558 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



He estimates that the volume of Eocene basalt here is at least 40,000 

 cubic miles. 



Petrographically the lavas are typical representatives of the tholeiitic magma 

 type (Kennedy, 1933). They are aphanitic rocks composed of monoclinic 

 pyroxene and labradorite set in a tachylitic base highly charged with magnetite 

 dust. Phenocrysts of augite or plagioclase appear in some flows, but the series 

 as a whole is characteristically nonporphyritic. Olivine is scarce or absent. Glass 

 commonly accounts for 20^10 per cent of the rock. Chlorophaeite is abundant 

 in some flows (Waters, 1955). 



Waters points out that the flows at the bottom of a continuous sequence 

 several thousand feet thick are of the same composition as those at the 

 top or in the middle, and concludes that progressive differentiation had 

 not occurred in the deep-seated magma chamber during the process of 

 eruption. In contrast, the thick sills after emplacement show magmatic 

 differentiation, and commonly consist of granophyric gabbro grading 

 downward into feldspathic gabbro. The lower portions of the sills are 

 rhythmically banded with layers of pyroxene and feldspar. 



The basalts have been described in part as spilites, and the albitization 

 in the Olympics has been pictured by Park ( 1944 ) as due to circulating 

 heated sea water through the pillowed lavas on the sea floor. Waters 

 ( 1955) does not reject this theory but believes it is not the entire explana- 

 tion. He says: 



Some dolerite sills, dikes, and subaerial flows are as thoroughly albitized as 

 the pillowed flows. Albite veins and albite overgrowths on detrital feldspars are 

 locally abundant in the graywackes and argillites that underlie the Olympic 

 flows. Furthermore, most lavas might be better described as ordinary green- 

 stones, zeolitized basalts, propylitized and saussuritized basalts, silicified basalts, 

 and chloritized basalts, instead of spilites. 



The Eocene basalts are underlain by thousands of feet of graywackes, 

 argillites, and tuffaceous sediments. In the writer's opinion the alteration of the 

 lavas to "spilites" and greenstones, and the simultaneous albitization, silicifica- 

 tion, and chloritization of the underlying sediments and intrusive bodies have 

 been produced by water, alkalis, silica, and other easily removable constituents 

 stewed from the slowly metamorphosing root of geosynclinal sediments as it 

 was downbuckled to form a tectogene. Fluids expelled from this metamorphos- 

 ing root rose along zones of mechanical deformation altering the overlying 

 volcanics and sedimentary rocks. This is essentially the same conclusion reached 

 by Gilluly (1935) after an extensive review of the spilite-keratophyre problem. 



California Field. Volcanic materials are observed in several of the 

 Tertiary formations of the Coast Ranges of California but by all odds 

 those of the Miocene are the most abundant, and are particularly well 

 known in the central and southern Coast Ranges. The volcanic rocks 

 are thickest in certain basins or around certain centers of volcanism, and 

 in the central Coast Ranges several thousand feet of rhyolite tuffs, augite 

 andesite, basalt, and olivine basalt flows occur in the San Luis Obispo— 

 Huasna basin. Thick sills of analcite diabase and numerous plugs of 

 andesite and rhyolite porphyries also occur. 



In the southern end of the Santa Lucia Range there are rhyolite tuffs and 

 flows and sills, flows of olivine basalt, often having a well-developed pillow struc- 

 ture, and numerous plugs of rhyolite porphyry. Rhyolite ash, basaltic peperites, 

 flows of basalt and numerous sills of analcite diabase occur in the Santa Cruz 

 Mountains. Thin rhyolite ash, flows and breccias of basalt, and diabase sills 

 are present in the Berkeley Hills, but they are not thick. Basalt flows occur in 

 the Miocene of the Point Arena region. Aside from bentonized ash there are 

 few volcanics in the Miocene in the San Joaquin Valley but there are numerous 

 flows in the Cuyama Valley and the Carrizo Plain. There is abundant evidence 

 that the volcanics were largely submarine; the tuffs and ashy sediments are 

 often fossiliferous and the flows are generally interbedded with sediments con- 

 taining marine fossils. It is possible that in some instances the volcanics accumu- 

 lated so rapidly that local evanescent volcanic islands were built up, especially 

 in the immediate vicinity of vents. 



No single description would fit all of the occurrences of Miocene volcanics 

 as the sequence and relative proportions of the various types vary somewhat. 

 However the usual sequence is rhyolite tuffs and flows, flows of andesite and 

 basalt, intrusions of sills and analcite and thomsonite diabase and intrusions of 

 plugs, sills and dikes of soda rhyolite and waning explosive activity. 



The sills of analcite diabase are an important and widespread phase of the 

 Miocene volcanism. . . . Some of the thicker sills show gravitational differentia- 

 tion and vary from a picrite at the base to a highly feldspathic diabase at the 

 top. Most of them show chilled margins of analcite basalt, usually vesicular 

 (Taliaferro, 1943b). 



In the southern Coast Ranges 2280 feet of Miocene volcanic rock is 

 exposed on San Miguel Island, 4700 feet on Santa Cruz Island, 8000 to ' 

 10,000 feet in the western Santa Monica Mountains and Conejo Hills, and 

 at least 2000 feet in the area northeast of Glendora. Many wells have 

 penetrated the same volcanics in the subsurface. Shelton ( 1954) estimates I 

 an average thickness of 1000 feet over an area of 700 square miles for; 



