Geology, Published Online on 29 July 2014 As Doi:10.1130/G35902.1

Geology, published online on 29 July 2014 as doi:10.1130/G35902.1

Geology

Rethinking turbidite paleoseismology along the Cascadia subduction zone

Brian F. Atwater, Bobb Carson, Gary B. Griggs, H. Paul Johnson and Marie S. Salmi

Geology published online 29 July 2014; doi: 10.1130/G35902.1

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Brian F. Atwater1, Bobb Carson2, Gary B. Griggs3, H. Paul Johnson4, and Marie S. Salmi4 1U.S. Geological Survey at University of Washington, Seattle, Washington 98195-1310, USA 2Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015-3001, USA 3Earth & Planetary Sciences Department, University of California–Santa Cruz, Santa Cruz, California 95064, USA 4School of Oceanography, University of Washington, Seattle, Washington 98195-7940, USA

ABSTRACT A 130ºW 120º A stratigraphic synthesis of dozens of deep-sea cores, most of British them overlooked in recent decades, provides new insights into deep- Southern Columbia limit of 50º Fraser R. sea turbidites as guides to earthquake and tsunami hazards along the ice sheet B Cascadia subduction zone, which extends 1100 km along the Pacific Full-length coast of North America. The synthesis shows greater variability ruptures in Holocene stratigraphy and facies off the Washington coast than WWashingtonashington was recognized a quarter century ago in a confluence test for seismic Fig. triggering of sediment gravity flows. That test compared counts of 3A Holocene turbidites upstream and downstream of a deep-sea channel Columbia R. Partial junction. Similarity in the turbidite counts among seven core sites pro- BFZ Cascadia ruptures vided evidence that turbidity currents from different submarine can- Channel Oregon yons usually reached the junction around the same time, as expected Seaward of widespread seismic triggering. The fuller synthesis, however, shows edge of distinct differences between tributaries, and these differences suggest megathrust California sediment routing for which the confluence test was not designed. The 40ºN synthesis also bears on recent estimates of Cascadia earthquake mag- PACIFIC OCEAN 19 4 1010 nitudes and recurrence intervals. The magnitude estimates hinge on Tally of ruptures stratigraphic correlations that discount variability in turbidite facies. N at 125ºW inferred for the The recurrence estimates require turbidites to represent megathrust 500 km past 10,000 yr earthquakes more dependably than they do along a flow path where Figure 1. A: Index map. Submarine canyons and channels are in turbidite frequency appears limited less by seismic shaking than black. Dot along Cascadia Channel locates core sites 6609-24 and by sediment supply. These concerns underscore the complexity of M9907-22, M9907-23, and M9907-25. BFZ—Blanco Fracture Zone; R.—river. B: Summary of earthquake history postulated by Goldfin- extracting earthquake history from deep-sea turbidites at Cascadia. ger et al. (2012).

