Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 8-3
Presentation Time: 9:00 AM-6:00 PM


GRAHAM-DAVIS, Jazzy M., STREIG, Ashley R. and PETERSON, Curt D., Department of Geology, Portland State University, 1721 SW Broadway Ave, Portland, OR 97201

We present new dendrochronologic age constraints from a buried tree slab collected below the marsh surface at Netarts Bay, Oregon. A ghost forest is apparent at low tide on the central-eastern margin of Netarts Bay. 120 meters inland from the bay front, incision into marsh exposures reveal buried trees protruding 1.43 meters below the marsh surface and extending from the fourth layer of buried marsh soils overlain by tsunami sands. We interpret this stratigraphic sequence as evidence for tectonic subsidence and abrupt paleotsunami inundation. Prior research at the southern marsh at Netarts Bay has correlated CSZ ruptures to the stratigraphic record with sediment cores that have a pattern of marsh soils overlain by a sharp contact to sand, transitioning to mud and then marsh soils again, bulk sample radiocarbon dating, and diatom analysis. Sediment cores collected at the ghost forest site show the same stratigraphic pattern indicative of CSZ related subsidence. We correlated ghost forest trees exposed along the bay with trees protruding from incised marsh stratigraphy with both differential GPS and lidar elevation measurements for several ghost forest trees. We collected a cross section slab from a buried ghost forest tree, sanded the slab, counted growth rings at a consistent decadal interval, and submitted select samples for C14 Accelerated Mass Spectrometry age dating. We wiggle match C14 ages and the known interval between tree ring samples to the C14 calibration curve using OxCal 4.3, to find a narrow age range for the death of the trees. We will compare the dendrochronologically constrained death of the trees with other on-shore and off-shore paleoseismic studies that constrained the timing of prehistoric CSZ ruptures. We hope to identify the species of the tree, and use information on its modern analog environment to estimate relative on-shore elevation drop from subsidence associated with the time constrained fourth Cascadia earthquake back identified along the Oregon Coast.