3. 



RESUME OF STRUCTURAL 

 GEOLOGY OF NORTH AMERICA 



MAJOR TECTONIC DIVISIONS 



Canadian Shield 



The Canadian Shield has been the great stable portion of the North 

 American continent since Proterozoic time. It consists of Precambrian 

 rock except along the southern margin of Hudson Bay, where Ordovician, 

 Silurian, and Devonian strata, about 1000 feet thick, occur and probably 

 continue northward under much of the bay. Small outliers of Paleozoic 

 strata, fossil affinities, and the absence of shore facies in many places 

 indicate that the Paleozoic formations were once much more widespread 



over the shield than now, and that they have been stripped off by a long 

 interval of erosion during the Mesozoic and Cenozoic eras. 

 Hudson Bay is an epeiric sea of fairly modern time. 



Central Stable Region 



The Central Stable Begion consists of a foundation of Precambrian 

 crystalline rock, which is a continuation of the Canadian Shield south- 

 ward and westward, and a veneer of sedimentary rock. The veneer varies 

 greatly in thickness from place to place, and several broad basins, arches, 

 and domes are present. A number of unconformities attest the rise of 

 the arches and their erosion, and of great transgressions and overlaps. 

 For the most part the strata have only gentle dips, and aside from the 

 slow and prolonged vertical movements that created the basins, arches, 

 and domes, the geologic province properly deserves the name, the Central 

 Stable Begion. It and the Canadian Shield compose the great stable in- 

 terior of the continent. 



The arches and basins developed chiefly in the Paleozoic era, but later, 

 during the Mesozoic and Tertiary, vast amounts of clastic sediments from 

 the evolving Cordilleran mountain systems were spread eastward over 

 the Paleozoic strata beyond the Missouri Biver as far as Lake Superior. 



In the southwestern corner of the Central Stable Begion a system of 

 ranges was elevated in Pennsylvanian time, and then during the late 

 Pennsylvanian, Permian, and Mesozoic it was largely buried. The ranges 

 are known as the Ancestral Bockies in Colorado and New Mexico, and 

 as the Wichita Mountain system in Kansas, Oklahoma, and Texas. The 

 Late Cretaceous and Early Tertiary Laramide structures were partly 

 superposed on the Ancestral Bockies in Colorado and New Mexico. 



Orogenic Belts of the Atlantic Margin 



The Paleozoic orogenic belts bound effectively the southern, as well as 

 the eastern, margin of the continent. The major belt is known as the 

 Appalachian, and it consists of an inner folded and thrust-faulted division 

 from Alabama to New York, and a metamorphosed and intruded division 

 from Alabama to Newfoundland. One major orogeny occurred in the 



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