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Paleomagnetism of Some Lake Superior Keweenawan Rocks

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Paleomagnetism of Some Lake Superior Keweenawan Rocks GEOLOGICAL SURVEY PROFESSIONAL PAPER 760 Paleomagnetism of Some Lake Superior Keweenawan Rocks By KENNETH G. BOOKS GEOLOGICAL SURVEY PROFESSIONAL PAPER 760 Remanent magnetization directions for lower and middle Keweenawan rocks fall into three groups that have their polarities alternately normal) reversed) and normal UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1972 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 72-600117 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 50 cents (paper cover) Stock Number 2401-2180 CONTENTS Page Page Abstract ___________________________________________ _ 1 Ironwood-Mellen area _______________________________ _ 12 Introduction ________________________________________ _ 1 Geologic setting _________________________________ _ 12 Description of results ____________________________ _ Acknowledgments ___________________________________ _ 3 16 Lowermost E:eweenawan lava flows ____________ _ Field and laboratory techniques _______________________ _ 3 16 Portage Lake Lava Series ____________________ _ Collection and measurement ______________________ _ 16 3 North Shore of Lake Superior ________________________ _ Sources of error _________________________________ _ 16 3 Geologic setting _________________________________ _ 16 Magnetic cleaning _______________________________ _ 4 Description of results ____________________________ _ 16 General geology _____________________________________ _ 4 North Shore Volcanic Group __________________ _ 16 E:eweenawPeninsula ________________________________ _ 5 Isle Royale lava flows _______________________ _ 18 Discussion of results _________________________________ _ Geologic setting _________________________________ _ 5 18 Lower Keweenawan _____________________________ _ Description of results ____________________________ _ 6 18 Middle E:eweenawan ____________________________ _ 20 Portage Lake Lava Series ____________________ _ 6 Portage Lake and North Shore sequences ______ _ 22 Rhyolite flows north of Lake Gogebic, Mich ____ _ 6 Quartz porphyry north of Lake Gogebic, Mich __ _ 22 Quartz porphyry north of Lake Gogebic, Mich __ _ 12 Upper Keweenawan _____________________________ _ 24 Rhyolite intrusion in sec. 4, T. 56 N ., R. 32 W ___ _ 12 Rhyolite intrusions __________________________ _ 24 Rhyolite intrusion in sees. 24 and 25, T. 56 N ., Summary and conclusions ____________________________ _ 27 R. 34 W _________________________________ _ 12 References cited _____________________________________ _ 40 ILLUSTRATIONS Page FIGURE 1. Generalized geologic map of the Lake Superior region__________________________________________________ 2 2. Simplified columnar section for the Portage Lake Lava Series near E:earsarge, Mich _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 7 3-15. Equal-area diagram showing directions of remanent magnetization for- 3. Flows of the Portage Lake Lava Series below conglomerate No. 14 near E:earsarge, Mich_____________ 8 4. Flows of the Portage Lake Lava Series between conglomerate No. 14 and conglomerate No. 15 near Kearsarge, Mich_________________________________________________________________________ 9 5. Flows of the Portage Lake Lava Series above conglomerate No. 15 near Kearsarge, Mich_ _ _ _ _ _ _ _ _ _ _ _ _ _ 10 6. Flows of the Portage Lake Lava Series northeast of those near E:earsarge, Mich_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 13 7. Middle E:eweenawan rhyolite flows north of Lake Gogebic, Mich__________________________________ 15 8. Quartz porphyry north of Lake Gogebic, Mich________________________________________________ 17 9. Rhyolite intrusion in sec. 4, T. 56 N., R. 32 W________________________________________________ 19 10. Rhyolite intrusion in sees. 24 and 25, T. 56 N., R. 34 W _ _ _ __ _ _ _ _ __ _ _ __ _ _ ___ _ ____ _ _ __ _ _ _ ___ ______ _ 21 11. Basal quartzite and lowermost 400 feet of South Trap Range lava flows near Ironwood, Mich_ _ _ _ _ _ _ _ 23 12. Middle E:eweenawan flows from the Chippewa Hill quarry, Michigan_____________________________ 25 13. Middle Keweenawan flows on the Black River near Algonquin Falls, Mich_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 26 14. North Shore Volcanic Group, Minnesota_______________________________________________________ 28 15. Middle E:eweenawan lava flows on Isle Royale, Mich___________________________________________ 30 16-18. Equal-area diagram showing- 16. Summary of mean directions of remanent magnetization for lower(?) E:eweenawan rock units________ 32 17. Directions of remanent magnetization for pre-E:eweenawan units near Ironwood, Mich., and