ML20151H275

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Package of Papers Presented at Quaternary Research Ctr Spring Symposium on 880506-08 in Seattle,Wa Re Holocene Subduction in Pacific Northwest
ML20151H275
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Trojan Nuclear Plant Document Control Desk Docket 50-344 June 15, 1988 l License NPF-1 At t ac hment B I

HOLOCENE SUBDUCTION l IN THE PACIFIC NORTHWEST I QUATERNARY RESEARCH CENTER

, SPP. LNG SYMPOSIUM i

! MAY 6 8,1988 e

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HOLOCENE SUBDUCTION IN THE MARSHRECORDSOFTHE SOUTHERN PAClFlC NORTHWEST CASCADIA MARGIN ... .... . .... ... . . ..... ... .. .10 4 0 A Sympoelum 12. Alan R. Nelson: IMPLICATIONS OF LATE Wey 6.s. 1ses HOLOCENE SALT. MARSH STRATIGRAPHY FOR Sponsored by the Quetornery Research GREAT EARTHOUAKE RECURRENCE ALONG Center, Univorelty of Weehlngton THE COAST OF SOUTH CENTRAL OREGON Seattle, Weehington " ' jo;$o 3 f*fjg"y'g'r':"' 'k5 $U't E U'UI5ENCE FHfDAY. MAY a r147 ARCHf7ECTURE HALil OF HOLOCENE SUBMERGENCE ALONG THE OREGON AND SOUTHERN WASHNGTON KEYNOTE LF.CTURES A S To o-m om o -- m - m.o o -- o -. -o RM

1. Plerte Saint. Amend: THE GREAT CHILEAN EARTHOUAKES OF 19 6 0. . . . . . . . . . .. .. . . . . .. . 8 : 3 0 E s cu s s io n----~ ~-- - -- ~ ~~ -- e . m11 d 5 Lu neh . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . 12 :0 0 B r e a k. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . 10:00 OEODESY
2. Hiroo Xanamori: SUBDUCTION ZONE 14. Michael Lloowski, James C. Savage, S E I S M O LOG Y. . . . . .. .. . . . . . . . . . . . ... . . ... . . . .. 10 :30 William H. Prescott: STRAIN ACCUMULATON IN WESTERN WASHINGTON Lunch........................................ .. 12.00 AND SOUTHWESTERN BR TISH COLUMBIA

,, _ , ,, ,,,,,,,,,,,,_,,,,,,,,_,,,_j.30

3. George Piafker: TECTONIC DEFORMATON 15. Paul Vincent: DEFORMATION OF THE RELATED TO GREAT SUBOUCTON ZONE OREGON COAST RANGE. 1920 1941...... 1:40 E A R T H Q U A K E S ..... . ....... .. . .. . . .... . ... .. . i .3 0 16. H.J. Melosh: FINITE ELEMENT MODELING OF STRAIN AND UPLIFT IN THE PACIFIC B , e a k . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3 : 0 0 "mm"ST - - - -- - - ' :5o -

nE

4. .t,ianD.Aiwaie,:

PRiuERS TRIP Buried HOtOCENE g'c, u,aia

- - - a--- - - ------ - ---

-- g WETuNDS ALONG THE JOHNS RIVER, SOUTHWEST WASHINGTON.. .... ... .... 3:30 PLATE TECTONICS D i s e u s s io n .. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. 4 :00

17. Gregory A. Devla: GEOLOGIC
5. Mary Reinhert: AOUATIC SAFETY.... . 4:15 COMPARISONS OF THE CASCADIA AND OTHER A d p u r n. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 :30 .SIMOR* SUBDUCTON ZONES .... ....... 2:45
18. Brian 1. R. Lewis: THE CASCADIA SUNDAY. W AY a r147 Architecture Hafn SUBDUCTON ZONE. SOME UNRESO(.VED COASTAL GEOLOGY P R O6 LE MS . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . 2 :5 5
6. Gary A. Carver: SEISMIC POTENTIAL OF 19. Cral S. Weaver: EVIDENCE FOR A THE GORDA SEGMENTOF THE CASCADIA SEGMENTATON BOUNDARYIN WESTERN WASHINGTON FROM THE OVEETS PJVER TO
7. SUBOUCTONZONE~5d Atan G. Hull: RADI k55b" 8EE"gf :30MO UNT R AINlE R.... ...... . .. . .. . ... ..... 3:05 PROBABLE COSEISMIC BURIED SOlt LAYERS FROM WASHINGTON STATE........ ....... 8:40 3
8. David K. Y uchl: PRELIMINARY TREE. O'***"""'"""""""""""""""""'"""15 D ' * " ' ' I' " " " " ' ' ' " " " " " " " " " "" " " " ' " 3 RNG DATNG LATE-HOLOCENE DENCE ALONG mE WASHNGTCH y................................................:50 2 R e rn SaHy E. B. Abous,
9. Mary A. Reinhart, and Joanne Thomae Sourgeole: TESTING THE TSUNAMI g d W. p, ,E,dmondeon,

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PQ'SEISMICITYIN THE ET SOUNDP A VC F 'LARGE. SCALE. [^g A EA D N SEDMS LANDWARD DRECTED PROCESSES. ... 9:00 2t EsteHe B. LeopoN':""EdNI' di E" 00 D i s c u s a io n . . .. . . . . . . . .. . . . .. . . .. . . . . . .. . .. . .. . .. 9 TILTING :15 OF LAKE WASHINGTON:

B r e a k . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .SEDIMENTARY . 1 0 : 00 AND POLLEN EVIDENCE

,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,43o

10. Wendy C. Grant: EVIDENCE FOR LATE 22 '

HOLOCENE SUBOUCTON EARTHOUAKES 0EF R DP T SED N THE TACOMA NARROWS,WASHNGTON RECORDED IN TIDAL MARSH DEPOSffS ALONG THE NEHALEM AND SALMON RIVERS. '"""""""""""""'"""""""""""""'420 NORTH E RN ORE GON .. . . . . . . . . . . . . .. .. . . . . . . .. 10.30

11. Curt D. Peterson and Mark l'* Dis cu s sion""" ' " ' " ~ " " " 4:30 Derienzo: DISCRIM! NATION OF FLOOD. Ad#am'"""*""'"'""""""""" *5:00 STORM AND TECTONIC EVENTS IN COASTAL l.

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. 1 THE GREAT CHILEAN EARTHQUAKES OF 1960 Pierre Saint Amand Post Office Box 532 i

Ridgecrest, California 93555 In May 1960, a series of earthquakes, tsunamis and volcanic h eruptions devastated large areas of southein Chile. I!plift and subsidence were wmmon along the Chilean Coast. Damage to industry, private property. utilities and communications was spread over an aie 600 by 150 kilometers. All this took pi ee in an area in v h>ch 'ie oldest inhabit. ants could not remember ever having felt an earthquake before, the last similar event was in 1835, as reported by Darwin. The earthquakes did over 400,000,000 dollars in damage, an appreciable portion of the gross national prc. duct of the country. I Non. th Ass, recovery was rapid and the economy of the area improved greatly owing to increased investment, and replacement of obsolete and marginal facilities.

