Surficial geology of the
eastern Juan de Fuca Strait
Antony T. Hewitt and David C. Mosher
A combination of 36 piston and vibro-cores
and over 2300 line-km of high-resolution seismic reflection data
(Huntec DTS) were
used to map four main surficial geologic units in the eastern
Juan de Fuca Strait (see Trackplot mapsheets for data control).
The reflection records have been broken into four main descriptive
units, based on their acoustic character.
Unit 1 (Bedrock)
Unit 1 is characterised by discontinuous,
but very high-amplitude upper surface reflectors and incoherent,
but strong internal reflectors (Figure
1). It displays a highly irregular, undulating topography,
and can be seen to dip steeply into the subsurface. Correlations
with coastal outcrops and with deeper seismic reflection data
show it to be bedrock. Figure 9
of the bathymetric relief map sheet shows an example of submarine
outcrop at Race Rocks as imaged by multibeam bathymetry.
Unit 2 (Diamict/Till)
Unit 2 rarely crops out at the seafloor,
but usually forms the acoustic basement in Huntec records, except
where unit 1 occurs. It is the most widely spread unit throughout
the Strait, but varies greatly in thickness. Internal reflectors
are incoherent to semi-coherent with occasional point reflectors
forming hyperbolae in seismic images (Figure
2). It crops out only on the shallow banks within the Strait
and along coastal exposures. These exposures and its echo-character
suggest it is till and/or diamict, most likely remnant from the
latest (Fraser stade) glaciation. It is referred to locally as
Vashon till. The shallow banks in the Strait are likely erosional
remnants of drumlins and terminal- and side-moraines. The above
map shows these features are composed of Unit 2.
Unit 3 (Glacial marine)
Unit 3 offlaps and overlies unit 2
and is present almost everywhere in the subsurface (Figures
1-4). The unit consists of two distinct seismic facies: a
facies described as "bedded", displaying discontinuous to semi-continuous
reflectors at its base and progressively more coherent, higher
amplitude, closely spaced reflectors towards its top; and a facies
described as "massive", characterized by acoustic transparency
(weak internal reflectors), or sometimes a few widely spaced reflectors.
The thickness of the unit varies, but is typically 10 to 50 m-thick.
In some seismic sections, typically in waters shallower than 60
m, a prominent reflector truncates unit 3, indicating an erosional,
angular unconformity (Figure 3).
This unconformity is interpreted as being caused by erosion during
the last sea level recession following retreat of the Fraser stade
glaciers (Figure 6). The western
area of the map region is dominated by the massive facies. Core
information indicates unit 3 is glacial marine sediment deposited
between 13,690 ± 50 and 10,720 ± 50 14C yr-B.P. during ice recession.
This package is locally referred to as the Capilano Formation.
The echo-character of the unit, or its seismic facies, is a function
of the proximity of the deposit to the melting ice margin.
Unit 4 (modern sediments)
Unit 4 represents the uppermost seismic
unit. It overlies previous units in a variety of unconformable relationships,
including offlap, downlap, and non-conformable drape (Figures
3 and 4). As in unit 3, this unit is further subdivided into
two facies based on acoustic character: bedded and massive. The
bedded facies occur in near-shore areas, on the lower slopes of
banks and islands within the Strait, and in the local basins between
them. In these locations, it forms wedge-shaped deposits that downlap
onto unit 3 (Figures 3 and 4). It
is typically less than 20 m thick, but locally reaches 100 metres
thick near Whidbey Island. The bedded facies sometimes laterally
grades into the massive facies. This facies is typically 0 to 10
m thick, and often has large scale (> 1 m) bedforms on its surface.
Extensive sand wave fields are found in the western half of the
study area, and on some bank tops. The largest of the observed bedforms
occur in a sand dune field just east of Victoria. Individual dunes
at this location reach a maximum 25 m in height with 500 m wavelengths
(Figure 5). These features are among
the largest individual bedforms in the world. Off Esquimalt Harbour,
this unit is formed into a series of "barriers" which
are believed to represent sea level stands during the Holocene transgression
(Figure 7). This unit is locally
referred to as the Salish Formation. In the eastern Juan de Fuca
Strait, unit 4 is composed mostly of reworked glacial marine and
diamict sediments from shallow banks and coastal exposures.
Reference citation:
Hewitt,
A.T., and Mosher, D.C., 2000. Surficial geology 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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