RESUME OF STRUCTURAL GEOLOGY OF NORTH AMERICA 



I . 



inner belt, and this in late Paleozoic time. Several orogenies beset the 

 outer belt: the earliest one of significance occurred at the close of the 

 Ordovician, the major one during the Late Devonian and the last one in 

 Pennsylvanian and Permian time. The Carboniferous orogenic belt in 

 the outer crystalline division is recognized on the north along the eastern 

 margin of New England, the Maritime Provinces, and Newfoundland. 



Volcanic rocks and great batholiths are important components of the 

 crystalline division of the Appalachian orogenic belt, but the inner folded 

 and thrust-faulted belt is comparatively free of them. Roth divisions are 

 made up of very thick sedimentary sequences which are characterized 

 as geosynclinal, in contrast to generally thinner sequences in the Central 

 Stable Region. 



The orogenic belt bordering the southern margin of the stable interior 

 is mostly concealed by overlapping coastal plain deposits. Where exposed, 

 as in the Ouachita Mountains of Arkansas and eastern Oklahoma, the 

 Arbuckle Mountains of south central Oklahoma, and the Marathon Moun- 

 tains of western Texas, it is a folded and thrust-faulted complex, some- 

 what similar to the inner Appalachian division. The crystalline division, 

 if it parallels the inner noncrystalline division, is nowhere exposed, but 

 deep wells through the coastal plain deposits have penetrated low-grade 

 metamorphic rocks. 



Orogenic Belts of the Pacific Margin 



The great complex of orogenic belts along the Pacific margin of the 

 continent has evolved through a very long time. The oldest strata recog- 

 nized from their fossils are Ordovician, and deformed strata of Pleistocene 

 age mark the belt in places from Mexico to Alaska. In Paleozoic time, the 

 Pacific margin of the continent was a volcanic archipelago in outward 

 appearance and internally a belt of deformation and intrusion. The 

 Permian, Triassic, and Early and Middle Jurassic were times of excessive 

 volcanism, and represent a continuation of essentially the same Paleozoic 

 conditions well into the Mesozoic. In Late Jurassic and early Late 

 Cretaceous time, intense folding and batholithic intrusions (Nevadan 

 orogeny) occurred which are now characteristic of large parts of the 



Coast Range of British Columbia, the rangei along the International 

 border in British Columbia, Washington, and Idaho, the Klamath Moun- 

 tains of southwestern Oregon and northern California, the Sierra Nevadi 

 of California, and the Sierra of Baja California. The same Nevadan ele- 

 ments may also continue into southern Mexico and eastward through 

 Central America. 



Following the orogeny, in California at least, a new trough of accumu- 

 lation and a new volcanic archipelago formed outside the Nevadan belt, 

 and a complex history of deformation and sedimentation tarries down 

 through the Cretaceous and Tertiary to the present, to result in the Coast 

 Ranges of Washington, Oregon, and California. 



Orogenic Belts of the Rocky Mountains 



During the complex and long orogenic history of the Pacific margin, 

 the adjacent zone inward was one of gentle subsidence and sediment 

 accumulation, comparatively free of volcanic materials, during the 

 Paleozoic. By Triassic time, the troughs of deposition along the Pacific 

 had become effectively separated by a medial, linear uplift from those 

 in the Rocky Mountain area, and in the Mesozoic much coarse debris 

 came from the uplift or geanticline and filled the basins in eastern British 

 Columbia, western Alberta, Idaho, western Wyoming, central Utah, and 

 southern Nevada. Orogeny from place to place along the eastern margin 

 of the geanticline cast several floods of conglomerate eastward during 

 the Cretaceous. 



The Paleozoic and all the Mesozoic sediments except the Upper 

 Cretaceous of the Rocky Mountains may be divided into thick geosyn- 

 clinal facies on the west and fairly thin shelf facies on the east. The line 

 dividing the two lies approximately along the west side of the Colorado 

 Plateau and thence runs northward through western Wyoming and 

 Montana to western Alberta. The shelf facies were part of the Central 

 Stable Region until the Late Cretaceous and Early Tertiary ( Laramide ) 

 orogeny in whose belts both geosynclinal and shelf facies were deformed. 

 The western division of the Laramide belt (in the miogeosyncline) is 

 characterized by folds, thrusts, and numerous small stocks. The eastern 