Grand Portage, Minn__________________________________________________________________________ 33 18. Summary of mean directions of remanent magnetization for middle E:eweenawan rock units__________ 34 19. Columnar sections showing comparison of chronological movements in inclination of the magnetic fields of the Portage Lake Lava Series and North Shore Volcanic Group________________________________________ 35 20. Columnar sections showing comparison of chronological movements in declination of the magnetic fields of the Portage Lake Lava Series and North Shore Volcanic Group________________________________________ 36 III IV CONTENTS Page FIGURES 21-23. Equal-area diagram showing- 21. Comparison of mean directions of remanent magnetization for lava flows near Kearsarge and on Isle Royale_________________________________________________________________________________ 37 22. Alternative directions of remanent magnetization for the quartz porphyry north of Lake Gogebic, Mich_ _ 38 23. Directions of remanent magnetization for rhyolite intrusions in T. 56 N., R. 32 W., and T. 56 N., R. 34 W ., near Kearsarge, Mich_ __ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ __ _ _ _ __ _ __ __ _ _ __ _ _ _ _ __ _ __ __ __ __ _ __ __ _ _ _ _ _ _ 39 TABLES Page TABLE 1. Partial demagnetization data for four sites on the Portage Lake Lava Series________________________________ 4 2. Paleomagnetic results for Portage Lake Lava Series near Kearsarge, Mich_ __ __ __ _ _ _ __ _ _ _ __ __ _ __ _ _ _ __ __ __ __ 11 3. Paleomagnetic results for miscellaneous flows of the Portage Lake Lava Series northeast of Kearsarge, Mich____ 14 4. Comparison of declination and strike along the Ashbed Flow in the Portage Lake Lava Series________________ 14 • 5-141 Paleomagnetic results for- 5. Middle Keweenawan rhyolite flows north of Lake Gogebic, Mich___________________________________ 16 6. Quartz porphyry north of Lake Gogebic, Mich___________________________________________________ 18 7. Rhyolite intrusion in sec. 4, T. 56 N., R. 32 W., east of Kearsarge, Mich____________________________ 20 8. Rhyolite intrusion in sees. 24 and 25, T. 56 N., R. 34 W., southwest of Kearsarge, Mich______________ 22 9. Basal quartzite and lowermost 400 feet of South Trap Range flows near Ironwood, Mich_ __ _ _ _ __ _ __ _ _ _ 24 10. Portage Lake lava equivalents at the Chippewa Hill quarry, Michigan______________________________ 27 11. Portage Lake lava equivalents on the Black River near Algonquin Falls, Mich_______________________ 27 12. North Shore Volcanic Group in Minnesota______________________________________________________ 29 13. Middle Keweenawan lavas on Isle Royale, Mich_________________________________________________ 31 14. Pre-Keweenawan rocks near Ironwood, Mich., and Grand Portage, Minn____________________________ 31 15. Summary of new paleomagnetic data for rock units in the Lake Superior region_____________________________ 31 16. Tentative correlation chart of Lake Superior Keweenawan units based on magnetic polarity sequences_________ 40 PALEOMAGNETISM OF SOME LAKE SUPERIOR KEWEENAWAN ROCKS By KENNETH G. BooKs ABSTRACT region has been the object of geologic investigations The averaged directions of remanent magnetization for the for more than a century, and the section of lava flows Keweenawan rock units investigated fall into three general group­ of Keweenawan age near the lake is probably as well ings that have their polarities alternately normal, reversed, and known as any such succession in North America. normal. The older normal polarity and subsequent reversed polarity groups are found in lower Keweenawan rocks and have Nevertheless, the positioning of some nonfossiliferous significantly different directions from the younger group with rocks in the geologic column has been hard to assess, normal polarity which characterizes middle Keweenawan rocks. because of inherent difficulties in determining time From all av..ailable paleomagnetic data for this area, it seems that relationships between extrusive rocks at some distance magnetization directions among various rock units in the lower from each other, and even more difficulty in relating Keweenawan around Lake Superior are characteristically scat­ tered, which may indicate long periods between volcanic events, different intrusive rocks of the same age. a rapid rate of change in magnetic field directions, or changes due Before radiometric dating methods were developed, to metamorphism. Less scatter of magnetization directions among most Precambrian rocks were correlated by comparison middle Keweenawan rock units may indicate a shorter time in­ of facies and petrographic similarities, unconformities, terval between volcanic events or a slower rate of change in mag­ netic field direction. Upper
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