Late Quaternary stratigraphy and seafloor geology of eastern Juan de Fuca Strait, British Columbia and Washington

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 modem 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 preexisting deposits that occur in banks and extensive coastal exposures. (C) 2001 Elsevier Science B.V. All rights reserved.