GEOLOGIC STRUCTURE 



85 



and folds especially near its western end, but the 

 Franciscan rocks on the whole are less deformed here 

 than in any other part of the district. 



The Bincon fault is a little less than a mile south 

 of the Limekiln fault and is nearly parallel to it for 

 several miles, but to the east the Eincon swings south- 

 eastward and becomes a part of the Berrocal fault 

 zone. Two other faults split southward from the 

 Eincon fault; the longest of these, the Soda Spring 

 fault, joins the Sierra Azul fault in the south-central 

 part of the district. The rocks of the Franciscan 

 group within the silvers bounded by these faults are 

 dominantly graywacke to the west and greenstone to 

 the east. They dip generally northward, but locally, 

 especially near the greenstone masses, they have er- 

 ratic dips. 



The southernmost fault, the Sierra Azul, which 

 separates rocks of the Franciscan group from the 

 Upper Cretaceous rocks of the Sierra Azul to the 

 south, also trends west-northwestward. Although it 

 has been mapped in the district for only a couple of 

 miles, it is believed to be a major fault that extends 

 along its projection both ways for several miles more. 



Within El Sombroso block, characterized by faults 

 trending about west-northwest, are several steep more 

 or less tabular bodies of serpentine. These are be- 

 lieved to have been emplaced largely along faults or 

 shear zones, although in places they obviously break 

 across beds lying between the major faults. 



FOLDS 



FOLDS IN ROCKS OF THE FRANCISCAN GROUP 



The folds and warps in rocks of the Franciscan 

 group in the New Almaden district differ greatly in 

 character. This is partly because they result from 

 forces applied at different times since their deposi- 

 tion, and partly because of differences in the strength 

 or competency of the various kinds of rocks involved. 

 Extensive flowage of the shale and other fine-grained 

 rocks accompanied by breaking of the more competent 

 graywackes complicates the more detailed structure 

 of many of the fekls. The large folds are generally 

 composite and contain smaller warps, folds, and faults 

 superimposed on the broader structures; when viewed 

 on a district-wide scale, however, they show a com- 

 paratively orderly though rather vague pattern. These 

 large folds are believed to have resulted from regional 

 forces. On the other hand, some of the small-scale 

 folds might have been formed during the original 

 deposition, others are related to movement along later 

 faults or to local intrusions, and still others merely 

 represent minor flexures on larger structures. 



Folds formed during deposition 



Folds that might have been formed at the time of 

 deposition are found throughout the district, in at 

 least three of the rock types of the Franciscan group. 

 The most striking examples of these possibly prein- 

 duration folds are exhibited by some of the bedded 

 chert bodies of the area. For example, a roadcut 

 about half a mile northwest of Bald Mountain ex- 

 poses the highly contorted chert shown in figure 17, 

 which is well stratified in beds that are only locally as 

 much as 8 inches thick; in these cherts there are in- 

 tricate folds measuring less than 3 feet in any dimen- 

 sion, overturned and offset by small faults along their 

 limbs. Such contortion seems to be restricted to the 

 chert and not shared by the surrounding clastic sedi- 

 mentary rocks; and furthermore, the relatively brittle 

 chert has neither been crushed nor broken along the 

 places of maximum deformation. This suggests that 

 the crumpling occurred shortly after the original 

 deposition of the chert and before it became hardened, 

 though similar folds can also form without rupture 

 under a thick cover. Similarly contorted well-bedded 

 radiolarian chert may be seen in a cut along the Day 

 tunnel road on Mine Hill a few hundred feet west of 

 its junction with the main road to the mine camp; 

 other excellent examples are observable underground 

 in the Day tunnel of the New Almaden mine, between 

 the portal and the cross cut to the Santa Eita shaft 

 (pi. 6). 



Other small folds probably contemporaneous with 

 the formation of the rocks of the Franciscan group 

 have been observed along the margins of exposures 

 of pillow basalt throughout the district. These are 

 only small puckers or plications, partly in the mate- 

 rial that fills the interstices between the pillows of 

 basalt and partly in the soft shales adjacent to them. 

 Such folds doubtless resulted from the movement of 

 the extruded lava against the soft muddy sediments; 

 these folds, of course, do not indicate deformation 

 during deposition. An example of such minor folds 

 may be seen in the pillow lavas on Los Capitancillos 

 Eidge, about 2,000 feet northwest of the northwest 

 corner of the area shown on plate 3. 



Local unconformities in the Franciscan group may 

 indicate minor tilting at the time of deposition, but 

 these are hard to detect in the field. The unconformi- 

 ties most commonly observed separate individual beds 

 or series of beds in single isolated outcrops or in the 

 walls of mine workings, but neither their extent nor 

 the amount of tilting involved can generally be deter- 

 mined. No large-scale unconformities or channels that 

 might indicate extensive deformation during the pe- 

 riod of deposition were found. 



