The Puget Lowland physiographic province consists of a broad, low-lying region situated between the Cascade Range to the east and the Olympic Mountains and Willapa Hills to the west. In the north, the beautiful San Juan Islands form the division between the Puget Lowland and the Strait of Georgia in British Columbia.
Lithologies, radiometric ages, and spatial relations indicate that some of the rocks of the San Juan Islands were accreted to North America prior to the Late Jurassic. Then, during the late Cretaceous, a major suture zone passed through the islands joining Wrangellia terrane of Vancouver Island to the San Juan-Cascade nappes. On the islands, the Cretaceous suture is called the Haro thrust zone. In addition to extensive structural dislocation within each terrane, the Late Cretaceous orogeny stacked nappes on top of each other by thrust faulting. Burial during this event produced low-temperature/high-pressure metamorphism characterized by the minerals prehnite, lawsonite, and aragonite.
The oldest rocks in the San Juan Islands are Devonian limestone, argillite, black shale, siltstone, and andesitic tuff. The limestones have been extensively mined for cement and other products. The early Pennsylvanian is represented by andesitic pyroclastic rocks containing limestone lenses and beds; fossils help identify the age of this unit. Pillow lavas and associated limestone containing Permian fusilinids and foraminifera also crop out on the islands; thick units of Permian ribbon chert are also known. The Triassic and Cretaceous sections are represented by clastic sedimentary rocks that have only rare limestone lenses. Ophiolites, gabbro, and associated volcanic rocks are interbedded with radiolarian argillite and chert of Jurassic age.
A major feature cross-cutting the lowland is the east-west Seattle-Bremerton high. Its surface expression consists of Tertiary bedrock exposures protruding from glacial till. From east to west, the outcrops are: coal-bearing Puget Group rocks along Interstate 405 and in Renton; Oligocene-Miocene Blakeley Formation marine sedimentary rocks at Seward Park, at the south end of Lake Washington; Blakeley Formation sedimentary rocks and Oligocene volcanic rocks along Interstate 5 at Boeing Field; Blakeley Formation rocks at Alki Point, West Seattle; Blakeley Formation sediments from the south end of Bainbridge Island to Bremerton; and, from Bremerton west, large outcrop areas of Eocene Crescent volcanic rocks.
A remarkable geophysical anomaly extends along the north side of the Seattle-Bremerton high; over a short distance one moves from a gravity high to one of the deepest gravity lows known. The gravity gradients are among the steepest recorded in North America. Whereas bedrock outcrops characterize the Seattle-Bremerton high, a very deep depression filled with unconsolidated sediments represents the low. A geotechnical drill hole in downtown Seattle penetrated 3,125 feet (1,139 meters) of these sediments. Tertiary outcrops at Boeing Field are just three miles to the south of the geotechnical bore hole. A major east-west fault parallel to Interstate 90 has been postulated as the boundary between the gravity low and the Seattle-Bremerton high.* Adding credence to this hypothesis is the highly deformed nature of the Tertiary sedimentary rocks within the bedrock high.
Tertiary sedimentary rocks that contain significant coal resources are exposed along the eastern margin of the Puget Lowland . In the northeastern Puget Lowland, coal was mined as early as 1853 from the Chuckanut Formation at Bellingham and nearby localities. During the period 1900-1918, when coal was the major source of fuel for heating and locomotives, numerous underground mines produced as much as 4 million tons of coal annually. In King County, underground coal mining was carried out under Renton only 10 miles from downtown Seattle. Rapid urbanization in eastern King and Pierce Counties is creating pressure to build atop hazardous coal mine lands that are subject to subsidence. Now all coal mining in Washington is carried out by open-pit mining methods. At Black Diamond in King County, the Pacific Coast Coal Co., Inc., John Henry No. 1 mine produced 115,000 tons of coal in 1989. The largest mine in the state is operated by PacifiCorp Electric Operations near the city of Centralia, south of Olympia; large open pits in the late Eocene Skookumchuck Formation produce 4.5 to 5.1 million tons of subbituminous coal annually.
Starting in the early Pleistocene, the Puget Lowland was subject to four periods of extensive glaciation. Geologists are using zircon fission-track dating methods and paleomagnetic studies in an attempt to date and correlate old glacial drift deposits such as the Puyallup Formation, Salmon Springs Drift, Orting Drift, and the Logan Hill Formation. These units represent glacial stages that are older that 0.74 million years. A glaciation hiatus existed between 200,000 and 740,000 years ago; no deposits of that age have been identified from the Puget Lowland.
During the late Pleistocene the climate cooled, and renewed glaciation again modified the Puget Lowland. Excellent cliff exposures of Whidbey Formation interglacial sediments and Possession till can be seen on Whidbey Island. Amino acid studies of wood and shells from glaciomarine drift and radiocarbon dating techniques have improved the correlation of these glacial units with other deposits across the Puget Lowland.
The Puget Lowland owes its present-day geomorphic features to the last continental glacier that covered the region--part of the Fraser Glaciation. Named the Cordilleran ice sheet, it consisted of two parts: one called the Puget Lobe, and the other named the Juan de Fuca Lobe. The ice sheet advanced from British Columbia 18,000 years ago to just south of Olympia; the entire Puget Lowland was covered by glacial ice. As during previous glaciations, streams and rivers draining the Cascades were dammed by the Cordilleran ice sheet. The waters were diverted south along the flanks of the Cascades, then around the terminus of the ice sheet south of Olympia, and finally out the Chehalis River valley to the Pacific Ocean. On outwash prairies south of Olympia, curious small hummocks developed. These have been given the name Mima Mounds. Numerous hypotheses have been proposed over the last 100 years on how these mounds formed--for example, creation by gophers, earthquakes, or permafrost. By 14,000 years ago the ice had retreated to Seattle. Large areas south of Seattle were being covered by recessional outwash sands and gravels that are part of the Vashon Stade. These sands and gravels are an important source of raw materials for concrete and other construction uses. At about the same time, thinning ice allowed marine waters to return to the Puget Lowland, and seawater lifted the ice and caused it to break up into berg ice over the entire region. Approximately 10,000 years ago, after a short-lived readvance in the northern Puget Lowland, the Cordilleran ice sheet disappeared, bringing the Ice Age to a close in this region.
Mima Mounds formed in glacial recessional outwash sands and gravels, south of Olympia, Puget Lowland.
Photo: Arthur M. Ritchie
Washington State, and the Puget Lowland in particular, is subject to earthquakes. More than 1,000 earthquakes are recorded by seismographs each year. Of these, ten or more produce sufficient shaking to be felt by the public. The epicenters for the last two major earthquakes were under Olympia (1949) and Seattle-Tacoma (1965) with Richter magnitudes 7.1 and 6.5, respectively. These major earthquakes are attributed to subduction of the oceanic Juan de Fuca plate under western Washington. On a scale of 0 to 4, with 4 being to highest risk, the Puget Lowland is classed as a Seismic Risk Zone 3.
* Note: This structure is now known as the Seattle Fault (2000)
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.