Composition, age, and depositional rates of shoreface deposits under barriers and beach plains of the Columbia River littoral cell, USA

Marine Geology 273 (2010) 62–82

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Marine Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r g e o

Composition, age, and depositional rates of shoreface deposits under barriers and beach plains of the Columbia River littoral cell, USA Curt D. Peterson a,⁎, Sandy Vanderburgh b, Michael C. Roberts c,d, Harry M. Jol e, Jim Phipps f, David C. Twichell g a

Geology Department, Portland State University, Portland, Oregon 97207-0751, United States Centre for Applied Arts and Sciences, Lethbridge College, Lethbridge, Alberta, Canada T1K 1L6 Department of Geography, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada V5A 1S6 d Department of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada V5A 1S6 e Department of Geography and Anthropology, University of Wisconsin Eau Claire, Eau Claire, Wisconsin 54702, United States f Grays Harbor College, Aberdeen, Washington, 98520, United States g United States Geological Survey, 384 Woods Hole Rd, Quissett Campus, Woods Hole, Massachusetts, 02543-1598, United States b c

a r t i c l e

i n f o

Article history: Accepted 18 January 2010 Available online 17 February 2010 Keywords: accommodation space Columbia River shoreface littoral cell Holocene transgression

a b s t r a c t The Columbia River littoral cell (CRLC) consists of four subcells (totaling 160 km in length) that are unique in the West Coast of the United States, in that they contain prograded barriers and beach plains, reaching 0.5–3 km in width (Fig. 1). The prograded beach deposits (1–5 ka in age) overlie shoreface deposits (1–8 ka in age), as identified in 18 ground penetrating radar profiles, and sampled from 24 boreholes. Two competing hypotheses were initially proposed to account for the origins of these unique, progradative shorelines: (1) cross-shore feeding by onshore wave transport of pre-Holocene sand from the submerged shelf, and (2) longshore dispersal of nearshore sand that was supplied to the littoral system by bedload sediment discharge from the Columbia River during the Holocene. The CRLC sand forming the shoreface deposits is fine (diameter 0.2 ± 0.02 mm) and rich in lithic fragments (20–40% by volume). Gravel and shell lag layers are uncommon in most of the CRLC shoreface deposits, but they show greater abundance locally near ravinement surfaces, tidal inlets, and in the Clatsop subcell, located south of the Columbia River mouth. Gravel and granule layers increase upsection in barriers south of the Columbia River and downsection in barriers at the northern end of the littoral system. These trends suggest different mechanisms of shoreface sediment feeding within the four subcells. However, borehole samples from all four subcells show the same sand provenance, i.e., post-glacial Columbia River sand, which is identified by high ratios of hypersthene:augite in heavy-mineral fractions. Selected shoreface sections were dated (0.5–8 ka) by AMS radiocarbon analysis of articulated-shell and wood fragments recovered from auger flights (3–22 m depth subsurface). Relatively young shoreface deposition (2.5 ka at − 6.5 m elevation NGVD88) in the Clatsop subcell south of the Columbia River shows a net-southward beach transport that fed shoreface and beachface progradation into deeper water. Older and deeper shoreface deposition (4.4 ka at − 7.1 m elevation) in the Long Beach subcell north of the Columbia River was a result of the filling of innermost-shelf accommodation space prior to beachface progradation. The total volume of shoreface sand deposited under the barrier spits and beach plains of the CRLC is estimated to be 6–7 km3 deposited since 6–8 ka. There was a net-northward transport of littoral sand (∼ 1 × 106 m3 year− 1) along the nearshore and inner-shelf; subsequently some of this sand was transported onshore to feed beaches of the northernmost subcells. Columbia River sand was also the source for the formation of the offshore shelf wedge above the transgressive ravinement surface, and for the filling of major tidal basins located north of the Columbia River. In summary, the unique progradational history of the CRLC barriers and beach plains derives from the combination of (1) longshore dispersal of fine sand discharged from the Columbia River during Holocene time, and (2) across-shore feeding of beaches at the northern end of the littoral system from fine sand carried north along the nearshore and the inner-shelf. © 2010 Elsevier B.V. All rights reserved.

1. Introduction

⁎ Corresponding author. Tel.: + 1 503 625 6740. E-mail address: [email protected] (C.D. Peterson). 0025-3227/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2010.02.004

The broad barriers and beach plains (1–3 km in width) of the Columbia littoral system (Ballard, 1964; Fig. 1) are unique among the high-energy coastlines of the Northeast Pacific Ocean. Two apparently

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Fig. 1. Location of Columbia River littoral cell (CRLC) study area, the four subcells (North Beaches, Grayland, Long Beach, and Clatsop) and Intervening Tidal Inlets to Grays Harbor, Willapa Bay, and the Columbia River. The map inset shows most the extent of the major tributaries to the Columbia River in the northwest United States and British Columbia, Canada. Map coordinates are given in latitude and longitude (degrees and minutes) and in UTM intervals (50 km intervals). Shelf contours of 10 m and 200 m depth are shown. The CRLC is bounded to the south by Tillamook Head, and extends north to at least Point Grenville.

competing hypotheses of sand supply to the Holocene barrier spits and beach plains of the Columbia River littoral cell (CRLC) emerged from preliminary work in this dynamic littoral system (Peterson et al., 1999; Gelfenbaum et al., 1999). One hypothesis emphasized longshore transport from the Columbia River mouth, whereas the other hypothesis emphasized across-shore feeding from sand deposits on the continental shelf. This paper addresses the apparent contradiction of these two hypotheses. Present-day conditions of fine sand supply (Ruggiero and Voigt, 2000) and high-wave energy (9–10 m peak deepwater-wave height; Tillotsen and Komar, 1997) in the CRLC make for a broad zone of littoral sand mobilization. Storm-driven, northward geostrophic flow on the inner-continental shelf (Sternberg, 1986) could widen the littoral transport corridor under peak storm conditions of combined flow. Such a broad zone of transport was probably active throughout the development of the CRLC system during the Holocene trans gression. The two hypotheses for sand supply to the accreted spits and beach plains of the CRLC are outlined below. Substantial shoreline

progradation, varying from 0.5 to 3 km in width, occurred during the late Holocene (5–1 ka) in the Long Beach, Clatsop, and Grayland subcells (Woxell, 1998) (Fig. 1). The onset of net shoreline progradation occurred sequentially with increasing distance north of the Columbia River. These trends indicate a southern sand source, i.e., from Columbia River sand discharge, in late-Holocene time. On the other hand, nearshore transport models based on modern wave climate and shoreline angle (Buijsman, 1998), indicate southerly or no net littoral drift in three of the four subcells, i.e. North Beaches, Grayland, and Clatsop. Transport predicted by those models requires northern sand sources and/or offshore sand sources to supply the prograding beaches in the northern littoral subcells. Those preliminary modeling results are contradicted by heavy-mineral tracers in modern deposits from the lower reaches of the Grays Harbor tidal basin. Modern sand supply to the lower Grays Harbor basin reflects hypersthene-rich sand from the Columbia River, located 80 km south of Grays Harbor (Scheidegger and Phipps, 1976). The question over sand supply in the CRLC system is further complicated by post-transgressive shelf deposits (10–30 m in thickness)

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located offshore of the widest barriers (Twichell and Cross, 2001; Twichell et al., 2010-this issue). In southeast Australia and the Netherlands post-transgressive shelf deposits are thin, typically less than 5 m subsurface depth, as a result of their erosion under conditions of net-onshore wave transport (Short, 1987; Van der Valk, 1996). Accordingly, it could be predicted that across-shore feeding from the CRLC offshore deposits would have preempted the thick post-transgressive deposits that are found in inner-shelf reaches near the Columbia River mouth (Twichell et al., 2010-this issue). To help resolve these apparent contradictions a drilling project was undertaken to sample and radiocarbon date the Holocene deposits in the CRLC barrier spits and beach plains (Herb, 2000). The depth and age of the transgressive unconformities, i.e. the waveeroded ravinement surfaces, under the barriers are discussed separately (Vanderburgh et al., 2010-this issue). In this paper we compare the depth (1–36 m) and age (1–8 ka) of the post-ravinement, shoreface deposits in the four subcells of the CRLC. Trace minerals in borehole samples and vertical trends in fine gravel abundance are also used to establish sand sources and nearshore transport directions in the CRLC. These trends reflect different relative

proportions of longshore feeding and across-shore feeding in the four subcells, thereby reconciling the apparent contradictions of longshore feeding and cross-shore feeding of shoreface deposits in the CRLC system. 2. Background 2.1. Transgressive unconformity and shoreface evironments Twichell and Cross (2001) used seismic reflection profiles to map the Holocene fill above the transgressive ravinement or flooding surfaces throughout the CRLC inner-shelf region. They showed that Holocene transgressive deposits vary from zero to a few meters in thickness offshore of the North Beaches subcell (Fig. 1). The posttransgressive sand wedge thickens to 5–10 m offshore of the Grayland subcell, and to several 10s of meters in thickness offshore of the Long Beach and Clatsop subcells (Figs. 1 and 2). Thicker Holocene fill occurs in incised channels that extend offshore from the mouths of the Columbia River, Willapa Bay, and Grays Harbor. Smaller tributary channels drained laterally into the larger trunk channels, which

Fig. 2. Isopach map of Holocene fill above the transgressive ravinement surface offshore of the CRLC barriers and beach plains. Fill thickness is shown in 10 m contour intervals, i.e., 10, 20, 30 and 40 m. The shelf isopach contours also provide a very approximate depth to the ravinement surface at the modern shorelines (see Twichell et al., 2010-this issue for details). Predicted ravinement surfaces under the modern beaches reach 30 m depth in the Clatsop subcell, 20 m depth in the Long Beach subcell, 10 m depth in the Grayland subcell, and only a few meters in depth in the North Beaches subcell, north of Grays Harbor. Bedrock is currently exposed in a shallow shoal area located in the inner-shelf offshore of Grays Harbor. Figure after Twichell et al., 2010-this issue.

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crossed the shelf during low-stand conditions. These ancestral valley channels were filled with Holocene fluvial and bay deposits (Peterson and Phipps, 1992; Baker et al., 2010-this issue) prior to being truncated by the subsequent surf zone ravinement. The deeper transgressive surfaces that were identified and mapped from the offshore seismic profiles were beyond the reach of shallow coring from shipboard (Twichell and Cross, 2001). These offshore ravinement surfaces were readily tied to apparent transgressive unconformities that were logged in water-well records from the prograded barriers and beach plains (Herb, 2000). The transgressive ravinement surfaces from the nearshore seismic profiles were projected to modern beaches and interiors of the prograded barriers. Indeed, the existence of these ravinement surfaces were confirmed by lithologic contacts observed from continuous auger drilling and by radiocarbon dating of returned samples from the lithologic contacts (Vanderburgh et al., 2010-this issue). The following sequence should be expected under the prograded barriers and beach plains, from bottom to top: (1) transgressive ravinement surface, (2) shoreface sand, and (3) abandoned foredune ridges and inter-ridge valleys. We also identify a specific facies within the shoreface as progradational beach face. The progradational beachface facies is based on low-angle, seaward dipping, en echelon reflections in ground penetrating radar (GPR) records (Peterson et al., 2010-this issue). Chaotic GPR reflections from the shoreface deposits below the prograded beachface facies are loosely termed here as ‘beach platform,’ following Smith et al. (1999). The ‘beach platform’ is assumed here to represent surf zone-to-innermost-shelf deposits in the CRLC. For the purposes of discussion in this paper we break the modern shelf settings into mid–outer shelf (greater than 50 m water depth), inner-shelf (less than 50 m water depth), and innermost shelf (less than 25 m water depth). These depth-related settings are based on grain-size breaks reported for the surface deposits of the modern shelf offshore of the CRLC (Twichell et al., 2010-this issue). In summary the Holocene shoreface deposits reach maximum thickness (1) near the ancestral Columbia River valley (Twichell et al.,

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Fig. 3. Sea-level curve (0–16 ka) for the CRLC based on reported 14C dated peat samples from Grays Harbor (Peterson and Phipps, 1992), Willapa Bay (Atwater et al., 2004), and the Columbia River (Baker et al., 2010-this issue). Averaged ages are given in thousand years (ka) before present, and elevations are in meters relative to NGVD88. See Table 1 for specific sample sources, ages, and depths. The NGVD88 datum is about 1 m below mean tidal level in the CRLC.

