A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in the central Cascadia subduction zone. Eight cutbank sites are reported that show evidence of large-scale fluidization features (≥10 cm width) including clastic sand dikes and intruded sand sills in Holocene overbank mud deposits. The targeted alluvial sequences, and hosted paleoliquefaction records, are of late Holocene age, as based on radiocarbon dating, flood silt thickness (≤4 m thickness), and minimal consolidation of dike sand (~1.5 ± 0.5 kg·cm-2 unconfined compressive strength). Two of the paleoliquefaction sites, which are separated by 150 km distance, overlap in age (175 - 500 yr BP) with the last great megathrust rupture (Mw 8.5 - 9.0) in the Cascadia margin, dated at AD 1700. The scarcity of exposed late Holocene paleoliquefaction sites in the Willamette River cutbanks motivated subsurface searches for thick basal sand deposits and overlying fluidization features, using floodplain geomorphological analyses, ground penetrating radar, and remote pole-camera scans of deep trench walls (3 - 4 m depth). The onset of large-scale fluidization features occurred in overbank mud deposits (2 - 3 m thickness) above unconsolidated sand bodies (≥2 m thickness) with unconfined compressive strengths of ~1.5 ± 0.5 kg·cm-2. We recommend geomorphically-targeted subsurface explorations rather than traditional cutbank searches for evidence of coseismic paleoliquefaction in high-gradient river valley systems.
Field investigations were undertaken in this study to test previous field reports, which stated that large-scale paleoliquefaction was absent in the latest Holocene deposits in the Willamette forearc valley of the central Casca- dia convergent margin [
In this study two reaches of the Willamette River were initially searched for evidence of large-scale fluidization features (≥10 cm width) in late Pleistocene deposits that are exposed in modern cutbanks. Following documentation of large-scale clastic sand dikes and sills in the late Pleistocene deposits, more focused searches for similar features were conducted in late Holocene deposits. Late Holocene paleoliquefaction sites were present but rarely exposed in the artificially modified riverbanks. New approaches were developed to search for subsurface evidence of large-scale paleoliquefaction. High-resolution topographic analyses, ground penetrating radar, and deep mechanical trenching were successfully used to find and test thicker sand beds and overlying overbank mud deposits for intruded clastic dikes and sills. An argument is made for subsurface investigations in high-gra- dient river valleys where bank stabilization efforts have preferentially covered rare sandy cutbank sequences. These results should have relevance to other high-gradient forearc valley settings where cutbank surveys for paleoliquefaction evidence have yielded negative or uncertain results.
Historic earthquakes in the Puget forearc valley of western Washington (
Location of Willamette and Puget forearc valleys (bold ellipses) are shown in the central Cascadia margin. Coseismic liquefaction (solid stars) occurred in historic time (1949, 1965, 2001) in the Puget Valley. Paleoliquefaction sites in late Pleistocene coastal deposits (solid squares), latest Holocene coastal sites (solid polygons, some numbered) and lower Columbia River sites (solid circles, lettered) are geo-referenced in Table 1. Unidentified coastal sites are from C. Peterson, unpublished data (2000-2010)
. Latest Holocene paleoliquefaction sites in the central Cascadia coast and in the lower Columbia River valley.