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References Saint-Amand, Pierre,1961, Los terremotos de Mayo Chile 1960:

Chir,a Lake, California, U.S. Naval Ordnancc Test Station, Technien. siticle 14, 39 p.

. 1963, a issue - Oceanographic, geologic,

, and enge w n e Chilean earthquakes of May e >

1960:

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.< ' ogical Society of America, v. 53,

d. .[ no. 6.

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TECTONIC DEFOsiMATION RELATED TO GREAT SUBDUCTION ZONE EARTHQJAKES George Plafker, U.S. Geological Survey Abstract Vertical and horizontal displacements associated with plate convergence at consuming plate ma; dins are the algebraic sum of interseismie, coseismie, and transient deformat-ions through a complete earthquake cycle on a time scale of tens to thousands cf years, Elastic and permanent deformations accumulated during the interseismic period are a function of coupling across the megathrust interface between the underthrusting oceanic crust and the upper plate, and of the direction, raie, and duration of relative plate motions. Coseismic deformations result fr6m seaward thrusting of the upper pl ate and depend upon dip of the megathrust, displacement along the megathrust, and the dip ant displacements along subsidiary faults that may break through the upper plate. Transient postseismic displacements may occur that result from re atively slow clastic strain release or creep deformation fol lowing an earthquake.

Coseismic regional sertical displacements typically involve a central broad asymmetric downwarp elongate parallel to the arc with a flanking zone of marked uplift on the sea-ward side, and a zone of relatively minor uplift on the landward side. The major zones of uplift and subsidence may extend from the trench to its assochted volcanic arc. in the 1960 Chile earthquake (Mw=9.5 the are over sn area of 85,000+ with km')shoreline deformation verticaloccurred for about displacements 1,050 to +5.7 km parallel to m and

, -2.3 m. In 191964 Alaska earthquake (Mw=9.2) deformation occurred for 950 km parallel

' to the are over 140,000+ km* with vertical displacements to +11.3 m and -2.3 m; two subsidiary thrust faults cut the surface with up to 7.9 m dip slip displacement. Both earthquakes generated major tsunamis due to sudden upl.eaval of the sea floor. Horizontal displacments determined by triangulation indicate a minimum of 20 m relative seaward displacement during the 1964 event. Interstismic vertical displacements in the reginn affected by both uplift and subsidence during the Alaska carthqt.ake are indicated by gradus! shoreline submergence of terrestrial vegetation and aboriginal sWs along the shores at average rates of 5-7 mm/yr. The submergence suggests a r gnificant component of downwarping in addition to elastic shortening and thickening of tl.: upper plate between coseismic displacements. Tide-gage data and lavellings show uansient changes 1

i and possible reversals (recovery) of 9-20% of the coseismic sertiesi displacements in 18 years after the 1964 earthquake.

Falcoseismic studies worldwide indicate recurrence intervals on the order of hundreds to thousands of years for great subduction rone earthquakes that involve shoreline deformation in Alaska, paleoseismic studies of a flight of 5 uplifted marine terraces

, at Middleton Island and of regional pre-1964 submerged shorelines along the mainland and j

islands of Prince William Sound indicate recurrence intervals for 1964-type earthquakes averaging 800 years with the last previous event about 1400 years ago.

4 Refe.esces h

Plafker,'leorse,1972 The Alaskan eart quake of 1964 and Chilean earthquake of 1960; Implication

  • for are tectonics: Journal of Geophysical Research, v. 77, no. 5, p.

901-93.

Plafker, George,1969, Tectonics of the March 27.1964, Alaska Earthquake: U.S.

Geological Survey Professional Pr.per 543-l,74 .

Plafker, George, and Savage, J.C.,1970, Mechanism of the Chilean earthquakes of May 21-22, 1960: Geological Society of America Bulletin, v. 24, p.1001-1030.

Plafker, George, and Rubin, Meyer,1978, Uplift history and carthquake recurrence as deduced from marine terraces on Middleton Island, Alaska,in Proceedings of Conference VI, Mett.odology for identifying seismic gaps and soon Mraak gaps: U.S. Geologica; Survey Open-File Report 78 943, p. 687-72.

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Buried Holocene wetlands along the Johns River, southwest Washington Brian F. Atwater U.S. Geological Survey at Department of Geological Sciences University of Washington AJ-20 Seattle, WA 98195 Low tides along this tributary of Grays Harbor expose as many as five ,

buried marshes and swamps younger than about 3100 sidereal years before present. These buried wetlands, the likes of which abound in coastal southwest Washington, seen best explained by the aperiodic recurrence of great earthquakes on the Cascadia subduction rone. Saturday's field trip 1

' permits scrutiny of six hypotheses on which this interpretation depends, ,

t (1) THE WETLANDS WERE BURIED BECAUSE OF SUBMERGENCE MORE LASTING THAN STORMS OR FLOODS. Well-preserved soils of buried wetlands bear rooted stems and leaves of the marsh herbs Deschampsia caespitnfa and Potentilla pacifica, or stumps of Sitka spruce and western red cedar--plants that i

flourish today only near or above highest-tide level. But mud that buried the wetlands commonly contains the below-ground stems (rhizomes) of Trirlochi; seritima or Carex lyngbyei, plants that today colonize intertidal mudflats (Weinmann and others, 1985). Collectively, such plant fossils in growth position show that burial entailed changing wetlands to ,

tideflats, with a relative sea-level rise on the cedcr of 0.5-2.0 m.  ;

(2) THE SUBMERGENCE, AN] SOME OF THE CONSEQUENT BURIAL, OCCURRED RAPIDLY. Submergence takir.g decades would not permit the preservation of stems and leaven on the herbs that are rooted in the buried wetlands.

(3) RAPID SUBMERGENCE RESULTED FROM SUBSIDENCE OF THE L.uT1, NOT FROM '

EUSTATIC OR ISOSTATIC RISE IN RELATIVE SEA LEVEL. Eustatic and isostatic  !

rise in sea level on other mid-latitude coasts has been sufficiently gradual in the past 5000 years for tidal wetlands to build upward space '

with the sea, thus pruducing peaty tidal-wetland deposits many meters i thick. Such deposits are unknown in coastal southwest Washington.

