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Although seismic profiles from mid-to-southern Hood Canal are referenced in the following section, the results focus primarily on seismic-stratigraphic unit facies, bedrock structure, and sediment thickness patterns seen on seismic profile tracklines 26 through 50 (Figure 1). Unlike the zigzag trackline layout of lines 1-25 (Figure 1), the data coverage in this area is reasonably dense and spatially uniform. This allows for realistic seismic unit interpretation and helps to verify lateral continuity among the individual seismic-stratigraphic units. The perpendicular crossing of tracklines in this area also serve to verify positional GPS fix, bathymetric, and seismic profile data. To simplify the description of seismic-stratigraphic units that follow, seismic profile figure captions and associated discussions use mean sea level (m. s. l.) as a vertical datum reference point. While tidal cycles in the survey area typically vary the water’s surface by up to 2 m, this is within the limits of seismic record resolution (about 2.3 m/pixel).
The bathymetry data acquired from NOAA is shown in two contour plots covering north latitude 47° 28¢ to 47° 38¢ and corresponding to tracklines 11-25 (Figure 20 and Figure 21). Both maps show steep basin walls along both eastern and western shorelines. On Figure 20, landslides along the western shoreline north of Eagle Creek (Carson, 1976b) move the deep axis of Hood Canal eastward. Further north, lines 15 and 16 outline the alluvial fan deposits of the modern Hamma Hamma River delta and remnant outwash sands and gravels of the Eldon Creek ice marginal delta (Thorson, 1989). A 600 m-long, 200 m-wide, north-northeast trending 170 m deep depression can be seen bordered by smaller bathymetric depressions. Further up the western shoreline anomalous depth contours adjacent to lines 20-22 probably represent submarine landsliding along the Fulton Creek ice-contact fan delta (Figure 21). A broad depression (> 170 m) is observed adjacent to the Fulton Creek delta and the canal bottom widens eastward at this point. As seen in Figure 20, steep basin walls line both east and west shorelines interrupted only by the undulating contours defining the Fulton Creek deltaic deposits. The stacked surface and contour bathymetric map shown in Figure 22 shows an irregular surface beneath the deeper waters of northern Hood Canal and southern Dabob Bay. Several depressions are observed near the center of the plot. Bathymetric highs are seen in the upper northeast corner of the lower plot ‘B’ where sediment is shedding from the unconsolidated glacial cliffs of southern Toandos Peninsula. Another prominent bathymetric high is observed in the NW corner of plot B and reflects outcropping Crescent Formation bedrock interpreted from this study and mapped by Gower and Yount (1991). The southwest corner of the plot deepens as it trends into the south-southwest extending axis of Hood Canal (see inset).
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Figure 20: High-resolution (about 130 data points/km), bathymetry covering 47° 28¢ to 47° 33¢ north latitude. This plot shows the steep sidewalls of Hood Canal coupled with several 160 m-deep closed depressions (hachured) along the canal axis. Similar to bathymetry further north (Figure 21), depressions are oriented parallel to the toe of remnant Eldon Creek ice-contact delta deposits. These deposits may have caused concentrated erosion and scour in these areas. Recent and older landslide deposits along the western shoreline (Carson, 1976b) appear to narrow the waterway.
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Figure 21: This bathymetric map covers 47° 33¢ to 47° 39¢ north latitude and again points out the steep-walled shoreline of Hood Canal. Between Hood Point and Tekiu Point (shown), the canal floor widens eastward to about 4 km in width. A 170 m-deep closed depression is observed along the toe of the remnant and more recent Fulton Creek alluvial deposits mapped as outwash sand, gravel, and till by Carson (1976b). The heterogeneous topography these deposits may suggest underwater landsliding or slumping.
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Figure 22: Bathymetry contour and surface plot. Whitened areas extending from the NW, NE and SW corners were ‘blanked’ during gridding of the contour and surface plot data to avoid extraneous contour lines. One-minute depth-positional data recorded on R. V. Thompson was used for both plots and resulted in 12-14 data points/km coverage.
Similar to seismic record observations made by Gower et al. (1985), a marked unconformity is observed throughout the dataset between lower faulted sedimentary and volcanic rocks assumed to be that of the Crescent Formation (Figure 23 to Figure 31). For this study two seismic-stratigraphic units, ‘Qh’; Quaternary and late-Holocene Glacio-Fluvial and Lacustrine Deposits, and ‘Cr’; Tertiary Crescent Formation, were designated based on their observed acoustic character, relationship with existing shoreline outcrops and other distinctive characteristics.
