Accommodation space controls on incised-valley sediment accumulation rates during the Holocene marine transgression (0–11 ka) in Grays Harbor, a large meso-tidal estuary, Washington, USA

Abstract

Holocene deposits (0–11ka) that were drill cored (275 samples from 17 sites) in the meso-tidal Grays Harbor estuary were analyzed for 1) sediment texture and source components and 2) calibrated radiocarbon age. These data were used with basin 10m-contour interval volumes to establish relations between rates of increasing basin accommodation space and sediment volume accumulation. Rates of increasing accommodation space (0.3–2.1×106m3yr−1) are based on interval volumes and a sea level curve (0–11.5ka) for the Grays Harbor basin. Total sediment accumulation rates (0.41–1.4×106m3yr−1) are computed for the same 10m-depth intervals (0–60m depth MSL) and corresponding 10m-interval sedimentation durations (0.7–5.0×103ka) from a sediment level curve. The rates of basin increasing accommodation space and total sediment accumulation are positively correlated (R2=0.81), but sediment accumulation lagged behind increasing accommodation space (23–37%) during early- and middle-Holocene time (7–10ka). Slowing rates of sea level rise after 8ka, permitted combined river and littoral inputs of sediment to reduce basin subtidal water depths (~10m) to intertidal water depths (~1m). The ratios between sand and mud components in the analyzed Holocene deposits are negatively correlated to basin water depth. Shallowing tidal flats promoted the export of river mud out of the estuary and the import of littoral sand into the central estuary areas. Variable rates of increasing basin accommodation space 1) directly controlled the total sediment accumulation rate in the evolving basin and 2) indirectly controlled the relative proportions of mud and littoral sand accumulation, due to changing water depths in the basin. The late-Holocene accumulation of littoral sand (52% of total sediment volume) is used to compute the net littoral sand sink (~100×106m3) in the Grays Harbor basin from a potential 1.0m rise of future sea level following predicted global warming. The contribution of littoral sand from adjacent barrier sand spits (20km in length) to fill the increasing accommodation space in the Grays Harbor estuary could result in 0.5–1.0km of shoreline retreat following a 1.0m rise in sea level.