chalcopyrite-cubanite rich veins appear to have formed by separation of a copper-rich liquid from a pyrrhotite solid solution at magmatic .temperatures. Late stage hydrothermal solutions deposited sulfides and gangue along fractures and in calc-silicate pods.
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    Movement of titanium across the Duluth Complex - Biwabik Iron Formation Contact at Dunka Pit, Mesabi Iron Range, northeastern Minnesota
    (1993-08) Muhich, Thomas G
    In the north half of the Dunka open pit iron ore mine (T60N-T61N, R12W) of northeastern Minnesota, the Biwabik Iron Formation (BIF) is in direct contact with the South Kawishiwi Intrusion (SKI) of the Duluth Complex. Progressive contact metamorphism of the BIF originally was thought to be isochemical (Bonnichsen, 1968), however this study presents evidence that titanium was introduced into the BIF from the SKI. Field and petrographic observations show that within 25 feet of the SKI the BIF has been highly altered. Textures include: (1) ilmenite intergrown in magnetite layers of the BIF, (2) layers of plagioclase, clinopyroxene, orthopyroxene and other commonly magmatic minerals in the BIF, and (3) symplectite of plagioclase-orthopyroxene and orthopyroxene- Fe-Ti oxide. Ilmenite forms partial pseudomorphs after magnetite in the altered BIF, as well as lamellae and rims around magnetite. Layers of magmatic minerals which are not found elsewhere in the iron formation in any abundance appear to be intergrown in magmatic textures. Within 25 feet of the iron formation, the SKI is also considered altered on the basis of: (1) increased amounts of magnetite, clinopyroxene, orthopyroxene, and amphibole, (2) loss of olivine, and (3) increased amounts of symplectite of plagioclase-orthopyroxene and orthopyroxene-Fe-Ti oxide. Magnetite in the altered SKI occurs in similar textures to the most altered BIF, with ilmenite partially forming pseudomorphs after magnetite. Loss of olivine and gain of pyroxene may be explained by the assimilation of quartz, which reacted with the olivine in the SKI magma to form pyroxene. Symplectites of orthopyroxene-plagioclase and orthopyroxene-Fe-Ti oxide are also present in the unaltered SKI, but in the altered SKI their abundance is much greater. Magnetite from altered BIF is higher in TiO2 (3.15 wt.% on average) than that in unaltered BIF (0.29 wt.% on average). Composition of titaniferous magnetite and ilmenite from both the altered BIF and SKI were used to calculate values of temperature of metamorphism/ intrusion and fO2 using the QUILF PASCAL program (Andersen, Linds.ley and Davidson, 1992). Values on a log fO2 - temperature plot appear above the fayalite-magnetite-quartz buffer curve, in the range of 550°C to 689°C and -15.2 to -18.8 log fO2 for both the most altered SKI and BIF. Whole rock analyses of altered BIF show gains of TiO2, V, Al2O3, CaO, Na2O, K2O, Ba, Rb, Sr, MgO, Cu, Ni, and H2O content, and loss of SiO2 and P2O5 compared to altered BIF. Present elevated amounts of ilmenite, plagioclase, sulfides, and actinolite correlate with these gains. Loss of SiO2 from the altered BIF correlates with the loss of quartz from the most altered BIF via assimilation into the SKI. Chemical components gained to the altered SKI include K2O, Rb, S, Fe2O3, and H2O. Gains in K2O, Rb and H2O correlate with increased biotite and amphibole content in the most altered SKI, especially hornblende, which is hydrous and contains some potassium. Ferric iron gained to the most altered SKI represents a change in oxidation state of the magma due in part to assimilation of some of the magnetite in the iron formation. Loss of TiO2 and MnO appear to be related to the formation of ilmenite in the most altered BIF. Oxygen isotope studies show that the altered BIF contains quartz which is lower in δ18O (δ18O = 10.82 parts per thousand) than quartz in altered BIF (δ18O = 15.21 parts per thousand, Perry and Bonnichsen, 1966). The altered SKI shows greater δ18O (7.00 to 9.40 parts per thousand) than the altered SKI (5.94 to 6.57 parts per thousand). Fingers of SKI magma assimilated quartz from the BIF, enriching the altered SKI in 18O, K2O, Fe2O3 , and LOI, whereas depleting the BIF of SiO2. Layers of magnetite in the altered BIF have been enriched in Ti, while preserving the original layering, indicating some diffusion or infiltration of titanium into the magnetite has occurred. Combination of infiltration of "fingers" of SKI magma into the BIF, and enrichment of BIF magnetite in titanium occurs only where the SKI is in direct contact with the BIF. None of the above textures, changes in mineralogy, or titanium enrichment occur where the Virginia Formation lies between the Duluth Complex and the Biwabik Iron Formation. Thus, textures, changes in mineralogy, and titanium enrichment in the most altered BIF must be caused by direct contact with the SKI of the Duluth Complex.
