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Eskers and Heavy Mineral Prospecting, Northeastern Minnesota

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Eskers and Heavy Mineral Prospecting, Northeastern Minnesota ESKERS AND HEAVY MINERAL PROSPECTING, NORTHEASTERN MINNESOTA A THESIS SUBMITTED TO THE FAOJLTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY Tir-IYI'HY REED BRcmN IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE JANUARY, 1988 ABSTRACT Mineral exploration in heavily glaciated terrain is difficult because a mantle of glacial drift generally covers the bedrock. Eskers and the heavy minerals they contain were studied to (1) determine their potential use as a prospecting tool, and (2) better understand the dynamics of esker formation. Studies emphasized paleohydraulics, origin of esker sediment, lithology, and mineralogy. 'IWo main types of eskers are recognized in northeastern Minnesota on the basis of their morphology, sedimentology, and origin. Continuous eskers form as a single, continuous segment when an ice sheet is stagnant, or nearly stagnant. Meltwater flowing through an ice tunnel deposits sediment in both the tunnel and onto an adjacent outwash pJ.ain. Melting of the tunnel walls induces inflow of the ambient ice which leads to the influx of rock debris. Seasonal fluctuations of meltwater discharge leave a sedimentary sequence characterized by a wide variety of grain sizes (medium-grained sand to boulders) and bedforrns (climbing ripples, massive beds, and planar, trough, and graded cross beds). Be2ded eskers are a series of consecutive segments, or beads, that are separated by swamps or lakes, and whose trends follow the low ground between the drumlins of the Toimi Drumlin Field. A limited range of bedforrns (massive beds and trough cross beds) and grain sizes (mostly bouJders) are exposed in the beaded eskers. Other sedimentary features may be present, but they are covered by the surrounding swamp deposits and not exposed. i Individual segments of the beaded esker system probably form at the margin of an actively retreating ice sheet as an annual deposition event. Average distances between successive bead crests imply an annual retreat rate of 1200 feet (365 m) per year. Stone counts, X-ray diffraction, and petrographic examinations of heavy mineral mounts were evaluated as drift prospecting tools. Similar results between stone counts in eskers and in the surrounding till indicate that the esker material is most likely derived from the basal ice debris. Concentration of the heavy mineral population by fluvial processes makes eskers and outwash better locations for sampling than till. A small split from each of the 97 bulk samples was analyzed by X-ray As no more than 5 or 6 minerals could be identified from the charts, it does not appear to be an efficient method for prospecting. In contrast, petrographic examination of heavy mineral mounts gave a much better indication of minerals present. However, no economic indicator minerals were identified with either method. The petrografhic studies were useful in differentiating glaciofluvial material deposited by the various lobes of ice. The St. Louis sublobe deposits, with a higher percentage of garnets, can be distinguished from the Rainy lobe and Superior lobe deposits, whose heavy mineral populations reflect the mafic bedrock over which the ice moved. ii ACKNCWLEDGMENTS A number of people helped make this thesis i::ossible but most of the credit goes to my wife, Val, who provided unlimited supp:>rt and encouragement during all stages of the work. I would like to thank my main advisor, Dr. Charlie Matsch, for his numerous suggestions and valuable input. Financial supp:>rt was partially provided by the Minnesota Department of Natural Resources, who originally suggested the project. Geologists Dennis Martin and Leon Gladen of the DNR are also thanked for their time and interest in the project. The suggestions by my thesis committee members, Drs. Ojakangas, Darby, and Schimpf, also appreciated. iii ESKERS AND HEAVY MINERAL PROSPECTING, NORTHEASTERN MINNESOTA Table of Contents ABSTRA.CI' ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 1 ••••••••••••••••••••••••••••••••••••••••••••••••••••••• 1 11 TABLE OF CONTENTS ••••••••••••••••••••••••••••••••••••••••••••••••••••••• iv LIST OF ILLUSTRA.TIONS ••••••••••••••••••••••••••••••••••••••••••••••••••• vi LIST OF FIGlJR.ES ••••••••••••••••••••••••••••••••••••••••••••••••••••••••• l..li LIST OF TABLES ••••••••••••••••••••••••••••••••••••••••••••••••••••••••• Vl 1 LIST OF PIATES ••••••••••••••••••••••••••••••••••••••••••••••••••••••••v111 I • I'N'rn.ODUCTION ••••••••••••••••••••••••••••••••••••••••••••••••••••• 1 Gener al s t. .. .1 I.oc a ti on ••••••••••• ·• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • . • • • . • • . • . • 2 Field me thOOs •••••••••••••••••••••••••••••••••••••••••••••••••• 4 Laboratory methods. ••••• 7 I I. REG GEOI.i()(;Y ••••••••••••••••••••••••••••••••••••••••••••••••• 8 Bedrock li thology •••••••••••••••••••••.