Grounding line processes of the southern Cordilleran Ice Sheet in the Puget Lowland
Published:January 01, 2017
Lauren M. Simkins, John B. Anderson, Brian P. Demet, 2017. "Grounding line processes of the southern Cordilleran Ice Sheet in the Puget Lowland", From the Puget Lowland to East of the Cascade Range: Geologic Excursions in the Pacific Northwest, Ralph A. Haugerud, Harvey M. Kelsey
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The northern Puget Lowland of Washington State, USA, provides an exceptional opportunity not only to examine grounding line processes associated with marine-based ice sheets, but also to relate subaerial outcrop to marine geological observations of grounding line landforms and sedimentary processes in Antarctica and the deglaciated Northern Hemisphere. During this trip, we visit outcrops that record the interaction of the Cordilleran Ice Sheet and its bed, starting with locations where the ice sheet slowly flowed across crystalline bedrock. We also visit locations where the ice flowed across unconsolidated deposits, allowing discussions of subglacial bed deformation and grounding zone wedge development. Evidence shows that grounding line retreat across Whidbey Island was punctuated by periods of grounding line position stability and local ice advance during the growth of multiple grounding zone wedges. We will discuss the criteria for identifying grounding zone wedges, including diamicton units with foreset bedding that downlap onto a regional glacial unconformity at the base, and are truncated at the top by localized unconformities indicative of ice advance across the foreset beds. Grounding zone wedge foreset beds are composed of debris flows sourced from a deformation till and from sediment transported to the grounding line by subglacial meltwater. The overlying surface unconformity is associated with a laterally discontinuous till and pervasive glacial lineations. Other field stops focus on iceberg scouring and evidence of subglacial meltwater drainage, as well as the transition from marine to subaerial conditions during retreat of the Cordilleran Ice Sheet from the northern Puget Lowland.
Ice sheet grounding lines resting on beds below sea level are dynamic environments that mark the transition from grounded to floating ice or, in the absence of an ice shelf, coincide with the calving line (Fig. 1A). For marine-based grounding lines to remain stationary, ice thickness must be greater than the surrounding water depth to prevent ice from floating off the bed (Fig. 1A; Alley et al., 2007); therefore, any process that changes ice thickness or water level at the grounding line will influence grounding line stability (e.g., Schoof, 2007). Observations of contemporary grounding lines of the Antarctic and Greenland ice sheets have led to some understanding of ice sheet sensitivity to a range of mechanisms and feedbacks (e.g., Anandakrishnan et al., 2007; Stearns et al., 2008; Pritchard et al., 2009; Scambos et al., 2011; Rignot et al., 2014). However, these observations are limited by the short period of instrumental observations from airborne, satellite, and through-ice acoustic measurements. Studying paleo-grounding lines allows richer temporal and spatial context and thus provides a more complete perspective on grounding line processes and behavior.
Observations of contemporary and paleo-grounding lines suggest that sediment accumulation at grounding lines can contribute to the stabilization of marine-based ice sheets. The resulting landforms include recessional moraines and grounding zone wedges. Whereas recessional moraines form in both marine and terrestrial ice sheet settings, grounding zone wedges are only observed in marine settings (Batchelor and Dowdeswell, 2015) and form in a range of water depths where ice is grounded below sea level. In Antarctica, contemporary grounding zone wedge growth by deposition of deformation till and meltout of englacial basal debris at grounding lines has been suggested as a mechanism to stabilize grounded ice by reducing the ice thickness necessary to offset buoyancy effects (Anandakrishnan et al., 2007; Alley et al., 2007; Horgan et al., 2013; Christianson et al., 2016). On formerly glaciated Antarctic and Northern Hemisphere continental shelves, landform construction facilitating ice advance at paleo-grounding lines is implied by prograded grounding zone wedge foresets and lineated topsets (Figs. 1B–1D) (e.g., Anderson, 1999; Mosola and Anderson, 2006; Nygård et al., 2007; Jakobsson et al., 2012; Batchelor and Dowdeswell, 2015; Bart et al., 2017). On the contrary, the growth of grounding zone wedges also contributes to the development of a local landward-dipping bed slope that might lead to destabilization of retreat across the landform once buoyancy at the grounding line is established. Despite their importance, the exact processes that form grounding zone wedges and the spatial and temporal scales to which they stabilize grounding lines are not well constrained.
