Geology of Washington - Olympic Mountains
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Geology of Washington - Olympic Mountains 

The Olympic Mountains, an extension of the Coast Range from Oregon, form the core of the Olympic Peninsula. The area is known for spectacular mountains, lush rain forests, and pristine coastlines. The peninsula is bordered by the Pacific Ocean to the west, the Strait of Juan de Fuca to the north, and Hood Canal to the east. The southern flanks of the Olympic Mountains adjoin the lowlands of Grays Harbor basin. The Olympic Mountains catch moisture-laden Pacific storms, causing an average of 140 inches of precipitation per year. Some locations have recorded nearly 200 inches of precipitation in one year. The area's highest point is Mount Olympus at 7,965 feet above sea level.

The Coast Range basement consists of Eocene Crescent Formation basalts, which were erupted close to North America in a marine setting. The formation consists mostly of thick submarine basalt flows such as pillow lavas. Locally, subaerial flows are preserved where islands formed during the Eocene. The Crescent Formation was deposited upon continentally derived marine sediments and is locally interbedded with early Eocene limy red sediments that now are limestones. During the middle Eocene, about 50 million years ago, the Crescent Formation was deformed due to accretion to North America. A thick sedimentary accretionary prism formed offshore at about this time. Sediments from the prism were thrust or underplated beneath the Crescent Formation. Uplift of the Olympics during middle to late Miocene time was possibly caused by arching of the subducting oceanic slab. The Crescent Formation was arched into a major antiform. Metamorphosed sedimentary rocks are now exposed in the core of the Olympics.

On the flanks of the Olympic Peninsula, marine nearshore clastic sedimentation continued throughout the Oligocene and early Miocene. By the middle Miocene, convergence of the Juan de Fuca plate with the North American plate accelerated to the point that sedimentary, volcanic, and metamorphic rocks along the west flank of the Olympics were broken, jumbled, and chaotically mixed to form a melange. This formation is known as the Hoh rock assemblage; its sedimentary blocks contain foraminiferal faunas from middle Eocene to Oligocene in age. Hoh melange rocks are exposed along 45 miles of the western coast. A pronounced angular unconformity marks the top of the Hoh, on which the marine Quinault Formation was deposited during the Pliocene

The north and east flanks of the Olympics were subject to erosion by continental ice sheets during the Pleistocene. Alpine glaciers also sculpted the Olympic Mountains. Thick deposits of sand and gravel fill valley bottoms and cover the coastal plains. Raised wave-cut platforms along the west coast indicate periods of uplift. Many of the cliffs along the west coast are capped by loess. In some areas, the coast was downwarped. Hoh melange rocks in a matrix of clay formed diapirs or piercement structures that were active through the Pliocene and Pleistocene. Offshore, they deform Holocene sediments.

Manganese deposits are associated with red limestone that is interbedded with Crescent Formation basalt. Copper is also known from the Crescent Formation, and pillow basalts yield beautiful zeolite specimens. Sedimentary rocks of the Hoh melange on the flanks of the Olympic Peninsula contain both oil and gas, as evidenced by shows in exploratory wells and seeps along the coastline.

The above text is modified from the following article: Lasmanis, Raymond, 1991, The geology of Washington: Rocks and Minerals, v. 66, no. 4, p. 262-277. © Copyright  Heldref Publications (Helen Dwight Reid Educational Foundation). Used with permission.

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Geologic Hazards Group
Geology & Earth Resources Division
Washington State Department of Natural Resources
Fax 360-902-1785

Dave Norman
State Geologist and Oil & Gas Supervisor


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