Macrotidal subarctic environment of Turnagain and Knik Arms, Upper Cook Inlet, Alaska: sedimentology of the intertidal zone.

An extensive sheet of silty sand crossed by tidal channels is exposed at low tide in Turnagain and Knik Arms, Alaska. Transportation and deposition of sediment composing this sheet is the result of strong tidal currents due to the maximum spring tidal range of approximately 11.4m. The initiation of the floodtide is accompanied by the occurrence of a tidal bore that travels at a speed of 4 m/sec or more up the tide channels. Virtually no macrofauna exist, and so the area provides an opportunity to study depositional processes and sedimentary structures without the masking effects of bioturbation. The intertidal deposits in Turnagain Arm contain a transitional upward-fining sequence which is analogous to point-bar and channel-bar successions described by other workers. This suite of sedimentary features and textures indicates changing current velocities and directions as bars emerge or are modified during ebb tide, and the vertical sequences could potentially be used to record intertidal bar erosion and aggradation by point- and channel-bar migration, a process carried out primarily by bed-load currents.-from Authors