INTRODUCTION Twenty-first century earthquakes are projected to take millions of 10,000 yr Cascadia history that specifies 19 full-length ruptures and lives (Holzer and Savage, 2013). Efforts to reduce the losses employ esti- a similar number of shorter southern ruptures (Goldfinger et al., 2012) mates of earthquake sizes and recurrence intervals. Paleoseismology has (Fig. 1B). This inferred history underpins tsunami scenarios (Priest et al., improved such estimates by extending earthquake histories thousands of 2010), fault-energy budgets (Goldfinger et al., 2013b), and giant earth- years into the past. quake probabilities (Kulkarni et al., 2013). Turbidite paleoseismology is based on observations that earthquakes In this paper, we reappraise deep-sea turbidites as guides to Casca- are among the triggers for sediment gravity flows that evolve downslope dia earthquake hazards. We bring together data from legacy cores off the into turbidity currents, and that shaking may cause slopes to fail in the Washington coast that previous paleoseismological studies overlooked. same few minutes at sites widely separated along fault strike, a coincidence We draw on physiography, stratigraphy, and sedimentology in disputing not expected of other potential triggering mechanisms (Talling, 2014). If the confluence test and in questioning the turbidite basis for influential turbidites indicative of earthquakes can be positively identified and cor- estimates of earthquake size and recurrence. related, their geographic extent would provide a measure of earthquake size, and their repetition would represent earthquake recurrence intervals. PHYSIOGRAPHY Settings shown to be conducive to turbidite paleoseismology include lakes Cascadia’s largest network of deep-sea channels originates in sub- in Switzerland and Chile (Strasser et al., 2013; Moernaut et al., 2014). Less marine canyons that notch Washington’s continental shelf (Figs. 1–31). favorable settings include parts of the continental slope off Sumatra, where The upper canyons debouch onto a lower continental slope ribbed with great earthquakes rarely trigger turbidity currents (Sumner et al., 2013). Pleistocene anticlines (Adam et al., 2004). The northern thalwegs from Interpretations of deep-sea turbidites are guiding hazard assessment Nitinat, Juan de Fuca, and Quillayute Canyons drain into Juan de Fuca at the Cascadia subduction zone, which extends 1100 km along the North Channel, which heads at the apex of Nitinat Fan and descends its eastern American Pacific coast (Fig. 1). The study of turbidite paleoseismology edge (Fig. 3A). Southern thalwegs emanating from Quinault, Grays, and began there with a confluence test for seismic triggering of turbidity cur- Willapa Canyons merge on the lower slope. Their shared outlet reaches rents (Fig. 2A). Holocene sequences of deep-sea turbidites were found to an abyssal confluence with Juan de Fuca Channel at the head of Cascadia be similar above and below a confluence of deep-sea channels, and this Channel, which drains southward between Nitinat and Astoria Fans and similarity was ascribed to abyssal merger of turbidity currents that had continues westward through the Blanco Fracture Zone. The network is been triggered simultaneously in different submarine canyons (Adams, 1990). Today, deep-sea turbidites provide the main basis for a proposed 1Figure 3 is provided as an oversize insert.

GEOLOGY, September 2014; v. 42; no. 9; p. 1–4; Data Repository item 2014286 | doi:10.1130/G35902.1 | Published online XX Month 2014 ©GEOLOGY 2014 Geological | September Society 2014of America. | www.gsapubs.org Gold Open Access: This paper is published under the terms of the CC-BY license. 1 Geology, published online on 29 July 2014 as doi:10.1130/G35902.1

A B Supporting information for Figure 3 includes an extended explana- N N tion (Table DR1 in the GSA Data Repository2), a core list (Table DR2), S S volcanic-ash data (Table DR3), notes on inferred flow paths (Table DR4), and logs of cores on Nitinat Fan and the Washington slope (Figs. DR1 and DR2). An index map to cores keys them by source (Fig. DR3). Figure DR4, showing plunge pools west of Willapa Bay, includes EM 122 mul- Continental Cascadia Fault tibeam bathymetry and acoustic backscatter data derived from Holbrook slope Channel rupture et al. (2012). Figure DR5 schematically relates findings in Figure 3 to the Path N1 confluence test of Adams (1990).