The acoustic character of this seismic-stratigraphic unit varies considerably yet is still markedly different from that of underlying inferred bedrock. A ubiquitous high-amplitude, 10-50 m-thick, strongly stratified, acoustically continuous reflector lies just below the sediment-water interface. Hood Canal gravity core and grab samples obtained 2.0 km north of the study area, show the stratified near surface layer to be composed primarily of coarse sand, silt, and clay-size particles (Roberts, 1974).
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Figure 23: Line-interpreted trackline 15 accompanied by detailed bathymetry. Black dots on the bathymetric plot denote one-minute time-positional fixes and are denoted by ticks on the seismic profile’s top axis. Hachured closed contours are depressions within the bottom sediments underlying Hood Canal. Overall shortening and/or strike-slip displacement may be represented in this figure. Laminar reflectors at 00:60 (about 375 m below m. s. l.) may be caused by water bottom multiples. The vertical exaggeration (V. E.) of the seismic profile is approximately 2.6 times.
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Figure 24: Trackline 16 and detailed bathymetry; interpreted. Strike-slip displacement may be producing the near vertical shear zones observed throughout this profile. It is not clear what seismic-stratigraphic unit is shown north of 01:14 (labeled Cr?). Strong multiples (M1 and M2) may indicate shallow bedrock in this area. The vertical exaggeration (V. E.) of the seismic profile is approximately 2.6 times.
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Figure 25: Trackline 18 profile and detailed bathymetry; interpreted. An obvious normal faulting pattern is shown in this profile. An estimated 500 m of horizontal extension has taken place over the area between 01:47 and 01:36. This extension may be accompanied by strike-slip displacement. Apparent folds are noted at about 200 m below the water’s surface and their apexes at 01:38 and 01:45. The vertical exaggeration (V. E.) of the seismic profile is approximately 2.6 times.
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Figure 26: Trackline 19 and associated bathymetry; interpreted. As with trackline 16 (Figure 24) it is unclear what seismic unit is represented east of 01:59. Shortening and/or strike-slip displacement may be indicated on this profile. The vertical exaggeration (V. E.) of the seismic profile is approximately 2.6 times.
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Figure 27: Trackline 41; interpreted. Horst and graben extensional structures are seen in this profile. An approximately 200 m vertical offset beneath 05:20 on the profile is very close to the Hood Canal-Discovery Bay fault trace mapped by Gower et al., (1985). A pressure ridge feature is seen at 05:41. The vertical line at 05:30 (labeled) is an editing artifact introduced when 8.5 X 11-inch profiles were merged to create this continuous profile. The vertical exaggeration (V. E.) of the seismic profile is approximately 2.6 times.
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Figure 28: Trackline 44; interpreted. This seismic profile was run from southwest to northeast and crosses the westward projected (N60W) Seattle Fault that terminates just 8 km to the east (see Figure 3). Faults extend from the bedrock to within about 10 meters of the sediment-water interface. The half-graben type features in this profile may indicate extension (e.g. 07:20-07:32). The vertical line at 07:25 (labeled) is an editing artifact introduced when merging postscript images to create this seismic profile. The vertical exaggeration (V. E.) of the seismic profile is approximately 2.6 times.
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Figure 29: Trackline 45; interpreted. This approximately 4 km-long line was run parallel to and 0.5 km off the southern Toandos Peninsula shoreline. It extends northwest to within < 1 km of Seal Rock Park beach (see inset). A classic graben feature centered at 07:58 GMT represents the greatest apparent vertical offset (about 200 m) observed in the dataset. With the exception of a small zone of shortening along the Hood Canal-Discovery Bay fault trace (07:58-08:02; about 200 m depth below m. s. l.) this area is characterized by horizontal extension. The vertical white gap at 07:53 represents about 30 sec of data loss during the changing of DAT tapes. The vertical exaggeration (V. E.) of the seismic profile is approximately 2.6 times.
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Figure 30: Trackline 47; interpreted. This seismic profile also shows vertical offset (about 100 m) proximal to the Hood Canal-Discovery Bay fault trace (09:29-09:30). Extensional horst and graben structures are noted on this 3.5 km-long trackline. Parallel and continuous reflectors between 09:16 and 09:30 (< 300 m) are unique and may represent proglacial lake deposits laid down during a slower period of glacial retreat. The origin of the apparent fold structure centered at 09:21 is discussed in the text. This may be the same 75 m bathymetric step noted by Harding et al. (1988b) and traced to an approximate 350 m vertical offset of the underlying Tertiary bedrock. The vertical exaggeration (V. E.) of the seismic profile is approximately 2.6 times.