Site | UTM 10 N (m) northing, easting | Setting | Hostlithology | Feature type | Width (cm) | Ref. No. |
---|---|---|---|---|---|---|
Coastal Sites | ||||||
1 | 5357760n379580e | bw | pm | cs, cd | 20 | [7] |
2 | 5219160n411820e | cb | psm | cs, sv | 20 | [22] |
3 | 5096270n429280e | bw | psm | cs, cd | 20 | [8] |
4 | 5083130n425620e | bw | pm | cd, cs | 30 | [9] |
5 | 4778040n387280e | cb | pm | cd, cs | 20 | [10] |
Columbia River Sites | ||||||
A | 5119080n454000e | cb | m | cd, cs* | 30 | [13] |
B | 5121450n457650e | cb | m | cd | 15 | [13] |
C | 5119120n467670e | cb | m | cd, cs* | 15 | [13] |
D | 5109700n479100e | cb | m | cd | 8 | [13] |
E | 5092100n511900e | cb | sm | cd | 10 | [13] |
F | 5045350n541300e | cb | sm | cd, cs | 6 | [13] |
G | 5045600n547000e | cb | sm | cd, cs | 5 | [13] |
H | 5044150n553350e | cb | ms | cd, cs | 6 | [13] |
I | 5052100n577000e | cb | sm | cd | 5 | [13] |
Notes: UTM 10T coordinates (m) NAD83. Coastal sites (numbers), Columbia River sites (letters). Settings include back-barrier wetland (bw), tidal marsh (tm), river cut bank (cb), floodplain (fp). Hosting deposit lithologies (Host) include unconsoldidated peat (p), mud (m), sand (s). Fluidization features include clastic dikes (cd) clastic sills (cs), sand volcano (sv). All feature widths are from trenches or cutbanks, except some deep clastic sills in vibracore (*). Feature width (cm) is the maximum observed for the first feature type (cd or sc) listed under the fluidization column. See
1949 and 6.5 in 1965. These intraslab or descending plate sources caused widespread liquefaction at ≤70 km distance from the epicenter [
Work on Cascadia subduction zone earthquakes in the mid-1980s revealed coastal coseismic subsidence events and associated paleotsunamis that recurred every several hundred years [
Coseismic fluidization features in late Holocene deposits have been widely observed during investigations of coseismic subsidence and paleotsunami inundation along the coast (
To address questions about the landward reach of strong shaking from earthquakes in the central Cascadia margin a search for paleoliquefaction evidence was initiated in the lower Columbia River valley (
This investigation of large-scale fluidization features, including clastic sand dikes and intruded sand sills in the Willamette Valley was initiated with a preliminary search of late Pleistocene deposits in modern cutbanks of the Willamette River. The late Pleistocene deposits were examined for paleoliquefaction evidence by floating a northern section and a southern section of the Willamette River (
After confirming the presence of large-sale paleoliquefaction sites in late Pleistocene deposits, further searches targeted late Holocene deposits in the Willamette River cutbanks. Exposures of Holocene deposits in cutbanks were predicted from geomorphic analyses of low terrace surfaces (NED 1/3 arc sec DEM) [
Paleoliquefaction sites that were identified in this study of the Willamette forearc valley include late Pleistocene (lettered) and late Holocene (numbered) sites along the Willamette River. Selected Willamette River cutbanks were examined over a distance of about 150 river kilometers. See Figure 1 for regional tectonic context of the Willamette forearc valley
Late Holocene cutbank deposits with large-scale clastic dikes and sills (≥10 cm widths) were photographed, geo-referenced (12 channel rtGPSepe ± 5 m), and measured for fluidization feature extents (along bank distances) and maximum widths (1 cm scaled rule). Deposit grain size ranges were estimated in the field from CANAM™ grain-size cards: vcU (1410 - 2000 µm), vcL (1000 - 1400 µm), cU (710 - 1000 µm), cL (500 - 710 µm), mU (350 - 500 µm), mL (250 - 350 µm), and fU (177 - 250 µm).
Due to the rarity of late Holocene sand deposits that were observed in exposed cutbank sequences the search methodologies were changed to subsurface investigations of abandoned or prograded low terraces for evidence of late Holocene paleoliquefaction. The subsurface methods are described in detail in a later section of this paper, but are also briefly outlined here. Late Holocene sites with the potential for large-scale paleoliquefaction (subsurface sand beds ≥2 m thickness) were identified using 1) ground penetrating radar (GPR) imaging (50 MHz 1000v Pro Ekko™ system) and 2) mechanical deep trenching (5 m bucket reach) with digital trench-wall scans (Go-Pro™ pole-camera). Trench walls showing digital-scan evidence of large-scale fluidization features, including sand intrusion pockets, clastic sand dikes and clastic sand sills with intruded sill contact boundaries [
Late Pleistocene paleoliquefaction sites were identified in modern cutbank exposures in the lower and upper sections of the Willamette River (
Part A. Large-scale sand volcano and feeder dike in late Pleistocene paleoliquefaction Site A. The sand volcano is truncated by overlying gravel deposits demonstrating syndepositional paleoliquefaction. Part B. Complex fluidization sequence including clastic gravel dike, clastic sand sill, and convoluted sand beds in late Pleistocene paleoliquefaction Site B. Site locations are shown in Figure 2 and are geo-referenced in Table 2
. Paleoliquefaction sites in the Willamette forearc valley, Oregon.