Former seascapes that have been uplifted and exposed by glacial isostatic adjustment may preserve paleo-grounding lines expressed as grounding zone wedges. Such a landscape exists in the northern Puget Lowland in Washington State, extending north to the San Juan Islands and south to Seattle (Fig. 2A).
This area has a rich, yet fragmented, history of Cordilleran Ice Sheet advance and retreat during the Quaternary. The southern Cordilleran Ice Sheet expanded into the Puget Lowland and split into two lobes, with the Juan de Fuca Lobe flowing west in the Strait of Juan de Fuca to the shelf edge (Herzer and Bornhold, 1982) and the Puget Lobe flowing south to a maximum terminus ~24 km south of present-day Olympia, Washington (Walsh et al., 1987; Figs. 2A, 3).
Shoreline erosion following post-glacial rebound in the northern Puget Lowland has resulted in spectacular exposures of marine strata that record the advance and retreat of the ice sheet. Demet (2016) observed landforms across Whidbey Island that resemble well-studied grounding zone wedges on deglaciated continental shelves. This study provides a detailed stratigraphic framework for interpreting the landforms as grounding zone wedges, based on published observations from contemporary and paleo-grounding lines. On this field trip, we visit outcrops that record ice flow and grounding line dynamics in the northern Puget Lowland (Fig. 2B) and discuss evidence for the interpretation of the Whidbey Island landforms as grounding zone wedges. With the first documented grounding zone wedges in subaerial outcrops, this region presents a unique opportunity to examine sedimentary processes acting at grounding lines, and how constructional landforms influence grounding line stability at a scale and resolution not achievable with geophysical and sediment coring methods.
The Puget Lowland is bounded on the west by the Olympic Mountains, which were uplifted by the oblique, northeastward subduction of the Juan de Fuca plate (Wells and Simpson, 2001). To the east, Cenozoic volcanic rocks of the Cascade magmatic arc and their underlying plutons were erupted upon and emplaced into a complex older basement (Tabor and Haugerud, 1999). This basement extends west into the San Juan Islands (Brown and Dragovich, 2003), where it is uplifted to form the north margin of the Puget Lowland. Northeast of the San Juan Islands lies the Fraser Lowland, which has a Quaternary history similar to that of the Puget Lowland. The Puget Lowland itself is dominated by Quaternary deposits, including glacial and interglacial strata (Blunt et al., 1987).
The Puget Lowland is dissected by west- to northwesttrending faults, including the southern Whidbey Island fault zone, which extends from the eastern Strait of Juan de Fuca, through Whidbey Island to the mainland between Seattle and Everett (Johnson et al., 1996, 1999; Sherrod et al., 2008). The southern Whidbey Island fault zone is expressed on the surface as a series of scarps and topographic lineaments (Sherrod et al., 2008) and, based on geomorphology, sediment deformation, and paleo-seismological evidence, has been active during the late Pleistocene and Holocene (Kelsey et al., 2004; Sherrod et al., 2008).
The Puget Lowland in the past 2.6 m.y. has seen at least six glaciations of the Cordilleran Ice Sheet (Booth et al., 2003). Due to deep local erosion during each advance of the Cordilleran Ice Sheet in the Puget Lowland, the Quaternary stratigraphy is laterally discontinuous and variable. Clague and James (2002) and Booth et al. (2003) provide overviews of the Quaternary glaciation of the Fraser and the Puget lowlands; numerous detailed geological maps of the region are indexed by the National Geologic Map Database (https://ngmdb.usgs.gov/mapview). Herein we focus on the northern Puget Lowland during the most recent (Marine Isotope Stage 2) glacial period, regionally termed the Fraser glaciation, which includes the older Vashon Stade, Everson Interstade, and the younger, regionally restricted Sumas Stade. During Fraser glaciation, the Cordilleran Ice Sheet first expanded from the Coast Mountains of British Columbia ~30,000 yr ago (Clague, 1981; Cosma et al., 2008). The ice sheet reached its maximum southern extent in the Puget Lowland around 17,000 yr ago during the Vashon Stade (Porter and Swanson, 1998). At this time, the Cordilleran Ice Sheet was up to 900 km wide and 2000 m thick (Wilson et al., 1958), with an estimated sea-level equivalent of 8.6–8.8 m (Seguinot et al., 2016).
