GEOLOGIC HISTORY OF THE SIERRAN GOLD BELT 

 By OLAF P. JENKINS 



The long chronicle of time recorded on this earth by its rocks and 

 pieced together by geologists to form a standard column of sequences, 

 is well represented in California. Indeed, the rocks of the Sierran Gold 

 Belt, which now have our attention, go a long way toward demonstrating 

 the entire historical section. 



Although the very early events of pre-Paleozoic and lower Paleozoic 

 time are indefinite in this part of the State, it is quite possible that during 

 this time of veiled antiquity a mountain range stood where the Sierra 

 Nevada now is located. Presence of such a range would have left little 

 chance for sediments to be deposited and therefore nothing in the way of 

 testimony, since geological testimony of history lies in the deposits which 

 are left behind. 



Marine deposited sediments with upper Paleozoic fossil corals and 

 other tropical organic life extend over a wide region in the northern 

 Sierra Nevada and prove the existence of an inland sea during that time. 

 Possibly it was only the western margin of the sea, but its waters cer- 

 tainly swept eastward across the present Great Basin area because more 

 complete sections of these sediments are found to the east in the interior 

 of Nevada and southeastern California. Rocks of Silurian, Devonian. 

 Carboniferous, and Permian periods representing all of the late portion 

 of the Paleozoic era. permit us to start out definitely with an epi- 

 continental sea as the beginning of our history of the rocks we may 

 encounter along Highway 49. 



The soft sands, muds, and marls of this early sea are now changed 

 to the form of very hard quartzites, slates, schists, cherts, limestones, and 

 marble. They are so old and have undergone so many episodes of loading, 

 compression, heating, crumpling, crushing, induration, recrystallization, 

 and infiltration of mineral bearing solutions that it is only by the most 

 skillful field and microscopic study that the origin of these rocks may 

 be interpreted. 



There is much evidence that volcanism was present during the broad 

 expanse of the 200 million years which elapsed while these Paleozoic 

 rocks were being laid down in the sea basin. The volcanic rocks which 

 are most in evidence are found in the upper Paleozoic. They are now 

 metamorphosed to greenstones and are so difficult to differentiate from 

 those of the later Mesozoic as to present one of the major problems to 

 geologic study. Fluctuations and movements of the earth 's crust caused 

 the sediments of this sea basin to be lifted and submerged several times 

 during its span of existence. 



The Mesozoic era which came after the Paleozoic is divided into an 

 early part, the Triassic, followed by the Jurassic, and finally the 

 Cretaceous. In the northern Sierra Nevada the Triassic is well repre- 

 sented by marine sediments, but there is not much to be seen of .them 

 along Highway 49. The period of tremendous importance in this region 



is the Jurassic, and especially the latter part of this period. Volcanic 

 activity was widespread throughout the entire province during upper 

 Jurassic. It may have been an ominous warning of the stupendous 

 mountain-making epoch that concluded the Jurassic and resulted in a 

 folded mountain range, the rocks of which were intruded throughout by 

 a molten granitic magma. Mineralized quartz veins, formed by the 

 filling and enlargement of cracks in the rocks with siliceous material, 

 followed the granitic intrusion represent the end product of this major 

 epoch of disturbance in California. All this took place over 100 million 

 years ago and the rocks of that period which we see exposed today were 

 probably buried then at a depth of at least two miles. 



Not all the rocks of the Upper Jurassic, however, are of eruptive 

 origin. There are the Mariposa slates, for example; these are marine 

 in origin. Some non-marine sediments were also deposited during the 

 Jurassic and there are many kinds of igneous intrusions associated with 

 the Upper Jurassic. Into the muds and other rocks of this period (also 

 of the Paleozoic), serpentine rocks bearing chromite were injected. These 

 ultra-basic intrusives antedate the granitic rock intrusions and may 

 have been introduced prior to folding of the sediments. 



The episode or episodes of folding present most interesting problems. 

 The whole scheme of affairs on this part of the earth was changed in 

 Upper Jurassic time, because the sea basin of long standing (hundreds of 

 millions of years) was destroyed. The strata were raised above sea level 

 and compressed into a complicated series of folds trending for the most 

 part northwest-southeast. As the granitic magma was intruded, large 

 sections of the earlier rocks disappeared. Perhaps they were assimilated 

 by the granite or pushed aside, or maybe the granite itself was manufac- 

 tured from the earlier rocks by melting. All we actually know, however, 

 is that where the granite is present now the earlier rocks are gone. In 

 many places the granitic rocks (largely granodiorite) include fragments 

 of earlier rocks as if floating within their firm crystalline structure. At 

 the southern end of the Mother Lode, at Mormon Bar, the earlier rocks 

 give way to granite, and thus thr belt of Mother Lode quartz veins is 

 terminated. 



The time interval required for the folding of all the sediments of 

 the Paleozoic. Triassic and Jurassic plus the intrusion of granite was 

 geologically not very long. It was a mountain-making epoch of a com- 

 paratively short span, all within the upper part of the Upper Jurassic. 

 The cooling, contracting, and releasing of mineral-bearing solutions by 

 the granitic rocks permitted quartz veins to fill the resulting fractures in 

 the rocks and the fractures previously formed by faulting during the 

 folding episode. 



All these rocks Paleozoic sediments, Triassjc and Jurassic sedi- 

 ments and volcanics, intrusive basic and ultra-basic rocks, intrusive or 



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