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| Professional Paper 1643: EVIDENCE FOR FAULTING ONSHORE, WHIDBEY AND CAMANO ISLANDS |
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The approximate locations of the Devils Mountain, Strawberry Point, and Utsalady Point faults on Whidbey Island are inferred based on projections from offshore seismic-reflection data (figs. 8-26), aeromagnetic data (figure 3), and onshore geologic mapping in the Skagit delta (Whetten and others, 1988). With this information, we examined exposures (figure 27) and subsurface data (figure 28, table 1) to more precisely locate faults and constrain fault histories, lengths, and slip rates. Additionally, virtually all coastal-bluff exposures on Whidbey Island and on the northeastern Olympic Peninsula have been examined for this and previous investigations (Johnson and others, 1996, among others) to provide a framework for recognizing structural deformation in Quaternary strata.
Subsurface information comes from lithologic logs of water wells (figure 28; table 1). Ground water is the primary source of water used on Whidbey and Camano Islands, and ground-water resources have been studied extensively (for example, Anderson, 1968; Cline and others, 1982; Jones, 1985; Sapik and others, 1988), with a focus on aquifer properties, subsurface stratigraphy, and saltwater intrusion. For this study, we used a data base, maintained by the Island County Health Department, that includes logs for more than 3,500 water wells. Stratigraphic interpretation of these logs can be problematic because (1) most wells are quite shallow and were logged by non-geologists, and (2) outcrops of Quaternary strata throughout the Puget Lowland reveal that substantial erosional relief can occur within and at the upper and lower contacts of glacially influenced depositional units (figure 6). Despite these potential problems, the water-well data base can have great value in stratigraphic investigations when (1) the lithologic logs are carefully selected based on quality of logging, (2) a large proportion of deeper wells (> 50 m) which penetrate several stratigraphic units are used; (3) the number of wells used for correlation on stratigraphic diagrams is large enough to mitigate problems associated with variable erosional relief and with poor logging in a few wells, and (4) information from boreholes can be correlated with nearby outcrops (figure 27). Table 1 is a list of wells (including depth and location) used to draw stratigraphic correlation diagrams for this investigation.
The Devils Mountain fault zone is not exposed on Whidbey Island. Based on projection of seismic-reflection data (figs. 8, 10), the southern fault of the zone (inferred to be the most active strand and the northern margin of the Everett basin) extends through an area characterized by large landslides and variable exposure on Whidbey Island’s east coast and a marshy lowland on the island’s west coast. Inland, the projected “basin-margin” fault strand is covered by vegetation and surficial deposits developed mainly on Vashon Drift (Pessl and others, 1989). No obvious topographic lineament is associated with the fault where it crosses Whidbey Island, in contrast to the conspicuous trace of the fault in the Cascade Range foothills to the east (figure 4).
The projection of the southern “basin-margin” fault in the Devils Mountain fault zone across Whidbey Island is on strike with a strand of the Devils Mountain fault mapped in the Skagit delta to the east, north of Goat and Ika Islands in Skagit Bay (figs. 2, 27; Whetten and others, 1988). Goat and Ika Islands are underlain by bedrock, however, and clearly lie north of the northern margin of the Everett basin. Moreover, an aeromagnetic anomaly that defines the basin margin (figure 3) in Skagit Bay lies just south of these islands, on the southern margin of a small, prominent, shallow (< 350 m), high-amplitude, northwest-trending aeromagnetic anomaly interpreted as mafic pre-Tertiary rock. Thus, the “basin-margin” fault appears to step south about 1,000 m in Skagit Bay. This step probably coincides with the steep gradient on the northeast margin of the high-amplitude aeromagnetic anomaly, and may represent a transfer zone or tear fault in the fault system. In this scenario, the inferred mafic rock is a basement block uplifted within this transfer zone.
Pleistocene strata are exposed near the fault along the east coast of Whidbey Island. Locality a (figure 27), south of the fault trace, consists of three outcrops exposed within 100 m in coastal bluffs or on the beach at low tide. Beds consisting of sand, organic-rich mud, and gravel strike N.25°-35° E. and dip 25°-36° NW. Detrital wood yielded an infinite radiocarbon date (>43,510 14C yr B.P.; SJ-00-7; table 2). About 200 m farther north (b in figure 27), similar sand and gravel facies exposed in a landslide scarp strike 70° and dip 17° NW.
