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Surficial Geology Map
 
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Themes visible in the above map: SURFICIAL GEOLOGY, LATITUDE/LONGITUDE, COASTLINE and LAND. Descriptions of the units are available in the UNIT field of the SURFICIAL GEOLOGY theme.

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

123°45' 40' 35' 30' 25' 20' 15' 10' 05' 123°00' 55' 50' 45' 40' 35' 30' 25' 20' 15' 122°10'