After the various terranes joined to form the

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rocks brought up from the mantle (serpentine), as well as granitic plutons. After the various terranes joined to form the superterrane, they were covered with sediments that became conglomerate, sandstone and shale. Region 4: North Cascades Superterrane The North Cascades Superterrane extends from the Methow Valley to the San Juan Islands and includes at least 10 separate terranes (crustal fragments from tectonic plates; Fig. 3). Some of these terranes represented volcanic island arcs, others were submarine fans consisting of sandstone, conglomerate, and shale, and other terranes were formed of ocean-floor basalt. This block of terranes docked with North America ~90 million years ago producing deformation, metamorphism, and igneous intrusions throughout the North Cascades. The increased pressure and temperature due to the collision caused parent rocks within the terranes to metamorphose to phyllite, schist, and gneiss (depending on the location and the parent rock). Granite plutons intruded the region between 90-65 Ma.
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Tree Fern Dawn Redwood Cinnamon Tree Sassafras Sycamore Swamp Cypress Palm trunk Palm fronds Figure 4. Fossils commonly found in the Chuckanut Formation near Bellingham, WA. Think about what the presence of plants such as palm trees indicates about the climate at the time of deposition. Lab #8: Geology of Washington 110 Region 5: Successor Basin Roughly 60 million years ago, after the major terranes had been added to the margin of North America, the landscape was gradually leveled to a broad flat plain by erosion (similar to Mississippi today). Big river systems flowed westward across this plain, which was gradually sinking, depositing layers of river cobbles/gravel, sand, mud, and abundant plant material (Fig. 4). The total thickness of these deposits exceeded thousands of meters and were eventually lithified into conglomerates, sandstones, siltstones, shales, and coal. The rocks that make up the Chuckanut Mountains near Bellingham, the Peshastin Pinnacles in the Wenatchee Valley, and the fossil-rich rocks around Republic are good examples of these rocks. This deposition continued until ~50 million years ago.
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Figure 5. Map of the Crescent basalts and Olympic Subduction Complex. Figure 6. Distribution of the Columbia River Basalts (shown in gray). Lab #8: Geology of Washington 111 Region 6: Olympic Subduction Complex During the deposition of the Successor Basin sediments, a major segment of the outer coast of Washington State was being rifted northward. This resulted in a huge sinking basin along the coastline that was filled with sediments, resulting in feldspar-rich lithic sandstone and shales. Soon thereafter an immense eruption of basaltic lava created big volcanic islands that grew along the coast of Washington and Oregon. As the Cascadia subduction zone (the modern subduction zone) developed, these rocks were pushed up into a horseshoe shaped dome that currently makes up the Olympic Mountains. Great wedges of sea-floor sediments were scraped off the descending plate to become the western part of the Olympics. These rocks are called the Olympic Subduction Complex (Fig. 5).
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