About 300 m north of the inferred fault trace (c in figure 27), Quaternary strata exposed in the upper part of steep coastal bluffs are subhorizontal (figure 29). The lowest beds at this locality consist of interbedded fine to coarse sand (plane-bedded and crossbedded), laminated silty mud, and peat. Based on their sedimentology, these strata are interpreted as interglacial alluvial-plain deposits. The peat yielded radiocarbon dates of >48,480 14C yr B.P. (SJ-98-17; table 2) and >40,130 14C yr B.P. (SJ-00-1; table 2). These strata, and the peat-bearing strata exposed at a, are correlated with the Whidbey Formation, the oldest known interglacial alluvial-plain deposits recognized on Whidbey Island and in the northern Puget Lowland (Easterbrook, 1994a, b). The correlation is based on three factors:
1. The lithology and sedimentology of these strata are identical to those of well-documented Whidbey Formation localities elsewhere on Whidbey Island (see, for example, Easterbrook, 1968, 1994b; Stoffel, 1981).
2. Subsurface data (figure 30, see next section) from this local northern Whidbey Island area show that strata occurring at similar elevations and with similar lithology are overlain by two units of interbedded “hardpan” (the term drillers use for dense clay-rich diamict), gravel, and sand interpreted as glacial drift. The Whidbey Formation predates both the Possession and Vashon Drifts, whereas the younger nonglacial Olympia beds predate only the Vashon Drift (figure 6). At c, the Whidbey Formation is overlain by crudely stratified gravel assigned to the Possession Drift and interbedded sand (parallel bedded and crossbedded) and silt assigned to the nonglacial Olympia beds (figure 6).
3. Peat from these strata is too old (> 48,480 14C yr B.P.) to yield reliable radiocarbon ages ( table 2), consistent with previous dating results from the Whidbey Formation (Blunt and others, 1987). In addition to the dates reported here, J.D. Dragovich (written commun., 2000) obtained four radiocarbon dates from the same unit at four localities within 2.5 km north of location c (figure 27) along Whidbey Island’s northeast coast. Two dates from wood samples similarly yielded infinite ages (> 43,790 14C and >37,100 14C yr B.P.), whereas an organic silty sand sample yielded a finite age of 41,380± 2,150 14C yr B.P. and organic material in silt yielded a finite age of 40,590 ± 530 14C yr B.P. These two finite ages from organic sediment are very close to the practical limit of radiocarbon dating, and they may have been contaminated by minute amounts of younger carbon.
East-west stratigraphic correlation diagrams on the north and south sides of the inferred trace of the Devils Mountain fault on northern Whidbey Island (locations, figs. 27, 28) are shown in figure 30. The diagram north of the fault includes 19 wells within 1.4 km of the inferred fault trace and one surface outcrop (figs. 29, 30A). Stratigraphic units inferred on the correlation diagram include pre-Whidbey Formation Pleistocene deposits, the Whidbey Formation, Possession Drift, Olympia beds, Esperance Sand Member, main body of Vashon Drift, and Everson Drift (figure 6). Lateral stratigraphic variation across this diagram probably reflects irregular scour and fill associated with the Possession and Vashon glaciations, and possible faulting within the block north of the Devils Mountain fault (figure 8A), but it may also reflect incorrect stratigraphic correlation. Interpretations on the correlation diagram south of the Devils Mountain fault (figure 30B), which includes data from 27 wells and one outcrop proximal to the inferred fault trace, suggest greater lateral stratigraphic continuity.
The stratigraphic correlation diagrams (figure 30A, B) were generated to provide constraints on vertical offset across the Devils Mountain fault. In this regard, lateral variability and potentially uncertain correlation of units above the Whidbey Formation make their utility as deformation markers problematic. The top of the interglacial Whidbey Formation is inferred to provide the most useful datum for the following reasons:
1. The top of the Whidbey Formation is a distinct contact in both outcrops (for example, Easterbrook, 1968) and subsurface data (figure 30), generally characterized by an abrupt upward transition from interbedded sand, silt, clay, and peat to overlying interbedded gravel, sand, and “hardpan.”
2. The Whidbey Formation is relatively thick and widespread on Whidbey Island (Easterbrook, 1968), and is present in every well that reaches its stratigraphic level. This occurrence reflects its deposition during a period of relatively high sea level (marine isotope stage 5; Muhs and others, 1994; Pillans and others, 1998), a condition which generates significant stratigraphic “accommodation space” (Jervey, 1988; Posamentier and others, 1988) in which sediments can accumulate and have high potential for preservation in the stratigraphic record. In contrast, younger (marine isotope stage 3) nonglacial Olympia beds (figure 6) are absent (due to nondeposition or erosion) in several wells and in most outcrops on Whidbey Island. Sea level at this time was much lower than at present and in stage 5 (Pillans and others, 1998), significantly limiting both stratigraphic accommodation space and preservation potential.