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    Stratigraphy and Hydrothermal Alteration of Archean Volcanic Rocks at the Headway-Coulee Massive Sulfide Prospect, Northern Onaman Lake Area, Northwestern Ontario
    (1985-10) Osterberg, Steven Arvid
    The Headway-Coulee massive sulfide prospect of northwestern Ontario is situated within the Superior Province of the Canadian Shield. Rocks at the prospect form part of the Archean Wabigoon greenstone belt and consist of an intensely hydrothermally altered succession of mafic and felsic volcanic and intrusive rocks. Subaqueously deposited pillowed and amygdaloidal to massive and autobrecciated mafic lava flows form a 1-2 km thick succession which is locally interlayered with, and overlies a thin sequence of felsic volcanic rocks. The felsic volcanic rocks are laterally limited (2 km) and are composed dominantly of bedded ash tuffs capped by massive to brecciated and flow-banded lavas. The tuffs are fine-grained, generally fragment-poor, and vary from laminated to thickly-bedded. An extensive polymictic diamictite deposit, which contains clasts of granite, mafic and felsic volcanic rocks, and iron formation, is interlayered with the felsic 1olcanic rocks and is believed to represent a debris flow deposit which had its source to the southwest of the study area. Based on their fine-grain size, limited lateral extent, and thin to thickly-bedded nature, the felsic tuffs are interpreted to be products of hydrovolcanic eruptions. Based on stratigraphic relationships the deposits are believed to have formed on the submerged flanks of two adjacent tuff cones. It is envisioned that the capping felsic lavas formed either under low water/magma ratio conditions as access of water to the erupting magma was restricted, and/or under high water/magma ratio conditions within a water flooded vent or on the submerged flanks of the cones. The majority of the volcanic rocks were intensely altered by hydrothermal solutions during the waning stages of felsic volcanism. Alteration in the rocks is relatively widespread and is subconcordant to stratigraphically conformable in distribution. The altered rocks have been subdivided into four distinct mineral zones. The zones, in order of formation and increasing alteration intensity, are: (1) least altered, (2) quartz-sericite, (3) iron chlorite, and (4) chloritoid. The progressive alteration of the rocks was studied by mass balance comparisons of the altered rocks and their less intensely altered, stratigraphic equivalents. These comparisons indicate that Al was generally immobile, and that volume losses during alteration range from 0 to approximately 50%; the largest volume losses occurred during alteration of the felsic ash tuffs. Major chemical trends involved in alteration of the rocks include large gains in K and loss of Na during sericitization, and generally addition of Fe, and loss of Ca and Na during formation of iron chlorite and subsequent development of the chloritoid alteration type. Based on the distribution of the alteration types as well as the alteration mineralogy and chemistry it is proposed that, by shallow circulation through porous volcanic rocks, an acidic, K-rich fluid evolved and caused widespread sericitization within the study area. Deeper circulation evolved an Fe-rich fluid which was discharged along synvolcanic faults from a pressurized reservoir at depth. The solution chemically reacted with the sericitized rocks to produce the iron chlorite assemblage, and the pre-metamorphic equivalent of the chloritoid assemblage. The chloritoid assemblage developed as pre-metamorphic, coexisting iron chlorite + hydrous Al--silicate became unstable and reacted to form chloritoid during regional greenschist facies metamorphism.