The northern Puget Lowland has numerous elongate landforms with relatively uniform orientations and spacing, interpreted as streamlined glacial lineations, flutes, and drumlins (Fig. 4A) (Thorson, 1980; Kovanen and Slaymaker, 2004). Preservation of these surface features suggests that they formed during the most recent glaciation. Lineation directions mark two major ice flow configurations: one to the south and another to the west and southwest. There is a clear example of southwest-oriented lineations overprinting lineations oriented to the south in northern Whidbey Island, suggesting that, at least at this site, southwest flow was established after an earlier phase of southward flow (Haugerud et al., 2003; Kovanen and Slaymaker, 2004).
The Lawton Clay (proglacial lacustrine deposit) and Esperance Sand (proglacial, largely fluvial deposit) members are the first indications of ice advance into the Puget Lowland during the Vashon Stade of the Fraser glaciation (Mullineaux et al., 1965). Glacial advance over the Esperance Sand Member is recorded by the deposition of the Vashon Till, a massive pebbly diamicton (e.g., Easterbrook, 1969; Domack, 1983). Together, the Lawton Clay Member, Esperance Sand Member, Vashon Till (including tills deposited during the retreat phase), and locally significant recessional outwash deposits constitute the Vashon Drift. Directly overlying the Vashon Till on Whidbey Island are glaciomarine deposits of the Everson Interstade (e.g., Armstrong et al., 1965; Thorson, 1980; Domack, 1983; Dethier et al., 1995), including the Everson Glaciomarine Drift of Easterbrook (1969, which record the deglaciation of the northern Puget Lowland. These deposits imply that sectors of the ice sheet were grounded below sea level in an isostatically depressed basin that has been subsequently uplifted above sea level (Thorson, 1980; Dethier et al., 1995). Radiocarbon-dated marine shells from the Everson Glaciomarine Drift yield calibrated ages that cluster around 15,500 yr ago (Dethier et al., 1995; Swanson and Caffee, 2001), although uncertainties in these ages result from a lack of constraints on the marine reservoir correction (Porter and Swanson, 1998). Additionally, cosmogenic exposure ages of bedrock and boulders across Whidbey and Fidalgo islands corroborate ice retreat from the northern Puget Lowland around 15,500 yr ago (Swanson and Caffee, 2001). The similarity between these ages and ages from lacustrine organics (Leopold et al., 1982; Anundsen et al., 1994) has been used to support rapid deglaciation following the Vashon Stade (Porter and Swanson, 1998; Booth et al., 2003). Localized glacial re-advances occurred in the Fraser Lowland during the Sumas Stade (~14,000–12,000 yr ago; Armstrong et al., 1965; Clague, 1992; Clague et al., 1997; Kovanen and Easterbrook, 2001; Kovanen, 2002), prior to rapid retreat of the remnant Cordilleran Ice Sheet back to its alpine sources (Margold et al., 2014).