(4) AT LEAST THE '90ST RECENT JERX OF SUBSIDENCE COINCIDED APPROXIMATELY WITH A LANDWARD DIRECTED SURGE OF SANDY WATER. This surge.

about 300 sidereal years ago, deposited sand arou.-d the stema and leaves of i

p. esempitosa and E. pacifica at the onset of marthland bur.'sl. Possible
analog
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the deposition of sand on freshly subsided chilean lowlands by the  ;

tsunami from the 1960 Chile earthquake (Wright and Mella, 1963).

(5) EACH JERE OF SUBSIDENCE INVOLVED LARGE AREAS. Individual buried wtclands along the Johns River extend tens to hundreds of meters in cutcrop. In addition, they ressable in stratigraphic sequence and i

radiocarbon age the buried wetlands o' most other estuarine areas in j coastal southwest Washington.

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YEARS.

(6) JERKS RECURRED AT IRREGULAR INTERVALS WHOSE AVERAGE IS ABOUT 600

' Aperiodic recurrence is indicated not only by radiocarbon ages but also by regional differences among buried wetlands in their maturity of i

soil and stage of forestation, i

Weinmann, Fred, and others, 1984. Wetland plants of the Pacific Northwest:

U.S. Corps of Engineers, Seattle, 85 p.

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Wright, Charles, and Mella, Arnoldo,1963. Modifications to the soil estterr of south-cen:Tal Chile resulting from seismic and associated  !

l phe.*omena during the period May to August 1960: Bulletin of the Seismobigical Society of America, v. 53, p.1367-1402.

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6 Seismic Pot:nti:1 of th] G:rda Segm:nt of the Cascadia Subduction Zone G:ry A. Carver, Humboldt State University, Arcata, CA 95521 g Assessing the seismic potential of the Cascadia subduction zone is especially difficult because the plate boundary lies well offshore along most of its length. Acoustic profiles of the sea floor across the subduction zone reveal a complex system of east dipping thrust faults and associated folds comprise a prominent fold and thrust belt parallellandward of the shallow dipping mega thrust. At its southern end the subduction zona converges with the northern California coast and the mega thrust approaches within 25 km o' Cape Mendocino. North of the Cape the fold and thrust belt extends onshore where it can be studied directly.

Deformation of Pleistocene and Holocene sediments and marine terraces in northern California records northeast southwest contraction across the 75 km wide portion of the fold and thrust belt exposed on land.

The contraction is represented by the growth of 15 major thrust faults and folds, and has averaged at least 20 25 mm/yr during the late Quaternary.

The contraction accommodates a large part of the Gorda North American plate convergence and is interpreted to reflect strong coupling across the upper 100120 km wide portion of the plate boundary.

Field relations indicating sudden displacement on the thrust faults and jerky growth of the associated foids demonstrate that the deformation has been coseismic. Paleoseismic investigations of the Little Salmon thrust show a late Holocene history characterized by individual slip events of at least 5 meters, and possibly 10 meters, at 400 to 600 year intmals; the most recent event occurred less than 415 C14 yrs B P. Carbon 14 dates from offset horizons in trenches across traces of the McKinleyville and Mad River faults show offsets of 3 to 4 meters recurring at intervals of a few thousand years. Raised marine terraces and associated terrace cover sediments at McKinleyville indicate two Jerky uplift evenu have effected this portion of the coast in the last 1170 C14 yrs, and buried saltmarsh peats record repeated episodes of late Holocene jerky subsidence where the axis of the Freshwater sync!ine intersects the margins of Humboldt bay. Growth ring patterns reflecting damage to trees on landslides along the Little Salmon thrust and buried trees on the raised Holocene terrace at Mckinleyville support an age estimate of about 300 yrs for the most recent subduction event. I In total, the geologic evidence strongly indicates tnat the Gorda segmer,t of the Cascadia subduction zone generates very large earthquakes. l These earthquakes are accompanied by thrusting and foloing in the I accretionary prism, and may involve simultr neous coseism:c growth of multiple faults and folds. Preliminary assessment of the recurrence interval and time since the last event suggest the Gorda segment of the subduction zone may be approaching the end of the present seismic cycle.

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RADIOCARBON AGE OF PROBABLE COSEISMIC BURIED SOIL I.AYERS FROM WASHINGTON STATE

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' ALAN G. HUL s UNIVERSITY OF CALIFORNIA, SANTA BARBARA

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I Stratigraphic and sedimentological data from exposures in .

coastal Washington have been interpreted to indicate that  !

i repetitive cycles of estuarine mud and thin organic horirons result i i

from sudden burial of subaeria'. wetlands during Cascadia Subduction  ;

i rene earthquakes. Regional synchroneity of these buried soils is j an important requirement to establish sudden burial. j t

i 91 radiocarbon dates from peat, twigs, cones, and roots have  !

i been measured from eight distinct organic layers. Each layer has i t

) from 3 to 27 dated 18 correlated exposures. samples taken from the uppermost part of up to i j

Large radiocarbon age ranges from each j layer indicate that instrumental and calibration errors are small compared to the variance in the population of sample ages and j

prevent calculation of weighted mean ages. Assuming minimal

! correlation, instrumental, and calibration errors the following mean ages for buried soils are obtained:

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1 m SAmre Mrs Act EJulgr, 1 27 263 +/-144 .

2 535 4 j 3 2 1730 +/ 180 i 27 1694 +/-185 4 810  !

< 5 8 2496 +/-117 382 10 2924 +/-234 l j 6 7 720 t

! COL /2 3187 +/-108 250 3 1027 +/-110  !

WTCH/1 7 260  !

1210 +/-256 900 l l

S i.mple radiocarbot, satistical analyses show that the population of  !

i ages for each layer has the same variance and all but  !

j two layers Layers 2 andare 3, significantly different (99.9% level) in mean age.  ;

1 alone. and COL /2 and WTCH/1 cannot be separated by dating l

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I Radiocarbon dating does not directly support sudden burial o.

soil layers, but does show that regional stratigraphic correlations are valid.

High standard errors reduce the accuracy of age ,

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determination for each layer and probably reflect the natural age range of organic materials preserved in the forest /setland t 3 envirement.

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8 FertTMIMRY TREE RING DATING OF IATE Is01DCENE SUBSIDENCE AIDNG THE WASHINC'10N COAST David K. Yamaguchi, Mountain Research Station, Univ of Colorado. Nederland, CO 80466, (303) 492 8841 The annual growth rings of trees vary in width in response to yearly variations in climate. Such ring width variation can be used to determine the death dates of dead trees by matching their ring width patterns with those of living trees (Fritts 1976).

I have been using this approach to determine limiting death dates for large western redcedar (Thuja plicata) snags rooted in buried wetland soili in estuaries along the southwestern Washington coast. The burial of the soils and the death of the cedars are hypothesized to have resulted from coseismic subsidence caused by a great earthquake on the Cascadia subduction zone (Atwa-ter 1987). Calibrated radiocarbon ages from the buried soils show that these areas subsided about 300 yr B.P. (Atwater et al. 1987). More precise ages vould be useful for testing the great earthquake hypothesis by determining if subsidence occurred synchronously throughout coastal southwest Washington at this time.

