BAJA CALIFORNIA AND SONORA SYSTEMS 



485 



California. The southern California area has been summarized in Chapter 

 17 to which the reader is referred for details. In brief, Larsen believes 

 that there are many bodies of metamorphosed rocks older than the 

 granitic rocks. Originally the sediments were shales, impure shales, and 

 sandstones. 



The argillaceous metasediments are chiefly on the west side of the main 

 batholithic masses and within them, and they are probably mostly Triassic 

 in age. The quartzites and coarse sericite schists are on the east side and 

 are probably Carboniferous in age. A body of mildly metamorphosed vol- 

 canics of Early Cretaceous age makes up part of the pre-intrusive complex 

 on the west. 



The batholiths and older metamorphic rocks are overlain by Upper 

 Cretaceous strata, and the date of the main intrusion is some time within 

 the Lower Cretaceous. 



Structure 



Nevadan System. The metamorphic rocks and granitic batholiths of 

 the Nevadan system probably make up the basement complex the entire 

 length of Baja California. The geologic map, Fig. 30.1, shows a single 

 great batholith extending halfway down the peninsula to the Desert of 

 Santa Clara, and numerous other granite bodies carry the Nevadan system 

 southeastward and end in the large batholith of the southern cape region 

 south of La Paz. Islands on both the east and west coasts are composed 

 entirely or in part of Nevadan complex. The Nevadan complex has been 

 described, so the following structural study will deal with the Cenozoic 

 folds, faults, and uplift. 



Anticlines and Synclines. Beal ( 1948 ) has mapped a long, gentle anti- 

 jcline and an almost equally long, gentle syncline in the southern half of 

 the peninsula. See map, Fig. 30.1. The syncline, known as the Baja Cali- 

 jfornia, extends from 31° N. Lat. southward for 600 miles to the isthmus of 

 La Paz. 



\* For the first 200 kilometers of its course it follows the Pacific Coast, first 

 Dffshore and then on land, with marine sediments dipping gently toward its 

 ixis. At 29° 30' N. Lat. near Bahia San Carlos, the syncline leaves the peninsula 

 and crosses Bahia Sebastian Vizcaino, enters the peninsula again in the north- 

 western part of Desierto de Santa Clara, and extends thence through the desert 



in nearly a straight line toward the Isthmus of La Paz. From the south pari of 

 the desert at 27° N. Lat. most of the marine sediments dip gently toward the 

 axis of the syncline, but, throughout much of this segment, these sediments are 

 overlain and piled high with Comondu and later volcanic debris. Numerous 

 local folds, some gentle, but others sharp, narrow wrinkles, were Found in the 

 trough of this great syncline. 



The anticline along the east coast is called La Gigante and extends from 

 Santa Rosalia southeastward about 200 miles to the Bay of La Paz. 



Detailed mapping will undoubtedly show the area through which the axial 

 line is drawn to be generally anticlinal and will probably disclose that the 

 uplift is made up of several discontinuous anticlines, and faulting has been a 

 factor in its elevation. Over nearly the entire distance the elevation of the area 

 has resulted in great coastal escarpments which rise steeply for hundreds of 

 feet from the gulf shore or the narrow coastal plain. 



Bahia Concepcion is definitely anticlinal as the Comondu rocks on both sides 

 dip gently in opposite directions, and the same rocks near Aguaje at the south- 

 ern end of the bay are folded into a well-marked anticline and several smaller 

 folds, with dips ranging up to 30°. North of Loreto (Comondu quadrangle) the 

 mountain shown as 2227 feet high has been forced up causing the Pliocene 

 sediments to dip away in all directions. This area lies on another anticline east 

 of the major uplift, but a few kilometers southwest granitic rocks are exposed 

 on the axis of La Gigante anticline with Comondu rocks arching over the 

 exposure. Southeast of Agua Verde (Santa Cruz quadrangle) the mountains 

 back of Punta San Marcial are distinctly anticlinal, and east of Rancho Carriza- 

 lito (Santa Catalina quadrangle) the Ysidro formation is arched over a small 

 mass of crystalline rocks. 



Faults. The great eastward-facing escarpment along the Sierras Juarez 

 and San Pedro Martir is believed to mark a fault zone which has been 

 called the San Pedro Martir by Beal (S.P.M. on map of Fig. 30.1). The 

 scarp is likened to that of the Sierra Nevada, and the fault zone is 

 thought to be continuous along the east side of the Peninsula Range into 

 southern California. Beal (1948) judges the vertical displacement to be 

 about 5000 feet at the 31st parallel. 



The western face of the Sierra Victoria of the southern cape region is 

 considerably steeper than the eastern, and is regarded by Beal as marking 

 a fault, called the La Paz (L.P. on map, Fig. 30.1). Submarine contours 

 northward in the gulf suggests a projection of the fault. Beal points out 

 that the Sierra Victoria trends northward obliquely across the peninsula 

 and stands apart as a distinct unit. It thus seems to require a structure 