Site | UTM-10 N (m) northing, easting | Site length (m) | Feature types | Feature depths x.x - x.x | Dike width (cm) | Sill width (cm) |
---|---|---|---|---|---|---|
Late Pleistocene | ||||||
A | 5000930n499910e | 50 | sv, cd, cs | 7.0 - 9.0 | 30 | 100 |
B | 5000850n499850e | 50 | cd, cb | 8.5 - 10.0 | 30 | - |
C | 4905000n483720e | 50 | sv, cd | 6.0 - 7.5 | 20 | 60 |
D | 4903540n485290e | 100 | cs, cd | 3.5 - 5.5 | 3-7 | 15 |
Late Holocene | ||||||
1 | 5074570n515610e | 100 | sb, cd | 2.5 - 4.5 | 30 | - |
2 | 5011430n508240e | 30 | cs, cd | 2.5 - 5.5 | 25 | 35 |
3 | 4992460n495510e | 20 | cs, cd | 3.0 - 5.5 | 15 | 30 |
4 | 4972640n489862e | 30 | cs, cd | 2.5 - 5.5 | 30 | 30 |
5 | 4952390n487990e | 20 | cs, cd | 4.5 - 7.0 | 20 | 35 |
6 | 4944010n494190e | 20 | cs, cd | 2.0 - 5.5 | 30 | 60 |
7 | 4934150n479572e | 15 | cs, cd | 0.5 - 4.5 | 20 | 35 |
8 | 4927000n482810e | 30 | cd, cs | 3.5 - 4.5 | 30 | 20 |
9 | 4904320n484430e | 50 | cs, cd | 3.0 - 4.0 | 20 | 30 |
Notes: UTM-10N coordinates (m) NAD83. Willamette River bank sites of late Pleistocene age (letters) and late Holocene age (numbers). Exposure length (m) is an estimate of contiguous riverbank distance containing fluidization features. Exposure depth (top-bottom) is the measured depth (m) of fluidization features relative to the terrace surface at the bank edge. Feature types include clastic sand dikes (cd), clastic sand sills (cs), sand boils (sb), convolute sand beds (cb). Maximum dike widths and maximum sill widths are in centimeters (cm).
The occurrences of large-scale clastic sand dikes and sills in the late Pleistocene deposits of the Willamette Valley were restricted to overbank mud deposits that were located directly above relatively thick sand beds (3 - 4 m in thickness). Such thick sand beds were rarely observed in the exposed cutbanks. The thicker sand beds occurred above basal gravels [
Cutbank searches for late Holocene paleoliquefaction sites in the Willamette forearc valley were started in the Columbia River at Sauvie Island (Site 1) in the Portland Basin (
Part A. Late Pleistocene deposits and small-scale fluidization features, (dikes ≤ 10 cm width) and sills (dotted lines), above a thin sand bed (~1.5 m thick) overlying basal gravel beds (dashed line) in a modern cutbank of the Willamette River at late Pleistocene paleoliquefaction Site D. Glacial Lake Missoula Flood rhythmites (GLMF silts) overlie the Post-Diamond Hill paleosol (dark Pre-GLMF soil). No fluidization features were observed in the GLMF silts. Part B. Late Holocene sequence of overbank mud overlying uncemented river gravel in a cutbank exposure (UTM-10N 4936940n, 482580e), located midway between late Holocene paleoliquefaction Sites 6 and 7. The rarity of thick sand beds (>2 m thickness) precluded paleoliquefraction and associated large-scale fluidization features at this site and in the great majority of other cutbank exposures of late Holocene sequences
Morpho-stratigraphic models (A-D) are shown for Willamette River cutbank sequences. Thin sand beds (≤2 m thickness) limit the abundance of paleoliquefaction sites and the heights of associated fluidization features in late Pleistocene deposits (A). The lack of thick sand beds between overlying mud deposits and underlying river gravels precludes paleoliquefaction evidence in most exposed late Holocene cutbank sections (B). Though rare in cutbank exposures, some late Holocene sequences do provide sufficient sand bed thickness (≥2 m) to permit coseismic paleoliquefaction and large-scale fluidization features (C and D). Artificial stabilization of the erodible sandy cutbanks in the unconsolidated late Holocene deposits (C and D) preferentially covered the sites with the greatest potential for fluidization evidence of paleoliquefaction
Part A. Truncated sand boil exposed in eroding northeast shoreline of Sauvie Island, Columbia River, OR near late Holocene paleoliquefaction Site 1. Part B. Cutbank exposure showing the stratigraphic relations between 1) truncated sand boils, 2) underlying feeder dikes (30 cm in width) and 3) overlying overbank mud deposits in Site 1, radiocarbon dated (RC) to 2180 - 2305 calibrated years BP (Table 3)
. Radiocarbon dates from latest Holocene paleoliquefaction sites.
