Accreted Terranes

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    MacKenzie Smith

    An important feature of West Coast geology and paleontology are accreted terranes. These are rocks, generally ocean island arcs, that get added to the continent by tectonic forces (subduction). Fossils from these localities often look nothing like those around them because they come from farther away from the continent. Additionally, they may be of a different age than the local rock. This often needs to be taken into consideration when interpreting a landscape. Examples of terranes include the Wrangellia Terranes in Alaska, the Olympic Mountain Terranes in Washington, the Izee and Suplee Terranes in Oregon and the Western and Eastern Arc Terranes of Oregon and Idaho. Have you found any interesting fossils from a terrane? Share below! Also feel free to share which ones you have been to or where you would like to go!

    Eleanor Gardner

    Hi, @mackenzie-smith!  I haven’t collected any fossils from accreted terrains in the northwest, but I am interested to learn more.  Could you share picture or two of what fossils from terranes in the northwest look like?  I’m thinking of the “squeezed” looking trilobites from the Conasauga shale in Georgia, which look squeezed due to low-grade metamorphic forces during regional mountain uplift.  However, that visual picture in my brain may be way off base for fossils from accreted terranes.

    MacKenzie Smith

    So there is actually no metamorphism that takes place in the bulk of the terranes. I would imagine some would happen along the margins but I don’t know if any fossils would survive that amount of pressure. Then again, it’s probably a case by case scenario. Attached is a Thalattosaur centrum from the Izee Terrane of Oregon (Carnian Triassic). The skeletons were found by Gloria Carr of the North America Research Group. Prep-work is being done at the Oregon Museum of Science and Industry and the fossils are being described at the University of Alaska. The fossils belong to the Oregon Museum of Natural and Cultural History at the University of Oregon. 3D prints of the fossils are also used at OMSI.

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