SOUTHERN ARIZONA ROCKIES 



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In the northern Dragoon Mountains andesite and quartz latite were 

 erupted on an erosion surface of mild relief developed after the deposition 

 of the Risbee sediments. These volcanics may be Late Cretaceous in age; 

 at least they preceded the strong thrusting. 



The most profound deformation of the area took place after the Bronco vol- 

 canics and Sugarloaf quartz latite were erupted. This involved great thrust 

 faults of northerly to northwesterly trend in the Dragoon Mountains and the 

 overturning of a section of the Bisbee formation fully 3 miles thick along the 

 eastern flank of this range. A gigantic breccia of fragments of nearly every 

 older formation exists in the Courdand and Gleeson areas. Refer to Fig. 26.6. 

 It suggests that the major fault was, in this section of its exposed course, advanc- 

 ing over the surface, producing the breccia by attrition of the overriding 

 thrust plate. Minor thrust fragments of this age are found in the Tombstone 

 Hills. . . . 



The Stronghold granite is younger than the thrusting and has domed the 

 thrust sheets slighdy. This doming does not appear, however, to account for 

 the emplacement of the granite, which is clearly transgressive. In the Tomb- 

 stone Hills the Schieffelin granodiorite seems also to be younger than all 

 important compressional stresses, as is the Uncle Sam porphyry. 



The sequence and pattern of Laramide (?) events in the Chiricahua 

 Mountains is instructive. The northwest course of the main structures 

 dominates the geologic map, whereas the giant thrust breccia in the 

 Dragoon Mountains leaves the trends uncertainly recognized there. Ac- 

 cording to Sabins (refer to Fig. 27.8), 



During the major post-Comanche to pre-Pliocene orogeny, strong southerly 

 to southwesterly horizontal compression caused the following tectonic sequence. 

 The autochthonous rocks along the northeast front of the range were overridden 

 from the southwest by the first thrust sheet. Strike-slip displacement along the 

 Emigrant fault cut the autochthonous block and the overlying thrust sheet, 

 which was separated into the Fort Bowie plate and the Wood Mountain 

 plate. The Fort Bowie plate was later folded to form the Marble Quarry 

 syncline and was truncated by the younger Fort Apache reverse fault. Finally, 

 the Whitetail plate overrode the Fort Apache fault block. 



Volcanic extrusions of approximately mid-Tertiary age then accumu- 

 lated on an erosion surface on the deformed strata. These were faulted 

 and tilted, possibly just prior to the deposition of the Gila conglomerate 

 in Pliocene time. Not only was the faulting the cause of the deposition 

 of the Gila conglomerate, but also probably the modern ranges were 



blocked out by it at this time. Some faulting continued after the Gila 

 conglomerate accumulated. 



The age of the thrusting cannot be more accurately placed than in the 

 Late Cretaceous (probably very Late Cretaceous) or Early to Mid-Tei 

 tiary, but when it and the stocks of post-thrusting age are compared to a 

 similar sequence of events in Utah, Nevada, Colorado, and central New 

 Mexico, one may logically point to a Laramide age. The folding of the 

 Upper Cretaceous beds of the Mexican geosyncline in Coahuila, Chi- 

 huahua, and Sonora is generally referred to as Laramide, and this broad 

 region continues the folding and thrusting of southeastern Arizona and 

 southwestern New Mexico southward. 



The picture described in the above paragraphs of the structure of the 

 ranges of southeastern Arizona is clarified by Jones ( 1961 ) , who recog- 

 nizes most of the ranges to be complex anticlines with Precambrian or 

 Triassic-Jurassic granite in their cores. The structural relief of some of 

 the uplifts is 25,000 feet. Some began to rise in Mesozoic time and con- 

 tinued intermittently through at least the Miocene. High angle reverse 

 faults define the flanks, and appreciably downslope mass movement has 

 occurred to form the low-angle thrusts. 



MESOZOIC AND CENOZOIC GEOLOGY 

 OF SOUTHERN ARIZONA 



The geology of the Ajo mining district of south-central Arizona in the 

 Sonoran desert is well known from the work of Gilluly (1946), and prob- 

 ably is representative of the geology of this general region. The main rock 

 units and events are listed in Fig. 27.2. A cross section is shown in Fig. 

 27.9. The only fossils found in the entire district are in blocks of lime- 

 stone in the Locomotive fanglomerate, presumably of about middle 

 Tertiary age. The fossihferous boulders are referable to the Devonian, 

 Mississippian, and Pennsylvanian, and hence when the fanglomerate was 

 being deposited outcrops of beds of these Paleozoic ages were probably 

 nearby in upland areas. The rocks and events, although their sequence 

 is relatively well established, are not dated by stratigraphic methods, 

 and the ages assigned are very tentative. 