N2 North-South Stratigraphic Contrasts S1 Holocene stratigraphy off Washington differs between north and S2 south, both on the continental slope and on the adjacent abyssal plain. S3 The differences involve numbers of successive turbidites and abundance of reworked Mazama ash. Erupted sometime between 7500 and 7800 yr ago, Mazama ash was delivered to the sea by the Columbia River (Peter- C Canyon on son et al., 2012) and to the deep sea by turbidity currents (Nelson et al., Barkley northern 1968). Offshore, the ash serves as a tracer of Columbia River sediment Thalweg Juan de Fuca Washington Quillayute slope (Fig. 3D), and its lowest stratigraphic occurrence provides a widely used Proposed spillover Quinault stratigraphic datum (Figs. 3A, 3B). southern Grays Off northern Washington, post-Mazama turbidites are few and volca- Lower reach of Washington slope nic ash contents are low (Figs. 3A, 3D). Core logs and supporting analyses Juan de Fuca Willapa Channel (Carson, 1971; Barnard, 1973; Goldfinger et al., 2012) are available for 11 Columbia River sites within or near Juan de Fuca Canyon and upper Nitinat Fan: 63-18, Cascadia Channel 63-19, 63-20, 39-8, 39-9, 29-23, 39-26, 39-27, M9907-5, 29-25, and 39-7. Edge of Washington N Confluence subduction coast 100 km The cores at all these sites show six or fewer post-Mazama turbidites. At of Adams (1990) zone M9907-5, where the likely number is between 3 and 6, Goldfinger et al. Figure 2. Contrasting views of turbidity current paths from tributaries (2012, p. 42–43) counted 13 by positing, implausibly, that turbidites high to Cascadia Channel that originate on northern (N) and southern (S) in the piston core overlie the uppermost turbidites of the associated grav- Washington continental slope. Black dot denotes core sites M9907- ity core (details in Table DR4, path N1). No reported ash content exceeds 11, M9907-12, 29-28, and 6705-2. A: Flows initiated at the same time 10% at any of the 11 sites. Only from site 63-17, between Quillayute and either merge or closely follow one another below a single confluence Quinault Canyons, did a sample from the northern Washington slope yield at the head of Cascadia Channel. Turbidites cored along lower Juan de Fuca Channel are monitors of shaking off northern Washington an ash content of 10%–35%. (Adams, 1990, p. 574; Goldfinger et al., 2012, p. 87–88). B: Flows ini- Off southern Washington, post-Mazama turbidites are comparatively tiated off southern Washington predominate, and reach lower Juan numerous, and reworked Mazama ash is widely abundant. These char- de Fuca Channel by way of spillovers (dashed paths S1, S2, and acteristics prevail on the slope west and south of Quinault Canyon, and S3). Shaking off northern Washington produces few if any turbidity currents in lower Juan de Fuca Channel or in Cascadia Channel. C: along channels on the adjoining abyssal plain: the lower reaches of Juan Skeletal index map. de Fuca Channel, the nearby outlet from southern canyons, and Cascadia Channel (Fig. 3). Some of the sequences contain a dozen or more post- Mazama turbidites (yellow numerals, Fig. 3A), and in many sequences the a palimpsest, shaped during Pleistocene glaciations and selectively over- maximum volcanic ash contents exceed 35% (Figs. 3A and 3B; individual written in the Holocene (Barnard, 1978, p. 110). analyses from five slope cores, Fig. 3D). Superimposed on these shared Turbidity currents were not always confined to the network’s thal- properties is pronounced variability in turbidite thickness, grain size, and wegs. Spillover paths to lower Juan de Fuca Channel are marked by abys- bedding, and in the thickness of intervening hemipelagic clay, near the sal plunge pools and sediment waves west of the Columbia River (Fig. head of Cascadia Channel (Fig. 3C). 3A), a source of Pleistocene hyperpycnal flows (Normark and Reid, 2003). An upper reach of Cascadia Channel transmitted Holocene flows SEDIMENTOLOGY that overtopped its banks (Griggs and Kulm, 1970). Sediment Supply to Submarine Canyons STRATIGRAPHY These north-south contrasts in deep-sea stratigraphy parallel trends in Holocene sediment supply on the Washington continental shelf. In the Data Sources and Syntheses north, the Holocene transgression isolated Juan de Fuca Canyon from Cascadia turbidite paleoseismology has roots in sedimentological sediment of the Fraser River (Williams and Roberts, 1989). Reworking studies that were unconcerned with earthquakes. Three of these studies of glacial drift on the ocean floor accounts for much of the Holocene defined deep-sea stratigraphy off the Washington coast, in the south near deposition in the Strait of Juan de Fuca and on the adjoining continental the head of Cascadia Channel (Griggs, 1969), in the north on upper Nitinat shelf (Herzer and Bornhold, 1982; Hewitt and Mosher, 2001). From the Fan (Carson, 1971), and along the continental slope (Barnard, 1973). We south, by contrast, Columbia River detritus has produced a mid-shelf silt draw on this coverage more fully than did Adams (1990) and Goldfinger of Holocene age that exceeds 20 m in thickness west of Willapa Bay and et al. (2012), who used data from one-quarter of the legacy cores in Figure thins to the northwest (Fig. 3A). Upper reaches of Willapa and Quinault 3 and overlooked the legacy cores off northern Washington. Canyons indent this mid-shelf silt (Nittrouer et al., 1979; Wolf et al.,

2GSA Data Repository item 2014286, Tables DR1–DR4 and Figures DR1–DR5, is available online at www.geosociety.org/pubs/ft2014.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