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    The Ore Petrology and Structural Geology of the Lower Precambrian Deer Lake Mafic-Ultramafic Complex, Effie, Itasca County, Minnesota
    (1973-06-30) Ripley, Edward Michael
    The Deer Lake Complex is a 13 km long by 3 km wide belt of Lower Precambrian ultramafic and gabbroic rocks located near the town of Effie, Itasca County, Minnesota. Two layered sills, averaging 200 m in thickness, a 122 m gabbroic sill, and two ultramafic lenses ranging in thickness from 15 to 40 m have been investigated by detailed mapping and drill core investigation. The igneous bodies are intrusive into a sequence containing metagraywackes, slates, argillites and mafic metavolcanic rocks. The volcanogenic sequence overlying the sills is on the order of 400 m in total thickness. Metamorphism of the Complex and associated host rocks has produced mineral assemblages of the greenschist facies. The layered sills contain a succession of basal chilled margin, peridotite, orthopyroxene clinopyroxenite, porphyritic two-pyroxene gabbro, nonporphyritic gabbro, quartz diorite cap rock, and upper chilled margin. The general structure of the sills and their petrographic and chemical characteristics indicate that they are differientiated from a basaltic magma by in situ gravitational mineral accumulation and subsequent nonaccumulative crystallization. Phase chemistry studies utilizing mineral and trace metal compositions from the layered sills of the Conplex indicate that their initial temperature of crystallization was near 1010 C. Sulfide and primary oxide phases are disseminated throughout the layered sills. Textural evidence and phase relations indicate that sulfide and oxide minerals within basal chilled margins formed from an immiscible sulfide-oxide liquid which segregated prior to sill emplacement. Nickel-rich sulfide minerals are concentrated in the lower portions of basal chilled margins with copper sulfide and oxide minerals located in overlying chilled units. Phase relations indicate that sulfide-oxide crystallization began at approximately 1070° C and ended about 960° C. Re-equilibration within sulfide phases occurred at temperatures below 600° C. The primary sulfide phases in the differentiated portions of layered sills range from nickel-rich in peridotite units through copper-rich at intermediate levels to iron-rich in upper layered units. Textural evidence and phase relations suggest that sulfides in peridotite and clinopyroxenite units crystallized from an immiscible sulfide-oxide liquid which segregated during silicate differentiation, whereas sulfides and oxides in upper gabbro units within the layered intrusives crystallized directly from the silicate magma in response to an increased oxygen activity. Textural features in quartz diorite cap rocks indicate that sulfides replaced silicate phases. The Complex has undergone Lower Precambrian isoclinal folding, with a maximum principal stress orientation of N45W. Gravity faulting followed as a result of stress relaxation producing grabens. Subsequent strike-slip faulting along a N40E trend resulted in 1100 m right lateral separation of Complex units. The final phase of observed deformation in the Deer Lake area is evidenced by an additional 100 m of movement of Middle Precambrian dikes along the strike-slip faults. Five characteristics commonly used to evaluate Ni-Cu sulfide ore bearing ultramafic rocks indicate that the Deer Lake Complex has little ore potential. These characteristics are: 1) the presence of layering in the differentiated sills; 2) the Fe-rich, Mg-poor nature of the parent magmas which produced the Deer Lake layered sills and ultramafic lenses; 3) the low nickel content of peridotite units and olivines at Deer Lake; 4) the absence of pyrite in sulfide assemblages of chilled margins and peridotites and 5) the low metal content of sulfide phases in ultramafic zones and chilled margins.
