Supplemental Information

Figure S1 –LiDAR images of WFZ east of Wallula Gap. A. LiDAR slope-map image

of the WFZ immediately east of the Columbia River at Vansycle Canyon. Refer to Figure

2C for regional context. B. Interpreted LiDAR image. Orange lines show the inferred

location of the Wallula fault based on geologic mapping, magenta lines are scarps

observed on LiDAR imagery, blue lines are possibly deflected streams of Mann and

Meyer (1993).

Figure S2 – Composite quarry logs from 1980 consulting project. Combined quarry

and trench logs from WPPSS studies conducted in 1980 (Woodward-Clyde Consultants

Inc, 1981). We stretched both logs to the same scale and matched them based on similar

contacts and faults observed on both logs. We interpret the contact between pre-MIS 2

glacial outburst flood deposits (silts and gravels colored light blue and orange) and

Pomona basalt and related units as an oblique reverse fault, probably subordinate to the

main fault marked in the outcrop by a vertical seam of cemented breccia. The ground

surface and uppermost contacts on the log are dashed, suggesting that the site was

disturbed prior to mapping in 1980. Reconnaissance fault mapping and trenching at The

Butte during the Washington Public Power Supply System (WPPSS) project in 1981

focused on a fault exposure in the quarry (Woodward-Clyde Consultants Inc, 1981). The

work included mapping a former quarry exposure (~12–20 m west of the present

exposure as measured using historical imagery in GoogleEarth™) and logging a trench

placed in the quarry floor adjacent to the former quarry face.

We created a composite log of the previous study by scanning and assembling

their logs of the trench face and quarry wall to similar scale, then digitizing the line work.

Our composite log provides a reasonable representation of the types of deposits and

deformation encountered in both exposures–the line work is identical to the original log

but we simplify their lithologic units. We group the flood deposits into three main units:

gravels, silts, and a thick calcified zone (k horizon) capping the sequence. In the previous

study, samples of carbonate from the calcified gravel yielded a minimum 230Th/U age of

19±1 ka (sample JB-7) and another sample (sample JB-8) yielded a minimum 230Th/U

age of 75±6 ka and a maximum 230Th/U age of 147±13 ka; these ages suggest that the

calcified zone formed between ~19 ka and ~160 ka years ago (Woodward-Clyde

Consultants Inc, 1981). We were unable to find any information in the consultant report

documenting where they obtained the samples. They noted that flood deposits containing

Mount St. Helens set S tephra [16.9– 13.75 ka cal BP] lap on the calcified gravels north

of the quarry. Contacts and bedding in the gravels and silts shown on the logs have

apparent dips of ~10˚ to the north, and the logs noted liquefaction features in several

places (Woodward-Clyde Consultants Inc, 1981). Tilted gravel and silt strata and

juxtaposition of these same strata against Pomona Member basalt suggests pre-MIS 2

deformation along the Wallula fault at Finley quarry. One south-dipping reverse fault

(F8) juxtaposes older flood deposits against Pomona Member basalts, while a second

south dipping reverse fault (F7) shows reverse separation of pre-MIS2 flood deposits.

Thin layers of flood gravels and younger colluvium bury both fault tips. Faults F1 and F2

bound a near-vertical seam of sheared basalt and carbonate cemented basalt breccia and

are the primary faults shown on the composite log.

Figure S3 – Raw color photograph of fault shown in Figure 10A.

Figure S4 – Raw color photograph of liquefaction feature shown in Figure 11A.

Table S1 - Detailed unit descriptions for 2016 trench log.

-118.90 -118.88 -118.86 -118.84 -118.82 -118.80

46.0

246

.04

46.0

6

Walla Walla River

Vansycle Canyon

N

to Columbia Riverto Columbia River

0 1 km

A

B

Figure S1

Deflected stream ofMann and Meyer (1993)

LiDAR scarp

Fault (dashed where inferred)

Edge

of L

iDAR

dat

a

Edge

of L

iDAR

dat

a

Bedding contact

Fault

Inferred fault

Inferred bedding contactPolished and striated clay surfaces

Loess

Calcified zone

GravelGravelSilt

Silt

SiltPomona Member

basalts and related units

Um

atilla Mem

ber basalts

Breccia

Dashed contacts along

disturbed surface?

CL-1 Brown/black silty clayCL-2 Black clayCL-3 Green clayC-1 Calcified zoneU-3 Compacted siltU-2 Gravelly silt and silty gravelU-1 Gray gravelsN-1 Clastic dikeN-2 White siltN-3 Silty sand/sand siltM-5 Silty gravelM-4 Bedded gravelM-3 Bedded silts and sandsM-2 Massive gravelsM-1 Basalt gravels

L-6 Clayey siltL-5 Sandy gravelL-4 Sandy silty clayL-3 Gravelly sandL-2 GravelsL-1 Clayey fine sand2H White tuff2G Weathered and sheared basalt2F-1 Basalt granules in silt matrix2F-2 Brecciated basalt2F-3 Basalt breccia in pink matrix2E-2 Breccia of Pomona Member2E-1 Breccia of Pomona Member2D Vesiculated Pomona Member basalts2A Fractured Pomona Member basalts