Within the northern Puget Lowland, subaerial exposure of glacial and glaciomarine deposits associated with the advance and retreat of the ice sheet result from post-glacial rebound (Thorson, 1989; Dethier et al., 1995; Clague and James, 2002; James et al., 2009) and coastal cliff formation owing to wave and stream erosion and slope failure. Little post-depositional erosion or reworking of Fraser glaciation deposits, with the exception of locally deformed sediments by active tectonism (e.g., Sherrod et al., 2008), occurred during or after uplift as evidenced by the preservation of surficial glacial landforms (Fig. 4A) (e.g., Booth and Hallet, 1993; Kovanen and Slaymaker, 2004). Capping the glacial and glaciomarine deposits are subaerial deposits, ranging from stream deposits to active soils, which are generally the thinnest units within the outcrops. On Whidbey Island, we have not observed shoreface deposits or associated sedimentary structures between the Everson Stade glaciomarine deposits and the subaerial deposits, most likely due to erosion by a transgressive or wave ravinement surface that would mark the transition from a submarine to subaerial environment. The elevation of marine limits in the northern Puget Lowland varies from ~125 m above sea level in the northern San Juan Islands, decreasing south to <30 m at the southern end of Whidbey Island (Thorson, 1981, 1989; Dethier et al., 1995; Kovanen and Slaymaker, 2004). Near Coupeville on Whidbey Island, highest post-glacial sea level was at 67 m elevation based on maximum elevations of glaciomarine drift (Polenz et al., 2005).
The glacial history of the Puget Lowland has been a topic of research for many decades, but little is known about processes that controlled ice sheet retreat. It has been suggested that marine incursion following the retreat of the Juan de Fuca Lobe led to the rapid retreat of the Puget Lobe (Thorson, 1980, 1981; Waitt and Thorson, 1983; Booth et al., 2003). Significant differences between outcrops in the stratigraphy of Vashon Stade and Everson Interstade deposits in the northern Puget Lowland imply variability in ice retreat behavior. Sedimentological analyses and airborne LiDAR (light detection and ranging) data lead us to suggest that grounding zone wedges are exposed in outcrop on Whidbey Island, and the stratigraphy records grounding line processes acting during ice retreat. The observations and interpretations summarized here are detailed in Demet (2016).
Surface slope maps of Whidbey Island reveal 11 asymmetric features (Fig. 4B) oriented approximately perpendicular to paleo–ice flow (Fig. 4A), which we consider possible grounding zone wedges. These features have foreset slopes of 4–10°, exhibit considerable variability in sinuosity of mapped break lines, and are overprinted by glacial lineations, similar to identified grounding zone wedges from other formerly glaciated continental margins like the examples shown in Figure 1. They are relatively small in comparison to most published grounding zone wedges, but are similar in size (length and height) to Antarctic paleo–grounding zone wedges observed with improved multibeam swath bathymetry data (shown in Figs. 1B–1C) (Halberstadt et al., 2016; Simkins et al., 2016; Demet, 2016), and to a contemporary grounding zone wedge at a West Antarctic ice stream grounding line (Anandakrishnan et al., 2007) (Fig. 5).
Detailed descriptions of outcrops at five of the potential grounding zone wedges (Fig. 4B; Demet, 2016) show foreset beds of diamicton that downlap onto an irregular unconformity, and topset beds that are truncated by more localized unconformities (as seen on this field trip) (Fig. 4C). The lower unconformity at the base of the foreset beds is only locally exposed, but at some locations it cuts into a variety of deposits including non-glacial beach, aeolian, estuarine, and fluvial deposits, as well as proglacial deposits. Above the foreset beds and associated unconformities is structureless diamicton that is stratigraphically associated with the glacial lineations exposed in the modern surface landscape (Fig. 4A). Relatively thin (<2 m) glaciomarine sediments cap the upper diamicton, but do not obscure the underlying glacial lineations. Lineations are not restricted to areas with grounding zone wedges and are apparently associated with the Vashon Till. This is consistent with observations from glaciated continental margins where lineations occur above and below wedges, indicating multiple generations of grounding line advance (e.g., Halberstadt et al., 2016).
We interpret the grounding zone wedges to have formed during deglaciation of Whidbey Island, not during advance of the ice sheet to its maximum extent, as suggested by the preservation of the landforms, the lack of continuity of the upper unconformities between outcrop sites, the restriction of the upper massive diamicton (till) to the top of the foreset beds, and the conformability of the upper till with overlying glaciomarine deposits. The interpretation of the Whidbey grounding zone wedges having formed during ice sheet retreat is supported by the lack of evidence that grounding zone wedges elsewhere are preserved during ice sheet advance phases, and abundant evidence that they are only preserved at glacial maxima positions and during deglacial phases (e.g., Dowdeswell and Fugelli, 2012; Halberstadt et al., 2016). Therefore, four criteria lead to the interpretation of the foresets and topset beds as grounding zone wedges formed during ice sheet retreat, consistent with observations of other documented grounding zone wedges (e.g., Figs. 1B–1D): (1) deposition above a glacial unconformity; (2) evidence of prograded foreset beds of till in the direction of paleo–ice flow; (3) localized unconformities and glacial lineations on the topset surface; and (4) capping by glaciomarine sediments.