Thus far, I have built a preliainary 610 yr long, ring width chronology from 6 modern old growth cedars on Long Island in Villapa Bay. In turn, by matching the ring patterns of subsided cedar snags with those of the long '

Island trees, I have determined A.D. 1618, 1642, and 16f limiting death dates for 3 snags at the head of the Crays Harbor estuary (70 k2. N of Long Island). '

Sic!1arly, I have established A.D. 1664 and 1684 limiting death dates for 2 snags in the Copali:: River estuary (90 km N of long Island). Statistical analyses of ring width data from these trees, which generally follow the nethodology of Yamaguchi (1956), show that the snag dates are highly signifi-cant: probabilities of obtaining similar levels of ring pattern matching by chance range from .001 to .0005, i

The A.D. 1618 84 dates provide only limiting dates for subs!dence and associated tree death because some external wood is missing from all collected 4 samples due to weathering. Other samples, not yet analyzed, havce been obtained from other snags at these sites, as well as from snags in the Palix R. and Crays R. estuaries (15 km N and 25 km SE of long Island, respectively).

Collectively, the a sites span 100 km of Vashington coastline.

I plan to expand this study to include sampling of other cedar snag-bearing subsided wetlands on the southwest Washington and northern Oregon '

coasts. To this end, I appeal to conference attendees to inform me of any observacions of large snags rooted on subs!ded wetlands, and their specific l locations. In return, I may be able to provide limiting dates for subsidence j at these sites. More importantly, such collaboration should peruit us to:

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1) rigorously test the hypothesis of synchronous subsidence in different estuaries about 300 yr ago; 2) closely date this inferred last great Cascadia -

earthquake and document the length of coastline it subsided; and (3) refine current estimates of its probable magnitude.

Atwater, B. F. (*.9 8 7 ) . Science 236, 942 944.

Atwater, B. F., Hull, A. C.

  • and Bevis, K. A. (1987). EOS 67, .

fritts H. C. (1976). "Tree Rings and Climate." Academic Press. New York.

Yamaguchi, D. K. (1986). Tree Rint Bulletin 46, 47 54  !

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. 9 TESTING THE TSUNAMI HYPOTHESIS AT WILLAPA BAY, WASHINGTON:

EVIDENCE FOR LARGE-SCALE, LANDWARD-DIRECTED PROCESSES MARY A. REINHART and Joanne Bourgeois :

Department of Geology, University of Washington, Seattle, WA.

The occurren'e of great-subduction earthquakes in the pacific Northwest and their related tsunamis has been postulated by several workers. Geologic evidence for rapid co-seismic subsidence of coastal wetlands (e.g., Atwater, 1987) at Willapa Bay consists of estuarine of seven supratidal surfaces alternating with beds mud, three of which are overlain by anomalous layers of sandy silt interpreted by Atwater to be tsunami-laid deposits.

In testing the tsunami hypothesis we must consider other mechanisms by which the sands could have been deposited:

1) seiches, tidal channels. 2) severe storms and 3) flooding and/or migration of To date, our analysis has been conducted in Willapa Bay and has focused on the sandy unit overlying the youngest buried wetland surface. This unit consists of planar-laminated sands alternating with muddy laminae, indicating deposition from suspension; commonly, a lamina of mud occurs directly above the buried peat, evidence that the coastal platform was submerged prior to deposition of the first sand.

The unit is capped by two thin laminae which can be correlated l regionally. Detailed mapping of the unit at the Niawiakun River shows river and that the to thins unit is thickest less (>60mm) near the mouth of the than 1 mm about 3 km upstream, and that surfaces than on the upstream sides. sides of point bar/ wetland the unit is thicker on the downstream )

that 1) The resulta of our preliminary work have led us to conclude scale; 2)the sandy unit was deposited by an event of regional

3) the event thewas depositional event was composed of several pulses, landward-directed; 4) flooding of the channel and/or migration of pattern of sand distribution.

tidal channels could not have produced this Analyses are underway to determine provenance the sediment.of the sand and to mecsure sottling velocities of REFERENCES l

! Atwater, B . F. , 1987, Evidence for great Holocene earthquakes along the outer coast of Washington State: Science, v.

p. 9 4 2-9 4 4. 236, Heaton, T.H. and Kanamori, H.,

1984, Seismic potential associated with subduction in the northwestern United States, Bulletin of theJ.H.J.,

Terwindt, seismological Society of America, v. 94, p. 9 3 3-941 and Breusers, H.N.C., 1972, Experiments on the origin of Sedimentology, bedding, flaser, lenticular, and sand-clay alternating

v. 19, p. 85-98

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10 Evidence for late llolocene subduction earthquakes recorded in tidal marsh deposits along the Nehalem and Salmon Rivers, northern Oregon Wendy C. Grant (U.S. Geological Survey at Geophysics Program, AK 50, University of Washington, Seattle, WA 9S195)

Subsurftee stratigraphy in tidal marshes along the Salmon and Nehalem Rivers in northern Oregon records at least one episode of sudden submergence that was probably caused by large thrust earthquakes on the Cascadia subduction zone Along the Salmon River, both core samples and streameuts are characterized by fine medium sand grading up into sandy silt and silt, and gradually (over several centimeters) becoming very organic rich, so that the upper part of the unit consists of peat or muddy peat with an abrupt upper contact. These peaty units vary from several up to about 10 cm in thick-ness and are,in some cases, overlain by a fine medium sand unit. This sand unit ranged (where present) frora a trace up to approximately 10 cm in thiekness and was overlain by silt and silty sand grading up into the present marsh surface. Along the Nehalem River, the tidal marsh subsurface stratigraphy is similar to that of the Salmon River with the difference that there is no 'paeket' of sand lying in direct contact over the peaty herizon.

Instead, the peaty horizon is abruptly truncated and overlain by organie poet sitt or sandy silt grading upward into the modern marsh surface.

At both the Nehalem and Salmon Rivers, the subsurface stratigraphy is interpreted to represent the sudden lowering of a high marsh environment into an unvegetated tidal flat as a result of a large thrust earthquake on the Cascadia subduction zone. The fine-medium sand ' packet' that lies on top of the peaty unit along the Salmon River shows a distribution that thins both up the valley and away from the river channel; this distribu-tion indicates a seaward source and is interpreted as resulting from a tsunami generated by the rupture process Prehminary radiocarbon dating of the peaty horizons and preserved plant thiremes both above and below the peat yields an approximate age of peat burial of 300-400 years before present at both the Nehalem and Salmon Rivers. This age is consistent with the age of the youngest buried unit at several sites on the Wuhington cout to the north (Atwater,1957). If the subsidence at the Salmon and Nehalem Rivers wu coincident in time with subsidence in southwestern Wuhington, the 240 km extent of the dowr.-

drepped fe atu res.

zone indicates that an earthquake of magnitude 8 5 could have generated these REFERENCES Atwater. B F. (1957) Evidence ser great llolocene carthquakes along the outer ecut of Wuhington state, Science 236, 9424 44.