2010-this issue), (2) within the tidal inlets, and (3) at the seaward edge of the prograded barriers (Herb, 2000; Vanderburgh et al., 2010this issue). The composition, age, and source of sand deposits from the shoreface sediment wedge under the CRLC barriers and beach plains are addressed in this paper.

Table 1 Radiocarbon-dated samples used for local sea-level curve, Columbia River littoral cell. Sample (reference)

Depth NGVD88 (m)

Present Cutbank Y (1) Cutbank W (1) Cutbank U (1) Cutbank S (1) Cutbank N (1) Cutbank L (1) Cutbank J (1) Bayc1-240 (2) Bayc1-395 (2) Dred7-387 (2) GH 17-05 (3) GH 15-10 (3) GH 17-12 (3) GH 17-16 (3) GH 15-24 (3) GH 2-35 (3) GH1-39 (3) CR W-24 (4) CR W-73 (4) CR W-112 (4)

1.0 0.5 0.5 0.0 − 0.5 − 0.5 −1.5 − 2.5 −1.7 −3.2 − 3.7 − 3.5 − 11.5 −19.0 − 28.0 −33.0 − 51.5 −56.0 −18 − 70 − 109

Material

AMS plant AMS plant AMS plant AMS plant AMS plant AMS plant AMS plant Bulk peaty AMS peaty AMS rooted Bulk peat Bulk peat Bulk peat Bulk peat Bulk peat Bulk shella Bulk wooda Mazama ashb AMS rooted AMS woodc

Age RCYBP CalibStD. 0 300 1100 1300 1700 2500 2800 3200 2730 ± 20 3950 ± 50 4470 ± 60 4670 ± 140 6570 ± 160 8360 ± 100 8800 ± 190 8940 ± 280 10,020 ± 190 12,810 ± 160 7700 11,550 ± 80 15,980 ± 90

Lab number Beta analytic

158069 158083 158074 20529 20527 20531 20532 20300 20282 20286

Time curve (ka) RCYBP 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Depth curve (m) NGVD88 1 0 −1 −2 −3 −5 −8 − 13 − 20 − 30 − 50 − 60 − 70 −85 − 90 − 100 − 110

131805 131808

NGVD88 estimated (+ 0.2 m MLLW) or − 1 m local MTL. (1) Buried wetlands Y-J in cutbanks from Niawiakum River, Willapa Bay (Atwater et al., 2004) and Johns River, Grays Harbor (Shennan et al., 1996) (Atwater et al., 2004). Presubsidence sea level taken (0–1 m) below wetland paleo-tidal elevation. (2) Willapa Bay and Grays Harbor vibracores (Peterson et al., unpublished data, 2000). Sea level taken at 0–1 m below marsh paleo-tidal elevation. (3) Grays Harbor drill core samples (Peterson and Phipps, 1992) Sea level taken at dated peat horizon elevations. (4) Warrenton Borehole, Columbia River mouth (Baker, 2002; Baker et al., 2010-this issue). a Onset of transgression at Grays Harbor mouth (4–6 m above Pleistocene contact). b Mazama ash in subtidal flat ∼3 m water depth at the time of deposition (Gates, 1994). c Onset of transgression in Columbia River mouth (1 m above Pleistocene contact) Baker et al. (2010-this issue).

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2.2. Relative sea-level curve A relative sea-level curve for the CRLC region has been constructed using published 14C-dated peat samples in drill cores taken in the Grays Harbor and Columbia River tidal basins and from tidal marshes in Willapa Bay (Table 1; Fig. 3). Sea level rose from a depth of about −110 m NGVD88 near the onset of transgression at 16 ka to a level of about −20 m at approximately 8 ka (Peterson and Phipps, 1992; Baker et al., 2010-this issue). High rates of sea-level rise during the early Holocene (2 cm year− 1; Fig. 3) resulted in rapid shoreline retreat across the continental shelf. Wave erosion at the shoreline during the transgressive ravinement truncated preexisting deposits from shoreface, alluvial, and/or lagoonal settings in the continental shelf (Vanderburgh et al., 2010-this issue). Beachface progradation in the CRLC did not commence until the late Holocene (5–1 ka) (Peterson et al., 1999) after the rates of sea-level rise had decreased to 0.1–0.2 cm year− 1. 2.3. Columbia River sand supply More than 200 dams on the Columbia River and its tributaries (Gelfenbaum et al., 1999) have possibly reduced bedload discharge in the lower Columbia River during the last half-century. While the modern Columbia River estuary is possibly a net sink for littoral sediments (Sherwood et al., 1990), in prehistoric times it was a major contributor of sand to the littoral system (Gates, 1994). The filling rates

of the Columbia River tidal basin fell from about 7–10 million m3 year− 1 in the mid-Holocene to half that amount in the late Holocene (Baker et al., 2010-this issue). The difference in fill rates is primarily attributed to declining accommodation space and increasing sediment bypassing the Columbia River tidal basin. Assuming no major changes in rates of sediment yield from the Columbia River drainage has probably delivered, on annual basis, a minimum of 3 × 106 m3 year− 1 of bedload to the CRLC in late-Holocene time. High wave energy conditions and abundant littoral sand supply at the Columbia River mouth have produced an asymmetric beach plain on the south side of the mouth, a 40 km long barrier-spit north of a bedrock headland on the north side of the mouth, and broad shoal area directly offshore of the mouth (Fig. 1). These sand bodies have been previously, though loosely, linked to a sand supply from the Columbia River (Ballard, 1964; Rankin, 1983). The inner-shelf sand located on either side of the Columbia River mouth, extending to shelf depths of 50 m, is locally enriched in heavy minerals (HMs), e.g., greater than 10% by weight (Fig. 4; Runge, 1966). Additional areas of HM enrichment are also reported just offshore of the Long Beach peninsula and in an irregular band located in the inner-shelf north of Grays Harbor (Venkatarathnam and McManus, 1973). Such areas of HM enrichment could signify light mineral winnowing under conditions of inner-shelf sand transport. The heavy minerals in the shelf deposits located immediately north of the Columbia River mouth have been traced to Columbia River sand

Fig. 4. Map of reported heavy-mineral (HM) anomaly (greater than 10% by weight of sand fraction) on the inner-continental shelf. HM enrichment in shelf sand deposits is reported for (1) the south and north sides of the Columbia River mouth, (2) the northern end of the Long Beach barrier, and (3) in an irregular band located north of Grays Harbor. HM enrichment patterns from modern shelf deposits in the CRLC are from Peterson and Binney (1988); data compiled from Runge (1966) and Venkatarathnam and McManus (1973).

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supply, as based on the similarity of their mineralogy to sand in the modern Columbia River (Scheidegger et al; 1971). By comparison, neither Willapa Bay nor Grays Harbor tributaries are large enough to have influenced the sand mineralogy in the lower reaches of their respective tidal basins (Scheidegger and Phipps, 1976; Peterson et al., 1984).

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3. Methods Herb (2000) analyzed 74 water-well logs from the CRLC barriers and beach plains to help target the depths to potential ravinement and flooding surfaces (Fig. 5) (Vanderburgh et al., 2010-this issue). Based on existing water well, borehole logs and offshore seismic

Fig. 5. Location of 74 reported water wells (small triangles), 24 study drill sites (solid circles) with abbreviated site names, and four projected ravinement sites on modern beaches (‘X’ symbols). Map coordinates are in UTM Sector10N (meters Northing and Easting). Drill-site location coordinates, elevations, and depths are presented in Table 3. Projected ravinement depths at selected beach sites are based on offshore seismic reflection records (Twichell et al., 2010-this issue) and onshore water-well logs (Herb, 2000).

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Fig. 5 (continued ).

reflection records (Twichell et al., 2010-this issue) 23 sites were selected for continuous sampling by solid-stem auger drilling with a Mobile B53 drill rig. The drill transects were spaced at about 10 km distances along the coast, with each across-shore transect established to sample modern beach, mid-barrier, and back-barrier settings. The completed drill sites were geo-referenced using dGPS and GIS othophoto heads-up display (±10 m WGS84) and LIDAR topography

(±0.5 m NGVD88; Daniels 2001). The NGVD88 datum is approximately one meter below local, mean tidal level in the CRLC. Solid-stem auger drilling of unconsolidated shoreface deposits is both rapid and reliable to depths of 20 m subsurface in the study area. Mud rotary drilling was used to reach greater depths in tidal inlet sites of Grays Harbor (55 m subsurface) (Peterson and Phipps, 1992), Willapa Bay (27 m subsurface) (NCOVE, this paper) and the Columbia

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River (113 m subsurface) (Baker et al., 2010-this issue). Ravinement surface depths were projected into several beach sites from offshore seismic lines (Herb, 2000; Twichell and Cross, 2001). These projections are used where unstable beach conditions prevented the use of the Mobile B53 drill rig to reach the greater depths (20–30 m depth subsurface) of the targeted ravinement surfaces. Holocene transgressive origins for the projected ravinement surfaces are based on the radiocarbon ages and measured sedimentation rates of shoreface deposits overlying the target ravinement surfaces. Samples recovered from the auger flights (1.5 m length) were photographed, logged to the nearest 2.5 cm length scale, and sampled for grain-size analysis and radiocarbon dating. Samples were split for sieving (0.25 phi intervals) and for separation of light and heavy minerals (0.150–0.250 mm size fraction) in Na-Polytungstate (specific gravity 2.95). Non-opaque, colored, monocrystalline, heavy-mineral grains were identified for their mineralogy under a polarizing petrographic microscope at 250× magnification. The ratios of two, specific pyroxene minerals, augite and hypersthene, are used to discriminate sand source provenances in this paper (Scheidegger et al., 1971; Venkatarathnam and McManus, 1973; Scheidegger and Phipps, 1976). These two heavy minerals (HMs) are similar in shape and density, so their relative abundances should not be biased by selective transport. Articulated bivalve shells, delicate single valves, and small wood fragments (0.5 cm diameter) were submitted to Beta Analytic Inc. for bulk or accelerator mass spectrometry radiocarbon dating (AMS 14C). The articulated and/or delicate bivalve shells were selected to minimize errors from the dating of remobilized materials. More robust shell fragments could have been remobilized from offshore deposits by shoreward wave transport. Such remobilized materials can yield ages that are older than those of the hosting shoreface deposits (Thoms et al., 1981). Conventional radiocarbon dates were calibrated by the local marine shell reservoir (Del-R 390 ± 25) and 14C production curves (shell and wood) using Calib4.4 (Stuiver and Reimer, 1993; Stuiver et al., 1998). The ranges of calibrated ages, at one sigma error, are portrayed in the