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    Stratigraphy and Hydrothermal Alteration of the Gagne Lake Prospect: An Occurrence of Volcanogenic-Type Massive Sulfides Near Mine Centre, Northwestem Ontario, Canada
    (1987-09) Davis, Douglas Stuart
    East-west trending, steeply dipping (85° +), overturned, Archean-age volcanic, sedimentary and intrusive rocks (sills) of predominantly greenschist-amphibolite transition facies grade are exposed in the Wabigoon Subprovince of northwestem Ontario, Canada approximately 1.5 km southeast of Gagne Lake. Volcanic rocks associated with the Gagne Lake prospect, a showing of volcanogenic massive sulfide-type mineralization (chalcopyrite, sphalerite, pyrite and galena), are primarily rhyolitic lava flows and pyroclastic (hydrovolcanic) rocks. The pyroclastic rocks serve as the host rock for the prospect. Mafic lava flows are interlayered with the felsic volcanics but constitute only a minor portion of the stratigraphy. Tonalitic and mafic sills comprise nearly 50% of the stratigraphy and range in time of emplacement from prior to formation of the Gagne Lake prospect to after intrusion of the Little Ottertail Lake Stock. Based on preserved primary textures and structures, the volcanic succession is thought to have formed under both subaerial and subaqueous conditions. Regional stratigraphic relationships suggest that the succession at one time formed part of an emergent volcanic island. Volcanic rocks and tonalitic sills underlying the prospect have been variably altered by hydrothermal solutions. Distribution and geochemistry of the altered rocks is such that four alteration assemblages can be defined: 1) least altered assemblage, 2) sericite (biotite)- chlorite-iron carbonate assemblage, 3) actinolite-chlorite-epidote assemblage and 4) dalmatianite (sericite, chlorite, iron carbonate). The alteration assemblages delimit a concentrically zoned alteration pipe below the prospect in which actinolite-rich rocks are enveloped by sericite or biotite-rich rocks and a stratigraphically semi-conformable zone of sericitc alteration within the hydrovolcanic rocks. A relatively small zone of dalmatianite (spotted alteration) envelopes the Gagne Lake prospect. Crosscutting relationships indicate that actinolite-rich rocks and dalmatianite formed at the expense of sericite and/or biotite-rich rocks. Alteration assemblages are believed to have formed by the circulation of hydrothermal solutions through the volcanic succession. Shallow circulating sea water reacted with felsic rocks and evolved into an acidic, potassium-rich brine. Reactions between this solution and felsic rocks in the field area produced the sericite/biotite-rich rocks by addition of potassium and magnesium to and leaching of calcium and sodium from the rocks. Deeper circulating solutions encountered mafic rocks at depth. Reactions between these fluids and the rocks produced a solution enriched in calcium, magnesium and iron. With ascent, this brine encountered sericite and biotite-rich rocks of the study area. The resulting reactions produced actinolite-rich rocks by addition of calcium, magnesium and iron to and leaching of potassium from the rocks. As this solution mixed with sea water near the water-rock interface, it became enriched in magnesium. Reactions between this magnesium-rich solution and sericite-rich rocks produced a chlorite-quartz alteration assemblage that became dalmatianite during prograde metamorphism.
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    Drift Lithology in Relation to Bedrock Geology, Long Island Lake Quadrangle, Cook County, Minnesota
    (1977-06) Everson, Curtis I
    Lithologic studies in northeastern Minnesota suggest that drift prospecting is a useful tool for mapping drift-covered bedrock. A detailed study of till clasts composition in the Long Island Lake Quadrangle revealed a significant relationship between drift lithology and bedrock geology. The Long Island Lake Quadrangle is a suitable area for this study for the following reasons: (1) outcrops are numerous enough to have allowed the construction of a detailed geologic map; (2) the area contains eight distinctive rock units; (3) the local bedrock experienced glacial erosion, indicated by the existence of glacially abraded and quarried outcrops. The distribution of glacial sediments, mainly till and outwash, were mapped and one hundred and one samples of drift were collected along traverses parallel to ice flow (perpendicular to strike of the bedrock). Both till and outwash contain a large quantity of local bedrock clasts in the size ranges greater than 1mm in diameter. Clasts smaller than 1mm are mainly minerals, and therefore not so diagnostic of local bedrock. As a test, boulders greater than 1 meter in diameter were used in the field for inferring bedrock contacts. These contacts were found to be within 60 meters (200 ft.) of contacts placed by outcrop mapping. Lack of local bedrock clasts in the smaller size fractions indicate either high resistance of local bedrock to crushing, or lack of opportunity for crushing because of short residence time in the glacial system (short distance transport). In either case, the fine-grained fraction therefore represents a contribution to the glacial load from more distant sources and the coarse-grained fraction represents a contribution form local sources.
