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major findings holocene to late pleistocene … · peat overlain by ~25 ft of fine-grained sand and...

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Table 1. Age control data from the map area. Lab uncertainty values include random errors and are one standard deviation (68% confidence interval), except where shown in quotation marks, indicating that we do not know how many standard deviations are included. 14C, radiocarbon analysis by liquid scintillation counting; AMS, radiocarbon analysis by atomic mass spectrometry; OSL, optically stimulated luminescence; laser Ar, laser-argon analysis; TL, thermoluminescence. New radiocarbon analyses from this study were performed by Beta Analytic, Inc. (Miami, Fla.) and are 'conventional' (that is, adjusted for measured 14C/13C ratio); OSL analyses were performed by University of Illinois at Chicago Luminescence Dating Research Laboratory. Radiocarbon age estimates are in radiocarbon years before 1950 (14C yr. B.P.); ages stated in ‘ka’ are in thousands of calendar years before 1950. Elevations are in feet above mean sea level, as estimated in this study using lidar data (supplemented by visual elevation estimates on bluffs) and referring to the North American Datum Conversion Utility (NADCON; http://geodesy.noaa.gov/TOOLS/Nadcon/Nadcon.html) for vertical datum shifts between lidar data and map elevations. Loc. no. estimate 14 C yr B.P. 14 C yr B.P. 14 C yr B.P. 14 C yr B.P. 13 C/ 12 C (‰) Analytical Age method Material dated Geologic unit Lab no. Elev. (ft) Reference Notes 1 >44,560 -26.8 14 C wood in peat above brown underclay Qguc Beta 212312 ~18 this report Sampled wood from peat bed from 3 ft exposure in mostly covered roadcut ~6 ft above East Point Drive where road starts to climb in elevation just past switchback; peat bed overlain by 2 ft of exposed bedded gray silt/clay; underlain by 8 in. exposed compact brown (under)clay 2 >45,000 -24.2 AMS probable detrital organic material Qguc (Qc) Beta 212313 ~35 this report Organic material occurs in 4-in.-thick zone in bedded clay/silt in sea cliff exposure ~28 ft above beach; organics overlie interbedded sand/silt unit; underlies ~10 ft of gray sand with tan silt 3 40.4 ±4.8 ka OSL* sand (core) Qc o ? UIC 1893 ~10 this report Core sample collected from 12-ft-thick massive grey sand unit 4.25 ft above beach (Columnar Section 4); sampled unit located just south of Mabana; N 48.09233, W 122.41367 4 124.8 ±14.2 ka OSL* sand (core) Qc w UIC 1910 ~9 this report Core sample collected from 6 in. sand bed located 3 ft above beach; medium-grained sand; collected from a 13 ft unit consisting of interbedded silty sand, sand, silt, and silty clay beneath ~60 ft of exposed till, 20 ft pebbly sand, 20 ft grey gravel, and 15 ft tan silt and sand in an 80 ft high bluff located on the southeast side of Lowell Point; N 48.12227, W 122.48470 5 >41,360 -28.4 14 C peat from 7-inch thick peat layer Qc w ? Beta 223835 ~16 this report Sample of 7-in.-thick peat bed exposed in sea cliff ~10 ft above high tide in 30-35 ft exposure of peat overlain by ~25 ft of fine-grained sand and silt and underlain by 10 ft of fine-grained sand and silt; peat located just south of Pebble Beach; N 48.06360, W 122.38512 6 >45,210 -28 14 C wood from blue-gray silty clay Qc w Beta 223836 ~6 this report Woody pieces collected from blue-grey silty clay ~8 in. above beach, southeast side of Lowell Point; silt located near the base of a 12-15 ft thick interbedded flood plain facies of interbedded silt, sand, and clay; overlain by ~60 ft of glacial till; N 48.12227, W 122.48470 Locations not understood well enough to plot (see Notes) 7 280 “±81” ka TL clay Qguc Easterbrook and others (1992) Clay beneath till correlated with Double Bluff Drift, at Pebble Beach, Camano Island 8 291 “±86” ka TL clay Qguc Berger and Easterbrook (1993) Clay beneath Double Bluff Drift at Pebble Beach, Camano Island 9 980 “±70” ka (plagioclase), 1040 “±40” ka (biotite) laser Ar pumice Qc pd ? Easterbrook (1994a) Pumice from cross bedded sand that is exposed between Double Bluff Drift and clays that were TL dated by Berger and Easterbrook (1993); sample location is at Pebble Beach, Camano Island * Analysis on 4 to 11 micron quartz grain fraction, by blue light excitation using a multiple aliquot regenerative dose method; blue light emissions measured with 3 mm thick Schott BG-39 and Corning 7-59 glass filters that block >90% of luminescence below 390 and above 490 nm; lab reports precision of analysis as “good to very good”. 4 1 6 2 3 5 Fig. 1 Qgta f ? af af Qgt v Qgt v Qgt v Qmw Qgo f Qgas v Qgas v Qgas v Qgas v Qgas v Qgas v Qgas v Qgof Qgo f Qgo f Qgo f Qgom e Qgd v Qm Qls Qb Qgas v Qmw Qmw? Qmw Qc Qls Qmw Qgt v Qc Qgo f Qc? Qc Qgt v Qgt v Qgt v Qp Qp Qp Qp Qp Qp Qp Qp Qp Qp Qp Qgo f Qgo f Qgo f Qgom e Qgd f Qgd f Qgom e Qgom e ? Qls e ? Qgas v Qgas v Qgas v Qgasv Qgas v Qgas v Qgas v Qgo f Qgo f Qgt v Qmw Qgtv Qgd f Qls Qga v Qga v Qgas v Qgd f Qgd f Qgt v Qgom e Qgas v Qls e ? Qgom e Qgom ee Qgo f Qgo f Qgof Qgo f Qgo f Qgo f Qgta f ? Qgom e Qgas v Qgas v Qgas v Qgas v Qgas v Qgas v Qgas v Qgas v Qgas v Qgdm e Qgdm e Qgta f Qgta f Qgas v Qgas v Qgas v Qgas v Qgas v Qgd f Qgta f Qgo f Qgo f Qgom e Qgas v Qc Qb Qb Qb Qc Qgas v Qgas v Qgas v Qgas v Qgt v af Qb af Qb Qc Qc? Qc af Qb Qgom e Qgom e Qls Qls Qmw Qmw Qgo f Qgo f Qgo f Qgo f Qgo f Qgo f Qgo f Qgo f Qgo f Qgo f Qmw Qmw Qgas v Qgt v Qmw af Qgt v Qgt v Qgt v Qgas v Qgt v Qgas v Qgas v Qgas v Qls Qls Qls Qls Qp Qgt v Qmw Qguc Qguc Qguc Qb Qb Qb af af Qgom e Qm Qb Qb Qguc Qgo f Qgo f Qgo f Qgo f Qgo f Qmw Qguc Qb af Qb af Qgt v Qgt v Qb Qb Qgt v Qguc Qgom e Qguc Qgdm e Qgo f Qgt v Qmw Qb Qguc af af Qgdm e Qgdm e Qgom ee Qgdm e Qgt f Qgt v Qb Qgom e Qgom e Qgas v Qgom ee Qgom ee Qgas v Qb Qgdm e Qb Qgo f Qgt v Qgd v Qga v Qgt v Qls Qgom e Qm af Qm Qu Qb Qm Qgdm e Qls Qgt v Qb af Qgo f Qgom e Qgo f Qgo f Qgom e Qgo f ? Qp? Qgas v ? Qgt v Qgo f Qgas v Qgdm e Qp Qp Qgas v Qgas v Qgas v Qgo f Qgas v Qgas v Qgom e Qp Qga v Qgo f Qgo f Qgas v Qgas v Qgom ee Qmw Qgas v Qgom e Qguc Qb Qgas v Qgas v Qb Qb Qgdm e Qgas v Qgas v Qgt v Qgas v Qgt v Qgt v Qb Qgas v Qgt v Qmw Qmw Qgo f Qgta f Qls Qgdm e Qgas v Qgo f Qp? Qgas v Qgas v Qgo f Qgas v Qb Qgo f Qgo f Qgasv Qc o ? Qco ? Qco ? Qc o ? Qc o ? Qc o ? Qco ? Qco ? Qc w ? Qgpt d Qc w ? Qgpt d Qcpd ? Qcw ? Qgt u Qgt u Qc w Qgpt d Qgta f Qgom e Qgt v Qgt v Qgt v Qgdm e Qgdm e Qgdm e Qgdm e Qgt v Qgt v Qgt v Qgt v Qgt v Qgt v Qgas v Qgom e Qgom e Qgt v Qgt v Qgt v Qgt v Qgas v Qgas v Qmw Qgtv Qgas v Qgt v Qgt v Qgt v Qgt v Qgt v Qgt v Qgt v Qgt v Qgt v Qgt v Qgt v Qgt v Qgt v Qgas v Qgas v S A R A T O G A P A S S A G E P O R T S U S A N ELGER BAY ZONE ISLAND WHIDBEY SOUTHERN ZONE ISLAND WHIDBEY SOUTHERN FAULT FAULT unmapped SOUTHERN WHIDBEY ISLAND FAULT ZONE stratified, fine-grained sand and silt covered elev. 90 ft 12 ft 60 ft 12 ft elev. 6 ft elev. 104 ft 15 ft 15 ft 15 ft 40 ft 13 ft elev. 6 ft elev. 6 ft 15 ft 8 ft 12 ft 48 ft elev. 89 ft Qc w ? Qc w ? Qc w ? fine-grained, stratified sand yellow gray coarse-grained sand with local clay diapirs gray, pebbly, silty diamict; upper 1 ft weathered tan stratified, crossbedded, gray, pebbly sand with discontinuous silty sand lenses stratified, fine-grained sand and silt x x x x Qgt v Qgas v Qc o? stratified, tan, silty sand (sparse pebbles) gray, cobbly gravel tan, crudely stratified, pebbly sand gray, compact, sandy, pebbly to cobbly drift and till nonglacial, interbedded silt, sand, and clay with organics Qgom e Qgo f ? Qgo f ? Qgt v Qc w OSL age: 40.4 ±4.8 ka (Table 1, loc. 3) OSL age: 124.8 ±14.2 ka (Table 1, loc. 4) 14 C age: >41,360 yr B.P. (Table 1, loc. 5) 14 C age: >45,210 yr B.P. (Table 1, loc. 6) unconformable contact unconformable contact Qc af Qgas v Qgom e Qgt u pebbly sand sand (poorly exposed) interbedded sand and silty sand sandy till fill and bulkhead elev. 116 ft 10 ft ~23 ft ~45 ft 20 ft 8 ft elev. 10 ft Qc Qc? Qgas v Qgt u Qc? Qc tan to gray sand gray till tan to gray, thinly laminated sand massive violet- gray sand gray sand with tan silt layers 14 C age: >45,000 yr B.P. on detrital organic material (Table 1, loc. 2) tan to gray interbedded sand and silt x x x x 40 ft 14 ft 10 ft 15 ft 10 ft 28 ft elev. 10 ft elev. 127 ft 122°22¢30² 122°30¢00² 48°07¢30² R 2 E R 3 E T 30 N T 29 N 02¢30² 25¢ 25¢ 122°22¢30² 48°00¢00² 122°30¢00² 48°00¢00² 27¢30² 05¢ 27¢30² R 2 E R 3 E T 30 N T 29 N T 31 N T 30 N 02¢30² 05¢ 12 2°22¢34.53² 20 525 525 99 532 5 5 Freeland TULALIP CAMANO CONWAY LANGLEY MUKILTEO UTSALADY FREELAND HANSVILLE NORDLAND STANWOOD COUPEVILLE MAXWELTON OAK HARBOR PORT LUDLOW JUNIPER BEACH CRESCENT HARBOR contour interval 20 feet 7000 FEET 1000 1000 0 2000 3000 4000 5000 6000 0.5 1 KILOMETER 1 0 0.5 1 0 1 MILE SCALE 1:24,000 Geologic Map of the Langley and Western Part of the Tulalip 7.5-minute Quadrangles, Island County, Washington by Henry W. Schasse, Michael L. Kalk, Bradley B. Petersen, and Michael Polenz February 2009 Lambert conformal conic projection North American Datum of 1927; to place on North American Datum of 1983, move the projection lines approximately 17.9 meters north and 94.2 meters east as shown by crosshair corner ticks Base map from scanned and rectified U.S. Geological Survey Langley 7.5-minute quadrangle, 1956, photorevised 1968 and from the scanned and rectified U.S. Geological Survey Tulalip 7.5-minute quadrangle, 1956, photorevised 1968 Shaded relief generated from a lidar bare-earth digital elevation model (available from Puget Sound Lidar Consortium, http://pugetsoundlidar.ess.washington.edu/); sun azimuth 315°; sun angle 45°; vertical exaggeration 6x Digital cartography and GIS by Anne C. Heinitz, Charles G. Caruthers, Elizabeth E. Thompson, J. Eric Schuster, and Robert H. Berwick Editing and production by Jaretta M. Roloff and Karen D. Meyers WASHINGTON DIVISION OF GEOLOGY AND EARTH RESOURCES GEOLOGIC MAP GM-69 Langley and Western Part of the Tulalip 7.5-minute Quadrangles February 2009 depth contours in feet—datum is mean lower low water APPROXIMATE MEAN DECLINATION, 2007 MAGNETIC NORTH TRUE NORTH 17°38¢ Suggested citation: Schasse, Henry W.; Kalk, Michael L.; Petersen, Bradley B.; Polenz, Michael, 2009, Geologic map of the Langley and western part of the Tulalip 7.5-minute quadrangles, Island County, Washington: Washington Division of Geology and Earth Resources Geologic Map GM-69, 1 sheet, scale 1:24,000. This geologic map was funded in part by the U.S. Geological Survey National Cooperative Geologic Mapping Program. Disclaimer: This product is provided ‘as is’ without warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of merchantability and fitness for a particular use. The Washington Department of Natural Resources and the authors of this product will not be liable to the user of this product for any activity involving the product with respect to the following: (a) lost profits, lost savings, or any other consequential damages; (b) the fitness of the product for a particular purpose; or (c) use of the product or results obtained from use of the product. This product is considered to be exempt from the Geologist Licensing Act [RCW 18.220.190 (4)] because it is geological research conducted by the State of Washington, Department of Natural Resources, Division of Geology and Earth Resources. © 2009 Washington Division of Geology and Earth Resources http://www.dnr.wa.gov/AboutDNR/Divisions/GER/ MAJOR FINDINGS Mapping of the quadrangles has resulted in the following improvements to and understanding of the geology of the area: • It provides the first geologic map of the area published at a scale of 1:24,000. • Lidar (laser swath mapping) has provided the means for improved geomorphologic interpretation of the landforms in the area, thus allowing more accurate mapping of surficial geologic units, especially those relating to the Everson interstadial period and the relative sea level high stand (sea level maximum) associated with that period. • New radiocarbon (14C) and optically stimulated luminescence (OSL) age estimates constrain the ages of some nonglacial sediments mapped in the sea cliffs of Camano and Whidbey Islands. • The combination of lidar and shoreline oblique aerial photographs of the shoreline bluffs have allowed us to more accurately locate geologic units exposed in the bluffs. GEOLOGIC SETTING AND DEVELOPMENT The map area is covered by 650 to 3300 ft of glacial and nonglacial sediment (Jones, 1999; Mosher and others, 2000; Johnson and others, 2001) above Miocene(?) bedrock (Johnson and others, 1996). The oldest unit in the map area consists of unnamed, apparently nonglacial sediments (unit Qc pd ?) that may pre-date the Double Bluff Glaciation, provided the overlying till is correctly assigned to the Double Bluff Drift (unit Qgpt d ). These sediments crop out along the lower west-facing bluffs on southern Camano Island (Fig. 1). Although tephras found within these sediments have been dated at about 1 Ma, they are probably detrital and thus do not represent the depositional age of the unit, leaving its correlation to a specific marine oxygen-isotope stage unresolved. Stage numbers and corresponding ages used in this report are as defined in Morrison (1991). The Double Bluff Drift is the oldest sediment of clearly glacial origin exposed in the map area and overlies unit Qc pd ? on Camano Island (Fig. 1). Identification of the Double Bluff Drift in the area is based on earlier studies of Easterbrook (1968). Booth and others (2004, fig. 2) correlate the drift to stage 6 (~185–125 ka), although Berger and Easterbrook (1993), Easterbrook and others (1992), Blunt and others (1987), and Easterbrook (1994a,b) report older ages for some exposures. Although we did not map Double Bluff Drift on Whidbey Island, a till of unknown age (unit Qgt u ) crops out in coastal bluffs just west of Langley (Columnar Section 2) and occurs stratigraphically below pre-Fraser nonglacial deposits (unit Qc) in this area. At this location, unit Qgt u could be the Double Bluff Drift, if overlying unit Qc correlates with the Whidbey Formation of interglacial stage 5 (~125–80 ka) (Blunt and others, 1987; Easterbrook, 1994a; Dragovich and others, 2005; Polenz and others, 2005). Easterbrook (1968) and Minard (1985) both suggest that nonglacial sediments exposed in the lower parts of east-facing bluffs along Saratoga Passage on Whidbey Island belong to the Whidbey Formation. However, in the absence of age control, we could not confidently identify units Qgt u (west of Langley) and Qc along Saratoga Passage on Whidbey Island as Double Bluff Drift and Whidbey Formation, respectively. We tentatively assign peat-bearing sands and silts that overlie the Double Bluff Drift on southern Camano Island near Pebble Beach to the Whidbey Formation. A single radiocarbon-infinite age estimate of >41,360 yr B.P. (Table 1, loc. 5; Columnar Section 3) did not allow us to confidently assign these sediments to the Whidbey Formation, although Easterbrook (1968) had previously assigned them to that unit. We limit ourselves to a tentative assignment because the above radiocarbon age estimate is also consistent with an Olympia age (stage 3, ~60–20 ka). We did not identify Possession Glaciation deposits (stage 4, ~80–60 ka) (Booth and others, 2004). Possession till may be present on Whidbey Island as suggested by Easterbrook (1968), who assigned till exposed in the bluffs south of East Point to the Possession Glaciation, but we cannot confidently assign this till (appearing in partial exposures 70 ft above the beach) to the Possession Glaciation and have therefore given it the designation Qgt u (Columnar Section 1). Two radiocarbon-infinite ages of >44,560 and >45,000 yr B.P. (Table 1, locs. 1 and 2; Columnar Section 1) south of East Point do not allow us to assign unit Qc confidently to either the Whidbey Formation or the Olympia nonglacial interval (stage 3, ~60–20 ka). The two aforementioned radiocarbon-infinite dates lie stratigraphically beneath the till (unit Qgt u ) in the vicinity of East Point, which is consistent with a Possession or older age for the till. However, sediments exposed in the lower bluffs between East Point and Rocky Point (1.5 mi to the west in the adjoining Freeland quadrangle), which appear to correlate stratigraphically with the nonglacial sediments south of East Point (unit Qc included within unit Qguc), have suggested affinities to the Olympia nonglacial interval (see discussion of unit Qc below under ‘Description of Map Units’). If correct, these affinities and the stratigraphic correlation between the section near East Point and that further south also suggest an Olympia age for the nonglacial deposits beneath the till mapped as Possession by Easterbrook. Such an association clearly conflicts with the assignment of the till to the Possession Glaciation. We conclude that the ages of both the till and the underlying nonglacial deposits remain unresolved. We mapped mostly Fraser Glaciation deposits (Armstrong and others, 1965) on upland surfaces and along some bluffs, and mass-wasting features along the remaining bluffs. The Fraser Glaciation correlates to stage 2 (~28–10 ka) (Booth and others, 2004). Advance outwash of the Vashon Stade (within the Fraser Glaciation) began to bury the Puget Lowland no sooner than about 20 ka if radiocarbon dates from northwest of the map area are accurate (Booth, 1994; Polenz and others, 2005; Johnson and others, 2001). Radiocarbon dates on detrital samples and thermoluminescence (TL) and OSL dates from southwest of the map area suggest that Fraser (Glaciation) outwash of the Vashon (and the preceding Evans Creek?) Stade(s) may have arrived earlier (Polenz and others, 2006). The Puget lobe of the Cordilleran ice sheet apparently arrived about 18 ka (~15,000 14 C yr B.P.)(Booth, 1991; Porter and Swanson, 1998; Booth and others, 2004; Polenz and others, 2005) and covered the map area with about 4000 ft of ice (Thorson, 1980, 1981, 1989). The end of the glaciation in the area is defined by a succession of events that began with grounded ice retreat alongside calving basins defining the northern margins of large proglacial lakes (Thorson, 1980; Porter and Swanson, 1998; Booth and others, 2004). Meltwater-driven incision of (typically relict) valleys that dissect the fluted uplands of southern Whidbey Island and western Camano Island (noted west of the map area by Polenz and others, 2006) indicates that as ice retreated at the end of the Vashon Stade, Whidbey Island and probably the western part of southern Camano Island were largely ice free, no longer submerged beneath ice-dammed lakes and not yet inundated by the Everson Interstade incursion of marine water. This seawater incursion raised base level for the meltwater streams to the glaciomarine limit (maximum relative seawater elevation), causing the lower portions of their valleys to be filled with fluvial and deltaic outwash before isostatic rebound began. Our interpretations of the local geomorphology place the glaciomarine limit at about 130 ft (at the southern quadrangle boundary) to 170 ft (at East Point), which is consistent with Thorson (1980, 1981), Dethier and others (1995), Easterbrook (2003), and Kovanen and Slaymaker (2004). Marine shells at Lake Carpenter (south of the map area) suggest that the Everson Interstade seawater incursion began before 14,610 14 C yr B.P., and freshwater gyttja (organic-rich lake-bottom sediment) from the same site suggests that it ended before 13,600 14 C yr B.P. (Anundsen and others, 1994), but that timeline conflicts with the apparent persistence of the Puget ice lobe in the central Puget Lowland through at least 13,800 14 C yr B.P. (Booth and others, 2004; Porter and Swanson, 1998), and various workers have favored or implied other dates for the start and (or) end of the incursion (Easterbrook, 1966a,b; Swanson, 1994; Dethier and others, 1995; Blunt and others, 1987; Porter and Swanson, 1998; Booth and others, 2004; Kovanen and Slaymaker, 2004; Polenz and others, 2005). The above age statements lack radiocarbon reservoir corrections, for which we refer interested readers to Hutchinson and others (2004). Drumlins formed by southward ice flow are overprinted with marine landforms (Kovanen and Slaymaker, 2004), such as terraces (typically units Qgom e and Qgom ee ), deltas (unit Qgom e ), and shorelines (typically unit Qgom ee ). Above the marine limit, upland surfaces are commonly dissected by relict valleys (Kovanen and Slaymaker, 2004) that expose a discontinuous, diverse, and mostly thin (0–15 ft) cover of lodgment till above thick sheets of apparent advance outwash sand (≤140 ft thick in a valley in the northwest corner of the map area) on Whidbey Island. We agree with Polenz and others (2006) that the valleys were cut by meltwater because: 1) valley formation required surface runoff in excess of that in the modern environment; 2) the valleys terminate at the marine limit, below which they are infilled with recessional outwash sand and marine delta deposits; and 3) the valleys lack active stream channels and some include dry, closed depressions where mass wasting from valley sidewalls has locally elevated the valley floor. STRUCTURE