3. As an alluvial-plain deposit, the top of the Whidbey Formation was a nearly horizontal surface across northern Whidbey Island following deposition. At the end of Whidbey deposition, paleo-relief on this surface in local areas like this small part of northern Whidbey Island would have been at most a few meters, reflecting erosion and deposition associated with fluvial channels and levees. Modern alluvial-delta plains of the Puget Lowland (such as the Skagit delta a few kilometers east of Whidbey Island) provide a useful analog.
4. In both correlation diagrams, the top of the Whidbey Formation is interpreted as a gently undulating though relatively horizontal surface; thus erosion into the Whidbey Formation by advance outwash facies of the Possession Drift appears to have been fairly limited and (or) uniform across this small area of northern Whidbey Island. Widespread preservation of this relatively horizontal surface in areas not within fault zones on northern Whidbey Island is documented by two stratigraphic diagrams in Easterbrook (1968; his figure 15 and plate 1-B) from the coastal bluffs south of Swantown (d1 and d2 in figure 27). These correlation diagrams show the top of the Whidbey Formation as a nearly horizontal surface for 4.6 km (elevation of 17 m) and 3.5 km (elevation of 10 m) of continuous exposure. However, elsewhere (such as f in figure 27), there is locally as much as 20 m of erosional relief on the top of the Whidbey Formation.
On the northern correlation diagram (figure 30A), our interpretation is that the top of the Whidbey Formation was penetrated by 14 wells for which it has a maximum elevation of 38.4 m and a mean elevation of 26.6 ± 5.8 m. On the southern correlation diagram (figure 30B), our interpretation is that the top of the Whidbey Formation was penetrated by 16 wells for which it has a maximum elevation of 26.9 m and a mean elevation of 14.7 ± 6.0 m. The elevation south of the Devils Mountain fault is similar to that observed in outcrops south of Swantown (d1 in figure 27; Easterbrook, 1968), whereas this contact is much higher north of the Devils Mountain fault. The difference between the maximum and mean elevations on the two correlation diagrams is 11.5 m and 11.9 m, respectively. The mean and standard deviation of the sum and difference of the mean depths (Spiegel, 1961) is 11.9 m ± 8.3 m. If one assumes that the amount of erosional relief on this datum is similar north and south of the projected fault trace, then these values provide a rough estimate for the amount of vertical separation along the Devils Mountain fault since deposition of the top of the Whidbey Formation.
There are no exposures where the trace of the Strawberry Point fault projects onto the west coast of Whidbey Island, about 1 km north of Rocky Point (figs. 27, 28). North of the projected trace, no exposures exist for several kilometers. Within about 1 km south of the inferred fault trace, Pleistocene glacial deposits occur in discontinuous bluff exposures, and pre-Tertiary bedrock and Holocene peat crop out at beach level at Rocky Point (g in figure 27). The bedrock consists of Upper Cretaceous graywacke, argillite, greenstone, and chert (Whetten and others, 1988) and is extensively faulted and foliated. These rocks have surprisingly low bulk magnetic susceptibility - 0.53 X 10-3 (SIU) based on 30 in-place measurements - and form a distinct and continuous west-northwest-trending aeromagnetic low over Rocky Point (figure 3). In contrast, Quaternary deposits that crop out adjacent to bedrock exposures yield susceptibilities that are roughly an order of magnitude higher.
Pleistocene strata (2 to 15-20 m thick) that overlie the bedrock at g (figure 27) consist of horizontal to very gently dipping (0°-2°) Everson Drift (figure 6) that yielded a 14C date on shells of 13,595 ± 145 14C yr B.P. (Pessl and others, 1989). Older Pleistocene strata, more than 450 m thick elsewhere on northern Whidbey Island (as in well 101, figure 28, table 1), are absent at this locality.
A woody peat is exposed south of the inferred fault trace at locality h (figure 27) for about 200 m along the beach at very low tides (< 40 cm below mean low low water). A fragment of tree branch (2 cm diameter) from this peat yielded a 14C date of 1,750 +/- 50 14C yr B.P. (SJ-98-14; table 2). This peat includes rhizomes of either seacoast bulrush (Scirpus maritimus) or hardstem bulrush (Scirpus acutus), both of which generally occur in the middle and upper parts of brackish or salt marshes (elevation > tide of 100 cm) (Cooke, 1997). The presence of these rhizomes is consistent with a relative sea-level rise of about 1.5 to 3 m since about 1.7 ka at this locality. This rise supports regional investigations (for example, Clague and others, 1982; Eronen and others, 1987) that suggest rates of sea-level rise of 1 mm/yr or less for the last few thousand years in Puget Sound and southern British Columbia.
There are an insufficient number of deep water wells flanking the Strawberry Point fault in this area to allow construction of well-constrained stratigraphic correlation diagrams either parallel to (as in figure 30) or across the fault in order to constrain fault slip.
