SRATIGRAPHY
AND SEISMIC STRATIGRAPHY
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Several distinct stratigraphic units occur onshore in this portion of
the northern Puget Lowland (fig. 6). Four different units, distinguished
on the basis of stratigraphic position and seismic stratigraphic facies
(for example, Sangree and Widmier, 1977; Stoker and others, 1997), are
imaged on offshore seismic-reflection data. These include pre-Tertiary
basement rocks, Tertiary sedimentary rocks, uppermost Pliocene(?) to Pleistocene
strata, and uppermost Pleistocene to Holocene strata.
Pre-Tertiary basement rocks
Pre-Tertiary bedrock consisting of the Jurassic Fidalgo Complex, which
includes ophiolite and tectonically mixed, variably metamorphosed, sedimentary,
volcanic, and plutonic rock is exposed north of the Devils Mountain fault
in the Skagit River delta, on northern Whidbey Island and Fidalgo Island,
and in the San Juan Islands (Whetten and others, 1988; Tabor, 1994; fig. 2). Ultramafic and mafic rocks of the Fidalgo ophiolite form notable magnetic
highs beneath the Skagit River delta, northernmost Whidbey Island, Fidalgo
Island, and the eastern San Juan Islands (figs. 2, 3; Whetten and others,
1980). In contrast, Mesozoic sedimentary rocks commonly form subdued aeromagnetic
anomalies, such as on southern San Juan Island and southwestern Lopez
Island (fig. 3). One small outcrop of pre-Tertiary rocks is exposed south
of the Devils Mountain fault on Whidbey Island at Rocky Point (fig. 2).
Pre-Tertiary bedrock, consisting of the Mesozoic Leech River complex and
Paleozoic to Mesozoic volcanic, plutonic, and metamorphic rocks, also
underlies southern Vancouver Island north of the Leech River fault (fig.
1; Roddick and others, 1979). Pre-Tertiary rocks are generally nonreflective
and represent acoustic basement on seismic-reflection profiles.
Tertiary sedimentary rocks
Tertiary sedimentary rocks crop out adjacent to the Devils Mountain fault
east of northern Whidbey Island within the Skagit River delta and in the
Cascade Range foothills (Whetten and others, 1988; fig.
1). Units include the Eocene, nonmarine Chuckanut Formation (Johnson,
1984a; Evans and Ristow, 1994) and the upper Eocene to lower Oligocene
marine to marginal marine rocks of Bulson Creek (Marcus, 1980) (fig.
6). These rocks are cut by numerous west- and northwest-trending faults
and are gently to steeply folded and locally overturned (Whetten and others,
1988; Dragovich and others, 2000). On industry seismic-reflection profiles,
Tertiary strata are characterized by relatively continuous, high-amplitude,
parallel to subparallel, moderate frequency reflections and can thereby
generally be distinguished from nonreflective pre-Tertiary basement.
Uppermost Pliocene(?) to Pleistocene strata
Onshore stratigraphy
Uppermost Pliocene(?) to Pleistocene deposits of the Puget Lowland 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 glacial drift units, three of which
are exposed on Whidbey Island (fig. 6). In the northern Puget Lowland,
glacial drift typically consists of till, outwash, and glaciomarine deposits.
Interglacial deposits are typically fluvial and deltaic, including peat.
The Double Bluff Drift is the oldest Pleistocene unit recognized on Whidbey
Island (Easterbrook, 1968, 1994b). It does not crop out on the northern
Whidbey Island area of this investigation, where it is inferred to occur
below sea level on the basis of stratigraphic correlation diagrams derived
from water-well logs. Deposition of the Double Bluff Drift is thought
to have occurred between about 250 and 130 ka (Blunt and others, 1987;
Easterbrook, 1994b) and probably coincides approximately with marine isotope
stage 6 (for example, Shackleton and others, 1983; McManus and others,
1994; Kukla and others, 1997).
