Tracking Prehistoric Cascadia Tsunami Deposits at Nestucca Bay, Oregon, USA I

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O Tracking Prehistoric Cascadia Tsunami Deposits at Nestucca Bay, Oregon, USA I

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o PHYSICAL PROPERTIES OF SAND 120 Robert C. Witter , Eileen Hemphill-Haley , and Roger Hart 130 o 125 o Queen Charlotte Three tsunamis triggered by fault INTRODUCTION. 1 2 BRITISH AB A N1 Sandy Deposit C N5 Sandy Deposit COLUMBIA Oregon Department of Geology and Mineral Industries Consulting Micropaleontology North South North South great earthquakes on the Cascadia subduction zone have 10 10 10 100 16 ND-B2, 107-110 cm Upton-06, 162-165 cm Upton -19, 116-118 cm Vancouver 50o o N1 sand N5 sandy mud, upper layer N5 sandy mud 50 S t N4 sandy layer N4 sandy layer r a Coastal Field Office, P.O. Box 1033, Newport, OR 97365 1871 Pickett Road 12 it % Volume % Volume Explorer o inundated Nestucca Bay, Oregon over the past 2000 years. m) River channel 0 0 0 f N5 sandy layer G Vancouver μ e CANADA ( Plate Island o 50 8 0 100 200 300 400 500 600 700 800 900 1000 0 100 200 300 400 500 600 700 800 900 0 100 200 300 400 500 600 700 800 900 1000 Continental rg i U.S.A. C a

o size n [email protected] McKinleyville, CA 95519 10 10 1000 10 ti WASHINGTON 4

n Mean particle S e The primary evidence includes layers of sandy sediment that Thickness (cm) LN-02, 43-45 cm Upton-06, 168-172 cm Upton-24, 96-98 cm e n N5 sandy layer a t w a shelf l N5 sandy mud a 0 0 N1 sandy mud N5 sandy mud, lower layer r s d lo p Seattle 0 500 1000 1500 0 500 1000 1500 e % Volume Puget North % Volume e 0 0 0 d bury tidal marshes submerged by earthquake-related Distance south of Little Nestucca River (m) Distance south of Little Nestucca River (m) Sound g America 0 100 200 300 400 500 600 700 800 900 1000 0 100 200 300 400 500 600 700 800 900 1000 0 100 200 300 400 500 600 700 800 900 1000 e Grays Harbor Plate Particle size (μm) Particle size (μm) o Willapa f 10 Bay subsidence. Additional tsunami evidence includes: the STRATIGRAPHIC PROFILES (A) Mean particle size versus distance from the Little Nestucca River channel for the Upton-06, 71-74 cm C a s River N1 sandy mud D Control Samples c mbia N4 and N5 sandy layers. In both cases, sandy layers become finer grained with a Colu

Pacific Ocean d

l i a e Portland % Volume 10 10 n Netarts Bay o increasing distance from the river channel. Mean particle size of sandy sediment from 0 Juan n 45 spatial extent of sandy deposits, clear trends in deposit Upton-19, 79-81 cm Upton-05, 63-66 cm o a s Nestucca Bay 0 100 200 300 400 500 600 700 800 900 1000 45 h

u C North South N4 sandy mud, middle layer mud overlying N4 sandy deposit deFuca b Salmon River the Little Nestucca River channel (open diamond) for comparison. (B) Sand layer

d Particle size (μm) a u

i Yaquina Bay

Plate c d

t Explanation

a i Alsea Bay % Volume o thickness versus distance from the Little Nestucca River channel for the N4 and N5

c 5 5 0 0 n

s thickness and mean particle size that decrease with

From Nelson et al., 2004 a B N4 Sandy Deposit 0 100 200 300 400 500 600 700 800 900 1000 0 100 200 300 400 500 600 700 800 900 1000 C Symbols Sedimentary units z sand layers as revealed in cores at Upton Slough.