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    Stratigraphy and Alteration of the Footwall Volcanic Rocks beneath the Archean Mattabi Massive Sulfide Deposit, Sturgeon Lake, Ontario
    (1984-03) Groves, David Alan
    Subaqueous mafic lava flows and breccias, mafic debris-flow and felsic pyroclastic-flow deposits, and felsic lavas form a 2 km thick succession beneath the Archean Mattabi massive sulfide deposit in northwestern Ontario. The lowermost 500 m is composed of massive amygdaloidal mafic flows, flow breccias, and heterolithic debris flows. Thin (<50m) amygdaloidal felsic lava flows and felsic block and ash deposits overlie the basal mafic flow sequence. This felsic horizon thickens both eastward and westward away from the Mattabi deposit and suggests the former existence of localized felsic vents on a broad shield volcano. Rocks interpreted to be mafic debris-flow and felsic pyroclastic-flow deposits lie above the felsic horizon and represent a change in eruptive style from lava extrusion to phreatomagmatic volcanism. The change is believed to be a result of a shallowing upward sequence and/or an increasing water/magma ratio. The mafic debris-flow deposits are massive to thick-bedded, poorly graded and composed of scoriaceous to amygdaloidal mafic clasts. Felsic pyroclastic-flow deposits include a) massive basal beds and overlying bedded ash tuff, b) well-bedded., graded lapilli tuff and c) massive pumice-rich beds. Felsic pyroclastic deposits intercalate and intermix with mafic debris flow deposits west of Mattabi and together these constitute the upper 500-600 m of the footwall succession. Massive pyroclastic beds truncate mafic debris-flow deposits and mark the culmination of explosive felsic volcanism prior to the ore-forming event. Massive pumiceous pyroclastic beds and quartz-porphyritic ash-flow tuff form the immediate mine footwall strata. Alteration within the footwall strata has been divided into four major mineralogical assemblages: 1) least altered (typical greenschist facies assemblages with moderate carbonatization), 2) iron carbonate-chlorite, 3) sericite and 4) chloritoid. Least-altered assemblage rocks are largely amygdaloidal mafic lava flows and mafic debris-flow deposits which are situated 4 to 5 km west of the Mattabi deposit. Iron carbonate-chlorite alteration is confined largely to felsic pyroclastic rocks and lavas within the upper 600 m of the footwall strata; sericitization is also widespread within these rocks. Chloritoid is developed in both sericite and iron carbonate-chlorite assemblage rocks. Mass balance computations indicate that constant volume has been maintained within all altered lithologies except iron carbonate-chlorite assemblages within felsic lavas; these rocks have undergone a 10 to 20% volume reduction. Iron carbonate-chlorite assemblage rocks display elemental gains of Fe, Mn and CO2, and losses of Si. Sericitization produces marked K and Rb gains at the expense of Na. Comparisons of iron carbonate-chlorite and sericite-assemblage rocks to similar chloritoid-bearing equivalents reveal no consistent elemental trends. It is envisioned that heated connate seawater/rock interactions within mafic lava flows and breccias produced a large reservoir of metal-rich hydrothermal solutions. Synvolcanic faulting allowed the rapid discharge of fluids from the reservoir. Diffuse, semiconformable alteration zones were developed in overlying felsic pyroclastic rocks and lavas as the solutions migrated upward to the seafloor surface. Focused discharge of fluids at several locations resulted in large-scale precipitation of iron sulfides on the seafloor.
