UNIVERSITY OF CALIFORNIA PUBLICATIONS 



BULLETIN OF THE DEPARTMENT OF 



GEOLOGY 



Vol. 9, No. 2, pp. 9-21 Issued June 30, 1915 



THE OCCURRENCE OF OLIGOCENE IN THE 

 CONTRA COSTA HILLS OF MIDDLE 



CALIFORNIA /<^^ 



BY 



BRUCE L. CLARK 



CONTENTS PAGE 



Introduction 9 



Agasoma gravidum Beds in the Vicinity of the Town of Walnut Creek 12 



Relation to Tejon (Eocene) 12 



Relation to Area montereyana Zone (Miocene) 13 



Faunal Relationship of Agasoma gravidum Zone to Area montereyana Zone 14 

 List of Species from Agasoma gravidum Zone of Contra Costa County, 



California 14 



Relation of Agasoma gravidum Zone to the Lower Miocene of Southern 



California 15 



Evidence Suggesting Correlation of the Fauna of the Agasoma gravidum 

 Zone of Contra Costa County, California, with that of the Oligoeene of 



Oregon and Washington 17 



Summary 21 



INTRODUCTION 



Heretofore only two marine stratigraphic groups, the Monterey 

 and the San Pablo, have been recognized in Contra Costa County above 

 the Tejon (Upper Eocene). In the vicinity of San Pablo Bay the 

 Monterey Group of Professor A. C. Lawson has a thickness of over 

 five thousand feet, consisting of nine formations, five of sandstone and 

 four of shale. In the past, three faunal zones have been recognized 

 in this group ; the lowest was described by Dr. J. C. Merriam as the 

 Agasoma gravidum zone, 13 the middle will be referred to here as the 

 Area montereyana zone. 1 The upper zone is known as the Scutella 

 breweriana zone. 



la Merriam, J. O, Univ. Calif. Publ. Bull. Dept. Geol., vol. 3, no. 16, p. 378, 

 1904. 



i Area montereyana Osmont is possibly the same as Area devincta Conrad. 

 If this determination should prove correct, the horizon should be known as the 

 Area devincta zone. 