Grant, W C. and D D McLaren (1957) Evidence for llolocene subduction earthquakes along the northern Oregon rout, EOS, 65,1239 i

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DISCRIStINATION OF FLOOD, STORh1 AND TECTONIC EVENTS IN 1

COASTAL h1ARSH RECORDS OF THE SOUTHERN CASCADIA h1ARGIN by Curt D. Peterson and hf ark E. Darienzo, College of Oceanography Oregon State University, Oceanography ADhUN BLDG ICM Corvallis, OR 973315503 phone (503) 754 2759 Regional climatie mechanisms of potential marsh burial have been investigated on a preliminary i basis in endmember marsh systems of the southem Cascadia margin. Deposition by Good or

, storm surge provides means by which marsh burial could possibly occur independently of tectonic rubsidence in some estuaries of the southem Cas:adis htargin. In an effon to identify potential marsh burial by Good deposition, selected marsh sites were cored to 3 5 ra depth in Good plain and estuarine marshes of the Little Nestucca, Alsea and Siltetz Rivers in nonhem Oregon. Sand deposits are associated with riverine tidal channels of each bay, providing abundant sand supply l to the donstream estuarine marsh systems. However, vertical sequences of 3 5 buried marsh j horizons either 1) lack sand espping layers or 2) are overlain by sand capping layers that show no indication of increasing thickness with increasing proximity to river channel axes or with increasing distance upstream. A thourough search for effects of the regional 1964 Good (100 year flood event)in modem marsh deposits of Alsea Bay (probably 300 years in age) showed no evidence of sand accumulatica or marsh burial associated with this historic Cooi Preliminary study results indicate that sand and coarse silt fall out of suspension upstream of estuanne marshes during maximum riverine floods when ebb flow is backed up by constricted tidal inlets. Seaward

transpon of river sand resumes in inter tidal or sub. tidal channels with decreasing discharge levels

{ and ficod sand deposition rarely if ever reaches supra tidal marsh settings.

J Sediment transpon and deposition by oscillatory and tirb'. eurrent now during storm surge conditions represent other potential mechanisms by which supra tidal ma shes might be buried by excessive sedimentation. Fresh water diatom assemblages in buried marsh horizons of Netans Bay i

establish supra tidal settings for the upper marsh surfaces. Detailed examination of sand capping j layers above 3 5 buried marsh sequences in exposed settings of Netarts Bay and Alsea Bay in

! nonhern Oregon, demonstrate a lack of any intemal cross bedding associated with traction current 1 transpon under sub-critical flow conditions. hiarsh burial sequences in exposed reaches of both I

J bays include sand and silt laminations fining up to silt and clay laminations between successive marsh layers. The burial sequences, up to 100 cm thick, are too thick to be formed by single storm i

events and yet they lack alternating sand and mud layers in the upper pans of the sequences w hich would be expected from deposition by repetitive storm events overprinted on a condition of rising sea level. Neither Good nor storm deposition can account for observed marsh burial sequences in i southem Cascadia co: stal marshes without prior tectonic subsid nee of marsh horizons to low 1

inter tidal elevations. The observed fining up burial sequences represent tectonic subsidence j follow ed by venical adiment accretion and/or tectonic emergence to supra tidal elevations.

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U IMPLICATIONS OF LATE HOLOCENE SALT-MARSH STRATIGRAPHY FOR GREAT i

EARTHQUAKE RECURRENCE ALONG THE COAST OF SOUTH CENTRAL OREGON Alan R. Nelson, Branch of Geologic Risk Assessment, U.S. Geological i Survey, PO Box 25046, MS 966 Denver, Colorado 80225 i

- Repeated, great plate-interface earthqur,kes have been postulated for the Cucadia subduction zone in western Washington and Oregon. The best evidence of the coseismic subsidence to be

. espected near the coast during great earthquakes is found in southwestern Washington where many l

esposures record repeated episodes of submergence of late Halccene marshes. Atwater and others i

have used consistent stratigraphic relationships, "C ages, and plant macrofossils from sequences of interbedded marsh peats and intertidal muds to show that the 6 marsh peats buried in the lut 4000 years throughout southwestern Washington were submerged suddenly. The late Holocene estuarine i

i record in the central part of the subduction zone in Oregon is more difficult to interpret; there are sery few good esposures, and coring at some of the 12 marsh sites investigated hu produced

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evidence of a gradual rise of late Holocene sea level while sea level rise appears to be jerky at other sites. l At sites in the eastern arms of Coos Bay and in the Umpqua River estuary, one probable ,

buried marsh surface is found in the upper 1.5 m of mest cores overlying 4 6 m of uniform mud.

In some cores both upper and lower contacts of peaty units are gradational, but in most cores the thickest peat bed hu a fairly abrupt upper contact suggesting sudden submergence of a marsh. A 1 spruce root from this buried surface in Shinglehouse Slough wu dated at 340 "C yrBP. One

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interpretation of this type of marsh sequence is that sediment deposition rates in most tioal inlets have been hw during all but the tut few hundred years of the late Holocene and that for this i renon no evidence (buried marsh surfaces) of earlier sudden submergence events has been ,

preserved. Another interpretation is that no sudden changes in sea level have occurred.

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1 Coring in South inlet, an arm of the Sluslaw River estuary, shows that 4 m of fairly uniform peat overlies 4 m of mud. This type of marsh sequence suggests that late Holocene relative sea-lesel rise was gradual with no ab.1pt changes in the type or rate of sedimentation. Subtle, gradual ;

lithologic changes within the peat section suggest only small, gradual changes in sea lesel. Abrupt' lithologic changes found in some cores farther up the valley of South inlet probably record stream flood esents. i i

in contrut to the abose records, at two sites in South Slough in western Coos Bay, cores show 6-8 abruptly buried marsh surfaces that are 0.41.2 m apart. Eatensive coring in a small marsh }

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" along Winchester Creek rescaled up to 8 buried marsh surfaces in sections 5 8 m thick. The 4  ;

best developed surfaces can be correlated across the intet. The uppermost buried surface hu a modern "C age; it must have been buried by sedimentation following diking of the marsh. Lower surfaces date at 460 (2.2 m) and 2880 (2.8 m) "C yrBP, indicating hir,hly non-uniform l

sedimentation rates. A core from Day Creek,4 km to the north, had a similar sequence of 6

> l buried surfaces. Thest Gtes are near the sais of the South Slough syncline, and tilted marine '

terraces on the west limb of the syncline document continued late Pleistocene folding of this structure. Thus, the South Slough buried surfaces may record local Holocene coseismic faulting or folding rather than regional deformation of the central Oregon co4st during great plate interface earthquakes, Alternatisely, sudden slip on flexure slip faults within the syncline might also occur primarily u a response to Ir ge subduction zone earthquakes.

