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Contributions to the Geology of Pine County, Minnesota MINNESOTA GEOLOGICAL SURVEY REPORT OF INVESTIGATIONS 60 CONTRIBUTIONS TO THE GEOLOGY OF PINE COUNTY, MINNESOTA Terrence J. Boerboom, Project Manager St. Paul, Minnesota 2002 iii This publication is accessible from the home page of the Minnesota Geological Survey (http://www.geo.umn.edu/mgs) as a PDF file readable with Acrobat Reader 4.0. Recommended citation— Boerboom, T.J., Project Manager, 2002, Contributions to the geology of Pine County, Minnesota: Minnesota Geological Survey Report of Investigations 60, 91 p. Cartography and other drafting by Philip Heywood Edited by Lynn Swanson Minnesota Geological Survey 2642 University Avenue West Saint Paul, Minnesota 55114-1057 Telephone: 612-627-4780 Fax: 612-627-4778 E-mail address: [email protected] Web site: http://www.geo.umn.edu/mgs ©2002 by the Regents of the University of Minnesota All rights reserved ISSN 0076-9177 The University of Minnesota is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, color, creed, religion, national origin, sex, age, marital status, disability, public assistance status, veteran status, or sexual orientation. iv CONTRIBUTORS E. Calvin Alexander, Jr. Professor, Department of Geology and Geophysics, University of Minnesota, Minneapolis Scott C. Alexander Junior Scientist, Department of Geology and Geophysics, University of Minnesota, Minneapolis Terrence J. Boerboom Scientist, Minnesota Geological Survey, St. Paul Val W. Chandler Acting Director, Minnesota Geological Survey, St. Paul; adjunct faculty, Department of Geology and Geophysics, University of Minnesota, Minneapolis Alan R. Knaeble Senior Scientist, Minnesota Geological Survey, St. Paul R.S. Lively Information Technology Supervisor, Minnesota Geological Survey, St. Paul C.J. Patterson Senior Scientist, Minnesota Geological Survey, St. Paul; adjunct faculty, Department of Geology and Geophysics, University of Minnesota, Minneapolis Anthony C. Runkel Senior Scientist and Chief Geologist, Minnesota Geological Survey, St. Paul; adjunct faculty, Department of Geology and Geophysics, University of Minnesota, Minneapolis Beverly L. Shade Director, Proyecto Espeleologico Purificacion, P.O. Box 8424, Austin, Texas, 78713 v INTRODUCTION This publication expands on information presented on the map plates of Part A of the Geologic Atlas of Pine County, Minnesota. The explanations given on those plates tend to be technical, and they include only that information necessary to read the maps. The papers included in this volume expand on the map explanations to help the user understand the geology and geologic framework of Pine County. The maps are intended only to provide general geologic information for use by (1) county residents; (2) resource managers and planners at various levels of government; (3) geologists, hydrologists, and others working in the county, and (4) those interested in learning about the geology of Pine County. Additional information is available in computerized data bases, such as the County Well Index, and in Arcview™ files that accompany Part A of the atlas. Because the maps in Part A of the atlas were constructed at a scale of 1:100,000, one must closely examine the data available for a given location before using them for detailed, site-specific studies, to determine the reliability of the geologic interpretations made for that area. vi CONTENTS CHAPTER 1— Bedrock geology of Pine County, Minnesota, by Terrence J. Boerboom, Anthony C. Runkel, and Val W. Chandler Introduction . 1 Paleozoic bedrock . 1 Precambrian bedrock . 3 Paleoproterozoic and Archean rocks . 3 McGrath Gneiss (Archean) . 3 Denham Formation and related rocks (Paleoproterozoic) . 4 Volcanic and sedimentary rocks of the Midcontinent rift system (Mesoproterozoic) . 6 Volcanic rocks (Mesoproterozoic) . 8 Basalt outcrops at Pine City . 8 Basalt outcrops at the Kettle River east of Hinckley . 9 Other bedrock outcrops . 10 Sedimentary rocks (Mesoproterozoic) . 10 Fond du Lac Formation . 10 Unnamed sedimentary units overlying the St. Croix horst . 11 Hinckley Sandstone . 14 Summary . 18 History of copper exploration . 18 References . 18 CHAPTER 2— History of glaciation in Pine County, Minnesota by C.J. Patterson and A.R. Knaeble Introduction . 21 Preglacial landscape . 24 Bedrock distribution and buried bedrock topography . 24 Thickness of Quaternary sediments overlying bedrock . 25 History of glacial advances . 25 Late Wisconsin glacial activity . 26 Glacial activity of the Superior lobe . 28 Emerald phase . 28 St. Croix phase . 28 Glacial Lake Lind . 28 Automba phase . 28 Beroun phase . 30 Hinckley, North Hinckley, Grindstone, and Sandstone phases . 30 Askov, Lookout Tower, Kerrick, and upper and lower Split Rock phases . 33 Nickerson phase . 