492 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



formed. Both the orogeny that resulted in their deposition and the 

 impulse that deformed them might, therefore, be considered parts of the 

 same phase until more information is available. 



Late Tertiary or Early Pleistocene Phase 



After the mid-Tertiary orogeny in the province of parallel ranges and 

 valleys and in the Sonoran Desert, the Baucarit formation was laid down 

 in the structural depressions between the uplifted mountain ranges. A 

 moderate recurrence of volcanic activity is indicated by the basalt flows in 

 the lower part of the formation. Interbedded with and overlying the 

 basalts are conglomerates which were doubtless laid down as coalescing 

 alluvial fans at the margins of the mountains. They contain fragments 

 derived from the cores of the ranges including boulders of granite. Similar 

 deposits overlying the yellow beds of Baja California have already been 

 mentioned. Beal describes two formations there, the lower Comondu vol- 

 canics and the overlying Salada formation, which appear similar to the 

 Baucarit, and correlative with it. The next structural phase postdates the 

 Baucarit, and is of varied aspect. There was renewed volcanic activity, 

 and the Baucarit formation was thrown into low folds and tilted. 



Quoting from King (1939): 



North of the 28th parallel, the rocks of each of the high mountain ranges, 

 from the crest of the Sierra Madre westward into central Sonora, were pushed 

 to the west on overthrust faults which partly overrode the Baucarit formation, 

 lying in the valleys next to the west. Some minor faults were thrust to the east. 

 The strong thrusting and the gende warping of this orogenic epoch suggest 

 that the strata of the mountains had already become so consolidated by previous 



folding and igneous intrusions that they could no longer yield to lateral pressure 

 by folding. The greater amount of thrusting north of the 28th parallel may be 

 due to the greater thickness of Paleozoic and Mesozoic sedimentary rocks in 

 that region. 



The normal faults extensively developed south of the 28th parallel and far- 

 ther west in central Sonora were somehow related to the thrust faults. At La 

 Colorada these offset the plane of an overthrust fault, but both here and to the 

 south they have the same north-northwest trends as the overthrusts and thus 

 may have taken their form from the same forces. In the province of parallel 

 ranges and valleys, the localization of overthrusts north of the 28th parallel and 

 of normal faults to the south of it suggests that the orogenic forces, although 

 dominandy compressional, produced local areas of tension. 



During rather recent geologic time, a mature erosion surface of low relief was 

 developed in the lava country along the crest of the Sierra Madre. After its for- 

 mation, the area was gready uplifted, and streams draining to the west deeply 

 intrenched their courses, forming the tremendous barrancas of the western flank 

 of the Sierra Madre. It is not entirely certain when this uplift took place, but 

 the great height of the surface above low country not far to the west strongly 

 suggests that it was raised by faulting on the west side of the plateau. This 

 faulting may have been the post-Baucarit thrust faulting, or it may have been a 

 renewed movement at a later time along the same trends. 



King cannot date the elevation of the lavas of the Sierra Madre Oc- 

 cidental with accuracy, but he believes the elevation was due to faulting 

 in post-Baucarit time. It seems probable, therefore, that the elevation 

 of the Sieras occurred at the same time as the sagging of the Gulf, and 

 that they are parts of the same fault block system. The differential move- 

 ment, as estimated from the bottom of the Gulf to the crest of the Sierras, 

 is about 12,000 feet. 



The position of Baja California in the regional tectonic plan is treated 

 in Chapters 29 and 31. 



M 



