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Phillips, A.C., 1993

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Bibliographic Reference

Phillips, A.C., 1993, Sedimentary processes at glacial marine deltas, Glacier Bay, Alaska: University of Illinois, Chicago, Ph.D. dissertation, 214 p., illust.

Abstract

Comparative studies of sedimentary processes at three meltwater-fed fjord deltas in Glacier Bay, Alaska, assessed source controls on sediment flux, modes of transport into the marine environment, and marine influences upon sedimentation. Suspended sediment is dispersed in turbid plumes within the estuarine surface layer. Hemipelagic sedimentation is enhanced by flocculation and pelletization, causing deposition of fine sediment--mainly silt--in proximal environments. Estimated in situ settling velocities were up to 8 times faster than Stokes Law predictions for a given grain size. Enhancement increased with sediment fineness. Tidal advection of settling sediment below the surface layer may complicate sediment pathways. Baselevel lowering on ebb tides shifts the focus of sedimentation from the delta plain to the prodelta. A graded silt-mud lamina is deposited with each ebb-flood phase. This drawdown effect is greatest during spring lower low water (LLW) when stream mouths are at the delta margin. Reentrainment of delta plain deposits magnifies fluvial sediment loads and bedload is delivered directly to the delta foreslope. Sediment trap catches within 6 km of delta plain margins were 2 - 5 times greater and coarser grained during spring tides than during neap tides. Drawdown effects are greatest for deltas with low-sloping plains and relatively low discharge. Other controls are superimposed. In summer 1988, fjord-head glacier advance in Wachusett Inlet uplifted older glacial marine sediments, creating an abundant new supply of muds to meltwater streams. Increasing hemipelagic accumulations through the summer correlated with meltwater stream suspended sediment concentrations (Crms), although neap-spring cycling of silt lamina deposition continued. Sediment gravity flows are intermittent but unremittent on delta foreslopes. They are largest and most frequent during spring LLW when bedload is delivered directly to delta margins, but occur at high tidal stages as well. In Queen Inlet, coalescing of sporadic delta-front slumps resulted in quasi-continuous turbidity currents in submarine channels that lead away from the delta. In flows measured in one channel 2.7 km seaward of the delta margin, velocities ranged from 0-120 cm cdot s-1, Crms reached 8.2 g cdot L-1, and durations were up to several hours. These relatively low density, low velocity flows may play important roles in maintaining channel form.

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