IGNEOUS PROVINCES IN WESTERN UNITED STATES 



559 



the volcanics of the southern Coast Ranges, and this would mean a volume 

 of approximately 140 cubic miles. 



Breccias and tuff breccias are most common but massive flows and 

 intrusions are prominent in the Conejo Hills and Glendora areas. In the 

 Conejo volcanic assemblage hornblende andesites occur at the base, and 

 above these generally are breccias, tuffs, and flows of augite andesite. 

 The upper part consists of flows of hypersthene basalt and olivine basalt. 

 The basalts probably thicken southward in the subsurface. The intrusives 

 in the area are chiefly diabase and hypersthene diabase (Shelton, 1954, 

 1955). 



The Glendora volcanics are largely andesites, but olivine basalt and 

 rhyolitic varieties are noted. In fourteen analyzed rocks the Si0 2 content 

 ranges from 47.23 to 75.50 percent, and the most common types contain 

 59 to 63 percent. Present knowledge of the province as a whole indicates 

 that andesites predominate among the extrusives, with basalt and dacitic 

 or rhyolitic rocks following in that order. The associated intrusive rocks 

 are dominantly basaltic or diabasic (Shelton, 1954). 

 Most of the volcanic rocks of this province are middle Miocene, but 



i some may be slightly older. Shelton concludes that much of the lava 

 was poured out on the sea floor or from vents close enough so that ac- 

 cumulation took place under water. Source fissures or vents have not 



i been recognized. The relation of volcanism to tectonism is striking in the 



| Los Angeles Basin. According to Shelton (1955): 



The Los Angeles basin is an area of locally derived Cenozoic sediments 

 II at least 25,000 feet thick, and as now exposed is a structural depression approx- 

 imately 60 miles long and 40 miles wide. The most pronounced cycle in its his- 

 jtory began in middle Miocene time and reached a climax of depth and localiza- 

 tion during the upper Miocene and Pliocene. The climax of Miocene volcanism 

 in southern California thus corresponds fairly closely with the beginning of the 

 period of maximum growth of the basin. 



Basalt Fields of Eastern Oregon and Washington 



The Blue Mountains are composed of central island-like masses of 

 Paleozoic and Mesozoic sedimentary rocks and intrusive masses with 

 flanking volcanic flows and tuffs. See Fig. 29.15. The north flank volcanics 

 ire older and consist at the base of the Clarno formation of late Eocene 



(Duchesnean) and early Oligocene (Chadronian) age. It consists of a 

 thick sequence of rhyolite and basalt flows with interlayered breccias and 

 varicolored tufts. Local unconformities are noted. See cross sections of 

 Fig. 36.2. Overlying the Clarno is the John Day formation of late Oligo- 

 cene and early Miocene age. It consists of colorful tuffs which in places 

 may grade into acidic flows and breccias. Overlying the John Day is the 

 Columbia River basalt which is now restricted to flows of mid-Miocene 

 age. They are widespread in northern Oregon and southeastern Washing- 

 ton. 



The section at Picture Gorge along the John Day River [D, Fig. 36.2] may 

 be considered as typical of this formation. Here it is situated between the John 

 Day formation and the Mascall formation. The basalt series appears to be 

 unconformable upon the John Day beds as shown by slight discordant relation- 

 ships over a wide area, but appears to be generally conformable with the 

 overlying Mascall formation. 



The Columbia River basalt poured out upon an area of varied relief. The 

 basalt flows in places tend to be thick where they filled irregularities in the 

 surface. The basalt flows are usually more massive and less columnar than 

 flows high in the formation. Some flows contain appreciable amounts of 

 olivine and weather more rapidly than the dense basalt higher in the section. 

 Zeolites are particularly common in some of the basal flows, particularly in 

 the Monument and Ritter quadrangles. 



The upper part of the Columbia River basalt characterized by "flow upon 

 flow" structure is by far the thicker and more widespread part of the formation. 

 Relatively parallel flows, commonly columnar, are visible for many miles 

 along the canyon walls of northeastern Oregon. The upper flows are character- 

 istically dark dense basalts with scoriaceous zones at the tops of each flow. 

 According to Waters (1955, p. 708) continuous sections of more than 5.000 

 feet of basalt are found in northeastern Oregon (Baldwin, 1959). 



Waters also calculates that about 35,000 cubic miles of basalt are present 

 in the field. 



The Mascall formation is largely made up of nearly white to buff 

 bedded tuffs. It is late Miocene in age. 



Following Mascall deposition the Columbia River basalts were folded 

 and faulted near the Blue Mountains as shown in Fig. 29.15, and then 

 eroded. On the erosion surface in mid-Pliocene time the Rattlesnake 

 formation was spread. It consists of gravels, tuffs, and silts with a bed of 

 welded tuff in the upper part. Uplift and moderate folding took place 