No demonstrably tectonic fault scarps have been mapped onshore, but the map area is tectonically active. The fault strands shown on this map are from Johnson and others (2000). The “Southern Whidbey Island Fault”, with possible Quaternary movement, was first inferred by Gower (1980). Johnson and others (1996) characterized the “southern Whidbey Island fault” as a long-lived transpressional zone that separates major crustal blocks. Brocher and others (2005) recently postulated that “the southern Whidbey Island fault…is a NW–SE oriented fold and thrust belt…[that accommodates] NE-directed crustal shortening”. Sherrod and others (2005a,b) showed the fault zone, about 95 mi long, extending from Vancouver Island to east of Seattle, with three to four main strands crossing Whidbey Island. Johnson and others (1996) noted that the Southern Whidbey Island fault zone has caused several historic shallow-crustal earthquakes and appears capable of generating earthquakes of surface-wave magnitude 7 or greater. Kelsey and others (2004) used Whidbey Island data to infer a moment-magnitude 6.5 to 7 earthquake on the fault zone about 3 ka. Sherrod and others (2005a,b) and Sherrod (2005) concluded that the Southern Whidbey Island fault zone “produced at least four events since about 16,400 years ago, the most recent after 2,700 years ago.” Johnson and others (2000) used seismic lines in Saratoga Passage to infer three northwest-trending unnamed faults between Whidbey and Camano Islands. Johnson and others (2000) suggest that the northern-most fault may be Pliocene to Pleistocene in age, while the other two may be of Pleistocene age. We show these faults, although we saw no compelling evidence for landward extensions. Although we did not recognize exposures anywhere in the map area that would provide compelling evidence of active faulting, we note an observation that is consistent with either of the above-mentioned interpretations (Johnson and others, 1996; Brocher and others, 2005) for the character of the Southern Whidbey Island fault zone—the topographically most elevated portions of southern Whidbey Island are systematically bracketed by the main mapped strands of the Southern Whidbey Island fault zone. We speculate that the lack of recognized fault scarps in the map area need not reflect a lack of surface-deforming fault events, but could result from diffusion of fault deformation in the thick, sandy deposits (notably unit Qgasv) that blanket much of the surface. DESCRIPTION OF MAP UNITS Postglacial Deposits HOLOCENE Fill—Clay, silt, sand, gravel, organic matter, rip-rap, and debris placed to elevate and reshape the land surface; includes engineered and nonengineered fills; shown where fill is readily verifiable, relatively extensive, and appears sufficiently thick to be geotechnically significant. Beach deposits—Sand and cobbles; may include boulders, silt, pebbles, and clay; pebbles and larger clasts typically well rounded and oblate; mostly well sorted; loose; derived from shore bluffs and underlying deposits and (or) carried in by longshore drift. Marsh deposits—Mostly soft to stiff, olive gray to gray silt and silty clay and bluish gray clay; commonly with lenses and layers of peat, muck, and other organic material; deposited in a saltwater or brackish marsh (lagoon) environment; deposits occur near highest tide and are covered with salt-tolerant vegetation and floated logs. We found unit Qm only on the Camano Island portion of the map area. HOLOCENE TO LATE PLEISTOCENE Qp Peat and freshwater marsh deposits—Organic and organic-rich sediment, typically in closed depressions; includes peat, muck, silt, and clay in and adjacent to wetlands; may locally grade down to and include the freshwater equivalent of unit Qm; thickness is highly variable, but bogs may contain more than 20 ft of peat and muck; queried where its interpretation was based largely on geologic setting and geomorphology with no supporting field data. Qmw Mass wasting deposits—Boulders, gravel, sand, silt, clay, and diamicton; generally unsorted, but locally stratified; typically loose; shown along mostly colluvium-covered or densely vegetated slopes that are demonstrably unstable or appear potentially unstable; commonly mapped where field conditions suggest landsliding, but evidence for landslides is ambiguous or weak; locally contains landslides and underlying units that either we could not map confidently or are too small to show at map scale; queried where its interpretation was based largely on geologic setting and geomorphology with no supporting field data. Qls Landslide deposits—Gravel, sand, silt, clay, and boulders in slide body and toe, includes exposure of underlying units in scarp areas; angular to rounded clasts; unsorted; generally loose, unstratified, broken, and chaotic, but may locally retain primary bedding; commonly includes liquefaction features; deposited by mass-wasting processes other than soil creep and frost heave; distinguished from unit Qmw by presence of unambiguous landslide features. Absence of a mapped slide does not imply absence of sliding or hazard. All shoreline bluffs in the map area are subject to episodic land sliding and bluff retreat, but many slides are too small to show at map scale, and most slide deposits are quickly removed by beach wave action. The most notable slides in the map area are located at Camano Head on Camano Island. Pleistocene Glacial and Nonglacial Deposits We used stratigraphic position, organic content, radiocarbon, OSL, and TL dating, fossil content, and provenance data to separate glacial from nonglacial deposits. Provenance was inferred from stratigraphic relations, age data, and sand-grain and gravel-clast composition as observed petrographically and in the field. Glacial deposits are dominated by northern provenance materials and therefore contain little or no Glacier Peak detritus (0–5%) and include “significant” granitic and metamorphic lithic clasts (Dragovich and others, 2005). Nonglacial deposits are distinguished by eastern provenance. We found some petrographic indications of possible ancestral Skagit and (or) Stillaguamish River provenance (hypersthene, hornblende, and possible Glacier Peak dacite) (Dragovich and others, 2005; Polenz and others, 2005; Joe Dragovich, Wash. Divn. of Geology and Earth Resources [DGER], oral commun., 2006), but agree with Terry Swanson (Univ. of Wash., oral commun., 2005) in suspecting that much of the sediment may be from an ancestral Snohomish River. Speculations of a Snohomish River source are based mostly on stratigraphic position and proximity to that basin; we lacked petrographic reference data from Snohomish basin sediments, but noted elevated levels of serpentinite and granitic and weathered volcanic rocks in some samples. That mix appears consistent with major bedrock units in the Snohomish River basin (Joe Dragovich, DGER, oral commun., 2006). DEPOSITS OF THE FRASER GLACIATION (PLEISTOCENE) Undivided Fraser Glaciation Qgd(f) Fraser drift, undivided—Composite of units deposited during the Fraser Glaciation; shown where map scale or exposure do not support stratigraphic division. Qgo(f) Recessional outwash—Mostly sand, but includes lenses and beds of pebble- and cobble-gravel, silt, sparse clasts of diamicton, and minor clay; gravelly facies tend to occur lower in the unit; loose; variably rounded; mostly moderately to well sorted; structureless to moderately stratified with medium to thick beds; forms either valley fill in relict, late glacial meltwater valleys or terraces above Everson Interstade marine shorelines; interfingers with adjoining Everson marine deltaic deposits (unit Qgom e ). We show crudely stratified pebbly sand and cobble gravel (which we assign to this unit) overlying Vashon till (unit Qgt v ) and underlying Everson glaciomarine deltaic outwash deposits (unit Qgom e ) cropping out ~50 to 80 ft above the beach at Lowell Point on Camano Island (Columnar Section 5). Most deposits within this unit probably date to Everson time, but the unit is assigned to the broader Fraser glacial period because some deposits may date to the Vashon Stade. Qgta(f) Ablation till—Unsorted, unstratified, and heterogeneous melt-out deposit of sand, silt, clay, gravel, diamicton, and patchy stratified proximal outwash deposits, ice-dammed lake sediment, and peat; loose to compact; typically forms geomorphically complex patchwork of melt-out deposits (material formerly carried by an ice mass, then left in place after the ice mass has melted away), outwash, closed depressions, erosional exposures of older (Fraser?) units, and incipient and larger drainages; queried where its interpretation was based largely on geologic setting and geomorphology with no supporting field data. Everson Interstade Qgdm(e) Everson Glaciomarine Drift—Clayey to silty diamicton with variable content of gravel clasts; also includes silt, clay, and sand; contains sparse shells, generally marine; dark gray where unweathered; mostly weathers to buff, but ranges to olive gray, ash gray, or white; commonly forms dry, vertical face with failure-prone vertical desiccation cracks with dark brown staining; massive to rhythmically bedded, commonly with sharp upper and lower unit-bounding unconformities (Domack, 1984); mostly loose and soft, but locally hard and compact; may resemble till (Domack, 1982, 1984; Domack and Lawson, 1985), but in general, till lacks fossils and glaciomarine drift has a finer-grained, smoother-feeling matrix, is less compact, and is more likely to be stratified. The unit is sea-floor sediment and consists mostly of glacial flour. Its diversity reflects proximity of the ice front (Domack, 1983; Dethier and others, 1995). Locally divided into: Glaciomarine drift, emergence (beach) facies—Sand and gravel, locally silty; loose; mostly structureless but may be stratified, laminated, or ripple crossbedded, with rare boulder lag deposits; typically only a few feet thick; occurs below the glaciomarine limit; underlain by units Qgdm e , Qgom e , or older sediments; represents emergence deposits that cap glaciomarine drift and may include terrestrial deposits (Domack, 1983; Dethier and others, 1995); includes characteristic but subtle benches that are paleo–beach berms at various elevations (Carlstad, 1992; Easterbrook, 2003; Kovanen and Slaymaker, 2004). Qgom(e) Glaciomarine deltaic outwash deposits—Mostly sand, with some sand–gravel mixtures with minor interlayered silt and silty sand; generally loose; maximum clast size limited to pebbles and small cobbles; west of Lone Lake on Whidbey Island this unit has a higher concentration of gravel, probably due to the proximity of paleo–side-channel erosion into a coarser facies of advance outwash (unit Qga v ); most deposits are at least a few tens of feet thick with a maximum observed thickness of about 130 ft near East Point on Whidbey Island; forms a marine delta–turbidite complex with a horizontally bedded, sandy sea-floor facies, an overlying delta-front foreset-bedded facies, and a capping deltaic top-set channelized facies. Qls(e?) Landslide deposits—Mix of till and outwash sand and gravel; identified as landslide based on surface morphology; queried because assigned to the Everson Interstade based only on field relations, low slope angle (~10% average), and subdued morphology. Vashon Stade Qgt(v) Till—Typically unweathered, unsorted mix of clay through boulder-size material (diamicton) deposited directly by ice; includes extensive areas of compact (advance outwash?) sand; compact, well-developed facies resemble concrete; locally ranges to loose in ablation till (also separately mapped as unit Qgtaf) and well-sorted in some sand-dominated areas; erratic boulders are common on the surface; gray where fresh; oxidizes yellowish brown; permeability very low in compact diamicton but locally high in sandy or loose facies; most commonly matrix supported; cobbles and boulders commonly faceted and (or) striated; may include flow banding; typically forms vertical faces in coastal bluffs; locally resembles unit Qgdm e ; lies stratigraphically between overlying units Qgdm e and Qgom e and underlying units Qga v and Qgas v ; may include unrecognized exposures of older till. Most till deposits have had their surface fluted by overriding ice and form a patchy and seemingly randomly distributed cover that varies from 0 to at least 200 ft thick, with 2 to 30 ft most common. Cliff exposures along the west shore of Whidbey Island, about 5 mi west of the map area, reveal that even where well developed and thick, lodgment till may locally pinch out across short distances, even in the center of a well-formed drumlin. Regional age data appear to constrain the age of the unit to between ~18 ka and the onset of the Everson Interstade. Qga(v) Advance outwash—Pebble to cobble gravel, sand, and some layers and lenses of silt and clay; may contain till fragments; gray to grayish brown and grayish orange; clasts typically well rounded, well sorted and clean; compact; mostly well stratified; very thinly to very thickly bedded; contains planar and graded beds, cut-and-fill structures, trough and ripple crossbeds and foresets; maximum thickness estimated at approximately 100 ft (separately mapped subunit Qgasv is locally thicker); deposited as proglacial fluvial (and deltaic) sediment; complete sections tend to coarsen upward; commonly overlain by unit Qgtv along a sharp contact and stratigraphically above units Qc and Qc o . The estimated age of the unit is ~18 to ≥20 ka. Locally divided into: Qgas(v) Advance outwash sand—Mostly lacustrine sand with layers of silt; thick exposures display well-developed crossbedding and cut-and-fill features that are typical of this unit; locally coarsens upward into gravel; thickness is typically 80 ft, with maximum estimated thickness of about 200 ft; extensive in subsurface; commonly forms angle-of-repose slopes along drainages and coastal bluffs; unit is the most widespread lithology of advance outwash occurring in the map area; queried where field identification was uncertain. Unit includes the Esperance Sand and possibly the Lawton Clay, the latter whose presence in the subsurface (on Whidbey Island) was noted in unpublished, site-specific geologic reports (Terry Swanson, Whidbey Geological Consulting, written commun., 2005) but not recognized by us in our mapping. Relict valleys are common on southern Whidbey Island (Polenz and others, 2006) and morphologically resemble many Camano Island valleys, suggesting that, like their Whidbey Island equivalent, these valleys on Camano Island are incised into unit Qgasv. They also typically lack modern streams, due to high permeability in the unit. Qgd(v) Vashon Drift, undivided—Composite of units Qgt v and Qga v (or Qgas v ); shown where map scale or exposure does not support stratigraphic division. Bluffs on Camano Island southeast of Elger Bay expose gray pebbly drift comprised of till and sand and gravel lenses, which grades southward to till (unit Qgt v ), overlying unsorted to moderately sorted gray pebbly sand and gravel (unit Qga v ). On Whidbey Island, south of East Point in mass wasting bluff exposures, unit consists of gray till (unit Qgt v ) overlying gray, crossbedded sand (unit Qgas v ). GLACIAL DEPOSITS OF UNKNOWN AGE (PLEISTOCENE) Qgt(u) Till of unknown age (columnar sections and line units only)—Unsorted mix of silt, sand, and gravel deposited dominantly as diamicton directly by glacier ice; dark gray; hard, compact with flow texture; pebble- to cobble-size clasts common; 15 to 20 ft thick where exposed at beach level for about 250 ft about ¼ mi west of downtown Langley (Columnar Section 2); lacks age control and is not assigned to Fraser glaciation because field relations suggest (but fail to demonstrate) older origin. Unit also assigned to a 15- to 20-ft-thick till exposed for several hundred feet in bluffs about 1300 ft south of East Point, where it overlies about 65 ft of nonglacial sediments (Columnar Section 1). DEPOSITS OF THE OLYMPIA NONGLACIAL INTERVAL (PLEISTOCENE) Armstrong and others (1965) defined “Olympia Interglaciation” as the “climatic episode immediately preceding the last major glaciation” and associated it with “nonglacial strata lying beneath Vashon Drift”. We associate those strata with stage 3 (~60–20 ka), but avoid the label “Olympia Interglaciation” because stage 3 is not a true interglacial period (as defined in fig. 4 of Morrison, 1991). Qc(o)? Nonglacial deposits (columnar sections and line units only)—An upper, tan to pale yellowish-gray, faintly to mostly nonbedded, fine- to coarse-grained sand and an underlying, laminated, gray to dark gray silt and clay (commonly wet); unit shown on map only at base of Columnar Section 4 just south of Mabana and as a discontinuous line unit near beach level on Camano Island, beginning ~4000 ft north of Camp Diana and continuing southward to Mabana. The upper, nonbedded sand is exposed nearly continuously from Mabana south to a point ~4000 ft north of Pebble Beach, where the unit has risen in elevation in the bluffs and can be traced no farther in the vegetation covering the bluffs. At the site of Columnar Section 4, the unit includes both the nonbedded sandy deposits and the underlying silt and clay. Diapirs and irregular intrusions (flame structures) of silt and clay (which we believe originated from the underlying silt-clay subunit, and which locally contain organic material [wood]) intrude the overlying nonbedded sand but do not extend into the crossbedded pebbly sands of unit Qgas v that are resting unconformably on unit Qc o ?. Both subunits thicken southward from Mabana, ranging from 10 to 20 ft thick for the nonbedded sand and 1 to 30 ft thick for the laminated silt and clay. Contact between the two subunits is not exposed because of the thick vegetation that covers most of the wet silt-clay subunit. Just south of Mabana (Columnar Section 4), we obtained an OSL age estimate of 40.4 ±4.8 ka (Table 1, loc. 3) from the nonbedded sand subunit. This exposure was formerly mapped as Whidbey Formation by Easterbrook (1968), apparently without direct age control. We query the unit (Qc o ?) because our tentative reassignment of the deposits to the Olympia nonglacial period is based only on a single OSL date, and the exposures that we saw lack stratigraphic age control that would assist in clarifying whether the deposits are properly associated with Whidbey or Olympia time. UNDIVIDED PRE-FRASER NONGLACIAL DEPOSITS (PLEISTOCENE) Qc Pre-Fraser nonglacial deposits, undivided—Sand, silt, clay, peat, some fine gravel, and rare medium gravel; compact; well stratified to massive; resembles sediments contained in nonglacial deposits of the Olympia nonglacial interval and Whidbey Formation mapped elsewhere on southern Whidbey Island (Polenz and others, 2006; Minard, 1985) and on Camano Island and is thought to be nonglacial, but may locally include unrecognized glacial material. Unit is primarily exposed in the generally east-facing coastal bluffs on Whidbey Island along Saratoga Passage that are prone to mass wasting. Unit is predominantly well-bedded, moderately well-sorted, tan silt and gray fine sand with sparsely occurring organic material. In the bluffs south of East Point, nonglacial sediments (included in unit Qguc) are shown as unit Qc (Columnar Section 1). Two infinite 14 C age estimates: >44,560 yr B.P. (on peat) and >45,000 14 C yr B.P. (obtained from detrital organic material) were collected from this unit (Table 1, locs. 1 and 2; Columnar Section 1). The ages are consistent with either an Olympia nonglacial or a Whidbey Formation (or older) origin. Additional age estimates and stratigraphic considerations west of East Point (Polenz and others, 2006) are weakly suggestive of an Olympia age for part or all of the exposures of unit Qc (included in unit Qguc) in the vicinity of East Point, but we cannot dismiss an older origin for at least part of the section, nor establish stratigraphic continuity along the section. We therefore assign all exposures of the pre-Fraser nonglacial deposits in the area to unit Qc (included in unit Qguc and not separately shown on the map to accommodate the presence in the same bluffs of apparent glacial deposits older than(?) Vashon till). Age control is also poorly constrained for unit Qc elsewhere in the map area on Whidbey Island. Farther south near Langley, organic matter collected from unit Qc has provided 14 C ages of ~35,000 yr B.P. (Terry Swanson, Whidbey Geological Consulting, written commun., 2005) suggesting that unit Qc in that area post-dates the Whidbey Formation and more likely represents sediments deposited during the Olympia nonglacial interval. Unit Qc lies stratigraphically below Fraser glacial deposits, but otherwise its age is unknown in the map area. INTERGLACIAL DEPOSITS OF THE WHIDBEY FORMATION (PLEISTOCENE) Qc(w) Whidbey Formation (columnar sections and line units only)—Sand, silt, and clay interbedded with peat and minor lenses of fine gravel; mostly weathered to a muted light-yellowish gray in exposed faces; includes flood plain and channel-sand facies. The flood plain facies, typically well stratified with characteristic wavy (ripple-marked) silt and clay beds, is slightly oxidized and forms prominent vertical bluffs. The channel-sand facies is typically clean, gray, and cross- to nonbedded. Peat beds vary in thickness from a few inches to a foot or more and are exposed in only a few scattered outcrops. Dacite clasts are a common component of sand and gravel facies and reflect an eastern provenance. Unit Qc w is mapped only on Camano Island. At southeast Lowell Point, we dated nonglacial sediments in two places (applying two different dating methods), which were separated from each other by only 20 ft laterally and 2.3 ft stratigraphically. A thin sand layer yielded an OSL date of 124.8 ±4.2 ka, and wood from a blue clay yielded an infinite 14 C date of >45,210 yr B.P. (Table 1, locs. 4 and 6; Columnar Section 5). Vashon till unconformably overlies the Whidbey Formation at this location, where the ice had apparently cut down through formerly overlying units to deposit the till directly on Whidbey Formation (Columnar Section 5). Farther south on Camano Island near Pebble Beach and northwest of Camano Head (Fig. 1), Easterbrook (1968) mapped nonglacial sediments above Double Bluff Drift (unit Qgpt d ) as Whidbey Formation. He also assigned similar deposits further north (where unit Qgpt d has dipped below sea level) to the Whidbey Formation. We dated a peat from nonglacial sediments near beach level just south of Pebble Beach (where Easterbrook [1968] formerly had mapped Whidbey Formation), which yielded an infinite radiocarbon date of >41,360 yr B.P. (Table 1, loc. 5; Columnar Section 3). We queried the unit in this area because of uncertainty surrounding its age. The radiocarbon date does not unambiguously identify the nonglacial sediments there as Whidbey Formation, and we have mapped Olympia nonglacial sediments (unit Qc o ?) to the north, where Easterbrook (1968) had mapped those sediments as Whidbey Formation. Although we could not demonstrate stratigraphic continuity between the exposures we mapped as unit Qc o ? to the north and those mapped as unit Qc w at Pebble Beach (line unit and Columnar Section 3), the gradual southward rise of unit Qc o ? suggests continuity, and we are left wondering if the deposits at Pebble Beach may also date to the Olympia nonglacial interval. DEPOSITS OF THE DOUBLE BLUFF GLACIATION (PLEISTOCENE) Double Bluff Drift (line units and Fig. 1 only)—Mostly till; gray, hard, compact; resembles Vashon till; drift includes gray cobble to boulder gravel that is uncemented and poorly sorted, with a silty matrix; grades northward to poorly exposed, nonbedded sand and pebble to cobble gravel that is locally iron-oxide stained; exposed at beach level with a south dip in a mass-wasted segment of sea cliffs. This unit is the oldest definitively glacial deposit recognized in the map area. Identification of the unit in the map area is based on prior mapping by Easterbrook (1968), who mapped Double Bluff at two places in the Camano Island portion of the map area: 1) northwest of Pebble Beach, where two beach-level exposures occur near each other, and 2) northwest of Camano Head in the lower and middle parts of the sea cliffs, where the drift consists mostly of compact gray till. Northwest of Pebble Beach, the more northerly exposure of till grades northward to glacial drift, which may include proglacial outwash of the Double Bluff Glaciation, but the details are obscured by vegetation and local mass wasting. The poor exposure makes it difficult to separate drift from overlying nonglacial deposits that were assigned by Easterbrook (1968) to the Whidbey Formation, and from the tentatively mapped Olympia nonglacial deposits (unit Qc o ?) that are exposed less than a quarter mile to the north. The deposits northwest of Camano Head that were mapped by Easterbrook (1968) as Double Bluff till are exposed for more than a quarter mile where till rises from beach level to mid bluff (Fig. 1). The till at this exposure overlies older nonglacial(?) sediments (unit Qc pd ?) but thins rapidly to the southeast, where it grades to a gravel unit in an area complicated by possible faulting. Berger and Easterbrook (1993) and Easterbrook and others (1992) used two thermoluminescence (TL) age estimates (291 ±86 ka and 280 ±81 ka; Table 1, locs. 7 and 8) to corroborate Easterbrook’s prior assignment of the exposures we included in this map unit to the Double Bluff Drift, but Berger and Easterbrook also cautioned about “the large TL analytical uncertainties”. The TL analyses were performed on clay that is exposed beneath the till (Berger and Easterbrook, 1993; Easterbrook and others, 1992). The two age estimates correlate well with a TL age of 289 ±74 ka from clay near the base of the Double Bluff Drift at its type section on southern Whidbey Island (Berger and Easterbrook, 1993). However, the age estimate for Double Bluff Drift at its Whidbey Island type section is tentative (Polenz and others, 2006), and we have little information regarding the reliability of the corresponding Camano Head age estimates. In the absence of direct age estimates, the section at Camano Head could have alternatively been interpreted as Olympia nonglacial deposits overlying Possession till above either Possession advance outwash or Whidbey Formation. UNNAMED DEPOSITS BENEATH DOUBLE BLUFF TILL (PLEISTOCENE) Nonglacial? deposits beneath Double Bluff till (line unit only)—Gray, coarse, pebbly, tangentially crossbedded sand (≤30 ft thick), containing concentrated bands of granule-size to very coarse sand-size (2–10 mm), white pumice grains (including a few sparse pebbles), sparse wood, and local pebble-gravel lenses; crops out (for ~1100 ft) northwest of Camano Head on Camano Island (Fig. 1) beneath deposits that were assigned by Easterbrook (1968) to the Double Bluff Drift—thus designating these (underlying sediments) as the oldest sediments exposed in the map area. The sediments were included with the Double Bluff Drift by Berger and Easterbrook (1993) and Minard (1985), although Easterbrook (1994a) seems to allow for the possibility that these sediments are separate from (predate) the Double Bluff Drift. We mapped these deposits as a separate unit because our field observations, petrographic characterization, and comparison of our data with observations of Peterson (2007) suggest a possibly nonglacial setting, although we can not rule out a (Double Bluff) glacial origin. Easterbrook (1994a) describes similar crossbedded sands, consisting almost entirely of pumice, south of Pebble Beach on Camano Island, which we presume to be part of the same unit. Laser-argon measurements on that pumice (lying stratigraphically between Double Bluff till and the TL-dated clay beneath the till [see unit Qgpt d ]) gave age estimates of 0.98 and 1.04 Ma on plagioclase and biotite, respectively (Easterbrook, 1994a; Table 1, loc 9). Easterbrook (1994a) suggested that the age discrepancy between the TL ages and laser-argon ages could be explained by the detrital nature of the pumice. He also noted that the concentration of the Camano pumice clasts argues against reworking of an older tephra. He further noted that major-element geochemistry of the Camano pumice, as determined by microprobe, did not match the geochemistry of the Lake Tapps tephra, which reportedly yielded fission-track age estimates ranging from 0.65 to 1.06 Ma (Westgate and others, 1987; Easterbrook, 1994a). He concluded that the lack of a chemical match suggests that the two tephras in this unit do not share the same origin, despite their similar age estimates. We collected a new sample of reworked pumice clasts from this unit (presumably southeast of Easterbrook’s sample) from a faulted(?) section of the lower bluffs 2000 ft northwest of Camano Head. The new sample was collected near the southeast end of the unit Qc pd ? exposure, where the unit disappears below sea level. We had the glass chemistry of our sample analyzed (13 clasts individually microprobe-analyzed) at Washington State University (see electron microprobe site). The results indicate that all 13 clasts came from the same tephra, and standard deviations for individual clasts are small. Comparison of the average of our analytical results with Easterbrook’s earlier samples from the same unit yielded a similarity coefficient of 0.74, which suggests no genetic relationship between the two tephra samples (Nick Foit, Wash. State Univ.,written commun., 2007). Assuming that the results of our chemical analyses and Easterbrook’s are both accurate, two unrelated tephras are present in the unit. Our samples were found to have similarity coefficients of 0.95 to 0.96 with tephras previously collected on Whidbey Island (Carlstad, 1992). The major-element chemistry of our tephra also yielded a similarity coefficient of 0.93 compared with the Lake Tapps tephra, suggesting that the best known match for both our Camano Island samples and those from Whidbey Island is the Lake Tapps tephra (Nick Foit, Wash. State Univ., written commun., 2007). Foit cautions, however, that the above similarity coefficients are still marginal. UNDIVIDED GLACIAL AND NONGLACIAL DEPOSITS OLDER THAN VASHON TILL (PLEISTOCENE) Qguc Undifferentiated deposits older than Vashon till—Locally includes sand, gravel, diamicton, silt, clay, and peat; compact; well stratified to nonstratified; composite of glacial and nonglacial deposits or of deposits of unresolved association; pre-dates Vashon till but may include Vashon advance outwash. Shown along coastal bluffs where poor exposure or map scale does not allow for more detailed subdivision. Peat and detrital organic matter from nonglacial sediments at East Point on Whidbey Island (Table 1, locs. 1 and 2) yielded radiocarbon-infinite age estimates that are discussed in the unit description for unit Qc. On Camano Island, unit Qguc includes nonglacial sediments Qc o ?, Qc w , and Qc pd ? and glacial sediments Qgpt d . These have yielded radiocarbon-infinite age estimates that are discussed under units Qc and Qc w . Undivided Quaternary Deposits Qu Pleistocene deposits, undivided—Locally includes sand, gravel, diamicton, silt, clay, and peat; ranges from loose to compact; well-stratified to nonstratified; composite of glacial and (or) nonglacial deposits; ranges in age (in coastal bluff exposures at Lowell Point [Columnar Section 5]) from Everson glaciomarine outwash deltaic deposits (unit Qgom e ) to Whidbey Formation (unit Qc w ); shown where map scale does not allow for more detailed subdivision. GEOLOGIC SYMBOLS Contact—Long dashed where approximately located; short dashed where inferred Fault, unknown offset—Dotted where concealed; dashed where inferred Geologic unit too narrow to show as a polygon at map scale Strand line (former shoreline) Arrow showing direction of landslide movement Age-date sample (radiocarbon) Age-date sample (optically stimulated luminescence) Electron microprobe site Figure location Columnar section location ACKNOWLEDGMENTS This geologic map was funded in part by the U.S. Geological Survey (USGS) National Cooperative Geologic Mapping Program, agreement nos. 05HQAG0085 for 2005–2006 (South Whidbey Island) and 06HQAG0035 for 2006–2007 (Camano Island). We thank Doug Kelly (Island Co. Health Dept.) and other Island County staff for geologic, hydrogeologic, geotechnical, and other assistance and records; Jerry Thorsen (DGER, retired) for supplying us his field notes on some sea cliff exposures on Whidbey Island and much of Camano Island, which both complimented and supplemented our field observations; Jon Peterson (M.S. candidate, Western Wash. Univ.) for assistance with our petrographic characterization and interpretation of sand samples, which attempted to match his systematic approach to distinguishing nonglacial from glacial deposits on Camano Island; Terry Swanson (Univ. of Wash.) for assistance with field work and some personal observations that we incorporated into our report; Nick Foit (Wash. State Univ.) for providing an electron microprobe glass chemistry analysis (at no cost) of the tephra near Camano Head and also providing similarity coefficient comparisons with other tephra chemistry; and Sam Johnson and Brian Sherrod (both USGS) for consultations on the tectonic setting. Thanks also to the Washington State Department of Ecology for permission to use proprietary shoreline aerial photos for our fieldwork, and to fellow DGER staff members Josh Logan and Joe Dragovich for technical assistance and reviews. Last, but not least, thanks to the uncounted people who permitted us to study and sample geologic exposures on their land and provided site-specific records and local expertise. REFERENCES CITED Anundsen, Karl; Abella, S. E. B.; Leopold, E. B.; Stuiver, Minze; Turner, Sheila, 1994, Late-glacial and early Holocene sea-level fluctuations in the central Puget Lowland, Washington, inferred from lake sediments: Quaternary Research, v. 42, no. 2, p. 149-161. Armstrong, J. 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K., 2005, Geologic map of the Oak Harbor, Crescent Harbor, and part of the Smith Island 7.5-minute quadrangles, Island County, Washington: Washington Division of Geology and Earth Resources Geologic Map GM-59, 2 sheets, scale 1:24,000. Easterbrook, D. J., 1966a, Glaciomarine environments and the Fraser glaciation in northwest Washington—Guidebook for First Annual Field Conference, Pacific Coast Section, Friends of the Pleistocene, September 24–25, 1966: [Privately published by the author], 52 p. Easterbrook, D. J., 1966b, Radiocarbon chronology of late Pleistocene deposits in northwest Washington: Science, v. 152, no. 3723, p. 764-767. Easterbrook, D. J., 1968, Pleistocene stratigraphy of Island County: Washington Department of Water Resources Water-Supply Bulletin 25, part 1, 34 p., 1 plate (in 4 parts). Easterbrook, D. J., 1994a, Chronology of pre-late Wisconsin Pleistocene sediments in the Puget Lowland, Washington. In Lasmanis, Raymond; Cheney, E. S., convenors, Regional geology of Washington State: Washington Division of Geology and Earth Resources Bulletin 80, p. 191-206. Easterbrook, D. J., 1994b, Stratigraphy and chronology of early to late Pleistocene glacial and interglacial sediments in the Puget Lowland, Washington. In Swanson, D. A.; Haugerud, R. A., editors, Geologic field trips in the Pacific Northwest: University of Washington Department of Geological Sciences, v. 1, p. 1J 1 - 1J 38. Easterbrook, D. J., 2003, Cordilleran ice sheet glaciation of the Puget Lowland and Columbia Plateau and alpine glaciation of the North Cascade Range, Washington. In Swanson, T. W., editor, Western Cordillera and adjacent areas: Geological Society of America Field Guide 4, p. 137-157. Easterbrook, D. J.; Berger, G. W.; Walter, R., 1992, Laser argon and TL dating of early and middle Pleistocene glaciations in the Puget Lowland, Washington [abstract]: Geological Society of America Abstracts with Programs, v. 24, no. 5, p. 22. Gower, H. D., 1980, Bedrock geologic and Quaternary tectonic map of the Port Townsend area, Washington: U.S. Geological Survey Open-File Report 80-1174, 1 sheet, scale 1:100,000, with 19 p. text. Hutchinson, Ian; James, T. S.; Reimer, P. J.; Bornhold, B. D.; Clague, J. J., 2004, Marine and limnic radiocarbon reservoir corrections for studies of late- and postglacial environments in Georgia Basin and Puget Lowland, British Columbia, Canada and Washington, USA: Quaternary Research 61, v. 2, p. 193-203. Johnson, S. Y.; Dadisman, S. V.; Mosher, D. C.; Blakely, R. J.; Childs, J. R., 2001, Active tectonics of the Devils Mountain fault and related structures, northern Puget Lowland and eastern Strait of Juan de Fuca region, Pacific Northwest: U.S. Geological Survey Professional Paper 1643, 45 p., 2 plates. Johnson, S. Y.; Mosher, D. C.; Dadisman, S. V.; Childs, J. R.; Rhea, S. B., 2000, Tertiary and Quaternary structures of the eastern Juan de Fuca Strait—Interpreted map. In Mosher, D. C.; Johnson, S. Y., editors; and others, Neotectonics of the eastern Juan de Fuca Strait—A digital geological and geophysical atlas: Geological Survey of Canada Open File Report 3931, 1 CD-ROM disk Johnson, S. Y.; Potter, C. J.; Armentrout, J. M.; Miller, J. J.; Finn, C. A.; Weaver, C. S., 1996, The southern Whidbey Island fault—An active structure in the Puget Lowland, Washington: Geological Society of America Bulletin, v. 108, no. 3, p. 334-354, 1 plate. Jones, M. A., 1999, Geologic framework for the Puget Sound aquifer system, Washington and British Columbia: U.S. Geological Survey Professional Paper 1424-C, 31 p., 18 plates. Kelsey, H. M.; Sherrod, Brian; Johnson, S. Y.; Dadisman, S. V., 2004, Land-level changes from a late Holocene earthquake in the northern Puget Lowland, Washington: Geology, v. 32, no. 6, p. 469-472. Kovanen, D. J.; Slaymaker, Olav, 2004, Relict shorelines and ice flow patterns of the northern Puget Lowland from lidar data and digital terrain modelling: Geografiska Annaler, Series A, Physical Geography, v. 86, no. 4, p. 385-400. Minard, J. P., 1985, Geologic map of the Tulalip quadrangle, Snohomish County, Washington: U.S. Geological Survey Miscellaneous Field Studies Map MF-1744, 1 sheet, scale 1:24,000. Morrison, R. B., 1991, Introduction. In Morrison, R. B., editor, Quaternary nonglacial geology—Conterminous U.S.: Geological Society of America DNAG Geology of North America, v. K-2, 672 p., 8 plates in accompanying case. Mosher, D. C.; Johnson, S. Y., editors; Rathwell, G. J.; Kung, R. B.; Rhea, S. B., compilers, 2000, Neotectonics of the eastern Juan de Fuca Strait—A digital geological and geophysical atlas: Geological Survey of Canada Open File Report 3931, 1 CD-ROM disk. Peterson, Jonathan, 2007, Petrographic signature for the Whidbey Formation: Western Washington University Master of Science thesis, 157 p. Polenz, Michael; Schasse, H. W.; Petersen, B. B., 2006, Geologic map of the Freeland and northern part of the Hansville 7.5-minute quadrangles, Island County, Washington: Washington Division of Geology and Earth Resources Geologic Map GM-64, 1 sheet, scale 1:24,000. Polenz, Michael; Slaughter, S. L.; Thorsen, G. W., 2005, Geologic map of the Coupeville and part of the Port Townsend North 7.5-minute quadrangles, Island County, Washington: Washington Division of Geology and Earth Resources Geologic Map GM-58, 1 sheet, scale 1:24,000. Porter, S. C.; Swanson, T. W., 1998, Radiocarbon age constraints on rates of advance and retreat of the Puget lobe of the Cordilleran ice sheet during the last glaciation: Quaternary Research, v. 50, no. 3, p. 205-213. Sherrod, B. L., 2005, Prehistoric earthquakes in the Puget Lowland, Washington [abstract]: Eos (American Geophysical Union Transactions), v. 86, no. 52, p. F1458-1459. Sherrod, Brian; Blakely, R. J.; Weaver, Craig; Kelsey, H. M.; Barnett, Elizabeth; Wells, Ray, 2005a, Holocene fault scarps and shallow magnetic anomalies along the southern Whidbey Island fault zone near Woodinville, Washington [abstract]: Eos (American Geophysical Union Transactions), v. 86, no. 52, p. F1438. Sherrod, B. L.; Blakely, R. J.; Weaver, Craig; Kelsey, Harvey; Barnett, Elizabeth; Wells, Ray, 2005b, Holocene fault scarps and shallow magnetic anomalies along the southern Whidbey Island fault zone near Woodinville, Washington: U.S. Geological Survey Open-File Report 2005-1136, 35 p. [http://pubs.usgs.gov/of/2005/1136/] Swanson, T. W., 1994, Determination of 36 Cl production rates from the deglaciation history of Whidbey Island, Washington: University of Washington Doctor of Philosophy thesis, 121 p. Thorson, R. M., 1980, Ice-sheet glaciation of the Puget Lowland, Washington, during the Vashon Stade (late Pleistocene): Quaternary Research, v. 13, no. 3, p. 303-321. Thorson, R. M., 1981, Isostatic effects of the last glaciation in the Puget Lowland, Washington: U.S. Geological Survey Open-File Report 81-370, 100 p., 1 plate. Thorson, R. M., 1989, Glacio-isostatic response of the Puget Sound area, Washington: Geological Society of America Bulletin, v. 101, no. 9, p. 1163-1174. Westgate, J. A.; Easterbrook, D. J.; Naeser, N. D.; Carson, R. J., 1987, Lake Tapps tephra—An early Pleistocene stratigraphic marker in the Puget Lowland, Washington: Quaternary Research, v. 28, no. 3, p. 340-355. Fig. 1 Qgt u 1 3 1 ) ( ) Qc Qga v Qguc Qb af Qgas v Qgd f Qgd v Qgdm e Qgo f Qgom e Qgom ee Qgt v Qgta f Qls Qls e? Qmw Qm Qp Qu Qc o ? Qgt u Qc w Qc pd ? Qgpt d Qc w Qc o ? Qgtu Figure 1. Photo looking east at west-facing bluffs on south Camano Island, 1 mi south of Pebble Beach. Lower unit Qc pd ? consists of 25 to 45 feet of south-dipping crossbedded nonglacial sand with local gravel lenses; unit becomes pumiceous to the south and pinches out about 900 ft south of photo. Gray lodgment till (unit Qgpt d [Double Bluff Drift] 40- to 45-ft thick) overlies unit Qc pd ?; till thins and grades to a cobbly pebbly gravel 800 to 900 ft to the south. Unit Qc w ? (Whidbey Formation) overlies the till (unit Qgpt d ) and consists of 40 to 50 ft of tan stratified sand and silt with sparse, thin gravel lenses. Upper part of bluff in photo may also include Vashon advance outwash sand (unit Qgas v and thin (1 ft or less) of Vashon till (unit Qgt v ) not shown in the photo. Qc w ? Qgpt d colluvium Qc pd ?

