MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA 



499 



Fault dies out 



Ford Cr 

 Throw of foult 

 I600' t thickness 

 of alluvium 



Bar nor d Cr 



Parr/shCr Centerville Cr 



Ward Cr * Hoi brook Cr. 



Throw of iouh 

 looo' 1 thickness 

 of o//uvium 



Mill Cr. 



Lake Bonne- 

 vrlle facets. 

 Salt Lake 

 salient 



Fig. 31. 7 Wasatch fault in the north central Wasatch Range and its relation to the erosion surfaces. 



the Great Basin region were accentuated, in part by warping, in part by 

 faulting. 



Next followed another long interval of repose, or relative repose, that 

 lasted through the entire Pliocene epoch. Only feeble eruptions took 

 place from time to time, and meanwhile the waters in the lava-covered 

 parts of the range reorganized themselves into new rivers and cut new 

 canyons, some of which attained depths of more than 1000 feet. 



Then, at the beginning of the Quaternary period the great uplift and 



tilting commenced that gave the Sierra Nevada its present great altitude. 



The summit peaks were raised to almost double their previous height, 



with Mount Lyell reaching more than 13,000 feet above sea level. At the 



same time, fracturing and faulting took place on an enormous scale. 



Owens Valley and other desert regions adjoining the range on the east 



ind south subsided, or else suffered but slight uplifts as compared with 



he mountain block; and so the Sierra Nevada came to stand out in its 



present imposing form, with gentle westward slope, sharply defined 



prest, and abrupt eastward-facing escarpment. Strangely, the volcanic 



jiccompaniments of this great upheaval and inbreaking of the earth's 



irust were not extensive in the immediate vicinity. Though molten ma- 



erial forced its way up repeatedly through fractures in or near the 



lone of faulting, and also through cracks in the Sierra block, the result- 



ig volcanic cones and lava flows were insignificant compared with 



hose elsewhere in the Great Basin and northward in Oregon and Wash- 



lgton. 



In the north-central Wasatch Mountains, the Wasatch fault broke and 

 Jisplaced an erosion surface of mid-Tertiary age. Most of the displace- 

 ment was attained by the early Pleistocene (Eardley, 1944). See Fig. 



31.7. Fresh scarps in the alluvium and across terminal moraines also 

 attest post-Wisconsin movements. 



Nolan believes that the best conclusion possible from present infor- 

 mation is that block faulting probably began in places in early Oligocene 

 time and has been more or less continuous ever since. Topographically 

 expressed faults, however, probably date back only to late Pliocene or 

 early Pleistocene, though earlier movements may have occurred along 

 then. 



Amargosa Chaos 



An immensely disordered complex occurs in the Death Valley region 

 which Noble (1941) has studied. See Fig. 31.8. In a centrally located 

 district 10 miles square, called the Virgin Spring area, he finds the prin- 

 cipal structure to be a flat thrust fault which originally followed approxi- 

 mately the contact of later Precambrian sediments and earlier Precam- 

 brian metamorphic rocks. On this thrust later Precambrian, Cambrian, 

 and Tertiary rocks have moved relatively westward for an unknown dis- 

 tance. The rocks of the overthrust plate are broken into innumerable 

 blocks and slices, which are thrust over one another to form an extremely 

 complex mosaic. This assemblage of blocks is named the Amargosa 

 chaos, and the flat fault upon which the chaos lies is named the Amargosa 

 thrust. The chaos is divided into the Virgin Spring, Calico, and Jubilee 

 facies. The Virgin Spring is characterized by blocks of late Precambrian 

 and Cambrian dolomite, marble, sandstone, quartzite, shale, and slate. 

 The Calico is made up almost wholly of Tertiary volcanic blocks, and 

 the Jubilee contains a much larger proportion of poorly consolidated 

 and broken-up material than the other two phases. The irregular blocks 



