COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM 



177 



Fig. 29.18. Section R-R' west of Tacoma in King County, Washington. See index map, Fig. 

 29.1. After Warren ef a/., 1945a. 



Late Miocene Phase 



After the deposition of the Astoria formation, the trough sediments of 

 Washington and Oregon were subjected to compression. As far as known, 

 mostly open folds resulted. Perhaps in places they were compressed so 

 as to have steep flanks or to be overturned. Examples are given in cross 

 sections R-R', and S-S', Fig. 29.18. Faults are not common, and where 

 present have been illustrated as the normal type. The fold axes that are 

 known to have originated in this late Miocene phase have been assembled 

 on the index map of Fig. 29.15. Through Washington, according to 

 Weaver, they pass in a west-northwest direction. The axes that Weaver 

 shows are those of very broad folds defined by the Vancouver Island-San 

 Juan Islands-northern Cascade upwarp and the Olympic-Newcastle 

 Hills-Cascade upwarp, with the intervening downwarp of the Strait of 

 Juan de Fuca. Also, the Columbia River lavas in the western part of the 

 basalt basin have been deformed into several northwest-trending broad 

 anticlines and synclines. A map by Warren et al. (1945b) just west of 

 Puget Sound (locality of section R-R', Fig. 29.18) shows the folds to be 

 small and rather tight, and they curve sharply from a west-northwest 

 direction to a southerly and southwesterly one. The area covered by the 

 new map is so small, however, in relation to that of the state and the 

 broader picture, that the significance of the local variations is not known. 



Section S— S' across the Coast Range From Cape Meares to Williamette River, Oregon. After 

 Warren et al., 1945b. The Tillamook volcanic series is probably equivalent to the Metchosin 

 volcanic series. 



The Miocene folds of the state seem to be of low to medium intensitv and 

 to trend generally to the northwest. 



In the Portland area of the Coast Range, the fold axes are gentle and 

 also extend in a northwest direction. They show a tendency to bend 

 southward and generally parallel the coast. Farther south in Oregon, 

 they parallel the coast, and some even trend to the southwest in the 

 northern Klamath Mountains. 



The Olympic Mountains uplift is ringed by a horseshoe-shaped ex- 

 posure of the Metchosin volcanics with the Solduc formation underneath 

 and presumably forming the core. The latter is more metamorphosed than 

 the Metchosin and consists of phyllites and argillites. It seems to have great 

 thickness. However, Oligocene fossils have been found in the area of 

 Solduc (?) rocks, and thus the simple dome structure is doubted. Park 

 (1950) concludes that the uplift contains steeply dipping thrust faults, 

 and considerable buckling, thus reducing the previous estimates of a very 

 great thickness for the Solduc. 



Late Pliocene and Early Pleistocene Phases 



Deformation at the close of the Tertiary and in the Pleistocene through- 

 out the Oregon and Washington region has been of the broad arching, 



