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Suggested citation: Sherrod, B.L., Barnett, E.A., Knepprath, Nichole, and Foit, F.F., Jr., 2013, 

Paleoseismology of a possible fault scarp in Wenas Valley, central Washington: U.S. Geological 

Survey Scientific Investigations Map 3239. (Available at http://pubs.usgs.gov/sim/3239/.)

Paleoseismology of a Possible Fault Scarp in Wenas Valley, Central Washington

By

Brian L. Sherrod,1

 Elizabeth A. Barnett,1 Nichole Knepprath,2 and Franklin F. Foit, Jr.3

2013

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3239

1

U.S. Geological Survey, Seattle, Wash.

2

U.S. Geological Survey, Menlo Park, Calif.

3

GeoAnalytical Lab, Washington State University, Pullman, Wash.

1

1

5

6

8BC

8BC

7

8Bw

8A

8A

2

3

4

North

South

Meters

Meters

0

0

1

2

3

1 2 3

Meters

Meters

0

0

1

2

3

1 2 3

B

A

Figure 3. The Hessler Flat trench in the Wenas Valley, central Washington, which was excavated in October 2009 and 

measured using a 1-meter grid. The location of the trench excavation (46°49’41.42”N., 120°40’0.59”W.) is shown in figure 1. 

A, Photomosaic of the Hessler Flat trench. B, Profile of the Hessler Flat trench stratigraphy. 

EXPLANATION

Soil or rock unit

Modern A soil horizon—Dark-brown, platy, undisturbed sandy silt. Uppermost part contains light-gray ash from 1980 eruption of Mount St. Helens

Modern Bw soil horizon—Brown, cobbly, silty sand between units 1 and 

8A. Only appears in hanging wall, or merges with units 5 and 6. Contact 

with unit 5 is obscured by trench rubble

Modern BC soil horizon—Tan, cobbly, silty, sand. Only appears above unit 

1 in northern end of trench

Upper loess or valley fill—Brown, sandy silt that is similar in appearance to 

unit 3, but is slightly lighter in color, contains some cobbles, and is slightly 

less compact. No stratification. Much softer than units 3 and 6. Only 

appears above unit 4, which separates it from unit 3

Upper cobble colluvium—Brown, silty to sandy cobble colluvium that is 

similar to unit 2, except with fewer cobbles. Contains more boulders and is 

a little less dense than unit 5 and is darker brown and finer grained than 

unit 3. Separated from unit 2 by unit 4

Upper bouldery cobble colluvium—Brown, silty to sandy cobble colluvium similar to unit 2 except that it is darker brown, contains fewer 

boulders, and is less compact than unit 2. Contact with unit 8Bw obscured 

by trench rubble

Buried soil—Distinctive dark-brown, silty soil containing few to no cobbles 

and pebbles. Present along the top of lower bench. Disappears beneath 

rubble north of unit 5. Thins out at southern end of trench. May have been 

disturbed by trench excavation and (or) was eroded away in places

Lower loess or valley fill—Mostly homogeneous brown, sandy silt. 

Contains very few cobbles. No stratification. Much softer than units 2 and 

4. Slightly darker and more compact than unit 7. Appears to onlap against 

unit 2. Separated from unit 7 in places by unit 4

Lower bouldery cobble colluvium—Hard-packed, brown, silty to sandy 

bouldery cobble colluvium. Includes soil and pieces of bedrock derived 

from unit 1. Contains tightly packed cobbles nearest to bedrock and 

boulders. Difficult to scrape. Darker brown than unit 8Bw. No color 

change across contact with unit 3, but unit 2 is more compact than unit 3

and its cobbles and boulders disappear in unit 3. Separated from unit 5 in 

places by unit 4

Weathered basalt bedrock—Highly weathered basalt with crumbly, 

dark-yellow weathering surface and fractured, dark-gray fresh surface. 

