MESOZOIC SYSTEMS ALONG THE PACIFIC 



267 



grades upward into the Mariposa (Taliaferro, 1942), but between the 

 Merced and Mariposa rivers, conglomerates are at the base of the 

 Mariposa. The pebbles are presumably from the underlying Amador. See 

 column 4, Fig. 17.2. 



The Mariposa formation consists of black slate and graywacke, with 

 which greenstone is closely associated (Knopf, 1929). Conglomerate 

 occurs locally, and sericite schist and limestone in a very few places. The 

 greenstone, because of its intimate interbedding with the normal sedi- 

 mentary rocks, is in many places an inseparable part of the formation, and 

 locally predominates in volume. The conglomerate contains a variety of 

 rocks, namely: quartz keratophyre (submarine lava flow origin), quartz- 

 ite, chert, quartz, aplite, and biotite granophyre. The last two point to 

 plutonic intrusions older than those of the Sierra Nevada ( Knopf, 1929 ) . 

 The graywacke contains grains of quartz, plagioclase, slate, quartzite, and 

 keratophyre (?). On the one hand they grade into slate and graywacke 

 slate, and on the other, by the presence of augite, into augite tuff. The 

 greenstones were principally augite basalt breccias, tuffs, and lavas, 

 now somewhat metamorphosed (Knopf, 1929). It appears that some of 

 jthe volcanics included by Knopf in the Mariposa are what Taliaferro 

 places in the Amador. 



j The great thickness of volcanics is a striking feature of practically all 

 Jurassic units in California and southwestern Oregon. The volcanic rocks 

 range from rhyolite to basalt, but augite andesites predominate. Practi- 

 pally all, if not all, are submarine, as they are interbedded with marine 

 Isediments (Taliaferro, 1942). Pyroclastics predominate over flows. Cen- 

 ters of volcanism have been recognized in the form of necks, both breccia 

 s and solid, and great accumulations of flows, tuffs, and very coarse brec- 



cias. 



I Intrusions of peridotite and dunite, now largely serpentinized, together 



with their closely associated gabbroic and diabasic differentiates, are com- 

 mon in the Jurassic of California and southwestern Oregon. They occur as 



r >ills, dikes, plugs, and large masses of undetermined form. The great ma- 

 jority were intruded prior to folding of the Jurassic sediments and before 

 the Sierra Nevadan batholith was emplaced. The basic intrusions of the 

 Mother Lode were serpentinized immediately after their emplacement 



(Knopf, 1929). They were slightly metamorphosed by the folding, and 

 greatly altered at the contacts of the granodiorite plutons. 



OREGON 



In central Oregon, a fairly complete Jurassic section has been described 

 by Lupher ( 1941 ) . He sets apart ten formations which range in age from 

 Early to Late Jurassic, perhaps to Early Cretaceous, and altogedier are 

 over 11,000 feet thick. These beds show only a succession of gentle emer- 

 gent and submergent movements. The lithology is in conspicuous contrast 

 to that of the Jurassic of the Sierra Nevada in lacking volcanics and having 

 only minor amounts of coarse elastics. It is nearly all sandstone and shale, 

 and in part it is very fossiliferous. 



The Oregon Jurassic rests with marked angular discordance on a base- 

 ment of highly folded Upper Triassic and Mississippian rocks. Some of the 

 beds called Upper Triassic may be Lower Jurassic, because a sequence 

 of shales, sandstones, and conglomerates, many thousands of feet thick, 

 overlies the fossiliferous Upper Triassic but underlies the great unconform- 

 ity. The folds in the Jurassic beds trend at divergent angles from those 

 of the Upper Triassic, and basic plutons now largely altered to serpentine 

 invade the Upper Triassic but not the Jurassic. It is, therefore, apparent 

 that an orogeny of considerable proportions is indicated. It will be re- 

 called that a similar unconformity separates two formations of Early Juras- 

 sic age in western Nevada, and it is evident, therefore, that the two may 

 be the same, perhaps with slightly different ages. It seems necessary, in 

 order to account for the different lithologies of the Jurassic beds of central 

 Oregon and those of the Sierra Nevada, to separate the central Oregon 

 beds from the volcanic belt by a nonvolcanic highland or Piedmont. See 

 the tectonic maps, Plates 11 and 12. The pebbles are cherts and lime- 

 stones, evidently from Paleozoic formations (Lupher, 1941). 



SOUTHERN CALIFORNIA 



Larsen (1948) has summarized the geology of the region southeast of 

 Los Angeles in southern California, especially in relation to the great 



