UNITED STATES DEPARTMENT OF THE INTERIOR Harold L. Ickes, Secretary GEOLOGICAL SURVEY W. C. Mcndenhall, Director Professional Paper 184 PRE-GAMBRIAN ROCKS OF THE LAKE SUPERIOR REGION A REVIEW OF NEWLY DISCOVERED GEOLOGIC FEATURES WITH A REVISED GEOLOGIC MAP BY C. K. LEITH, RICHARD J. LUND, AND ANDREW LEITH UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON : 1935 For salo by the Superintendent of Documents, Washington, D.C. ............. Price 60 cents (paper cover) CONTENTS Page Abstract. _________-____________^__.-___-_._________._--__ _--...-_.--_-__-__-_--___---________-_____._____._ 1 Introduction. ________________________:___________________________!________ _ __________.__________________.____' 1 Principal changes from the old mapping.._______________________________________________________________________ 2 Michigan and Wisconsin.________:_____-____--_--__----_----_--___-__----___-_-_-_--__-___.___.___________ 3 Minnesota. ___-__-_-__-_-____._-______________-_---___-__-----_------..------__-_____-______-__-._______._ 5 Canada.________--l_________________-__________-...-_--____-______---________,--_________________________ 7 General succession..__________________________________________________________________________________________ 7 Cretaceous, Ordovician, and Cambrian rocks_________________________________________________________________ 7 Keweenawan series.___________--__.._________-_-_--__--_-_-.___--_-_-___-___-___---___-__.________________ 7 Huronian series____-_-___-_--------__--.i--_-----_--_---------------_-_---------_-_-_-__--_-___-__________ 8 Basement complex (Keewatin and Laurentian series) _-------___-_____---_-_---___-_-__________-_______.-.. 9 Correlation. _______________ _"_____________________---_-_-l-_-_-_-_-___-_____ _______________________________ 9 Keweenawan series_____..__-__-_--_---__-----__-------_--------_----------_---_---_------_-_---_-_-__-____._.. 10 New correlation of the Upper and Middle Huronian groups..______________________________________________________ 13 Michigan and Wisconsin,_____-_-________-_-__-_-_--------_-------_-_-_---___-----___--_-.________________ 13 Minnesota..._ ____________________________-__-_-_----------_-_-----_-_-_--__-_-___________-___.__________ 15 Age of the Knife Lake series and its equivalents_--_---_--------------------------_-__-___-_-_--------_-_________. 16 Relations to the unquestioned Huronian rocks__-__------__-_---__--_-_----___-_-_-_-_-_________.__________.. 16 Relations to the Keewatin series_______________-_--------------___---_--_-_-_-___-______-_________________ 17 Three periods of granitic intrusion.___________________-----._-_-_-_-------_^_-_-__------_--_-_____._____________ 18 Minnesota_ _---_--_____-_________________--_l__--_-_-__^___________-_-___--____________________________ 18 Wisconsin and Michigan. _______-_:________-_______-_----_ __---___________________________________________ 19 North shore of Lake Superior.__________---__-___--------_-_-_-----------__-__-_-_--__-____-__._______.____ 20 East shore of Lake Superior.________----_-__--------__-_-_----_---------------------_-----_--___-________. 20 General comments..______________________________--_----_-____-__-______-_-__-___-_____________________ i _ 20 Origin of the iron-formation.__.____---_-___-_-_-------------------_-_-----------_---_-_-_-__----__-__.________ 21 Different hypotheses,.___--___________--_---_-----------_-.---------------_---_-_---___--_-_-__-__________ 21 Iron bacteria and algae__________-____--__--__---_-------------------------_-_-_-_--__--_______-_____.._.__ 23 Conclusion ________________-_.-__--__---_-__------_----------------------------------_-__-________________ 23 Oxidation of the iron-formation and concentration to ore._-_----_-_---_____-____--_-_-____.__________________._____ 23 Agency involved.__.______________--_---_-_---_--_---------------------------_-___----___---_-_____..__._ 23 Restatement of processes of ore concentration._______________________________________________________________ 24 Bibliography __ ___._________..___--_-_---------_-------------------- _-----------__-----__--_-____--_________ 26 Index _____._-_._-_--.__.___-_____-- - --- ----- ----- ___ --_-__-_..__.____.