Site/sample | Sample/depth (m) | Radiocarbon age ± 1 syr BP | Calibrated 2 syr BP | Beta # |
---|---|---|---|---|
CRSR1 | Wood/0.6 | 250 ± 70 | 134 - 489 | B76446 |
CRSR3 | Wood/0.7 | 410 ± 70 | 308 - 537 | B241088 |
Site 1/Sauvie1 | Peat/1.3 | 2280 ± 30 | 2180 - 2305 | B373489 |
Site 5/Luck1 | Char/4.5 | 3840 ± 30 | 4150 - 4405 | B373490 |
Site 7/MT5_185 | Char/1.85 | 490 ± 30 | 510 - 620 | B329859 |
Site 7/MT8_320 | Peat/3.20 | 510 ± 30 | 510 - 550 | B329860 |
Site 7/MT11_70 | Wood/0.70 | 510 ± 30 | 510 - 550 | B329861 |
Site 7/MT11_133 | Char/1.33 | 520 ± 30 | 510 - 620 | B328685 |
Notes: Sample site coordinates are shown in
located just above the vented sand layers at the top of the sand boils. The sampled yielded a radiocarbon date of 2180 - 2305 (
Cutbank searches were extended south of the Portland Basin along the Willamette River where floodplain analyses indicated active channel erosion of late Holocene accretionary bank sequences (
Large-scale clastic dikes and sills were generally not discovered in unexcavated Holocene cutbanks in the lower reaches of the Willamette River, due to the presence of mud drapes, artificial fill or stone revetment or rip- rap that covered the unstable cutbanks. Shallow excavations (0.25 - 1.0 m depth) in covered cutbanks revealed clastic sills (10 - 30 cm thick) and dikes (10 - 15 cm thick) at Site 3 (
Part A. Covered cutbank at Site 2 showing stratigraphic levels of late Holocene clastic dikes and sills. See Figure 8 for close-up photos of clastic dike (Dike) and clastic sill (Sill) in the middle and upper levels of the cutbank. Part B. Close-up photo of basal clastic gravelly-sand dike and clastic sand sill intruding into mud overbank deposits (above) and iron-hydroxide (Fe) coated sand beds (below) at 4.5 - 5.5 m subsurface depth
Part A. Clastic sand dike feeding clastic sand sill in middle level (3.0 m subsurface depth) of cutbank in Site 2. Part B. Clastic sand sill intruded into upper level (2.7 m depth) of overbank mud deposits in Site 2
The fluidization features in Site 5 terminated just below a pair of soil A-horizon paleosols at subsurface depths of 4.0 - 4.5 m. A radiocarbon sample from 4.5 m depth in Site 5 yielded an age of 4150 - 4405 (
Lower cutbank heights (3 - 4 m above low water level) in the upper reaches of the Willamette River reduced the potential for observing substantial vertical extents of clastic dikes and stacked clastic sills. Though large- scale fluidization features were identified in the upper reaches of the Willamette River, as exemplified in paleoliquefaction Sites 8 and 9 (
A test locality for subsurface paleoliquefaction investigations (Site 7) became available in conjunction with an
Part A. Clastic sand sill (25 cm thickness) in late Holocene paleoliquefaction in Site 5. The shallowest dikes in Site 5 reached charcoal bearing overbank mud deposits (4.5 m depth), radiocarbon dated at 4150 - 4405 (Table 3). Part B. Clastic sand dike and adjoining sill (60 cm width) in late Holocene paleoliquefaction Site 6. Intruded sand in the thick sill yielded unconfined compressive strengths of 0.75 - 1.25 kg∙cm−2
archaeological survey for a municipal wastewater reclamation project on the east bank of the Willamette River in Corvallis, Oregon (
Mechanical trenching (4 m depth subsurface) and down-trench pole-camera scans documented 1) cross-sec- tion lithology, 2) absence or presence of paleosols, and 3) absence or presence of fluidization features. Fluidiza-