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corresponding tables by a midpoint and two end points. The midpoint is not presumed to be the center of the age probability curve for the sample. The two end points represent the age range of the sample assuming one unit of standard error. The use of articulated, and/or delicate bivalves and twig fragments proved to minimize bias from the potential dating of remobilized materials. Sample dates were neither reversed downcore nor widely spread in age, within the shallower, stratigraphic sections in adjacent drill sites (Herb, 2000). Any reversal of dates downcore would have implied a mixing of older remobilized materials within the younger deposit, requiring the use of the youngest date to constrain the age of the deposit. Ground penetrating radar (GPR) profiles (∼100 m length) were collected using a PulseEkko 100 MHz 400v system at 18 drill sites to establish depths to the bases of the prograded beachface deposits (Peterson et al., 2010-this issue). The GPR profiles readily discriminated between the seaward dipping reflectors of prograded beachface deposits (Jol et al., 1996) and the chaotic, subhorizontal reflectors of the underlying ‘beach platform’ (Smith et al., 1999) (Fig. 6). Midbarrier and back-barrier drill sites were also profiled with PulseEkko 50 MHz systems to target the basal transgressive unconformity prior to drilling. An abrupt loss of GPR signal penetration coincided with mud or mudstone contacts marking the transgressive unconformity below the sandy shoreface deposits in mid-barrier and back-barrier drill sites (see ground penetrating radar profiles in the Results section below). The 50 MHz GPR lacked the resolution to distinguish sedimentary structures, but did provide target depths of ravinement surfaces in the mid-barrier and back-barrier drilling locations. 4. Results 4.1. Ground penetrating radar profiles GPR records from 18 west–east profiles (Table 2) were used to discriminate between prograded beachface facies, and the underlying

Fig. 6. Ground penetrating radar (GPR) record from a back-barrier site in the Long Beach subcell. The GPR records show seaward dipping beachface reflections above discontinuous, chaotic reflections of the beach platform, i.e., surf zone and innermost-shelf deposits. Beachface reflections dip to the west (towards figure left). Distance in meters (80–300 m) for this portion of the GPR profile is shown at figure top. A travel/time depth calibration scale for the entire profile (maximum 0–18 m depth below highest profile elevation) is shown at figure right. The travel/time depth calibration is based on a common midpoint measured velocity of 0.08 m ns− 1. The top and bottom of the GPR beachface facies occur at the relative depths of 9 and 15 m respectively, yielding a beachface deposit thickness of 6 m at this site.

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Table 2 Depths of beach facies and target ravinement surfaces from GPR records of CRLC drill sites. Site name

GPR line

Frequency MHz

Surface elevation (m) NGVD88

Beach facies subsurface Depth (m)

Beach facies depth (m) NGVD88

Target ravinement Depth (m)

Target ravinement (m) NGVD88

ROOS COPR1 OYHUT1 WEST1 GRBE GOUL NCOV SMIT OYST BAYR KLIP MCHU PCYD 67TH SUNS GLEN DELR DELM

ROOS1RA COPR1R1 OHYU1R1 WEST1R1a CANN1TB GRAY1TA WARR1R1 BOGD1TA OYSD1TA BAY1TA 227D1TA 1135TA PCYD1TA 67D1TA SUNS1RA PERK1R23a DELR DELD1TA

100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

3.4 5.8 4.0 5.0 5.9 4.6 5.3 5.5 5.4 3.3 6.3 3.7 5.6 4.0 4.4 4.0 3.0 5.3

1.5 7.0 6.0 6.0 5.5 5.0 7.0 4.5 6.0 6.0 7.0 5.0 6.0 6.0 5.0 6.0 4.0 6.0

1.9 −1.2 − 2.0 − 1.0 0.4 − 0.4 − 1.7 1.0 −0.6 − 2.7 − 0.7 − 1.3 − 0.4 − 2.0 − 0.6 − 2.0 − 1.0 − 1.3

2 8 – – – N 12b – 11 – 9 17 14 – 10 – 11 – 12

1.4 − 2.2 – – – N − 7.4b – − 5.5 – − 5.7c − 10.7 − 10.3 – − 6.0 – – – −6.7

and 50 and 50 and 50 and 50

and 50

and 50

Velocity = 0.8 m/ns based on averaged CMP tests from the CRLC barriers and beach plains. a Nearest GPR line to drill site, all other GPR lines directly over drill site. b Deeper than limiting value. c Flooding surface.

‘beach platform,’ i.e. innermost-shelf or tidal inlet deposits, at the study drill sites. The prograded beachface facies range from 1.5 to 7 m in thickness, and average 5.5 m for all 18 profiles. The base of the GPR beachface facies range from an elevation of +1.9 m NGVD88 for a beach site seaward of a recently eroded sea cliff (ROOS) in the North Beaches

subcell to −2.7 m NGVD88 (BAYR) at the landward backedge of the broad, Long Beach barrier. An average thickness of 6 m for the prograded beaches of the Grayland, Long Beach, and Clatsop subcells does not include abandoned foredune ridges (5–20 m in height) (Cooper, 1958; Rankin, 1983) which are not directly addressed in this paper.

Table 3 Shoreface drill sites, elevations and section depths in the Columbia River littoral cell. Core data for most drill sites are from Herb (2000). Drill site

MOCLVC MOCL HIGH2 ROOS3 COPR2 COPR1 Projected OYHUT1 AIRP4a WEST1a GRBE GOUL NCOV SMIT OYST BAYR Projected KLIP Projected MCHU Projected PCYD 67TH Projected WARRb SUNS GLEN DELR DELM

Location

Surf zone Beach Beach Beach Beach Historic foredune Beach Mid-barrier Back-barrier Beach Beach Mid-beach plain Beach Back-beach plain Front-barrier Back-barrier Beach Mid-barrier Beach Back-barrier Beach Mid-barrier Back-barrier Beach Back-barrier Beach Back-beach plain Beach Back-beach plain

UTM coordinates WG84 Northing

Easting

5232040 5231817 5230296 5225267 5218322 5218502 5204626 5207679 5205398 5193198 5184604 5178448 5177600 5177720 5155500 5149177 5146154 5146288 5138496 5137083 5133395 5133774 5133592 5112046 5113671 5105427 5106088 5099877 5101122

407750 407985 408392 409371 410281 410603 410851 411446 413303 414142 416169 417956 416337 418669 418725 420786 418722 420150 418480 421729 418097 419192 420795 425211 428773 427241 429208 427973 429937

Surface elevation (m) NGVD88

Ravinement surface (m) NGVD88

Shoreface thickness (m)

0.6 2.9 2.9 3.4 3.0 5.8 4.3 4.0 2.8 5.0 5.9 4.6 5.3 5.5 5.4 3.3 4.0 6.3 4.2 3.7 4.1 5.6 4.0 4.2 2.6 4.4 4.0 3.0 5.3

N − 3.0 1.4 1.3 2.0 − 4.3 − 1.7 (−25) − 8.0 − 11.2 − 30.5 − 8.9 − 7.1 − 22 (− 20) − 2.4 − 16.3 − 5.8c (− 20) −11.6 (− 30) − 5.3 (− 25) − 8.6 − 6.0 (− 30) − 8.9 N − 17 (− 30) − 9.3 N −10.5 (− 33) − 6.6

3.6 1.5 1.6 1.4 7.3 7.5 29.0 12.0 14.0 35.5 14.8 11.7 27.3 7.9 21.7 9.1 24.0 17.9 34.0 9.0 29.0 14.2 10.0 34.0 11.5 34.0 13.3 36.0 11.9

Projected = beach sites where ravinement surface elevations are projected from cross-section ties between offshore seismic reflection records and onshore water-well records (Herb, 2000; and Twichell et al., 2010-this issue). (xx) = projected depths of ravinement surfaces from offshore seismic reflection records and onshore water-well records. a Core data for AIRP4 and WEST1 are from Peterson and Phipps (1992). b Core data for WARR are from Baker (2002) and Baker et al. (2010-this issue). c Flooding surface.

C.D. Peterson et al. / Marine Geology 273 (2010) 62–82

Ten borehole sites were profiled by GPR to target potential preHolocene contacts below the shoreface sand deposits. Depths to the ‘target’ transgressive ravinement surfaces in the barrier and beach sites range from 2 to 17 m subsurface (Table 2). Ravinement surface depths at beach sites in the southern two subcells, Long Beach and Clatsop, exceeded the GPR penetration of 15–20 m depth subsurface. Onshore projections of the deeper ravinement surfaces (20–30 m depth) from nearshore seismic profiles were used to compliment the 20 m depth penetration records of the 50 MHz GPR in the deepest shoreface sections under the modern foredune ridges of the Clatsop and Long Beach subcells.

71

4.2. Borehole logs Holocene shoreface deposits were recovered in 23 boreholes and one vibracore site drilled in the CRLC barriers and beach plains (Fig. 5). The base of the Holocene shoreface, i.e., ravinement surface, is quite variable in subsurface elevation, ranging from 1 to −33 m NGVD88 (Table 3). Ravinement surfaces are shown for four representative boreholes in Fig. 7. Shoreface deposits average about 15 m in thickness in mid-barrier sites (drill sites OYHUT 12 m, GOUL 12 m, KIIP 18, PCYD 14 m) and about 10 m in thickness near backedges (AIRP4 13 m, SMIT 8 m, BAYR 9 m, MUCHU 9 m, 67TH 10 m, WARR

Fig. 7. Representative drill-site section logs from OHYUT (North Beaches), GRBE (Grayland), OYST (Long Beach) and SUNS (Clatsop). See Fig. 5a,b for borehole locations. Borehole elevations are in meters (NGVD88). Apparent breaks in dominant grain size were calibrated in the field with micrometer grain-size cards and hand-lens, i.e., mud, fine sand, medium sand, coarse sand, granule, and gravel. Detailed grain-size analyses for these and other borehole samples are presented in Table 4. Radiocarbon dates from these boreholes and others are presented in Table 6. Section logs are redrawn from Herb (2000).

