26 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



greater Algoman orogeny ( see Fig. 4.2 ) . The latter occurred about 2500 

 m.y. ago, although the very ancient dates range from 2200 to 2600 m.y. 

 The name Algoman is here used for the belt of ancient dates through the 

 southern part of the Canadian Shield. It has been variously called the 

 Keewatin and Superior by other writers. 



The Algoman and the Slave (also called Yellowknif e ) provinces are 

 the oldest known and possibly parts of the original nucleus of the con- 

 tinent. They have a high ratio of lavas to sediments which are of the 

 graywacke facies, presumably deposited in geosynclinal basins. The 

 Churchill province is considered an orogenic belt by which the two 

 nuclei were welded together (J. Tuzo Wilson, 1949, 1954). See also 

 Farquhar and Russell (1957) and Lowdon (1960). 



A belt of Huronian rocks extending from Minnesota through Wisconsin 

 into Michigan and lying south of the main Algoman belt has ages of about 

 1700 m.y. Goldich et al. ( 1961 ) call it the Penokean orogenic belt, and 

 the name has been applied in this book to adjacent regions on the south- 

 west in the United States and on the northwest in Canada. 



The Grenville subprovince of M. E. Wilson approximately is postulated 

 as an orogenic belt about 1000 m.y. old. Its deformed front borders 

 directly on the Algoman province. Southeast of the Grenville belt are 

 the Taconic and Acadian orogenic belts, about 400 and 300 m.y. old, 

 respectively. 



Eighty-three isotopic age analyses on biotite, K-feldspar, and whole- 

 rock samples from forty-five localities, using both K-Ar and Rb-Sr 

 methods have been made on igneous rocks and a few metasediments in 

 the Sudbury-Blind River area of the Grenville belt by Fairbairn et al. 

 (1960). 



The numbers obtained, forming an almost continuous age spectrum from 

 1.0 b.y. to 2.2 b.y., are correlative with widespread and repeated diastrophism 

 in the region. Whole-rock analyses of igneous material, where available, show 

 higher ages than coexisting minerals in most examples, and there is reason to 

 believe that these are close approximations to the true age. There is consider- 

 able evidence by both K-Ar and Rb-Sr methods, of orogenic events at ap- 

 proximately 1.0 b.y., 1.2 b.y., and 1.6 b.y. 



The oldest igneous rock found thus far is the Copper Cliff "rhyolite" (2200 

 m.y.), which intrudes the basal section of a thick series of conformable 



metasediments and volcanics southeast of Sudbury. At Quirke Lake granite in 

 the basement, uncomformably beneath U-bearing pebble beds, is 2050 m.y. 

 old. As the time of uranium mineralization in these Huronian sediments is 

 placed at 1700 m.y., and gabbro which intrudes them may possibly be older 

 than 1800 m.y., their deposition must have been in the age bracket 1800-2050 

 m.y. 



ARCTIC STABLE REGION 



South of the orogenic belt of northern Greenland and Ellesmere Island 

 and north of the Precambrian Canadian Shield is a stable region com- 

 posed of a Precambrian crystalline basement with a veneer of nearly 

 horizontal Paleozoic sedimentary rocks. It includes most of the Arctic 

 islands, and the shallow sea-covered areas between. See the Geologic 

 Map of North America or the Geologic Map of Canada. The Precambrian 

 rocks of the shield extend northward into Baffin and Devon islands, and 

 exposed extensively in Melville and Boothia peninsulas, but the Paleozoic 

 blanket indicates that much, if not all, of the Arctic islands region 

 (also called Arctic Archipelago) and the northern part of the Canadian 

 Shield were submerged at times during the Paleozoic. The part south 

 of the fold belt ( Chapter 35) has suffered only gentle vertical movements 

 since the Proterozoic, and is therefore part of the great stable interior of 

 the continent. The Precambrian crystalline rocks extend southward into 

 the United States under a veneer of Paleozoic sedimentary rocks com- 

 monly called the Central Stable Region. It seems appropriate, therefore, 

 to speak of the similar northern geologic province as the Arctic Stable 

 Region. 



PRECAMBRIAN PROVINCES OF THE UNITED STATES 



Isotope Age Determinations 



Recent age determinations fall into a pattern that marks successive 

 orogenic belts in the central, southern, and western states of the United 

 States, and these are shown in Fig. 4.3. The ages pertain to rocks gen- 

 erally called Archean or basement complex. In Arizona, Utah, Idaho, and 

 Montana, younger and much less metamorphosed strata rest unconform- 