Seismic-reflection data (figs. 8, 10, 13) suggest that the Strawberry Point fault changes from a discrete fault into a broad fault zone as it crosses Whidbey Island from west to east (figs. 2, 27, 28). On central Whidbey Island, the trace of this zone is covered by vegetation and surficial deposits developed mainly on Vashon Drift and recessional outwash (Pessl and others, 1989). There are no obvious topographic lineaments associated with the fault zone on existing topographic maps in this area. Furthermore, the number of deep water wells flanking the Strawberry Point fault in this area is insufficient to construct stratigraphic correlation diagrams either parallel to or across the fault in order to constrain fault location or slip. The location of the fault zone and its components on central Whidbey Island is thus somewhat speculative, and is based primarily on aeromagnetic data (figure 3; Blakely and Lowe, 2001).
Seismic-reflection data (figure 8) suggest that the Strawberry Point fault is a zone comprising several splays along Strawberry Point on the east coast of Whidbey Island. Within this zone, there is significant deformation of upper Pleistocene strata discontinuously exposed in coastal bluffs (figure 31 ). There are also a significant number of lithologic logs from water wells adjacent to the coastline (table 1, figure 28). Together, the outcrops, lithologic logs, and seismic-reflection profiles provide the data needed for a stratigraphic correlation diagram across Strawberry Point that provides some constraints on fault-zone interpretation (figure 32 ).
Easterbrook (1968), Hansen and Easterbrook (1974), and Blunt and others (1987) have described aspects of the stratigraphy and palynology of the nonglacial Olympia beds exposed at Strawberry Point, and reported dates ranging from 22,700 to 47,600 14C yr B.P. The unit consists mainly of mud and peat but also includes coarse sand, gravel, and redeposited peat that may have been deposited in Skagit River floodplains and paleochannels, graded to a lower sea level. Younger deposits (Esperance Sand Member and main body of Vashon Drift) locally overlie these strata in the coastal bluffs (figure 31 A). The nonglacial Olympia beds are underlain by a variably thick unit of diamict, gravel, and sand assigned to the Possession Drift, in turn underlain by a fine-grained unit consisting of interbedded ripple- and plane-laminated sand, silty mud, and peat. This lower fine-grained unit occurs in the axis of a gentle anticline on the southeast coast of Strawberry Point and in faulted and folded strata on the northeast coast. These older strata are assigned to the Whidbey Formation because (1) radiocarbon dates yield infinite ages (see next paragraph and table 2), (2) they underlie the well-dated Olympia beds and Possession Drift (figure 6), (3) lithologies are similar to those described for the Whidbey Formation at nearby Whidbey Island localities (e.g., Easterbrook, 1968), and (4) no similar nonglacial units older than the Whidbey Formation have been identified in this part of the Puget Lowland. Pebbly diamict that underlies the Whidbey Formation in Strawberry Point water wells is considered pre-Whidbey Formation glacial drift (figs. 6, 32).
Figures 27 and 28 show where the four faults identified on seismic-reflection data (figure 8) are projected to intersect the coastline. South of fault 1, the exposed Pleistocene section (figure 31 A) is folded in a gentle anticline with the Whidbey Formation in its core at sea level, an interpretation supported by well data (figure 32 ). Outcrops of Whidbey Formation and Possession Drift on the south limb of the fold dip 5°, and outcrops of Possession Drift and nonglacial Olympia beds on the north limb of the fold (in figure 27) dip 7°. Peat in the Whidbey Formation exposed at beach level at i yielded a radiocarbon date of >44,320 14C yr B.P. Both low-tide beach exposures and the Figure 32 correlation diagram suggest that dips on the north limb of the fold may be steeper than 7° in the Whidbey Formation and older strata, but this interpretation is based on outcrop with minimal stratigraphic context and projection of only one well (No. 65), and is thus tentative. The section exposed at sea level on the north limb of the fold extends upward from the Whidbey Formation through thin Possession Drift into well-dated, nonglacial, peat-bearing Olympia beds (Hansen and Easterbrook, 1974) and is about 30 m thick, providing a minimum estimate for the amount of vertical fold growth since the top of the Whidbey Formation was deposited. For this investigation, we obtained radiocarbon dates of 37,190 ± 550 at i, 29,270 ± 340 14C yr B.P. at j and 25,770 ± 310 14C yr B.P. at k from the Olympia beds (figure 27). Exposures of the Olympia beds are flat-lying and nearly continuous north of the gentle fold and south of the projected trace of fault 1 (from j to k in figure 27).