n OREGON Pacific e Coos Bay Vertical exaggeration x100 10 10 20 sharp contact (<3 mm) upland soil/pasture Upton-05, 69-72 cm Upton-19, 81-84 cm LN-01, Little Nestucca River Plate increasing distance inland, the presence of brackish-marine Coquille River N4 sandy mud, upper layer N4 sandy mud, lower layer Sixes River clear contact (3-10 mm) peaty sediment 10 active channel sediment Thrust fault at Engineered plate boundary Older 4 levee sandy laminae 4 % Volume 0 0 % Volume 0 Other faults diatoms within the deposit and normally graded layers levee peaty/muddy sediment 0 100 200 300 400 500 600 700 800 900 1000 0 100 200 300 400 500 600 700 800 900 1000 0 100 200 300 400 500 600 700 800 900 1000 Spreading ridge low marsh plant, T. maritima Debris 200-m isobath 14 mud A B C 10 10 20 Deep-sea channel probable correlation of contacts flow Gorda Upton-05, 72-74 cm Upton-24, 57-59.5 cm NS-2, Nestucca Spit dune sand Volcano within each deposit. C age ranges for the youngest tsunami deposit N4 sandy mud, middle layer N4 sandy mud Earthquake evidence Plate Humboldt Bay possible correlation of contacts sandy sediment 10 Tsunami deposit % Volume o 14 % Volume 0 0 0 Cape Mendocino 40 <0.3 calibrated C age x1000 yr BP debris flow deposit Elevation (NAVD 88, m) f 40o CALIFORNIA sand span the date of the most recent Cascadia earthquake 3 Tidal 3 0 100 200 300 400 500 600 700 800 900 1000 0 100 200 300 400 500 600 700 800 900 1000 0 100 200 300 400 500 600 700 800 900 1000 p 0 100 200 300 San Outcrop slough Core number Andreas 30 10 10 20 km fault 36 Upton-05, 74-78 cm Upton-25, 73-77 cm NS-1, Nestucca Spit beach sand 120 o and tsunami in 1700. Sediment cores and a single tidal 6 26 130 o o Ditch 27 N4 sandy mud, lower layer N4 sandy mud 10 125 ~0.6 m compaction 29 31 LN2 15 13 19 21 23 25 32 34 % Volume 35 % Volume Tectonic setting of the Pacific northwestern U.S. showing the 22 28 33 NS-1 200 µm NS-2 200 µm NR-1 200 µm 0 0 0 outcrop define the spatial limit of the 1700 tsunami deposit, 2 4 11 16 18 20 24 2 0 100 200 300 400 500 600 700 800 900 1000 0 100 200 300 400 500 600 700 800 900 1000 100 200 300 400 500 600 700 800 900 1000 <0.3 1 2 5 7 8 9 12 Cascadia subduction zone and other plate boundaries, Quaternary <0.3 17 DEr F N1? r 10 10 20 faults in the North American plate, and the location of the study N1 r m Upton-19, 77-79 cm Upton-34, 81-84 cm NESK-P1, Neskowin beach sand which extended at least 4.4 km inland. The widespread ? ? N2 N4 sandy mud, upper layer N4 sandy mud ? 1.2 10 site at Nestucca Bay in northwestern Oregon. The deformation N3?NN2?3? N3 % Volume 1.3 1.41.4 1.3 % Volume 0 0 0 front (barbed line) is defined by bathymetry where the abyssal m) 88, (NAVD Elevation MLLW MLLW extent of the 1700 deposits makes storm surges, and waves 1 1.3 1.61.6 1.8 N4 1 0 100 200 300 400 500 600 700 800 900 1000 0 100 200 300 400 500 600 700 800 900 1000 0 100 200 300 400 500 600 700 800 900 1000 plain meets the continental slope and is inferred to represent the N4N4 11.6.6 p Particle size (μm) Particle size (μm) Particle size (μm) r r surface projection of the Cascadia thrust fault. Open and closed superimposed on them, an unlikely explanation. Physical N5 Particle size (μm) analyses by volume percent for selected samples of sandy sediment from surface sites and cores in the circles represent sites with evidence for prehistoric Cascadia Little N5 LN-01 200 µm LN-02 200 µm ND-B 200 µm Nestucca N6 Nestucca Bay area. (A) Size distributions for sandy deposits overlying contact N1 come from two cores and an outcrop on the earthquakes and tsunamis. Closed circles also mark sites with attributes similar to beach and dune sand indicate an 0 0 GHI deposits interpreted to record tsunami inundation caused by a M9 River N7 western margin of the Little Nestucca River (LN-02). Results indicate mean particle size of the deposits decreases inland. (B) Channel Sediment Size distributions for sandy deposits overlying contact N4 come from 5 cores along Upton Slough. (C) Size distributions for Cascadia earthquake on January 26, 1700 (Satake et al., 1996). ocean-ward source and preclude river flooding. cores Debris N8 flow r sandy deposits overlying contact N5 come from 3 cores along Upton Slough. (D) Particle size analyses for samples of sandy deposit -1 -1 f sediment from the beach (NS-1, NESK-P1), dune (NS-2) and the western channel margin of the Little Nestucca River (LN-01) 0 200 400600 800 1000 1200 1400 provide a basis for comparison with samples from outcrop and cores. Analyses of mud sampled from core 5 (Upton-05) provide Distance (m) particle size data for sediment deposited in a muddy tidal flat. Upton 6C 200 µm Upton 5D 100 µm Upton 25 100 µm