••.•.•••••••••••••••.••. 8 Bedrock topography ••••••••••••••••••••••••••••••••••••••.••••• 11 Quaternary geology •••••••••••••••••••••••••••••.•••••••••••••• 12 III .ORIGIN OF ESKERS •••••••••••••••••••••••••••••••••••••••••••••••• 17 C-ener al s tate:rrent •••••••••••••••••••.•.•....•••..•.••••.•••••• 17 Paleohydr auli cs ••••••••••••••••••••••••••••••••••••••••••••••• 18 Sununa ry • • • • . • • . • . • • • • • • • • • • • • • • • • • • • • • • • • • • . • • • . • • • • • • • • • • • 2 3 N. ESKER TYPES IN MINNESOTA •••••••••••••••••••••••••••••••••••••••• 2 4 Gener al s .•••••••••••••••••••••••••••••••••••••••••••• 24 Continuous eskers •••••••••.•••••••.••••••••••••••••••••••••••• 24 Char acteri sti cs •••••••••••••...••.••••••••••••••••••••.••• 24 The Clcx:.Juet esker ••••••••••••••••••••••••••••••••••••••••••••• 26 I..c:>cation and morphology ••••••••••••••••••••••••••••••••••• 26 Sedi mentology •••••.•••.•••••••••.••.•••.••••••••••••.•.••• 27 Li tho logy ••••••••••••••••••••••••••••••••••••••••••••••••. 3 2 Origin of esker sediment ••••••••••••••••••••••••••.•.••••• 32 Sununary ••••••••••••••••••••••••••••••••••••••••••••••••••• 3 8 Be c3Cied e ske rs .•••••••.••••••••.•••.••••••.•.• ••••••••••• 40 Char acteri sti cs •••.•••••••••.••••••••• • .40 Origin of beaded eskers ••••••••••••••• • .42 iv The Hulligan Lake esker ••••••••••••••••••••••••••••••••••••••• 44 I.ocati on and ITlOrphology- ••••••••••••••••••••••••••••••••••• 44 Sedi nientology-••••••••••••••••••••••••••••••••••••••••••••• 45 Li 'tl'l.ology- ••••••••••••••••••••••••••••••••••••••••••••••••• 46 Origin of esker sedirnent •••••••••••••••••••••••••••••••••• 48 .SlIITlITlary ••••••••••••••••••••••••••••••••••••••••••••••••••• 5 0 V. DRIFT PROSPECTING •••••••••••••••••••••••.•••••.••••••••••••••••• 52 IntrOOuction •••••••••••••••••••••••••••••••••••••••••••••••••• 52 Stone counts •••••••••••••••••••••••••••••••••••••••••••••••••• 54 <£ner al s taterrent ••••••••••••••••••••••••••••••••••••••••• 54 SlIITlITlary ••••••••••••••••••••••••••••••••••••••••••••••••••• 7 0 He CJ!\TY mi ner al s tudi es ••••••••••••••••••••••••••••••••••••••••• 7 3 General statem=nt ••••••••••••••••••••••••••••••••••••••••. 73 Jigging procedures •••••••••••••••••••••••••••••••••••••••• 76 X-r ey di f fraction ••••••••••••••••••••••••••••••••••••••••• 8 0 Procedures •••••••••• r•••·······························80 Results •••••••••••••••••••••••••••••••••••••••••••••••• 81 Optical studies ••••••••••••••••••••••••••••••••••••••••••• 8 3 Procedures ••••••••••••••••••••••••••••••••••••••••••••• 8 3 Mineral descriptions ••••••••••••••••••••••••••••••••••• 85 Results •••••••••••••••••••••••••••••••••••••••••••••••• 87 Ge- och emi s try • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 8 9 Sunnnary ••••••••••••••••••••••••••••••••••••••••••••••••••• 8 9 VI • CONCLUSION'S ••••••••••••••••••••••••••••••••••••••••.••••••••••••• 9 3 Gener al s taterrent •••••••.•••.••••••••••••••.•••••.•.. •• 93 Problems ••••••••••••••••••••••••••••••••••••••••••••• . • • 95 Suggestions for further drift prospecting • • • • • • • • • • • !' ••••••• • • 96 BIBLicx;R.APHY •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 98 Appendix I: Sample numbers, locations, and weights •••••••••••••••••.•••.A-1 Appendix II: Mineralogies determined by X-ray diffraction •••••••••••••• A-4 v LIST OF ILLUSTRATIONS Figures 1. Location of study area, shown by cross hatching, northeastern Minnesota . ••..•.•.•••••••••••.•....•.•....•.•.•.•.•...•.•.•......•••. 3 2. Generalized map of bedrock geology, after Sims (1972). Study area outlined . ............................................................ 9 3. Generalized map of Quaternary geology showing areas covered by major ice advances, after Hobbs and Goebel (1982). Arrows indicate ice flow direction . ......................................................... 13 4. Photograph of the exI.X>sed cross section through the Cloquet esker. Flow direction is toward the viewer ••••••••••••••••••••••••••••••••• 28 5. Photograph of a sketch drawn from the above photo (Figure 4) •••••••• 28 6. Stratigraphic column for the Cloquet esker. Interpretations after Mi all (1978) ............•..•.•...................................... 29 7. Hydraulic criteria for bedforms based on stream I.X>wer (after Allen, 1968) .............................................................. 33 8. HyI.X>thetical model of a continuous esker showing meltwater flowing through a subglacial tunnel and deI.X>Siting sediments in an esker and onto an outwash plain (top), and the present, exI.X>sed topographic re lati onshi ps (bottOITl) •••••••••••••••••.•••••••••.•.••••••.••••.••.•
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