SOUTHERN ARIZONA ROCKIES 



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I Fig. 27.1. Index map of Arizona showing the central mountain region and the desert region 

 : (Sonoran Desert). Both mountains and desert regions are part of the Basin and Range physio- 

 graphic province. The chief mining districts are also shown together with mountains for which 

 1 cross sections are given in Figs. 27.6, 27.7, and 27.9. 



separated by relatively wider basins than those of western Utah and 

 -Nevada. Hence, the inclusion of the Sonoran Desert of Arizona in the 

 Rasin and Range province from a structural point of view must be made 

 with reservations. The crisp boundaries imparted to ranges by block 



faulting are generally absent, and if the region is one of extensive block 

 faulting, then the faults are older than those in Utah and Nevada, and 

 erosion has beaten the fault scarps back considerable distances to form 

 broad flanking pediments. The desert floors are in need of gravity surveys 

 to delimit buried faults and their patterns, if such exist. Some reports 

 refer to late Tertiary normal faults as Rasin and Range faults, but other 

 reports do not use the term Rasin and Range. 



PALEOZOIC AND MESOZOIC BASINS 



The paleotectonic maps of Figs. 5.1 through 5.8 show that the Trans- 

 continental Arch dominated most of Arizona in Paleozoic time. Except in 

 the southeastern corner of the state (Figs. 27.2 and 27.3), the deposits are 

 thin or absent, and it does not seem possible that they could have affected 

 in any major way the pattern of later Mesozoic and Tertiary structures. 



Triassic and Jurassic sedimentary rocks are absent in the Mountain 

 Region and Sonoran Desert and hence events which may have occurred 

 during these times cannot be accurately dated stratigraphically. Lower 

 Cretaceous rocks are present in southeastern Arizona as part of the Mexi- 

 can geosyncline, and most igneous and deformational events there can be 

 dated either as pre-Lower Cretaceous or post-Lower Cretaceous. Late 

 Cretaceous strata also occur in places there, and assist further in dating of 

 events. However, over most of the Sonoran Desert and Mountain Region, 

 outside of this southeastern part of the state, the ages of rock masses and 

 structures are poorly known. The history is eventful, and the sequence 

 of events can be established but few of them accurately dated. 



The Gila conglomerate whose oldest fossils to date are early Pliocene 

 (Anderson et al., 1958), but which for the most part probably is late 

 Pliocene (Knechtel, 1936), is widespread, and serves as an upper dating 

 plane. Events between the Cretaceous ( generally Lower Cretaceous ) and 

 the Pliocene must be spaced or interpolated according to the best judg- 

 ment of the researchers concerned. 



USE OF TERMS, LARAMIDE AND NEVADAN OROGENIES 



Recause of the inability of geologists to date accurately events in 

 southern Arizona during the Mesozoic and Cenozoic, the terms Nevadan 