STUDY AREA Simplified stratigraphic profile correlating mud-over-peat or sand-over-peat contacts inferred to reflect sudden rises in relative sea level in cores along Upton Slough. Solid and dashed black lines Selected photographs of sand grains under 3.5x and 10x magnification using a petrographic microscope. (A and B) mark sharp (<3 mm) and clear (3-10 mm) lithologic contacts, respectively. Gradual lithologic contacts lack either line symbol. Gray dashed lines labeled N1 through N8 correlate contacts inferred Examples of well-rounded beach (NS-1) and dune (NS-2) sand. Conspicuous pyroxene (p) and rock fragments (f) are to record sudden episodes of relative-sea level rise. Bold numbers indicate preferred ages for contacts N1, N3 and N4, rounded to the nearest hundred years, based on calibrated 14C ages. Core 425000mE 430000mE 425000mE 430000mE TSUNAMI DEPOSITS visible in sample NS-2. (C) Sand sampled from the Nestucca River is subrounded. (D) Very fine, predominantly elevations determined by RTK GPS survey have <2 cm vertical error. We infer compaction of ~0.6 m lowered the north pasture, located behind large engineered levees that bar tidal flooding. Sample Quartz opaque White grains/feldspar Iron-oxide stained grains Rock fragments Dark grains angular sand from the Little Nestucca River channel near Upton Slough contains rare larger (>300 μm) rounded Sample Mica Type Cape Haystack MLLW, mean lower low water tidal datum estimated from tide gage data for the Little Nestucca River. quartz grains (r) probably redeposited by tidal currents. (E and F) Samples of sand above contact N1 from cores close Rock A B C Kiwanda to river channels contain rounded quartz grains (r), although they are more abundant in sample ND-B and rare in Neskowin beach Nestucca Spit beach Beach and dune deposits Large sample LN-02. A rounded pyroxene (p) occurs in sample ND-B and a mica (m) grain is visible in sample LN-02. (G) landslide Nestucca Spit dune ! ! Rare, subrounded (r) quartz grains were identified in sand overlying contact N5 in core 6. (H and I) Sand overlying Horn!! Creek North South ! ! 33 Nestucca River channel River channel sediment ! ! contact N4 lacks rounded grains and consists of abundant angular quartz grains and rare rock fragments (f). Note !! ! 2.0 0.0 Simplified Lithology Pacific City Pacific City 23 25 Nestucca River debris layer Peat Mud change in scale, lower right of photos. “Nestucca Duck” core B, N Nestucca River N 19 Sand layer in core ND-B Martella 5 Muddy peat Sandy mud sand N1 000m 000m ! 05 NS-1 ! Ranch 05 N2N2 50 50 peaty mud black Peaty mud Sand NS-2 ! !! “Nestucca Duck” Little Nestucca River channel River channel sediment ¤£101 ¤£101 soil Upland soil Mud, sand and NNR-1R-1 site black N3N3 Core ND-B sand sandy mud N1? black gravel soil soil Rooted mud Little Nestucca River outcrop, 1.5 N2N2 0.5 sand N1 Sand layer in outcrop dark brown * 1.2 soil Symbols LN-1 and LN-2 outcrop N4N4 North knife on Core 6, sand N1 T. maritima Wood fragments ! contact N1 Core 6, sand N5 (upper) Pacific Ocean !! !! Nestucca Spit ! !!! “Saltmarsh” site Detrital forest litter Nestucca Bay Nestucca Bay ! * !!! peat Core 6, sand N5 (lower) !!! ! peaty mud MLLW !! !! Low marsh plant, T. maritima Little Nestucca River !! ! !!!! 1.3 !! peaty mud N4N4 11.4.4 Core 5, mud ! ** Area of ! 1.0 1.8 1.0 <0.3 Calibrated 14C age x1000 yr BP CHRONOLOGY OF TSUNAMIS Little Nestucca River ** Fig. 3 N5N5 Core 5, sand N4 (upper) N N 1.6 beach from Increasing distance Core 5, sand N4 (middle) Tsunami 000m Upton Slough 000m T. maritima * * * 1.6 ** 00 00 * *