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    Early Precambrian Bedrock Geology of the Northwest Angle, Lake of the Woods County, Minnesota
    (1979-07) Zamzow, Craig Edward
    The Northwest Angle is located in northwestern Minnesota, bounded by the Lake of the Woods, Manitoba, and Ontario. About 4 km2 of outcrop scattered over 250 km2 area was investigated, during ten weeks of field work. The Northwest Angle contains four major rock units: a supracrustal unit; a tonalitic unit; a granitic unit; and a mafic dike unit. The supracrustal rocks show evidence of amphibolite prograde metamorphism; they are well foliated, and lineated. The foliation is given by compositional banding while the lineation is caused by the alignment of hornblende prisms in the plane of foliation. Tonalitic rocks show evidence of amphibolite grade metamorphism, the degree of foliation is variable. The granitic and mafic dike rocks show evidence of deuteric alteration; most outcrop areas contain massive rock, but two of the granite outcrop areas, both in the northwest part of the peninsula are foliated. The supracrustal rocks appear to be composed of intermediate-mafic, calc-alkalic and tholeiitic volcanics mafic to ultra mafic hypabysal intrusions, and sediments which are mineralogically similar to the volcanics. The plutonic rocks of the area show varied metamorphic effects, but maintain an igneous texture overall. The structure of the area is complex. The predominate structural grain is northeast - southwest and is produced by foliations found in the supracrustal, and tonalitic units. The granite and supracrustal rocks which crop out in the northwest part of the peninsula have foliations trending northwest to southeast. It appears that northeast trending isoclinal folding of the supracrustal rocks is responsible for the northeast trending structural grain; northwest trending structural features are attributed to detachment, or rotation of large crustal blocks, during plutonic emplacement. The apparent order of geologic events is: (1) deposition of supracrustal rocks; (2) isoclinal folding of supracrustal rocks along a northeast trend; (3) emplacement of tonalitic intrusives; (4) emplacement of granitic intrusives; (5) emplacement of large mafic dikes. Isoclinal folding of the supracrustal rocks may be comtemporaneous with tonalitic emplacement.
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    Geology of a Lower Proterozoic Volcaniclastic Sequence near Wausau, Marathon County, Wisconsin
    (1986-06) Reichhoff, Colin Lee
    A sequence of Lower Proterozoic (1859 m.y.) volcanic and volcaniclastic rocks is exposed in the vicinity of Brokaw, north of Wausau, in Marathon County, Wisconsin. These rocks were mapped in detail and sampled for petrographic study. This sequence has been metamorphosed to lower greenschist facies and is only mildly deformed, dipping to the west at 10-30°. Based on rock type, the study area can be divided into three segments. The easternmost and westernmost segments are poorly exposed; they consist mainly of intermediate to felsic lava flows and pyroclastic rocks. Basaltic rocks are only minor components of these two segments. The central segment contains the best exposures, and consists mainly of sedimentary and pyroclastic rocks. The stratigraphy of this segment, from oldest to youngest, consists of red sandstones and pebble conglomerates, thinly bedded siltstones and tuffs, pyroclastic deposits and greenish-black sandstones and conglomerates. The greenish-black and red sandstones and conglomerates are interpreted as fluvial sediments deposited by a braided river system. Paleocurrent study of the greenish-black sandstones indicates the sediment source was to the E-SE. The red sandstones are composed mainly of felsic volcanic rock fragments and quartz with only small amounts of plagioclase; plutonic rock fragments are very minor components. The greenish-black sandstones contain more felsic to intermediate volcanic rock fragments and roughly equal amounts of quartz and plagioclase; plutonic rock fragments and quartzite grains represent 1-3% of these sediments. The modes of the two sandstone units, when plotted on QFL and QmFLt diagrams (after Dickinson and Suczek, 1979) indicate the sediment was derived from a magmatic arc-type setting. An increase in the percentage of plutonic and quartzite grains in the greenish-black sandstones, compared to the red sandstones, suggests an increased depth of erosion and the input of material from outside the magmatic arc. The thinly bedded siltstones and tuffs represent lacustrine sediments associated with the fluvial system. Three pyroclastic units are present in the southern half of the central segment, and are apparently unconformably overlain by the greenish-black sandstones. These pyroclastic units overlie the thinly bedded siltstone and tuff unit and include; a unit varying from lithic-rich at its base to crystal-rich near its top, a block-and-ash flow, and a welded dacite tuff. The association of fluvial and lacustrine deposits and the dominance of ash and lapilli tuffs, with only minor pyroclastic breccias, suggests this sequence represents an intermediate-source facies (Fisher and Schmincke, 1984) within the volcanic field. The intermediate-source facias is comparable to the dispersal facies of Dickinson (1974) and is found at distances > 5 km from the central vent. The depositional setting for this sequence of rocks is interpreted as a small restricted basin within a continental margin magmatic arc (intra-arc basin). These rocks were deposited following the main phase of deformation associated with the Penokean Orogeny.