Chuckanut Formation, Bellingham, WA
The Eocene Chuckanut Formation consists of a phenomenally thick sequence (9000 m!) of alluvial sandstone, conglomerate, mudstone, and coal, originally deposited in flood plains in subsiding basins near the coast of Washington- or at least, where the coast was around 50,000,000 years ago. Sediment sources were the highlands of the Rocky Mountains and the southern interior of British Columbia. Deposition was prior to growth of the Cascade arc or subduction-accretion of the Crescent Terrane (broadly, the Olympic Peninsula rocks). Orogeny of the Cascade Range effectively shut off the sediment supply. Docking of the Crescent Terrane deformed the Chuckanut sedimentary rocks. Most people know that Chuckanut rocks are found in west-central Whatcom County. However, scattered units of these rocks also extend along the Darrington-Devil’s Mountain fault zone through Skagit and Snohomish Counties. Visits to sites in these less well-known localities will be included here as well. A succinct introduction to the Chuckanut is “Geology and paleontology of the early Tertiary Chuckanut Formation” by G. Mustoe, R. Dilhoff, and T. Dillhoff, 2007. This appears in the 2007 Cordilleran GSA Field Guide, Floods, Faults, and Fire: Geological Field Trips in Washington State and Southwest British Columbia.
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