438 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



even older. The most extensive and probably thickest is the Datil field, 

 which is described in Chapter 36. In the mountain region of Ari- 

 zona, the Laramide ( ? ) stocks had been exposed by erosion and then were 

 covered unconformably by the lavas. The Tertiary in general was a time 

 of prolonged volcanic activity from place to place, the lavas were more 

 acid than those of the Cretaceous, and widespread block faulting was 

 prevalent. See charts, Figs. 27.2 and 27.3. 



A third group of mineral deposits is associated with the Tertiary lavas. 

 The districts that belong to this class are listed in the following table. The 

 ore deposits are in the form of fault veins that cut the lavas. The veins are 

 generally crustified, shallow in depth, and contain adularia. Gold is the 

 chief ore mineral. 



Age of Ore Deposits in Arizona and New Mexico 



Precambrian 



Nevadan 



Laramide 







Late Tertiary 



Jerome-Prescott 



Bisbee? 



Magma 







Mogollon 



Pecos 



Patagonia? 



Globe 







Steeple Rock 





Red Bed 



Miami 







Lordsbury? 





copper 



Ray 







Stanley Butte 





deposits? 



Christmas 

 Morenci 

 Tombstone? 

 Twin Buttes 

 Magdalena 

 Santa Rita-Fierro 

 Pinos Altos 

 Tyrone 

 85 Mine? 







Oatman? 

 Ajo? 

 Silver Bell? 







Silver-manganese 



metalliza- 









tion below sha 



le 



beds in 









southwest New 



Mexico; 









Silver City, Cooks 



Peak, 









Kingston, etc. 









TERTIARY NORMAL FAULTING 



Everywhere, it seems, in the mountain and desert regions of Arizona, 

 high-angle faults cut and offset the bedrock. They trend in many direc- 



tions. They both predate and postdate the Gila conglomerate of Pliocene- ] 

 Pleistocene age; some predate the Laramide orogeny, some are part of it, 

 but the majority postdate it, and are Middle and Late Tertiary. 



Either because of block faulting, regional warping, or both, the central 

 and southern part of Arizona became an area of aggradation in late Ter- 

 tiary time, and stream and lake sediments, in places 10,000 feet 

 thick, accumulated in the lower areas. The deposits, though given various 

 names in several local areas, are best known as the Gila conglomerate. 

 Mild volcanic activity accompanied the sedimentation, and lava flows are 

 locally present in and on top of the formation. Relative uplift of the ranges 

 and subsidence of the intermontane trough areas continued intermittently 

 into Quaternary time, and the Gila formation is tilted, faulted, and locally 

 folded. Where uplifted, it is trenched, and the material eroded from it 

 and other sources has been deposited as a relatively thin veneer of Qua- 

 ternary terrace and stream alluvium. 



Tertiary volcanic rocks are nearly everywhere, and in one place or an- 

 other represent continuing volcanic activity down to the time of the In- 

 dians. A resume of the Tertiary volcanic activity throughout the Tertiary 

 and Quaternary in the mountain and desert regions of Arizona and in the 

 Colorado Plateau from the point of view of age, distribution, and com- 

 position is a very inviting study. 



In general, the ranges trend northerly in southeastern Arizona and 

 northwesterly in the central and southwestern part. These directions are 

 probably due to the major Late Tertiary faults. Considerable time has 

 elapsed since the last major movements, because extensive pediments 

 have formed across many faults and true fault scarps are few. 



CONCLUSIONS REGARDING TECTONIC HISTORY 



In southern Arizona sometime during the Triassic and Jurassic, the 

 Paleozoic and Precambrian rocks were folded, intruded by granitic stocks 

 or small batholiths, and deeply eroded. The orogeny is tentatively corre- 

 lated with the early Nevadan of central and western Nevada and 

 California, and the belt of orogeny is recognized to extend from Arizona 

 to south-central Coahuila in the site of the Late Jurassic Coahuila penin- 



