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PETROLOGY, STRATIGRAPHY AND SEDIMENTATION OF THE MIDDLE PROTEROZOIC BAYFIELD GROUP, NORTHWESTERN WISCONSIN A THESIS SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY Kent Fred Adamson IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE December, 1997 © Kent Fred Adamson, 1997 Abstract The Bayfield Group (Middle Proterozoic, Lake Superior region) is composed of three formations which are, from oldest to youngest, the feldspatholithic to lithofeldspathic Orienta Formation; the quartz-arenitic Devils Island Sandstone; and the feldspatholithic Chequamegon Formation. The average QFL percentages are 69/17 /14 for the Orienta Formation, 95/4/1 for the Devils Island Sandstone, and 79/14/7 for the Chequamegon Formation. The large amounts of quartz and appreciable feldspar suggest that the primary provenance was older Early and Archean rocks. Diagenetic action on the Bayfield Group is evidenced by authigenic quartz, hematite, kaolinite, zeolite, and minor amounts of potassium feldspar. The average pore space of the sandstones in the Bayfield Group decreases with depth. Dissolution of potassium feldspar is the major factor in the development of minor secondary porosity. Accessory minerals show that "normal" zircon is the most prevalent form found within the non-opaque, non-micaceous suite. The presence of "normal" zircons suggests that a potential provenance was Keweenawan volcanics; some residence time in previously deposited rift sediments (such as the Oronto Group) may have occurred. The minor amount of other zircon varieties support the framework grain analyses that older Early Proterozoic and Archean sources were shedding major amounts of sediment into the depositional basin. The Bayfield Group was deposited in a transgressive-regressive lacustrine to fluvial environnment in the waning stages of the Midcontinent Rift System. The Orienta and Chequamegon Formations represent fluvial environments of deposition while the Devils Island Sandstone was likely deposited in a nearshore lacustrine depositional environment. The paleocurrent trend for the Devils Island is toward the east and southeast while paleocurrent trends for the Orienta and Chequamegon Formations are toward the northeast. The exposed sections suggest that the near-shore environment was restricted in comparison to the fluvial systems that were transporting sediment towards the central rift valley. 111 Table of Contents Abstract 111 Table of Contents lV List of Figures lX List of Plates Xl List of Tables XlV Acknowledgements xv INTRODUCTION 1 Purpose of Study 2 Methods 4 Previous Work 5 REGIONAL GEOLOGY 7 Archean Rocks 9 Early Proterozoic 10 Middle Proterozoic 11 Keweenawan Supergroup 11 Grenville Province 13 Paleozoic and Mesozoic 14 Quaternary 14 Regional Structure 15 South Shore Geophysical Data 16 Review of Upper Keweenawan Paleomagnetic Analysis 21 Age of the Bayfield Group - Conclusions 23 lV PETROLOGY - FIELD DESCRIPTIONS 24 Orienta Formation 24 Devils Island Sandstone 28 Chequamegon Formation 33 Fond du Lac Formation 35 Hinckley Sandstone 35 MINERALOGY AND PETROLOGY 38 Introduction 38 Sample Preparation 38 Operational Definitions 39 Quartz 39 Feldspars 40 Lithics 41 Miscellaneous Grains 43 Cement/Matrix 44 General Petrographic Characteristics and Descriptions 45 Orienta Formation 45 Devils Island Sandstone 48 Chequamegon Formation 52 Diagenesis of the Bayfield Group 55 Paragenetic Sequence ----------------- 57 Texture 58 Porosity 59 Modal Analysis of Thin Sections 63 v Heavy Minerals 78 Sample Preparation 78 Operational Definitions 80 Modal Analysis of Heavy Minerals 84 Orienta Formation 87 Devils Island Sandstone 87 Chequamegon Formation 88 KEY AREAS IN THE BAYFIELD GROUP 91 Orienta Formation at Middle River 91 South Landing, Devils Island ----------------- 97 Northern Sand Island 101 Geologic Well Logs 105 PALEOCURRENT ANALYSIS 115 Introduction 115 Orienta Formation 118 Devils Island Sandstone 118 Chequamegon Formation 118 PROVENANCE 123 Introduction 123 Paleocurrent Evidence 124 Petrologic Evidence and Interpretation 126 Accessory Mineral Evidence and Interpretation 127 Fond du Lac Formation Data 127 Hinckley Sandstone Data 128 Vl Interpretation 128 SEDIMENTATION AND ENVIRONMENTS OF DEPOSITION 130 Fluvial Systems 130 Nearshore Lacustrine Systems 133 Laminae Sands 134 Forms of Evidence Present 135 Interpretation 138 Orienta Formation 138 Devils Island Sandstone 138 Chequamegon Formation 140 Fond du Lac Formation 140 Hinckley Sandstone 141 TECTONic!SEDIMENTARY MODELS 142 Model 1 - Orienta Formation is equal to the Chequamegon Formation 142 A. Synclinal Structure 142 B. Faulting 144 Model 2 - Orienta Formation is not equal to the Chequamegon Formation 145 CONCLUSIONS 148 REFERENCES CITED 153 Appendix A - Field Data. 160 Appendix B - Outcrop Locations. 