The projected traces of both faults 1 and 2 are covered by vegetation. However, between these faults, flat-lying massive diamict and crudely stratified gravel and sand crop out extensively (l1 in figure 27) in low coastal bluffs, with a maximum exposed thickness of 5-6 m. These strata have an inferred glacial origin based on their texture, and underlie a fine-grained interval in a nearby water well (64, figs. 28, 32; table 1). In the simplest interpretation, the strata in these outcrops are Possession Drift and the overlying fine-grained strata correlate with the Olympia beds. In this scenario (hypothesis 1), vertical offset on fault 1 has raised the basal contact of the Olympia beds about 3-9 m on the north side of the fault. The vertical separation on the top of the older Whidbey Formation based on well logs (figs. 28, 32), also south side down, is about 10-15 m. Alternatively, the coarse-grained section that occurs at sea level between faults 1 and 2 could underlie the Whidbey Formation. In this less likely scenario, south-side-down vertical separation on fault 1 increases to about 40–60 m.
Fault 2 coincides with a significant dip change in coastal outcrops. The last exposure south of the fault (l2, ≈20 m south of the fault shown in figs. 27 and 28) consists of flat-lying, crudely stratified sand and gravel assigned to the Possession Drift. These strata are cut by a thrust fault that strikes N. 77° E., dips 45° NW., and has apparent displacement of 1-2 m (figure 31 B).
The first exposures north of fault 2 (m1 in figs. 27, 32) consist of a continuous 33-m-thick section of sand, silt, and peat assigned to the Whidbey Formation that strike N. 35° E., and dip 42°-48° NW (figure 31 C). Peat from these strata yielded a radiocarbon date of > 44,000 14C yr B.P. (figure 27, table 2). The section is cut by numerous fractures that generally trend NE. (10°-50°) and dip steeply (>70°) to the southeast. Offsets of 1-15 cm are common on these fractures, and range up to 100 cm. These Whidbey Formation strata are overlain by a 10-m-thick covered interval, then a continuous 60-m-thick section of sand and gravel assigned to the Possession Drift. These beds have the same structural attitude as the underlying Whidbey Formation and form part of one continuous northwest-dipping stratigraphic section that is more than 100 m thick and extends from the base of the coastal bluff to elevations of 15-20 m on the bluff face. Farther north between faults 2 and 3, sand and gravel outcrops (m2 in figure 32 ) have similar structural attitude, appear continuous with the underlying section, and are also correlated with the Possession Drift. The thickness of strata between faults 2 and 3 is not accurately portrayed in figure 32, which plots the distribution of strata relative to the elevation at which they occur.
The outcrops on opposite sides of the projection of fault 2 suggest a minimum of about 10 m of north-side-up vertical separation along the structure. Determining a more precise value for this separation is problematic for three reasons: (1) Uncertainty exists in the correlation of the section that occurs between faults 1 and 2; (2) Well data are lacking in the critical area just south of the fault (between wells 61 and 62 in figure 32 ), thus the thickness of Possession Drift and the elevation of the top of the Whidbey Formation immediately south of fault 2 are unknown; (3) Projecting outcrop dips into the subsurface cannot be done with certainty. Cumulative vertical separation on faults 1 and 2 on the top of the Whidbey Formation thus appears to be a minimum of 20-25 m.
The projected trace of fault 3 is also covered. Three outcrops of Pleistocene strata occur between the inferred projections of faults 3 and 4 on Strawberry Point (figs. 27, 32). At the southern locality (n1 in figs. 27, 32), a ≈4-m-thick section of interbedded sand and mud dips 24° NW. These strata are correlated with the Whidbey Formation based on lithology and sedimentology, and because they are at the same elevation as a very thick (as much as 100 m) subsurface section of mainly clay and silt penetrated by nearby water wells (figs. 27, 28). The nonglacial Olympia beds are not known to reach comparable thickness anywhere in the region (for example, Dragovich and Grisamer, 1998; D.J. Easterbrook, written commun., 2000). A small sample of detrital wood from these strata yielded a radiocarbon date of 44,740 ± 910 14C yr B.P., an age very close to the practical limit of radiocarbon dating. The sample could have been contaminated by minute amounts of younger carbon, and the date is here considered to be “infinite.” Forty meters to the north (n2 in figure 32 ) across a covered interval, a 5-m-thick section of sand and gravel assigned to the Possession Drift, based on lithology and stratigraphic position, has a similar structural attitude. Thus, the Whidbey-Possession stratigraphic contact, exposed in the structural domain between faults 2 and 3, appears to be repeated between faults 3 and 4.