72

C.D. Peterson et al. / Marine Geology 273 (2010) 62–82

11.5 m, DELM 12 m). Holocene deposits of littoral sand reach maximum thickness near tidal inlets (WEST 35 m, NCOV 27 m, OYST 22 m) and in the Clatsop subcell (SUNB 34 m, DELR 36 m). The wavedominated tidal inlet deposits are transitional with shoreface deposits of the barrier beaches, both consisting of fine, well-sorted sand. The Holocene shoreface deposits narrow and thin dramatically just north of Grays Harbor, e.g., from south to north in the North Beaches subcell, CORP1 = 7 m thick, HIGH2 = 2 m thick, and MOCL2 = 1 m thick. The range of shoreface deposit thickness (1 to 36 m) beneath the prograded barriers and beach plains is a function of available accommodation space above the corresponding ravinement surfaces (Table 3). The ravinement surfaces under the barriers show regionalscale shallowing north of Grays Harbor (Vanderburgh et al., 2010-this issue). Twichell et al. (2010-this issue) observed similar trends of shallowing ravinement surfaces north of Grays Harbor, as recorded in offshore seismic profiles. The pre-Holocene shelf topography that influenced the morphology of the Holocene ravinement surface resulted from (1) increased tectonic uplift north of Grays Harbor, and (2) down cutting of ancestral river valleys in the vicinities of the Columbia River mouth, Grays Harbor and Willapa Bay (Peterson et al., 1999). See Vanderburgh et al. (2010-this issue) for more details on ravinement surface discrimination in auger borehole logs. 4.3. Shoreface grain-size analysis Lithologic logs were constructed for representative Holocene sections in each of the four subcells (Fig. 7). The beach and innershelf deposits of the four subcells are dominated by fine sand. There were no apparent lithologic distinctions, such as grain-size differences, or relative abundance of mica flakes, granules, shell fragments, or driftwood between the upper and lower sections of the shoreface deposits in the Grayland, Long Beach and Clatsop subcells. Only the GPR records were able to discriminate between the prograded beachface facies and underlying ‘beach platform’ facies in the CRLC. Shell hash and granule fragments occur, though rarely, as erosional lag laminae (0.5–1 cm in thickness) in the shoreface deposits of the Long Beach subcell. Granule layers (1–10 cm in layer thickness) are common in lower sections of the Grayland subcell and in the upper sections of the Clatsop subcell. Pebbles and cobbles are abundant in the basal beach sections of the North Beaches subcell, but are nearly absent in the near surface deposits of the recently accreted beach face. Shoreface samples acquired at 0.5–1.5 m intervals downhole, were sieved for siliclastic grain-size distribution. The down-section trends in sand–gravel ratio and mean sand size are summarized in Table 4. Gravel abundances in the CRLC shoreface deposits generally range from 1 to 20% by weight where present. Most of the shoreface samples show small accumulations of mud (3–10% by weight). Some of the mud might have originated from post-depositional disaggregation of weak lithic fragments. However, many intact mud-laminae ‘drapes’ and ‘infiltration bands’ were observed in undisturbed augerring samples from the shoreface deposits including both the beach platform and prograded beachface facies. The significant mud content in some of the high-energy shoreface deposits possibly originates from the Columbia River plume that reverses in seasonally in longshore direction and/or from biogenic production in the nearshore. The mean grain sizes of the sieved sand fractions of the CRLC Holocene shoreface deposits range from 0.14 to 0.70 mm in diameter (Table 4). However, about 90% of the borehole samples represent a narrower range of sand sizes (0.18–0.23 mm sample mean sizes) within the sand fraction. There is relatively little change in sand size in either the downhole or across-barrier traverses. There is no apparent distinction in sand grain size or sorting (standard deviation) between the upper and lower sections of the shoreface deposits (Tables 2 and 4) at the study drill sites. The small range in auger-sample sand size is consistent with small variations in sand size reported from the modern

beaches (Peterson et al., 1994; Ruggiero and Voigt, 2000) and from the modern innermost shelf, i.e. to about 25 m water depth (Twichell and Cross, 2001). 4.4. Heavy-mineral analyses of sand provenance Sand samples from three of the deeper boreholes, i.e. WEST1 to 27 m depth NGVD88 (Grayland subcell), OYST to 16.2 m depth (Long Beach subcell) and SUNS to 15.9 m depth (Clatsop subcell) were analyzed for light- and heavy-mineral tracers of sand source. Quartz and feldspar are abundant (60–80 wt.%) in the sand samples, but volcanic lithic fragments are common (20–40 wt.%) in all samples. Two non-opaque, colored heavy minerals, including hypersthene and augite, are used to discriminate between sand sources from CRLC coastal streams and the late-Holocene littoral sand supplied by the Columbia River (Table 5). These two pyroxene minerals are of similar shape and density, and have been previously used to discriminate between (1) basalt provenances (high in augite), and (2) the Cascade volcanic arc drained by western tributaries to the Columbia River (high in hypersthene) (Scheidegger et al., 1971; Scheidegger and Phipps, 1976). Baker et al. (2010-this issue) have shown that the Columbia River sand provenance changed from eastern provenance (augite-rich HM fraction) to western arc provenance (hypersthenerich HM fraction) with transition from glacial to post-glacial conditions. The western tributaries of the Columbia River have dominated the mineralogy of the lower Columbia River since about 9 ka. Littoral sand deposits derived from the Columbia River in middle- to late-Holocene time are identified by high hypersthene:augite ratios (0.6–0.8) and are referred to here as ‘post-glacial’ Columbia River sand. Heavy-mineral analyses from the three boreholes (WEST1, OYST and SUNS) range from 0.6 to 0.9 for the hypersthene:augite ratio (Table 5). The downhole analyses of hypersthene:augite ratio for the three borehole sites show affinities for ‘post-glacial’ Columbia River sand supply. None of the samples showed significant sand supply from potential augite-rich sources, such as the rivers draining the Olympic Range to the north of Grays Harbor, and/or sand from the mid-to-outer continental shelf located offshore and north of Grays Harbor (Venkatarathnam and McManus, 1973). The Holocene barriers, together with their underlying tidal inlet and inner-shelf sand deposits, were derived from Columbia River sand delivered to the CRLC in middle- to late-Holocene time. There is no evidence that pre-Holocene or ‘glacial period’ sources of Columbia River sand contributed to the CRLC Holocene shoreface deposits. 4.5. Shoreface age dating Shoreface deposits in 16 boreholes and adjacent vibracore sites are dated by (1) radiocarbon (31 samples; Table 6), (2) established paleoshoreline positions (33 positions; Peterson et al., 2010-this issue), and (3) projected ravinement surface depths (SUNS and DELR at ∼ 30 m depth NGVD88) (Fig. 3). The depths and ages of shoreface samples from the barriers and beach plains are plotted relative to the CRLC sea-level curve (Fig. 8). The onset of shoreface deposition under the CRLC barriers and beach plains started in the Clatsop and Long Beach subcells at 8–9 ka (OYST −17 m NGVD88 at 8325–8389 calRCYBP). Shoreface vertical accretion did not occur above the shallower ravinement surfaces of the Grayland and North Beaches subcells respectively, until after 5 ka (GRBE −7.5 m at 4250–4430 calRCYBP) and after 1–2 ka (OHYUT1 −7.1 m at 485–535 calRCYBP). The timing of the onset of shoreface deposition under the CRLC barriers and beach plains was dependent on pre-Holocene topography, sand supply, and sand retention within the shoreface, as will be outlined below. The oldest and deepest Holocene shoreface deposits were found in the Long Beach and Clatsop subcells, both located on either side of

C.D. Peterson et al. / Marine Geology 273 (2010) 62–82

73

Table 4 Grain-size distributions from shoreface deposits under CRLC barriers and beach plains. Elev MOCLVC 0.3 MOCL 2.4 HIGH2 2.6 1.1 ROOS 2.2 COPR2 2.4 1.1 − 2.5 − 3.6 COPR1 5.2 3.3 0.5 − 1.4 OYHUT1 2.7 − 1.3 − 2.7 − 6.0 AIRP4 2.3 − 1.7 WEST1 3.5 −1 −6 −10.5 −15 −19.5 −24 GRBE 4.6 1.3 − 1.7 −5.5 GOUL 4.2 0.8 −3 − 6.8 SMIT 4.2 0.5 NCOV 4 0.2 − 3.4 OYST 2.8 −0.9 − 4.8 − 6.3 − 10.0 − 11.1 BAYR 2 0.1 − 1.8 − 4.9 KLIP 5.7 2.5 − 1.3 − 5.1 −7.7 −11.0 MCHU 3.1 1.8 − 0.1