50 site 50

inundation Intertidal stratigraphic sequences in five Core 5, sand N4 (lower)

Pacific Ocean Æ Ä Æ line Ä 130 130 0 0.5 N5N5 (m) Depth Core 19, sand N4 (upper) Sand layers in cores ¤£101 N6N6 cores from the Upton Slough transect. along Upton Slough Core 19, sand N4 (middle) meters 0.5 1.5 Dashed gray lines correlate sharp Explanation Core 19, sand N4 (lower) Core ND-B Outcrop LN-2 Core 6 mud-over-peat or sand-over-peat contacts, Core 19, sand N5 Calibrated 14C age at 2 012 012 Photographs of 5-cm diameter sediment cores and an outcrop exposure showing sharp numbered N1 to N7, inferred to reflect Core 24, sand N4 Elevation (NAVD 88, m) 88, (NAVD Elevation N7N7 Kilometers Kilometers lithologic contacts inferred to record the 1700 Cascadia earthquake and tsunami at sudden episodes of relative sea level rise Core 24, sand N5 Offshore turbidites WillapaCopalis Bay, Grays River, Harbor,Columbia and River, CannonWA Beach, OR Netarts Bay, OR Nestucca(this Bay, study) OR Salmon River, OR South Slough,Coos Bay, OR 14 black AD 2000 Maximum C age on Core 25, sand N4 Nestucca Bay. Scales to right of cores in cm and inches. Stem bases from sharp contacts peaty mud caused by coseismic subsidence during great 0 S Map of Nestucca Bay estuary and surrounding Explanation 0.0 2.0 detrital sample in both cores and the outcrop limit the time of submergence to <300 years before 1950 , Cascadia earthquakes. Sandy sediment OF-I1MT N1 1 1 South Core 34, sand N4 uplands showing the locations of 57 sediment cores Y 1 Local site label for earthquake Beach and dune sand Holocene dune sand which is consistent with regional evidence for the 1700 Cascadia earthquake and overlying peaty tidal deposits are T1 examined for evidence of sand layers deposited by T2 and tsunami evidence 0 20 percent Alluvial and estuary Tertiary volcanic and tsunami. 542 m 118 m 62 m 394 m interpreted to record sediment transported tsunamis or river floods. (Left) Topographic deposits sedimentary rock OF-II 2MT N2? by tsunamis. Bold numbers indicate 2 Type of Stratigraphic Evidence Frequency of constituent grains in sandy sediment from cores, outcrop and surficial deposits around hillshade (USGS 10 m DEM) of the Nestucca Bay -0.5 Distance between 2.5 ! Surface sediment preferred ages for contacts N3 and N4 based 3MT3MT Sediment core core sites T3 N3 Offshore turbidite deposit Nestucca Bay. Samples are arranged vertically according to sample site proximity to the beach: area showing the Oregon tsunami inundation line sample AB C Contacts 14 W OF-IIIOF-III 2 2 on calibrated C ages. Contacts N3, N6 and 3 AD 1000 samples at the top of the list are beach samples; samples at the bottom of the list are far from the used to restrict new development along the coast (Olmstead, 2003; Priest, 1995). (Right) Simplified geologic Sharp (<3 mm) 1000 Undated, insufficient evidence N7, evident in core 33, were not observed in for great earthquake origin beach. Analyses of sandy layers from cores are arranged vertically from north (top) to south (bottom). map of the Nestucca Bay area showing core sites (black dots) and sites sampled for sandy surface sediment Clear (3-10 mm) gravelly sand N4 3 Gradual (>10 mm) all cores. MLLW, mean lower low water T4 4MT In general, quartz and mica sand components increased with increasing distance from the beach. We (white squares). U 4 3 Coseismic subsidence stratigraphy -1.0 3.0 tidal datum estimated from tide gage data for found no mica in samples of either beach or dune sand. Beach, dune and river channel sediment had the Little Nestucca River. S N5 4 Coseismic subsidence stratigraphy higher components of dark sand grains relative to sandy layers in cores far from the beach. T5 4 with tsunami deposit sandy mud sandy mud OF-IV Tsunami deposit