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i j ARCHAEOLOGICAL EVIDENCE OF HOLOCENE SUBMERGENCE ALONG i THE OREGON AND SOUTHERN WASHINGTON COAST i

Rick Minor

Heritage Research Associates, Inc. I 1997 Garden Avenue Eugene, Oregon 97403 2
Evidence from archaeological sites along the Oregon and  !

l southern Washington coast has the potential to contribute to the j study of Holocene earthquakes and subduction in the Pacific '

Northwest. Although the antiquity of human occupation (as  ;

demonstrated by radiocarbon dating) has been pushed back to circa i 8000 B P, the great majority of the archaeological sites so far  ;

) investigated in this coastal region were occupied within the last I

3000 years. A significant change in sea level is reflected in the

4 archaeological record around 3000 BP; this change apparently '

] coincided with the formation of extensive dune systems at this time.  !

Continuing relative rise in sea level is indicated in the occurrence of i j archaeological sites with submerged cultu.al deposits occupied in late i

) prehistoric and early historic times. Available archaeological

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l evidence shows that submergence has occurred along the Oregon and l

southern Washington coast in the late Holocene.  !

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1 Strain Accumulation in Western Washington and Southwestern British Columbia h!. Lisowski, J.C. Savage, W.H. Prescott ( All at USGS, hienlo Park, CA 94025), and H.

Dragert (Pacific Gececience Centre, P.O. Box 6000, Sidney, D.C. VSL 4B2)

Analysis of repeated measurements of geodetic networks in western Washington and southwestern British Columbia indicates active horizontal deformation along tne Casen-dia rubduction zone, with maximum contraction approximately parallel to the N71*E convergence direction of the Juan de Fuca plate given by the NUVEL-1 global plate model. Across the Strait of Juan de Fuca the avernge shear strrdn rate (dilatation not determined) is 0.19 0.06 prad/yr between 1802 and 1950 and 0.14 0.00 rad /yr be-tween 1942 and 1950 with maximum contraction in a N70*Et10' direction. Across the Puget Lowlands near Seattle the average principal strain rates (extension reckoned peni-tive) are 0.03 0.02 pstrain/yr and -0.03 & 0.01 pstrain/yr with maximum contraction in a NGO*E 7' direction. In the Olympic h!ountains the average principal strain rates are 0.07 0.00 pstrain/yr and -0.00 i 0.00 strain /yr with maximum contraction in a N55'E 0' direction. The orientation of the principal strains and the average rate of shear strain accumulation (i = is - 4 )2 in the networks can be roughly reproduced by a simple dislocation model of the Cascadia subduction zone that has the shallow portion of the plate interface locked to a pceition near the Washington coastline.

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16 I 1

A Finite Element Study of Strain and Uplift in the Pacific '

j Northwest i

H. J. MELOSH '

Lunar and Planetary lab and Department of Geosciences, University of Arizona, Tucson,

} AZ 85721  !

Considerable controversy has surrounded the interpretation of geodetic measurements in the Pacific Northwest. On the one hand Ando and Balazs (1979) argue that uplift in l

western Washington indicates that the Juan de Fuca plate is subducting aseismically. On i the other hand Lisowski et al. (1987) have measured a compressive strain rates ranging i

' between 0.03 and -0.11 pstrain/yr in the Puget trough and vicinity that they argue ,

< indicates accumulating stress on a locked fault. I constructed a finite element model of the 4

i, Juan de Fuca plate and western Washington to study both uplift and strain in a consistent i

manner. The two dimensional modelincorporates the known geophysical structure of the  :

j region, including the bend in the slab at about 40 km depth. Earthquake dislocations are introduced via an efficient split node technique; ster.dy slip is simulated by either internal l

) slide lines or presenbed split node displacement rates. A large number of models was i  !

investigated to determine the slip configuration that best fits both the uplift and horizontal l,  !

strain data.

) None of the runs with steady slip were able to reproduce both the observed uplift

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] and compressive strain. Steady slip models that fit the uplift data predict a compressive strain more than 20 times smaller than observed. Models in which the fault is presently j -

locked fit the data best, although they predict a rather complex pattern of uplift and strain.

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ne evidence for coseismic contal downwarping presented by Atwater (1987) requires that  !

the coseismic slip drops to zero in the vicinity of the coast. The fault may be slipping

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aseismically beneath the Puget Trough and fanhcr inland. Recarrence times on the order of j l 300 to 500 years seem to fit the data best. In the models investigtted so.far, the best fits

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1 are obtained in the last 95% of the canhquake cycle. Such models must be tested by further

) measurements of contemporary strain and uplift .

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REFERENCES i J {

Ando, M. and Balazs, E. I. (1979). J. Geophys. Res. 84, 3023 3028.

Atwater, B. (1987). Science 236,942 944  !

Loowski, M., Savage, J. C., Prescott, W. H., and Dragert, H. (1987). EOS 68, l 1240. l' l l

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GEOLOGIC COMPARISONS OF THE CASCADIA AND OTHER "SIMILAR" SUBDUCTION ZONES

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Davis, Gregory. A., Dept. Geol. Sci., Univ. So, Califomia. Los Angeles, CA 90089 0740 Geologic comparisons have been made between the Cascadia subduction ' zone (CSZ) and six other :ones deemed most similar to the CSZ in terms of youthful age (t) of

' subdacting crust and rate of plate convergence (v). e.g. Heaton and Hartzell (1986,  !

1987). Such comparisons neither prove nor disprove the potential for great  !

' carthquakes along the JdF/NA plate interface, but do provide a scientific basis for i concluding that subduction along the CSi may be occurring in a manner unlike that i of the seismogenic zones with which it has been compared. Specifically, geologic

) differences with the six zones support the conclusion that slow subduction of the hot. -

i topographically smooth, and sediment laden JdF plate may be occuring by stable '

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sliding rather than stick slip behavior. The table below summarizes t and v l

parameters of the 7 subduction zones, and cites reajer geologic differences of each  ;

i with the CSZ. Values for two independently derived measures of coupling between converging plates l

] Jarrard's (1986) strain class numbers based largely on geologic i

characteristics of the upper plate, and alpha (the seismic slip rate / relative total plate i convergence rate) j

-- are considerably lower fcr the CSZ than for s.ly of the other

  • zones. Such low values are consistent with geologic arguments that W, the downdip l j width of the seismogenic plate interface, may be very narrow or nonexistent for CSZ.

