Geology of Washington - Northern Cascades
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Geology of Washington - Northern Cascades 

The Cascade Range is part of a vast mountain chain that extends from British Columbia to northern California. It separates the coastal Pacific lands from the interior of North America. The Cascades consist of an active volcanic arc superimposed upon bedrock of Paleozoic to Tertiary age. Pliocene to recent uplift has created high topographic relief. As a result, the Cascades form an effective barrier to moisture carried eastward by the prevailing Pacific winds. This has a great effect on the productivity of the land. The Puget Lowland receives 30 to 50 inches of rain annually, while the western slopes of the Cascade mountains get over 80 inches of precipitation a year. In contrast, there are large parts of southeastern Washington that get less than 10 inches of precipitation annually.

A major northwest-southeast structural break separates the Washington Cascades into northern and southern portions. In a general way, the structure follows the trace of Interstate 90 between Seattle and Ellensburg. The North Cascades consist of jagged mountains with numerous glaciers and are composed predominantly of Mesozoic crystalline and metamorphic rocks. The South Cascades contain mainly Tertiary to Holocene volcanic rocks. In the north, the structural fabric is extremely complex because of the unrelated "rock packages", called terranes, that have been brought in contact with each other by strike-slip and thrust faults. The North Cascades are also known for mylonite development, extensive areas of crushed and jumbled exotic rocks called melange, and plates of rock thrust over each other.

The eastern boundary of the North Cascades province is the Pasayten fault zone, while the Puget Lowland lies to the west. Further, the North Cascades are an extension of the Coast Crystalline tectonic belt from British Columbia. A north-south regional strike-slip structure, called the Straight Creek fault, divides the North Cascades into contrasting eastern and western portions.

In the northeastern Cascades, northwest-southeast striking faults of mid-Cretaceous and Eocene ages form contacts between adjacent geologic blocks. Rocks on either side of these faults are unrelated, thus prompting geologists to describe each group of these rocks individually as a terrane. More than 15 terranes have been described. These terranes were brought together by subduction of the northeast Pacific (Wrangellia) against North America during the Mesozoic.

The Methow Valley contains the thickest stratigraphic section of Cretaceous sedimentary rocks in the state. The Cretaceous consists of 29,000 to 60,000 feet of marine black shales, sandstones, conglomerate, and arkose. The top of the section is marked by red-purple sedimentary rocks. Protolith ages of adjacent terranes are Permian to Jurassic. Some of these terranes, such as in the Chelan mountains, contain limestone, basalt, tonalite-diorite, gabbro, and ultramafic pods.

Late Cretaceous plutons subsequently intruded the terranes. Extensive metamorphism and deformation took place during the Late Cretaceous and locally extended into the Eocene. Original rocks were metamorphosed to schists, phyllites, slates, argillites, marble, and gneiss. During the Eocene, the North Cascades were intruded by plutons such as the potassic Golden Horn batholith, which is known to collectors for its rare minerals. During that time, there was extensive deformation, movement along faults, and fluvial sedimentation in pull-apart basins. Coal in the Roslyn area formed in such a basin.

The northeastern Cascades are known for mineral deposits of various geologic ages. Triassic calc-silicate gneiss, biotite schist, and marble were hosts to the state's largest copper producer, the Holden mine in Chelan County. It produced 207 million pounds of copper along with zinc, silver, and gold from 1938 to 1957. There is a porphyry copper deposit in a Cretaceous stock near Mazama. The largest copper deposit in the state is at Miners Ridge in Snohomish County. There, drilling has outlined a porphyry copper resource of 1.9 billion tons averaging 0.334% Cu and 0.015% MoS2. The host is the Miocene Cloudy Pass pluton dated at 21 Ma. This deposit will not be developed because it is within the Glacier Peak Wilderness Area. At Wenatchee, the Cannon mine is currently the second largest underground gold mine in the country. The stockwork mineralization is of the epithermal type and is associated with Eocene rhyodacite intrusions.

West of the Straight Creek fault, the northwestern Cascades are characterized by thrust plates stacked one on another. This thrust regime extends to the west into the San Juan Islands of the Puget Lowland. Marine sedimentary rocks and basalts of Permian to Jurassic age were involved in the tectonic deformation. The spectacular Twin Sisters mountains are made up of dunite. This ultramafic mass, measuring 4 miles wide and 11 miles long, of probable Jurassic age, contains the largest olivine reserves in the United States. During the Late Cretaceous orogeny, all the rocks were metamorphosed. Some, such as the Easton schist, were metamorphosed to high-pressure, low-temperature blueschist facies. Cretaceous subduction tectonism produced extensive regional zones of melange and mylonite accompanied by prehnite and pumpellyite metamorphism. In the northwestern Cascades, active olivine mining is taking place at Twin Sisters, and Permian limestone is mined in Whatcom County.

During the Eocene, active deformation caused the formation of pull-apart basins. West of Mount Baker, the Chuckanut Formation was deposited in one of these rapidly subsiding basins. The Chuckanut is over 16,000 feet thick, one of the thickest nonmarine sequences in North America. The Chuckanut is known for large fossil palm fronds.

Throughout the Cascades, Oligocene and Miocene plutons cut and altered older rock types. These intrusives are the plutonic counterparts of volcanic rocks in the southern Cascades. They are high-level intrusives, and some are very large, such as the Oligocene Snoqualmie batholith, which extends over ten townships. Some contain explosion breccias. Near Mount Baker, Eocene coals were altered to anthracite by a pluton. There are also scattered remnants of Neogene tuffs, volcanic breccias, and associated sedimentary rocks in the northwestern Cascades.

Former mining camps are scattered throughout the northwestern Cascades. Copper, silver, and gold were the main metals recovered. Breccia pipes associated with Oligocene to Miocene shallow plutons contain not only copper but also molybdenum and tungsten. These pipes are favored by collectors of quartz and pyrite crystals. The Spruce claim in the Snoqualmie batholith in King County is a world-class mineral locality. This batholith also contains a large porphyry copper deposit.

The magmatic arc in the North Cascades is dominated by two Quaternary stratovolcanoes. Mount Baker is the most impressive, with a summit rising to 10,781 feet above sea level; after Mount St. Helens, it is the second most active volcano in the state. Steam and gas emissions were common during the 1970s. The second stratovolcano, Glacier Peak, has a 10,451-foot summit. These volcanic cones are probably less than 1 million years old. Tephras from Glacier Peak are used to correlate Quaternary sediments in the Columbia Basin. Eruptions on both volcanoes were accompanied by pyroclastic flows and lahars. Pumice deposits are known in small pockets near these mountains.

The current rugged topography is a result of Holocene glaciation, which has greatly modified the North Cascades. This region contains the greatest concentration of alpine glaciers in the United States outside of Alaska.

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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