WASHINGTON STATE DEPARTMENT OF NATURAL RESOURCES WASHINGTON GEOLOGICAL SURVEY Casey R. Hanell—State Geologist

GEOLOGIC MAP OF WASHINGTON GEOLOGIC UNITS 123° 122° 121° 120° 119° 118° CANADA 117° BRITISH COLUMBIA ITs ITv ‡s 49° Unconsolidated Deposits ‡s ƒv ‡s Qg ‡s ITv Ti Ti ƒi ƒs ƒs ‡s ‡s Qg ƒs pTm ITv Ti Quaternary sediments, dominantly ƒs ƒi ƒs Ti ƒi ƒv p�s Qs pTm ITv ƒs ‡s ITs Qv ƒi Ti ‡s ITv nonglacial; includes alluvium and Bellingham ITs Ti ITv ƒs ƒi Qg ITs Ti Mt.Baker p�s ‚s Ti ƒv ‡s pTm Ti volcaniclastic, glacial outburst flood, ‡i ‡s Ti ƒi ‡s ITv ‡i ITs ITs ITs ‡s pTm ƒv ƒi ITv ITs Qg eolian, landslide, and coastal deposits ƒs ƒs ‡s u ITv ITv ƒs ITs ‚s ƒv ƒs Qg pKm ƒi Ti pTm ‡s ƒi ƒs ƒv pTm � ITs Qg Quaternary sediments, dominantly u ƒs ‡s ITv p s S ƒs ‡i Qg 395 ITs glacial drift; includes alluvium tra Qg pKm Ti ƒi ƒi it ƒ ƒs ITv p�s ƒi 48°30 ' of ‡s v pKm � ITs Ju ƒv ƒv ƒs Ti ƒi p m an ƒs ƒv ITs Ti ƒi Sedimentary Rocks De ƒv ƒs ƒv p�s ‡s Fuc Qg ITv ITs pKm ‡s Qg � ƒi uTs a ƒs pKm ‡m ƒi ƒi p s uTs Upper Tertiary (Pliocene–Miocene) ITs Qg Ti Ti ITv ƒi ƒv � ƒi ƒi ITs ITs ‡s p s uTv Qg ITv Ti pTm ƒi pKm c ƒi p�s ITs ITv Port Angeles ƒs Glacier pKm ‡s p�m Lower Tertiary (Oligocene–Paleocene) 5 Peak Ti pKm ƒi ‡s Ti Qv pKm Qg uTvc ‡s ƒi Ti 101 ITs ITv ITv p�s ƒs 48° ‚s ƒi Mesozoic 101 ITv ITs Ti ƒi Ti Ti ITs Qg Everett ƒi ƒi Ti uTvc 2 p�m Forks ITv ITv Ti ƒi Ti ‡s ‡s Qs 48° Qg ITs ITs ITs ƒi Qg Ti Ti ‚s Mesozoic–Paleozoic ITs ƒs ‡s ƒi ITv Qg pKm Ti Ti uTvc pKm pTm ƒi ƒi uTs Mt. Olympus ‚s 97 Qs Qg Ti ITv 395 uTv ‡s Paleozoic Qg Ti ƒi ITs ‡s c pTm

Puget Qg ƒi Qg uTv pKm pTm ITs 2 ‡m Qs p�s uTs ITv ƒi uTvc p�s Precambrian 405 ITv N A E C O C I F I C A P ITv ƒi uTvc Qs Qs SPOKANE ƒi pKm p�m uTvc 2 Qs p�m ITs Bremerton SEATTLE ITs � Qs p�s Sound p m ƒs u Qs p�s uTvc Volcanic Rocks ITs ITv Qs ƒi ITs ITv u ƒi Qs Qs ITv ITs pTm p�s Qv uTs Mt.Stuart Ti uTvc Qs ƒi 47°30 ' Quaternary ITv Ti ƒs u Qs Qs 90 IDAHO ITv ITs ITs Wenatchee Qs Qs Qg 90 Qs Qs Qs QPv Quaternary–Pliocene Qg pKm Qs uTvc uTv c Qs Qg ITs ITv ITv ITs Qs uTs ITs Qs uTv Qg Qg Upper Tertiary (Pliocene–Miocene) ITs Ti ITs 97 ITs TACOMA uTv ƒv p�s pKm Qg uTv Qg uTvc uTvc c uTvc Columbia River Basalt Group Qs ITs 5 Qs Qs Qs Qs Qs uTv uTs uTvc Ritzville p�s uTs uTs ƒs uTvc Qs 47° Aberdeen uTs ITv ITv Olympia Qg ITs ITv Qg Ellensburg Moses Lake Qs uTvc Lower Tertiary (Oligocene–Paleocene) Hoquiam ITv 47° Ti uTv pKm Qs 195 101 uTv ƒv Mesozoic 12 ITs ITs 90 uTs Mt. Rainier ITv Qs uTv Qs ITs ITv uTs ITv 395 c Qs ITs Ti uTvc uTvc Qs ITv ITs Qv uTs Qs p�s p�m Ti Qv uTs Intrusive Igneous Rocks uTs ITs Qs Centralia Qg Ti ITv Pullman uTv uTs ITv Chehalis Qv Qs Ti Qs uTvc ITs uTv uTs Ti uTs 82 uTs Tertiary ITv ITv Qs uTvc Qs ƒi ITs Qv Qv Qs ITv Ti Ti Qs uTvc ƒi ITs 12 QPv Qs Yakima Qs Mesozoic ITs Qg ƒs Qs ITs ITv ITs Qs Qs Ti Qg QPv uTs Qs 46°30 ' ‡i uTvc ITv uTs Paleozoic Qs uTs Qs Qs uTs ITv ITv 12 Qs Qs ITs uTs Qs uTvc uTvc ITs ITv Ti uTv ITv QPv uTvc Qs Mt. Richland Metamorphic Rocks ITs ITv Qs uTs ITs Qs Pasco uTvc uTv Adams Qs bia c Mt. St. Helens QPv uTvc pTm Pre-Tertiary olum Kelso Qs Kennewick C Qs R ITv Qv Qs uTvc Qs Qs uTs Qv uTvc pKm Pre-Cretaceous Qs 97 82 46° uTs QPv Qs ƒv 5 ITv Qs Walla Walla 124° 46° ‡m Paleozoic 123° Qv 117° ITv uTvc 119° 118° uTs Ti Qs River p�m Precambrian Qs QPv Qv uTvc uTs Qs Goldendale EXPLANATION Qs Ti Qv u Ultramafic rocks uTv Qv uTs 120° Qv bia Detachment fault—blocks Vancouver Qs Colum Contact Note: Some pre-Tertiary sedimentary and uTs uTs OREGON on upper plate 121° volcanic rock units include low-grade 122° Strike-slip fault—arrows show QPv Fault metamorphic rocks. Ages assigned to 0 40 miles relative movement metamorphic rocks are protolith ages. Anticline—dotted where http://www.dnr.wa.gov/geology 0 50 kilometers Thrust fault—sawteeth http://www.dnr.wa.gov/geologyportal on upper plate concealed THE GEOLOGY OF WASHINGTON STATE