2 www.gsapubs.org | September 2014 | GEOLOGY Geology, published online on 29 July 2014 as doi:10.1130/G35902.1

1999), and Quinault Canyon is known to receive modern shelf sediment 1971). Facies variability also impedes fine correlations among Holocene during storms (Carson et al., 1986). turbidites near the head of Cascadia Channel (Fig. 3C) and between lower Juan de Fuca Channel and lower Cascadia Channel (Atwater and Griggs, Flow Paths to the Lower Juan de Fuca Channel 2012, their figure 5). These congruent patterns in turbidite stratigraphy and sediment supply cast doubt on a longstanding assumption about the provenance PALEOSEISMOLOGY of Holocene turbidites in lower Juan de Fuca Channel, on the abyssal plain off southern Washington (location in Fig. 2C). Adams (1990) and Confluence Test Goldfinger et al. (2012) assumed that these turbidites represent flows that The confluence test of Adams (1990) asks whether turbidity currents originated in canyons off northern Washington (paths N1 and N2; Fig. 2A; off Washington were triggered regionally by earthquakes or locally by other Table DR4). This routing requires flows that deposited few turbidites and means (Fig. 2A; Fig. DR5A). The regional earthquake hypothesis fails if little Mazama ash on upper Nitinat Fan to have produced numerous turbi- the numbers of post-Mazama turbidites differ greatly among thalweg sites dites and abundant Mazama ash farther downstream (Fig. 3A). above and below the head of Cascadia Channel. In the test design, flows It is unlikely that numerous flows from northern canyons would have from northern canyons aggrade lower Juan de Fuca Channel sites 6705-2 bypassed all core sites off northern Washington. Juan de Fuca Canyon is and 6705-6; flows from southern canyons register on the slope at 53-18 lined with dozens of slumps (Carson and McManus, 1969) that subsequent and downstream at 6705-5; and flows from both north and south reach the Holocene flows have not removed. In contrast, on the southern Washing- confluence site 6508-K1 and are capable of reaching Cascadia Channel ton slope, erosive flows are evidenced by a scarcity of slumps and by the sites 6509-15 and 6609-24 (Figs. 1A, 3A, and 3B). Citing similarities in presence of unconformities in canyon thalwegs (at 53-19, M9907-13, and turbidite counts among these 7 sites, Adams (1990) concluded that each 63-08), and these features coexist with Holocene turbidites about as numer- of 13 post-Mazama earthquakes shook the entire region, north and south. ous and ash-rich as those in lower Juan de Fuca Channel (Figs. 3A and 3B). The previously overlooked cores off northern Washington confound We therefore propose that Holocene flows from canyons off the this venerable result (Fig. 2B; Fig. DR5B). Regional seismic triggering southern Washington slope spilled into lower Juan de Fuca Channel (Fig. fails a rerun confluence test in which post-Mazama turbidites are few off 2B, paths S1–S3). The most important of the proposed paths would enable northern Washington (range of 0–6) but numerous off southern Washing- flows from upper Quinault Canyon to reach the abyssal plain as far north ton (range of 8–14; Fig. 3). The test design also fails if Holocene flows as site 48-8, the northernmost known occurrence of numerous ash-rich from northern canyons rarely reached lower Juan de Fuca Channel. turbidites in Juan de Fuca Channel. This path, S1, is evidenced by stratigraphy and appears consistent with heavy-mineral assemblages. Earthquake Size Holocene flows using path S1 are evidenced by 8 post-Mazama tur- Geologic dating usually lacks the time resolution to show whether a bidites and abundant ash at 53-14, on the lower slope midway between long fault broke as a whole during the seconds or minutes of a single earth- upper Quinault Canyon and Juan de Fuca Channel. To account for these quake, or whether it ruptured piecemeal in a series of lesser earthquakes turbidites, westward discharge from the upper canyon would continue distributed across days, years, or even decades (Nelson et al., 1995). The directly across an