Geophysical and sedimentological studies within lower Cook Inlet, Alaska, have revealed acoustic facies relations and sedimentologic depositional environments typical of large, tidally dominated estuarine systems along the Pacific margin of the United States. Since 1976, detailed high resolution seismic and sidescan sonar surveys, bottom underwater television, and bottom photography, together with sediment sampling, in lower Cook Inlet, have delineated six major depositional environments: high-energy tidal flat; trough-edge platform; trough slope; tidal trough (channel); channel-mouth plateau; and seaward progradational ramp. Within these environments are found lithofacies ranging from sand patches, sand ribbons, and mixed cobble-sand "hard bottom" to sand-wave bodies of oth large- and small-scale and to sand-wave and shell-lag complexes. These facies appear to be primarily controlled by the modern hydrodynamic regime and the availability of sediment within lower Cook Inlet.

In lower Cook Inlet at present, sands and gravels are being deposited while older glacial sediments are being winnowed. Marine transgression since glaciation has resulted in a more energetic tidal environment in the present than existed in the past. Geophysical (evidence shows that deposition has occurred over a preexisting glacial topography consisting of an irregular surface cut by numerous shallow channels. In other areas, banks of till lightly covered by recent sediments appear to nearly crop out on the seafloor. Throughout lower Cook Inlet, however, modern sediments average approximately 30 to 40 m in thickness.

Four primary acoustic depositional facies are recognized in the shallow subsurface sediments. The upper two acoustic facies can be correlated with the modern lithofacies within lower Cook Inlet. Facies A, considered to represent unsorted tills, overlies the eroded glacial surface and is up to 75 m thick. It has a characteristically nearly transparent acoustic appearance on high resolution geophysical records. Overlying Facies A is Facies B which is characterized by a strong acoustic reflection. This facies is very thin and is thought to represent glacially derived outwash gravels and cobbles. Facies C overlies Facies B and it is acoustically identified by its pattern of multiple horizontal reflectors which is thought to represent a succession of alternating layers of silt and sand. Fa ies C is considered to have been deposited when lower Cook Inlet was a quiet bay. The uppermost acoustic facies, Facies D, appears limited to trough, plateau, or ramp environments and is composed of large-scale sand-wave complexes. Modern analogous sand-wave complexes can be presently found in parts of lower Cook Inlet.

Willipa Bay, Oregon, and San Francisco Bay, California, exhibit modern sediment facies which are similar in some ways to those described for lower Cook Inlet. Comparison of these Pacific margin embayments in terms of hydrodynamics, sediments, constructional history, and topography has led to a general facies model for high-energy tidally dominated estuarine systems.

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