474 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



project out to the San Clement basin and escarpment on the basis of 

 the epicenters and submarine topography (Allen et al., 1960). 



COAST RANGES OF OREGON AND WASHINGTON 



Geomorphic and Geologic Provinces of Oregon and Washington 



Figure 29.15 has been prepared to show the geomorphic provinces of 

 Oregon and Washington, and in a broad way the geologic divisions. The 

 Klamath, Rlue, and Northern Cascade Mountains, and the Okanogan 

 Highland have been referred to in Chapters 6 and 17. They are made up 

 chiefly of the Nevadan complex. The trends in the Klamath Mountains 

 veer northeastward as they pass under the Tertiary volcanics and are 

 generally thought to find a continuation in the Blue Mountains. Most of 

 the sedimentary rocks of the Blue Mountains are unmetamorphosed, and 

 this is puzzling because the rocks of the Nevadan complex elsewhere are 

 fairly crystalline. The large Idaho batholith lies east of the Blue Moun- 

 tains and appears to make up a knot at the intersection of the Sierra 

 Nevada-Klamath-Blue arc and the British Columbian Coast Range arc 

 with its great batholiths. The basement geology of Oregon and Washing- 

 ton is thus believed to be the Nevadan complex at the junction region 

 of two great arcs. It evolved as a Paleozoic and early Mesozoic eugeo- 

 syncline. In Late Jurassic and Mid-Cretaceous time folding, meta- 

 morphism, and batholithic intrusions brought its history to a climax. 

 The Tertiary Coast Ranges and the extensive volcanic fields developed 

 thereafter. 



As in California the Coast Ranges are bordered on the east by a 

 general depression, known in Oregon as the Willamette River Valley, 

 and in Washington as Puget Sound. The two are referred to as the Wil- 

 lamette-Puget depression or Willamette-Puget Sound depression. On 

 the east of the depression are the Cascade Mountains, made up of 

 volcanic rocks. They are divided into the Western Cascades and the High 

 Cascades as shown in Fig. 29.15, and are treated fairly extensively in 

 Chapter 36. 



East of the Cascade Mountains and surrounding the islands of pre- 

 Tertiary rocks in the Blue Mountains are vast Tertiary volcanic fields. 



North of the Blue Mountains and including part of them is the Columbia 

 River basalt field, and south of the Blues are several geomorphic province-, 

 all underlain by volcanics, sometimes collectively referred to as the 

 Malheur field. The southern lavas are generally younger than the north- 

 ern. The Columbia and Malheur fields are outlined in Chapter 33. 



Divisions of Coast Ranges 



The Coast Ranges of Oregon and Washington are a coherent unit 

 geologically, because their formations are probably all Tertiary and they 

 have been deformed as a unit. The northern end is composed of the 

 Olympic Mountains, a domal uplift supporting the highest peaks of the 

 Coast Ranges, with Mount Olympus 7954 feet above sea level. The 

 canyons of the Olympic Mountains have been heavily glaciated. 



At the northern end of the Coast Ranges of Oregon, just south of the 

 Columbia River and west of the city of Portland is another uplift in 

 which a core of fairly old rocks (middle Eocene) relative to those of the 

 ranges elsewhere is exposed. 



Stratigraphy 



Selected sections of the Cenozoic rocks of the Coast Ranges of Oregon 

 and Washington are given in Fig. 29.16. They are taken from Weaver's 

 (1945a,b) extensive study with the western Oregon section modified 

 according to Baldwin ( 1959 ) and Wilkinson ( 1959 ) . The idealized cross 

 sections, A- A' and B-B' of Fig. 29.17, attempt to restore the deposits to 

 their condition before the late Miocene folding. 



At the beginning of Tertiary time, according to Weaver ( 1945 ) , a vast 

 erosion surface existed in eastern and western Washington in the manner 

 of a coastal plain. It had been carved chiefly in the rocks of the Nevadan 

 orogenic belt. Early in the Eocene, the plain began to subside, and the 

 earliest deposits filled the broad valleys of the extensive erosion surface 

 The Swauk formation of eastern Washington may be a fresh-water de- 

 posit in the upper part of one of these valleys, and the Solduc formation 

 of the Olympic Mountains may be the marine equivalent. Both of these 

 formations were folded somewhat and eroded before the overlying vol- 