Gr

Sa

Mu

sM

sSD

Elev

Gr

Sa

Mu

sM

sSD

− 1.6

0

99

1

0.19

0.044

0.034

2.1

0

94

6

0.142

0.171 0.231

0.044 0.223

2.3

0

91

9

5

0.168

0.04

1.8

31

66

97 94 92 84

6 5 7 3

0.18 0.19 0.203 0.274

0.054 0.051 0.126 0.317

2.2 0.2 − 2.6

0 0 8

0 0 0 19

89 95 94 78

11 5 3 3

0.173 0.184 0.201 0.245

0.05 0.037 0.061 0.245

4.5 1.5 − 0.1

0 0 1 1

97 96 94 94

3 4 5 5

0.178 0.185 0.187 0.221

0.031 0.054 0.083 0.207

0 19

10 78

90 3

– –

– –

0 0 1 7 0 0 3

97 98 96 92 98 96 95

3 2 3 1 2 4 2

0.18 0.25 0.21 0.55 0.22 0.19 0.42

1 3 0 0

92 95 98 79

7 2 2 21

11 0 0 0

69 94 97 87

8 0

Elev

Gr

Sa

Mu

sM

SSD

− 3.0

5

93

2

0.188

0.07

0.04

1.6

26

72

2

0.175

0.054

0.16

0.03

2

0

95

5

0.177

0.031

3

0.389

0.479

92 94 85

8 6 7

0.18 0.155 0.31

0.046 0.16 0.303

1.7 − 0.8 − 3.3

0 1 2

91 94 95

9 5 4

0.193 0.177 0.218

0.204 0.191 0.17

0 11 3

92 85 81

8 4 16

0.174 0.229 0.215

0.041 0.153 0.134

3.9 0.9 − 0.8

0 1 15

92 96 77

8 3 8

0.179 0.201 0.183

0.039 0.094 0.123

1.3 − 1.6 − 3.6 − 7.1

0 5 0 30

97 92 97 67

3 3 3 3

0.171 0.184 0.184 0.446

0.03 0.54 0.056 0.419

0 − 2.4 − 5.3

0 10 3

97 85 96

3 5 1

0.184 0.24 0.176

0.038 0.162 0.062

0.8 −3.2

0 18

99 79

1 3

– –

– –

− 0.2 − 4.7

0 0

100 98

0 2

– –

– –

0.07 0.15 0.24 0.8 0.08 0.06 0.49

3 − 2.5 − 7.5 − 12 −16.5 − 21 −25.5

0 0 22 3 0 0 1

97 97 76 96 98 99 97

3 3 2 3 2 1 2

0.19 0.19 0.89 0.32 0.21 0.37 0.45

0.06 0.06 1.15 0.49 0.08 0.29 0.43

0.5 −4.5 −9 −13.5 − 18 − 22.5 − 27

0 0 32 1 0 2 36

99 98 63 91 98 97 61

1 2 5 8 2 1 3

0.17 0.19 1.14 0.21 0.22 0.39 1.51

0.04 0.04 1.29 0.27 0.09 0.4 1.18

0.168 0.194 0.218 0.168

0.032 0.07 0.083 0.054

3.4 0.8 − 3.0 −7.2

0 0 0 0

92 89 86 95

8 11 14 5

0.165 0.197 0.174 0.198

0.03 0.051 0.045 0.075

2.1 − 0.4 − 4.3

0 0 0

88 93 90

12 11 10

0.171 0.188 0.168

0.034 0.055 0.051

20 6 3 13

0.467 0.189 0.192 0.214

0.44 0.044 0.098 0.07

3.3 −0.5 − 4.3

1 0 0

97 95 98

2 5 2

0.158 0.206 0.174

0.057 0.088 0.056

2.1 − 1.7 − 5.6

0 0 0

91 97 95

9 3 5

0.18 0.201 0.171

0.039 0.078 0.053

89 0

3 66

0.547 0.171

0.482 0.05

3 −0.8

0 0

58 47

42 53

0.159 0.21

0.063 0.097

1.7 − 2.1

0 0

35 96

65 4

0.166 0.184

0.046 0.065

0 0 0

92 97 97

8 3 3

0.17 0.203 0.201

0.036 0.047 0.061

2.8 −1 −4.7

0 0 0

99 95 100

1 5 0

0.191 0.206 0.192

0.03 0.076 0.035

1.5 −2.3 − 5.7

0 0 1

99 95 74

1 5 25

0.194 0.192 0.182

0.034 0.058 0.093

0 0 0 0 0 0

95 95 87 92 97 81

5 5 13 8 3 19

0.187 0.234 0.216 0.229 0.189 0.162

0.037 0.058 0.061 0.113 0.041 0.039

1.6 −2.2 − 5.4 −7.3 − 10.2 − 12.4

0 0 0 0 5 0

83 95 92 90 85 89

17 5 8 10 10 11

0.195 0.205 0.233 0.206 0.239 0.188

0.05 0.051 0.077 0.056 0.228 0.047

0.5 − 3.5 − 6.0 − 8.6 − 10.6 − 16.2

0 0 0 0 4 0

85 89 83 90 82 95

15 2 17 10 14 5

0.205 0.205 0.199 0.195 0.238 0.204

0.057 0.055 0.062 0.048 0.216 0.057

0 0 0 0

91 82 89 77

9 18 11 23

0.187 0.202 0.211 0.193

0.036 0.055 0.055 0.065

1.4 − 0.5 − 3.0 −5.6

0 0 0 0

90 80 81 84

10 20 19 16

0.201 0.202 0.201 0.201

0.04 0.056 0.049 0.06

0.8 − 1.1 − 3.7

0 0 0

87 97 91

13 3 9

0.193 0.24 0.19

0.042 0.05 0.046

0 0 0 0 0 0

92 89 93 95 84 90

8 11 7 5 16 10

0.277 0.187 0.227 0.205 0.192 0.194

0.233 0.04 0.055 0.056 0.05 0.064

5 1.2 − 2.6 − 6.0 − 8.9 − 11.5

0 0 0 0 0 0

75 83 82 92 85 87

25 17 18 8 15 13

0.19 0.181 0.201 0.211 0.21 0.196

0.039 0.04 0.07 0.081 0.07 0.076

3.8 −0.0 − 3.9 − 6.4 − 10.2 −11.7

0 0 0 0 0 0

91 82 93 86 63 90

9 18 7 14 37 10

0.185 0.197 0.204 0.198 0.17 0.195

0.039 0.054 0.048 0.05 0.045 0.05

0 0 0

90 86 89

10 14 11

0.172 0.199 0.2

0.043 0.051 0.052

2.6 1.2 −1.4

0 0 0

94 86 88

6 14 12

0.185 0.185 0.205

0.065 0.047 0.047

2.4 0.3 −2.0

0 0 0

91 95 83

9 5 17

0.186 0.185 0.181

0.042 0.041 0.038

0

100

0

0.195

0.06

0

85

15

0.158

0 37

93 53

7 10

0

95

0 1 1 13

(continued on next page)

74

C.D. Peterson et al. / Marine Geology 273 (2010) 62–82

Table 4 (continued) Elev

Gr

Sa

MCHU − 2.6 0 85 − 4.5 0 82 PCYD 4.3 14 79 1.4 0 83 − 2.0 0 94 −5.8 0 82 67TH 2.7 0 91 −1 0 87 −4.9 24 64 WARR correct for NGVD88 1.1 1 9 −2.6 0 95 −7.1 0 93 GLEN 2.7 0 96 − 1.1 0 88 −4.3 20 69 −7.4 0 91 SUNS 3.1 0 91 −0.6 0 89 −3.6 8 83 −6.6 0 96 −8.9 1 84 −10.9 0 84 − 13.1 0 88 −15.0 0 79 DELM 4 19 76 0.2 0 83 − 3.6 0 90 6.1 0 87 DELR 1 9 80 −2.1 1 92 −6.3 0 95 − 9.3 12 82

Mu

sM

sSD

Elev

Gr

Sa

Mu

sM

sSD

Elev

Gr

Sa

Mu

sM

SSD

15 18

0.209 0.182

0.069 0.042

−3.3

0

89

11

0.194

0.049

− 3.9

0

96

4

0.196

0.042

7 17 6 18

0.228 0.171 0.247 0.197

0.19 0.041 0.061 0.054

3.1 0.5 −3.3 − 7.1

0 0 0 0

94 86 92 88

6 14 8 13

0.198 0.205 0.222 0.198

0.045 0.051 0.063 0.054

1.8 − 0.8 −4.6 −8.6

0 0 0 0

91 90 89 74

9 10 11 26

0.197 0.224 0.206 0.217

0.044 0.055 0.052 0.119

9 13 12

0.179 186 0.187

0.035 0.045 0.11

1.5 − 2.3 − 5.5

0 0 0

87 94 83

17 7 17

0.17 0.18 0.167

0.035 0.035 0.036

0.2 −3.6

0 0

80 93

20 7

0.18 0.17

0.042 0.033

90 5 7

0.104 0.218 0.201

0.409 0.79 0.72

− 0.4 − 4.1

0 0

50 91

50 9

0.189 0.215

0.68 0.737

−1.1 −5.6

0 0

96 89

4 11

0.212 0.191

0.796 0.725

4 12 11 9

0.189 0.182 0.284 0.197

0.038 0.047 0.248 0.055

1.5 − 2.3 − 4.9 − 8.7

0 0 0 0

88 90 86 85

12 10 14 15

0.175 0.2 0.196 0.193

0.043 0.046 0.054 0.051

0.2 −3.6 −6.2

0 0 0

87 94 85

13 6 15

0.192 0.233 0.19

0.047 0.066 0.052

9 11 9 4 15 16 12 21

0.214 0.268 0.229 0.222 0.239 0.158 0.181 0.18

0.056 0.151 0.094 0.076 0.171 0.043 0.103 0.061

1.9 − 1.9 − 4.6 − 7.0 − 9.6 − 12.1 − 13.3 − 15.4

7 0 3 4 0 0 0 0

78 89 78 86 86 82 85 92

15 11 19 10 14 18 15 8

0.228 0.254 0.278 0.302 0.158 0.15 0.17 0.204

0.094 0.086 0.191 0.232 0.064 0.05 0.063 0.054

0.6 −3.2 −5.8 −8.3 − 10.3 −12.5 − 14.7 − 15.9

0 0 0 0 5 0 0 0

86 96 87 89 80 99 85 93

14 4 13 11 15 1 15 7

0.212 0.215 0.212 0.228 0.196 0.174 0.166 0.166

0.056 0.058 0.068 0.081 0.198 0.046 0.045 0.046

5 17 10 13

0.697 0.19 0.209 0.201

0.527 0.044 0.053 0.052

2.8 −1 −4.9

0 0 0

93 84 84

7 16 16

0.193 0.188 0.195

0.046 0.046 0.065

1.5 −2.3 − 5.8

0 0 0

86 91 96

14 9 4

0.182 0.202 0.22

0.042 0.047 0.057

11 7 5 6

0.216 0.278 0.26 0.215

0.15 0.121 0.15 0.18

0.5 − 4.6 −7.2 −9.7

0 0 0 0

91 96 87 89

9 4 13 11

0.221 0.216 0.202 0.157

0.068 0.055 0.064 0.038

− 0.8 − 5.9 − 8.4 − 10

1 0 1 0

91 86 88 92

8 14 11 8

0.201 0.205 0.21 0.158

0.07 0.063 0.098 0.05

Elev = m NGVD88; Gr = wt.% gravel (N 2 mm diameter); Sa = wt.% sand, Mu = wt.% mud (b0.063 mm); sM = sand mean size (mm diameter); sSD = sand standard deviation (± mm diameter).

ancestral Columbia River mouth (Fig. 8). These subcells are characterized by (1) deep ravinement surfaces, e.g., projected 20–30 m depth NGVD88 under the youngest beaches, and (2) substantial distances of shoreline progradation (2–3 km) (Fig. 5). In contrast, a shallow shelf platform north of Grays Harbor (Fig. 2) generally delayed the arrival of the transgression in the North Beaches subcell. A lack of sand supply, or perhaps a lack of sand retention, north of Grays Harbor precluded any significant shoreface accretion there until nearly historic time (Fig. 8). The age of the transgressive ravinement surface and the subsequent shoreface deposition should roughly correspond to the sealevel curve for a given elevation. Three borehole samples appear to be anomalously young, relative to the sea-level curve (Fig. 8). Two samples in the Grayland subcell, at −30 and −10 m elevation NGVD88, are from the WEST1 borehole, located near the Grays Harbor tidal inlet. A probable explanation is that post-transgressive, lateral migrations of the tidal inlet likely eroded and backfilled barrier deposits with younger materials there. The other anomalously young shoreface sample (4.3 ka) was recovered in the Clatsop subcell at −16 m elevation NGVD88; this important anomaly is discussed later under shoreface ages and sedimentation rates in the Discussion section. A grouping of late-Holocene deposits (6–4 ka) from the Long Beach subcell appears to be too ‘shallow’ (0 to + 6 m elevation NGVD88) relative to the CRLC sea-level curve (Fig. 8). These deposits represent early beachface deposition at several meters above the existing sea level during the earliest shoreline progradation in the

CRLC. Modern beachface deposits reach 4–5 m above mean tidal level in the CRLC (Peterson et al., 1994). Conversely, a grouping of latestHolocene deposits (1–0 ka) from the North Beaches, Grayland, and Clatsop subcells appear to be ‘too deep’ (−1 to − 16 m NGVD88) relative to the CRLC sea-level curve. These deposits are generally from modern beach boreholes in the Clatsop and North Beaches subcells. The ‘late’ ages in the youngest Clatsop and North Beaches drill sites apparently represent a lag in the most recent shoreface accretion in these subcells. These apparent lags in shoreface accretion in the Clatsop and North Beaches subcells are addressed in more detail under shoreface ages and sedimentation rates in the Discussion section. 5. Discussion 5.1. Variation in shoreface facies under barriers and beach plains Holocene deposits that were drilled along the backedges of the CRLC barriers and beach plains (Fig. 4) record a variety of depositional environments. The backedge deposits include (1) fine sand in prograding beachface facies at all sites, (2) fine gravel in overwash fans on the Grays Harbor north barrier (AIRP4 −1.7 to − 3.2 m), (3) muddy sand from infilling bay inlets below the prograded beachface facies in the Willapa Bay barrier (BAYR −3.0 to −5.6 m), (4) muddy sand in back-berm lagoons or debris flow fans at the bases of abandoned sea cliffs (SMIT 3.0 to −0.8 m), and (5) muddy peat from bay wetlands that buried bay shorelines during transgressive bay

C.D. Peterson et al. / Marine Geology 273 (2010) 62–82

75

Table 5 Heavy-mineral indicators of sand provenance in CRLC barrier boreholes. Grayland subcell WEST1 Depth (m)

Hypersthene: augite ratio

NGVD88 3.5 3 0.5 −1 − 2.5 − 4.5 −6 − 7.5 −9 − 10.5 − 12 − 13.5 − 15 − 16.5 − 18 − 19.5 − 21 − 22.5 − 24 − 25.5 − 27

Long beach subcell OYST Depth (m)

