BAJA CALIFORNIA AND SONORA SYSTEMS 



491 



Within the barranca section (great gorges indenting the west-facing 

 escarpments) not only are the plateau summits largely destroyed by 

 erosion, but the volcanic rocks are also broken by faults that belong to 

 later deformational phases. 



Mid-Tertiary Phase 



Parallel Ranges and Valleys. After the early Tertiary eruptions, there 

 was a vigorous phase of mountain making that is known principally in the 

 province of parallel ranges and valleys and in the Sonoran Desert. Within 

 the plateau section of the Sierra Madre, the volcanic rocks were only 

 gently folded, and over wide areas they still remain nearly horizontal. 

 This gentle folding contrasts with the strong disturbance of the Creta- 

 ceous and other Mesozoic rocks, where they can be observed beneath, 

 and indicates that the Laramide orogeny was greater than the mid- 

 Tertiary in the Sierra Madre proper. 



Farther west, as in the province of parallel ranges, folds and thrust 

 faults occur that can be assigned to the mid-Tertiary deformation, which 

 here exceeds the Laramide. The mountains probably began to assume 

 their present form at this time. 



The ranges are generally bordered by faults. North of the 28th parallel, 

 the faults are high- and low-angle thrusts. To the south, steep normal 

 faults predominate. They are not all, however, of the mid-Tertiary dis- 

 turbance; some are late Tertiary. 



Accompanying the mid-Tertiary orogeny were vast intrusions of granite 

 ;and other plutonic rocks, which ascended through the Paleozoic and 

 iMesozoic rocks and, in places, penetrated the early Tertiary volcanics. 

 (Some of the faulting started at this time, because several of the thrust 

 jfaults that break the early Tertiary volcanic rocks of the barranca section 

 |are cut off by granite intrusions. See fourth cross section from bottom of 

 jFig. 30.6. 



Sonoran Desert Province. North of latitude 28° 30' N., a large propor- 

 tion of the detached mountain ranges in the Sonoran desert province con- 

 sists of Paleozoic and Mesozoic sedimentary rocks. To the south in the 

 unmapped area that extends to the coast, they consist of volcanic rocks 

 jand granite. The ranges of sedimentary and volcanic rocks appear to be 



only detached roof pendants in a vast granite batholith or group of 

 coalescing batholiths (King, 1939). They probably represent the lowest 

 part of the roof at the end of the period of intrusion. Nearly all the non- 

 granitic rock in the ranges is cut by apophyses of granite. 



The granite intrusions have complicated the pregranite structure of the 

 sedimentary and volcanic rocks by metamorphosing and shattering them 

 close to the contact and by jointing them excessively for some distance 

 from the contact. Alternations of competent and incompetent strata, such 

 as are found in parts of the Paleozoic and the Jurassic Barranca formation, 

 shows such a confusion of dips and small faults that it is very difficult to 

 work out the main structural features. Only the most massive, resistant 

 formations, such as the Permian limestone and the upper part of the 

 Barranca formation, show the structure clearly; and even these only at 

 some distance from the nearest granite contact (King, 1939). Despite 

 these confusing relations, King finds some of the larger features of the 

 structure plain. The mountains in part are clearly upfaulted. Some still 

 preserve the form of tilted fault blocks, although considerably modified 

 by erosion. Some of the depressions are downfaulted, and some over- 

 thrusting is present in the Sonoran Desert. 



Baja California. The bulk of the Tertiary formations in Baja California 

 are the result of orogeny to the east in Sonora. This is particularly true of 

 the "yellow beds" (Darton terminology). They are present in great 

 volume and coarsen eastward. From the relations that Darton depicts, the 

 yellow beds are the great orogenic deposit in the southern half of Baja 

 California, and if they are the late lower and middle Miocene Ysidro 

 formation of Beal, then the mid-Tertiary orogeny of King in Sonora is 

 probably dated by them. 



In Sonora itself, the next youngest formation after the disturbance is the 

 Baucarit of late Pliocene or Quaternary age. It occupies the depressions 

 between ranges and probably was deposited some time after the orogeny. 

 See second section from the top of Fig. 30.6. The yellow beds were up- 

 turned in places, eroded, and then covered with sands, conglomerates, 

 agglomerates, and basalt flows. Since these capping deposits are late 

 Miocene (?) and Pliocene in age (Beal, 1948), it appears that the yellow 

 beds were deposited in a hurry and then immediately somewhat de- 