.Zcg (Jarrard. Plate ace Convereence Geolocie differences with Cascadia S. Z.  ;

'S6 strain class) it trench ratefernkr) t Cascadia 8 Ma 3. 4 .. .... Alpha = 0.3 (Kanamerl & Astiz, '85) for (3 4a) t = 1015 Ma; 0.0 to < 0.3 (this analysis). [

SW Japan k 17 24 2.93.7...... Major topographic asperities on subductirig i q (5) plate (abandoned spreading ridge); no j l active volcanic are; upper and lower plates l in possible subhorizontal contact beneath l Shikoku and Honshu; alpha = 0.88 S Ch;le 1.0. i 5 35 9............. Very high convergence rate; 1960 M 9.5 t (5) {

i event initiated in subduction zone where 1

oldest crust is being subducted; accretionary l

prism very narrow (<100 km); W may be an-  ;

omr'ously wide; alpha = I (Peterson/Seno '84) i l SW Columbia S 17 8...-.......... High convergence rate; zone is highly seis-  !

! (6) mogenic; major asperities (an abandoned -

i spreading ridge and Camegie Ridge ) on sub- l ducting plate make this a collisional zone;  ;

upper plate under strong compression; adj. (

Andean mts. very high (6 km); alpha = 0.85.

Rivera 8 10 2. 3 . . . . . . . . .. . . . . Topographic asperities (seamounts) on i

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Rivera plate; subducting plate has only thin sediment cover; trench is w ell defined with l only localized ponded sediments; narrow (?)

accretionary prism = wide (?) W; alpha = ?. I N Cocos 3 20 5 . 9 . . .. . . . . . . . . . . Zone is exceedingly seismogenle (42 M 7+

1 (6)

' events since 1900); subducting plate has only thin sediment cover; trench is well defined

] with only loeilized ponded sediments; very narrow accretionary prism (35 km) may j

produce anomalously wide W; alph a = 0.5.

Tierra del Fuego 8 10 2 .1 . .. . . . . . . ... . . . No active volcanic are opposite northem

(?)

part of zone where an active spreading ridge

' ridge was subducted during past 6 Ma; thermal characteristics cf this aseismic ses-

) ment presumably anomalous; alpha = ?.

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. 18 .

'Ihe Cascadia Subisction Zone, i

Sme unresolved problems.

Brian T.R. Iewis School of Oceanography University of Washington  ;

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j The recurrence tine of subduction eart&Taakes depends on at least the follcwing. '1he strength of the driving force, the gewetry of the subduction zone, tertperatures in the subdu:: tit.n zone, and asperities on the ,

thrust fault.

j b If the driving force is the negative bouyancy of the sinking plate then l

, to ascertain its strength, we nust know the history of subriuction in the ,

i area, the gemetry, terTerature and densities in the sinking slab, and the i' viscosity unknown.

of the meditan into which it is sinking. All of these are largely 4

4 The gecretry of the subduction rene is inportant because the frictional .

f forces resisting sutdoction depend on the mass of the overlying mterial, l that is the gewetry. 'Ihis gemetry is largely unknown, but probably not l i l constant along the Oregon / Washington margin and this could influence the 4

recurren:e tine.  !

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TerTeratures alcog the thrust zene are clearly important in determining whether the material is in the brittle or plastic regire, and therefore ,

j whether subiaction is aseismic or not. Redirent heat flow values predict l

' very high twTeratures based on conductive heat ficw. If advection of heat by porous flow is significant the taTeratures could be nuch lower. i t

AsTerities can alter the recurrence tire significantly. He know very l little ator. asperities in general, and little about the Cascadia area 1

specifically. Are the Oly@ic nountains an asperity ? l I

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Paleo3elsmicity in the Puget Sound Area as recorded in sediments from Lake Washington Robert KaIlin School of Oceanography, University of Washington WB 10, Seattle, WA 98195 Sally Abel.ta, Department of Zoology, University of Washington NJ 15, Seattle, WA 98195 Estella Leopold, Quatemary Research Center, University of Washington KB 15. Seattle, WA 98195 Patrick Williams, Lamont Doheny Geological Observatory, Palisades NY 109M In tectonleally active regions, lacustrine sediments potentially can provide a proxy record of paleoseismicity, because of disturbances in mass Oux and sediment texture due to mass wasting and gravity Cow processes during and after strong ground motion. In 1929, a moderate canhquake on the Grand Banks caused major submarine slum sing and large scale turbidity currents on the Atlantic continental margin (Heeren and Swing,1952). In Kenal Lake, Alaska, the great canhquake of 19M initiated seiching and delta destabilization, resulting in massive slides, debris Dows, and turbidites (McCulloch,1966).

Similarly, major canhquake activity in western Washington might be expected to cause slides and slumping on the steep sides of fiords, estuaries and lakes of the Puget Lowlands. Major increases in sediment supply to lake bottoms might also occur due to seismically induced landslides in the catchment basins as well as seiching and delta destabilization along lake margins. The effects of these dieuptions would be to introduce changes in sediment type, concentration, and texture which are in disequilibrium with ambient sedimentation conditions.

In this research, we have been evaluating the history of seismic activity in the Puget Sound region as recorded by turbidites and other rapid changes in mass Oux in sediments from Lake Washington. Fonning the western boundary of Seattle, the Lake Washington basin is underlain by a thick sequence of blue glacial clay ofindeterminate thickness, overlain by 7 17 m of Holocene limnic peat or gyttja less than 13,400 yrs in age (Gould and Budinger,1958; Leopold et al,19532). The limnic section contains the Mazama ash (6S50 >Ts BP) and distinctive varved layers which serve as key marker beds. Ee lake contains three sunken forests with standing trees that were presumably emplaced by massive block slides. Two of the submerged forests, lying at the nonh and southwest ends of the lake, have been radiocarbon dated at 1160 yrs BP and 2750 ps BP, respectinly. These events are synchronous with postulated seismic acuvity along the Washington coast w hich caused rapid submergence of marsh peats (Atwater,1987). The younger event is also coeval with faulting reponed on the Olympic Peninsula (Wilson, et al.,1979).

X radiographs and whole core magnetic susceptibility (K) profiles of 10 piston cores and 14 grasity cores from the lake have been used to delineate the spatial and te.nporal vanatons in sedimentation. Since the sedimen:s' magnetic properties are very sensitive tu changes in the concentration and grain size of magnetic minerals, susceptibility rneasurements are an extremely useful remote sensing technique for intercore correlanon and rapidly identifying lithologic and textural changes. The K profiles show remarkably consistent downcore pattems which can be readily correlated throughout the lake. A characteristic series of magnetic peaks is observed that correspond exactly with ash horizons and X ray opaque intervals containing tenigenous luutes, presumably turbidites.

