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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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