COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM 



475 



canics were poured out. These outpourings have been called the Tean- 

 naway volcanics in eastern Washington, the Metchosin volcanics in 

 western Washington, and the Tillamook and Siletz volcanics in western 

 Oregon. 



The basal Eocene volcanics are a voluminous deposit. They originally 

 formed a vast lava field that extended from Vancouver Island 500 miles 

 southward to the Klamath Mountains and from a line considerably west 

 of the present coast 150 miles inland. Their minimum average thickness 

 was 3000 feet. According to Weaver, the volume of these volcanics was 

 greater than the Columbia plateau basalts. They consist mainly of an- 

 desitic and basaltic flows with tuffs, agglomerates, and numerous in- 

 trusive plugs and dikes. The latter crosscutting intrusions, Weaver 

 believes, were the vents of much of the volcanic material. 



Ry the close of the Metchosin volcanism, a narrow north-south trough 

 formed with its axis in the approximate position of the present Willamette- 

 Puget Sound depression, and its sediments extended westward into the 

 site of the modern Coast Ranges. After the volcanic eruptions 8000 to 

 14,000 feet of sediments were deposited. They make up the Puget group 

 of the Seattle region, the Cowlitz formation southward in Washington, 

 and the Tyee sandstone and Coaledo formation in Oregon. 



The basal volcanics remained emergent in a narrow peninsula that 

 projected southward from Vancouver, with the trough to the east. In 

 early Oligocene time the peninsula submerged in part, and sediments 

 were deposited directly on the Metchosin volcanics there; farther east 

 they rest on the late Eocene strata of the trough. Ry late Oligocene, the 

 peninsula area had sagged so much that 8000 feet of sandstone and shale 

 had accumulated. Again in middle Miocene time, over 4000 feet of sand- 

 stone and shale, the Astoria formation, were deposited in the Coast Range 

 area. 



During Miocene times, great quantities of lavas were coming to the 

 surface through numerous vents and fissures, especially in the areas of 

 the Columbia plateau and the present Cascade Mountains. These flows 

 fingered out westward, but north of Portland they are particularly abun- 

 dant and form about 50 percent of the Astoria formation (Weaver, 1945). 

 See cross sections A-A' and R-R', Fig. 29.17. 



o 

 o 



UJ 

 2 

 UJ 



o 

 o 



_l 

 a 



WESTERN 

 OREGON 



MARINE TERRACE 

 OEPOSITS 



DEVELOPMENT OF 

 MT. HOOD, ETC. 



OIASTROPHISU 



EMPIRE SS 



OIASTROPHISM 



Minimum 



ASTORIA FM. 



uu 



NYE SHALE 



TUNNEL POINT 

 SS. 



BASTENDORFF 

 SH. 



COALEDO FM. 



TYEE SS 



WESTERN 

 WASHINGTON 



MARINE TERRACE 

 OEPOSITS 



GLACIAL OEPOSITS 



DEVELOPMENT OF 

 MT. RAINIER, ETC 



MONTESANO 

 SS. 



Illlllllllllll 



OIASTROPHISM 

 Illlllllllllll 



ASTORIA FM. 



UPPER 

 TWIN RIVER 



LOWER 

 TWIN RIVER 



lllllllll ll l l ll 



LINCOLN FM. 



KEASEY SH. 



COWLITZ FM. 



CRESCENT FM. 



TILLAMOOK 8 



SILETZ RIVER, MECH0S|N 



VOL. SERIES VOLCANICS 



EASTERN 

 WASHINGTON 



OLACIAL ANO 

 ALLUVIAL DEP 



ELLENSBURG 



COLUMBIA RIVER 

 VOLCANICS 



YAKIMA BASALT 



UPPER 

 KEECHELUS 



LOWER 

 KEECHELUS 



ROSLYN FM. 



TEANNAWAY BAS 



I l l I I I I I I I II I I I I 

 SWAUK 



EASTERN 

 OREGON 



VOLCANICS AND 



ALLUVIUM 



VOLCANICS 



RATTLESNAKE 



MASCALL 



COLUMBIA 

 RIVER LAVAS 



UPPER 

 JOHN DAY FM. 



LOWER 

 JOHN DAY FM. 



CLARNO 



Fig. 29.16. Representative stratigraphic sections of the Tertiary in Washington and Oregon. 

 After Weaver, 1944. 



