14 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Laramide division extended through the shelf region of central and 

 eastern Wyoming, central Colorado, eastern Utah, and central New 

 Mexico, and is characterized by large, elliptical uplifts. 



The Laramide belt of orogeny extends southward through Mexico, 

 where thick sediments of the Mexican geosyncline of Upper Jurassic and 

 Cretaceous age are fairly tightly folded. The same belt of orogeny is 

 believed to veer eastward through Central America. 



Following well after the Nevadan and Laramide orogenies of western 

 North America, an episode of high-angle faulting occurred, that created 

 the Great Basin physiographic province and gave sharp definition to 

 many of its ranges and to those of central and western Mexico. The 

 high-angle faults were superposed on both the Nevadan and Laramide 

 belts; most of them are Late Tertiary in age and some are still active. A 

 long zone of the faults extends northward from central Utah to British 

 Columbia and probably beyond to Yukon Territory to form a belt of 

 trenches with local relief of 3000 to 5000 feet. The faults cut the older 

 folds and thrusts both discordantly and concordantly, and the activating 

 forces appear deep-seated. 



Coastal Plains 



Following the Appalachian orogeny in Triassic time, the outer meta- 

 morphosed division was broken by a belt of high-angle faults that has 

 been traced discontinuously from South Carolina to the Bay of Fundy, 

 between New Brunswick and Nova Scotia. Long and narrow downfaulted 

 basins trapped thick series of generally red elastics. The Triassic lowland 

 of Maryland, New Jersey, and Pennsylvania, and the central lowland of 

 Connecticut are the best known of the basins. 



The eastern extent or breadth of the Appalachian orogenic system 

 and the nature and condition of the crust that lay east of it are not 

 known, but the continental margin had begun to subside, at least by 

 Early Cretaceous time, if not before. The peneplained surface on the 

 crystalline rocks has been traced eastward under a Cretaceous and 

 Tertiary sedimentary cover to a depth of 10,000 feet, which is near the 

 margin of the present continental shelf. Most sedimentary units of the 

 cover dip gently and thicken like a wedge oceanward as far as they have 



been traced by deep drilling and by seismic traverses. The zone of 

 Cretaceous and Tertiary overlap on the older rocks of the eastern con- 

 tinental margin is known as the Atlantic Coastal Plain, but because the 

 same sediments continue out beyond the present ephemeral shore line, 

 the submerged part belongs to the same province. Coastal plain sedi- 

 ments are known to exist in Georges Bank off Rhode Island and prob- 

 ably make up part of, or all, the shallow continental shelf to and 

 including the Banks of Newfoundland. 



The Gulf Coastal Plain is continuous with the Atlantic Coastal Plain, 

 and counting its shallowly submerged portions, it nearly encloses the 

 Gulf of Mexico. The oldest known sediments of its marginal areas are 

 Permian. The Mississippi, Rio Grande, and other rivers draining the 

 interior of the continent have deposited a great weight of sediments at 

 their mouths and the crust has subsided along the Texas, Louisiana, and 

 Mississippi coast to the extent of 25,000 to 30,000 feet. 



Deep drilling in Florida and the Bahamas indicates that the coastal 

 plain province extends southeastward almost to the orogenic belt of 

 Cuba and Hispaniola. 



Canadian Arctic 



The Precambrian rocks of the Canadian Shield are overlapped on the 

 north by nearly flat-lying sedimentary strata of Paleozoic age. Basins 

 and arches are recognized in this province as in the Central Stable 

 Region of the United States. North of the arches and basins is a fold 

 belt developed in geosynclinal sediments. The fold belt extends across 

 northern Greenland, northern Ellesmere Island and farther to the south- 

 west through other islands of the Arctic Archipelago. Folding first oc- 

 curred in pre-Pennsylvanian time. After erosion a voluminous sequence of 

 Pennsylvanian to Tertiary sediments accumulated, and then these were 

 somewhat folded in Tertiary time. A narrow Tertiary coastal plain is 

 terminated on the north by the Arctic Ocean basin. 



Alaska 



Alaska continues the broad and complex western cordillera across to 

 Asia, and has had basically the same history but with variations and 

 singular details. 



