Sediment dispersal and accumulation in an insular sea: deltas of Puget Sound
Webster, Kristen L.
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Small rivers carry several million tons of sediment annually into Puget Sound and the Strait of Juan de Fuca. Once delivered to the marine environment, processes in the water column and on the seabed dictate dispersal, deposition and accumulation of these particles. To investigate these mechanisms, water-column measurements, including long-term bottom-boundary-layer time-series, water-column profiles and shipboard velocity, and seabed sampling, such as sediment cores, multibeam bathymetry and seismic reflection profiling, were collected from 2007-2009 on the Elwha and Skagit River deltas. Tidal currents at both study sites were strong and capable of dispersing muds to more distal portions of the delta. At the Skagit delta the intertidal topset had strong ebb tidal currents that exported most of the Skagit River mud rapidly beyond the topset. The mud found on the flat was limited spatially, near channels and at the outer flat edge and temporally, following high discharge. Physical processes drive shear stresses that act on the seabed to mobilize sands and muds and rework the seabed at various frequencies and depths: on a semi-diurnal tidal timescale, both channel and flat seabeds are reworked to 1-2 cm; and over a decadal timescale, lateral channel migration acts to rework the seabed to 1-2 m, making the limited mud deposits available for export. The Skagit muds were rapidly transported > 10 km into two distinct distal basins; southward within the deep quiescent basin of Saratoga Passage mud accumulates up to 10 mm y-1 for a total of ~0.5 MT annually, and northward within the high-energy unconstrained basin of Rosario Strait some mud accumulates in small local deposits, but much is likely broadly dispersed. Throughout Puget Sound deltas typically form bay head (muddy progradation) or sidewall (broad mud dispersal), and the Skagit River dispersal system has both characteristics. The Elwha River delta is a coarse-grained sidewall delta and tidal currents act to broadly disperse muds. Over the past 10,000 years the dominant sediment sinks have shifted from foreset beds to alongshore spits. This was likely related to variations in sea level as a result of deglaciation and sediment supply. As sea-level rise slowed slightly, wave-driven longshore transport began to actively shape delta morphology and created paleospits, which formed successively one at a time, stepwise as sea-level rose. These sediment sinks were formed under similar processes as the modern spit, Ediz Hook. It is shown that deltas in insular seas can form complex deposits, and this has implications for interpreting the geologic record in areas of constrained basins. In the modern environment, basin morphology and tidal currents determine accumulation patterns. Over longer timescales, the effects of previous glaciations have a major role in delta evolution.
- Oceanography