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 Oregon and Washington, USA

Abstract

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).

Late Holocene Geomorphology of the Columbia River Estuary, Oregon and Washington, USA

“…Sedimentation rates of mud and peaty mud in overbank deposits from protected tidal marshes in Cathlamet Bay (JohnD) and Grays Bay (CRGB07) are much lower (1.6-1.8 m ka -1 ). Those overbank sedimentation rates in the protected tidal marshes more closely follow the relatively low-rates of latest-Holocene sea level rise in the estuary (Peterson et al, 2013). …”

“…During subsequent spring months of river flooding Jay & Naik, 2011;Jay et al, 2011) the strong asymmetries in riverine-tidal discharges in the shallow shoals and tidal channels transported sand seaward to the Pacific Ocean. These unusual circumstances permitted the shallow Columbia River estuary to bypass bed load to the marine side (Gates, 1994;Baker et al, 2010, Peterson et al, 2013) and yet to remain unfilled by upper-intertidal overbank deposits in the central axes and in the larger lateral embayments for the last several thousands of years.…”

“…The very-high sedimentation rates in some of the sand shoals greatly exceed rates of expected relative sea level rise (~ 0.1 cm yr -1 ) as projected into the last 100 year interval from the latest Holocene sea level curve Peterson et al, 2013). If such high sedimentation rates occurred widely in the central axis of the estuary during latest Holocene time (0 -2.5 ka) then the shallow subtidal shoals would have converted to intertidal marsh islands and/or to supratidal floodplains prior to historic time.…”

“…The Columbia River estuary was supplied with abundant sand (> 5 million m 3 yr -1 ) during late Holocene time (Peterson et al, 2013), but the estuary failed to infill with upper-intertidal overbank deposits. Measured sedimentation rates in the central channel axes and lateral embayments (Table 4) were substantially greater than coeval rates of sea level rise (Figure 12).…”

“…These conditions, therefore, likely encouraged interpretations that the lower estuary serves as a net sink for littoral sand entering the river mouth in deep channels (Walter, 1980;Sherwood & Creager, 1990). However, long-term declines in sedimentation rates from mid-Holocene to late-Holocene time in the lower Columbia River valley (Peterson et al, 2013) demonstrate substantial bypassing of river sand (about 5 million cubic meters per year) to the Pacific Ocean. The Holocene depositional filling of the inner-shelf and the late Holocene progradation of barrier beach plains on either sides of the Columbia River mouth are attributed to the substantial throughput of bed load from the Columbia River to the marine side .…”