outside bend at the canyon outlet and would rise ~200 10,000 yr earthquake history proposed by Goldfinger et al. (2012) seem- m from there to pass through a saddle in an anticlinal ridge (Fig. 3A). The ingly overcomes this ambiguity, because it includes 19 full-length rup- upper portions of turbidity currents are known to diverge from canyons or tures but no instance of serial rupture (Fig. 1B). This inference of long channels at outside bends (Piper and Normark, 1983). Off California, split ruptures hinges, however, on centimeter-scale turbidite-bed correlations flows have exited Monterey Canyon 150 m above its floor more than 12 in which each correlated bed represents a pulse of seismic shaking that times in the past 3000 yr (Fildani et al., 2006), and flows have overtopped was felt during the same few minutes along hundreds of kilometers of the a wall 280 m high at an outside bend of Eel Canyon (Lamb et al., 2008). subduction zone (Goldfinger et al., 2012, p. 135–136). Correlation this Holocene turbidite sand of lower Juan de Fuca Channel contains exact does not accord with the variable turbidite facies noted above, and it heavy-mineral assemblages consistent with sources on upper Nitinat Fan would not be expected of earthquake motions that vary along fault strike, or Vancouver Island (Goldfinger et al., 2013a). However, such assem- of slope failures that lag the shaking, or of tributaries of unequal length blages are also known from volcanic rocks of the Olympic Mountains that issue flows at different times (Atwater and Griggs, 2012, p. 13–17). (Carson, 1971, p. 81) and from surficial sand on the inner shelf north As interpreted by Goldfinger et al. (2012), Holocene turbidites of the of Grays Harbor (Venkatarathnam and McManus, 1973). Furthermore, lower Juan de Fuca Channel help define rupture length by representing assemblages derived from Vancouver Island may be available for rework- megathrust rupture off northern Washington. If, however, these turbidites ing, both along path S1 in Pleistocene turbidites derived from the Cordil- reflect spillover from canyons off southern Washington, they do not reli- leran ice sheet and along lower Juan de Fuca Channel in slide debris (Fig. ably show whether the fault broke north of those canyons. 3A; Fig. DR4). Earthquake Recurrence Turbidite Facies and Fine-Scale Correlation Submarine mass movements need not correspond dependably with Facies variations impede stratigraphic correlation among cores near earthquakes (Völker et al., 2011; Goldfinger et al., 2012, p. 138; Sumner the head of Cascadia Channel (Fig. 3C). The uniformly thin, sand-poor et al., 2013). Off northern Washington, Holocene turbidites from Juan de turbidites in Juan de Fuca Channel at site 6705-2 contrast with the thick, Fuca Canyon compare to less than one-third of the 19 Holocene ruptures sandy turbidites along the southern thalweg at 6705-4 and 6705-5. The posited to have extended beneath it (Figs. 1B and 3A). Conversely, off core sites nearest the head of Cascadia Channel, 6705-6 and 6508-K1, southern Oregon, turbidites outnumber megathrust earthquakes at a local- show elements of both. ity where the region’s most recent megathrust earthquake (A.D. 1700) pre- It has been proposed that sandy units several centimeters thick cor- dates three turbidites of unknown cause (Goldfinger et al., 2013c, p. 2141). relate among cores taken hundreds of kilometers apart in mostly uncon- In the earthquake history of Goldfinger et al. (2012), the Cascadia nected channels and basins along the Cascadia subduction zone (Gold- plate boundary breaks twice as often off northern California and southern finger et al., 2012). In Pleistocene deposits, turbidite correlations this fine Oregon as off Washington and British Columbia (Fig. 1B). This estimate have been made along a North Atlantic levee (Hesse, 1995), but are statis- requires one-to-one correspondence of turbidites and earthquakes off tically precluded by variable facies on Nitinat Fan (Carson and McManus, Oregon and Washington (Goldfinger et al., 2012, p. 92, 112), in contrast

