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PACIFIC PENINSULA 



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STRUCTURAL GEOLOGY OF NORTH AMERICA 



GULF 



SONORA DESERT AND 

 PARALLEL RANGES 



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SIERRA MADRE 

 OCCIDENTAL 



PLEISTOCENE 



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SALAOA FM. AND ", 



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BAUCARIT SE05. * V0LCANIC3 



LATE TERTIARY 



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YELLOW BCDJ OR YSIORO FM. 



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MID-TERTIARY OROGENY^- 





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BY LATE CRETACEOUS 



Fig. 30.6. Evolution of the provinces of western Mexico from late Cretaceous time to the 

 present. Diagrams are highly idealized. 



height; faulting and folding probably occurred along the east side of the penin- 

 sula; and further movement may have occurred along the Ceralbo and Bellenas 

 fault zones and along unmapped faults in the adjacent sea bottom. 



Some of the islands probably continued as independent structural units, the 

 upward or downward movement of one not necessarily being coincident with 

 or dependent upon the movement of another or with that of the peninsula as a 

 whole. This is indicated by steeper dips in the Comondu on many of the islands 

 than on adjacent parts of the peninsula, though some or much of this deforma- 

 tion may have occurred in the Pleistocene. The Comondu often shows more 

 deformation on the east coast than farther west, which suggests that the major 

 structural forces were more effective on that side. If true, that may have re- 

 sulted in the first westward tilting of the peninsula. 



The Pliocene history is complex and not well understood (Real, 1948). 

 Pliocene deposits in places indicate sea-level deposition, but since now 

 observed at elevations over 1000 feet, late Pliocene or Pleistocene uplift 

 must be postulated. In places the uplift is believed to be post-Comondu 

 but pre-Pliocene. 



An area of greater significance is the known marine Pliocene at Santa Rosalia, 

 which has been elevated to about 500 feet, possibly more. According to in- 

 formation from Mr. Ivan F. Wilson, the underlying Comondu formation occurs 

 in a series of fault blocks cut by southwesterly dipping faults. As these faults 

 are probably of pre-Pliocene age, the sierra southwest, composed principally of 

 Comondu rocks and rising to more than 5000 feet, must have reached nearly 

 that elevation at the time of the post-Comondu uplift and deformation, but in 

 pre-Pliocene time. It is doubtful if any major change in the relative elevations 

 of the Comondu and Salada areas has been caused by erosion and deformation. 



In the diagrams of Fig. 30.6, the post-Salada and post-Raucarit disastro- 

 phism is indicated as due to compression, and the Gulf of California 

 had not yet come into existence. According to Real, however, some fault- 

 ing had probably occurred in mid-Tertiary time, and not all of the down- 

 faulting of the gulf and the uplift of the peninsula took place in the I 

 Pleistocene, as illustrated. It is certain, however, that a great deal of the 

 displacement that shaped these major elements is post-Salada. 



Quaternary Phase. The submarine canyons on the continental margin 

 have been regarded as of subaerial erosion, and hence to represent a great 

 emergence, according to Real, in postfaulting time. This does not seem 

 necessary, however, because when once deeply submerged, the form re- 

 mains little changed, and the canyons may be of considerable antiquity. 

 Retter understood is evidence of a great Pleistocene submergence. Ter- 

 races and marine shells lead Real to conclude that 



. . . there seems little doubt that the sea level rose at least 1600 feet, and 

 Wittich (1920) believed it rose to about 3000 feet. If the depression of 1600 

 feet was uniform throughout the full length of Baja California, the peninsula 

 would have been only about two-thirds its present length with a string of is- 

 lands extending southeastward. 



Johnson (1924), in his study of the fauna and flora of Baja California, states 

 "For some reason the fauna and flora were subjected to a crisis during Pleisto- 

 cene, and all but a few vertebrates were destroyed." This wholesale destruction 

 might have resulted from the submergence of the peninsula, indicated by the 

 presence of sea shells at considerable heights. 



