TEMBLOR AND MONTEREY FORMATIONS 



71 



Woodring, Pseitdophragmina (Proporocyclina) psila 

 (Woodring), Gypsinal sp., and Operculinoides sp. 

 He writes (written communication, November 7, 

 1946) : "These three lots represent a single horizon, 

 that from which Woodring originally described the 

 two species listed above." The horizon referred to is 

 the Sierra Blanca limestone (Nelson, 1925) of the 

 Santa Ynez Range in southern California believed by 

 Woodring (1930, p. 145-170; 1931, p. 371-387) to be 

 "probably well down in the middle Eocene or in the 

 upper part of the lower Eocene." The fossil content 

 of this limestone, which is remarkably like that of the 

 limestone in the New Almaden area, was also studied 

 by Keenan (1932, p. 53-84), who described it as con- 

 taining echinoid fragments, small brachiopods like 

 Terebratalia, oyster-shell fragments, Discacyclina, 

 Globigerina, Nodosaria, nummulitoid Foraminifera, 

 bryozoans, and calcareous algae. Keenan placed its 

 age as "younger than Martinez (lower Eocene or 

 Paleocene) and older than Tejon at its type locality, 

 probably either upper Meganos or lower Domengine." 

 A third variety of limestone that is characterized 

 by abundant tests of the large orbitoid Discocyclina 

 is exposed in the low hills east of the point where 

 Guadalupe Creek first reaches the Santa Clara Valley. 

 It forms rounded, locally knobby outcrops on which 

 the Discocyclma stand out in relief, in an area where 

 no other rocks are exposed. The weathered surfaces 

 are light tan to brownish gray in color, but the fresh 

 rock is dark gray or grayish brown. This limestone 

 is well stratified into beds about 3 feet thick, but in- 

 dividual beds are massive or somewhat brecciated. In 

 some outcrops the limestone is largely made up of 

 closely packed Foraminifera tests, but in others the 

 fossils are more sparsely scattered through a matrix 

 of finely crystalline calcite, together with small scat- 

 tered angular grains of plagioclase, microcline, ortho- 

 clase, and pyrite. Glauconite also commonly occurs 

 interstitially to the fossils, in fine aggregates up to 

 several millimeters in diameter. No fragments of fer- 

 romagnesian minerals were found in this rock. This 

 limestone was studied by Schenck (1929, p. 224-227), 

 and is the type locality of DiscocycUna californica 

 Schenck. Other forms that Schenck described as oc- 

 curring in this limestone are "numerous smaller 

 Foraminifera, bryozoans, calcareous algae (cf. Ar- 

 chaeolithothamnum) , nummulitic Foraminifera, Gyp- 

 sina, perhaps a stellate Discocyclina, gastropods, pele- 

 cypods, crustaceans, and an occasional brachiopod." 

 The fossil content suggests that this limestone is a 

 little younger than the strata in the Santa Teresa 

 Hills; because of its isolated position and poor expo- 

 sure, this cannot be verified by stratigraphic relations. 



TEMBLOR AND MONTEREY FORMATIONS 



Sedimentary rocks of early, middle and late Miocene 

 age, including fossiliferous clastic limestone, conglom- 

 erate, coarse-grained sandstone, and siliceous and dia- 

 tomaceous shales, interbedded with a few lenses of 

 volcanic rocks, occur in the central and northwestern 

 parts of the New Almaden district. They are assigned 

 on the basis of lithology and a few fossils to the wide- 

 spread Temblor and Monterey formations, which in 

 other areas include early to late Miocene deposits. As 

 the two formations intergrade, the placing of the con- 

 tact is somewhat arbitrary; in mapping, the Monterey 

 shale was regarded, in accordance with the suggestion 

 of Bramlette (1946, p. 3-5), as including the lowest 

 beds of the typical white porcelaneous shales, and the 

 rocks lying below are regarded as a part of the Tem- 

 blor formation. 



The maximum aggregate thickness of the Miocene 

 sedimentary rocks in the district, as determined from 

 cross sections, is about 3,800 feet (pi. 1) ; but, because 

 of the probable time overlap of different facies, it is 

 possible that the sedimentary rocks now exposed were 

 not quite so thick in any one place. On the other 

 hand, the upper part of the section is everywhere un- 

 conformably overlain by other formations or removed 

 by erosion so that the full thickness of the Monterey 

 shale cannot be determined. 



The rocks of Miocene age in the New Almaden dis- 

 trict crop out in two main areas occupying about 5 

 square miles. One area is a broad band that extends 

 eastward from Los Gatos through the low hills along 

 the margin of Santa Clara Valley for nearly 7 miles. 

 The other area, which contains several segments iso- 

 lated by faulting, is a narrow band trending southeast- 

 ward; its northwest end is 1 mile west of the Guada- 

 lupe mine, and its southeast end 1 mile west of 

 Almaden Canyon. In some places, one of them east 

 of the Senator mine, these rocks can be seen to lie 

 unconformably on the older rocks of the district, but, 

 for the most part, they form blocks bounded by faults. 

 They are generally thrown into fairly open folds ex- 

 cept close to faults, where they are steeply tilted or 

 crumpled. 



Temblor formation 



The Temblor formation consists largely of calcite- 

 cemented conglomerate and medium- to coarse-grained 

 arkosic sandstone, the latter containing, in some places, 

 numerous fragments of pelecypods, oysters, and bar- 

 nacles; the upper part of the formation, however, is 

 more heterogeneous than the lower, and contains, in 

 addition to the rocks mentioned, some chocolate-col- 

 ored organic shale, greenish-gray shale, and glauconitic 