36 Glacial activity of the Grantsburg sublobe . 37 Pine City and West Rock phases . 37 Glacial Lake Grantsburg . 37 Late- and postglacial activity . 37 Glacial Lake Nemadji and the Kettle River . 37 Glacial Lake Duluth and the Brule and St. Croix Rivers . 38 Snake River and Cross Lake . 38 Wind erosion . 40 References . 40 vii CONTENTS continued CHAPTER 3— Investigation of stream-like magnetic anomalies in Pine County, Minnesota, by Val W. Chandler, Terrence J. Boerboom, and R.S. Lively Introduction . 43 General geology . 43 Aeromagnetic survey method . 43 Sandstone area . 45 Grindstone Lake area . 45 Sources of magnetic anomalies . 49 Model studies . 50 Conclusions . 52 References . 52 CHAPTER 4— Karst features in Pine County, Minnesota by Beverly L. Shade, Scott C. Alexander, and E. Calvin Alexander Introduction . 55 Karst features . 56 Background . 56 Global distribution of features . 56 Previous work . 56 Are the sinkholes in Pine County karst features? . 58 Observations . 59 Sinkholes . 59 Sinkhole D222 . 59 Sinkhole D144 . 60 Sinkhole D127 . 61 Sinkhole D355 . 64 Streamsinks . 64 Springs . 65 Caves . 65 Composite features . 65 Interpretation . 65 Sinkhole distribution relative to bedrock type . 65 Sinkhole distribution relative to depth to bedrock . 67 Sinkhole distribution relative to glacial features . 68 Sinkhole formation . 68 Collapse sinkholes . 68 Subsidence sinkholes . .68 Composite features . .69 What is not a sinkhole? . 69 Acknowledgments . 72 References . 72 viii CONTENTS continued APPENDIX A— Geophysical and geologic logs of core recovered from five boreholes in Pine County, Minnesota . 73 Explanation . .73 Minnesota Geological Survey borehole log MGS-PCR1 . 74 Minnesota Geological Survey borehole log MGS-PCR2 . 76 Minnesota Geological Survey borehole log MGS-PCR3 . 78 Minnesota Geological Survey borehole log MGS-PCC1 . 79 Minnesota Geological Survey borehole log MGS-PCC2 . 81 APPENDIX B— Inventories of known karst features in Pine County, Minnesota . 83 Appendix Table B1. Pertinent information on 262 sinkholes in Pine County, Minnesota . 84 Appendix Table B2. Pertinent information on 24 streamsinks in Pine County, Minnesota . 90 Appendix Table B3. Pertinent information on 31 streamsinks in Pine County, Minnesota . 91 ix COUNTY ATLAS PLATES The following is a listing of plates prepared for the Geologic Atlas of Pine County, Minnesota, Part A. The list includes short descriptions of the information contained on each plate, as well as suggestions as to how the various plates might be used for planning purposes. The use cited for each plate represents a single suggestion. It is left to atlas users to envision ways in which information from the various plates can be brought together to achieve their goals. 1 The Data-Base Map shows the location of factual data, for example, water-well logs, exploratory drill holes, and outcrops. Other information, such as well depth and stratigraphy, is directly accessible through Arcview� data files. The Data-Base Map can be used to evaluate the reliability of the geologic interpretations shown on other maps of the atlas. 2 The Bedrock Geology Map and Cross Sections shows the types of bedrock that are likely to be encountered in a given area, either beneath glacial materials or at the land surface, and the attitude of structural features like bedding and metamorphic foliation. Knowledge of the type of bedrock present below the land surface is essential when considering the likely quantity or quality of ground-water resources or the possibility of finding rock suitable for aggregate production. 3 Supplemental Data on Bedrock Geology and Geophysics shows the bedrock geology of some areas in the county where abundant outcrops permitted detailed mapping that could not be portrayed at the 1:100,000 scale of the geologic map on Plate 2. The plate also includes geophysical maps that were integral to interpreting the bedrock geology. The plate can be used by those who want to understand how geophysical information was used in the construction of Plates 2 and 6 (see description below), and for those interested in examining the geology of the areas portrayed in detail. For example, the understanding of materials encountered in drilling water wells can be better understood by examining similar rocks in outcrop. 4 The Surficial Geology plate shows the geology of the land surface. It can be used to gain general knowledge of the character of material at the surface—for example, sand-rich versus clay-rich material might be important in consideration of well-head protection areas. 5 Quaternary Stratigraphy shows the geology of Quaternary materials (unconsolidated sediment deposited by glacial activity) in cross sectional views. It is intended to demonstrate the
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