436 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



the Chico Shunie could correlate in age with the Juniper Flat granite 

 and other related plutons, and the Cornelia with the Stronghold granite 

 and Schieffelin granodiorite. 



GEOLOGY OF WEST-CENTRAL ARIZONA 



The Artillery Mountains near the west end of the mountain region 

 has been studied by Lasky and Webber (1944), and the rock units and 

 succession of events there are probably representative of the general 

 area. The stratigraphy is briefly described below, and the relation of 

 rock units to the structural events is shown in Figs. 27.2 and 27.9. 



Of particular interest are two thrust sheets, one of which occurs in the 

 Artillery Mountains and one in the Chemehuevis Mountains just south- 

 east of Needles (see Tectonic Map of the United States). Both sheets 

 overrode toward the Colorado Plateau and are probably Laramide in 

 age. 



The Laramide orogeny seems to have been in two phases, first an up- 

 lift that furnished the lower Eocene (?) conglomerate, arkose, sandstone, 

 shale, and limestone beds to a northwestward-trending trough, and then 

 the thrusting that brought the Precambrian and Paleozoic (?) sedimen- 

 tary rocks on top of the lower Eocene (?) beds. These lower Eocene beds 

 have a structural setting similar to the conglomerates, arkosic sandstones, 

 and claystones of the New Water Mountains farther south. 



The Artillery Mountains thrust is overlapped by the Miocene (?) vol- 

 canics. Minor normal faulting then formed a graben in which lower Plio- 

 cene ( ? ) sediments and an overlying basalt accumulated. After the graben 

 basin had become integrated into a regional drainage system, the upper 

 Pliocene (?) conglomerate was deposited. The Pliocene (?) rocks were 

 then folded into a shallow composite syncline that parallels the northwest- 

 ward trend of the basin, and that now occupies the valley between the 

 Artillery and Rawhide Mountains. 



The folded rocks along either side of the valley, together with the over- 

 lying Pleistocene (?) basalt, are broken by northwestward-trending nor- 

 mal faults, which presumably are the effect of renewed movement along 

 older fault zones. See cross sections of Fig. 27.9. 



Rock Units of the Artillery Mountains Manganese Area 



Thickness, Feet 



0-350 plus 



0-250 plus 



Recent: 



Talus deposits, and gravel and sand along the present drainage. 



Erosional Unconformity 

 Later Pleistocene: 



Pediment gravel and valley fill. 



Angular Unconformity 

 Earlier Pleistocene (?): 



Massive, fine-grained to vesicular glassy basalt. 



Angular Unconformity 

 Upper Pliocene (?): 



Largely light to dark red, poorly sorted conglomerate with dis- 

 continuous bedding. Includes a prominent basalt member in the 

 southwestern part of the area. 



Erosional Unconformity 

 Lower Pliocene (?): 



Massive aphanitic vesicular basalt 



Alluvial fan and playa deposits— fan-glomerate, conglomerate, sand- 

 stone, siltstone, mudstone, clay, and limestone; in part gypsiferous. 

 The principal manganese-bearing formation. 



Angular Unconformity 

 Miocene (?): 



Tuffs, breccias, and flows, rhyolitic to andesitic. 



Angular Unconformity 

 Lower Eocene (?): 



Conglomerate, arkose, sandstone, shale, limestone, and a little 

 clay, with some tuff and a widespread basalt member; in large 

 part highly indurated. 



Angular Unconformity 

 Paleozoic (?): 



Limestone with minor quantities of shale and quartzite in part 

 metamorphosed. 



Angular Unconformity 



Precambrian: 



Granite, gneiss, microbreccia, and subordinate schist, including 

 some monzonitic rock in the Rawhide and Buckskin Mountains that 

 may be of post-Cambrian age. 





0-2000 plus or minus 





0-1500 plus or minus 



1800 plus 



2500 plus 





