24 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Erm 

 (HfYeen) 



Penod-Syilem 



Major Sequence 



Formation 





Orogeny 



Intrusive Rocka 



Ptlcoxoic 



C»mbnmn 









Grenville 

 . Penokean 



. Algoman 

 -Lauxentiaa 





(0* b,y.) 

 (11 bjr.) 



lati mtawa 



Keweenawan 



Hinckley sandstone 

 Fond du Lac sandstone 



North Shore volcanic Undivided 

 group 





Duluth complex, sills at DulurJi, Beaver Bay 



complex, Logan intrusive* 







Puckwunge 



Sioux quartxite (?) 



= Thomson 

 Gunflint 



Granite: St. Cloud Red, Rockville (?) granite at 



Granite Falls, Bellingham (?) 

 Gneiss: McGralh. Montevideo (?) 



(1.7 bj.) 



Haiooiu 



Animikie froup 



Virginia argHlile = Rove 

 Biwabik iron-formation = 



Tonalites: St Cloud Gray. Warman, Hillman. 

 Freedbem, Montevideo 



MwMlr Preewrabrita 



Pokegama quartxile 





Granite: Gold Island, Giants Range, Sacred 











«•» bjr.) 

 (fbjr.) 



Early Pircambriao 



Tuxttskaraiao 



OnUnui 



Knife Lake group 

 Keevatin group 



Undivided 



Soudan iron- formation 



Ely greenstone 



Gneiss: Giants Range, Vermilion, Morton 

 Saganaga granite, Grassy Island lonalile (?) 





Coulchiching (?) 



Undivided 











Older rocka 







Fig. 4.2. Stratigraphic succession and geochronology of the Precambrian of Minnesota. Repro- 

 duced from Goldich ef a/., 1961. 



west, the Southern, the Timiskaming, and the Grenville. The provinces 

 and the subprovinces thus defined represent natural divisions and the 

 limits to which attempts should be made to correlate rock units. Wilson 

 recommends separate names for formations, series, or intrusive bodies 

 within each of these divisions at least for the present. 



Geologists in recent culminating studies in the iron and copper region 

 of Lake Superior recognize a threefold division of the rocks ( Grout et ah, 

 1951, James, 1955, and Goldich et ah, 1961). The Precambrian of Minne- 

 sota is classified by Goldich et ah after many radioactivity age determina- 

 tions, as shown in Fig. 4.2. 



Previously, in 1934, a committee of the Royal Society of Canada on 

 stratigraphical nomenclature had recommended that Precambrian time 

 be divided into two eras, Archean and Proterozoic, and since then this 

 classification has been used on most geologic maps issued by the 

 Geological Survey of Canada. M. E. Wilson in 1958 contends that the dual 

 classification is still the best and includes the Middle Precambrian rocks 

 of Grout and James in the Proterozoic. In all provinces of the Canadian 

 Shield a profound unconformity is known, by reference to which the 

 rocks can be divided into two great groups (M. E. Wilson, 1958). The 

 standard of reference, for instance, is the rock succession on Lake Timis- 



kaming where the Huronian (Cobalt series) rests "with great uncon- 

 formity" on granite — the Laurentian. 



The Archean rocks consist of clastic sediments and various volcanic 

 rocks conformably interbedded, and even though extremely old they are 

 so little affected by metamorphism in places that original sedimentary 

 structures are clearly visible. In many other places they are meta- 

 morphosed to various degrees. According to Pettijohn (1943) one may 

 study the bedding in certain argillites in the finest detail, and the asso- 

 ciated volcanics show pillow structures, amygdules, spherulitic structures, 

 the same as in lavas of much later geologic time. Metamorphism is mainly 

 of the low-grade variety, and orogeny has left the very ancient rocks of 

 many areas untouched. Recognizing the near-absence of metamorphism 

 in places, however, it must also be understood that enormous volumes of 

 intrusive igneous rocks occur, and estimates have been made that these 

 intrusive rocks constitute as much as 80% of the shield. The great bulk 

 of these are granites of various types, with relatively small but important 

 amounts of basic rocks such as gabbro, norite, and peridotite. Need- 

 less to say, much metamorphism has occurred and gneisses and schists 

 (migmatites) are extensively developed. 



The Archean sediments of the southern Canadian Shield are mainly gray- 

 wacke. Much conglomerate, a litde slate, and still less iron-bearing formation 

 are also present. Excessive thickness, especially of the conglomerates, abundance 

 of graded bedding, rarity of cross-bedding and absence of ripple mark, the 

 graywacke nature of the arenaceous beds, the absence of true quartzites and 

 limestones, and the scarcity of normal argillaceous sediments, and the associa- 

 tion with greenstones and tuffs are all the earmarks of a geosynclinal facies of 

 sedimentation (Pettijohn, 1943). 



In particular, these types characterize the eugeosyncline, and since they 

 are repeated in later Precambrian rock series, it is little wonder that 

 confusion in correlations has resulted. 



In eastern Ontario and adjacent parts of Quebec the oldest rocks are 

 sedimentary gneisses associated with great thicknesses of crystalline 

 limestone and a little basic metavolcanics. These rocks are termed the 

 Grenville series. They appear to have been originally shales, sandstones, 

 limestones, and some lavas, but owing to the intense metamorphism, they 

 are now biotite schists and sillimanite-garnet gneisses, vitreous quartzite, 

 and crystalline limestones. 