tion features were widely developed at Site 7 in mechanical trench wall exposures in MT4, MT5, MT10 and MT11. The large-scale fluidization features (>10 cm width) included sand intrusion pockets, clastic sand dikes and clastic sand sills. Clastic dikes and sills, up to 15 cm in width, were intruded into overbank mud deposits from 3.4 m depth to 0.4 m depth, as recorded in continuous scans of deep mechanical trench walls in MT10 (
The trenches with the largest fluidization features (10 - 20 cm in MT5 and 20 - 30 cm width in MT11) were lengthened and widened [
The paleoliquefaction locality at Site 7 provided an opportunity to test the abundance and scale of fluidization features against underlying sand bed thickness for a discrete event in a single morphostratigraphic terrace setting (
Relations between basal sand bed thickness and the abundance and scale of fluidization features that were intruded into overlying overbank mud deposits in the latest Holocene paleoliquefaction Site 7 (
. Summary of cutbank and mechanical trench lithology and fluidization features in latest Holocene paleoliquefaction Site 7.
CB/trench | Mud depth (m) | Sand depth (m) | Gravel depth (m) | Dike/sill abundance | Dike max. width (cm) |
---|---|---|---|---|---|
CB16 | 0.0 - 3.5 | 3.5 - 4.0 | 4.0 - 4.5 | None | 0 |
CB17 | 0.0 - 2.0 | 2.0 - 4.0 | >4.0 | Rare | 2 |
CB18 | 0.0 - 3.0 | 3.0 - 4.0 | >4.0 | None | 0 |
CB19 | 0.0 - 3.0 | 3.0 - 4.0 | 4.0 - 5.0 | None | 0 |
MT1 | 0.0 - 3.7 | 3.7 - 4.0 | > 4.0 | None | 0 |
MT2 | 0.0 - 2.3 | - | 2.3 - 3.6 | None | 0 |
MT3 | 0.0 - 1.6 | 1.6 - 2.7 | 2.7 - 2.9 | None | 0 |
MT4 | 0.0 - 1.4 | 1.4 - 3.7 | >3.7 | Common | 5 |
MT5 | 0.0 - 3.0 | 3.0 - 5.0 | >5.0 | Common | 10 |
MT6 | 0.0 - 2.8 | 2.8 - 3.1 | 3.1 - 3.3 | None | 0 |
MT7 | 0.0 - 2.8 | 2.3 - 4.0 | >4.0 | None | 0 |
MT8 | 0.0 - 2.9 | 2.9 - 3.2 | 3.2 - 3.5 | None | 0 |
MT9 | 0.0 - 3.2 | 3.2 - 3.4 | 4.4 - 3.7 | None | 0 |
MT10 | 0.0 - 3.2 | 3.2 - 5.0 | >5.0 | Common | 10 |
MT11 | 0.0 - 1.7 | 1.7 - 5.5 | >5.5 | Abundant | 20 |
Notes: Mud and sand depths recorded in trench walls (0 - 4.0 m depth). Depth to gravel, when below trench bottom (4.0 m depth) estimated from GPR profiles. Fluidization feature abundance (cutbank of trench wall surface area) none (0%), rare (1% - 4%), common (5% - 25%), abundant (>25%). Maximum dike width measured with 1 cm scaled rule in trench wall. Data are from Minor and Peterson [
dization features (5% - 25% surface area coverage in trench walls) and 2) clastic sand dike widths ≥10 cm. In one trench (MT11) the most abundant fluidization features and the widest clastic sand dikes (20 cm width) occurred directly above sand beds of 3 - 4 m thickness. The trench walls in MT11 showed a silt overburden thickness of 1.7 m. These values are comparable with similar deposit thicknesses reported for paleoliquefaction sites that resulted in ground damage from historic earthquakes [
The results from the latest Holocene paleoliquefaction Site 7 (
The maximum widths of clastic sand dikes are plotted against position (north-south) in the Willamette forearc valley for both late Pleistocene and late Holocene paleoliquefaction sites (
traced along a cutbank exposure (600 m in length) at Site 1 (
Relative to the high-gradient Willamette Valley the New Madrid seismic zone, USA, produced more abundant sand blows in Mississippi River localities, up to ~100 km distances from upperplate earthquake epicenters (Mb 7.0 - 7.4 in 1811 - 1812) [
The rare occurrences of fluidization features observed in the late Holocene cutbanks in the Willamette Valley relative to some other seismic regions could reflect 1) lower paleoseismic energies, 2) a lack of thick basal sand beds in the high-gradient river system, and/or 3) preferential cover of sandy accretionary bank deposits in the artificially-stabilized river cutbanks. Geotechnical testing of identified latest Holocene paleoliquefaction localities, such as at Sites 1, 2, 5, 7 and 9 (
