GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



dreds of feet across are shown on the geology map 

 of the district (pi. 1) and the more detailed geologic 

 map of the New Almaden mine area (pi. 3). Where 

 a large area is considered, folds of this magnitude 

 seem to make no regular pattern, and no simple con- 

 cept of folding seems to account satisfactorily for the 

 position and attitude of such beds. They seem to 

 represent yield to local stresses whose direction is 

 diverted from the normal stress direction by the large 

 difference in stress transmission of the competent and 

 incompetent rocks. 



Drag folds adjacent to faults occur throughout the 

 district, but an entire fold can rarely be observed in 

 a single outcrop. The presence of these folds is indi- 

 cated on the map by the patterns showing the distri- 

 bution of the rocks adjacent to faults, and some of the 

 folds are best shown in cross sections. On the south 

 side of the Soda Spring fault there are drag folds 

 probably as much as 100 feet wide, as is shown on 

 section A-A', plate 1. Another drag fold of com- 

 parable size is indicated by the surface trace of a 

 cherty limestone bed adjacent to a minor fault in the 

 deep canyon 1,250 feet southwest of the modern re- 

 duction plant on Mine Hill. 



Another type of small fold in sedimentary rocks 

 of the Franciscan group, apparently related to, and 

 probably caused by, intrusions of serpentine, may be 

 observed at a few places in the New Almaden mine 

 adjacent to serpentine bodies. An easily accessible 

 example of such crumpling can be seen in a short 

 crosscut from the Day tunnel south of the Cable raise 

 (pi. 6), where graywacke and shale are in contact with 

 silica-carbonate rock derived from serpentine. Be- 

 cause the serpentine was intruded plastically, rather 

 than as a peridotite magma, its contacts are not very 

 different from faults, and the folds formed along 

 them are similar to drag folds formed along faults. 



FOLDS IN POST-FRANCISCAN ROCKS 



Folds in the post -Franciscan rocks of the district 

 are much more clearly defined, simple, and regular in 

 shape than those in Franciscan rocks, and they are 

 more readily worked out because of the more numer- 

 ous outcrops and the greater consistency of observed 

 attitudes (fig. 62). Most of the folds are large and 

 open and have comparatively gentle dips on the limbs, 

 although in exceptional places, along faults for ex- 

 ample, the younger rocks are drag folded like those 

 of the Franciscan group. The deformation of the 

 post -Franciscan rocks has been accomplished by sim- 

 ple folding, unaccompanied by the extensive rock 

 flowage that is so typical of the deformation of the 

 rocks of the Franciscan group. 



Upper Cretaceous rocks of the Sierra Azul 



Folds in the Upper Cretaceous rocks of the Sierra 

 Azul have been mapped in much less detail than those 

 in any of the other post -Franciscan formations. To 

 judge from the few dips observed, the beds along the 

 divide near the south boundary of the district strike 

 northward to northwestward and in general dip at 

 moderate angles to the northeast, whereas in the east- 

 ern and western tributaries of Almaden Canyon, ad- 

 jacent to the Sierra Azul fault, they strike northwest 

 and dip southwest. As the attitudes in this unit are 

 comparatively consistent over large areas, they prob- 

 ably indicate a broad synclinal structure. 



Upper Cretaceous rocks of the Santa Teresa Hills 



Folds in the Upper Cretaceous rocks of the Santa 

 Teresa Hills are indicated by attitudes observed in 

 the sandstone and by the surface traces of contacts 

 between sandstone and shale. The large mass of these 

 rocks in the Santa Teresa Hills is broadly folded into 

 a syncline whose axis plunges to the west. The posi- 

 tion of the axis of this fold is readily plotted through 

 an area extending westward from the quarry near the 

 west end of the hills to the place where it passes be- 

 neath the alluvium of Alamitos Creek, but east of the 

 quarry the fold gets lost in poorly exposed shale. The 

 flanks of this fold are cut off by parallel east -trending 

 faults that lie about 1.500 feet apart, Lut the eastward 

 extension of the south limb is indicated by the easterly 

 strike and gentle northerly dips of the beds exposed 

 in the fault-bounded blocks. 



Eocene rocks 



The broad syncline which involved the rocks of Late 

 Cretaceous age in the Santa Teresa Hills is shared by 

 the Eocene rocks, which overlie them with only a 

 minor unconformity. Two smaller well -defined folds, 

 a syncline and an anticline, lie near the crest of the 

 ridge west of the Santa Teresa mine. Their axes, 

 which are traceable for less than half a mile, trend 

 northwestward, and their limbs have dips of less 

 than 45. 



Miocene formations 



The folds in the rocks of the Temblor and Monterey 

 formations of Miocene age have been mapped in more 

 detail than those in any of the other formations in the 

 district, owing to the abundance of exposures, the dis- 

 tinctness of the bedding, and the presence of mappal'le 

 stratigraphic units. These folds are regular and clearly 

 defined, and are comparatively extensive, though many 

 are cut off by faults. The most extensive is a syncline 

 that makes up the central part of Blossom Hill and 

 the hill north of the Senator mine. This fold trends 

 eastward and is nearly symmetrical. The steepest dips 



