Depth to the base-of-Quaternary
in the eastern Juan de Fuca Strait region
Samuel Y. Johnson, Susan B.
Rhea, David C. Mosher, Robert B. Kung, and Shawn V. Dadisman
This map shows the depth to the base
of Quaternary strata in the eastern Juan de Fuca Strait region
and is a companion to the map showing the thickness of Quaternary
strata. For offshore areas, this value is determined entirely
on the basis of interpretation of seismic-reflection data. These
data include high-resolution and conventional seismic-reflection
surveys of the U.S. Geological Survey and Geological Survey of
Canada as well as parts of four conventional industry seismic-reflection
surveys. Tracklines and data-acquisition
parameters are described elsewhere in this report. Examples of
seismic reflection data from which these maps were generated can
be accessed through the trackline or
the structure map sheets or by activating
the following links: PGC96006,
SHIPS, USGS
and Industry. For onshore areas, we relied on geologic
mapping of Tabor and Cady (1978), Roddick
and others (1979), Muller (1983), and
Whetten and others (1988), and on logs of a
few deep boreholes (Brocher and Ruebel, 1998;
Rau and Johnson, 1999). North and northeast
of the southern Whidbey Island and Leech River faults (Neotectonics
Map), Quaternary strata are mainly underlain by pre-Tertiary
meta-sedimentary, volcanic, plutonic, and metamorphic rocks. South
of these faults, Quaternary strata are underlain by Tertiary sedimentary
and volcanic rocks (Roddick and others, 1979;
Johnson and others,1996). The base of Quaternary
strata is typically most distinct on conventional industry seismic-reflection
data, where it is recognized on the basis of contrasts in seismic
facies (e.g., Mitchum and others, 1977; Johnson
and others, 1996).
Quaternary strata are generally characterized
by low to moderate amplitude, discontinuous to continuous, irregular
hummocky, divergent, and parallel reflections. Internal truncation,
onlap, and offlap of reflections are all common. In contrast,
reflections from underlying Tertiary strata have higher amplitude,
are more continuous, and are typically parallel to subparallel.
Where Tertiary rocks are folded, this contact is generally an
angular unconformity that may pass laterally into a disconformity.
Underlying pre-Tertiary rocks are generally non-reflective. Examples
of conventional seismic-reflection data that show the base-of-Quaternary
horizon in the eastern Juan de Fuca Strait are in Johnson
and others (1996) and Figures
7, 8, and 14.
On high-resolution seismic-reflection
profiles, this contact and the contrast in seismic facies between
Tertiary and Quaternary strata may not be as distinct (e.g., Figures
3, 6, 9,
13, and 16).
For these profiles, the location of the contact is generally based
on projection from nearby conventional industry profiles or on
the basis of locally distinct unconformities and onlapping surfaces.
Once the contact at the base of the Quaternary was identified
at one or more locations on individual high-resolution profiles,
it was traced across the profiles to yield fairly complete regional
coverage.
This map shows both the onshore and
offshore data points used to generate contour surfaces. For onshore
areas, data points are mainly the boundaries of exposed bedrock
and depths from a few deep boreholes. For offshore data points,
we converted two-way travel time to depth by assuming velocities
of 1500 m/sec for the water column and 1800 m/sec for Quaternary
strata. The data were contoured using the following smoothed inverse-distance
weighting function with a distance decay value of d-5, a radius
of 12 km, and a cell size of 100 m:

In this function, z = the elevation
of the base of the Quaternary section, d = distance to a data
point, i = distance to all data points within the radius, and
r = radius.
Quaternary deposits in the Puget Lowland
of western Washington comprise a stratigraphically complex basin
fill of glacial and interglacial deposits that are locally as
thick as 1,100 m (Yount and others, 1985;
Jones, 1996). Easterbrook
(1994a, b) described six distinct glacial-drift units, three
of which are exposed on Whidbey Island. Onland glacial and interglacial
strata in the eastern Juan de Fuca Strait region consist mainly
of till, fluvial channel and floodplain deposits, and coarse-
to fine-grained glaciomarine deposits. Given the physiography
of the eastern Juan de Fuca Strait, it is likely that much of
the Quaternary strata imaged on offshore seismic-reflection profiles
consists of recessional glaciomarine drift. Present day seafloor
morphology is largely governed by these drift deposits (Mosher
and Johnson, 2000). Many of the Quaternary stratigraphic units
exposed onshore are well dated (Yount and others,
1980; Easterbrook, 1994a, b), however
determining the age of the Quaternary deposits imaged on seismic-reflection
data in Puget Lowland waterways is problematic. No boreholes have
penetrated the submerged section, and multiple pulses of subglacial
scour and subsequent filling make correlation with adjacent, dated
units on land untenable. We did not recognize specific sequences
in the offshore Quaternary data that could be correlated with
different glaciations, and we think it likely that much of the
Quaternary strata in the subsurface of the eastern Juan de Fuca
Strait was deposited during retreat of the most recent (Fraser)
glaciation.
The depth to base of Quaternary is
inferred to be controlled by both tectonic structures and large-scale
erosion and deposition associated with multiple Quaternary glaciations.
Maximum depths occur in two locations. The first area is beneath
Camano Island in the southeastern part of the map region, where
Quaternary strata reach a thickness of more than 1,000 m. This
trough is part of the Everett basin (Johnson
and others, 1996), a structural low bounded by the southern
Whidbey Island fault to the southwest and the Strawberry Point
and Utsulady Point faults (Johnson and others,
2000) to the north. This area corresponds to both a gravity
and aeromagnetic low (Gravity Map, Aeromagnetic
Map). The second area where the depth to the base of the Quaternary
reaches as much as 1,000 m lies in the southeastern Juan de Fuca
Strait, north of the Miller Peninsula and eastern Quimper Peninsula.
This trough lies southwest of and within the northwest portion
of the southern Whidbey Island fault zone and is cut by several
northwest-trending faults (Neotectonics Map).
This region overlies an aeromagnetic high (Aeromagnetic
Map) and does not form a distinct gravity anomaly (Gravity
Map). Thus it appears that this region is underlain partly
by relatively shallow, dense magnetic rocks (inferred Crescent
Formation) and is not a long-lived structural low like the Everett
basin. Instead, the increased depth to the base of the Quaternary
section in this area suggests either a site of locally enhanced
glacial erosion and (or) a structural inversion.
With the exception of the Whidbey
and Camano Island areas, the depth to the base of the Quaternary
section is less in onshore areas than in offshore areas. Depths
in the offshore area north of the Devils Mountain, Strawberry
Point, and Utsulady Point faults (Neotectonics
map; Johnson and others, 2000) are
notably lower than in areas south of the fault, strongly suggesting
Quaternary structural control.
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References
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Reference citation:
Johnson, S.Y., Rhea, S.B., Dadisman, S.V., and Mosher, D.C., 2000.
Depth to the base-of-Quaternary in the eastern Juan de Fuca Strait
region, 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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