GEOLOGY OF THE LOWER GILA REGION, ARIZONA. 



193 



ternary age. The rock is more weathered than 

 the basalts of known Quaternary age and is 

 beheved to be Tertiary. It was clearly 

 erupted after the Granite Wash ?Tills came 

 into existence. 



Large portions of the "N'ulture Mountains, 

 the ^'\Tiite Tank Mountains in Maricopa 

 County, the Palomas Mountains, the Mari- 

 copa Mountains, and other ranges lie above 

 any known lava flows and probably never 

 were covered by such flows. The character 

 of the topography of areas of the basal com- 

 plex from which the lavas have but recently 

 been removed and the irregular lower con- 

 tact of the lavas that are exposed in many 

 places in the mountains show that the surface 

 upon which they were poured out was hilly if 

 not actually mountainous. This is especially 

 well shown in the Gila Bend and Eagle Tail 

 mountains. 



In adjoining parts of Arizona there is con- 

 clusive evidence of pre-Tertiary faulting. 

 Mountain building -* attended by faulting 

 took place during the Mesozoic era in what 

 is now Cochise County. Bryan ^^ has found 

 evidence of a similar period of faulting in 

 Pinal and Pima counties. Ransome ^'' showed 

 that faults of pre-Tertiary age probably exist 

 in the Globe district, and this inference has 

 been confirmed by later work in the Old 

 Dominion mine.-' 



FAULTS YOUNGER THAN THE TERTIARY LAVA. 



There is abundant evidence, both within 

 and near the region covered by this report, of 

 normal faulting subsequent to the eruption 

 of the Tertiary lavas. The lavas have been 

 broken into numerous l)locks that dip in 

 various directions and are bounded by faults 

 with various strikes. Almost every range 

 containing Tertiary lavas has obvious exam- 

 ples of such fault blocks. In some localities, 

 as at Saddle Mountain, there are more or less 

 heterogeneous groups of fault blocks. In 

 others the whole mass of lavas composing the 



" Schrader, F. C, Mineral deposits of the Santa Rita and Patagonia 

 Mountains, Ariz.: U. S. Ctcol. Survey Bull. 6S2, p. 77, 1915. 



"^ Bryan, Ivirk, The Papago country, Ariz.: U. S. Ceol. Survey Water- 

 Supply Paper — (in preparation). 



26 Ransome, F. L., Geology of the Globe copper district, Ariz.: U. S. 

 Geol. Survey Prof. Paper 12, p. 104, 1903. 



2^ Bjorge, G. N., personal communication. 



range has been lifted, relative to the blocks on 

 either side, with but little change in the hori- 

 zontal attitude of the beds. The Eagle Tail 

 Mountains and probably also the S. H. Moun- 

 tains are of this character. The structure in 

 both ranges is complicated by cross faults 

 which have ])roken portions of the lai'ge block 

 into smaller tilted blocks. Probably the 

 Plomosa, Big Horn, Castle Dome, and other 

 ranges are built up, in part or wholly, of such 

 horst-like blocks l^roken by cross faults and 

 carved by erosion, but the blocks of lava ai'e 

 now comparatively small, and many of them 

 are tilted, so that the evidence as to the char- 

 acter of the originally dominant structure is 

 obscure. Probably the faults of this period 

 followed in part the lines of weakness devel- 

 oped during the pre-Tertiary crustal move- 

 ments, but it is also probable that faulting 

 along entu-ely new planes took place. Most 

 of the ranges either trend approximately' N. 

 50° W. or N. 50° E. or show a combination 

 of these two directions. The strike of the 

 ranges near Colorado River is more nearly 

 north than that of most of those farther east. 

 This is true both of those that strike west of 

 north and those that strike east of it. The 

 trends of the ranges doubtless correspond to 

 the strikes of the major faults in them. Mmor 

 cross faulting in other directions also took 

 place. 



QUATERNARY FAULTS. 



Probably no formations of laiown Quater- 

 nary age in this region are involved in large- 

 scale faults. Minor earth movements broke 

 and tilted the partly consolitlated strata of 

 older Quaternary alluvium in the Gila Bend 

 Mountains and elsewhere. Probably some 

 movement took place along the fault planes 

 formed during the previous period of crustal 

 disturbance. Lee '" found evidence of con- 

 siderable Quaternary faulting at Mesa and 

 Tempe. This movement lowered some of the 

 valley fill in this vicinity below sea level, as 

 is shown by the log of a deep well at Mesa. 

 Ransome ''' and others have shown that the 

 Gila conglomerate is faulted in many places in 

 the mountains east of PhoenLx. 



2» Lee, W. T., Undergrotmd waters of Salt Kiver valley, Ariz.: U. S. 

 Geol. Survey Water-Supply Paper 136, p. 115, 1905. 

 " Ransome, F. L., op. cit., p. 104, 