From the size and lithologies of the turbidites and their areal distribution throughout the lake, we conclude that these deposits are riot due to Gooding or local slides but must

. . o represent basinwide disruptions, wthaps related to seismic acdvity. Preliminary results indicate thst one such layer may save been deposited in responte to the 1160 yr BP slide that aroduced the sunken forest in the lake's north end. In post.Mazama time (<6850 yrs LP), more than fourteen such events are observed, tentadvely suggesting recurrance intervals of less thsn 500 years. The significance of these sedimentary disturbances and their possible relationship to palcoseismicity and paleoclimate will be discussed.

Gould. H.R. and Budinger, T.F.,1958. Control of sedimentation and bottom configuration by convection currents, Lake Washirigton, Washington, Journ. Mar. Res., 17:183 197.

Heezen B.C. and Ewing, M. Turbidity currents and submarine slumps armi the 1929 Grand Banks earthquake, Amer.Joum Sci.,250:849 873.

Leopold, E.B., Nickman, R., Hedges, J.I., and Ertel, J.R ,1982. Pollen and lignin records of Late Quatemary vegetation, Science,218:13051307.

McCulloch, D.S.,1966. Slide induced waves, seiching and ground fracturing caused by the Earthquake of March 27,1964 at Kenal Lake, Alaska, U.S.O.S. Prof. Paper 543B.

I i An abstract for the Symposium on "Holocene Subduction in the Pacific Northwest"  !

l May 6 8,1988, sponsored by the Quaternary Research Center, University of Washington  !

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. . 21 AESTRACT POSTGLACIAL TILTING OF LAKE WASHINGTON: SEDIMENTARY AND POLLEN EVIDENCE Estella P. Leopold Apparent tilting of the Lale Washington basin in postglacial time is based on the difference in elevation of subsurface volcanic ash deposited at the la6e outlet (at Renton) and that at Mercer ulough marsh in lacustrine sediment. The sites are 10 km apart, a r. d the difference elevations of the tephca is 5 m.

Mullineaux's (1970) work established that the building of a fan (

36 m an thickness) bw the Cedar River followed the deposition of n.artne shell beds that F. art a period about 13,000 wears ago when the

) La6 e Washington basin was connected to Puget Scand. A volcanic ash lawer occurring at about midsection is 00 meters above the highest marine shell beds. Intermittent gravel, sands and peat suggest that the fan was deposited an shallow water. He suggested the ash was of mid Holocene age and probably represented the Marama tephra.

At Mercer Slough the position of the Ma:ama ash (7000 yr E . P. ) at 13 m below the present marsh surface and 7 m below present sealeveli at occurs in lacustrine silts (Leopold 1986). In contrast the purported Ma:ama ash an the Renton enres as about 10 m below present sealevel (Mullaneau: 1970), some 5 m lower than the Mercer tephra. A line connecting the ash layers at the two points slopes southward 5m in 10 km. Since the late outlet at the time of deposition could not have been lower than the late se d a r.e n t at Mercer S l ou r, h , the ba s a r, has proballw endured defermatA n.

Accor di ng to Thorson's flGEl) net postelacial deformation curve the Mercer Slough site 5,hould heve been deformed 10 m more than that at Aenton. The position of the subsurface volcanic ashes accounts fcr half of that amplstude. This cculd mean that half of this deformation t ool place in the last 7000 years.

At Mer cer Slcugh the occurrence of shallow weter and marsh peets began a b e .. t 2m ebove the Ma:ama ash and contane to the butface.

These seJimente and their abundant pollen of marEh planth anC1catt thet it. . lak e level w a r, near thc peat surface at Mercer Slough throu". mort of the late Holocene; the data Indicatt thct the l e s te l t-c, ate Washargtor, wcs rarang slowls curing mac and Icte Halcc(nr t i m e~ . During that tir+ alluviation cf the fan cf th( Cedar F 14 ee r built to its present It<cl.

L e o s.o l d . E.F. IE. Per t n r.en ce- of La6 e Wa t. h i n g t o n to postgIacac1 rase an sealevel. Geol. So:. A., Ar.nual Meeting, Abst.

Thor son, R.M., it 1. Isostati; effects cf the lawt glactation in t he Fuget Lowland, Weth .,g t on. U.S. Geological S u r e ,- nrer-Fale Report El-??O.

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Deformed Pleistocene Sediments of the Tacmoa Narrows, Washington Albert A. Eggers, Barry Goldstein: Geology Department, University of Puget Sound.

Tacoma, Washington 98416 Detotled outcrop meoping (l 500) of pre-Fraser age () 15 ke) glzle! sodiments exposed in wwe-cut benches et Pt. Evans and Pt Deffence of the income Narrows shows thet these rocks were mildly to strongly oeformed during the Pleistocene. The rocks at Pt Defiance, unconformebly underhvg Yaston Drift, crop out along 2 km of wave-cut bench and consist of rhythmicelty tie 0deo dropstone-beering silt units which are mildly folded (500 maximum dips) and multiply faulted (displacements usually of a few em) Orientation date on stuctures et Pt.

Defiance suggests aestwest compression. Th rocks et Pt Evans, unconformabtv 'e.oerlying Vashon D*tft and cropping tot along more than 1 km of wwe-cut bench, constst of ,nterbackso3 diamicton tieJs and drcetone-bearing slit units which are complex 1y and intenstvely folded and faulted. Deformation of the Pt. Evans rocks is melenge-like and is character 12nd ty closed and opt fol&, vertical at worturned beds, and high-angle faults. Continutty of the sectments along the wwe-cut bench for more than r, kilometer indimtes the oeformation was not produced by landslidng. (rtentation dote oii Pt. Evans structures shows complex deformation, and no single axis of compression. Comoertson of orientation dete on Pleistocene structures et Pt Defiance, Pt. Evans, and et other localttles within the Pugrt Lowland to Yeshon ice flow erections and to the or1entation of Tertlery structures in and around the Puget Lowlan1 Indicates the structures in Pleistocene rocks et Pt. Evans and Pt. Def tencc, and perhaps et other localities in the Puget Lowland, might hwe been producsd tectonically, and not ty gleclotectonte and/or syndeposttional prcoesses The literature, unpubitsNes data, and personal communimtions reveel the existance cf numerous other sites where deformes Quaternry sediments ar* Womed in the Puget Lowlerd Although no systemttic region 41 slu@ criticelty Weluating these structures has been cbne, structures obserYed in Pleistocene sediments of tha Pupt Lowlard era commonk attt'lbuted to glaciotectonic/rrndepositionel prmmm We suggest that observ6d deformation in the Quaternarv sediments of the 9uget Lowland cannot be assumed .

as gleclotectonte/syndepositionel tr, origin, that many of these structures might be of tectonic

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origin, and that their stu@ might prwide veludte insight into the seismicity of the Puget Lowland ,

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