Spheroidally weathers into subangular boulders and cobbles. Intact 

bedrock material forms “matrix” for spheroidally weathered, boulder-sized 

clasts. Upper contact coincides with benched wall of trench and soil units 

8BC, 8Bw, and 8A

Basalt clast

Contact—Dashed where inferred

Ash—From Mount St. Helens eruption in 1980

8A

8Bw

8BC

7

6

5

4

3

2

1

F1

F2

F3 F4

F5

F6

F7

F8

F9

6ABb

6ABb

3 3

4

4

4

4

4

4

5

5

5

5

5

6ABb

6ABb

6ABb

7Bw

7Bw

8ABw

8ABw

8ABw

9A

9A

9A

2a

2b

2c

2d

2d

2d

2d

2d

2d

2d

2d

2d

2d

2d

1

2e

2e

2e

2e

2e

2e 2e

2e

2e

2f

2f

2f

2f

2f

2f2

3

5?

7Bw (coarse-grained)

7Bw (coarsed-grained)

7Bw (coarse-grained)

Trench bench

Trench bench

Trench bench

Sheared

Sheared

47,000 yr B.P.

Meters

North

South

0

0

1

2

3

4

5

6

1 2 3

B

A

Meters

EXPLANATION

Soil unit—See table 1 for full unit descriptions

Sandy silt with ash

Sandy silt

Sandy silt

Sandy clay

Gravelly silty sand

Sandy silt

Sandy silt with volcaniclastic fragments

Clay with volcaniclastic fragments

Sandy silt with lapilli

Cobbly sandy silt

Sandy silt

Gravelly silty sand

Sandy silt

Gravelly silty sand

Sandy silt

Primary stratigraphic contact — 

Dashed where inferred

Unconformities

Upper

Middle

Lower

Fault—Arrow indicates relative motion

Edge of prismatic soil structure

Location of ash sample and date. yr 

B.P., years before present

47,000 yr B.P.

F2

9A

8ABw

7Bw

6ABb

5

4

3

2f2

2f

2e

2d

2c

2b

2a

1

Figure 2. The McCabe Place trench in the Wenas Valley, central Washington, which was 

excavated in October 2009 and measured using a 1-meter grid. The location of the trench 

excavation (46°51’7.06”N., 120°42’46.28”W.) is shown in figure 1. A, Photomosaic of the McCabe 

Place trench. B, Profile of the McCabe Place trench stratigraphy. Electron microprobe analysis 

of lapilli contained in unit 2f indicates a similarity coefficient of 0.98 with the Mount St. Helens 

set C tephra, which has an age of around 47,000 years before present (B.P.) (Berger and 

Busacca, 1995). 

Unit

(see 

figure 2)

Lithology Matrix texture Color

Roots1 Clasts

Stratification

Soil horizon2

Cement

Lower contact 

Deformation Genesis

Quantity Size Orientation

Cobbles 

(percent 

of unit)

Pebbles 

(percent 

of unit)

Angularity Support Horizon Horizon 

suffix

Structure

Distinctness Relief

Grade Size Type

9A Sandy silt with 

ash

Silty clay to 

silty clay 

loam (ashy) 

7.5YR 3/2 Few

Few

Very fine

Medium to coarse

Subvertical to vertical

Vertical

NA <1 Subangular to 

subrounded

Matrix NA A NA Moderate

Weak

Fine to medium

Very fine

Platy

Prismatic

NA Abrupt Smooth to 

wavy

NA Modern A horizon; contains ash from 1980 

Mount St. Helens eruption.

8ABw Sandy silt Sandy clay 

loam

7.5YR 2.5/2 Few Medium to coarse Vertical NA <1 Subangular to 

subrounded

Matrix NA B w Weak to moderate

Weak

Medium to coarse

Fine

Prismatic

Subangular blocky

NA Gradual Smooth Cut by a normal fault (youngest event in trench)Colluvium deposited by faulting event that cut 

unit 7Bw.

7Bw Sandy silt Sandy clay 

loam

7.5YR 3/3 Few Medium to coarse Vertical NA <2 Subangular to 

subrounded

Matrix NA B w Weak Fine Subangular blocky NA Abrupt to 

clear

Smooth Cut by normal faults Colluvial deposit that buried unit 6ABb. 

6ABb Sandy clay Sandy clay 10YR 3/2 Few Medium Vertical NA NA Subangular to 

subrounded

Matrix NA BA b Moderate to strong

Moderate to strong

Medium to coarse

Medium to coarse

Prismatic

Subangular blocky

Carbonate(?) Abrupt Smooth Possibly folded in center 

of trench; cut by several 

normal faults

Soil formed on preexisting scarp, then buried 

by colluvium (unit 5).

