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Refining Paleoproterozoic Sedimentary Sequence Boundaries

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Refining Paleoproterozoic Sedimentary Sequence Boundaries Refining Paleoproterozoic Sedimentary Sequence Boundaries in East-Central Minnesota, Carlton County: Implications for Source, Age, Correlations, and Tectonic Histories A thesis submitted to the Kent State University Graduate College in partial fulfillment of the requirements for the degree of Master of Science by Scott W. Scheiner December, 2012 Thesis written by Scott W. Scheiner B.S. Kent State University, 2009 M.S. Kent State University, 2012 Approved by ________________________________, Advisor Dr. Daniel K. Holm ________________________________, Chair, Department of Geology Dr. Daniel K. Holm ________________________________, Dean, College of Arts and Sciences Dr. Raymond A. Craig ii TABLE OF CONTENTS LIST OF FIGURES.............................................................................................................v LIST OF TABLES..............................................................................................................vi ACKNOWLEDGEMENTS...............................................................................................vii SUMMARY.........................................................................................................................1 Chapter I. Introduction..................................................................................................3 Animikie Basin in Minnesota......................................................................5 Lake Superior Banded Iron Formations.......................................................9 Tectonic Model of the Penokean Orogeny................................................11 Southern Margin of Animikie Basin..........................................................14 II. New Developments....................................................................................17 Yavapai Magmatism..................................................................................17 Yavapai Metamorphic Signature...............................................................18 Yavapai Structures.....................................................................................20 Yavapai Sedimentation..............................................................................21 Field Description of Bedrock Geology: Southern Margin of Animikie Basin..........................................................................................................21 New Hypothesis.........................................................................................22 III. Field Sampling...........................................................................................25 Lithologic Variation Across Holst’s Line..................................................26 Preparation of Thin Sections......................................................................27 Thin Section Analysis................................................................................27 Geochemistry.............................................................................................30 Geochemical Analysis Techniques............................................................30 Geochemical Results..................................................................................31 Isotope Analysis.........................................................................................35 Preparation of Isotope Samples.................................................................38 Isotope Results...........................................................................................38 iii Discussion..................................................................................................41 IV. A New Interpretation.................................................................................45 A New Tectonic Model..............................................................................49 Implication for Inventory of Structures.....................................................51 REFERENCES..................................................................................................................53 iv LIST OF FIGURES Figure 1. Paleoproterozoic supracrustal sequences in the Lake Superior region....................4 2. Stratigraphy of major iron ranges in Minnesota and Ontario..................................6 3. Simplified geologic map of east-central Minnesota................................................8 4. Aeromagnetic map of east-central Minnesota.......................................................10 5. Schematic cross-section of proposed Penokean orogeny tectonic evolution........13 6. Large fold structure at Thomson Dam within Carlton County, Minnesota...........16 7. Bedrock geology map of Carlton County, east-central Minnesota........................23 8. Schematic synopsis of tectono-sedimentary models along the southern margin of the Animikie Basin................................................................................................24 9. Photomicrographs of SS10-2a, SS10-4, SS10-5, and SS10-6...............................29 10. La/Sc vs. Ti.Zr ratio plot........................................................................................34 11. Sc/Cr vs. La/Y ratio plot........................................................................................34 12. Th vs. Zr plot..........................................................................................................36 13. Sc vs. V plot...........................................................................................................36 14. CaO vs K2O+Na2O plot.........................................................................................37 15. εNd(Age) vs. Age plot...........................................................................................43 16. Th/Sc vs εNd plot...................................................................................................44 17. Composite stratigraphic column of the Animikie Basin........................................46 18. Proposed tectono-sedimentary formation of the Animikie Basin..........................48 19. New tectonic interpretation....................................................................................50 v LIST OF TABLES Tables 1. Major and trace element geochemistry of samples from Carlton County, Minnesota.........................................................................................................32 2. Nd isotopic composition of Carlton County, Minnesota samples...................40 vi Acknowledgements I owe many thanks to Daniel Holm for his guidance and dedication through all stages of this project. I greatly appreciate Tathagata Dasgupta’s guidance on elemental and isotopic analysis, and for connecting me with Syracuse University where my samples were analyzed. I would like to thank Terry Boerboom, David Hacker, and Donald Palmer for their help in the review process. I appreciate the help and support I received from everyone in the KSU Geology Department. I cannot thank my family and friends enough for the love, support, and encouragement they provided during this process. I would especially like to thank my loving wife, Aubrey, for her everlasting patience and support. Also, to my parents, Ed and Lisa, for their unwavering support in everything I have undertaken. Funding for this research was provided by: Kent State University Department of Geology (School of Hard Rocks Mineral Resources Award), and The Institute of Lake Superior Geology Student Research Grant. vii Summary The Animikie Basin in north-eastern Minnesota is usually interpreted as a Penokean (1875-1835 Ma) foredeep deposit consisting mainly of 2 to 5 km thick turbidite sequences underlain by banded iron formations and quartzite conglomerates, all deposited in a foreland basin north of the accreted Penokean arc terrain. Much of the Animikie basin sequence is only very weakly metamorphosed and only mildly deformed. However, along its southern margin in Carlton County, east-central Minnesota, the sedimentary sequence is strongly deformed into what are interpreted to be refolded nappes. The twice deformed sequence occurs south of a similar sedimentary sequence that has undergone a single deformation. The contact between these sequences, although nowhere exposed, was interpreted by Holst (1985) as a Penokean thrust fault and all of the deformation assumed to be Penokean in age. This contact is commonly referred to at Holst’s Line. A wealth of new data collected over the last ~15 years document Yavapai-age magmatism, metamorphism, sedimentation and deformation overprinting the Penokean orogen. Metamorphic ages that that post-date the Penokean orogeny and detrital zircon ages indicate that some of the Paleoproterozoic sedimentary rocks may be younger than previously thought. Based on this new evidence, I propose Holst’s Line to represent an unconformity separating sedimentary foredeep packages of different ages and test this hypothesis using geochemical and Sm/Nd isotopic data. 1 2 Geochemical analyses were obtained on 10 samples divided into two groups based upon location north or south of Holst’s Line. Relative proportions of major elements varied little within and across these groups. Trace element ratio plots of La/Sc vs Ti/Zr, Sc/Cr vs La/Y, Th vs Zr and Sc vs V were created with the hope of distinguishing between different tectonic settings for the source of the sediments. The results show a concentrated grouping of data from the southern samples and a much larger spread from northern samples, suggesting a more variable source for the northern
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