Hypersthene: augite ratio

NGVD88 0.8 0.9 0.8 0.6 0.8 0.8 0.6 0.8 0.7 0.8 0.9 0.8 0.7 0.7 0.8 0.8 0.7 0.7 0.8 0.8 0.7

2.8 1.6 0.5 0.9 −2.2 −3.5 −4.8 −5.4 −6.0 −6.3 −7.3 −8.6 −10.0 − 10.2 − 10.6 − 11.1 −12.4 −16.2

Clatsop subcell SUNS Depth (m)

Hypersthene: augite ratio

NGVD88 0.8 0.8 0.7 0.8 0.9 0.8 0.9 0.8 0.7 0.8 0.8 0.6 0.8 0.7 0.8 0.8 0.7 0.8

3.1 1.9 0.6 − 0.6 − 1.9 − 3.2 − 3.6 − 4.6 − 5.8 − 6.6 − 7.0 − 8.3 −8.9 − 9.6 − 10.3 − 10.9 − 12.1 −12.5 − 13.1 − 13.3 − 14.7 − 15.0 − 15.4 − 15.9

0.7 0.8 0.8 0.7 0.9 0.8 0.7 0.8 0.8 0.7 0.9 0.8 0.8 0.7 0.8 0.8 0.8 0.6 0.8 0.7 0.7 0.8 0.7 0.8

Hypersthene:augite ratio by point count (N100 grains). Columbia River Sand Mineralogy (Baker et al., 2010-this issue). Potential sand sources: Lower Chehalis River = hypersthene:augite 0.1–0.2; Humptulips River = hypersthene:augite b0.1; Willapa River = hypersthene:augite b0.1; Early Columbia River 15 ka = hypersthene:augite 0.2; Columbia River 12 ka = hypersthene:augite 0.2; Columbia River 9 ka = hypersthene:augite 0.8; and Late Columbia River 6 ka = hypersthene:augite 0.8.

flooding (WARR 1.1 to − 0.4 m and AIRP4 2.3 m) (Table 4). The onset of beachface progradation in both the barriers and beach plains of the CRLC is characterized by substantial variability between and within drill sites with respect to lithofacies development. With shoreline progradation of several hundred meters the shallowest deposits are organized into beachface facies (Smith et al., 1999) and abandoned foredune ridges (Cooper, 1958; Rankin, 1983). The lithologies of these deposits are uniformly dominated by well-sorted sand (∼90% by weight) that is fine upper in size (0.177–0.250 mm; Table 4). Ponds or peat bogs developed in some of the abandoned interdune-ridge valleys following a gradual rise in groundwater level (Peterson et al., 2007). Thin soils have developed on the older foredunes (Peterson et al., 2006). Episodic accretion and catastrophic retreat of the beachface facies have been correlated to subduction-zone tectonic cycles (500 year mean recurrence interval) in the study area (Meyers et al., 1996; Peterson et al., 2010-this issue). Unlike the barrier backedge drill sites the shallowest shoreface deposits under the mid- and front-barrier sites are characteristically uniform in lithology. 5.2. Source of littoral cell sand Heavy-mineral (HM) analyses of sand samples from three boreholes in the Grayland, Long Beach, and Clatsop subcells have high ratios of hypersthene:augite (Table 5). These ‘hypersthene-rich HM fractions’ extend down to ravinement surfaces dating to 6 and 8 ka in the Grayland and Long Beach subcells, respectively (Table 6). The high hypersthene:augite ratios (0.6–0.8) in the WEST1 and OYST boreholes rule out sand supply to the Grayland and Long Beach subcells from potential sand sources that have augite-rich HM fractions. Both the local coastal rivers and the outer-to-middle shelf sands are characterized by high augite:hypersthene ratios in the HM fraction (Scheidegger and Phipps, 1976; Venkatarathnam and McManus, 1973). The only other possible source of sand to the CRLC shoreface is the Columbia River, which currently yields ‘hypersthene-rich HM

fractions.’ How then was the Columbia River sand delivered to the northern subcells? Venkatarathnam and McManus (1973) report hyperstheneenriched HM fractions in sand from modern deposits of the innershelf extending from Willapa Bay to the northern end of the CRLC at Point Grenville (Fig. 9). The hypersthene-enriched HM fractions in modern deposits of the inner-shelf represent middle- to lateHolocene bedload discharge from the Columbia River that was transported north along the inner-shelf. The depositional age of the surface sand samples from the inner-shelf were not established by Venkatarathnam and McManus (1973). However, the Grays Harbor tidal inlet borehole (WEST1) was dominated by the supply of hypersthene-rich HM fractions since at least 8 ka (Tables 5 and 6). Nearshore sand supply to the Long Beach and Grayland cells has been dominated by northward longshore supply of Columbia River sand since the middle Holocene, that is from at least 8 ka to the present. A disruption of the pattern of hypersthene-rich HM fractions in the inner-shelf deposits just north of Grays Harbor (Fig. 9; Venkatarathnam and McManus, 1973) might reflect diverted sand transport around a shallow bedrock feature located offshore of Grays Harbor (Fig. 2). The bedrock shoal would have likely acted as a headland or offshore island during the lower sea-level conditions of the middle Holocene. A heavymineral anomaly (HM greater than 10% by weight) also stretches north from the same bedrock shoal area on the inner-shelf (Fig. 4). The HM anomaly suggests non-uniform transport conditions associated with northward transport around or over the bedrock shoal located in the inner-shelf offshore of Grays Harbor. Venkatarathnam and McManus (1973) report diminished, but significant, hypersthene factor loading in samples from the innermost-continental shelf at the northernmost end of the CRLC (Fig. 9). These innermost-shelf patterns of hypersthene dispersal indicate a recent loss of some post-glacial Columbia River sand from the CRLC, northward around Point Grenville. Elevated hypersthene-values also occur in sand deposits from middle shelf reaches located north of

76

C.D. Peterson et al. / Marine Geology 273 (2010) 62–82

Table 6 Ages of shoreface deposits below CRLC barriers and beach plains arranged north to south, then east to west and surface to depth. Sample

Depth (m) NAVD88

Northing (UTM)

Easting (UTM)

Facies

Source

Type

Measured C14 (±1sYBP)

C13/C12 ratio o/oo

Swash bar MOCLVC-362 Backshore Lower beach MOCL1-50 Dune ridge Lower beach COPR1-285 Backshore Lower beach OYHUT1-336 OYHUT1-436 OYHUT1-489 Dune ridge Lower beach WEST1 WEST1 Backshore Lower beach GRBE-310 GRBE-418 GRBE-518 Beach plain Peat GoulRd Upper beach Lower beach GOUL-470 Foredune Lower beach NCOV-384 Foredune Lower beach OYST-492 OYST-864 Peat bog Peat S. cran Upper beach (1)Cran98 Lower beach KLIP-706 Beach plain Upper beach Lower beach MCHU-416 MCHU-514 Beach plain Upper beach Lower beach 67TH-201 67TH-397 Beach plain Peat TinkLk Upper beach Lower beach PCYD-600 Backshore Lower beach SUNB-312 SUNB-431 SUNB-827 Projected Beach plain Rankin#3 Upper beach Lower beach DELM-438 Backshore Lower beach DELR-366 Projected

0.6 −3.0 2.9 1.6 1.6 5.8 −1.2 −1.4 4.0 −2 −4.5 −7.1 −8.4 5 −1 −10 −29.5 5.9 0.4 −2.0 −4.7 −7.3 4.6 3.9 3.5 −0.4 −7.3 5.3 −1.7 −4.5 5.4 −0.6 −7.1 −16.5 6.0 5.4 5 2.0 −1 −11.6 3.7 3.4 −1.3 −6.8 −9.3 4.0 4 −2 −1.1 −6.1 5.6 4.9 4.7 −0.4 −9.6 4.4 −0.6 −3.5 −6.5 −16.6 (− 30) 5.3 4 4 −1.3 −5.8 3.0 −1 −6.3 (− 33)

5232040

407750

S S B B R B B S B B S S NR B B S S B B S S S

Active Shell Active

AMS

640 ± 40

− 3.4

Shell Active Event A Shell Post1880 Pre1880 Shell Shell Wood Active Post1880 Shell Shell Active Post1880 Shell Shell Peat

AMS

114 ± 0.5%

− 0.1

AMS

1300 ± 40

+ 1.5

AMS AMS AMS

520 ± 40 860 ± 40 33,310 ± 230

0.0 1.0 − 29.3

BULK BULK

3630 ± 120 5640 ± 90

− 1.1 − .7

4020 ± 120 6040 ± 90

AMS AMS BULK

1120 ± 40 1040 ± 40 3940 ± 70

0.0 + 0.9 − 26.6

1550 ± 40 1460 ± 40 3920 ± 70

P B B NR B B S B B S R

Wood Event C Event D Wood Active Post1880 Shell Active Pre1880 Wood Wood

AMS

–

–

AMS

7420 ± 60

− 26.5

7400 ± 60

AMS

460 ± 40

+ 1.0

870 ± 40

BULK AMS

3980 ± 70 7530 ± 60

− 24.7 − 25.7

3980 ± 70 7520 ± 60

P B B B R B B B E F B B B S R

Peat Event D Wood Event E Peat

AMS

–

–

1450 ± 50

BULK

2430 ± 60

− 25.4

2420 ± 60

5231820

407990

5218500

410600

5207490

411550

5193000

5182750

5178320

414600

416480

1000 ± 40

108 ± 0.5%

1730 ± 40

930 ± 40 1290 ± 40 33,240 ± 230

418100

5177550

416440

5155220

418950

5146150 5137720 5146150 5137820 5146050

420100 419720 420100 419820 420100

5136800

421870

5133420

Conventional C14 (±1sYBP)

421150

5133420 5133840

419500 419360

5133420

419500

5105300

427490

0.6

5101000 5103271

430150 430110

5099800

428360

910 ± 50

BULK

4700 ± 70

− 24.8

4710 ± 70

Event I Event J Wood Wood

BULK AMS

5240 ± 60 39,350 ± 530

− 25.4 28.2

5240 ± 60 39,300 ± 530

Event H Event I Wood Wood

AMS AMS

4750 ± 40 4660 ± 50

− 25.4 − 27.0

4750 ± 40 4630 ± 50

AMS

–

–

AMS

7080 ± 110

− 26.4

7060 ± 110

AMS BULK AMS

160 ± 40 2530 ± 60 3870 ± 50

− 25.4 − 25.5 − 26.8

160 ± 40 2520 ± 60 3840 ± 50

BULK

2785 ± 85

BULK

4280 ± 80

− 25.9

4260 ± 80

AMS

2410 ± 10

− 27.5

2370 ± 60

P B B NR B B S S S R

Peat Event D Event E Shell Active Post1880 Wood Wood Wood

P B B bR B B S R

Peat Event G Event H Peat Active Post1880 Wood

860 ± 50

Calibrated C14 (±1sYBP) (0.0 ka) 265 ± 25

(0.0 ka) (0.0 ka) (0.3 ka) 870 ± 35 (0.0 ka) (0.1 ka) 185 ± 70 510 ± 25 Pleistocene (0.0 ka) (0.1 ka) 3525 ± 141 6050 ± 103 (0.0 ka) (0.1 ka) 695 ± 35 637 ± 23 4340 ± 90 (0.0 ka) 840 ± 71 (1.3 ka) (1.7 ka) 8252 ± 77 (0.0 ka) (0.1 ka) 77 ± 67 (0.0 ka) (0.1 ka) 4430 ± 190 8357 ± 32 (0.0 ka) 1343 ± 44 (1.7 ka) 2527 ± 177 (2.5 ka) 5450 ± 130 (0.0 ka) (4.7 ka) (5.0 ka) 5970 ± 45 Pleistocene (0.0 ka) (4.0 ka) (4.7 ka) 4545 ± 1035 5390 ± 75 (0.0 ka) 793 ± 91 (1.7 ka) (2.5 ka) 7882 ± 97(RM) (0.0 ka) 140 ± 140 2612 ± 127 4317 ± 87 (8–9 ka)a (0.0 ka) 2872 ± 93 (3.2 ka) (4.0 ka) 4733 ± 13 (0.0 ka) (0.1 ka) 2519 ± 185 (8–9 ka)a

Sample depth is relative to 0 m NAVD1988 (+ 0.2 m MLLW). Position UTM coordinates WG84. Sample type: material (marine shell or wood) or correlated earthquake scarp (x.x ka) (Peterson et al., 2010-this issue). Shoreface facies: b R, just above ravinement; R, at ravinement; N R, just below ravinement; F, flooding surface; P, peat bog; B, beach; S, shelf; E, pre-ravinement lagoonal. Shell marine reservoir corrections based on delta R 390 ± 25. RM = possibly remobilized (too old?). a

Estimated age of projected ravinement surface based on relative sea-level curve.

