Modern surface morphology
of the eastern Juan de Fuca Strait
David
C. Mosher, Robert Kung and Antony T. Hewitt
The bathymetric/relief map represents
a compilation of historic hydrographic field sheet data and multibeam
bathymetric data. The field sheet data are derived from single
beam echosounding by the Canadian
Hydrographic Service (CHS) and the National
Oceanographic and Atmospheric Administration (NOAA) of the
U.S. The NOAA field sheet data are available digitally through
the National
Geophysical Data Center . The CHS field sheet data are available
digitally from Nautical Data International.
Multibeam bathymetric data were collected from the Victoria waterfront
during cruise PGC98004 and subsequent surveys by the CHS and the
Acoustic
Data Analysis Centre - Pacific of the Department of National Defense.
The overall shape of the Strait is
bedrock controlled, enhanced by glacial deposition and scouring
during numerous Pleistocene glaciations. Water depths range to
250 m in the study region. The present-day seafloor relief is
quite variable as a result of the many shallow banks and intervening
deep troughs. These features may result from glacial deposition
and erosion during the Pleistocene, or may, in part, be tectonically
controlled. The banks are glacial drumlins, and side- and end-moraines
(Figure 1). The upper surfaces
of some of the shallower banks have been eroded during lower relative
sea level stands. A set of "braided" channels is apparent, emanating
from Puget Sound at Admiralty Inlet, likely resulting from outwash
during melting of the Puget Lobe glacier, but perhaps tectonically
controlled as well. Draping of the topography is obvious to the
east and west of Whidbey Island, resulting from reworking (erosion
and redeposition) of abundant Pleistocene sediment comprising
much of Whidbey Island and surrounding land-mass.
Multibeam sonar data are available
only along the Victoria waterfront (Figures
2-8) and off of Race Rocks.
These data provide a scale of resolution impossible to achieve
by conventional sounding techniques and show many features, anthropogenic
and natural, that otherwise would not be observed. These data,
combined with limited sidescan sonar data, indicate that it is
likely much of the seafloor is covered by bedforms, resulting
from reworking of material by strong tidal and estuarine circulation
currents within the strait. The largest of these bedforms is a
set of massive sand dunes just east of Victoria (Figure
3), in which dune height is up to 25 m. Paleoshorelines are
also obvious on the multibeam data (Figure
4) as are pipelines, cables, and other sources of debris.
Reference citation:
Mosher, D.C., Kung,
R., and Hewitt, A.T., 2000. Modern surface morphology of the eastern
Juan de Fuca Strait, in: Mosher, D.C. and Johnson, S.Y. (Eds.),
Rathwell, G.J., Kung, R.B., and Rhea, S.B. (Compilers), Neotectonics
of the eastern Juan de Fuca Strait; a digital geological and geophysical
atlas. Geological Survey of Canada Open File Report 3931
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