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A combination of shallow sediment cores and high-resolution seismic-reflection data were used to identify and map four main surficial geologic units in eastern Juan de Fuca Strait. These units comprise bedrock (unit 1), till/diamict (unit 2), glacial-marine (unit 3), and post-glacial sediments (unit 4). Bedrock crops out only in nearshore environments off Vancouver Island. Diamicton forms the numerous morainal and drumlin-like banks in the region. A series of banks, including Middle and Hein Banks, and the offshore extension of Dungeness Spit, divide the Strait into two major areas based on the dominant surficial sedimentary units; post-glacial sediments are most common to the east and glacial-marine sediments to the west. Glacial-marine sediments are represented by several seismo-lithologic subunits that suggest progression from ice-proximal to ice-distal depositional environments. Radiocarbon dates indicate these sediments are a product of the last phase of glacier retreat. There is relatively little modern sediment input to eastern Juan de Fuca Strait, so most post-glacial sediments are thought to consist of material reworked from glacial outwash or eroded from shallow bank deposits. This conclusion is supported by radiocarbon ages from the post-glacial sediments, most of which are early Holocene or younger. Also, there are sandwave fields, sediment drifts, and barrier-spits that appear to be actively maintained. The source of sediment to these features is probably the pre-existing deposits that occur in banks and extensive coastal exposures. |
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