Beta #

146887

129512

129521

129513 129520 146882

20957 20284

129516 129511 129531 122810

129524

129515

148095 129514 122809 129528 129519

129525 129518

146886 129523 122811

131122

146884 129529 129522

129532

146880

C.D. Peterson et al. / Marine Geology 273 (2010) 62–82

77

5.3. Trends in sand-fraction grain size

Fig. 8. Plot of sample depths (meters NGVD88) and corresponding ages (ka) from shoreface and beachface deposits developed above transgressive ravinement surfaces in the study area boreholes. Samples are grouped by subcell (North Beaches, Grayland, Long Beach, and Clastsop). Deposit ages are based on small, articulated bivalve shells, delicate single valves, and small fragments of wood. The relative sea-level curve for the CRLC is also plotted to 40 m depth for comparison to the shoreface development. Ravinement surface ages are not included in this figure. Sample depth and age data are from Table 6.

Point Grenville. These mid-shelf patterns of hypersthene dispersal could indicate that some Columbia River sand has bypassed the CRLC to feed the northern Washington shelf since the middle Holocene. Such a loss of Columbia River sand from the northern end of the CRLC could help explain the lack of prehistoric shoreface progradation north of Grays Harbor in the North Beaches subcell. The potential volume of Columbia River sand lost to the northern Washington shelf during the Holocene transgression is not known. However the volume could be large, based on the broad dispersal of hypersthene trace minerals in middle- and inner-shelf deposits from the northern Washington shelf (Venkatarathnam and McManus; 1973). The lack of glacial-period Columbia River sand from eastern-basin provenances in the Clatsop and Long Beach subcells raises questions about the fate of the Glacial Lake Missoula flood sediments that were carried down the Columbia River (Bretz, 1969). These glacial flood sediments were intermittently delivered to the continental slope until at least 13 ka (Benito and O'Connor, 2003; Normark and Ried, 2003). During the last glacial maximum (LGM) the Columbia River valley graded to a sea level some ∼ 120 m below present sea level (Twichell et al., 2010-this issue), and would have extended across the inner-shelf to the Astoria Canyon head (Fig. 4). A broad topographic depression in the continental shelf was centered on the ancestral Columbia River valley. The broad valley system would have directed much of the littoral sand downslope to the Astoria canyon during low-stand conditions (P. Cowell, pers. comm., 2000). The incised shelf topography leading directly to the Astoria Canyon apparently precluded the storage of large volumes of glacial-period Columbia River sand on the inner-shelf outside of the ancestral Columbia River valley.

The averaged sample means for the sand-size fractions from three representative drill sites in the CRLC are: 0.209 ± 0.041 mm SUNS (Clatsop subcell; 24 samples), 0.207 ± 0.021 mm OYST (Long Beach subcell; 18 samples) and 0.183 ± 0.0174 mm GRBE (Grayland subcell; 11 samples) (Table 4). Intervals from the top halves of the North Beach drill sites (22 samples from MOCL, HIGH2, ROOS, COPR and OYHUT1) averaged 0.175 ± 0.0125 mm in mean grain size. These results show a subtle grain-size fining of shoreface sand with increasing distance north in the CRLC system. Localized coarsening of the sand-size fraction occurs at tidal inlets, such as Grays Harbor 0.403 ±0.355 mm (WEST 21 samples) and near the ravinement surface in the North Beaches 0.30 ± 0.10 (7 samples) (Table 4). There are also interesting differences in up-section trends of sand grain size on opposite sides of the Columbia River mouth. Whereas averaged mean grain size increases upsection in SUNS (Clatsop subcell), e.g., 0.187 ±0.026 mm bottom half and 0.229 ± 0.043 top half, there is no significant change in grain-size upsection in OYST (Long Beach subcell) where the values are 0.208± 0.017 mm bottom half and 0.206± 0.025 mm in the top half. The subtle, vertical trends in sand grain size are consistent with minor gravel abundance in the shoreface deposits (Table 4 and Fig. 7). Bedload discharged from the Columbia River mouth (Baker et al., 2010-this issue) drifted south in the nearshore of the Clatsop subcell prior to being segregated into coarser fractions in beachface deposits and finer factions in lower shoreface and/or inner-shelf sections. By comparison, the lower and upper sections of the shoreface deposits from the Long Beach and Grayland subcells show no size segregation, indicating a common source of fine sand supply. The base of the North Beaches shoreface, e.g., north of Grays Harbor, reflects some local coarse sediment supply, from the adjacent eroded sea cliffs. Gravels released from the transgressive retreat of the North Beaches sea cliffs were transported south to the Grays Harbor tidal inlet (WEST1) since the mid-Holocene (Tables 4 and 6). However, the recently-prograded beach deposits of the North Beaches subcell are derived almost entirely from fine sand supplied via Columbia River discharge, located about 100 km to the south of the COPR1 and ROOS drill sites (Fig. 4a). 5.4. Shoreface deposit ages and sedimentation rates The backedges of the Long Beach barrier and the Clatsop beach plains respectively, began to accrete vertically by 5–6 ka (MCHU −6.8 m NGVD88 at 5925–6015 calRCYBP) and 4–5 ka (DELM − 5.8 m at 4720–4746 calRCYBP) (Table 6; Fig. 4b). Beachface progradation in these two subcells was underway by 4–5 ka at (DELM, 67TH, and MCHU). Vertical accretion of the lower sections of the shoreface deposits from the Long Beach subcell occurred rapidly, reaching −7.1 m elevation by 4240–4620 calRCYBP at OYST. By comparison, nearly the same elevation of deposition in the Clatsop subcell, i.e. at −6.5 m SUNS, was not reached until 2485–2739 calRCYBP. Vertical accretion of the innermost-shelf deposits largely preceded the beachface progradation in the Long Beach subcell. In the Clatsop subcell the inner-most shelf accretion, under the modern beaches, appears to have just kept pace with beachface progradation (Fig. 8). Shoreface sedimentation rates are calculated for dated depth intervals from eight beach drill sites in the study area (Tables 6 and 7). The North Beaches subcell shows a very-late onset of net deposition in the innermost shelf, i.e. after 1 ka at the mid-barrier OYHUT site. The rate of shallow shoreface sedimentation at OYHUT increased from 0.8 cm year− 1 (depth interval, −4.5 to −7.1 m) to 2.8 cm year− 1 (depth interval, −2.0 to −4.5 m) just before historic beach accretion reached that site (Table 6) (Woxell, 1998). Similar increasing rates of shallow shoreface sedimentation are observed in the Grayland subcell (GRBE) and the Clatsop subcell (SUNS) just prior to historic beachface

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C.D. Peterson et al. / Marine Geology 273 (2010) 62–82

Fig. 9. Simplified map of (A) clinopyroxene factor loadings, and (B) orthopyroxene and epidote factor loadings in surface sand samples analyzed from the northern Washington shelf. The Q-mode factor loadings for clinopyroxene factor (0.8–0.4) as generalized here, are based primarily on augite abundance (0.8 = high augite abundance). The loadings for orthopyroxene and epidote factor, ranging from 0.8 to 0.2, as generalized in this figure, are based largely on hypersthene content (0.8 = high hypersthene abundance) (Venkatarathnam and McManus, 1973). The resulting contours of ‘augite’ and ‘hypersthene’ factor loadings in the heavy-mineral fractions (HMs) of the inner-shelf sand are discussed in detail in the text. Augite-rich HM fractions, derived from local Coast Range Rivers and/or ‘glacial period’ Columbia River sand is found in the middle to outer shelf regions, located offshore of the Grayland and North Beaches subcells. Hypersthene-rich HM fractions, derived from ‘post-glacial’ Columbia River discharge is most concentrated (factor loading 0.8) in the inner-shelf, located directly offshore of Grayland and the North Beaches. Lesser hypersthene enrichment (factor loading 0.4) occurs north of Point Grenville in both a mid-shelf band, and in the innermost-shelf deposits of the northern Washington shelf between Point Grenville and the Hoh River. Smaller hypersthene factor loadings (0.4–0.2) are reported to continue north of the Hoh River on the northern Washington shelf (not shown here) (Venkatarathnam and McManus, 1973). Venkatarathnam and McManus, 1973.

progradation at those beach drill sites. The brief increase in sedimentation rates of shallow shoreface deposits at these sites is likely related to rapid historic beachface progradation in those parts of the CRLC during the early–middle 1900s (Phipps, 1978; Peterson et al., 1999). Shoreface deposits of the Long Beach and Clatsop subcells show consistent trends of decreasing shoreface sedimentation rate with decreasing depth. Shoreface sedimentation rates decreased from 0.24 cm year− 1 (depth interval, − 7.1 to − 16.9 m) to 0.15 cm year− 1 (depth interval, − 0.6 to −7.1 m) at OYST in the Long Beach subcell (Table 7). The early decline in shoreface sedimentation rate upsection at OYST reflects the early filling of shoreface accommodation space north of the Columbia River (Fig. 10). Northward sand transport in the shoreface was required to supply the high sedimentation rates north of the Columbia River during the middle-Holocene period of rapid sea-level rise (Fig. 8). The sedimentation rates are relatively low for the deeper shoreface deposits at SUNS and DELR in the Clatsop subcell, being generally less than 0.5 cm year− 1 at depth intervals from −30 to −6.5 m (Table 7). These low rates could reflect decreased sediment supply rates, and/or lower sediment retention rates in shoreface deposits located south of the Columbia River. Episodic stripping of innermost-shelf deposits of the Clatsop subcell by net-northward transport around the Columbia River ebb-tidal delta could account for lagging sedimentation rates south of the Columbia River (Fig. 10). Such a net-northward transport offshore of the Clatsop subcell would be consistent with north-directed transport of fine sand in the innermost shelf of the Long Beach subcell, as proposed above.

