486 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



such as the postulated La Paz fault, which he considers pre-Tertiary 

 in age. 



The submarine topography of the Gulf of California will be described 

 immediately, and a downfaulted origin postulated. The Ceralbo fault is 

 the major dislocation visualized. 



Darton ( 1921 ) believed the major orogeny of the peninsula in Tertiary 

 time resulted in the tilting of the long block upward on the east side and 

 the sinking of the gulf, as diagrammed in Fig. 30.5. This presumably 

 is the overall picture, but Real adds three other structural elements, 

 namely, the long, gentle folds and the diagonal La Paz fault, the major 

 faults indicated by the submarine topography along the west side 

 of the peninsula, and regional uplifts and submergences in late Cenozoic 

 time. The submarine topography is treated separately in Chapter 32 and 

 the regional vertical movements in the following tectonic history. 



Tectonic History 



The following resume of the tectonic history of Raja California is com- 

 posed of quotations from Real's (1948) memoir. 



Cretaceous Phase. The earliest record of the Cretaceous in Baja California 

 is the San Fernando formation, which, insofar as it is known, was deposited only 

 on the western slopes of the peninsula. Its lower stratigraphic limit is not known. 

 The area over which the formation occurs probably was subjected to erosion 

 during a long period before the deposition of the Rosario sediments and was 

 extensively intruded during that time, which in places almost obliterated the 

 sedimentary character of the series. No intrusions of the same type were ob- 

 served to cut the younger Rosario formation. 



The base of the Rosario formation was not seen as it probably lies under the 

 ocean, and the series may be much thicker than indicated by the exposures. 

 . . . During the deposition of the Rosario formation considerable areas of the 

 San Fernando formation and of the earlier metamorphic rocks stood above 

 water; erosion, whether shoreline or by streams, was principally in such rocks; 

 and parts of the Rosario formation were also above sea level while sediments of 

 the same series were being deposited. . . . 



Following the deposition of the Rosario formation, the strata were locally 

 distorted, but where observed, they were not usually folded sharply nor faulted. 

 These structural phenomena suggest compression and folding while the sedi- 

 ments were but slighdy loaded and before they had been completely indurated. 

 . . . The diastrophic activity resulted not only in the mild folding and partial 

 erosion of the Rosario sediments, but also marked an important emergence ex- 



tending as far south along the west coast as 28° N. Lat., because the succeeding 

 Eocene beds north of that parallel were laid down in a sea which transgressed 

 over a rugged topography in which many kinds of rocks were exposed. 



Early Tertiary Phase. The Paleocene and Eocene periods were marked by 

 an important subsidence during which the sea, with some protruding insular 

 areas, covered the western flanks of the peninsula. ... It appears that the sea 

 may have first occupied the coastal regions of the northern part of the peninsula 

 from about 31° 30' N. Lat. southward to the 27th parallel. . . . The southern 

 margin of the sea in Baja California at that time appears to have been near the 

 isthmus of La Paz, and the sea may have extended across the isthmus to the 

 present lower gulf. . . . [See Fig. 30.5.] 



The back country must have been of moderate elevation, well watered, thus 

 supporting large streams, and the climate was tropical as indicated by the 

 faunas. 



An emergence near the close of Eocene time marks the beginning of a period 

 of erosion and local folding of the Tepetate formation. The contact between 

 Eocene beds and the overlying Miocene appears to be almost conformable 

 where observed near the axis of the Baja California syncline, but in the Western 

 Cape region the unconformity between the Tepetate and Miocene is more im- 

 portant, indicating that the earlier movement along the western marginal uplift 

 continued in the post-Eocene. 



If the sandstones at Santa Gertrudis east of Desierto de Santa Clara prove to 

 be Tepetate in age, they probably represent the eastern limit of the formation 

 in that area. They are overlain direcdy by Upper Miocene volcanics, thus in- 

 dicating post-Eocene pre-Lower Miocene uplift along the axis of the peninsula 

 near the eastern marginal uplift. 



After the deposition of the Tepetate and before Miocene time, volcanism of 

 some importance must have broken out, for the granitic rocks underlying the 

 Ysidro beds in the Southern Cape region are intruded and in places covered by 

 volcanic rocks; furthermore, east of San Ignacio Lagoon the basal light con- 

 glomerate of the Ysidro formation, resting on the Tepetate with slight un- 

 conformity, contains pebbles of volcanic rock. 



Mid-Tertiary Phase. The depressed area (in early and middle Miocene) 

 probably covered the synclinal region from a point in the desert area northwest 

 of Purisima and southeastward to the Isthmus of La Paz; it probably was 

 bordered on the west by the uplifted granitic areas at Bahia de Magdelena, 

 which protruded as islands in the sea. The eastern extension of the marine in- 

 vasion may have occupied the east coast of the peninsula from Punta San 

 Marcial to La Paz and extended well into the adjacent gulf. . . . This possible 

 eastward marine transgression, insofar as known, is the first Tertiary sea to have 

 occupied any part of the gulf coast, except for the period during which the 

 Cornwallius beds were deposited. [See Fig. 30.5.] 



The upper Ysidro submergence in the southern area appears to have been a 

 continuation of that which allowed the deposition of the lower shale member. 

 It was important and widespread — much more so than the preceding. The 