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Page 1: MAJOR FINDINGS HOLOCENE TO LATE PLEISTOCENE … · peat overlain by ~25 ft of fine-grained sand and silt and underlain by 10 ft of fine-grained ... lab reports precision of analysis

Table 1. Age control data from the map area. Lab uncertainty values include random errors and are one standard deviation (68% confidence interval), except where shown in quotation marks, indicating that we do not know how many standard deviations are included. 14C, radiocarbon analysis by liquid scintillation counting; AMS, radiocarbon analysis by atomic mass spectrometry; OSL, optically stimulated luminescence; laser Ar, laser-argon analysis; TL, thermoluminescence. New radiocarbon analyses from this study were performed by Beta Analytic, Inc. (Miami, Fla.) and are 'conventional' (that is, adjusted for measured 14C/13C ratio); OSL analyses were performed by University of Illinois at Chicago Luminescence Dating Research Laboratory. Radiocarbon age estimates are in radiocarbon years before 1950 (14C yr. B.P.); ages stated in ‘ka’ are in thousands of calendar years before 1950. Elevations are in feet above mean sea level, as estimated in this study using lidar data (supplemented by visual elevation estimates on bluffs) and referring to the North American Datum Conversion Utility (NADCON; http://geodesy.noaa.gov/TOOLS/Nadcon/Nadcon.html) for vertical datum shifts between lidar data and map elevations.