Unit Legend2 meters

Top of trench

Bottom ofquarry wall

F1F2

F7F8

Strike - N65ºWDip = ~90º

North South

Footwall units Hanging wall units

Fault zone units2C Breccia of Pomona Member2B Breccia of Pomona Member1B Breccia of Umatilla Member in carbonate matrix1A Breccia of Umatilla Member in carbonate matrix

1 Umatilla Member basalts

L LoessCo-2 Colluvium

Capping units

Figure S2

L

C-1

U-3

C-1U-1

U-1

C-1a

U-3b

U-3aU-3 U-2

U-2 U-1Co-2

Co-2

Co-2

M-2M-2

M-3M-3 M-2

M-1

U-1 L-5

L-4L-2

L-1

L-2

L-3

M-4M-4

M-5

M-3

M-3

12

2A

2H

2D

2F-1 & 2F-2

2E-1

CL-2

CL-3

CL-1

1A

1A

1B

2C

2B

2A

2A

2A

N1/N2

2G

2G

2E-1

2F-3

Figure S3

Figure S4

Table DR1. Descriptions of stratigraphic units at Finley quarry. Unit Lithology Matrix Texture Munsell

Color % Cobbles % Pebbles Clast

Angularitya Clast Support Sedimentary Structure b Soil Structure

19 Cobbly gravelly silt to fine-medium well-bedded sand interbedded with silt laminae

Silt clay loam A-SR Matrix Parallel bedding and lamination in places

18 Turf Silt clay loam 10YR5/3 <2 <5 A-SR Matrix Massive 1, gr, f 17 Dark brown silt, well sorted with occasional

gravel clast Silt clay loam 10YR5/3 <2 <5 A-SR Matrix Massive 1, gr, f; 1, pr, f-m

16 Light brown silt, well sorted with occasional gravel clast

Silt clay loam 2.5Y6/3 <2 ~10 A-SR Matrix Massive 1, sbk, f

15 White silt (tephra) Silt clay loam 2.5Y7/2 <2 <10 A-SA Matrix Massive 1, sbk, f-m; carbonate present

14 Basalt clasts with a light brown silt matrix Silt clay loam 2.5Y6/3 ~15 ~20 A-SA Clast (tapering to matrix)

Massive to fining upward Matrix is massive; clasts have carbonate coatings

13 Light brown silt, well sorted with occasional gravel clast

Silt clay loam 2.5Y6/3 ~2 ~5 A-Umatilla; R-Pomona and exotics

Matrix Massive 1-2, sbk, f-m; carbonate filaments

12 Basalt clasts with a light brown silt matrix Silt loam (loess) 2.5Y6/3 30 40 A-SA Clast (tapering to matrix)

Massive to fining upward Some carbonate coatings on clasts but could be relict

11 Silt with cobles Silt clay loam 2.5Y6/4 40 10 A-Umatilla; SR-Pomona and exotics

Matrix 1-2, sbk, f-m; carbonate cement in places; carbonate rinds on clasts

10 Pink volcaniclastic cobbly sandy silt Silt loam to clay loam

7.5YR7/3 10 25 SR-R Matrix Coarse planar bedding in places

1-2, sbk, f; indurated; carbonate filaments

9 Sandy silt with cobbles Sandy clay 5Y6/3 ~5 ~60 SA-SR Clast to matrix Mostly massive but grossly fines upward

8a Tan to light brown (buff) sandy silt with carbonate clasts

Silt loam? <1 R Matrix

8 Greenish gray sandy silt Silt loam 5Y6/3 ~2 5-10 SR Matrix Layers of brownish fine sand/silt have planar bedding

1, pr-co, vf ;1-2, sbk, f-m; carbonate on some ped surfaces

7 Indurated silty clay with rare clasts of vesicular basalt (Pomona Member)

Silty loam 2.5Y2.5/1 <1 SR Matrix 2-3, sbk, f-c Carbonate veins 1-2 mm thick, random throughout unit

6/6a Tan to dull white, carbonate cemented breccia; clasts are 100% Umatilla Member; grain size of basalt clasts decreases towards F1 (from cobble and pebble size to pebble-granule) to form a ~5-10-cm thick seam of gouge with vertical shearing and banding. 6a has abundant shears.

Cemented carbonate matrix

dull white to tan in color

10 65 Angular Clast to matrix

5 Yellow orange sandy silt with abundant cobbles of light gray vesicular basalt (Pomona Member; no Umatilla Member clasts observed)

Silty loam to clay loam

7.5YR7/4 ~40 ~20 SA-SR Clast to matrix

5a Yellow orange sandy silt with pebbles of light gray vesicular basalt (Pomona member; no Umatilla clasts observed)

Silty loam 5Y8/2 ~60 SA-SR Clast to matrix

4 White tuff, silty fine-medium sand sized tephra, original bedding broken into clasts

5Y8/11 SA-SR Clast Lamination, cross bedding

3 Sandy silt with cobbles (all clasts Pomona Member)

Silty loam to clay loam

2.5Y5/4 30 20 SA-SR Matrix 1, gr, vf

2 Deeply weathered basalt to cobble conglomerate (basalt clasts with little to no matrix)

Silt loam to loam ~15 ~80 A-SA Clast

1 Black to dark gray basalt, aphanitic, a few samples have small phenocrysts(<<1mm) of plagioclase and olivine, weathers tan to dark brown.

a A - angular; SA – subangular; R – rounded; SR – subrounded b Soil structure nomenclature follows Schoeneberger et al. (1998)