86 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



Uplift during deposition may also be indicated by 

 conglomerate beds that contain pebbles derived from 

 erosion of other parts of the same formation. Else- 

 where in the Franciscan rocks of the Coast Ranges 

 there are beds of conglomerate or breccia that seem 

 to demonstrate local uplift, erosion, and deposition 

 (Taliaferro, 1943b, p. 140-143) . In the New Almaden 

 district, conglomerates are of rare occurrence in the 

 Franciscan group, and those that have been found 

 show by their general lack of pebbles of Franciscan 

 rocks that they were not derived in any large part, 

 if at all, from a reworking of deposits laid down in 

 Franciscan time. 



In summary, the evidence provided by the chert and 

 the minor unconformities suggests a little warping 

 during the deposition of the rocks of the Franciscan 

 group, and the presence of extrusive lavas, which 

 must have come from nearby vents even though none 

 of these were found, also requires some crustal insta- 

 bility. However, no uncontestable evidence for de- 

 formation during deposition was found, and although 

 the area of deposition must have been subsiding, it 

 seems unlikely that it was being strongly deformed 

 while the thick sequence of Franciscan rocks was be- 

 ing built up. 



Folding after deposition 



Folds in the rocks of the Franciscan group resulting 

 from regional or local forces applied after the rocks 

 were deposited have been formed at repeated inter- 

 vals. The broader folds are comparatively nor- 

 mal, though complicated by superimposed structural 

 features, but the character of the tighter or smaller 

 folds depends largely upon the competence of the 

 folded rock and the difference in competence of adja- 

 cent beds (figs. 59-61). Especially incompetent rocks 

 are the shale, siltstone, greenstone tuff, and the closely 

 associated, but slightly younger, serpentine involved 

 in the later folding. Other rocks, including some of 

 the graywacke, chert, limestone, and diabasic green- 

 stone, are comparatively competent, and where they 

 occur in large masses, they form rather cohesive units 

 more susceptible to faulting than to small-scale fold- 

 ing. Where the more competent 'rocks are folded with 

 interbedded incompetent rocks, they have commonly 

 been broken into blocks which are embedded in a 

 plastically deformed matrix. In such material the 

 softer rocks are largely folded and show evidence of 

 having flowed whereas the harder ones are faulted. 

 Much of the Los Capitancillos block contains such 

 material, and the rocks in the Ben Trovalo fault 7.0710 

 and the alta bordering the serpentine provide out- 

 standing examples. 



FIGURE 59. Plastically deformed and sheared rocks of the Franciscan 

 group as exposed In workings of the New Almaden mine. White 1> 

 tuffaceous greenstone, gray Is chiefly graywacke, and black Is siltstone 

 and shale. 



Parallel or isoclinal folds involving rocks of the 

 Franciscan group and large enough to be major fea- 

 tures are not a prevalent type of structure in the New 

 Almaden district. One such well-defined major fold, 

 however, is the irregular and crenulated anticline that 

 underlies Los Capitancillos Ridge. Many of the data 

 on the form of this anticline near Mine Hill are de- 

 rived from the detailed maps of the New Almaden 

 mine and cross sections prepared from these map~: 

 the details of its structure as related to the ore liodies 

 are discussed in the description of the mine, hegin- 

 iiinir on page 128. This antic-line has a breadth from 

 one flank to the other of about li/ 2 miles, and its axis 