5 Gravelly silty 

sand

Loamy sand 10YR 4/4 Few Very fine to medium Vertical 40 20 Subangular to 

subrounded

Matrix NA CB NA Weak Fine Prismatic Carbonate(?) Clear Wavy Bounded by unconformities; 

cut by normal faults

NA

4 Sandy silt (Too indurated 

for texture 

analysis)

2.5Y 7/4 Few Very fine to fine Subvertical <1 <2 Subangular to 

subrounded

Matrix NA BA b Weak Coarse Prismatic Carbonate(?) Clear to 

gradual

Smooth to 

wavy

Cut by normal faults; bounded by unconformitiesPossible buried soil.

3 Sandy silt with 

volcaniclastic 

fragments 

(Too indurated 

for texture 

analysis)

10YR 4/2 Few Very fine Subvertical <2 <3 Subangular to 

subrounded

Matrix Possibly 

graded

CB NA Moderate Coarse Prismatic Carbonate(?) Clear Wavy Dips to north; eroded by unconformity; cut by normal 

faults

NA

2f2 Clay with 

volcaniclastic 

fragments 

Silty clay 2.5Y 5/4 Few Very fine to fine Vertical to subvertical NA 2 Subangular to 

subrounded

Matrix NA CB NA Massive NA NA NA Abrupt Irregular and 

(or) broken

Offset by a normal fault NA

2f Sandy silt with 

lapilli 

Sandy clay 

loam

2.5Y 6/4 Few Very fine Subvertical NA NA NA NA NA CB NA Weak Medium Granular NA Clear Irregular Unit is both folded and 

faulted at 8 m

Electron microprobe analysis of lapilli and ash 

sample indicates correlation with layer Cy in 

Mount St. Helens tephra set C (Mullineaux, 

1996).

2e Cobbly sandy 

silt

(Too indurated 

for texture 

analysis)

2.5Y 6/3 Few Very fine to fine Subvertical 30 20 Subangular to 

subrounded

Matrix Slight imbricationCB NA Weak Medium Prismatic Slight (carbonate?) Clear Wavy Dips 20° to 35° to the north, 

possibly folded at 6 to 7 m

Alluvial-fan deposit.

2d Sandy silt (Too indurated 

for texture 

analysis)

2.5Y 6/4 Few Very fine Subvertical NA 2 Subrounded Matrix NA CB NA Weak Fine Prismatic Very indurated 

(carbonate?)

Clear Smooth Dips 20° to 25° to the north NA

2c Gravelly silty 

sand

Sandy clay 

loam

2.5Y 6/3 Few Very fine to fine Subhorizontal 15 15 Subangular to 

subrounded

Matrix Slight imbricationCB NA Weak Medium Prismatic Very weak NA Wavy Dips 20° to 25° to the north NA

2b Sandy silt Sandy clay 

loam

2.5Y 6/4 Few Very fine Vertical <1 <1 Subangular Matrix NA CB NA Moderate Fine Prismatic NA Abrupt Abrupt Dips 20° to 25° to the north Fine-grained alluvial-fan deposit.

2a Gravelly silty 

sand

Sandy clay 2.5Y 7/2 Few Very fine Subvertical 8 12 Subangular to 

subrounded

Matrix NA C NA Weak Fine Granular(?) NA NA Smooth Dips approximately 25° to 

the north

Debris-flow and (or) alluvial-fan deposit.

1 Sandy silt Sandy clay 

loam

2.5Y 7/3 Few Very fine to fine Subvertical 1 ≈5 Subangular to 

subrounded

Matrix Massive C NA Weak Fine Granular(?) NA Not visible Smooth None observed NA

1

For unit 9A, two types of roots were present.

2

For units 9A, 8ABw and 6ABb, two types of soil horizons were present.

Table 1. Descriptions of units in McCabe Place trench.

