490 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



region, but the submerged island is now separated from Ceralbo by a deep, 

 narrow basin with its bottom 6000 feet below sea level and a sill depth of 4800 

 feet. Immediately northwest of Isla Ceralbo there is a smaller submerged hill 

 about the same size as Ceralbo; its crest lies only 600 feet below sea level, and 

 it may originally have been a part of the same mountain mass. If facts can 

 finally be collected on the structure of the gulf floor east and northeast of the 

 Southern Cape region, they will probably show that the deep basin immediately 

 east of San Jose del Cabo has been caused by north-south faulting parallel to 

 the La Paz fault, and that the deep narrow basins farther north owe their origin 

 to northwest-southeast faulting, with the same structural trend as the gulf 

 trough. 



In seeking for the cause of the broad, deep basins in the gulf below the 28th 

 parallel, one may conjecture that they are probably structurally depressed, 

 wedge-shaped blocks, bounded by faults. Ballenas Channel and the depression 

 east of Isla Ceralbo, both of which appear to be grabens, may have originated 

 in the same way. If they were deep troughs with open ends, instead of elongated 

 steep-sided basins, their unusual depths might have been attained by the erosive 

 action of the gulf currents. It appears that their great depth as basins, however, 

 can logically be explained only by assuming the basins to be the apices of 

 structurally depressed wedges. 



Tertiary sediments in the Salton basin and farther northwest may be 

 very thick, and Real suggests that basement rocks under the north end of 

 the gulf trough may be 25,000 to 30,000 feet below sea level. It is gen- 

 erally recognized that the Colorado delta has contributed much toward 

 filling the trough and making the present floor shallower. 



SIERRA MADRE OCCIDENTAL 



Early Tertiary Phase 



According to R. E. King (1939), the Sierra Madre Occidental takes 

 form south of the international boundary by the coalescing of mountain 

 ranges which, in southern New Mexico and Arizona, are more or less 

 isolated. South of the boundary, the plains between the mountains be- 

 come narrower, and the volcanic rocks spread out in a broad plateau. 

 The western edge of the plateau, at an elevation of 6000 feet or more, 

 breaks off toward the Gulf of California in lofty escarpments which are 

 trenched by most impressive gorges. West of the Sierra Madre proper, 

 high ranges are separated by long, narrow valleys. Still farther 

 west, bordering the gulf, low mountains are separated by broad 



plains, as in the Basin and Range province of southwestern United States. 



The three geomorphic divisions have been called, by King, the Sierra 

 Madre Occidental province, the province of parallel ranges and valleys, 

 and the Sonoran Desert province. 



The Sierra Madre Occidental has generally been assumed to be a struc- 

 turally simple plateau of flat-lying lavas overlying a basement of sedi- 

 mentary rocks and ancient granites, but a reconnaissance survey by 

 R. E. King ( 1939 ) has added greatly to our knowledge of the region and 

 revealed a complex structural history. The rocks studied by King have 

 been much folded and faulted and are intruded by numerous plutons 

 of fairly large size. There are several periods of deformation, but only 

 those of the Tertiary can be deciphered with any assurance. Two un- 

 conformities in the Tertiary mark times of important mountain building. 

 The structural features produced by the Tertiary episodes of deformation 

 trend in general north-northwest, and produce a conspicuous alignment of 

 rock outcrops and ridges. 



The effects of the Laramide revolution have already been mentioned 

 in connection with the Mexican geosyncline. See lower two sections of 

 Fig. 30.6. Early Tertiary volcanic rocks spread out over much of the sur- 

 face of western Sonora but reached their greatest development in the 

 plateau section of the Sierra Madre Occidental. See third section from 

 bottom of Fig. 30.6. In the plateau section, the underlying Mesozoic rocks 

 are probably greatly deformed, for such disturbance is evident along the 

 western edge of the plateau and in the few inliers within the plateau and 

 in the Nevadan type rocks of Baja California. The later or post-volcanic 

 deformations strongly expressed to the west in the parallel ranges and 

 Sonoran Desert have, however, scarcely affected this region. Over wide 

 areas, the volcanic rocks are more than 5000 feet thick and are flat or 

 gently tilted. They consist of flows and pyroclastics with basalts dominant 

 in northern Sonora (Imlay, 1939) and more acidic types most voluminous 

 in central Sonora (King, 1939). The volcanic layers were then uplifted 

 epeirogenically thousands of feet, evidently, because an erosion surface 

 developed to maturity on them. It is now deeply dissected by the present 

 cycle of erosion (King, 1939). See fourth section from bottom, Fig. 30.6. 



Toward the west the plateau gradually loses its structural simplicity. 