181 Appendix C-1 - Orienta Formation Modal Analyses. 183 Appendix C-2 - Devils Island Sandstone Modal Analyses. 185 Appendix C-3 - Chequamegon Formation Modal Analyses. 187 VH Appendix C-4 - Converted QFL and QmFL Percentages from Myers (1971b). 189 Appendix C-5 - Pore space variance. 190 Appendix D - Heavy Mineral Modal Analyses. 191 Appendix E. Paleocurrent Rose Plots. 192 Appendix F. Sand Point Geologic Well Log. 200 Appendix G. Sand Point Well modal analyses. 202 Vlll List of Figures 1 Stratigraphic column for Middle Proterozoic rocks of the Midcontinent Rift System in the Lake Superior region. 1 2 QFL diagram of Oronto and Bayfield Groups showing compositional maturity. 3 3 Basement tectonic provinces in the North American midcontinent. 7 4 Geologic map of the Midcontinent Rift System, showing the Bayfield basin and study area. 8 5 Schematic geologic interpretation of seismic profiles across axis of Midcontinent Rift System, offshore Lake Superior. 18 6 Interpreted reflection profile along line C (shown on Figure 4). 19 7 Geologic map of the Midcontinent Rift System. 20 8 Suggested Apparent Polar Wander path from 1500 to 800 Ma. 22 9 Strikes and dips of Bayfield Group strata. 37 10 Classification of sandstones. 63 11 Orienta Formation QmFLt plot of 18 modal analyses. 69 12 Orienta Formation QFL plot of 18 modal analyses. 70 13 Devils Island Sandstone QmFLt plot of 41 modal analyses. _____ 71 14 Devils Island Sandstone QFL plot of 41 modal analyses. 72 15 Chequamegon Formation QmFLt plot of 17 modal analyses. 73 16 Chequamegon Formation QFL plot of 17 modal analyses. 74 17 Bayfield Group mean modes, QmFLt. 76 18 Bayfield Group mean modes, QFL. 77 19 Outcrop locations for heavy mineral analysis. 79 20 Sketch map of the Middle River at Moonshine Road outcrop area. 94 21 QmFLt diagram for Middle River Orienta samples. 95 lX 22 Structure section on steeply dipping Middle River beds. 96 23 Map of Devils Island showing samples from location 110. 99 24 QmFLt diagram for Devils Island Sandstone, location 1 lOD. 100 25 Sketch map of Sand Island. 103 26 QmFLt diagram for Devils Island Sandstone (location 117). 104 27 Percentages of quartz and lithics in Sand Point well. 109 28 Percentages of feldspar in Sand Point well. 110 29 Maps showing paleocurrent trends in pre-volcanic quartz, interflow sandstones, Oronto Group, and Bayfield Group. 116 30 Paleocurrent directions for Orienta Formation. 120 31 Paleocurrent directions for Devils Island Sandstone. 121 32 Paleocurrent directions for Chequamegon Formation. 122 33 Compositional plots of key sandstone suites. 125 34 The eight basic architectural elements in fluvial deposits. 132 35 Sketch map of how the Midcontinent Rift System may have appeared during Keweenawan sedimentation. 136 36 Sketch map of Northwestern Wisconsin showing the location of brownstone quarries and seacaves. 141 37 A cross section depicting the Devils Island Sandstone as the core of a syncline. 143 38 A cross section depicting the Devils Island Sandstone deposited adjacent to a small-throw fault. 144 39 Sketch of a cross section of the Bayfield Group showing a theoretical depositional pattern for the sediments. 147 x List of Plates Plate ·1 - Orienta Formation 26 A. Large cross-bed sets within the Orienta Formation at Amnicon Falls, location 4. B. Ripple marks in the Orienta Formation at Copper Creek, location 17. Plate 2 - Orienta Formation 27 A. Trough cross-bedding in the Orienta Formation at Copper Creek, location 14. B. Steeply dipping beds of Orienta Formation on Middle River and Moonshine Road, location 33. Plate 3 - Devils Island Sandstone 30 A. Ripple marks and mudcracks on the same bed, Devils Island Sandstone, Devils Island, location 11 OM2. B. Mudcracks in the Devils Island Sandstone, Devils Island, location 1 lOPl. Plate 4 - Devils Island Sandstone 31 Cross-bedding in Devils Island Sandstone, Devils Island, location 11 OAl. Plate 5 - Devils Island Sandstone 32 Seacaves of Devils Island Sandstone, Devils Island, location 11 OS. Plate 6 - Chequamegon Formation 34 A. Channel in the Chequamegon Formation, Hermit Island. B. Large lensoid massive bodies (channels?) on North Twin Island. Plate 7 - 46 Photomicrograph of sandstone - Orienta Formation. Plate 8 - 47 Photomicrograph of sandsone - Orienta Formation. Xl Plate 9 - 49 Photomicrograph of sandstone - Devils Island Sandstone. Plate 10 - 50 Photomicrograph of sandstone - Devils Island Sandstone. Plate 11 - 51 Photomicrograph of sandstone - Devils Island Sandstone. Plate 12 - 53 Photomicrograph of sandstone - Chequamegon Formation. Plate 13 - 54 Photomicrograph of sandstone - Chequamegon Formation. Plate 14 - 61 Photomicrograph of sandstone - Orienta Formation. Plate 15 - 62 Photomicrograph
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