Loc.no. estimate

14C yr B.P.

14C yr B.P.

14C yr B.P.

14C yr B.P.

13C/12C(‰)

Analytical Age method Material dated

Geologic unit Lab no.

Elev.(ft) Reference Notes

1 >44,560 -26.8 14C wood in peat above brown underclay

Qguc Beta 212312 ~18 this report Sampled wood from peat bed from 3 ft exposure in mostly covered roadcut ~6 ft above East Point Drive where road starts to climb in elevation just past switchback; peat bed overlain by 2 ft of exposed bedded gray silt/clay; underlain by 8 in. exposed compact brown (under)clay

2 >45,000 -24.2 AMS probable detrital organic material

Qguc(Qc)

Beta 212313 ~35 this report Organic material occurs in 4-in.-thick zone in bedded clay/silt in sea cliff exposure ~28 ft above beach; organics overlie interbedded sand/silt unit; underlies ~10 ft of gray sand with tan silt

3 40.4 ±4.8 ka OSL* sand (core) Qco? UIC 1893 ~10 this report Core sample collected from 12-ft-thick massive grey sand unit 4.25 ft above beach (Columnar Section 4); sampled unit located just south of Mabana; N 48.09233, W 122.41367

4 124.8 ±14.2 ka

OSL* sand (core) Qcw UIC 1910 ~9 this report Core sample collected from 6 in. sand bed located 3 ft above beach; medium-grained sand; collected from a 13 ft unit consisting of interbedded silty sand, sand, silt, and silty clay beneath ~60 ft of exposed till, 20 ft pebbly sand, 20 ft grey gravel, and 15 ft tan silt and sand in an 80 ft high bluff located on the southeast side of Lowell Point; N 48.12227, W 122.48470

5 >41,360 -28.4 14C peat from 7-inch thick peat layer

Qcw? Beta 223835 ~16 this report Sample of 7-in.-thick peat bed exposed in sea cliff ~10 ft above high tide in 30-35 ft exposure of peat overlain by ~25 ft of fine-grained sand and silt and underlain by 10 ft of fine-grained sand and silt; peat located just south of Pebble Beach; N 48.06360, W 122.38512

6 >45,210 -28 14C wood from blue-gray silty clay

Qcw Beta 223836 ~6 this report Woody pieces collected from blue-grey silty clay ~8 in. above beach, southeast side of Lowell Point; silt located near the base of a 12-15 ft thick interbedded flood plain facies of interbedded silt, sand, and clay; overlain by ~60 ft of glacial till; N 48.12227, W 122.48470

Locations not understood well enough to plot (see Notes)

7 280 “±81” ka TL clay Qguc Easterbrook and others (1992)

Clay beneath till correlated with Double Bluff Drift, at Pebble Beach, Camano Island

8 291 “±86” ka TL clay Qguc Berger and Easterbrook (1993)

Clay beneath Double Bluff Drift at Pebble Beach, Camano Island

9 980 “±70” ka (plagioclase), 1040 “±40” ka (biotite)

laser Ar pumice Qcpd? Easterbrook (1994a) Pumice from cross bedded sand that is exposed between Double Bluff Drift and clays that were TL dated by Berger and Easterbrook (1993); sample location is at Pebble Beach, Camano Island

* Analysis on 4 to 11 micron quartz grain fraction, by blue light excitation using a multiple aliquot regenerative dose method; blue light emissions measured with 3 mm thick Schott BG-39 and Corning 7-59 glass filters that block >90% of luminescence below 390 and above 490 nm; lab reports precision of analysis as “good to very good”.

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ISLAND FAULT ZONE

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x x x x

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OSL age: 40.4 ±4.8 ka(Table 1, loc. 3)

OSL age: 124.8 ±14.2 ka (Table 1, loc. 4)

14C age: >41,360 yr B.P. (Table 1, loc. 5)

14C age: >45,210 yr B.P. (Table 1, loc. 6)

unconformable contact

unconformable contact

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SCALE 1:24,000

Geologic Map of the Langley and Western Part of the Tulalip7.5-minute Quadrangles, Island County, Washington

by Henry W. Schasse, Michael L. Kalk, Bradley B. Petersen, and Michael Polenz

February 2009

Lambert conformal conic projectionNorth American Datum of 1927; to place on North American Datum of 1983, move the projection

lines approximately 17.9 meters north and 94.2 meters east as shown by crosshair corner ticksBase map from scanned and rectified U.S. Geological Survey Langley 7.5-minute quadrangle,

1956, photorevised 1968 and from the scanned and rectified U.S. Geological Survey Tulalip 7.5-minute quadrangle, 1956, photorevised 1968

Shaded relief generated from a lidar bare-earth digital elevation model (available from Puget Sound Lidar Consortium, http://pugetsoundlidar.ess.washington.edu/); sun azimuth 315°; sun angle 45°; vertical exaggeration 6x

Digital cartography and GIS by Anne C. Heinitz, Charles G. Caruthers, Elizabeth E. Thompson, J. Eric Schuster, and Robert H. Berwick

Editing and production by Jaretta M. Roloff and Karen D. Meyers

WASHINGTON DIVISION OF GEOLOGY AND EARTH RESOURCESGEOLOGIC MAP GM-69

Langley and Western Part of the Tulalip 7.5-minute QuadranglesFebruary 2009

depth contours in feet—datum is mean lower low water

APPROXIMATE MEANDECLINATION, 2007

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17°38¢

Suggested citation: Schasse, Henry W.; Kalk, Michael L.; Petersen, Bradley B.; Polenz, Michael, 2009, Geologic map of the Langley and western part of the Tulalip 7.5-minute quadrangles, Island County, Washington: Washington Division of Geology and Earth Resources Geologic Map GM-69, 1 sheet, scale 1:24,000.

