PRECAMBRIAN TECTONIC PROVINCES 



23 



ithe shield are Devonian and Silurian, and are chemical deposits or fine 

 elastics. Along the southern margin of Hudson Bay is a fairly large area 

 pf flat-lying Devonian, Silurian, and Ordovician sedimentary rocks, and 

 :rom fossil studies it seems probable that the Manitoba, Hudson Bay, and 

 Michigan Devonian deposits were once continuous (G. M. Ehlers, per- 

 sonal communication). The thickness of the Devonian and Silurian south 

 af Hudson Bay is at least 1000 feet, but their extension northward under 

 ;he Bay's waters is not known. It can easily be imagined that they are 

 continuous to Coats and Mansel islands at the entrance of Hudson Bay 

 ind thence to the nearly horizontal Paleozoic strata of Southampton 

 island and the Arctic Archipelago. If continuous, one wonders if some- 

 where in that large area the beds are not thick and form a trough or 

 basin, perhaps similar to the Michigan basin. In fact, basins and arches 

 have been recognized in the far north, and are described in Chapter 40. 



It has been thought until lately that the Canadian Shield was com- 

 paratively free of epeiric seas in the past; but now, by the discovery 

 pf a number of small erosional remnants of Paleozoic strata far within 

 'the crystalline rocks (W. Sinclair and J. Tuzo Wilson, personal com- 

 jmunications ) , it is believed that large areas were blanketed by sediments. 

 Perhaps very little escaped submergence. What seems more important is 

 'that no orogenic belts developed across it during all of post-Proterozoic 

 time. The same is true with some exceptions of the stable region of the 

 United States. 



In the iron ore belt of central Labrador (the Redmond iron deposit) 

 downfaulting of a trench occurred in early Late Cretaceous time, and in 

 jit various argillites and ferric concretionary deposits accumulated. The 

 Redmond deposit is in a basin 1 mile long, 1000 feet wide, and 600 feet 

 deep. Abundant plant fossils in certain beds serve to date the deposits 

 |and the faulting. The extent of the Cretaceous faulting is not known 

 !(R. A. Blais, 1959). 



From simple map examination, it looks probable that Greenland was 

 part of the Canadian Shield until Cretaceous time when, perhaps, a 

 Cretaceous trough extended as far north as Disco Island. Greenland was 

 further severed from the shield either by Tertiary downfaulting or by 

 drifting apart. See Chapter 40. 



Geologic Provinces 



The Canadian Shield until recently has been difficult of access, and 

 this with the extensive "bush" cover has made geologic exploration 

 pensive of energy and slow. The advent of airplanes and aerial photos 

 has hastened the work immensely, and a beginning has now been made 

 in analyzing the composition of the great Precambrian shield. But the 

 time has not yet arrived, according to M. E. Wilson, when the vast region 

 can be broken down into divisions with confidence. He draws approxi- 

 mate boundaries between five provinces (see map, Fig. 4.1), namely, 

 the Western or Churchill, the Ungava, the Arctic Island, the Greenland, 

 and the St. Lawrence. The last is divided into subprovinces, the North- 



Fig. 4.1. Geologic provinces of the Canadian Shield best suited at present for individual forma- 

 tional names. After M. E. Wilson, 1958. 



