Paleoglaciological Context of Rögen Moraine, Northeastern Minnesota

PALEOGLACIOLOGICAL CONTEXT OF ROGEN MORAINE, NORTHEASTERN MINNESOTA A THESIS SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY MARGRETTA SOPHIA MEYER IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE May, 2008 i © Margretta Sophia Meyer 2008 ii Table of Contents Table of Contents i List of Illustrations ii Acknowledgements iii Introduction 1 Study Site 2 Glacial History 5 Rogen Moraine 11 Hypotheses for Formation 24 Methods of Investigation 41 Results 43 Discussion 49 Conclusion 61 References 64 i List of Illustrations Fig. 1. Location of study area 4 Fig. 2. Phases of glaciation in Minnesota 9 Fig. 3. End moraines and other features in northeastern Minnesota 9 Fig. 4. Geographic distribution of Rogen moraine 13 Fig. 5. Rogen moraine morphological characteristics 16 Fig. 6. Jigsaw-like matching of Rogen moraine 17 Fig. 7. Association with other landforms 22 Fig. 8. Shear and stack model 26 Fig. 9. Two-step/precursor ridge model 28 Fig. 10. Thrust stacking and lee-side cavity fill model 29 Fig. 11. Catastrophic subglacial flood model 31 Fig. 12. Extensional fracturing model 33 Fig. 13. Field map 39 Fig. 14. Photos of till in field area 40 Fig. 15. Rose diagrams of moraine and drumlin orientations 42 Fig. 16. Grain size classification 43 Fig. 17. GPR profile 45 Fig. 18. Coulomb fracture criteria 48 Fig. 19. Isabella ribbed moraine extension 49 Fig. 20. Plan and cross-sectional view of boudins 58 ii Acknowledgements Thank you to my examining committee, Howard Mooers, Stacey Stark, and Keith Brugger. Thank you particularly to Howard for being patient. Tim Demko provided valuable GPR field assistance and I am grateful to Nigel Wattrus for processing the GPR data. Thanks to Vicki Hansen for references on boudins. The UMD Geology Department provided funds for fieldwork expenses, and the UMD Visualization and Digital Imaging Lab provided summer support in 2005. Many thanks to Sarah Davidson for camping out, chipping through compacted subglacial till, enduring repetitive banjo practice, and doing controlled experiments on botanical tick repellent. My family and friends, especially my fellow graduate students, without whom I certainly would not have finished this, have kept me sane through your support and encouragement. And lastly, T.C. Chamberlin for inspiration. iii Introduction Subglacial landforms are important features of continental glaciations, yet have traditionally posed problems for researchers simply because of the difficulty of physically investigating or modeling the subglacial environment. Modern continental ice sheets hide the formation of any analogous structures. Active alpine glacier settings feature recently exposed glacier beds but do not behave like larger scale continental ice sheets. We are therefore confined to working with the geomorphological records left by the most recent continental glaciation; these landforms are the key to understanding not only their mechanics, but the behavior of the glacier in creating them. Rogen, or ribbed, moraines are enigmatic ridges found throughout the northern parts of areas covered by the Laurentide, Fennoscandian, and Irish Ice Sheets. Transverse to ice flow and of uncertain origin, they are generally assumed to be subglacial landforms. Many hypotheses for their formation have been suggested over the last century. These range from marginal features, ripple marks from catastrophic subglacial floods, to a wide variety of subglacial deformation. Although prevalent throughout Canada, there are few examples of Rogen moraine in the United States. Rogen moraine in northeastern Minnesota thus offers a unique perspective on both potential modes of formation and paleoglaciology of the Late Wisconsin Rainy Lobe. The formation of this area of ribbed landscape is constrained to a narrow window of time compared to other areas located more centrally within the Laurentide Ice Sheet (LIS). Ice advanced over this area of northeastern Minnesota shortly before the Last Glacial Maximum (LGM) and retreated shortly thereafter, also offering further constraint on the glaciological conditions. 1 Eskers and striated bedrock provide little evidence that cold-based conditions existed in this area. The Rogen landscape is developed in an area of thin till, down- glacier from ice-scoured Canadian Shield bedrock and up-glacier from a thicker drumlinized till sheet. The abrupt change in basal boundary conditions along this flowline may have resulted in initiation of extensional flow and Rogen moraine formation. This transition poses intriguing questions as to the behavior of the Laurentide Ice Sheet along this flowline and suggests that in this setting, a transition in the glacial substrate may be responsible for Rogen moraine genesis. Study site The study area is located in central Lake County, MN, immediately north of the town of Isabella and Minnesota State Highway 1 within the Tofte district of the Superior National Forest. This area was glaciated repeatedly by the Rainy Lobe of the LIS during the Late Wisconsin