EOCENE ROCKS 



69 



sil identified by L. W. Stephenson, of the U.S. Geo- 

 logical Survey, as B acuities chicoenpis Trask was 

 found in a concretion just above a small saddle 0.38 

 mile east of the summit of the 430-foot hill southwest 

 of the Santa Teresa mine. In addition, two casts 

 found in the sandstone of the Stanford University 

 buildings have been identified by S. W. Muller as 

 Turritella chicoensis Gabb. Both of these fossils are 

 found in the Chico formation (Coniacian to Cam- 

 panian) at its type locality, but they hardly seem to 

 justify correlation over so great a distance. 



The stratigraphic thickness of this unit in the New 

 Almaden area, as determined from cross sections, is 

 at least 1,200 feet ; but the total original thickness 

 was greater, for its base is not exposed and its upper 

 limit is an unconformable contact with rocks of middle 

 Eocene age. The thickness even of the rocks that are 

 exposed is uncertain, because the unit is cut by faults 

 of small but unknown displacement. Moreover, an 

 exact measurement of the exposed thickness in any one 

 place would have little significance, for the sequence 

 is characterized by lenticular sandstone beds that thin 

 out within short distances along their strike. 



Possible correlation and age 



Although isolated specimens of the rocks of Upper 

 Cretaceous age from the Sierra Azul and the Santa 

 Teresa Hills might be easily confused, the differences 

 between the rock of the two areas are considerable. 

 The Sierra Azul sequence contains thick beds of con- 

 glomerate, which are lacking in the Santa Teresa 

 Hills. The fresh feldspathic sandstones of the two 

 formations look much alike in hand specimens, but 

 they are readily distinguished in the field because they 

 weather differently and have different types of expo- 

 sure. The graywacke found in the Sierra Azul weath- 

 ers to form hard angular pieces of reddish or pinkish 

 color, whereas the arkosic sandstone in the Santa 

 Teresa Hills weathers to a tan-colored and generally 

 friable rock; the graywacke of the Sierra Azul is 

 poorly exposed, whereas the sandstone of the Santa 

 Teresa Hills generally forms prominent outcrops of 

 spheroidally weathered and casehardened boulders. 

 The shales of the two formations differ chiefly in their 

 mode of occurrence: in the Sierra Azul they are gen- 

 erally interbedded with many thin layers of gray- 

 wacke, whereas the Santa Teresa Hills sequence con- 

 tains thick strata of homogeneous shale. 



The two formations generally differ, also, in the 

 character of their folding. The rocks in the Sierra 

 Azul are so tightly folded that they show dips as high 

 as 70, whereas the beds of the Santa Teresa Hills 

 rarely dip so steeply as 45. This, however, expresses 

 only a part of the difference in degree of deformation, 



for in finer detail the Sierra Azul rocks are much more 

 crumpled and are cut by a multitude of small frac- 

 tures and faults that are not found in the rocks of the 

 Santa Teresa Hills. 



Because of these differences, the two formations are 

 believed to be distinct, even though both contain fos- 

 sils indicating Late Cretaceous age. The differences 

 in their lithology and deformation suggest that the 

 Sierra Azul sequence is the older. Perhaps this se- 

 quence corresponds to the Pacheco group of Taliaferro 

 (1943a, p. 130-134), and the Santa Teresa Hills se- 

 quence corresponds to a part of his Asuncion group, 

 which was deposited after his Santa Lucia orogeny. 



EOCENE BOCKS 



A sedimentary sequence, of early middle Eocene age, 

 consisting of fissile shale, fossiliferous limestone, and 

 coarse-grained sandstone underlies the central part of 

 the Santa Teresa Hills and a group of low hills east 

 of the point where Guadalupe Creek emerges from the 

 mountains onto Santa Clara Valley. Because this se- 

 quence occupies less than a square mile and is very 

 poorly exposed, it is not assigned a formational name 

 in this report. In the Santa Teresa Hills it lies un- 

 conformably on the previously described sediments of 

 Late Cretaceous age with a small angular discordance, 

 and it is overlain only by alluvium. Its thickness 

 cannot be accurately determined, but it appears to be 

 at least 900 feet thick. The limestone is much like 

 the Eocene Sierra Blanca limestone of Nelson (1925) 

 of the Santa Ynez Mountains, in Southern California, 

 and the entire sequence is probably equivalent in age 

 to the upper part of the Meganos formation or the 

 Capay formation of Weaver and others (1944). The 

 nearest known exposures of Eocene rocks represent 

 the equivalent of the Domengine or Capay formation 

 northeast of Morgan Hill, described by Gilbert (1943, 

 p. 640-646). 



The lowest part of the Eocene sequence in the Santa 

 Teresa Hills consists chiefly of shale, and unexposed 

 layers of shale are probably intercalated between the 

 younger beds of sandstone. Shale underlies most of 

 the eastern part of these hills, but even here it is 

 nowhere exposed ; it forms subdued grass-covered hills 

 riddled with ground-squirrel burrows, and the largest 

 observable shale fragments are found in the material 

 excavated from these holes. The shale is light tan, 

 fissile, and exceptionally powdery. Examined under 

 the microscope, it appears to consist of clay minerals, 

 calcite, some limonite, and only a few grains of quartz 

 and feldspar. 



Sandstone makes up most of the unit in the western 

 part of the Santa Teresa Hills and also in the low 



