OTHER UPPER CRETACEOUS ROCKS 



65 



chiefly in the Santa Teresa Hills in the northern part. 

 As the two units are nowhere in contact and have 

 yielded only a few fossils, none of which are closely 

 limited in range, their relative age is not known. The 

 writers, however, regard the rocks of Sierra Azul as 

 being the older, because they are more indurated and 

 more intensely deformed than the rocks in Santa Te- 

 resa Hills. 



Uppper Cretaceous rocks of the Sierra Azul 



The Upper Cretaceous rocks found in the Sierra 

 Azul consist of several thousand feet of interbed- 

 ded conglomerate, feldspathic graywacke, and shale. 

 Within the mapped area graywacke constitutes more 

 than half of the unit, and conglomerate and shale 

 each constitute a little less than a fourth, but shale is 

 much more abundant farther south. These rocks un- 

 derlie only a small area along the northern slope of 

 the Sierra Azul in the south-central part of the dis- 

 trict (pi. 1), but, because they extend south into Santa 

 Cruz County, their total area of outcrop is much 

 larger than is indicated by the map accompanying this 

 report. As these rocks are poorly exposed, and con- 

 tain few fossils within the district, and as they are 

 being studied by others in the area of better expo- 

 sures to the south, we have not assigned a formational 

 name to the unit. 



The fresh graywacke is medium grained and light 

 colored, but where it is weathered, it is speckled with 

 white feldspars in a red matrix. The grains are sub- 

 angular and subrounded and are only moderately well 

 sorted. Locally, they are admixed with small peb- 

 bles or fist-sized clay balls. The principal minerals 

 observed in the single thin section that was made are 

 quartz, orthoclase, and albite; these are accompanied 

 by a little muscovite, biotite, chlorite, and calcite. 

 Staining tests made on a half-dozen specimens indicate 

 orthoclase amounts to from 5 to 20 percent. Frag- 

 ments of chert and mafic lava amount to about 10 

 percent, and some of the feldspars show myrmekitic 

 intergrowths. The feldspathic graywacke differs from 

 that of the Franciscan group in being slightly cal- 

 careous and in containing much more orthoclase, more 

 matrix, and more clayey material in the matrix. It 

 is poorly exposed, but is somewhat better exposed, 

 on the average, than the graywacke of the Franciscan 

 group; on hillsides it forms characteristic brushy 

 slopes that are generally mantled with talus, and on 

 hilltops it yields a reddish or pinkish soil. 



The conglomerate forms beds commonly more than 

 10 feet thick, and in places these occur in groups sepa- 

 rated by foot-thick beds of graywacke. Locally, the 

 conglomerate beds crops out in bold relief, but more 



commonly they are subdued and give rise to a red- 

 dish bouldery soil. Some .of the boulders are as much 

 as 1 foot in diameter, but the average is between 2 

 and 3 inches. The result of a pebble count made on 

 the conglomerate exposed on a ridge road just south 

 of the serpentine body near Mount Umunhum is 

 shown in table 14, and a similar count from a con- 

 glomerate of the Franciscan group is given for com- 

 parison. The 2 conglomerates are notably different 

 in the proportion of igneous and sedimentary rock 

 pebbles; igneous rocks make up 90 percent of the 

 pebbles in the conglomerate of the Sierra Azul, but 

 less than 50 percent of those in the conglomerate of 

 the Franciscan group. Only 16 of the 100 pebbles 

 identified in the Sierra Azul conglomerate could pos- 

 sibly be derived from the Franciscan group, and it is 

 likely that most of these are not. The matrix of the 

 conglomerate of the Sierra Azul is similar to the gray- 

 wacke with which it is interbedded, but in many places 

 it is more silicified. The conglomerate is much frac- 

 tured; in the more silicified parts the fractures cleave 

 the pebbles and in the unsilicified parts they break 

 around them. Calcite fills some of the fractures and 

 locally replaced part of the matrix. 



TABLE 14. Pebbles from conglomerate in the Upper Cretaceous 

 rocks of the Sierra Azul and the Franciscan group, Santa Clara 

 County, Calif. 



Rock 

 Sedimentary rocks: 



Sandstone and quartzite. 



Chert 



Conglomerate 



Slate., 



Sierra Franciscan 

 Azul group 



Number of Number of 

 pebbles pebbles 



32 



21 



1 



Total sedimentary rocks 



Igneous rocks: 



Granite 



Aplite 



Quartz porphyry 



Volcanic rocks with quartz phenocrysts- _ 

 Volcanic rocks with feldspar phenocrysts 



and no quartz phenocrysts 



Diabase 



Greenstone without phenocrysts 



54 



32 



35 



7 



11 



Total igneous rocks . 

 Vein quartz 



91 



1 

 3 

 9 



20 

 3 

 9 



45 

 1 



Total pebble count. 



100 



100 



The shale in the Upper Cretaceous rocks of the 

 Sierra Azul is exposed in the New Almaden area in 

 few places other than artificial cuts. In some road- 

 cuts one may see beds of shale less than 1 foot thick 

 rhythmically interbedded with graywacke, as is shown 