Determining the amount of vertical separation across fault 3 is problematic because of extensive cover between coastal exposures and the presence of just one useful well in the domain between faults 2 and 3. If one assumes that the contact between the Whidbey and Possession is repeated and that the structural dips in coastal exposures are continuous through the noncovered areas, then the amount of vertical separation needed to produce the repeated contact is about 80-120 m.
Outcrop data suggest that the section might be repeated not only across fault 3 but also between faults 3 and 4 by a fault not apparent on the seismic-reflection data. About 10 m of fractured, west-dipping (28°) sand and mud tentatively interpreted as Whidbey Formation is exposed at locality o (figure 31 D); alternatively, these strata could have been deposited during the Olympia nonglacial interval and thus form the upper part of a continuous stratigraphic section, extending from the Whidbey Formation to Olympia beds, in the southern part of the domain between faults 3 and 4. The beds at o are cut by west-northwest-striking (≈70°) subvertical fractures.
Outcrop locality p occurs along the projected trace of fault 4 (figure 27) and is the last bluff exposure on the northeast coast of Strawberry Point. The outcrop consists of 4-5 m of highly fractured friable sand (figure 31 E, F) overlain by pebbly diamict. Fractures have variable northwest to northeast strikes (50° to 340°), dip 30°-90°, are lined with clay, and commonly displace stratigraphic markers 2-10 cm. These strata are tentatively interpreted as uppermost Whidbey Formation overlain by Possession Drift. Well data suggest possible minor north-side-up slip on fault 4 (≈10 m) following deposition of the Whidbey Formation, but the amount is poorly constrained.
We interpret the faulting and folding within the outcrops at Strawberry Point to be tectonic in origin. This structural zone coincides precisely with the fault zone recognized a few hundred meters offshore on seismic-reflection data (figs. 5A, 8, 27). Similarly deformed Pleistocene strata are extremely rare in the Puget Lowland (among others, Gower and others, 1985; S.Y. Johnson, unpub. data, 1994-2000). Deformation by landsliding can be ruled out for several reasons: (1) Dipping sections are continuous vertically for as much as 100 m and laterally to the limits of exposure – they have not been disrupted or broken apart by gravity sliding, as is the case for many small recent landslides in the Strawberry Point area; (2) Dipping sections cross major stratigraphic contacts between mud- and sand-dominated units. These unit boundaries typically form the failure planes for Puget Lowland landslides. (3) The strike of dipping sections is perpendicular to the trend of the modern coastal bluffs, not parallel to the bluff free face as would be expected for latest Pleistocene or Holocene landslides. (4) Topography along Strawberry Point indicates the presence of many small landslides derived from the upper part of the bluff, but nothing remotely similar to the size of the observed dipping sections. Also, the dipping section between faults 2 and 3 extends to the top of the bluff; hence no source for a possible massive landslide exists in that location.
The coincidence with the offshore fault zone and the rare occurrence of deformed Pleistocene strata elsewhere in the Puget Lowland argue against a glaciotectonic origin (for example, Aber and others, 1989; Hart and Boulton, 1991; Hart and Roberts, 1994) for deformation in Strawberry Point outcrops. Beds are not deformed in the thin-skinned fold-and-thrust style that characterizes glaciotectonic deformation, and no obvious glaciotectonic landforms (such as composite ridges or push moraines) occur at Strawberry Point. Moreover, typical glaciotectonic features such as listric faults, small-scale folds, chevron folds, tectonic pods and boudins, augens, and pressure shadows are absent; secondary faults and fractures are generally subvertical and not low angle; and tectonic fabric is not widely distributed throughout the zone.
The Utsalady Point fault projects onto Whidbey Island's west coast across a sandy beach and lowland south of Rocky Point (figs. 27, 28), an area of no exposures. The closest exposures north of the fault are at Rocky Point (g in figure 27), where pre-Tertiary basement rock is overlain by Everson Drift (figure 6; Pessl and others, 1989). Easterbrook (1968) described the closest coastal bluff exposures to the south, 4 km away along bluffs south of Swantown (d1 in figure 27). These exposures consist of the Whidbey Formation overlain by Esperance Sand Member with no intervening Possession Drift or nonglacial Olympia beds (figure 6).
Nearby offshore seismic-reflection profiles (figs. 10, 13) indicate about 200-300 m of down-to-the-south vertical separation on the base of the uppermost Pliocene(?) and Pleistocene section across the Utsalady Point fault. On land, this contact is at an elevation of a few meters above to 50 m below sea level north of the fault (figure 33 ). No wells south of the fault (as deep as 300 m) encountered basement. The thicker Pleistocene section south of the fault appears to consist almost entirely of pre-Whidbey Formation glacial and interglacial deposits (figure 33). The geometry of the Utsalady Point fault on offshore seismic-reflection data strongly suggests that the noted onshore relief on the base-of-uppermost Pliocene(?) to Pleistocene surface is created by a near-vertical fault bounding two subhorizontal surfaces and not by a gentle subsurface slope between Rocky Point and well 100 (figs. 28, 33). The top of basement also deepens to about 160 m about 1 km north of the Utsalady Point fault, indicating a possible south-side-up fault splay in this area, consistent with offshore seismic-reflection data (figure 10).
