THE GEOLOGY OF THE PACIFIC NORTHWEST
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.
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.
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.
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.
© San Juan Islands Conservation District 2016