84 



University of California Publications in Geology [Vol. 



Markley Formation 



Only one of the Oligocene formations found on the north side of 

 Mount Diablo can with certainty be correlated with beds to the west 

 of the mountain; this is the Kirker formation, the tuffs of which are 

 believed to be contemporaneous with the tuffs of the Sobrante anti- 

 cline, and to which the name Kirker tuffs has been applied. The 

 lithology and thickness of the beds between the Tejon (Eocene) and 

 the Kirker formation, as found to the north of the mountain, is very 

 different from that of the San Ramon formation, which in the 

 Concord Quadrangle occupies the same stratigraphic position. This, 

 together with the fact that the known fauna in these beds to the north 

 of the mountain is meager, makes it seem best to give them a differ- 

 ent formational name ; though, as will be brought out later, it is very 

 probable that they are contemporaneous, at least in part, with the San 

 Ramon formation. The name Markley formation is proposed for 

 this portion of the section. 



The Markley formation has a thickness of approximately 3300 

 feet; it consists of a heterogeneous assemblage of beds which are 

 mostl3 r of shallow-water origin. The lower two thousand feet of 

 deposits are predominantly sandstone, the remaining portion being 

 composed of alternating layers of clay-shale, sandy shale and sand- 

 stone. The basal bed of the Markley formation rests with apparent 

 conformity upon the Tejon, the uppermost beds of which in this 

 vicinity consist of a clay-shale. This shale, as seen at the old town 

 of Somersville, has a thickness of about five hundred feet. 



The lower two thousand feet of the Markley formation is unfossil- 

 iferous. These beds are apparently conformable with the Eocene 

 deposits below and with the fossiliferous Oligocene above ; therefore 

 the period to which they belong might be an open question. The 

 writer in a former paper 04 included these lower beds in the Tejon 

 (Upper Eocene). Later work, however, has caused him to believe 

 that they are probably Lower Oligocene in age for the reason that 

 thejr apparently grade up into strata containing an Oligocene fauna, 

 and have a lithology which is similar to the sandstones immediately 

 above these fossiliferous beds and different from any of the sandstones 

 of the Tejon of this general region. 



Lithology. — The first distinct lithologic member of the Markley 

 formation, commencing at the base, consists of about seven hundred 



Clark, B. L., The Neocene section at Kirker Pass, Univ. Calif. Publ., Bull. 

 Dept. Geol., vol. 7, no. 4, pp. 49-52, 1912. 