Using the top of the interglacial Whidbey Formation as a younger datum (discussed above, Devils Mountain fault subsurface data) for determining possible offset on the Utsalady Point fault is problematic on the figure 33 correlation diagram because of the apparent absence of Possession Drift on this part of Whidbey Island. In these wells, the top of the Whidbey Formation may be in contact with lithologically similar strata of the nonglacial Olympia beds or with the Esperance Sand Member as in the Swantown coastal bluff exposures (d1) at the south end of the profile. In either case, the top of the Whidbey Formation is not an obvious surface on lithologic logs of wells adjacent to the fault. Well data are thus consistent with Quaternary offset on the Utsalady Point fault in this area but do not provide the data needed to estimate the amount of post-Whidbey Formation offset.
Across central and eastern Whidbey Island, the Utsalady Point fault is covered by vegetation and by surficial deposits developed mainly on Vashon Drift and recessional outwash (Pessl and others, 1989). No obvious topographic lineament is associated with the fault zone on existing topographic maps, and there is an insufficient number of deep water wells flanking the fault to construct stratigraphic correlation diagrams either parallel to or across the fault that might constrain location and slip. Thus, the location of the Utsalady Point fault as it crosses and possibly bifurcates on central and eastern Whidbey Island is speculative. Constraints on the location shown in figures 2, 27 and 28 include (1) the locations of faults on offshore seismic-reflection profiles (figs. 9, 10, 13), (2) weak aeromagnetic anomalies (figure 3), (3) one deep test well south of the fault on the Maylor Peninsula (No. 101, figure 28, table 1), which encountered bedrock at an elevation of about 475 m below sea level, and (4) a 500-m-long continuous exposure of flat-lying, locally fractured but unfaulted glacial and interglacial strata on the southeast coast of Polnell Point (q in figure 27).
One splay in the central part of the Utsalady Point fault zone is exposed in the coastal bluffs at Utsalady Point on Camano Island (r in figure 27). Strata are exposed almost continuously for 250 m along the coast and consist of three principal lithologies (figs. 34, 35): (1) laminated silty mud and sand with rare pebbles and organic detritus; (2) fine to medium sand and less common silty mud in which primary bedding has been largely destroyed by convolute stratification, pillar structures, and other soft sediment deformation features; and (3) lenses and wedges of gravel, pebbles, cobbles, and boulders. The geometries of units and the mix of lithologies suggest variable depositional environments within channels, floodplains, and possibly lakes of an alluvial plain, and postdepositional mixing of sandy strata by processes associated with liquefaction. Sparse fine-grained organic matter disseminated within sandy silt beds a few meters apart yielded radiocarbon dates of 21,100 ± 150 and 15,190 ± 220 14C yr B.P. (SJ-99-1A, 1B; table 2). The difference between the two dates is larger than expected given their close stratigraphic position. Both samples are low in carbon and it is possible that the age of one or both samples is too young due to minor contamination.
These dates place the strata within the latter part of the Olympia nonglacial interval (figure 6), and they indicate that these strata are younger than the Olympia beds exposed across Saratoga Passage on the southeast coast of Strawberry Point (figure 27, table 2; Hansen and Easterbrook, 1974). In the Tacoma area of southern Puget Sound, Troost (1999) reported radiocarbon dates as young as 15,110 14C yr B.P. for Olympia beds. The older of the two ages from Utsalady Point is similar to many reported for the Coquitlam Drift, a pre-Vashon glacial formation exposed about 100 km to the north near Vancouver, B.C. (Hicock and Armstrong, 1981), and to the pre-Vashon Evans Creek Drift, deposited by valley glaciers extending from the Cascade Range to the edge of the Puget Lowland (Armstrong and others, 1965).
Beds at the Utsalady Point locality are deformed into a broad northwest-trending fold. Structural dip ranges from 24° (figs. 34, 35) on the northeast limb of the fold to 10° on the southwest limb. The fold axis is cut by a vertical east-southeast (120°) trending fault (figs. 33, 34B, C). This fault truncates irregular pods and wedges of massive sand and pebbles in the middle of an ≈8-m-wide zone in which primary bedding has been destroyed by faulting and soft-sediment deformation. The fault is characterized by a 20-30 cm wide zone of highly sheared sediment, which contains both rotated and broken clasts. Eight fractures adjacent to the master fault have a mean strike and dip of 127° and 84° SW., respectively.