Grounding zone wedge foreset beds on Whidbey Island are composed of matrix-supported diamictons that suggest sediment transport at paleo–grounding lines was dominantly by debris flows sourced by subglacial sediment delivery to the grounding line. The debris flow deposits are interbedded with laterally discontinuous, cross-stratified sand and gravel and thinly laminated silt and clay with scattered dropstones, which are interpreted as channelized and sediment-laden meltwater plume deposits that emanated from the subglacial environment to the grounding line. Additionally, thin alternating sand and mud laminae are likely tidal couplets, suggesting tidal influence at these paleo–grounding lines. The Whidbey Island grounding zone wedges are capped by glaciomarine deposits, which are characterized by a range of sediment types from diamictons deposited in close proximity to the grounding line, to pebbly mudstones deposited far from the grounding line. Increased marine influence upsection is evidenced by improved grain-size sorting and presence of marine fossils.
The grounding zone wedges are truncated by glacial unconformities that, in some cases, extend beyond the topset-foreset break and over the foreset slope, but cannot be traced to nearby outcrops. This indicates that ice advanced across its own grounding zone wedge, further suggesting that grounding zone wedge growth played an important role in stabilizing the grounding line. The prevalence of grounding zone wedges (five exposed in outcrop and six additional inferred from the surface topography) is strong evidence for the retreat of the ice sheet being interrupted by stillstands and even local ice advance. Based on relative and absolute age constraints, Antarctic grounding zone wedges similar in size are thought to form over decades to centuries (intermediate-range, Fig. 5; Anandakrishnan et al., 2007; Simkins et al., 2017). Assuming comparable grounding line sediment fluxes (for which constraints are lacking), the Whidbey Island grounding zone wedges likely formed over similar timescales and, thus, represent stillstands of decades to centuries or less if sediment fluxes were higher. Spacing of grounding zone wedges on Whidbey Island decreases from south to north, from several kilometers to hundreds of meters as the grounding line approached the region of bedrock exposure (Fig. 4B), suggesting reduced rates of grounding line retreat in the vicinity of less to non-deformable substrate. Demet (2016) suggests that the ice sheet here did not instantaneously retreat due to a marine incursion, but rather experienced a punctuated retreat that was influenced by the growth of stabilizing grounding zone wedges. Rapid relative sea-level fall should have also stabilized the grounding line during ice sheet retreat; however, our stratigraphic observations are unlikely to record this.
This field trip is intended as a one-day trip leaving from and returning to Seattle with lunch planned at Stop 4; however, accommodations are available on Whidbey Island for extended trips. The coordinates are referenced to the WGS84 datum. Dragovich et al. (2000, 2005) and Polenz et al. (2005) provide recent 1:24,000-scale geologic maps of the areas we visit.
Stop 1. Mount Erie (48.4541° N, 122.6252° W)
From Seattle, drive north on Interstate 5. At Burlington, just north of Mount Vernon, take Highway 20 West toward Anacortes/Skagit Airport. After 11.7 miles, use the left lanes to follow 20 West toward Avon cutoff. Drive 1.8 miles and take a right on Campbell Lake Road. Continue straight onto Heart Lake Road and take a slight right to continue on Heart Lake Road at Lake Erie grocery. After 1.4 miles, make a slight right on Ray Auld Drive and then an immediate sharp right on Erie Mountain Drive. Follow the road to the top of Mount Erie. There is parking and, just to the east of the antennae station, an overview platform where we will view the northern Puget Lowland and discuss the regional glacial landscape.
