Peterson, C.D.; Minor, R., and Baker, D.L., 2013. Accommodation space controls on the latest Pleistocene and Holocene (16-0 ka) sediment size and bypassing in the Lower Columbia River Valley: a large fluvial-tidal system in In this study, we establish the roles that increasing basin accommodation space have on sediment size and bypassing in the transgressive fill (16-0 ka) in the submerged Lower Columbia River Valley (LCRV). The antecedent forearc valley (225 km in length, 4-8 km in width, and 60-115 m in axial valley depth) is characterized by high sediment supply rates (10-15 million t y À1 ) but no delta at its mouth to the Pacific Ocean. Core sample sediment textures (N ¼ 1600) are analyzed from 3000 m of borehole sections in 58 representative boreholes to characterize the ancestral valley fill: 57% sand, 17% muddy sand, 12% sandy mud, and 14% mud (total fill volume ¼ 77 km 3 ). Decreasing mud endmember texture from between 30 and 60% (early Holocene) to 18% (late Holocene) is directly related to (1) declining rates of increasing basin accommodation space and (2) increasing sediment bypassing through the LCRV. Both conditions result from decreasing rates of sea level rise (15.0-1.5 mm y À1 ) in Holocene time. The overall coarsening of sediments, both upvalley and upsection, is controlled by declining rates of increasing basin accommodation space (5.66 million-0.57 million m 3 y À1 ) during middle to late Holocene time. Differences between middle and late Holocene sediment accumulation rates yield bedload bypassing rates of 5-6 million m 3 y À1 , under assumed conditions of constant sediment supply rates in middle to late Holocene time. ADDITIONAL INDEX WORDS: Basin accommodation space, incised valley, Holocene transgression, sediment size, sediment bypassing.Figure 2. Ancestral valley depth contour map of transgressive fill, seismic profiles (lines), and geotech boreholes (dots) in the LCRV. Seismic lines include Uniboom (dotted), sparker (solid line), and air gun (dashed line). Radiocarbon-dated deep boreholes (solid squares and rectangle) include Warrenton-Skipanon LNG, Clatskanie LNG, and I-5 CRC. Ancestral valley depth contours of 0-to À110-m elevation NAVD88 (10-m intervals) are from Baker et al. (2010).

Fire hearths associated with prehistoric Native American occupation lie within the youngest buried lowland soil of the estuaries along the Salmon and Nehalem rivers on the northern Oregon coast. This buried soil is the result of sudden subsidence induced by a great earthquake about 300 years ago along the Cascadia subduction zone, which extends offshore along the North Pacific Coast from Vancouver Island to northern California. The earthquake 300 years ago was the latest in a series of subsidence events along the Cascadia subduction zone over the last several thousand years. Over the long term, subsidence and burial of prehistoric settlements as a result of Cascadia subduction zone earthquakes have almost certainly been an important factor contributing to the limited time depth of the archaeological record along this section of the North Pacific Coast.

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Netarts Bay is the setting of one of the largest concentrations of late prehistoric Native American settlements on the tectonically active Oregon coast. A prehistoric site (35TI74) exposed by sea cliff erosion in 1998 at the south end of the Netarts littoral cell contained a stratigraphic record of activity by prehistoric Native Americans and interbedded paleotsunami deposits. The nearfield paleotsunamis were produced by great Cascadia earthquakes (Mw8.5 ± 0.5) emanating from the Cascadia Subduction Zone offshore, as previously documented in episodically buried tidal marsh surfaces at the south end of Netarts Bay and in many other estuaries in the region. The cultural deposits at 35TI74 reflect multiple reoccupations by Native Americans at this barrage creek flood plain site between paleotsunami runup events (≥7.5 m elevation) at 1.3 ka, 1.1 ka, 0.8–0.9 ka, and 0.3 ka. The conditions that led to the exposure of 35TI74 are traced to erosion of a protective beach dune ramp at the south end of the Netarts littoral cell, placing the site within reach of progressive storm‐surf erosion. The diminishing geoarchaeological record at 35TI74 represents a microcosm of the sea cliff erosional processes that threaten vulnerable prehistoric shoreline archaeological sites along much of the Pacific Northwest Coast.

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