Vertical offset on this fault appears to be less than a few meters based on the projected elevation of the contact between silty mud and sand facies on opposite sides of the mixed zone adjacent to the fault. The pebbly pods on the east side of the fault have probably been intruded from a source bed lower in the section. The source for the pebble wedge on the west side of the fault is not obvious; it may also have been intruded from lower in the section (requiring transport from out of the plane of the exposure), or alternatively, it could be the depositional fill of a large fissure that formed along the fault. Water-well data (figure 37) show no nearby tills at this elevation, so formation and filling of this pebble wedge by nontectonic subglacial processes is unlikely.
In the bluff outcrops about 50 to 150 m west of this fault on the west limb of the fold, a 6-m-thick sandy horizon displays large-scale soft-sediment deformation (figure 35D). Features include numerous dikes and pillar structures, convolute stratification, ball-and-pillow structure, and intrusion by large dikes of sandy silt to gravel that extend up through as much as 13 m of section. Subvertical fractures with a mean strike of 305° (n=16) are common on the west limb of the fold.
As with the Pleistocene strata exposed at Strawberry Point (see above, “Strawberry Point fault, east coast of Whidbey Island), we infer that the faulting and folding of beds at Utsalady Point is tectonic in origin. The coincidence with the fault zone recognized offshore in Saratoga Passage is striking (figure 9), and such deformation of Pleistocene strata in the Puget Lowland is extremely rare (Gower and others, 1985; S.Y. Johnson, unpub. data, 1994-2000). Landsliding or glaciotectonic origins can be ruled out for most of the same reasons cited for the Strawberry Point fault.
Two faults are exposed in a vegetated roadcut 900 m southeast (128°) of the fault exposed at Utsalady Point (s in figure 27). Strata cut by these faults consist of sand and gravel and are inferred to be Vashon glaciofluvial advance deposits, correlative with the Esperance. One fault (figure 36A) is subvertical, strikes 95°, juxtaposes massive gravel (on the south) and plane-stratified sand (on the north), and has minimum vertical displacement of 150 cm. The second fault (figure 36B) occurs 15 m to the southeast, strikes 0° and dips 67°E, juxtaposes massive gravel (on the east) and stratified sand (on the west), and similarly has minimum vertical displacement of 150 cm. These exposed faults indicate southeastward continuation of the Utsalady Point fault zone, consistent with the trend of aeromagnetic anomalies (figure 3).
Figure 37 shows a stratigraphic correlation diagram across part of the Utsalady Point fault zone based on lithologic logs from water wells, with the bluff exposure and dates for the Olympia beds at Utsalady Point (figs. 27, 34, 35; location r) providing important stratigraphic control. The outcrop section at r correlates with a fine-grained interval in the subsurface that is much thicker than any known section of Olympia beds from this part of the Puget Lowland. This suggests that the Possession Drift is missing from this local area and that Olympia beds directly overlie the lithologically similar interglacial deposits of the Whidbey Formation. Because of this lithologic similarity, there is no basis for inferring the Whidbey-Olympia contact in the subsurface near this locality, and no obvious stratigraphic marker is present to constrain vertical offset on the fault exposed at Utsalady Point.
The southwestern splay in the Utsalady Point fault zone projects onto the correlation diagram between wells 106 and 107 (figs. 28, 37). The stratigraphy of wells southwest of this projected fault differs from those to the northeast in several ways. (1) Pre-Whidbey Formation glacial drift is inferred to be present at the bottom of one deep well on the southwest side of the fault but was not encountered at similar depths by wells northeast of the fault. (2) The Possession Drift is inferred to be present in five of six wells southwest of the fault; the Possession is apparently absent in the five wells northeast of the fault. (3) West of the fault, the elevation of the inferred top of the Whidbey Formation is more than 15 m, east of the fault it is below sea level at outcrop locality r (figs. 27, 34, 35, 37). (4) The top of the Olympia beds in six wells southwest of the fault has a mean and maximum elevation of 56.7 ± 7.9 m and 66.4 m in contrast to 26.9 ± 3.8 m and 30.8 m for three wells northeast of the fault. This difference is much less, only 11.3 m, in the two wells closest to and on opposite sides of this fault (106 and 107). These contrasts cumulatively suggest that the southwest side of the fault has moved up relative to the northeast side. This is opposite to the sense of throw along the Utsalady Point fault on western and central Whidbey Island. Vertical offset on this fault is not obvious on the nearby (≈2.5 km) offshore seismic-reflection profile in Saratoga Passage (figs. 5, 9).
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