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Figure 31: Trackline 48; interpreted. A generally symmetrical "V"-shaped valley is seen on this 2.6 km-long profile run roughly parallel to tracklines 45 and 47 (Figure 29 and Figure 30 respectively). The figure inset extends from just below the water bottom down to about 400 m and details an inferred pressure ridge structure between 09:44 and 09:50. This is one of several shortening structures noted east of the Dosewallips River delta. The vertical exaggeration (V. E.) of the seismic profile is approximately 2.6 times.
Beneath this 10-50 m-thick surface layer glacio-marine sediments are weakly discontinuous to partially stratified, parallel to sub- parallel, and exhibit low to medium amplitude reflections. Equally represented, especially toward the steep basin sidewalls, are seismic stratigraphic units with chaotic, unstratified, mounded and/or hummocky character and variable reflection amplitude. Similar acoustic characteristics were noted by Johnson et al. (1994) in sediments along the southwest margin of the Seattle basin and interpreted to be "non-marine" in origin. Along the thalweg of Hood Canal, sediments commonly appear disrupted and structureless (e.g. trackline 18; Figure 16, 01:40 to 01:42, < 0.4 km below sea level).
Throughout the dataset otherwise coherent seismic reflectors are segmented and vertically offset. For example, blind faults (faults that do not break the surface) on tracklines 19 (Figure 26) and 48 (Figure 31) extend vertically through up to several hundred meters of Quaternary section. Many of these faults originate below the interpreted Tertiary bedrock (‘Cr’) erosional surface and extend to within 10-20 meters of the sediment-water interface.
This seismic-stratigraphic unit varies slightly in acoustic character. It is considerably less chaotic than the fluvio-glacial deposits making up the Qh seismic unit and displays consistent medium-to-high-amplitude, sub-parallel to low angle reflections along its erosional surface. This consistent character allows this seismic unit to be easily traced laterally between adjoining profiles. Limited internal reflectors, strong record multiples, and rare interbeds of parallel, finely laminated reflectors are seen below the Crescent Formation’s erosional surface. Individual beds within these laminar reflector sequences are < 10 m-thick, often faulted, and sometimes appear to be rotated (e.g. Figure 29; trackline 45, 0743-0750). Where these laminae are part of large-scale fault-bounded blocks they allow rotational components to be inferred. The strong multiples present along this unit’s erosional surface indicate considerable differences in acoustic impedance exist. Along the inferred Gower et al. (1985) fault trace (Figure 3) bedrock reflectors are commonly terminated abruptly and frequently accompanied by substantial apparent vertical displacement (see Figure 29).
Quaternary sediments (Qh), up to 300 m thick, fill an asymmetric V-shaped valley overlain by parallel 50-70 m-thick, continuous reflectors. The 160 m closed contour depression (see Bathymetry) may be structurally controlled. Relative to the chaotic, discontinuous, and disrupted seismic-stratigraphic units observed in adjacent profiles, Quaternary fill on this profile appears acoustically continuous and subdued with only limited near-surface faulting and sediment shear zones. On the west-southwest side of the profile, 30-50 m-thick, parallel, continuous, near surface reflectors may represent distal deltaic deposits along Eldon Creek (Figure 20). Interpreted bedrock faulting patterns are seen at 00:55 and 00:60 and extend down to depths of about 0.6 km below mean sea level. Vertically offset reflectors in this profile may represent sediment shear zones that have migrated toward the surface. Faulting patterns may indicate east-west crustal shortening.
This approximately north-south trending profile begins along Hood Canal’s steep eastern sidewall (01:06) and runs for about 1.0 km along the northeast distal toe of the Eldon Creek delta (01:14-01:20; see Figure 20). Observed between 01:06 and 01:13 is a 1.0 km-wide, V-shaped erosional channel bounded to the S-SE and N-NW by interpreted bedrock faulting. This may be a half-graben structure. A 100 m-thick section of discontinuous, somewhat chaotic reflectors are observed along the sidewalls of this relatively narrow valley feature. Just below the canal bottom, a 10-20 m-thick ‘hard’ reflector defines the bathymetric high north of 01:12. Underlying this hard reflector is a seismic unit (Cr?) that exhibits strong record multiples. Its subdued acoustic character resembles interpreted Crescent Formation bedrock observed elsewhere along Hood Canal’s western shoreline. Minor shortening may be accompanied by strike-slip faulting and/or sediment subsidence within the V-shaped erosional channel.
