Deposition and Preservation of Estuarine Sediment, Turnagain Arm, Cook Inlet, Alaska

Cook Inlet of south-central Alaska is a funnel shaped estuary that splits into two branches at Anchorage. Turnagain Arm is the hypertidal (commonly exceeding 9m) west-east trending extension. The inlet formed from a drowned glacial valley that was subsequently filled with tidal deposits of silt and fine sand. The tidal system is semidiurnal with a prominent diurnal inequality. There are also variations due to spring and neap tides. Turnagain Arm is home to a tidal bore generated during spring tides that can reach heights of up to 2m and travel at speeds of up to 5m/s. Current reversals can be dramatic with ebb tidal velocities of 6m/s changing to flood velocities of 10m/s over a period of a few minutes. During the initial flood tide, highly turbid water can rise as fast as 10cm/min. This combination of elements results in a highly dynamic depositional setting. Measurements taken in the inner estuary during several neap-spring cycles in the summers of 2007-08 documented deposition upon mud bars of as much as 2.5cm per tidal event. Conversely, erosion of up to 6.2cm per tidal event has been measured. High rates of deposition together with the extreme fluctuation of water levels result in various types of soft sediment deformation. High rates of deposition also result in the random orientations of fine-grained particles which when disturbed can create dangerous quicksand conditions. Some portions of the inner estuary are only submerged during the extreme high tides. Observations at study area sites during the same two week neap-spring cycles show that less than sixty percent of all high tides are high enough to submerge some bars. Even if submerged, deposition does not always occur. Such a high percentage of non-depositional events has real implications when interpreting tidal cyclicity of the rhythmites found at these sites.