Grain size data compilation and parameters of sediment samples; lower Cook Inlet, Alaska, 1976 through 1979

Abstract

Lower Cook Inlet is the southern part of the large tidal inlet that connects Anchorage with the Pacific Ocean (fig. 1). During the summer field seasons of 1976, 1977, 1978, and 1979, the U.S. Geological Survey under contract with the Bureau of Land Management through the National Oceanic and Atmospheric Administration conducted geo-environmental studies in lower Cook Inlet (note Table 1) (Bouma and Hampton, 1976; Bouma and others, 1978, Hampton and Bouma 1979, 1980). The USGS research vessel SEA SOUNDER was used to obtain continuous seismic reflection profiles and to collect seafloor sediment samples. In addition, bottom samples were taken by the University of Alaska during benthic biological studies aboard the R/V MOANA WAVE (April 1976) and the NOAA vessel MILLER FREEMAN (October1976, Feder,1977). Textural analyses were carried out on sediment samples taken at 117 station locations. Sample collection was influenced by the nature of the substrate, texture, equipment malfunction and weather. Figure 2 presents a map with station locations. Station locations and other data are tabulated in Appendix I. A modified Van Veen grab sampler, capable of taking an undisturbed surface sample of 40X60X30 cm was used to collect unconsolidated sediment. In addition, a 3-m gravity corer and a 2-m vibracorer were used at selected sites. The top ten centimeters of each sample were used for grain size analysis. Most samples were taken from bedform fields that occupy much of central lower Cook Inlet. Water depths in the lower inlet range from 40 to 170 m, averaging approximately 70 m. Strong tidal currents are present and surface water velocities of 3-5 knots (150-250 cm/sec) are common during flood and ebb tides (Bouma and Hampton, 1978). The bottom of central lower Cook Inlet is almost completely covered by sand. Bedrock material can be observed at the seafloor on seismic records in isolated areas. The sand occurs in bedform patterns; textural data are important when determining the relationship between the currents and migrating sand bodies patterns. Immediate and long-term effects of sand movement over and around obstructions placed on the seafloor should be studied to prevent damage or destruction to the sensitive ecosystem of lower Cook Inlet. This report presents textural grain-size data that have been gathered for lower Cook Inlet samples, the method of collection, and analytical processes. The data in Appendix 2 can be used to compare calculated parameters in order to properly assess the sedimentary environment of lower Cook Inlet.