IGNEOUS PROVINCES IN WESTERN UNITED STATES 



000 



plutons in western Nevada are included. The pre-Franciscan meta- 

 morphosed sedimentary and igneous rocks exposed in the Coast Ranges 

 of California seem to belong to a metamorphic belt such as was intruded 

 by the batholiths in Chile, and may be west of the true batholithic belt. 



In Oregon, Washington, Idaho, and southern British Columbia the belt 

 is immensely wide — more so than at any other place. It has been pointed 

 out in Chapter 17 that this region marks the intersection of two great 

 arcuate segments of the Cordillera of western North America. The 

 maximum width measured from the Cascade Range to the east side of the 

 Idaho batholith is over 400 miles (650 kilometers). Farther north in 

 southeastern Alaska and adjacent British Columbia it is about 300 miles 

 wide, depending upon interpretations. By way of comparison, the 

 Andean batholithic belt ranges from 40 to 70 miles in width. 



In composition the great bulk of the Sierra Nevada batholith ranges 

 from granodiorite to granite, with granodiorite indicated by some as the 

 most voluminous, but quartz monzonite by others. See Chapter 17. 

 Tonalite is said to be the dominant batholithic rock of southern Cali- 

 fornia. 



POST-BATHOLITHIC PROVINCES OF THE BATHOLITHIC BELT 



Cascade Volcanic Complex 



Divisions. The Cascade Range is a post-batholithic volcanic complex 

 in Oregon and southern Washington (see map, Fig. 36.1), but in northern 

 [Washington and its continuation as the Coast Range of British Columbia 

 ■lit consists of the Nevadan complex. The central and southern volcanic 

 ,part may be classed as an andesite orogenic belt province, and is divisible 

 iinto the Western and the High Cascades. 



Extrusive Rocks. According to Williams ( 1957) the Western Cascades: 



<L . . consists of gently folded volcanic rocks ranging in age from late Eocene 

 *!to late Miocene. Most of the topography here is mature and there are no 

 traces of original volcanic forms. The High Cascades, on the other hand, 

 jconsist of younger volcanic rocks that are virtually undeformed; most of the 

 topography there is constructional and the original forms of the volcanoes, even 

 though modified by glaciation, are easy to visualize. Other important contrasts 

 distinguish the two belts. The thick volcanic accumulations of the Western 

 Cascades are mainly products of fissure eruptions that produced extensive 



plateaus. Hence there are few eroded plugs marking the conduits of large 

 volcanoes; instead, eruptive fissures are marked by narrow dikes of irregular 

 trend. The High Cascades, on the contrary, were built almost wholly by 

 eruptions from central craters so that clusters of large, coalescing cones were 

 formed, many of which have been dissected by glaciers so as to reveal their 

 feeding pipes. Finally, whereas the High Cascade volcanoes grew almost 

 entirely by effusions of basalt and basaltic andesite, the rocks of the Western 

 Cascades were produced by much more varied eruptions. Moreover these older 

 rocks range in composition from rhvolite to basalt, and the lavas are inter- 

 calated with heterogeneous sheets of explosion debris, ranging from coarse 

 agglomerates to fine tuffs, as well as with layers of tuffaceous sediment. 



The Western Cascade belt averages approximately 50 miles in width, and 

 the volcanic rocks are as much as 13,000 feet thick. Beneath the High Cascades, 

 these rocks must interfinger with equivalents of the Clarno, John Day, Colum- 

 bia River, and Mascall formations, which are exposed on the plateau to the 

 east. 



The High Cascade volcanoes probably began to erupt about the beginning 

 of the Pliocene epoch, and almost all of them were broad shield volcanoes built 

 by quiet outpourings of gray olivine basalt and subordinate flows of oliviue- 

 bearing basaltic andesite. Explosive activity contributed little to their growth 

 until the final stages when the summit craters of many shields were capped by 

 steeper cones of fragmental ejecta. Glacial erosion has modified the shapes of 

 all these volcanoes; indeed, most of them have been reduced to radiating ridges 

 separated bv glacial cirques. The parasitic cones on their flanks have been all 

 but demolished. The fragmental cones on their summits have been denuded 

 until the more resistant fillings of their central pipes have been left standing 

 as gigantic monoliths, like miniature Matterhorns. 



The earliest High Cascade lavas were erupted from a north-south chain of 

 volcanoes close to the present edge of the Western Cascades. It seems more- 

 over, that these volcanoes lay on or near the base of an eastward-facing erosion 

 scarp cut in the rocks of the Western Cascade sequence. In places, this buried 

 scarp was between 1,500 and 3,000 feet high, and where it was steepest and 

 straightest it was almost certainlv the result of faulting. As the volcanoes gained 

 in height and the crest of the scarp was lowered by erosion, more and more 

 of the High Cascade lavas were able to flow westward, inundating the scarp 

 and spreading beyond on to a surface of low to moderate relief cut in the older 

 volcanic rocks. 



The bulk of the High Cascades, as noted already, consists of Pliocene and 

 Pleistocene olivine basalts and olivine-bearing basaltic andesites erupted from 

 flattish shield volcanoes, and in places discharge of similar lavas continued until 

 very recent times. But during the Pleistocene epoch several large, steep-sided, 

 composite cones of andesite and dacite were built either on the tops of the older 

 shields or in the depressions between them. The South Sister, for example is 

 made up of three parts. Its lower part is an eroded basaltic shield volcano 

 capped by a steeper cone composed of andesitic and dacite lavas, whereas its 



