Seismic Refl ection Profiling and Tectonic Evolution of the Midcontinent Rift in Lake Superior L.D. McGinnis and M.G. Mudrey, Jr. WISCONSIN GEOLOGICAL AND NATURAL HISTORY SURVEY MISCELLANEOUS PAPER 91-2 2003 WISCONSIN GEOLOGICAL AND NATURAL HISTORY SURVEY James M. Robertson, Director and State Geologist John W. Attig, geologist Susan L. Hunt, graphic artist William G. Batten, geologist Mindy C. James, publications manager Kenneth R. Bradbury, hydrogeologist Marcia J. Jesperson, Map Sales associate Bill C. Bristoll, information manager Kathy A. Kane, computer specialist Bruce A. Brown, geologist Irene D. Lippelt, water resources specialist Lee Clayton, geologist Frederick W. Madison, soil scientist Michael L. Czechanski, cartographer M.G. Mudrey, Jr., geologist Donna M. Duffey, Map Sales associate Stanley A. Nichols, biologist (emeritus) Timothy T. Eaton, hydrogeologist Deborah L. Patterson, cartographer Thomas J. Evans, geologist Roger M. Peters, subsurface geologist Stephen J. Gaffi eld, hydrogeologist Kathy Campbell Roushar, cartographer Madeline B. Gotkowitz, hydrogeologist Apichart Santipiromkul, information processing David J. Hart, hydrogeologist consultant Ronald G. Hennings, hydrogeologist (emeritus) Curtis L. Thomas, geotechnician Rilla M. Hinkes, offi ce manager Virginia L. Trapino, program assistant Thomas S. Hooyer, geologist Alexander Zaporozec, hydrogeologist (emeritus) Kathie M. Zwettler, administrative manager plus approximately 10 graduate and undergraduate student workers. RESEARCH ASSOCIATES Gregory J. Allord, USGS George J. Kraft, Central Wis. Groundwater Center Mary P. Anderson, UW –Madison James T. Krohelski, USGS Michael F. Bohn, Wis. Dept. of Nat. Res. Michael D. Lemcke, Wis. Dept. of Nat. Res. Stephen M. Born, UW–Madison J. Brian Mahoney, UW–Eau Claire Philip E. Brown, UW–Madison Kevin McSweeney, UW–Madison Charles W. Byers, UW–Madison Christine Mechenich, Central Wis. Groundwater Center William F. Cannon, USGS David M. Mickelson, UW–Madison Douglas S. Cherkauer, UW–Milwaukee Donald G. Mikulic, Ill. State Geol. Survey Kevin Connors, Dane Co. Land Conserv. Dept. William N. Mode, UW–Oshkosh Robert H. Dott, Jr., UW–Madison (emeritus) Maureen A. Muldoon, UW–Oshkosh C. Patrick Ervin, Northern Ill. Univ. Robert E. Pearson, Wisc. Dept. of Transportation Daniel T. Feinstein, USGS James O. Peterson, UW–Madison Robert F. Gurda, Wis. State Cartographer's Offi ce Jeffrey K. Postle, Wis. Dept. Ag., Trade & Consumer Nelson R. Ham, St. Norbert Coll. Protection Mark T. Harris, UW–Milwaukee Kenneth W. Potter, UW–Madison Karen G. Havholm, UW–Eau Claire Todd W. Rayne, Hamilton Coll. Randy J. Hunt, USGS Daniel D. Reid, Wis. Dept. of Transportation Mark D. Johnson, Gustavus Adolphus Coll. Allan F. Schneider, UW–Parkside (emeritus) Joanne Kluessendorf, Weis Earth Science Museum J. Antonio Simo, UW–Madison James C. Knox, UW–Madison Kent M. Syverson, UW–Eau Claire Jeffrey A. Wyman, UW–Madison The Wisconsin Geological and Natural History Survey also maintains collaborative relationships with a number of local, state, regional, and federal agencies and organizations regarding educational outreach and a broad range of natural resource issues. Seismic Refl ection Profiling and Tectonic Evolution of the Midcontinent Rift in Lake Superior L.D. McGinnis and M.G. Mudrey, Jr. WISCONSIN GEOLOGICAL AND NATURAL HISTORY SURVEY MISCELLANEOUS PAPER 91-2 2003 Published by and available from Wisconsin Geological and Natural History Survey 3817 Mineral Point Road • Madison, Wisconsin 53705-5100 ☎ 608/263.7389 FAX 608/262.8086 www.uwex.edu/wgnhs/ James M. Robertson, Director and State Geologist ISSN: 0731-7603 This report is an interpretation of the data available at the time of preparation. Every reasonable effort has been made to ensure that this interpretation conforms to sound scientifi c principles; however, the report should not be used to guide site-specifi c decisions without verifi cation. Proper use of the report is the sole responsibility of the user. The use of company names in this document does not imply endorsement by the Wisconsin Geological and Natural History Survey. Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, University of Wisconsin–Extension, Cooperative Extension. University of Wisconsin–Extension provides equal opportunities in employment and programming, including Title IX and ADA requirements. If you need this information in an alternative format, contact the Offi ce of Equal Oppor tunity and Diversity Programs or the Wisconsin Geological and Natural History Survey (☎ 608/262.1705). Mission of the Wisconsin Geological and Natural History Survey The Survey conducts earth-science surveys, fi eld studies, and research. We provide objective scientifi c information about the geology, mineral resources, water resources, soil, and biology of Wisconsin. We collect, interpret, disseminate, and archive natural resource information. We communicate the results of our activities through publications, technical talks, and responses