[Color terminology is taken from Munsell Color, Inc. (2002). Terminology regarding texture, roots, clasts, soil-horizon features, and distinctness of contacts is taken from Schoeneberger and others (2002). For lower contact 

distinctness, the categories are determined by the thickness of the transition zone between units, as follows: Abrupt, 0.5 to < 2 cm; Clear, 2 to <5 cm; Gradual, 5 to <25 cm; Diffuse, >15 cm. Abbreviations and symbols are as 

follows: b, buried genetic horizon; cm, centimeters; NA, not available; w, incipient color or pedogenic structure development and minimal illuviation; ≈, approximately; <, less than]

DISCUSSION

In October 2009, two trenches excavated across an 11-kilometer-long scarp at Wenas 

Valley in central Washington exposed evidence for late Quaternary deformation. Lidar 

imagery of the Wenas Valley illuminated the west-northwest-trending, 2- to 8-meter-high 

scarp as it bisected alluvial fans developed at the mouths of canyons along the south side 

of Umtanum Ridge (fig. 1A). The alignment of the scarp and aeromagnetic lineaments 

suggested that the scarp may be a product of and controlled by the same tectonic structure 

that produced the magnetic lineaments (Blakely and others, 2011). Several large 

landslides mapped in the area demonstrated the potential for large mass-wasting events in 

the area (fig 1A). In order to test whether the scarp was the result of an earthquakegenerated surface rupture or a landslide, trenches were excavated at Hessler Flats and 

McCabe Place (fig. 1B). The profiles of bedrock and soil stratigraphy that underlie the 

scarp in each trench were photographed, mapped, and described (figs. 2, 3; table 1), and a 

sequence of depositional and deformational events was established for each trench (tables 

2–4).

The exposure in the McCabe Place trench revealed a sequence of volcaniclastic 

deposits (units 1–3) overlain by soils (units 4, 6 and 9) and alluvial deposits (units 7 and 

8), separated by three unconformities. Six normal faults and two possible reverse faults 

deformed the exposed strata. Crosscutting relations indicated that up to five earthquakes 

occurred over the last approximately 47 thousand years on a blind reverse fault, and a 

microprobe analysis of lapilli contained in unit 2 suggested that the earliest faulting 

occurred after the deposition of Mount St. Helens ash layer Cy 47,000 years before 

present (B.P.).

The Hessler Flat trench exposure revealed a weathered bedrock (unit 1) that abuts 

loess and colluvium deposits (units 3 and 2, respectively) and is overlain by soil (unit 8), 

an upper sequence of loess (unit 7), and colluvium (unit 6). The latter two units bury a 

distinctive paloesol (unit 4). 

REFERENCES CITED

Berger, G.W., and Busacca, A.J., 1995, Thermoluminescence dating of late Pleistocene 

1oess and tephra from eastern Washington and southern Oregon and implications for 

the eruptive history of Mount St. Helens: Journal of Geophysical Research, v. 100, 

no. B11, p. 22,361–22,374. (Also available at 

http://www.agu.org/journals/jb/v100/iB11/95JB01686/.)

Blakely, R.J., Sherrod, B.L., Weaver, C.S., Wells, R.E., Rohay, A.C., Barnett, E.A., and 

Knepprath, N.E., 2011, Connecting the Yakima fold and thrust belt to active faults in 

the Puget Lowland, Washington: Journal of Geophysical Research, v. 116, no. 

B07105, 33 p., doi:10.1029/2010JB008091.

Mullineaux, D.R., 1996, Pre-1980 tephra-fall deposits erupted from Mount St. Helens, 

Washington: U.S. Geological Survey Professional Paper 1563, 99 p. (Also available 

at http://pubs.usgs.gov/pp/p1563/.)

Munsell Color, Inc., 2009, Munsell soil color book: Grand Rapids, Mich., Munsell Color, 

Inc., not paginated.

Schoeneberger, P.J., Wysocki, D.A., Benham, E.C., and Broderson, W.D., eds., 2002, 

Field book for describing and sampling soils (version 2.0): Lincoln, Nebr., Natural 

Resource Conservation Service, National Soil Survey Center, 228 p., available only 

online at http://soils.usda.gov/technical/fieldbook/.

1 Emplacement of basalt.

2 Development of soils (units 8A and 8Bw) on surface of basalt (unit 1), which 

weathers into cobbles and boulders now found on the scarp surface.

3 Possible folding or faulting (no fault surface is visible), which uplifts the 

northern side of the weathered basalt (unit 1) and results in the development 

of a scarp.