GEOLOGY | September 2014 | www.gsapubs.org 3 Geology, published online on 29 July 2014 as doi:10.1130/G35902.1 with the likely turbidite deficit off northern Washington and the observed Hesse, R., 1995, Long-distance correlation of spillover turbidites on the western surplus off southern Oregon. levee of the Northwest Atlantic Mid-Ocean Channel (NAMOC), Labrador Sea, in Pickering, K.T., et al., eds., Atlas of deep water environments: Archi- tectural style in turbidite systems: London, Chapman and Hall, p. 276–281. CONCLUSIONS Hewitt, A.T., and Mosher, D.C., 2001, Late Quaternary stratigraphy and seafloor Holocene turbidity currents have partly overwritten a Pleistocene chan- geology of eastern Juan de Fuca Strait, British Columbia and Washington: nel network off Washington. The resulting turbidite stratigraphy reflects lati- Marine Geology, v. 177, p. 295–316, doi:10.1016/S0025-3227(01)00160-8. Holbrook, W.S., Kent, G., Keranen, K., Johnson, P., Trehu, A., Tobin, H., Caplan- tudinal differences in Holocene sediment supply to canyon heads and proba- Auerbach, J., and Beeson, J., 2012, Cascadia fore arc seismic survey: Open- ble Holocene spillover from canyons on the southern Washington slope. The access data available: Eos (Transactions, American Geophysical Union), stratigraphy has provided important estimates of earthquake magnitudes and v. 93, p. 521–522, doi:10.1029/2012EO500002. Holzer, T.L., and Savage, J.C., 2013, Global earthquake fatalities and population: recurrence intervals, but these estimates hinge on simplifying assumptions Earthquake Spectra, v. 29, p. 155–175, doi:10.1193/1.4000106. about flow paths, stratigraphic correlation, and record completeness. The Kulkarni, R., Wong, I., Zachariasen, J., Goldfinger, C., and Lawrence, M., 2013, way forward requires renewed attention to sediment supply, flow initiation, Statistical analyses of great earthquake recurrence along the Cascadia sub- downstream pathways, and uncertainty in turbidite correlations. duction zone: Seismological Society of America Bulletin, v. 103, p. 3205– 3221, doi:10.1785/0120120105. Lamb, M.P., Parsons, J.D., Mullenbach, B.L., Finlayson, D.P., Orange, D.L., and Nit- ACKNOWLEDGMENTS trouer, C.A., 2008, Evidence for superelevation, channel incision, and forma- We are particularly indebted to John Adams, Danny Brothers, Jake Covault, tion of cyclic steps by turbidity currents in Eel Canyon, California: Geological Audrey Dallimore, Chris Goldfinger, Joan Gomberg, Ross Heath, Sam Johnson, Society of America Bulletin, v. 120, p. 463–475, doi:10.1130/B26184.1. 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4 www.gsapubs.org | September 2014 | GEOLOGY Geology, published online on 29 July 2014 as doi:10.1130/G35902.1 127ºW 126º 125º 124º A B 0 C Maximum extent Juan de Fuca Lower Juan Trunk path Loudoun Distance Water 1 N2 Canyon de Fuca from southern of Juan de Fuca Juan de Fuca N1 Canyon BBritishritish CColumbiaolumbia along longitudinal profiles depth Channel canyons lobe VE x25 (km) 2 Channel Vancouver Island 5 63-18