Parallel, 10-15 m thick reflectors directly below the sediment-water interface are continuous, and overlie up to 300 m of acoustically discontinuous, folded, disrupted, and generally chaotic Quaternary sediments. The fold-like feature at 01:38 is truncated to the northwest (01:40) and may have at one time been laterally continuous to 01:48. Within the Quaternary fill, sheared sediment and blind faults extend from 400 m below the water surface to within 30 m of the canal bottom. A disrupted zone between 01:40 and 01:42 may be a stress transfer zone for bedrock displacement below.
The interpreted 50° east-dipping bedrock erosional surface (Cr) projecting beneath this Quaternary sediment fill is extensively faulted with 50-100 m of apparent vertical offset. Acoustically, this interpreted bedrock unit has no apparent hard reflectors, and is somewhat subdued. The reflector draping and infilling the fault-block "V’s’ at 01:44 and 01:46 (about 300 m depth) may represent dense basal till similar to that observed along Seal Rock Park beach (Figure 14B). It is not clear whether the normally faulted blocks also reflect strike-slip displacement or remnant thrust fault structures. Overall, this profile displays approximately 500 m of horizontal extension.
A 170-meter deep closed bathymetric depression (hachured; left figure) is associated with an apparent half-graben fault-bounded structure between 01:57 and 02:00. This depression may also indicate settlement has taken place in this area. Several blind faults offset Quaternary sediments that infill the eastern half of this half-graben (e.g. 01:57; about 250 m depth), originate as deep as 0.6 km, and terminate 30-50 m below the sediment-water interface. The 35-40° east-dipping Tertiary bedrock (Cr) displays prominent multiples and acoustic character similar to that seen elsewhere. Laminar, parallel, reflector sequences are observed below the interpreted Crescent Formation erosional surface (between 01:50 and 01:54). These may represent marine sedimentary or conglomerate interbeds (Figure 9). Although not observed east-northeast of 02:00, the east-dipping bedrock reflector may continue eastward at a depth of about 500 m and underlie the bathymetric high between 02:00 and 02:05. If this bathymetric high is instead a broad, folded Crescent Formation bedrock or crustal shortening may be indicated.
This southwest trending, 5 km-long trackline begins along the western side of Toandos Peninsula and ends adjacent to the Dosewallips River delta. Horst and graben bedrock faulting dominates this profile. Vertical offset at 05:21 corresponds spatially with the Hood Canal-Discovery Bay fault trace (Figure 3). The down dropped graben feature between 05:12 and 05:21 is the same graben feature seen on trackline 45 (Figure 29). The trackline crosses the structure at an oblique angle and causes it to appear much wider. Quaternary fill (Qh) varies from 25 m to a maximum of 270 m in thickness. The inverted "V"-shaped structure at 05:41 resembles that of a pop-up and/or pressure ridge; the possible result of crustal shortening. The top of the apparent fault block located between 05:09 and 05:12 may instead represent a higher density lens or clay layer. However, the continuation of the Crescent erosional surface beneath this structure at about 400 m, is not observed. The apparent top of this horst feature may simply be an artifact of a water bottom multiple. Major structural faults at 05:21, 05:31 and 05:38 are oriented approximately perpendicular to the projected Seattle fault zone (see Figure 3) and extend to depths of 0.6 km below the water surface. A highly disrupted sediment-shear zone is seen between 05:30 and 05:39.
This seismic profile was run from southwest to northeast and crosses the westward projected Seattle Fault terminating just 8 km to the east (Figure 3). Numerous near-vertical faults are observed in both the interpreted Quaternary sediments (Qh) and the underlying bedrock. Asymmetric "V"-shaped valleys and possible half grabens are seen to the southwest and northeast of 07:21. The highly disrupted bedrock topography exhibited by this profile may be fault splays from the Seattle fault, particularly between 07:05 and 07:22. The undulated bottom bathymetry between 07:06 and 07:16 may be related to bedrock outcrops or rock avalanches from the western shoreline of the SW-NE trending promontory (see inset) that forms Seabeck Bay to the east (Figure 1). Navigational charts for this area of Hood Canal show large boulder outcrops along the promontory shoreline adjacent to this trackline.