This geologic map was funded in part by the U.S. Geological Survey National Cooperative Geologic Mapping Program.

Disclaimer: This product is provided ‘as is’ without warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of merchantability and fitness for a particular use. The Washington Department of Natural Resources and the authors of this product will not be liable to the user of this product for any activity involving the product with respect to the following: (a) lost profits, lost savings, or any other consequential damages; (b) the fitness of the product for a particular purpose; or (c) use of the product or results obtained from use of the product. This product is considered to be exempt from the Geologist Licensing Act [RCW 18.220.190 (4)] because it is geological research conducted by the State of Washington, Department of Natural Resources, Division of Geology and Earth Resources.

© 2009 Washington Division of Geology and Earth Resourceshttp://www.dnr.wa.gov/AboutDNR/Divisions/GER/

MAJOR FINDINGS

Mapping of the quadrangles has resulted in the following improvements to and understanding of the geology of the area:• It provides the first geologic map of the area published at a scale of 1:24,000.• Lidar (laser swath mapping) has provided the means for improved geomorphologic interpretation of

the landforms in the area, thus allowing more accurate mapping of surficial geologic units, especially those relating to the Everson interstadial period and the relative sea level high stand (sea level maximum) associated with that period.

• New radiocarbon (14C) and optically stimulated luminescence (OSL) age estimates constrain the ages of some nonglacial sediments mapped in the sea cliffs of Camano and Whidbey Islands.

• The combination of lidar and shoreline oblique aerial photographs of the shoreline bluffs have allowed us to more accurately locate geologic units exposed in the bluffs.

GEOLOGIC SETTING AND DEVELOPMENT

The map area is covered by 650 to 3300 ft of glacial and nonglacial sediment (Jones, 1999; Mosher and others, 2000; Johnson and others, 2001) above Miocene(?) bedrock (Johnson and others, 1996). The oldest unit in the map area consists of unnamed, apparently nonglacial sediments (unit Qcpd?) that may pre-date the Double Bluff Glaciation, provided the overlying till is correctly assigned to the Double Bluff Drift (unit Qgptd). These sediments crop out along the lower west-facing bluffs on southern Camano Island (Fig. 1). Although tephras found within these sediments have been dated at about 1 Ma, they are probably detrital and thus do not represent the depositional age of the unit, leaving its correlation to a specific marine oxygen-isotope stage unresolved. Stage numbers and corresponding ages used in this report are as defined in Morrison (1991).

The Double Bluff Drift is the oldest sediment of clearly glacial origin exposed in the map area and overlies unit Qcpd? on Camano Island (Fig. 1). Identification of the Double Bluff Drift in the area is based on earlier studies of Easterbrook (1968). Booth and others (2004, fig. 2) correlate the drift to stage 6 (~185–125 ka), although Berger and Easterbrook (1993), Easterbrook and others (1992), Blunt and others (1987), and Easterbrook (1994a,b) report older ages for some exposures. Although we did not map Double Bluff Drift on Whidbey Island, a till of unknown age (unit Qgtu) crops out in coastal bluffs just west of Langley (Columnar Section 2) and occurs stratigraphically below pre-Fraser nonglacial deposits (unit Qc) in this area. At this location, unit Qgtu could be the Double Bluff Drift, if overlying unit Qc correlates with the Whidbey Formation of interglacial stage 5 (~125–80 ka) (Blunt and others, 1987; Easterbrook, 1994a; Dragovich and others, 2005; Polenz and others, 2005). Easterbrook (1968) and Minard (1985) both suggest that nonglacial sediments exposed in the lower parts of east-facing bluffs along Saratoga Passage on Whidbey Island belong to the Whidbey Formation. However, in the absence of age control, we could not confidently identify units Qgtu (west of Langley) and Qc along Saratoga Passage on Whidbey Island as Double Bluff Drift and Whidbey Formation, respectively.

We tentatively assign peat-bearing sands and silts that overlie the Double Bluff Drift on southern Camano Island near Pebble Beach to the Whidbey Formation. A single radiocarbon-infinite age estimate of >41,360 yr B.P. (Table 1, loc. 5; Columnar Section 3) did not allow us to confidently assign these sediments to the Whidbey Formation, although Easterbrook (1968) had previously assigned them to that unit. We limit ourselves to a tentative assignment because the above radiocarbon age estimate is also consistent with an Olympia age (stage 3, ~60–20 ka).

We did not identify Possession Glaciation deposits (stage 4, ~80–60 ka) (Booth and others, 2004). Possession till may be present on Whidbey Island as suggested by Easterbrook (1968), who assigned till exposed in the bluffs south of East Point to the Possession Glaciation, but we cannot confidently assign this till (appearing in partial exposures 70 ft above the beach) to the Possession Glaciation and have therefore given it the designation Qgtu (Columnar Section 1). Two radiocarbon-infinite ages of >44,560 and >45,000 yr B.P. (Table 1, locs. 1 and 2; Columnar Section 1) south of East Point do not allow us to assign unit Qc confidently to either the Whidbey Formation or the Olympia nonglacial interval (stage 3, ~60–20 ka). The two aforementioned radiocarbon-infinite dates lie stratigraphically beneath the till (unit Qgtu) in the vicinity of East Point, which is consistent with a Possession or older age for the till. However, sediments exposed in the lower bluffs between East Point and Rocky Point (1.5 mi to the west in the adjoining Freeland quadrangle), which appear to correlate stratigraphically with the nonglacial sediments south of East Point (unit Qc included within unit Qguc), have suggested affinities to the Olympia nonglacial interval (see discussion of unit Qc below under ‘Description of Map Units’). If correct, these affinities and the stratigraphic correlation between the section near East Point and that further south also suggest an Olympia age for the nonglacial deposits beneath the till mapped as Possession by Easterbrook. Such an association clearly conflicts with the assignment of the till to the Possession Glaciation. We conclude that the ages of both the till and the underlying nonglacial deposits remain unresolved.

We mapped mostly Fraser Glaciation deposits (Armstrong and others, 1965) on upland surfaces and along some bluffs, and mass-wasting features along the remaining bluffs. The Fraser Glaciation correlates to stage 2 (~28–10 ka) (Booth and others, 2004). Advance outwash of the Vashon Stade (within the Fraser Glaciation) began to bury the Puget Lowland no sooner than about 20 ka if radiocarbon dates from northwest of the map area are accurate (Booth, 1994; Polenz and others, 2005; Johnson and others, 2001). Radiocarbon dates on detrital samples and thermoluminescence (TL) and OSL dates from southwest of the map area suggest that Fraser (Glaciation) outwash of the Vashon (and the preceding Evans Creek?) Stade(s) may have arrived earlier (Polenz and others, 2006). The Puget lobe of the Cordilleran ice sheet apparently arrived about 18 ka (~15,000 14C yr B.P.)(Booth, 1991; Porter and Swanson, 1998; Booth and others, 2004; Polenz and others, 2005) and covered the map area with about 4000 ft of ice (Thorson, 1980, 1981, 1989).

The end of the glaciation in the area is defined by a succession of events that began with grounded ice retreat alongside calving basins defining the northern margins of large proglacial lakes (Thorson, 1980; Porter and Swanson, 1998; Booth and others, 2004). Meltwater-driven incision of (typically relict) valleys that dissect the fluted uplands of southern Whidbey Island and western Camano Island (noted west of the map area by Polenz and others, 2006) indicates that as ice retreated at the end of the Vashon Stade, Whidbey Island and probably the western part of southern Camano Island were largely ice free, no longer submerged beneath ice-dammed lakes and not yet inundated by the Everson Interstade incursion of marine water. This seawater incursion raised base level for the meltwater streams to the glaciomarine limit (maximum relative seawater elevation), causing the lower portions of their valleys to be filled with fluvial and deltaic outwash before isostatic rebound began. Our interpretations of the local geomorphology place the glaciomarine limit at about 130 ft (at the southern quadrangle boundary) to 170 ft (at East Point), which is consistent with Thorson (1980, 1981), Dethier and others (1995), Easterbrook (2003), and Kovanen and Slaymaker (2004).

Marine shells at Lake Carpenter (south of the map area) suggest that the Everson Interstade seawater incursion began before 14,610 14C yr B.P., and freshwater gyttja (organic-rich lake-bottom sediment) from the same site suggests that it ended before 13,600 14C yr B.P. (Anundsen and others, 1994), but that timeline conflicts with the apparent persistence of the Puget ice lobe in the central Puget Lowland through at least 13,800 14C yr B.P. (Booth and others, 2004; Porter and Swanson, 1998), and various workers have favored or implied other dates for the start and (or) end of the incursion (Easterbrook, 1966a,b; Swanson, 1994; Dethier and others, 1995; Blunt and others, 1987; Porter and Swanson, 1998; Booth and others, 2004; Kovanen and Slaymaker, 2004; Polenz and others, 2005). The above age statements lack radiocarbon reservoir corrections, for which we refer interested readers to Hutchinson and others (2004).

Drumlins formed by southward ice flow are overprinted with marine landforms (Kovanen and Slaymaker, 2004), such as terraces (typically units Qgome and Qgomee), deltas (unit Qgome), and shorelines (typically unit Qgomee). Above the marine limit, upland surfaces are commonly dissected by relict valleys (Kovanen and Slaymaker, 2004) that expose a discontinuous, diverse, and mostly thin (0–15 ft) cover of lodgment till above thick sheets of apparent advance outwash sand (≤140 ft thick in a valley in the northwest corner of the map area) on Whidbey Island. We agree with Polenz and others (2006) that the valleys were cut by meltwater because: 1) valley formation required surface runoff in excess of that in the modern environment; 2) the valleys terminate at the marine limit, below which they are infilled with recessional outwash sand and marine delta deposits; and 3) the valleys lack active stream channels and some include dry, closed depressions where mass wasting from valley sidewalls has locally elevated the valley floor.

STRUCTURE

No demonstrably tectonic fault scarps have been mapped onshore, but the map area is tectonically active. The fault strands shown on this map are from Johnson and others (2000). The “Southern Whidbey Island Fault”, with possible Quaternary movement, was first inferred by Gower (1980). Johnson and others (1996) characterized the “southern Whidbey Island fault” as a long-lived transpressional zone that separates major crustal blocks. Brocher and others (2005) recently postulated that “the southern Whidbey Island fault…is a NW–SE oriented fold and thrust belt…[that accommodates] NE-directed crustal shortening”. Sherrod and others (2005a,b) showed the fault zone, about 95 mi long, extending from Vancouver Island to east of Seattle, with three to four main strands crossing Whidbey Island. Johnson and others (1996) noted that the Southern Whidbey Island fault zone has caused several historic shallow-crustal earthquakes and appears capable of generating earthquakes of surface-wave magnitude 7 or greater. Kelsey and others (2004) used Whidbey Island data to infer a moment-magnitude 6.5 to 7 earthquake on the fault zone about 3 ka. Sherrod and others (2005a,b) and Sherrod (2005) concluded that the Southern Whidbey Island fault zone “produced at least four events since about 16,400 years ago, the most recent after 2,700 years ago.”

Johnson and others (2000) used seismic lines in Saratoga Passage to infer three northwest-trending unnamed faults between Whidbey and Camano Islands. Johnson and others (2000) suggest that the northern-most fault may be Pliocene to Pleistocene in age, while the other two may be of Pleistocene age. We show these faults, although we saw no compelling evidence for landward extensions.

Although we did not recognize exposures anywhere in the map area that would provide compelling evidence of active faulting, we note an observation that is consistent with either of the above-mentioned interpretations (Johnson and others, 1996; Brocher and others, 2005) for the character of the Southern Whidbey Island fault zone—the topographically most elevated portions of southern Whidbey Island are systematically bracketed by the main mapped strands of the Southern Whidbey Island fault zone. We speculate that the lack of recognized fault scarps in the map area need not reflect a lack of surface-deforming fault events, but could result from diffusion of fault deformation in the thick, sandy deposits (notably unit Qgasv) that blanket much of the surface.

DESCRIPTION OF MAP UNITS

Postglacial Deposits

HOLOCENE

af Fill—Clay, silt, sand, gravel, organic matter, rip-rap, and debris placed to elevate and reshape the land surface; includes engineered and nonengineered fills; shown where fill is readily verifiable, relatively extensive, and appears sufficiently thick to be geotechnically significant.

Qb Beach deposits—Sand and cobbles; may include boulders, silt, pebbles, and clay; pebbles and larger clasts typically well rounded and oblate; mostly well sorted; loose; derived from shore bluffs and underlying deposits and (or) carried in by longshore drift.

Qm Marsh deposits—Mostly soft to stiff, olive gray to gray silt and silty clay and bluish gray clay; commonly with lenses and layers of peat, muck, and other organic material; deposited in a saltwater or brackish marsh (lagoon) environment; deposits occur near highest tide and are covered with salt-tolerant vegetation and floated logs. We found unit Qm only on the Camano Island portion of the map area.

HOLOCENE TO LATE PLEISTOCENE

Qp Peat and freshwater marsh deposits—Organic and organic-rich sediment, typically in closed depressions; includes peat, muck, silt, and clay in and adjacent to wetlands; may locally grade down to and include the freshwater equivalent of unit Qm; thickness is highly variable, but bogs may contain more than 20 ft of peat and muck; queried where its interpretation was based largely on geologic setting and geomorphology with no supporting field data.

Qmw Mass wasting deposits—Boulders, gravel, sand, silt, clay, and diamicton; generally unsorted, but locally stratified; typically loose; shown along mostly colluvium-covered or densely vegetated slopes that are demonstrably unstable or appear potentially unstable; commonly mapped where field conditions suggest landsliding, but evidence for landslides is ambiguous or weak; locally contains landslides and underlying units that either we could not map confidently or are too small to show at map scale; queried where its interpretation was based largely on geologic setting and geomorphology with no supporting field data.