__-_._ 33 ILLUSTRATION PLATE 1. Geologic map of the Lake Superior region and structure sections- _ In pocket. INSERT Page Correlation of formations in the Lake Superior region__ 10 n PRE-GAMBRIAN ROCKS OF THE LAKE SUPERIOR REGION A REVIEW OF NEWLY DISCOVERED GEOLOGIC FEATURES, WITH A REVISED GEOLOGIC MAP By C. K. LEITH, RICHARD J. LUND, and ANDREW LEITH ABSTRACT significance of the terms, which implied that the fEparchean Detailed kno\vledge of the geology of the pre-Cambrian rocks intervals of two widely separated districts were equivalent in of the Lake Superior region has been greatly augmented since time, has been abandoned. the publication of Monograph 52 by the United States Geologi­ The question of the origin of the iron-formations and the cal Survey iui 1911. The authors have attempted in the pres­ subsequent concentration of the iron ores is reviewed, and it ent report to assemble as much of this new information as pos­ is concluded that the theories presented in Monograph 52 are sible, and to give the reader a birdseye view of the geology essentially correct. The iron in the iron-formations is thought as seen in the light of all studies made there up to the pres­ to be derived principally from volcanic sources and possibly ent time. Detailed descriptions are not presented, and the to a lesser extent from,weathering; the concentration of the emphasis is placed upon the major problems of correlation. iron ores is due to oxidation and leaching of silica by down­ A revised nuip of the region, with cross sections, accompanies ward-moving surface, water. the report, bringing up to date a knowledge of the areal geol­ A'bibliography of literature bearing on the pre-Cambrian ogy of the region, which otherwise could be gained only by con­ geology.of the xegiori which has appeared since the manu­ sulting some 150 maps contained in almost as many separate script of, Monograph 52 \ya.s completed, in 1910, concludes the local reports, many of which are unpublished. report. ,.---,..... Principal changes fromo the map accompanying Monograph INTRODUCTION 52 are summarized, the most striking of which, aside from Around Lake-Superior,-in the- States of Michigan, changes resulting from shifts in correlations, are found in Can­ ada, northern Minnesota, and northern Wisconsin areas which Wisconsin, and. Minnesota" and the Province of On­ up to 1911 had not received the detailed study..which.has been tario, is-an area of pre-Cambrian rocks representing given them since that time. " , part of :the south margin of the great pre-Cambrian The geologic succession is, in ascending order, (1) the shield of North'.America., It yields about 85 percent Keewatin series, intruded by the Laurentian granite; (2) the of the\iron ore of; the United States and 10 percent of Knife Lake series (which may be Lower Huronian), intruded its copper. Production in this region is confined to by the Algoman granite; (3) the Huronian series, including the Lower, Middle, and Upper Huronian groups; (4) the' Ke- United States territory. ' The region has produced weenawan series, intruded by the Killarney granite, the exact more 'than 1,500,000,000 tons of iron ore, and tax com­ age of which is doubtful; and (5) the Cambrian, Ordoyician, mission estimates for Minnesota, Michigan, and Wis­ Cretaceous, and Pleistocene rocks. The series and- groups consin show-about the same amount in reserve. Exten­ named are separated by unconformities. sion of< known deposits (particularly in depth) and New views in respect to correlation of the pre-Cambrian rocks are discussed at length. The most noteworthy of these further discoveries will add. to this figure. If pro­ are (1) the correlation of the major iron-formations of the duction continues at the rate established since the Mesabi, Gogebic, Marquette, and Menoininee ranges as of World War, exclusive of the present depression, the Negaunee (Middle Huronian) age, and the iron-formations of peak of capacity for production will be passed in per­ the Cuyuna, Iron River, Florence, and Crystal Falls districts haps 25 years, after which there will be a falling-off as of Upper Huronian age; (2) the recognition of "the uncer­ tainty surrounding the age of the Knife Lake series, with an due to beginning exhaustion of .the Mesabi range of inclination to view it as pre-Lower Huronian, but pointing .out Minnesota, which has been the principal producer. the distinct possibility of its being Lower Huronian; (3) the The region has also produced about 4,000,000 tons of recognition and better delimitation of three, great periods of metallic copper, but the output of copper is already granitic intrusion Laurentian, Algoman, and Killarney the waning, because of low grade, great depth, and high Laurentian of wide distribution throughout the region, the cost. .,.-:. Algoman confined largely to the north shore of-Lake Superior, and the Killarney confined largely to the south shore. - Copper mining has- continued since 1844 in the One important change from Monograph 52 is in the use of Keweenaw district of Michigan. Iron mining began the terms " Archeau " and " Algonkian." In the present report in .the Marquette. .district of Michigan in 1848 and they designate two rock types a lower or basement group, slowly - spread over other parts of the region. The largely igneous, which is not divisible