Seismic-stratigraphic units observed on this profile, especially those interpreted as upper Crescent Formation bedrock, serve as the central "type-locality" for bedrock interpretations on subsequent seismic profiles. Local geologic features support the bedrock interpretation. These include 30° east dipping upper Crescent Formation basalt outcrops (about 1.0 km northwest of 08:07) along the beach of Seal Rock Park (Figure 14B) and an existing isopach map that shows a relatively thin layer of sediment (£ 10 m) covering bedrock in this area (Yount et al., 1985). On the seismic record, this bedrock unit (Cr) is seen dipping beneath the Quaternary sediments of Hood Canal between 08:00 and 08:07. This reflector is also clearly seen on the 3.5 kHz sub-bottom record (Figure 19).
The correspondence between both the seismic and sub-bottom records is central to the interpretation of bedrock seismic units throughout the study. As seen in other seismic records, the apparent vertical offset (08:00-08:01; 180 m) of the bedrock seismic reflector coincides remarkably well with the Hood Canal-Discovery Bay fault line (Figure 3). Although its erosional surface may be at a shallower depth of 0.4 km, the dashed line along the top of the graben feature between 07:55 and 08:01 corresponds to the graben structure seen on trackline 41 (Figure 27: 05:12-05:21). Blind faults are seen within sediments infilling the graben structure and may represent shear zones. Some of these blind faults terminate within 20-30 m of the sediment-water interface. A series of fold structures are observed between 07:58 and 08:02 (0.2 km depth) and may be the result of horizontal shortening.
Southeast of 07:56, sediments shedding from the south and southwest-facing unconsolidated glacial cliffs of Toandos Peninsula create a bathymetric high at 07:28-07:50. This bathymetric high can also be observed on the bathymetry plots shown in Figure 22. Sediments forming this feature extend and drape beneath sediments along the eastern shoreline of Dabob Bay. These are top-lapped to the west by a 200 m thick layer of horizontal and continuous, parallel reflectors between 07:50 and 07:56. The western edge of a crustal block lies below the bathymetric high at 07:49 (about 150 m depth). This block may control the western shoreline of Toandos Peninsula which defines Dabob Bay’s eastern shoreline further north (see inset). The deeper crustal block between 07:49 and 07:54 (Cr?) shows apparent vertical displacement of 150 m and is possibly rotated relative to the crustal block further east (07:44-07:50). Further to the southeast, a reflector at 200-260 m below sea level (07:38-07:46) defines two vertically stacked shallow basin structures infilled with up to 200 m of sediments. East-dipping parallel reflectors at 07:47 to 07:49 (200-250 m depth) may be Crescent Formation interbeds. Overall, this profile indicates horizontal extension of an estimated 250 meters with small shallow zones of horizontal shortening.
Acoustically continuous, parallel, and folded sediments fill a box-shaped basin feature between 09:16 and 09:30. This basin-like feature is bordered to the southeast and northwest by zones of discontinuous, sub-parallel, chaotic reflectors. Again, a good correlation exists between bedrock vertical displacement of 75-100 m at 09:31 and the inferred Hood Canal-Discovery Bay fault trace. A fold or mounded structure is seen with its axial plane at 09:21. This bathymetric high may be a southern continuation of a bathymetric high extending from the SW corner of Toandos Peninsula (07:49; Figure 29). The origin of this feature is curious. At first glance its morphology resembles a glacial moraine, however its continuous, parallel, bedding suggests sub-aqueous deposition. Its orientation roughly perpendicular to regional ice flow may suggest an esker. However, an esker would have been eroded and/or buried during or shortly after glacial retreat. The acoustically continuous nature and shallow stratigraphic position of this feature would however argue against rapid deposition.
A generally symmetric "V"-shaped valley or half-graben structure is seen on this profile. Deep Quaternary sediment fill with variable acoustic character is observed down to depths of > 0.4 km below sea level. The Quaternary seismic section overlying the valley axis (09:52-10:02) displays a generally consistent pattern of continuous to weakly discontinuous reflectors. Parallel interbeds are seen southeast of 09:56 but may represent discontinuous water bottom multiples. More chaotic, hummocky, and discontinuous reflectors are seen northwest of 09:52 and in the disrupted Quaternary sediments overlying faulted bedrock close to the Hood Canal-Discovery Bay fault zone. Blind faults in the area show 50 m of vertical displacement and extend from depths of 500 m below sea level to within 10 to 20 m of the Hood Canal water bottom. Three bedrock faults located along the "V" valley centerline (09:56-09:59) may define a structural bend in the 20-25° east-dipping Crescent basalt. A pop-up or pressure ridge shown in the figure inset is similar to the shortening structure seen on trackline 41 (Figure 27; 05:41). Note the offset of bedding along the 09:47 centerline of this structure. On the figure inset, a distinct change in acoustic character can be seen below the interpreted erosional surface (dashed; 200 m below sea level). Crescent Formation dip angles are consistent with the 20-30° angles shown by Carson (1976a) along the western shoreline between Quatsap Point and Seal Rock Park beach (Figure 1).