The interglacial alluvial- and delta-plain deposits of the Whidbey Formation
(Easterbrook and others, 1967; Easterbrook, 1968, 1969; Heusser and Heusser,
1981) overlie the Double Bluff Drift on Whidbey Island. The Whidbey Formation
is locally more than 60 m thick and is widespread in the central and northern
Puget Lowland (Easterbrook, 1968, 1994b). Amino-acid analyses of shells
and wood at 13 localities suggest an age of 100±20 ka for the Whidbey
Formation; four thermoluminescence ages range from 102±38 to 151±43
ka (Blunt and others, 1987; Berger and Easterbrook, 1993). These dates
indicate a correlation with interglacial marine isotope stage 5 (for example,
Shackleton and others, 1983; McManus and others, 1994; Kukla and others,
1997), and pollen data indicate internal climatic fluctuations occurred
during its deposition (Heusser and Heusser, 1981; T. A. Ager, written
commun., 2000). We therefore infer that the top of the Whidbey Formation
(where it is not significantly eroded) has an age corresponding to the
end of this stage (and not the age of a stage 5 substage), about 80 ka.
Our investigations suggest that the top of the Whidbey Formation can be
recognized and traced in the northern Whidbey Island area using water-well
logs (see “Evidence for Faulting Onshore, Whidbey and Camano Islands”),
and therefore provides a potential marker for estimating both amounts
and rates of recent deformation.
The Possession Drift discontinuously overlies the Whidbey Formation (fig. 6). At its type locality on southern Whidbey Island, it thins laterally
through a distance of about 400 m from a thickness of 25 m to an unconformity
between the Whidbey Formation and younger sediments (Easterbrook, 1968).
Stratigraphic correlation diagrams from northern Whidbey Island based
on water-well logs (see “Evidence for Faulting Onshore, Whidbey
and Camano Islands”) suggest a similarly variable occurrence and
thickness for this unit. Radiocarbon dates from the Possession Drift yield
infinite ages (> ~45,000 14C yr B.P.); amino acid analyses of marine
shells in Possession Drift at several localities suggest an age of about
90-75 ka (Blunt and others, 1987). Based on these ages and those from
underlying and overlying strata, the Possession Drift appears correlative
with marine isotope stage 4 (for example, Shackleton and others, 1983;
McManus and others, 1994) and was probably deposited between about 80
and 60 ka .
The informally named nonglacial alluvial- and deltaic Olympia beds locally
overlie Possession Drift or older deposits in the central and northern
Puget Lowland (fig. 6). Outcrop and water-well data (see “Evidence
for Faulting Onshore, Whidbey and Camano Islands”) indicate these
strata have variable thickness (as much as ˜ 25 m) and are locally
absent on northern Whidbey Island. To the northeast in the Swinomish Island-Skagit
River delta area, Dragovich and Grisamer (1998) and Dragovich and others
(1998) suggested that Olympia beds are as thick as 50 m based on interpretations
of subsurface data. Olympia beds were probably deposited between about
60 and 17 ka (Fulton and others, 1976; Blunt and others, 1987; Clague,
1994; Easterbrook, 1994b; Porter and Swanson, 1998; Troost, 1999). Hansen
and Easterbrook (1974) and Easterbrook (1976) described a thin sequence
of inferred glacial deposits within the Olympia beds at Strawberry Point
(fig. 2), which they consider evidence for the ˜34,000-40,000 14C
yr B.P. “Oak Harbor stade of the Possession glaciation.” Fulton
and others (1976) and Clague (1978) argued, however, that an ice advance
at this time was unlikely because of a lack of evidence to the north in
southern British Columbia for correlative ice occupation.
Strata associated with the Fraser glaciation on northern Whidbey Island
include advance outwash of the Esperance Sand Member of the Vashon Drift,
the main body of the Vashon Drift, and recessional deposits of the Everson
Drift (Easterbrook, 1968). These units have variable thickness but collectively
are commonly 50-100 m thick or more. Radiocarbon dating suggests that
this stratigraphic “package” in the northern Whidbey Island
area was deposited between about 17,000 and 12,000 14C yr B.P. (Easterbrook,
1968, 1969; Blunt and others, 1987; Dethier and others, 1995; Porter and
Swanson, 1998). Hicock and Armstrong (1981) presented evidence for a ˜21,000
14C yr B.P. pre-Vashon, early Fraser till-bearing unit (Coquitlam Drift)
to the north in southern British Columbia, but ice from this early Fraser
stade apparently did not reach the northern Whidbey Island area.
