BAJA CALIFORNIA AND SONORA SYSTEMS 



483 



sandstone 



Agglomerate,, 

 conglomerate, 

 a no" igneous 



Ocean 



ii^^gS^^^^"^^^ 1 '? — : -^y£7Iow ^ beds' y 



jondstone 



conQ 10 _^tk jgneou* 



Ocean 



10 



15 



MILES 



Fig. 30.4. Sections across parts of northern and central Baja California. After Darton, 1921. 



Stratigraphy 



Beal (1948) records: 



The rocks of Baja California consist of ( 1 ) unaltered marine sedimentary 

 ^ocks ranging in age from Cretaceous to Pleistocene; (2) a series of sedimentary 

 vrocks of probable Cretaceous age exhibiting varying degrees of alteration; (3) 

 ^extrusive rocks, principally of andesite and basalt; and (4) intrusive rocks 

 consisting principally of quartz diorite and granodiorite, which have intruded 

 and metamorphosed older rocks, the age of which is not definitely known. 

 L| The Cretaceous is represented by the San Fernando and the Rosario forma- 

 tions. The San Fernando formation, Lower and early (?) Upper Cretaceous in 

 age . . . consists of a series of slates, conglomerates, quartzite, limestone, and 

 sandstone, with varying amounts of associated intrusive and extrusive rocks; 

 some parts of the series are only slightly, but others greatly metamorphosed. 

 The younger Rosario formation (Upper Cretaceous) is unconformable on the 

 ,San Fernando formation. It consists of unaltered red and gray shale, brown 

 sandstones, and conglomerates on the Pacific Coast near Rosario. . . . 



The Tertiary is divided into the following formations: the Tepetate (Paleo- 

 cene to Eocene in age) . . . composed generally of yellow to brown silt and 



sandstone; the San Gregorio formation, Oligocene (?) to Lower (?) Miocene 

 in age . . . resembling in some respects the Monterey shales of California: the 

 Ysidro formation, late Lower or Middle Miocene, or both, in age, comprising 

 a lower member of shales, in part diatomaceous, and an upper member of 

 light-colored sandstone and shale; the Comondu formation. Upper Miocene 

 (?) in age, composed mainly of agglomerates, tuffs, and lavas; and the Salada 

 formation (Pliocene), and consisting mainly of yellow marine sandstone and 

 shale. 



Following are the outstanding geologic features in the different districts of 

 Baja California: 



1. The Northern district is characterized by a high, westward-sloping block 

 of crystalline rocks, which appear to owe their elevation to profound faulting 

 along the east side. The axial mountains have a granitic core, flanked on both 

 sides by gneisses, schists, and slates probably of pre-Crctaceous age; the bed- 

 rock complex is overlain on the west side by irregularly metamorphosed rocks of 

 Cretaceous age, which are themselves overlain by unmetamorphosed marine 

 sediments of Cretaceous and Tertiary ages; these rocks do not appear in any 

 outcrop of importance on the east side. The crystalline rocks arc prominent but 

 decrease in elevation as far south as the 28th parallel, die southern boundary 

 of the Northern district. 



