10 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



GENERAL GEOLOGY 



The rocks of the New Almaden district range in 

 age from Late Jurassic to Recent, but they represent 

 a depositional history that was interrupted by several 

 major breaks. The oldest assemblage consists of the 

 Iierterogeneous Franciscan group of Late Jurassic and 

 Cretaceous age, which occupies nearly three-fourths 

 of the district. It contains graywacke, siltstone, mafic 

 volcanic rocks, chert, limestone, and minor amounts 

 of metamorphic rocks, including glaucophane schists. 

 This assemblage is regarded as typical of eugeosyn- 

 clinal accumulations in erogenic belts, and, as might 

 be expected, the rocks nearly everywhere exhibit 

 small-scale folds and shears and show abundant evi- 

 dence of rock flowage. Intrusive into the rocks of 

 the Franciscan group are many tabular bodies of 

 serpentine, a few of which are of special interest be- 

 cause they have been hydrothermally altered during 

 the late Tertiary to silica-carbonate rock the host 

 rock for the quicksilver ore bodies. 



Upper Cretaceous deposits include two groups of 

 rocks differing in both lithology and degree of de- 

 formation. The more deformed group, and prob- 

 ably the older, consisting of several thousand feet 

 of conglomerates, graywacke, and shale, is exposed 

 only in the higher parts of the Sierra Azul in the 

 south-central part of the district. The other unit of 

 Upper Cretaceous age, consisting of gently folded 

 sandstone and shale, is found in the northern part of 

 the area, principally in the Santa Teresa Hills. This 

 unit may be the equivalent of part of the Chico for- 

 mation, but because of the uncertainty of correlation, 

 it is herein designated as the Upper Cretaceous rocks 

 of the Santa Teresa Hills. Overlying this unit is a 

 relatively thin sequence of limestone, sandstone, and 

 shale of middle Eocene age, which is so poorly ex- 

 posed that it has not been assigned a formation name. 



The next record of sedimentation is furnished by 

 rocks of lower, middle and upper Miocene age. These 

 are at least 3,800 feet thick, and grade upward from 

 conglomerate and sandstone to diatomaceous shale. 

 The older part of this sequence has been referred 

 to the Temblor formation, whereas all beds above the 

 lowest diatomaceous shale are included in the Mon- 

 terey shale. Some included felsic volcanic material 

 reveals igneous activity in the area during the middle 

 Miocene. 



The post-Miocene deposits consist largely of al- 

 luvium, which fills the Santa Clara Valley, floors 

 major canyons, and in places lies along the base of 

 the foothills as perched gravels. Largely on the 

 basis of their dissection and topographic position these 

 gravels have been assigned to two formations. The 



older deposits have been correlated with the Santa 

 Clara formation of Pliocene and Pleistocene ajre. 

 whereas the younger ones are grouped as Quaternary 

 alluvium. 



The structural features of the district consist of 

 folds and faults trending westward or northwestward. 

 The oldest rocks in general dip northeastward, but 

 they have yielded to deformational forces by exten- 

 sive crumpling, folding, flowing, and faulting. The 

 major faults are believed to have largely strike-slip 

 displacement, and they are generally marked by shear 

 zones rather than single planes of slippage. The 

 most notable of these shear zones the Ben Trovato, 

 which trends obliquely through the central part of 

 the district attains a width of more than a half a 

 mile. The rocks of intermediate age, though also 

 deformed, have remained more cohesive and form 

 simple continuous folds cut by narrow faults. The 

 young alluvial formations are little tilted, and show 

 traces of faults only by topographic scarps less than 

 100 feet in height. 



The following sections describe the 21 distinctive 

 cartographic rock units mapped in the district. Some 

 of these contain only a single kind of rock, whereas 

 others, such as the greenstone of the Franciscan group, 

 contain several distinct but related lithologic types. 

 Correlations between isolated exposures of rocks of 

 the different cartographic units were made largely 

 on the basis of lithologic similarity, as fossils were too 

 rare in nearly all of them to be of much aid. 



FRANCISCAN OKOUP 



The oldest and most extensive assemblage of rocks 

 in the district has been assigned primarily on the 

 basis of lithology to the Franciscan group, which in- 

 cludes rocks of both Jurassic and Cretaceous age. 

 All the diverse rock types commonly found in this 

 group throughout the California Coast Ranges occur 

 in the New Almaden district, and their relative 

 abundance and general lithology in this district are 

 believed to be typical of the central part of the 

 Coast Ranges. The group consists mainly of medium- 

 to fine-drained graywacke (p. 13) and dark shale; it 

 contains a somewhat smaller amount of generally 

 altered mafic volcanic rocks usually classed as "green- 

 stones,'' and small amounts of conglomerate, lime- 

 stone, and chert. Another rock, serpentine, which 

 accompanies the group in many places, is considered 

 by sonic as a part of the group: but it is somewhat 

 younger, because it is intrusive into the other rocks. 

 Because of the distinctive character of the group, 

 both lithologieally and structurally, and because of 

 its importance to California geologists and quicksilver 



