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University of California Publications in Geology [Vol. 11 



Belation op San Lorenzo Beds to North of Mount Diablo to Those 



to the West 



The contrast between the different sections of the San Lorenzo 

 beds, just described, is so striking that it brings up the question, what 

 were the conditions of deposition that would cause such a difference ? 

 Below, in diagrammatic form, the sections are placed side by side, 

 and lines drawn from one to the other to indicate the writer's ideas 

 as to the" probable relationships of the three most important sections. 



None of the formations to the west of the mountain are comparable 

 in thickness and lithology to the Markley formation. The few fossils 

 found in the diatomaeeous shales of this formation show that the beds 

 are of Oligocene age. This alone, perhaps, should not be sufficient 

 evidence for saying that these beds were contemporaneous in deposi- 

 tion with those of the San Ramon formation. In general, the lithology 

 of the San Ramon beds is very different from that of the Markley ; 

 however, south of Walnut Creek we find, a little above the middle of 

 the San Ramon formation, the thin band of siliceous shale which 

 possibly represents the westward continuation of the light-colored 

 diatomaeeous shale of the Markley formation. 



The greater thickness of the Markley formation, compared with 

 that of the San Ramon formation, together with the fact that the 

 two sections are separated by only a few miles, suggests the prob- 

 ability that deposition began in the area to the north of Mount Diablo 

 before it did in the area to the west. The distribution of the marine 

 Oligocene to the south of Mount Diablo shows that these deposits were 

 laid down in a long north-and-south trough or geosyncline, the axis of 

 which extended from the southern end of the San Joaquin Valley 

 northward along the most eastern line of major folding in the Coast 

 Ranges to at least as far north as the area under discussion. This is 

 shown by the distribution of the Oligocene diatomaeeous shale (the 

 Kreyenhagen shale), which is found in a long, narrow area between 

 Coalinga and Mount Diablo. It has already been suggested that the 

 narrow band of diatomaeeous shale in the Markley formation repre- 

 sents the northward extension of the same conditions of sedimentation 

 as those found farther to the south, i.e., during the period of deposition 

 of the Kreyenhagen shales. The presence of the great thickness of 

 arkosic sandstone and lignitic shale, both below and above the diatom- 

 aeeous shale to the north of Mount Diablo, shows that this Oligocene 

 trough was shallower at its northern end than in the region to the 

 south. As has already been suggested, it seems probable that these 