Table 7 Summary of lower-shoreface sedimentation rates under the CRLC barriers and beach plains. Interval elevation (m) MOCLVC 0.6 to − 3.0 COPR1 −1.2 to − 1.4 OYHUT1 − 2.0 to − 4.5 − 4.5 to − 7.1 WEST1 − 1 to − 10 −10 to − 29.5 GRBE 0.4 to − 2.0 − 2.0 to −7.3 OYST − 0.6 to −7.1 − 7.1 to −16.9 SUNS − 0.6 to −3.5 − 3.5 to −6.5 − 6.5 to −12.4 − 12.4 to − 16.6 − 16.6 to (− 30)* DELR − 1 to − 6.3 − 6.3 to (− 33)*

Interval distance (m)

Interval age RCYBP (ka)

Interval time (ka)

Sedimentation rate (cm/yr)

3.6

0.00 to 0.27

0.27

1.4

0.2

0.30 to 0.87

0.57

b 0.1

2.5 2.6

0.10 to 0.19 0.19 to 0.51

0.09 0.32

2.8 0.8

9.0 19.5

0.10 to 3.53 3.53 to 6.05

3.43 2.52

0.3 0.8

2.0 5.3

0.10 to 0.69 0.69 to 4.34

0.59 3.65

0.34 0.14

6.5 9.4

0.10 to 4.43 4.43 to 8.36

4.30 3.90

0.15 0.24

2.9 3.0 5.9 4.2 13.0

0.00 0.14 2.61 4.22 4.32

0.14 2.61 4.22 4.32 8.5*

0.14 2.47 1.61 0.10 4.00

2.07 0.12 0.37 4.2 0.32*

5.3 27.0

0.10 to 2.52 2.52 to 8.5*

2.42 6.00

0.22 0.45*

to to to to to

C.D. Peterson et al. / Marine Geology 273 (2010) 62–82

79

Fig. 10. Diagram of shoreface cross sections at OYST (Long Beach) and SUNS (Clatsop) comparing sediment depth and age relations between early filling of shelf accommodation space (OYST) and late filling of shelf accommodation space (Clatsop) relative to onset of beachface progradation. Depth elevations (m) are in NGVD88. Distance is taken from the modern beach (0 km) to the barrier or beach plain backedge. Shoreface deposit ages and elevations are taken from Table 6. Across-shore correlation lines are diagrammatic.

In summary, the low rates of early shoreface sedimentation south of the Columbia River and the high rates of early shoreface sedimentation just north of the Columbia River reflect a generally north-directed transport of fine sand in what must have been the inner-shelf at the time of middle-Holocene transgression. There is no evidence that remobilization of pre-transgressive shelf deposits played a role in Holocene shoreface progradation in the CRLC system.

5.5. Shoreface volumes under the Holocene barriers and beach plains The Holocene shoreface deposits in 18 drill sites and 5 projected beach sites from the prograded barriers and beach plains, from OYHUT to Tillamook Head, average 20 m in thickness (Table 3). Multiplied by the alongshore length (120 km) and an average acrossshore width (2.5 km) the 20 m thick shoreface deposits, including wave-dominated tidal inlets, yields a conservative volume of 6 km− 3. The beaches north of OYHUT (40 km length) and the abandoned foredune ridges 5–20 m in height in the four subcells increase that volume by about 0.2 to 0.5 km− 3. The estimated total shoreface and

dune-ridge volume (6–7 km− 3) under the prograded barriers and beach plains accumulated within the last 8–6 ka (Table 6). The annual rate of total shoreface and beachface deposition, under the accreted barriers and beach plains is on the order of 1 × 106 m3 year− 1 since 7 ka (Herb, 2000). Contemporaneous filling of Willapa Bay and Grays Harbor (Peterson and Phipps, 1992) and the shelf offshore of the barriers (Twichell et al., 2010-this issue), and the shelf north of the CRLC (Fig. 9), also required sand supply from northward transport. That additional sand supply must have substantially exceeded the 1 × 106 m3 year− 1 needed to construct the shoreface under the relatively narrow barriers and beach plains. The longshore transport extended across, what was at the time, the innermost shelf. The fine sand supply largely filled the available shoreface accommodation space and then forced beachface progradation in the northern subcells. The beach and foredune deposits, in the three northern subcells total a volume bounded by 2.5 km in width, 85 km in length, and 5–7 m in thickness. Over the last 5 ka, the barrier progradation in the three subcells was fed by combined longshore and onshore transport averaging 2.5 × 105 m3 year− 1 using the volume and time estimates

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C.D. Peterson et al. / Marine Geology 273 (2010) 62–82

Fig. 11. Diagram of proposed bedload dispersal paths in the CRLC system. Columbia River sand is throughput out to the Columbia River ebb-tidal delta where combined wave and shelf currents transport fine sand north in a broad transport zone. Early filling of the shelf accommodation space proceeded beachface progradation in the Northern subcells (see Fig. 8). Fine-medium sand and small pebbles are transported south from the Columbia River along the southeast trending Clatsop subcell beaches. Fine sand is lost to the offshore, where net-northward transport on the inner-shelf bypasses the sand around the Columbia River ebb-tidal delta. Low rates of sand delivery and/or retention offshore the southern Clatsop subcell prohibited early filling of shelf accommodation space south of the Columbia River. Fine sand transport in the nearshore and innermost-shelf north of the Columbia River supplied sand to the northern subcells, eventually filling the inner-shelf and tidal basins, and then forcing the progradation of the beach face. The amount of Columbia River sand lost from the CRLC to the northern Washington shelf during the Holocene has not been evaluated.

above. The beachface progradational feeding represents about one quarter of the total supply needed to fill the combined shoreface volumes under the prograded barriers and beach plains of the northern subcells. That is to say, roughly three times as much sand volume was needed to fill the innermost shelf as that needed to force the progradation of the beach face within the area covered by the prograded barriers and beach plains.

5.6. Sand dispersal and accumulation in the shoreface High wave energy and reversing directions of wave approach have distributed sand north and south of the Columbia River mouth. However, substantial asymmetries are apparent in total sand volume and in the early rates of shoreface accretion from opposite sides of the Columbia River. The sequential filling of shoreface accommodation space beneath the prograded barriers and beach plains of the CRLC proceeded with increasing time from the southern Long Beach and Clatsop subcells northward to the Grayland subcell, then further north to the North Beaches subcell. The filling of innermost-shelf accommodation space was largely completed in the Long Beach subcell prior to beachface progradation. By comparison, beachface progradation was contemporaneous with filling of the innermost shelf in the Clatsop subcell (Fig. 10). These asymmetries are important because they reflect mechanisms of net-transport that were superimposed on the high-energy diffusion of fine sand from the Columbia River mouth.

We use the vertical sequences of (1) sand provenance, (2) sediment grain size, and (3) sedimentation rate in key boreholes, as discussed above to establish the mechanisms of littoral sand supply and shoreface accretion in the CRLC. Bedload discharge from the Columbia River split into two dispersal paths. One path led to the outer ebb-tidal delta of the Columbia River (Fig. 11). Combined wave resuspension, nearshore currents, and innermost-shelf currents then transported fine sand in a broad zone northward from the Columbia River mouth. A second dispersal path led south from the Columbia River mouth, along the Clatsop shoreline (Fig. 11). A south-southeast trending shoreline in the Clatsop subcell enhanced southward wave attack, resulting in net-southward transport at the shoreline. The southsoutheast shoreline trend in the Clatsop subcell was maintained by a prograding bar and spit complex at the mouth of the Columbia River, and possibly by the loss of some fine sand to the inner-shelf throughout the remainder of the Clatsop subcell. The steep shoreface gradient in the Clatsop subcell (Fig. 10) likely facilitated some across-shore transport to the innermost shelf. Rapid accretion of the shoreface in the Clatsop subcell was prevented by a net-northward transport and bypassing of fine sand around the Columbia ebb-tidal delta. Localized HM enrichment in inner-shelf surface deposits offshore of the Clatsop subcell (Fig. 4) indicates that winnowing and loss of sand from the Clatsop inner-shelf might be ongoing. Without significant sand supply from offshore or around Tillamook Head (Fig. 4) the southern end of the Clatsop subcell is susceptible to future beach erosion. Such erosion could follow

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expected decreases in sand discharge from the impounded Columbia River tributaries, and harbor dredge spoil disposal in offshore deepwater sites (see Columbia River sand supply in Background section). Net-northward longshore transport of fine sand in the shoreface north of the Columbia River rapidly filled available accommodation space there (Fig. 10). The northern transport then supplied fine sand to prograding barriers and beach plains of the northern subcells by longshore transport and across-shore feeding from the innermost shelf (Fig. 11). Basal shoreface sections of the northernmost subcells, North Beaches and northern Grayland, contain gravel from eroding sea cliffs. However, the recently-prograded beachface deposits in those subcells were supplied by fine sand transported north from the Columbia River. The net-northward migration of Columbia River sand (1) bypassed at least two major tidal inlets, including Willapa Bay and Grays Harbor, (2) filled available accommodation space in the innershelf and tidal basins, and (3) then forced the progradation of barrier and beach plains of the northern subcells. Some Columbia River sand has likely been lost from the North Beaches subcell, due to northward bypassing of Columbia River sand to the inner-shelf of northern Washington (Fig. 9). A potential loss of fine sand from the North Beaches subcell to the northern Washington shelf could eventually lead to localized, shoreline retreat in the North Beaches subcell, in the advent of decreased sand supply from the more southern subcells. 6. Conclusions (1) Heavy-mineral analyses of shoreface sand beneath the accreted barrier and beach plains of the Columbia River littoral cell (CRLC) demonstrate that the dominant sand source was from Columbia River bedload discharge since mid-Holocene time. (2) Transgressive ravinement surfaces, as interpreted from nearshore seismic profiles, and water-well borehole logs, were verified by drilling and radiocarbon dating of returned samples. The deeper ravinement surfaces (15–30 m depth) are of middleHolocene age, as based on radiocarbon dating of fragile shell and wood fragments in overlying shoreface deposits. Beachface deposits (typically 5–7 m depth) in the oldest barriers do not record net progradation until late-Holocene time. (3) Grain-size trends indicate that the prograded barriers and beach plains north of the Columbia River mouth were supplied by longshore transport of fine sand in a broad zone that extended north of the Columbia River ebb-tidal delta. A much narrower nearshore transport carried sand south of the Columbia River mouth into the Clatsop subcell. (4) Shallow gradients of shoreface progradation in the Long Beach subcell reflect early filling of available accommodation space in the innermost-shelf north of the Columbia River mouth. Low delivery rates and/or possible stripping of innermost-shelf deposits south of the Columbia River mouth has maintained a steeper gradient of recent shoreface progradation in the Clatsop subcell. (5) A net-northward transport of fine sand in the nearshore and inner-shelf of the Columbia River littoral cell will likely continue into the future. In the event of decreased sand discharge from the Columbia River, due to tributary impoundments and offshore dredge spoils disposal, a potential stripping of shoreface sands from the Clatsop and North Beaches subcells could result in localized shoreline retreat in those subcells. Acknowledgements John C. Kraft and Derald G. Smith provided inspirations to several of these authors to work out the details of the prograded barriers and beach plains of the SW Washington coast. We thank April Herb for her early efforts in this logistically challenging study. Andrew Zachary put in long hours assisting with the auger and mud-rotary drilling. This

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