Offshore stratigraphy
Two distinct seismic units occur above pre-Tertiary or Tertiary “basement”
in the eastern Strait of Juan de Fuca. On both industry and higher resolution
seismic-reflection data, the lower of these two units consists of seismic
facies typical of glacial deposits (Davies and others, 1997). Characteristics
include discontinuous, variable-amplitude, parallel, divergent, and hummocky
reflections, with common internal truncation, onlap, and offlap of reflections.
Based on this seismic facies and on stratigraphic position, this seismic
unit is inferred to comprise uppermost Pliocene(?) to Pleistocene deposits
(excluding uppermost Pleistocene postglacial deposits). Given the physiography
of the eastern Strait of Juan de Fuca, it is likely that much of this
unit consists of recessional glaciomarine drift. Present-day seafloor
morphology is largely governed by these drift deposits (Hewitt and Mosher,
2001; Mosher and others, 2001). We did not recognize any internal sequences
within the inferred uppermost Pliocene(?) to Pleistocene section that
could be traced across the region and might correlate with eustatic fluctuations
associated with multiple glacial and nonglacial intervals. We attribute
this lack of internal stratigraphy to irregular and large-scale glacial
erosion and deposition.
Identification and age of the base of the uppermost Pliocene(?)
to Pleistocene section
The base of the inferred uppermost Pliocene(?) to Pleistocene seismic
unit is typically most distinct on conventional industry seismic-reflection
data (fig. 5b), where it is recognized on the basis of contrasts in seismic
facies (Johnson and others, 1996, 1999). As described in the preceding
paragraph, these mainly Quaternary strata are generally characterized
by low- to moderate-amplitude, discontinuous to continuous, irregular
hummocky, divergent, and parallel reflections, with common internal truncation,
onlap, and offlap. In contrast, pre-Tertiary rocks are nonreflective,
and 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.
On high-resolution seismic-reflection profiles (fig.
5a, c), this seismic-unit
contact and the contrast in seismic facies between older “basement”
and mainly Quaternary strata are generally less distinct. For these profiles,
the location of the contact is generally based on projection from nearby
conventional industry profiles (fig. 5b) or
on the basis of locally distinct unconformities and onlapping surfaces.
Once the contact at the base of the mainly Quaternary section is identified
at one or more locations on individual high-resolution profiles, it
can generally be traced across the profile based on reflection continuity.
Complete regional coverage is accomplished by thus iteratively combining
the industry and high-resolution data. Figure 7 is a contour map based
on these data that shows the depth to the base of the uppermost Pliocene(?)
to Pleistocene section in the northeastern Strait of Juan de Fuca.
Knowing the age of the base of the uppermost Pliocene(?) to Pliocene section
in offshore data is important because it provides a potential marker for
estimating rates of deformation. Determining this age is, however, problematic.
No boreholes have penetrated the submerged section, and multiple pulses
of deep subglacial scour and subsequent filling in offshore areas (for
example, Booth, 1994) suggest that the age of the surface may vary locally
and that correlation with adjacent dated units on land is untenable. For
this investigation, we infer that this surface has a maximum age of ˜2
Ma, coinciding with the age of the first glaciation for which there is
evidence in the Puget Lowland (Easterbrook, 1994a, b), slightly older
than the Pliocene-Pleistocene boundary (Gradstein and Ogg, 1996). However,
because of repeated deep glacial erosion, we think it probable that the
oldest deposits in this unit imaged on many seismic-reflection profiles
are much younger.
Uppermost Pleistocene and Holocene strata
Variable-amplitude, parallel, and continuous reflections that fill in
local basins bounded by Pleistocene bathymetric highs characterize the
uppermost seismic unit recognized in the eastern Strait of Juan de Fuca.
East of Whidbey Island, these sediments are inferred to be clay and silt
derived from the Skagit River (fig. 2). In the
eastern Strait of Juan de Fuca where there are no major inputs of postglacial
terrestrial sediment, the uppermost Pleistocene to Holocene basin fill
probably consists largely of glacial recessional deposits variably reworked
by strong tidal currents.
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