Qls Landslide deposits—Gravel, sand, silt, clay, and boulders in slide body and toe, includes exposure of underlying units in scarp areas; angular to rounded clasts; unsorted; generally loose, unstratified, broken, and chaotic, but may locally retain primary bedding; commonly includes liquefaction features; deposited by mass-wasting processes other than soil creep and frost heave; distinguished from unit Qmw by presence of unambiguous landslide features. Absence of a mapped slide does not imply absence of sliding or hazard. All shoreline bluffs in the map area are subject to episodic land sliding and bluff retreat, but many slides are too small to show at map scale, and most slide deposits are quickly removed by beach wave action. The most notable slides in the map area are located at Camano Head on Camano Island.

Pleistocene Glacial and Nonglacial Deposits

We used stratigraphic position, organic content, radiocarbon, OSL, and TL dating, fossil content, and provenance data to separate glacial from nonglacial deposits. Provenance was inferred from stratigraphic relations, age data, and sand-grain and gravel-clast composition as observed petrographically and in the field. Glacial deposits are dominated by northern provenance materials and therefore contain little or no Glacier Peak detritus (0–5%) and include “significant” granitic and metamorphic lithic clasts (Dragovich and others, 2005). Nonglacial deposits are distinguished by eastern provenance. We found some petrographic indications of possible ancestral Skagit and (or) Stillaguamish River provenance (hypersthene, hornblende, and possible Glacier Peak dacite) (Dragovich and others, 2005; Polenz and others, 2005; Joe Dragovich, Wash. Divn. of Geology and Earth Resources [DGER], oral commun., 2006), but agree with Terry Swanson (Univ. of Wash., oral commun., 2005) in suspecting that much of the sediment may be from an ancestral Snohomish River. Speculations of a Snohomish River source are based mostly on stratigraphic position and proximity to that basin; we lacked petrographic reference data from Snohomish basin sediments, but noted elevated levels of serpentinite and granitic and weathered volcanic rocks in some samples. That mix appears consistent with major bedrock units in the Snohomish River basin (Joe Dragovich, DGER, oral commun., 2006).

DEPOSITS OF THE FRASER GLACIATION (PLEISTOCENE)Undivided Fraser Glaciation

Qgd(f) Fraser drift, undivided—Composite of units deposited during the Fraser Glaciation; shown where map scale or exposure do not support stratigraphic division.

Qgo(f) Recessional outwash—Mostly sand, but includes lenses and beds of pebble- and cobble-gravel, silt, sparse clasts of diamicton, and minor clay; gravelly facies tend to occur lower in the unit; loose; variably rounded; mostly moderately to well sorted; structureless to moderately stratified with medium to thick beds; forms either valley fill in relict, late glacial meltwater valleys or terraces above Everson Interstade marine shorelines; interfingers with adjoining Everson marine deltaic deposits (unit Qgome). We show crudely stratified pebbly sand and cobble gravel (which we assign to this unit) overlying Vashon till (unit Qgtv) and underlying Everson glaciomarine deltaic outwash deposits (unit Qgome) cropping out ~50 to 80 ft above the beach at Lowell Point on Camano Island (Columnar Section 5). Most deposits within this unit probably date to Everson time, but the unit is assigned to the broader Fraser glacial period because some deposits may date to the Vashon Stade.

Qgta(f) Ablation till—Unsorted, unstratified, and heterogeneous melt-out deposit of sand, silt, clay, gravel, diamicton, and patchy stratified proximal outwash deposits, ice-dammed lake sediment, and peat; loose to compact; typically forms geomorphically complex patchwork of melt-out deposits (material formerly carried by an ice mass, then left in place after the ice mass has melted away), outwash, closed depressions, erosional exposures of older (Fraser?) units, and incipient and larger drainages; queried where its interpretation was based largely on geologic setting and geomorphology with no supporting field data.

Everson Interstade

Qgdm(e) Everson Glaciomarine Drift—Clayey to silty diamicton with variable content of gravel clasts; also includes silt, clay, and sand; contains sparse shells, generally marine; dark gray where unweathered; mostly weathers to buff, but ranges to olive gray, ash gray, or white; commonly forms dry, vertical face with failure-prone vertical desiccation cracks with dark brown staining; massive to rhythmically bedded, commonly with sharp upper and lower unit-bounding unconformities (Domack, 1984); mostly loose and soft, but locally hard and compact; may resemble till (Domack, 1982, 1984; Domack and Lawson, 1985), but in general, till lacks fossils and glaciomarine drift has a finer-grained, smoother-feeling matrix, is less compact, and is more likely to be stratified. The unit is sea-floor sediment and consists mostly of glacial flour. Its diversity reflects proximity of the ice front (Domack, 1983; Dethier and others, 1995). Locally divided into:

Qgom(ee) Glaciomarine drift, emergence (beach) facies—Sand and gravel, locally silty; loose; mostly structureless but may be stratified, laminated, or ripple crossbedded, with rare boulder lag deposits; typically only a few feet thick; occurs below the glaciomarine limit; underlain by units Qgdme, Qgome, or older sediments; represents emergence deposits that cap glaciomarine drift and may include terrestrial deposits (Domack, 1983; Dethier and others, 1995); includes characteristic but subtle benches that are paleo–beach berms at various elevations (Carlstad, 1992; Easterbrook, 2003; Kovanen and Slaymaker, 2004).

Qgom(e) Glaciomarine deltaic outwash deposits—Mostly sand, with some sand–gravel mixtures with minor interlayered silt and silty sand; generally loose; maximum clast size limited to pebbles and small cobbles; west of Lone Lake on Whidbey Island this unit has a higher concentration of gravel, probably due to the proximity of paleo–side-channel erosion into a coarser facies of advance outwash (unit Qgav); most deposits are at least a few tens of feet thick with a maximum observed thickness of about 130 ft near East Point on Whidbey Island; forms a marine delta–turbidite complex with a horizontally bedded, sandy sea-floor facies, an overlying delta-front foreset-bedded facies, and a capping deltaic top-set channelized facies.

Qls(e?) Landslide deposits—Mix of till and outwash sand and gravel; identified as landslide based on surface morphology; queried because assigned to the Everson Interstade based only on field relations, low slope angle (~10% average), and subdued morphology.

Vashon Stade

Qgt(v) Till—Typically unweathered, unsorted mix of clay through boulder-size material (diamicton) deposited directly by ice; includes extensive areas of compact (advance outwash?) sand; compact, well-developed facies resemble concrete; locally ranges to loose in ablation till (also separately mapped as unit Qgtaf) and well-sorted in some sand-dominated areas; erratic boulders are common on the surface; gray where fresh; oxidizes yellowish brown; permeability very low in compact diamicton but locally high in sandy or loose facies; most commonly matrix supported; cobbles and boulders commonly faceted and (or) striated; may include flow banding; typically forms vertical faces in coastal bluffs; locally resembles unit Qgdme; lies stratigraphically between overlying units Qgdme and Qgome and underlying units Qgav and Qgasv; may include unrecognized exposures of older till. Most till deposits have had their surface fluted by overriding ice and form a patchy and seemingly randomly distributed cover that varies from 0 to at least 200 ft thick, with 2 to 30 ft most common. Cliff exposures along the west shore of Whidbey Island, about 5 mi west of the map area, reveal that even where well developed and thick, lodgment till may locally pinch out across short distances, even in the center of a well-formed drumlin. Regional age data appear to constrain the age of the unit to between ~18 ka and the onset of the Everson Interstade.

Qga(v) Advance outwash—Pebble to cobble gravel, sand, and some layers and lenses of silt and clay; may contain till fragments; gray to grayish brown and grayish orange; clasts typically well rounded, well sorted and clean; compact; mostly well stratified; very thinly to very thickly bedded; contains planar and graded beds, cut-and-fill structures, trough and ripple crossbeds and foresets; maximum thickness estimated at approximately 100 ft (separately mapped subunit Qgasv is locally thicker); deposited as proglacial fluvial (and deltaic) sediment; complete sections tend to coarsen upward; commonly overlain by unit Qgtv along a sharp contact and stratigraphically above units Qc and Qco. The estimated age of the unit is ~18 to ≥20 ka. Locally divided into:

Qgas(v) Advance outwash sand—Mostly lacustrine sand with layers of silt; thick exposures display well-developed crossbedding and cut-and-fill features that are typical of this unit; locally coarsens upward into gravel; thickness is typically 80 ft, with maximum estimated thickness of about 200 ft; extensive in subsurface; commonly forms angle-of-repose slopes along drainages and coastal bluffs; unit is the most widespread lithology of advance outwash occurring in the map area; queried where field identification was uncertain. Unit includes the Esperance Sand and possibly the Lawton Clay, the latter whose presence in the subsurface (on Whidbey Island) was noted in unpublished, site-specific geologic reports (Terry Swanson, Whidbey Geological Consulting, written commun., 2005) but not recognized by us in our mapping. Relict valleys are common on southern Whidbey Island (Polenz and others, 2006) and morphologically resemble many Camano Island valleys, suggesting that, like their Whidbey Island equivalent, these valleys on Camano Island are incised into unit Qgasv. They also typically lack modern streams, due to high permeability in the unit.

Qgd(v) Vashon Drift, undivided—Composite of units Qgtv and Qgav (or Qgasv); shown where map scale or exposure does not support stratigraphic division. Bluffs on Camano Island southeast of Elger Bay expose gray pebbly drift comprised of till and sand and gravel lenses, which grades southward to till (unit Qgtv), overlying unsorted to moderately sorted gray pebbly sand and gravel (unit Qgav). On Whidbey Island, south of East Point in mass wasting bluff exposures, unit consists of gray till (unit Qgtv) overlying gray, crossbedded sand (unit Qgasv).

GLACIAL DEPOSITS OF UNKNOWN AGE (PLEISTOCENE)

Qgt(u) Till of unknown age (columnar sections and line units only)—Unsorted mix of silt, sand, and gravel deposited dominantly as diamicton directly by glacier ice; dark gray; hard, compact with flow texture; pebble- to cobble-size clasts common; 15 to 20 ft thick where exposed at beach level for about 250 ft about ¼ mi west of downtown Langley (Columnar Section 2); lacks age control and is not assigned to Fraser glaciation because field relations suggest (but fail to demonstrate) older origin. Unit also assigned to a 15- to 20-ft-thick till exposed for several hundred feet in bluffs about 1300 ft south of East Point, where it overlies about 65 ft of nonglacial sediments (Columnar Section 1).

DEPOSITS OF THE OLYMPIA NONGLACIAL INTERVAL (PLEISTOCENE)

Armstrong and others (1965) defined “Olympia Interglaciation” as the “climatic episode immediately preceding the last major glaciation” and associated it with “nonglacial strata lying beneath Vashon Drift”. We associate those strata with stage 3 (~60–20 ka), but avoid the label “Olympia Interglaciation” because stage 3 is not a true interglacial period (as defined in fig. 4 of Morrison, 1991).

Qc(o)? Nonglacial deposits (columnar sections and line units only)—An upper, tan to pale yellowish-gray, faintly to mostly nonbedded, fine- to coarse-grained sand and an underlying, laminated, gray to dark gray silt and clay (commonly wet); unit shown on map only at base of Columnar Section 4 just south of Mabana and as a discontinuous line unit near beach level on Camano Island, beginning ~4000 ft north of Camp Diana and continuing southward to Mabana. The upper, nonbedded sand is exposed nearly continuously from Mabana south to a point ~4000 ft north of Pebble Beach, where the unit has risen in elevation in the bluffs and can be traced no farther in the vegetation covering the bluffs. At the site of Columnar Section 4, the unit includes both the nonbedded sandy deposits and the underlying silt and clay. Diapirs and irregular intrusions (flame structures) of silt and clay (which we believe originated from the underlying silt-clay subunit, and which locally contain organic material [wood]) intrude the overlying nonbedded sand but do not extend into the crossbedded pebbly sands of unit Qgasv that are resting unconformably on unit Qco?. Both subunits thicken southward from Mabana, ranging from 10 to 20 ft thick for the nonbedded sand and 1 to 30 ft thick for the laminated silt and clay. Contact between the two subunits is not exposed because of the thick vegetation that covers most of the wet silt-clay subunit. Just south of Mabana (Columnar Section 4), we obtained an OSL age estimate of 40.4 ±4.8 ka (Table 1, loc. 3) from the nonbedded sand subunit. This exposure was formerly mapped as Whidbey Formation by Easterbrook (1968), apparently without direct age control. We query the unit (Qco?) because our tentative reassignment of the deposits to the Olympia nonglacial period is based only on a single OSL date, and the exposures that we saw lack stratigraphic age control that would assist in clarifying whether the deposits are properly associated with Whidbey or Olympia time.

UNDIVIDED PRE-FRASER NONGLACIAL DEPOSITS (PLEISTOCENE)

Qc Pre-Fraser nonglacial deposits, undivided—Sand, silt, clay, peat, some fine gravel, and rare medium gravel; compact; well stratified to massive; resembles sediments contained in nonglacial deposits of the Olympia nonglacial interval and Whidbey Formation mapped elsewhere on southern Whidbey Island (Polenz and others, 2006; Minard, 1985) and on Camano Island and is thought to be nonglacial, but may locally include unrecognized glacial material. Unit is primarily exposed in the generally east-facing coastal bluffs on Whidbey Island along Saratoga Passage that are prone to mass wasting. Unit is predominantly well-bedded, moderately well-sorted, tan silt and gray fine sand with sparsely occurring organic material. In the bluffs south of East Point, nonglacial sediments (included in unit Qguc) are shown as unit Qc (Columnar Section 1). Two infinite 14C age estimates: >44,560 yr B.P. (on peat) and >45,000 14C yr B.P. (obtained from detrital organic material) were collected from this unit (Table 1, locs. 1 and 2; Columnar Section 1). The ages are consistent with either an Olympia nonglacial or a Whidbey Formation (or older) origin. Additional age estimates and stratigraphic considerations west of East Point (Polenz and others, 2006) are weakly suggestive of an Olympia age for part or all of the exposures of unit Qc (included in unit Qguc) in the vicinity of East Point, but we cannot dismiss an older origin for at least part of the section, nor establish stratigraphic continuity along the section. We therefore assign all exposures of the pre-Fraser nonglacial deposits in the area to unit Qc (included in unit Qguc and not separately shown on the map to accommodate the presence in the same bluffs of apparent glacial deposits older than(?) Vashon till). Age control is also poorly constrained for unit Qc elsewhere in the map area on Whidbey Island. Farther south near Langley, organic matter collected from unit Qc has provided 14C ages of ~35,000 yr B.P. (Terry Swanson, Whidbey Geological Consulting, written commun., 2005) suggesting that unit Qc in that area post-dates the Whidbey Formation and more likely represents sediments deposited during the Olympia nonglacial interval. Unit Qc lies stratigraphically below Fraser glacial deposits, but otherwise its age is unknown in the map area.

