1016] 



Nomland: Fauna from the Lower Pliocene 



201 



In a recent paper written essentially to show the economic possi- 

 bilities of this region by R. W. Pack and W. English 5 the Jacalitos 

 and Etchegoin are grouped together as upper Miocene. In this paper, 

 however, no faunal study is given of the relation of the Jacalitos to 

 the other formations. 



Description op the Section on Jacalitos Creek 

 The present paper is a study of the type section of Arnold and 

 Anderson on Jacalitos Creek, and of the beds on Jasper Creek and 

 Waltham Creek which are traceable directly into the type section. 

 The description of the lithology and fauna may therefore be taken 

 as representative of the Jacalitos at the type locality. 



The area studied has in late geologic time been subjected to numer- 

 ous structural disturbances. Tangential stresses have caused the beds 

 to be folded into open anticlines and synclines. The axes of these 

 folds are generally parallel to the major structural features of the 

 district. A fault zone having a northwest-southeast trend cutting 

 across the Diablo Range has greatly complicated the structure. Tbis 

 fault zone has the same general direction as the San Andreas fault 

 a few miles to the west. Cross-faults are also numerous. These are, 

 however, usually not of as great magnitude as those in a northwest- 

 southeast direction. Due to faulting, some of the Tertiary formations 

 are not exposed for considerable intervals. Both the determination 

 of drainage lines and the elimination of formations by faulting are 

 well shown along the upper part of Jacalitos Creek. For several 

 miles this creek follows a fault of comparatively recent age, displacing 

 the Santa Margarita ( 1) and the lowest portion of the Jacalitos beds. 

 On following this in some places the lowest Jacalitos is found to be 

 absent, while in others the Santa Margarita ( ?) shale has been entirely 

 faulted out. 



According to Arnold and Anderson, the thickness of the Jacalitos 

 beds exposed at Jacalitos Creek is 3800 feet. The thickness of the 

 section measured by them is probably too great, as beds apparently 

 of Santa Margarita ( ?) age are included in it. The separation of these 

 beds would, however, diminish the total thickness but slightly, the 

 Santa Margarita ( ?) having an exposure of only a few hundred feet. 



On Jacalitos Creek the Pliocene strata comprise essentially an 

 alternation of soft coarse sandstone and conglomerate. In many of 



■> U. S. Geol. Surv., Bull. 581-D, 1914. 



