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 11.4 m. In Turnagain and Knik Arms, 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 tidal channels. In intertidal environments in many parts of the world, physical processes of sedimentation are typically influenced by surface fauna; sedimentary structures in some zones are altered or obliterated by the activities of the infauna. However, in Turnagain and Knik Arms, virtually no macrofauna exist, and so the area provides an opportunity to study depositional processes and sedimentary structures without the masking effects of bioturbation. Suspended-sediment values for Turnagain and Knik Arms are greatest in the summer months and nearest low-tide stages. Ice floes form during winter months and, in combination with the freezing of intertidal sediment to several meters of depth, have a significant effect on sediment dispersal and on the deepening and increased stability of the tidal channels. The intertidal sediment is mainly very well sorted, medium to very fine sand and coarse silt, with a notable paucity of gravel, coarse sand, and clay. Sediment accumulates in sandbars oriented parallel to the axis of the estuary. Textural and facies patterns reflect decreasing current energy, first, as a function of distance from the mouth of the estuary and, second, as a function of topographic elevation on the sandbars. 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. The base of the sequence contains parallel-laminated sand that is overlain by mixed parallel- and cross-laminated silty sand overlain by cross-laminated silty sand. At the top of the sequence is a unit of organically rich parallel-laminated silt and sandy silt. 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.