G E O L O F G Y O T A N N E D M M T I R N E A R P A E L D I N N O D G U E S R T R O Tracking Prehistoric Cascadia Tsunami Deposits at Nestucca Bay, Oregon, USA I E S 1 937 1 2 1 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. o 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 m) 88, (NAVD Elevation 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 NN2?N3?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 1.61.6 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.