and lies to the west of the complex interlobate junction of the Rainy and Superior Lobes (Fig. 1). The Rogen moraine trends northwest-southeast at an angle of approximately 300˚ and is primarily confined to the northern halves of the Mitawan Lake and Sawbill Landing 7.5" 1:24,000 USGS quadrangles, with a few ridges in the Gabbro Lake, Quadga Lake, and Slate Lake East quadrangles. These ridges are visible on both 1:24,000 scale topographic maps and on shaded-relief DEM of the area. The landscape is heavily vegetated and marked by wetlands or parallel streams between the Rogen moraine ridges. Although the glacial landforms are the predominant features there are scattered patches of scoured bedrock and extremely large boulders incorporated into the till, primarily locally-derived gabbro and granophyre from the Middle 2 a b Duluth c w o e fl ic Fig. 1. (a) DEM of field area with ice flow direction. White line is Minnesota State Highway 1. (b) Field area (rectangle) in relation to regional geography. (c) Example of topographic map expression of Rogen moraine ridges , section lines indicate scale.. Proterozoic Duluth Complex. The northern border of the study site is evident from digital elevation models as it essentially ends at the bedrock-controlled geomorphology of the Boundary Waters Canoe Area Wilderness, whose scoured and jointed bedrock provides for the abundant lakes (Fig. 1.). Up-ice of the Isabella area the ice was flowing over crystalline bedrock of the Canadian Shield and Duluth Complex. Ice moving over bedrock must move via the process of regelation and enhanced creep and thus flows slowly. The Rogen moraine is developed on thin till between the striated bedrock and the thick till of the Toimi drumlin field, which lies further down ice (Wright,1969, 1972). The till of the Toimi drumlins ranges from 50 to over 200 feet thick as determined by rotasonic drilling (Hobbs, 1992). The till that makes up both the Rogen moraine and the drumlins is extremely bouldery and its texture is rocky sandy loam to loamy sand, with a grain size distribution of the less than 2mm fraction at 69% sand, 29% silt, and 2% clay (Lehr and Hobbs, 1992). It is known as the Independence Till (Wright, 1969). Late Glacial chronology is controlled primarily by the relative dating of landforms, with a few radiocarbon dates as anchors. It is uncertain when the Rogen moraine was formed because of the improbability of obtaining a date on subglacial sediments. The positions behind the Vermillion and Highland moraines can therefore be used to constrain a maximum formation to the period during the last glacial maximum in northeastern Minnesota. 4 Glacial History Historical Glacial Investigations in Northeastern Minnesota Continental ice sheets advanced and retreated over the upper Midwest for much of the last million years. Most of Minnesota was covered by ice at some point except for the southeastern portion, part of the Driftless Area. Pre-Wisconsin glacial deposits are found in northeastern Minnesota, but not much is known about their distribution and history. Northeastern Minnesota was continuously covered by ice from the earliest Late Wisconsin ice advance at approximately 27-29ka (Clayton and Moran, 1982; Mooers and Lehr, 1997) until about 11ka by the Rainy and Superior lobes of the LIS (Fig. 2.). The earliest formal studies of glacial deposits in northeastern Minnesota were conducted by Upham (1894) who identified a series of moraines across the state. He identified the Vermillion moraine, as the 12th moraine, although he did not define its entire length. Winchell, as the first Minnesota state geologist, organized systematic mapping of the glacial geology of Minnesota. Along with Upham and others, Winchell (1899) was one of the first to map large portions of northeastern Minnesota and describe the surficial deposits. Todd (1898) postulated two lobes of ice, the Lake Superior lobe, which flowed along the axis of Lake Superior, and the Red River lobe, which advanced from the west. Elftman (1898) suggested two lobes for the northeastern portion of Minnesota because of observed till differences and provenances; he named these the Superior and Rainy lobes. The Rainy lobe refers to the ice flowing from the Rainy River area. Leverett (1932) proposed that northeastern Minnesota was glaciated by three separate lobes of ice. He suggested that the earliest drift in the area was the result of ice 5 from the Patrician ice center located in the Hudson Bay Lowlands between the Keewatin and Labradorean ice accumulation centers, and that this drift was deposited by a lobe of ice which was a combination of the Rainy and Superior lobes. He also proposed that a later advance of both lobes resulted in the interlobate area to the east of Isabella. In terms of the overall glacial history of Minnesota, the modern understanding began with Wright, who identified multiple phases of glaciation (Wright, 1964) and was the first to identify and interpret the tunnel valleys, drumlins, and eskers (Wright and Ruhe, 1965).

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