INTERGLACIAL DEPOSITS OF THE WHIDBEY FORMATION (PLEISTOCENE)

Qc(w) Whidbey Formation (columnar sections and line units only)—Sand, silt, and clay interbedded with peat and minor lenses of fine gravel; mostly weathered to a muted light-yellowish gray in exposed faces; includes flood plain and channel-sand facies. The flood plain facies, typically well stratified with characteristic wavy (ripple-marked) silt and clay beds, is slightly oxidized and forms prominent vertical bluffs. The channel-sand facies is typically clean, gray, and cross- to nonbedded. Peat beds vary in thickness from a few inches to a foot or more and are exposed in only a few scattered outcrops. Dacite clasts are a common component of sand and gravel facies and reflect an eastern provenance. Unit Qcw is mapped only on Camano Island. At southeast Lowell Point, we dated nonglacial sediments in two places (applying two different dating methods), which were separated from each other by only 20 ft laterally and 2.3 ft stratigraphically. A thin sand layer yielded an OSL date of 124.8 ±4.2 ka, and wood from a blue clay yielded an infinite 14C date of >45,210 yr B.P. (Table 1, locs. 4 and 6; Columnar Section 5). Vashon till unconformably overlies the Whidbey Formation at this location, where the ice had apparently cut down through formerly overlying units to deposit the till directly on Whidbey Formation (Columnar Section 5). Farther south on Camano Island near Pebble Beach and northwest of Camano Head (Fig. 1), Easterbrook (1968) mapped nonglacial sediments above Double Bluff Drift (unit Qgptd) as Whidbey Formation. He also assigned similar deposits further north (where unit Qgptd has dipped below sea level) to the Whidbey Formation. We dated a peat from nonglacial sediments near beach level just south of Pebble Beach (where Easterbrook [1968] formerly had mapped Whidbey Formation), which yielded an infinite radiocarbon date of >41,360 yr B.P. (Table 1, loc. 5; Columnar Section 3). We queried the unit in this area because of uncertainty surrounding its age. The radiocarbon date does not unambiguously identify the nonglacial sediments there as Whidbey Formation, and we have mapped Olympia nonglacial sediments (unit Qco?) to the north, where Easterbrook (1968) had mapped those sediments as Whidbey Formation. Although we could not demonstrate stratigraphic continuity between the exposures we mapped as unit Qco? to the north and those mapped as unit Qcw at Pebble Beach (line unit and Columnar Section 3), the gradual southward rise of unit Qco? suggests continuity, and we are left wondering if the deposits at Pebble Beach may also date to the Olympia nonglacial interval.

DEPOSITS OF THE DOUBLE BLUFF GLACIATION (PLEISTOCENE)

Qgpt(d) Double Bluff Drift (line units and Fig. 1 only)—Mostly till; gray, hard, compact; resembles Vashon till; drift includes gray cobble to boulder gravel that is uncemented and poorly sorted, with a silty matrix; grades northward to poorly exposed, nonbedded sand and pebble to cobble gravel that is locally iron-oxide stained; exposed at beach level with a south dip in a mass-wasted segment of sea cliffs. This unit is the oldest definitively glacial deposit recognized in the map area. Identification of the unit in the map area is based on prior mapping by Easterbrook (1968), who mapped Double Bluff at two places in the Camano Island portion of the map area: 1) northwest of Pebble Beach, where two beach-level exposures occur near each other, and 2) northwest of Camano Head in the lower and middle parts of the sea cliffs, where the drift consists mostly of compact gray till. Northwest of Pebble Beach, the more northerly exposure of till grades northward to glacial drift, which may include proglacial outwash of the Double Bluff Glaciation, but the details are obscured by vegetation and local mass wasting. The poor exposure makes it difficult to separate drift from overlying nonglacial deposits that were assigned by Easterbrook (1968) to the Whidbey Formation, and from the tentatively mapped Olympia nonglacial deposits (unit Qco?) that are exposed less than a quarter mile to the north. The deposits northwest of Camano Head that were mapped by Easterbrook (1968) as Double Bluff till are exposed for more than a quarter mile where till rises from beach level to mid bluff (Fig. 1). The till at this exposure overlies older nonglacial(?) sediments (unit Qcpd?) but thins rapidly to the southeast, where it grades to a gravel unit in an area complicated by possible faulting. Berger and Easterbrook (1993) and Easterbrook and others (1992) used two thermoluminescence (TL) age estimates (291 ±86 ka and 280 ±81 ka; Table 1, locs. 7 and 8) to corroborate Easterbrook’s prior assignment of the exposures we included in this map unit to the Double Bluff Drift, but Berger and Easterbrook also cautioned about “the large TL analytical uncertainties”. The TL analyses were performed on clay that is exposed beneath the till (Berger and Easterbrook, 1993; Easterbrook and others, 1992). The two age estimates correlate well with a TL age of 289 ±74 ka from clay near the base of the Double Bluff Drift at its type section on southern Whidbey Island (Berger and Easterbrook, 1993). However, the age estimate for Double Bluff Drift at its Whidbey Island type section is tentative (Polenz and others, 2006), and we have little information regarding the reliability of the corresponding Camano Head age estimates. In the absence of direct age estimates, the section at Camano Head could have alternatively been interpreted as Olympia nonglacial deposits overlying Possession till above either Possession advance outwash or Whidbey Formation.

UNNAMED DEPOSITS BENEATH DOUBLE BLUFF TILL (PLEISTOCENE)

Qc(pd)? Nonglacial? deposits beneath Double Bluff till (line unit only)—Gray, coarse, pebbly, tangentially crossbedded sand (≤30 ft thick), containing concentrated bands of granule-size to very coarse sand-size (2–10 mm), white pumice grains (including a few sparse pebbles), sparse wood, and local pebble-gravel lenses; crops out (for ~1100 ft) northwest of Camano Head on Camano Island (Fig. 1) beneath deposits that were assigned by Easterbrook (1968) to the Double Bluff Drift—thus designating these (underlying sediments) as the oldest sediments exposed in the map area. The sediments were included with the Double Bluff Drift by Berger and Easterbrook (1993) and Minard (1985), although Easterbrook (1994a) seems to allow for the possibility that these sediments are separate from (predate) the Double Bluff Drift. We mapped these deposits as a separate unit because our field observations, petrographic characterization, and comparison of our data with observations of Peterson (2007) suggest a possibly nonglacial setting, although we can not rule out a (Double Bluff) glacial origin. Easterbrook (1994a) describes similar crossbedded sands, consisting almost entirely of pumice, south of Pebble Beach on Camano Island, which we presume to be part of the same unit. Laser-argon measurements on that pumice (lying stratigraphically between Double Bluff till and the TL-dated clay beneath the till [see unit Qgptd ]) gave age estimates of 0.98 and 1.04 Ma on plagioclase and biotite, respectively (Easterbrook, 1994a; Table 1, loc 9). Easterbrook (1994a) suggested that the age discrepancy between the TL ages and laser-argon ages could be explained by the detrital nature of the pumice. He also noted that the concentration of the Camano pumice clasts argues against reworking of an older tephra. He further noted that major-element geochemistry of the Camano pumice, as determined by microprobe, did not match the geochemistry of the Lake Tapps tephra, which reportedly yielded fission-track age estimates ranging from 0.65 to 1.06 Ma (Westgate and others, 1987; Easterbrook, 1994a). He concluded that the lack of a chemical match suggests that the two tephras in this unit do not share the same origin, despite their similar age estimates. We collected a new sample of reworked pumice clasts from this unit (presumably southeast of Easterbrook’s sample) from a faulted(?) section of the lower bluffs 2000 ft northwest of Camano Head. The new sample was collected near the southeast end of the unit Qcpd? exposure, where the unit disappears below sea level. We had the glass chemistry of our sample analyzed (13 clasts individually microprobe-analyzed) at Washington State University (see electron microprobe site). The results indicate that all 13 clasts came from the same tephra, and standard deviations for individual clasts are small. Comparison of the average of our analytical results with Easterbrook’s earlier samples from the same unit yielded a similarity coefficient of 0.74, which suggests no genetic relationship between the two tephra samples (Nick Foit, Wash. State Univ.,written commun., 2007). Assuming that the results of our chemical analyses and Easterbrook’s are both accurate, two unrelated tephras are present in the unit. Our samples were found to have similarity coefficients of 0.95 to 0.96 with tephras previously collected on Whidbey Island (Carlstad, 1992). The major-element chemistry of our tephra also yielded a similarity coefficient of 0.93 compared with the Lake Tapps tephra, suggesting that the best known match for both our Camano Island samples and those from Whidbey Island is the Lake Tapps tephra (Nick Foit, Wash. State Univ., written commun., 2007). Foit cautions, however, that the above similarity coefficients are still marginal.

UNDIVIDED GLACIAL AND NONGLACIAL DEPOSITS OLDER THAN VASHON TILL (PLEISTOCENE)

Qguc Undifferentiated deposits older than Vashon till—Locally includes sand, gravel, diamicton, silt, clay, and peat; compact; well stratified to nonstratified; composite of glacial and nonglacial deposits or of deposits of unresolved association; pre-dates Vashon till but may include Vashon advance outwash. Shown along coastal bluffs where poor exposure or map scale does not allow for more detailed subdivision. Peat and detrital organic matter from nonglacial sediments at East Point on Whidbey Island (Table 1, locs. 1 and 2) yielded radiocarbon-infinite age estimates that are discussed in the unit description for unit Qc. On Camano Island, unit Qguc includes nonglacial sediments Qco?, Qcw, and Qcpd? and glacial sediments Qgptd. These have yielded radiocarbon-infinite age estimates that are discussed under units Qc and Qcw.

Undivided Quaternary Deposits

Qu Pleistocene deposits, undivided—Locally includes sand, gravel, diamicton, silt, clay, and peat; ranges from loose to compact; well-stratified to nonstratified; composite of glacial and (or) nonglacial deposits; ranges in age (in coastal bluff exposures at Lowell Point [Columnar Section 5]) from Everson glaciomarine outwash deltaic deposits (unit Qgome) to Whidbey Formation (unit Qcw); shown where map scale does not allow for more detailed subdivision.

GEOLOGIC SYMBOLS

Contact—Long dashed where approximately located; short dashed where inferred

Fault, unknown offset—Dotted where concealed; dashed where inferred

Geologic unit too narrow to show as a polygon at map scale

Strand line (former shoreline)

Arrow showing direction of landslide movement

Age-date sample (radiocarbon)

Age-date sample (optically stimulated luminescence)

Electron microprobe site

Figure location

Columnar section location

ACKNOWLEDGMENTS

This geologic map was funded in part by the U.S. Geological Survey (USGS) National Cooperative Geologic Mapping Program, agreement nos. 05HQAG0085 for 2005–2006 (South Whidbey Island) and 06HQAG0035 for 2006–2007 (Camano Island). We thank Doug Kelly (Island Co. Health Dept.) and other Island County staff for geologic, hydrogeologic, geotechnical, and other assistance and records; Jerry Thorsen (DGER, retired) for supplying us his field notes on some sea cliff exposures on Whidbey Island and much of Camano Island, which both complimented and supplemented our field observations; Jon Peterson (M.S. candidate, Western Wash. Univ.) for assistance with our petrographic characterization and interpretation of sand samples, which attempted to match his systematic approach to distinguishing nonglacial from glacial deposits on Camano Island; Terry Swanson (Univ. of Wash.) for assistance with field work and some personal observations that we incorporated into our report; Nick Foit (Wash. State Univ.) for providing an electron microprobe glass chemistry analysis (at no cost) of the tephra near Camano Head and also providing similarity coefficient comparisons with other tephra chemistry; and Sam Johnson and Brian Sherrod (both USGS) for consultations on the tectonic setting. Thanks also to the Washington State Department of Ecology for permission to use proprietary shoreline aerial photos for our fieldwork, and to fellow DGER staff members Josh Logan and Joe Dragovich for technical assistance and reviews. Last, but not least, thanks to the uncounted people who permitted us to study and sample geologic exposures on their land and provided site-specific records and local expertise.

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Berger, G. W.; Easterbrook, D. J., 1993, Thermoluminescence dating tests for lacustrine, glaciomarine, and floodplain sediments from western Washington and British Columbia: Canadian Journal of Earth Sciences, v. 30, no. 9, p. 1815-1828.

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Fig. 1

Qgtu

1

3

1

)

( )

Qc

Qgav

Qguc

Qb

af

Qgasv

Qgdf

Qgdv

Qgdme

Qgof

Qgome

Qgomee

Qgtv

Qgtaf

Qls

Qlse?

Qmw

Qm

Qp

Qu

Qco?

Qgtu

Qcw

Qcpd?

Qgptd

Qcw

Qco?

Qgtu

Figure 1. Photo looking east at west-facing bluffs on south Camano Island, 1 mi south of Pebble Beach. Lower unit Qcpd? consists of 25 to 45 feet of south-dipping crossbedded nonglacial sand with local gravel lenses; unit becomes pumiceous to the south and pinches out about 900 ft south of photo. Gray lodgment till (unit Qgptd [Double Bluff Drift] 40- to 45-ft thick) overlies unit Qcpd?; till thins and grades to a cobbly pebbly gravel 800 to 900 ft to the south. Unit Qcw? (Whidbey Formation) overlies the till (unit Qgptd) and consists of 40 to 50 ft of tan stratified sand and silt with sparse, thin gravel lenses. Upper part of bluff in photo may also include Vashon advance outwash sand (unit Qgasv and thin (1 ft or less) of Vashon till (unit Qgtv) not shown in the photo.

Qcw?

Qgptd

colluviumQcpd?

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