Nestucca Bay AD 1950) age (before Calibrated sandy mud North South 0 LN-1 ABCEFGHD 2000 Insufficient local evidence for (! peat peat 0 0 142-24780 20 0 20 213 6(! great earthquake origin ¤£101 Explanation LN-2 N1? (! Symbols km north of Nestucca Bay km south 5 (! N2N2 (! peat sharp contact (<3 mm) CONCLUSIONS. Stratigraphic sequences of intertidal facies 4(! 0.9 (! (! 1 N3N3 1 (! clear contact (3-10 mm) 14 0 0.5 Debris (! sandy laminae Comparison of calibrated C ages and coastal evidence for great Cascadia earthquakes and tsunamis over the last two millennia at seven sites in Little Nestucca River flow Upton Slough profile(! (Fig. M) (m) Depth beneath the margins of Nestucca Bay archive a history of relative sea level (! Kilometers deposit probable correlation of contacts southwestern Washington and northwestern Oregon (modified from Nelson et al., 2004). Black rectangles represent ages for offshore turbidite (! possible correlation of contacts 14 (!(! Core 5 Core 19 Core 24 N4N4 ! 2 2 14 19 (!( 0.9 calibrated C age x1000 yr BP deposits inferred to record strong shaking during prehistoric Cascadia earthquakes (Goldfinger et al., 2009). Arrows indicate maximum limiting C (! (! ! (! change in response to the earthquake deformation cycle as well as the (! ( (! A Depth (m) (! ages based on dates of detrital plant fossils and herbaceous stems rooted in marsh sediment. Ages for evidence at sites in southwestern Washington 25 (! Sediment N5?N5? Sedimentary units (! 23 (! DE F (! Upton Slough cores upland soil sandy sediment (!(! (! N6?N6? are from Atwater et al. (Atwater et al., 2003). Ages from coastal sites in Oregon come from Cannon Beach (Witter, 2008), Netarts Bay (Darienzo et (! (! Saltmarsh profile (Fig. N) (! 3 3 effects of tsunamis, but they may not capture the complete record of great (! (! peaty sediment laminated sandy (! Debris flow (! al., 1994; Nelson et al., 1995; Shennan et al., 1998), Salmon River (Nelson et al., 2004; Nelson et al., 1995), and South Slough near Coos Bay (Nelson et (! Laminated sandy and mud deposit G peaty/muddy (! muddy sediment (! Vertical exaggeration x100 sediment debris flow al., 1998; Nelson et al., 1996). Local evidence for earthquake-related subsidence is lacking for one or more stratigraphic contacts at Cannon Beach, 34 (! deposit Cascadia earthquakes and tsunamis at Nestucca Bay over the past 2000 33 mud (! 4 4 Netarts Bay and Coos Bay (e.g., Shennan et al., 1998). (! 0200 400 600 (! Distance (m) years. This inference is supported by equivocal evidence for one or two sandy mud sandy mud mud Simplified stratigraphic profile correlating mud-over-peat or sand-over-peat contacts inferred peaty horizons buried between the times of the 1.2 ka and 1700 tsunamis. to reflect sudden or gradual rises in relative sea level in cores along the “Saltmarsh transect” in peat peat the Nestucca Bay Refuge. Vertical axis depicts depth, in meters, from the surface. peat These buried marsh deposits,which largely lack distinctive, laterally Map covering part of the Nestucca Bay National Wildlife Refuge and the Little Nestucca continuous sandy deposits, probably reflect small changes in relative sea River showing core locations along two transects: one along Upton Slough and another

within a saltmarsh east of Highway 101 restored by the U.S. Fish & Wildlife Service. Core 25 Core 33 Core 34 level that may have been caused by a number of processes, including

Sharp lithologic contacts (contact N4) in six cores from Upton Slough inferred to mark BIOSTRATIGRAPHY earthquakes, climate variation and changes in the configuration of the coseismic subsidence and tsunami deposition caused by a Cascadia earthquake that occurred approximately 1.2 ka. estuary (Nelson et al., 1996). In addition, older sharp lithologic contacts recognized in stratigrphic sequences along Upton Slough lack the lateral Core 25 Inferred Core 33 Inferred Core 33 Inferred