to inquiries from the public. These activities support informed decision-making by government, industry, business, and individual citizens of Wisconsin. ii | Miscellaneous Paper 91-2 ABSTRACT 1 INTRODUCTION 1 REGIONAL GEOLOGIC SETTING 4 OVERVIEW OF SEISMIC REFLECTION DATA 8 DATA ACQUISITION AND PROCESSING 10 DESCRIPTION AND INTERPRETATION OF SEISMIC PROFILES 11 ACKNOWLEDGMENTS 20 REFERENCES 20 TABLE 1. Summary of length of seismic lines 11 FIGURES 1. Outline map of Lake Superior showing location of Argonne–Grant Norpac reflection seismic lines and GLIMPCE reflection seismic lines 2 2. Simplified map of the Midcontinent Rift defined from gravity sur­ veys 4 3. Detailed structural interpretation of the Superior Zone of the Mid- continent Rift System 5 4. General geology of the western Lake Superior Basin part of the Mid- continent Rift System 6 5. General stratigraphic section in the western part of the Lake Superior Basin 7 PLATES 1. LS-04 6. LS-18 11. LS-45 2. LS-08 7. LS-25 12. LS-47 3. LS-10 8. LS-26 13. LS-53 4. LS-12 9. LS-36 14. LS-57 5. LS-15 10. LS-43 15. LS-69 Profiling and Evolution of Midcontinent Rift in Lake Superior| iii ABSTRACT An orthogonal grid of 1837.784 line km of seismic refl ection profi les in Lake Superior, which covers approximately 20 percent of the length of the Midcontinent Rift, provided a comprehensive data set that assisted structural and stratigraphic analysis of the rift beneath the lake. The data allowed us to identify several distinct basins and interconnecting troughs, a large anticlinal structure, and major boundary faults and folds. In addition, we were also able to revise locations and characteristics of previously known boundary faults and their relationship to rifted blocks. Integration of information obtained from outcrops with data reported here indicated that the rift is associated with as many as four major strike faults, including the Douglas Fault, the Keweenaw Fault, the Isle Royale Fault, and a fault along a hinge line on the north flank of the basin south of the Isle Royale Fault. The Douglas and Keweenaw Faults are subparallel strike faults bounding the rift and sag basins on the south flank of the rift graben. Strike faults and hinge lines form the rift graben boundaries. INTRODUCTION In this report we present deep reflection seismic profiles in Lake Superior, shot by Grant Norpac, Inc., and leased by Argonne National Laboratory to be made available to the geological research community. The locations of all deep seismic profiles shot in Lake Superior, including Great Lakes International Multidisciplinary Program for Crustal Evolution (GLIMPCE) data (see Behrendt and others, 1988) and some profiles shot, but not leased, are shown in fi gure 1. The initial interest in this proprietary seismic data stemmed from the recognition that late Precambrian rock may contain recoverable petroleum (Dickas, 1986) and that basins of rift origin are noted worldwide for their hy­ drocarbon productivity. Following their discovery and exploitation on other continents, rift basins of Precambrian age have only recently been recognized as potential sources of hydrocarbons in North America. Although petroleum reserves have not been identified along the Middle Proterozoic Midcontinent Rift System, oil seeps emanate from the Nonesuch Formation in the White Pine mine on the Keweenaw Peninsula (Dickas, 1986). More recent interest in the seismic data has focused on the evolution of large-scale continental rift basins. Proprietary seismic data suggested that major revisions to the geometry of the Midcontinent Rift were necessary; as a result, Cannon and others (1989) supplemented that proprietary data with publicly acquired seismic data. Since the acquisition of these data, several deep petroleum tests have been made that help in the understanding of the stratigraphy and tectonic set­ 92° 00’ 91° 00’ 90° 00’ 89° 00’ 88 ° 00’ 87° 00’ 86 ° 00’ 85° 00’ rift axis 0 50 100 km 49° 00’ ANL--GNI (published) 49° 00’ GLIMPCE contractor (unpublished) ONTARIO depth of basin A 4 to 6 km Thunder > 6 km Fault Bay oyale B CANADA e R Isl 25 U.S.A. G 48° 00’ 43 Isle 48° 00’ 45 Royale C 47 53 y alle MINNESOTA 57 k V roc 04 Marquette 15 ed F B e ld g Basin 26 o d i W R e t it l 69 h au W F aw Keweenaw 47° 00’ 12 n 47° 00’ ee Peninsula DULUTH Allouez ew 08 10 K BAYFIELD Basin Porcupine Whitefish 18 Mountains Point ult 36 Au Sable as Fa MARQUETTE ugl Point Do Chequamegon MICHIGAN MUNISING Bay WISCONSIN Lake OwenFault 92° 00’ 91° 00’ 90° 00’ 89° 00’ 88 ° 00’ 87° 00’ 86 ° 00’ 85° 00’ Figure 1. Outline map of Lake Superior showing location of Argonne–Grant Norpac (ANL–GNI) reflec tion seismic lines (numbered) and GLIMPCE reflection seismic lines (alphabetic). Shaded areas show depth to base of Keweenawan sedimentary rock section. For conversion from travel time to depth, an average sedi mentary rock velocity of 4.5 km/s was assumed. Sedimentary rock section incorporates the composite facies (mixed sedimentary rock and volcanic rock unit).