3 63-19 39-8 STRAIT OF JUAN DE FUCA 63-20 0 100 km 39-6 39-27 Barkley Core 48-8 S3

100 km ≈ 3 hr at 10 m/s 6705-2 6705-6 Slide of López Canyon site M9907- 6705 6705 6508 6705 6705 CONTINENTAL SHELF 6508-K1 et al. (2010) M9907-12 -2 -6 -K1 -5 -4 Height and distance 100 m along transverse profiles. View upstream. VE x50 10 km 1613 13- 3- 0? 3 1 4 CONTINENTAL SLOPE 14 6? Nitinat 3 4 Canyon ABYSSAL PLAIN 7 Hypothetical paths of Holocene 7 Levee ~100 m high turbidity currents that aggraded lower 29-23 N1 Juan Juan de Fuca Channel 39-9 de Fuca 1 m 48ºN 0 N1 From Nitinat and Juan de Fuca 1 Piston core

0 Canyon core Trigger 3 63-19 Canyons Juan de Fuca Channel 3? 63-20 N2 From Quillayute Canyon 39-8 Olympic S1 from Quinault Canyon 0? Across slope 23 45 10 0.5 1-10 1-10 1-10 0 39-26 4 Quillayute Mountains to vicinity of 39-6 and 48-8 39-27 3h 10pm 8q 6q 13pm 7pm 7pm Canyon west of NITINAT FAN 63-18 S2 Through plunge pools Willapa Canyon (detail, Fig. DR4) 3-6? S1 S2 S3 S3 Across sediment waves between Adams (1990) Confluence of 0 M9907-5 Quinault 6705-4 and 6705-6 29-25 Canyon Quinault N2 Canyon 0 x 0 M9907-10 Pleistocene South edge of Willapa Canyon 39-7 Saddle ~200 m outflow from WashingtonWashington 1 map area in A D 2 68-27 above canyon mountain 5 2 >35 Northern floor to SE glaciers Cascadia Channel sites 63-18, 63-17 10-35 14 3 Silt 53-18 Clay Sand 1-10 63-19, 63-20 53-19 S1 63-08 -25 total of 28 8 4 6705-1 68-26 53-18 6705-4

1 6705-5 6705-8 samples M9907-13 M9907 6508-K1

1 53-14 6705-13 M9907-14 6509-15 0 R 6609-24 39-6 Grays Pleistocene Turbidite >35 Southern 48-8 and -9 Canyon outflow from 47º10-35 sites 53-14, ≥6 maximum 53-17, 53-18, Puget lobe Turbidite count 1-10 13 13 10 8 14 13 12? 1463 Hemipelagic

Volcanic ash (%) Volcanic 53-22, 63-10 by age unit 1 2 clay total of 97 M9907-11 Grays 6 0 samples and -12 53-17 Deposit age 53-19 Mazama ash Harbor 13-14 interpreted by Holocene 0 1 29-28 R Royse (1967) as Post-Mazama 1-10 >35 2-5? 10-35 1 5 10 6705-26705-2 Pre-Mazama R airfall Willapa Bay Sand (%) Samples Pleistocene 1 m 2 Quaternary 8 Willapa Canyon 39-24 Detail, Fig. DR4 Ash abundance

S2 Thickness in core R 63-10 2 Maximum % of 326-36 35 3 5 >35 sand fraction Lowest turbidite Holocene 6 6 nd in core 6508-K1 6705-6 R 4pm 2h 13h 13pm Samples (pm, 10 silt post-Mazama; that contains 6705-7 on bank SS33 53-22 6705-10 in side channel h, Holocene; abundant Confluence in volcanic ash confluence test M9907-13 q, Quaternary) of Adams (1990) 6705-1 33 20 0 58 6705-4 63-08 COLUMBIA 14 12? 15pm 5pm 6h* 10pm * Samples from lowest Holocene turbidite only 6705-10 R 6705-7 6508-K1 M9907-14 Pleistocene RIVER 8 R floods from 6705-5 Montana Flow path Slide debris along Juan de Fuca Channel Figure 3. Inferred paths of 8 and Utah Thalweg of canyon or channel x Core site—Holocene thickness >1 m ( ) or sediment gravity flows 46º 6705-9 on levee ~80 m high Astoria Canyon Proposed spillover path <1 m ( ). x, log unavailable originating between Nitinat and Oregon Distributary on Nitinat Fan—Chiefly relict Volcanic ash (maximum percent, shown only Willapa Canyons. Details in Plunge pool where measured systematically) Table DR1. A: Plan view. B: Cascadia Channel Core data plotted along Sediment wave >35 35 (on continental slope, in range 10-35) bathymetric profiles. C: 6705-14 ASTORIA FAN Holocene silt on continental shelf—E-W Stratigraphy along and near N extent and maximum thickness (range 6- 10 (on continental slope, in range 1-10) 10 Turbidites younger than Mazama ash lower Juan de Fuca Channel. D: 20 m). Black tail, E or W limit not reached Volcanic-ash abundance in 13 post-Mazama turbidites 120-200 km farther 14 Number counted Ice-sheet limit—Dashed where inferred R Rhythmic and likely >6; ash not identified post-Mazama deposits on the down Cascadia Channel in cores 6509-15; 0 100 km from drift, dotted where interpolated 13 M9907-22, -23, and -25; and 6609-24 continental slope. Rethinking turbidite paleoseismology along the Cascadia subduction zone—Figure 3 Atwater et al. Supplement to Geology, v. 42, no. 9 (September 2014)