AB C wetland Fresh taxa Mid high to marsh (less saline) Sandy or muddy tidal flat/channel Brackish-marine or planktonic tychoplanktonic Fresh wetland wetland Fresh taxa Mid high to marsh (less saline) Sandy or muddy tidal flat/channel Brackish-marine or planktonic tychoplanktonic Low marsh Low saline) (more Fresh wetland wetland Fresh taxa Sandy or muddy tidal flat/channel Brackish-marine or planktonic tychoplanktonic Mid high to marsh (less saline) Low marsh Low saline) (more (64-112 cm) paleoenvironment (54-101 cm) paleoenvironment (140-190 cm) marsh Low saline) (more paleoenvironment Contact number Lithology Depth (cm) Contact number Lithology Depth (cm) Contact number Lithology Depth (cm) 140 uncertain continuity and biostratigraphic evidence necessary to confidently attribute

muddy tidal flat muddy tidal flat them to Cascadia events. Therefore, the evidence we have uncovered at 70 tsunami deposit 60 N4 sandy mud high salt marsh tsunami deposit Nestucca Bay stresses the importance of understanding the variability of sandy mud sandy mud N4 150 OLDER SANDY DEPOSITS. Two older sandy high salt marsh tidal channel? ????? N6 creation and preservation thresholds that control whether a particular site peat high to low deposits record tsunamis that inundated the bay about 1.2 ka 80 salt marsh 70 fresh to fresh- peat peat brackish marsh holds complete or incomplete geologic record of great Cascadia and 1.6 ka – times that correspond to widespread evidence for 160 high to low salt marsh earthquakes and tsunamis. (References available upon request) great Cascadia earthquakes and tsunamis at adjacent estuaries 90 80 Core 6 Core 5 Core 19 muddy tidal flat and in offshore turbidite records. Both deposits meet multiple high to low salt marsh 170 DE F low salt marsh or low salt marsh to criteria used to infer a tsunami origin, although the physical tidal flat tidal flat ACKNOWLEDGMENTS. George Priest encouraged us to pursue an 100 uncertain sandy, 90 investigation of tsunami deposits near Pacific City to provide an empirical check on properties of the sand resembled sediment from the sandy flats brackish-marine tidal flat or uncertain environment subtidal channel tsunami deposit? N5 180 uncertain N5 N7 tsunami inundation modeling. T. Manning of the Tillamook County Sheriff’s Office and of the Little Nestucca River rather than beach or dune sand mud peaty mud high salt marsh? wet, fresh water high salt marsh marsh or swamp sandy mud the Tillamook County Board of Commissioners endorsed the study. We thank R. Lowe over 5.5 km away. The physical characteristics of older sandy 110 100 peat peat 0 20 percent 0 20 percent and K. White for access to the Nestucca Bay National Wildlife Refuge. We also owe 190 layers reflect sediment sources along the tsunami flow path. peat Simplified Lithology Contacts thanks to S. Martella and R. Hurliman for access to private land. C. Folger and T. DeWitt Peat Mud Sharp (<3 mm) Frequency of diatom taxa, grouped by inferred paleoenvironment, from analyses of 11 0 20 percent Peaty mud Sandy mud Gradual (>10 mm) samples from a 5-cm-diameter Russian-type sampler at core 33 along Upton Slough. trained Witter in particle size analysis and the U.S. Environmental Protection Agency at Diatom frequency is expressed as a percentage of total diatoms counted in each sample. The Frequency of diatom taxa, grouped by inferred paleoenvironment, from analyses of 9 Frequency of diatom taxa, grouped by inferred paleoenvironment, from analyses depth of contacts N4 and N5 in the Russian-type core were approximately 10 mm deeper samples from a 5-cm-diameter Russian-type sampler at core 33 along Upton Slough. Hatfield Marine Science Center provided use of its laser particle size analyzer at no cost Core 24 Core 25 Core 33 of 9 samples from a 5-cm-diameter Russian-type sampler at core 25 along Upton than the same contacts observed in gouge cores used to construct stratigraphic profiles. Diatom frequency is expressed as a percentage of total diatoms counted in each to this project. R. Van Hoy provided tide gage data for the Little Nestucca River. This Slough. Diatom frequency is expressed as a percentage of total diatoms counted in Photograph of Russian-type core at left; core lithology shown to the left. sample. The depths of contacts N6 and N7 in the Russian-type core differed by <20 Sharp lithologic contacts (contact N5) in six cores from Upton Slough inferred to each sample. The depths of contacts N4 and N5 in the Russian-type core were 10 mm compared to contacts observed in gouge cores used to construct stratigraphic study was funded by a grant from the U.S. Geological Survey’s National Earthquake mark coseismic subsidence and tsunami deposition caused by a Cascadia earthquake to 20 mm deeper than contacts observed in gouge cores used to construct profiles. Photograph of Russian-type core at left; core lithology as shown to the left. that occurred approximately 1.6 ka. stratigraphic profiles. Photograph of Russian-type core at left. Hazard Reduction Program, award number 08HQGR0076.