Large Dunes and Other Bedforms in Lower Cook Inlet, Alaska

Abstract

ABSTRACT The surficial geology of lower Cook Inlet, Alaska, reveals: numerous fields containing bedfroms of various sizes and types, a smooth bottom, sand patches, or boulders. High-velocity tidal currents exceeding 3–4 knots (150–200 cm/s) dominate the hydrodynamic environment. The fields normally are elongate in the axial direction of the inlet, parallel to the dominant tidal flow. Lengths of the fields normally exceed several kilometers; their widths range from a few hundred to a few thousand meters. Although ebb and flood surface currents in lower Cook Inlet are typically about 3.5 to 4 knots, currents of 5 to 6 knots (250–300 cm/s) are not exceptional. In the 25 km-wide constriction between upper and lower Cook Inlet, tidal currents may peak at 9 knots or higher. The most abundant group of bedforms are sand waves, some as large as sand ridges. Normally, smaller sand waves ride on their flanks, and opposing asymmetry may indicate opposite transport directions for the large and small waves. Dunes are less common and have not been seen covering large fields. Sand ridges, having their crests parallel to the current flow, are about 10 m high and have spacings of 300 to 400 m. Other bedforms observed are sand ribbons and sand patches, and gravel and boulder fields. Shallow parallel grooves, partly filled with small dunes, were observed on sonographs. Seismic studies indicate that these grooves occur where steeply dipping strata reach the surface and are sculpted by differential erosion. Although nothing is known about the migration of these various bedforms, the oppositely oriented asymmetry of large sand waves and the smaller superimposed ones suggests some degree of transport. Bottom television observations, revealing active motion in the form of small current ripples, support bottom transport. An understanding of these various bedforms and other bottom characteristics and their distribution and movement is essential to future oil and gas development, in particular to the safety of exploration/production platforms and pipelines, and to biological studies, primarily those dealing with benthic organisms. INTRODUCTION Lower Cook Inlet is the south part of the large tidal inlet that connects Anchorage with the Pacific Ocean (Fig. 1). Pacific waters, moving in part as the Alaska Current in a counterclockwise motion along the Kenai Peninsula, enter Cook Inlet from the southeast via Kennedy and Stevenson Entrances. The flood current moves predominantly along the eastern shores of Cook Inlet, whereas ebb waters flow mostly south along the western side of the inlet. High-resolution shallow-penetration seismic subbottom profiling systems -Uniboom and 3.5 kHz were used on board the USGS R/V Sea Sounder in June-July 1976 to obtain environmental geologic information about the surficial geology (Bouma and Hampton, 1976; Hampton and Bouma, 1976). During this survey, large bedforms were found in many areas (Fig. 1). In some of these areas, a side-scan sonar survey and bottom television observations were made.