BAJA CALIFORNIA AND SONORA SYSTEMS 



481 



PALEOCENE 



EOCENE 



OUGOCENE 



PLIOCENE 



Fig. 30.5. Paleogrography of Baja California during the Tertiary, after Durham and Allison, 1960. Ruled 

 areas denote land. The Oligocene beds of Beat are earliest Miocene on the basis of the megafauna, 

 according to Durham and Allison. 



iPurisima region was again submerged. The San Ignacio area and probably 

 much of Desierto de Santa Clara, much of the Western Cape region, and part 

 of Isla Cedros suffered their first Miocene submergence. The northern limit of 

 ithis sea may have been some place north of the 28th parallel. The eastern limit 

 of the sea extended along the west side of the sierras, beginning not far west 

 of Las Tres Virgenes, and crossed the peninsula to the gulf coast near Punta 

 San Marcial. 



Some structural considerations indicate that much of the lower gulf was 

 occupied by the Ysidro sea. For example, the Southern Cape region probably 

 was an elevated block from early Cretaceous, as it appears that in Eocene, and 

 probably in Cretaceous time, the Isthmus of La Paz marked the southernmost 

 'limit of marine invasion, and no sedimentaries are known to have been de- 

 tposited on it until Ysidro time. 



The La Paz fault is deeply significant from the standpoint of the geologic 

 history of the gulf and of the peninsula. Downthrow on the west side allowed 

 ■the deposition of Tertiary and perhaps Cretaceous beds from the Isthmus of 

 La Paz northwestward, and the northerly extension of the fault may have been 

 a factor in severing the peninsular structural block from the old land mass. . . . 



After the deposition of the Ysidro formation the peninsular area was elevated; 

 its western margin may have been roughly coincident with the western marginal 

 juplift and the eastern side bounded by the ancestral gulf over a part of its 



length, but parts of the near-shore insular area, from about 27° 30' X. Lat. to 

 La Paz west of the Ceralbo fault zone, were still a part of the land area. The 

 northern half, which stood above water during Ysidro time, was further ele- 

 vated, and areas of Ysidro sediments were elevated sufficiently to allow consid- 

 erable erosion, especially along the margins of the peninsular area. The syncli- 

 nal and some other areas appear to have suffered but minor erosion, as at many 

 places there is littie evidence of unconformity between the Ysidro and the 

 overlying Comondu rocks. 



Late Tertiary Phase. Volcanism broke out in late Miocene time and, in 

 places, has continued down to the present. The Comondu formation of the 

 peninsula is thick and made up of many kinds of rocks of volcanic origin. 

 The northern half of the pensinsula must have been out of water, but the 

 southern half was largely a site of deposition. The volcanism is probably 

 related to the Baucarit sedimentation and volcanism of Sonora. See second 

 from top section in Fig. 30.6. 



The Baja California syncline was gentiy depressed; the Isthmus of La Paz 

 elevated; the Western Cape region became a part of the peninsula, if formerly 

 separated from it; the northern half of the peninsula had not reached its present 