Washington consists of a diverse collection of rocks that tells an amazing geologic history. The deepest rock in Washington, called “basement”, consists mostly of terranes accreted to North America over the last 200 million years. These basement terranes are overlain by a variety of sedimentary and volcanic rocks that add detail to the history.

At the eastern edge of Washington State are exposures of Paleozoic North America. These rocks are overlain by metamorphosed sedimentary rocks dated around 1.46 billion years ago (unit pŒm). These are the oldest rocks that have surface exposures in the state. Overlying the oldest rocks are metamorphosed sedimentary and volcanic rocks, dated around 700 million years (unit pŒs).

For much of the Paleozoic (540 to 250 million years ago), the western coast of North America was tectonically inactive and bordered an ancient ocean. Some Paleozoic rocks in northeast Washington, including quartzite and conglomerate (unit ‡s), indicate river, coastal, and ocean environments.

By 250 million years ago, the first of several subduction zones formed along the western edge of Washington.

The onset of subduction brought the first of what would be many arrivals of exotic terranes. The first accreted terrane included a collection of already accreted volcanic islands, collectively known as the Intermontane Superterrane. The collision of this terrane around 170 million years ago caused metamorphism and magmatism throughout the region.

Following the arrival of the first superterrane, the western edge of Insular Intermontane Washington hosted the Superterrane Superterrane prehistoric ocean, the STRAIGHT CREEK FAULT Methow ocean. Ice sheet limit Old Marine sand and mud North built upon the ocean America floor (unit ƒs), later to Siletz-Crescent be thrust eastward with terrane the arrival of another accreted terrane.

The complex patchwork that is the Insular Superterrane arrived throughout the Modern Mesozoic, between volcanics Missoula 250 and 60 million Floods Columbia River years ago. Intermittent Basalts volcanic arcs contributed plutons that intruded the accreted terranes during this Modern volcanics time. Terrane boundary Columbia River Basalts Tectonic rearrangement beginning Poorly understood basement rock Missoula Flood deposits around 60 million years ago exerted a northward push that created extensive north–south strike-slip faulting through the middle of the North Cascades. Right-lateral motion along these faults, notably the Straight Creek fault, resulted in approximately 90 km of displacement. At the same time, extension created and exposed metamorphic core complexes in the Okanogan Highlands (unit pTm) and the metamorphic and intrusive igneous rocks of the North Cascades terranes (unit pKm). About 50 million years ago, the final major addition to Washington had arrived. The Siletz-Crescent terrane (unit ITv) was an exceptionally large chunk of basaltic islands and ocean floor. When it collided with North America, subduction temporarily ceased.

By 40 million years ago, subduction resumed west of Siletzia, resulting in another volcanic arc and uplifting rocks of the Cascade Range. By 17 million years ago, the Yellowstone Hot Spot caused the eruption of the Columbia River Basalt Group, the youngest continental flood basalt eruption on Earth. These eruptions ended by 6 million years ago, and they covered vast areas of southeastern Washington, Oregon, and Idaho (unit uTvc). During these eruptions, continental rifting in the Basin and Range and northward drift of much of California caused clockwise rotation and deformation of the Pacific Northwest, creating the Yakima fold and thrust belt. Rotation about a pole near the northeast corner of Oregon is still ongoing.

This rotation likely contributed to the onset of the modern Cascade arc ~10 million years ago. Volcanism and uplift of the mountain range introduced stratovolcanoes that are still active today (unit Qv). The mostly basaltic Boring Volcanic Field was also active beginning about 2.7 million years ago.

Pleistocene cooling brought broad continental ice sheets across the northern half of the state (unit Qg). Repeated glacial advances and retreats carved the modern landscape, including the Puget Sound and surrounding lowlands. Massive glacial lakes were dammed by ice and episodically breached during this time, releasing the enormous Missoula Floods that spread across eastern Washington to the western coast, traversing the Columbia River. Washington Department of Natural Resources—WA Geological Survey [email protected] • 360.902.1450 • https://www.dnr.wa.gov/geology