Mount Erie represents a crudely sculptured crag-and-tail composed of bedrock with overriding glacial striations indicating ice flow toward the southwest (Fig. 6). These striations occur at elevations up to 385 m above the adjacent landscape, where Thorson (1980) showed the ice was >1350 m thick. During the advance of the ice sheet, Mount Erie, along with other bedrock highs in the area, likely acted as sticky spots where ice flow was relatively sluggish. However, just south of Mount Erie on the northern part of Whidbey Island is the transition between bedrock and sedimentary strata, where elongate landforms suggest basal substrate was deformable and erodible and likely facilitated accelerated ice flow due to ice movement across a deforming bed.
Stop 2. Rocky Point (48.3278° N, 122.7000° W)
Follow the road back down the mountain to Lake Erie grocery at the intersection of Heart Lake Road and Rosario Road and make a sharp right on Rosario Road. Continue on Rosario Road for 3.3 miles, then turn right onto Highway 20 W. Stay on the highway for 8 miles and turn right on W. Ault Field Road, which continues straight into W. Clover Valley Road. Turn right on Rocky Point Road and continue for 0.8 miles. Turn into a small parking lot on the right. The parking lot is ~100 m south of the western prolongation of the Holocene Utsalady Point fault scarp trenched by Johnson et al. (2003). Walk north ~2000 ft along the beach to an exposure of striated bedrock and continue just around the corner to view an outcrop of Vashon Stade and Everson Interstade stratigraphy.
This is an excellent location to discuss grounding line pinning on bedrock relief features (Fig. 7A). Exposures of striated bedrock record two ice flow directions, representing a temporal shift in ice flow. Figure 4B shows a couple of curved ridges near this location, which Demet (2016) interprets as grounding zone wedges based on prograded foreset beds of diamicton and bounding erosional surfaces visible along the beach 600 m north of Rocky Point; however, grounding zone wedge topset and foreset beds are not visible at the field trip-stop. A relatively thick massive diamicton, interpreted here as till, accumulated on a bedrock high at the location of one of the curved ridges. Continue past the bedrock exposure around the corner of the point to see fluvial deposits (perhaps proglacial outwash of the Esperance Sand, or possibly an older unit) overlain by massive diamicton that we identify as Vashon Till (Fig. 7B). A second diamicton unit is present above a horizontal unconformity above the Vashon Till (Fig. 7B), and is likely part of the Everson Glaciomarine Drift. The top of the section consists of soil that formed following subaerial exposure of the outcrop.
Stop 3. North West Beach (48.2978° N, 122.7250° W)
From the parking lot at Rocky Point, continue straight (east) on Rocky Point Road for 0.4 miles and turn right on N. Transmitter Road. After 0.6 miles, turn right on Crosby Road (continues straight to W. Beach Road) and drive 2.2. miles. Pull over at the parking spot on the right.
We will observe massive diamicton (Vashon Till) that is cut by a highly irregular erosional surface (Fig. 8). The sediment that fills this scour is a diamicton with considerable soft sediment deformation and is interpreted as an iceberg turbate that formed following grounding line retreat at this location (Domack, 1983). The diamicton is conformably overlain by deposits of the Everson Glaciomarine Drift. In the field, note the discrete beds of sand and gravel, which are interpreted as intense events of ice-rafting, depositing dropstones and outsized clasts within the Everson Glaciomarine Drift.
Stop 4. Beach North of Fort Casey (48.1759° N, 122.6870° W)
Turn right onto W. Beach Road from the parking lot and continue straight onto NW Beach Road. Turn left onto W. Libbey Road and take a right on Highway 20 W for 3.5 miles. Turn right onto S. Main Street, which continues to S. Engle Road, for 3.1 miles to Camp Casey Conference Center. Advance permission is required to park at Camp Casey. Lunch will be eaten at Camp Casey prior to walking north along the beach for ~1 mile to the locale.