4 Deposition of colluvium (unit 2), which incorporates boulders from weathered 

basalt (unit 1). 

5 Deposition of silty loess (unit 3), which fills the valley and onlaps against the

scarp.

6 Formation of soil (unit 4) on surface of colluvium (unit 2) and loess (unit 3).

7 Possible north-side-up folding or faulting event, which refreshes scarp, causes 

further erosion of the weathered basalt (unit 1), and disrupts the soil (unit 4).

8 Deposition of colluvium (units 5 and 6), which incorporates boulders from 

weathered basalt (unit 1) mostly into unit 6.

9 Deposition of loess (unit 7), which fills the valley. 

10 Development of modern soil horizon (unit 8A) develops on surface. Soil units 

8Bw and 8BC continue to develop. Upper part of unit 8A contains ash from 

1980 eruption of Mount St. Helens tephra (shown by dashed gray lines on 

Table 3. Inferred sequence of depositional and deformational events documented in 

the Hessler Flats trench.

figure 3).

600

600

600

600

600

700

700

700

700

800

800

800

900

900

900

1000

1000

1000

1100

1100

1200

1200

1100

1000

700

800

900

1000

1100

1200

1300

1400

120.75° 120.70° 120.65° 120.60° 120.55°

46.85° 46.80°

Figure 1B

Cleman Mountain

Wenas Creek

Umtanum Ridge

Yakima, 25 km Ellensburg, 23 km

A

Seattle

Ellensburg

Yakima

WASHINGTON

Columbia River

Snake River

STUDY

AREA

Columbia River

AREA OF MAP

EXPLANATION

0 5 KILOMETERS

Topographic contour line— 

Contour interval is 50 

meters

Topographic scarp

Bedding-plane scarp

Magnetic lineament

Trace of bedding

Location of trench 

Landslide

0 0.5 1 1.5 2 KILOMETERS

UMTANUM RIDGE

Hessler Flats

trench (figure 3)

McCabe Place

trench (figure 2)

Wenas Creek

Wenas Lake

B

Figure 1. Maps showing study area and vicinity. A, Regional shaded-relief map (derived from a 10-meter digital elevation model) of the Wenas Valley area of central Washington, 

showing scarps, bedding traces, landslides, magnetic lineaments, and trench locations (Blakely and others, 2011). km, kilometers. B, Shaded-relief map generated from a lidar 

image of the area showing the location of the Wenas Valley scarp (white arrows) and trenches (red lines).

1 Deposition of units 1, 2a, 2b, 2c, 2d, 2e, 2f, 2f2, and 3 (see table 1 for 

unit numbers and descriptions).

2 Erosion leading to development of lower angular unconformity 

(shown by orange line in figure 2); formation of soil (unit 4).

3 Normal slip on faults F1, F3, F4, F8, and F9; notably—

 Fault F4 with a minimum of 83 centimeters (cm) of vertical

 separation

 Fault F3 with a minimum of 15 cm of vertical separation.

 Note that all faults may have lateral motion; strata thicknesses

 do not match up across faults.

4 Erosion leading to development of middle unconformity (shown by 

purple line in figure 2).

5 Deposition of unit 5 (inferred to be colluvium). Unit 5 at northern end 

of the trench could be of a different generation than similar deposits at the southern end.

6 Erosion leading to development of upper unconformity (shown by 

green line in figure 2).

7 Development of soil (unit 6ABb).

8 Slip on fault F5 with a minimum of 65 cm of vertical separation.

9 Deposition of colluvium (unit 7Bw). Unit 7Bw buries unit 6ABb in 

lower half of trench. Unit 6ABb continues to develop in top of 

trench (from fault F2 northward).

10 Movement on fault F2 with a minimum vertical separation of 

approximately 87 cm. Faults F6 and F7 could have moved at the 

same time as fault F2 but with substantial lateral motion; units 

between F6 and F7 do not match those on either side and have 

different colors and lithologies.

11 Deposition of unit 8ABw.

12 Development of the modern A soil horizon (unit 9A). Upper part of 

unit 9A contains ash from the 1980 eruption of Mount St. Helens.

Table 2. Inferred sequence of depositional and deformational events 

documented in the McCabe Place trench.