The thickness of Quaternary sediment (‘Qh’) beneath tracklines 26 and 40-50 (Figure 1) was calculated using a P-wave velocity of 1500 m/s (Figure 32). Glacial deposits vary considerably in lithology, consolidation, and density. Because of this, the modeled sediment thickness should be considered minimum estimate (assuming velocities do not exceed 1800 m/s). Actual thickness may vary by an estimated ± 25%.
Generally, sediment thickness throughout Hood Canal reflects changes in bedrock topography (Figure 33). In the northern study area (tracklines 26-50; Figure 1), several bathymetric lows overlie deep graben features. Where marine sediments are actively infilling depressions, this relationship is less apparent. For example, landsliding glacial deposits that form bathymetric highs formed by can be observed extending from the south-southwest facing cliffs of southern Toandos Peninsula (Figure 22 and Figure 29). Much of these unconsolidated deposits are probably tidally re-mobilized, transported away from the shoreline area, and actively infill bathymetric depressions.
Quaternary sediment between southern Dabob Bay and Quatsap Point varies in thickness from a maximum of 380 m to a thin layer of less than 5 m covering outcropping bedrock southwest of the Dosewallips River delta and southeast of Seal
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Figure 32: This isopach map shows the sediment thickness observed between the Hood canal bottom and bedrock erosional surface shown in Figure 33. Quaternary glacio-marine fill reaches an estimated 350 m along the middle of the N-NE trending Dabob Bay and along Hood Canal’s centerline (SW quadrant of the figure). Basalt outcrops (white-filled closed contours) are observed along the western shoreline and reflect the steeply east dipping seismic units on the proceeding interpreted profiles.
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Figure 33: Stacked plots of surface bathymetry and bedrock topography which combines ship tracklines (A), Hood Canal bottom bathymetry (B) and the interpreted Crescent Formation bedrock erosional surface (C). B and C are plotted at the same vertical exaggeration. This figure illustrates the considerable vertical relief and asymmetry of the bedrock surface underlying the low-relief Quaternary glacio-marine and alluvial fill bathymetric surface. The N60W projected Seattle fault is shown (see Figure 3). The deepest point on the SW corner of plot C approximately locates Hood Canal’s centerline as it continues S-SW below Quatsap Point (Figure 1).
Rock (Figure 19). Sediments overlying the bedrock surface thin rapidly toward the western side of Hood Canal and are thickest along the deep axis of Hood Canal (e.g. trackline 45; Figure 29).
The bedrock surface is highly irregular and displays prominent relief (Figure 33C). The erosional surface of this east-dipping reflector is observed projecting beneath thick glacial sediments infilling Hood Canal (e.g. Figure 19). The eastward, down-dip projection of this seismic unit is commonly terminated by near vertical fault structures paralleling the axis of Hood Canal. These faults are more prevalent along the western half of Hood Canal. Down-dropped bedrock blocks control the shape of asymmetrical "V"-shaped valleys and half-grabens. The deepest points of these valleys are associated with up to 180 m of vertical fault offset along the thalweg of Hood Canal. Fault-bounded blocks, some up to 0.75 km in width, form both highly irregular (trackline 41; Figure 27) and more classic normal faulting patterns (trackline 18; Figure 25). Near-vertical faults may indicate strike-slip displacement. The widest part of Hood Canal, south of Toandos Peninsula, is dominated by extensional patterns.
Unlike the western side of Hood Canal, bedrock does not appear to directly control the steep west-dipping slopes along the eastern shoreline of Hood Canal. The apparent low impedance contrast along the bedrock erosional surface yields weak acoustic reflectors. This makes interpretation difficult, particularly where Quaternary sediments may be thicker in southern Hood Canal.
Near-surface bedrock highs are observed primarily along the western half of Hood Canal and correspond closely to Bouguer gravity highs mapped by Finn et al. (1991) (Figure 2). Bedrock topographic lows generally relate to bathymetric depressions (e.g. Figure 24 and Figure 33). This bathymetric relationship may indicate either simple subsidence via sediment loading or possibly vertical bedrock displacement.