The Fort Casey section is the only location where we observe till from a pre-Vashon advance (Fig. 9A). It comprises the base of the section to the left of the glaciotectonic thrust and is overlain by non-glacial deposits that include sand and gravel, which are overlain by well-sorted sand with beach and eolian sedimentary structures. Together, these non-glacial deposits compose a shallowing-upward succession indicating a relative sea-level fall and, ultimately, subaerial exposure. These deposits are cut by an unconformity that records the Vashon Stade advance across this location (Fig. 9A). Large and small glaciotectonic features occur beneath the unconformity, including soft sediment folds and thrust faulting. Above the unconformity is diamicton with prominent foreset bedding (Fig. 9A).
Foreset beds are interbedded with laterally discontinuous, cross-stratified sand and gravel and thinly laminated silt and clay with scattered dropstones, interpreted as meltwater deposits and proximal glaciomarine deposits, respectively. Foreset bed dips range from 3° to 34°, which indicates a viscous, matrix-supported sediment with downslope transport via debris flows. Individual debris flow units are commonly separated by thin beds of finely laminated sands and muds, without ice-rafted material, that we interpret as turbidites and meltwater deposits.
The foreset beds are truncated by an irregular erosional surface that is interpreted as a glacial unconformity caused by a localized advance of grounded ice. Overlying this upper unconformity is a massive diamicton, interpreted as topset till. This unit grades over a thickness of 1 m into a pebbly mud that exhibits upward fining and increased sorting (Fig. 9B) and an increase in marine shell abundance interpreted as Everson Glaciomarine Drift, and records a conformable contact between Vashon Till and Everson Glaciomarine Drift.
Stop 5. Penn Cove 1 (48.2386° N, 122.7073° W)
Take S. Engle Road/S. Main Street and turn left on Highway 20 W. Continue for 4.5 miles, then turn right onto Arnold Road and right onto Riepma Avenue. Park in the lot at the end of the road.
We walk east on the beach from the parking lot, to see a gravel-rich outwash unit overlain by diamicton with large sedimentary clasts (Fig. 10A). The clasts are laminated siltstones with fractures that are filled with the surrounding gravel and sand (Fig. 10B), implying high-pressure conditions syn-deposition. This unit is interpreted as a subglacial meltwater deposit with large rip-up clasts. Numerous subglacial meltwater channels, now exposed subaerially, are observed in the surface topography of the northern Puget Lowland (Booth and Hallet, 1993; Booth, 1994).
Optional Stop 6. Penn Cove 2 (48.2401° N, 122.6803° W)
From the parking lot on Riepma Avenue, turn right onto Penn Cove Road. Drive 1.3 miles and park on the right at the intersection of Penn Cove Road and N. Monroe Landing Road. Walk ~1000 ft to the west on the beach until you get to the cliffs.
This stop provides an opportunity to examine the Everson Glaciomarine Drift in greater detail than is possible at previous stops because of its exposure directly at the beach (Fig. 10B). As you walk along the exposures in the cliff, note variations in pebble (ice-rafted debris) content and marine shells. The Everson Glaciomarine Drift is composed mainly of terrigenous silt with scattered dropstones, and scarce marine shells. This suggests a marine setting where subglacial meltwater delivered large volumes of silt to marine basin, and contrasts with the modern glaciomarine deposits of Antarctica, which contain significant biogenic material and, with the absence of surface melting, less material transported by meltwater.
End of Trip
From optional Stop 6, continue onto N. Monroe Landing Road from the parking lot and turn left on W. Arnold Road. After 1.6 miles, turn left onto Highway 20 W. In 10.3 miles, continue straight onto Highway 525 S and make a right to stay on Highway 525 S just before the Clinton-Mukilteo ferry station. The ferry will take ~30 min to get to the mainland. Once off the ferry, follow Highway 525 S/Mukilteo Speedway for 8.5 miles and then take the exit onto Interstate 5 S to Seattle.
The authors thank those who have conducted research on the glacial history of the Puget Lowland over the past several decades, as well as J. Wellner, R. Minzoni, and R. Haugerud who reviewed this field guide.
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From the Puget Lowland to East of the Cascade Range: Geologic Excursions in the Pacific Northwest
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This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.
Earth Sciences Sector Contribution Number: 20100093.