An important finding of this study is the success and efficiency of subsurface exploration for latest Holocene paleoliquefaction in the Willamette forearc valley. Thick sand deposits (>3 m thickness), above which evidence of large-scale fluidization might be found, are rarely exposed in cutbanks in the Willamette River (total search distance 150 river kilometers). In addition to the general lack of thick sand bed deposition in the high-gradient Willamette River system two historic factors contributed to the rarity of cutbank exposures of late Holocene sandy accretionary banks. These artificial factors include 1) extensive shortening or straightening of the historic Willamette River course and 2) selective bank protection or bank-fill in the least-resistant (sandy) riverbank sections [
To overcome the bias of covered unconsolidated sandy riverbanks, subsurface investigations were conducted to search for evidence of paleoliquefaction in flood plain settings that were not excavated or covered by historic cutbank modifications (
Paleoseismic events represented in the Willamette forearc valley could have originated from seismic sources in the upperplate, the descending lowerplate, and/or the interpolate megathust, as have been documented in western Washington (
The ages of late Holocene paleoliquefaction in the Willamette Valley are constrained by radiocarbon dating of youngest hosting deposits at four paleoliquefaction sites including sites in the Columbia River, Site G (
The youngest late Holocene paleoliquefaction event dated in the Willamette Valley at Site 7 post-dates the age of the hosting deposits (~500 yr BP) but predates the historic record of seismicity in western Oregon. Specifically, no large earthquakes were reported in northwest Oregon during the last 175 years [
Regardless of the potential coseismic sources in northwestern Oregon the widespread presence of large-scale fluidization features in late Holocene deposits of the Willamette River (
Large-scale fluidization features including clastic sand dikes and intruded sand sills are present in late Holocene flood plain deposits in the Willamette Valley in northwestern Oregon. At least two latest Holocene paleoliquefaction sites, separated by 150 km distance, overlap in age (circa AD 1500 - 1840) with the last great earthquake (Mw 8.5 - 9.0) in the Cascadia subduction zone at AD 1700. The rarity of thick sand beds (>3 m thickness) in the high-gradient gravel-dominated Willamette River, together with artificial cover of the erosion-prone sandy accretionary banks, has resulted in very rare cutbank exposures of late Holocene paleoliquefaction sites. Subsurface explorations for paleoliquefaction evidence successfully utilized floodplain geomorphic analyses, ground penetrating radar surveys, and deep mechanical trenching to find and document subsurface evidence of latest Holocene paleoliquefaction. The subsurface paleoliquefaction sites are readily accessible to geotechnical testing for paleoliquefaction susceptibility and estimation of paleoseismic ground accelerations in the Willamette forearc valley in the central Cascadia margin.
Subsurface investigations and radiocarbon dating at Site 7 were carried out during a geoarchaeological survey, for a proposed wastewater reclamation plant by Heritage Research Associates under contract to Kennedy/Jenks Consultants for the City of Corvallis, Oregon. Galen Peterson assisted with ground penetrating radar (GPR) surveys and performed the trench wall scans with the remote pole-camera system. Ron Bush performed the GPS surveys for the GPR profile endpoints and for the mechanical trenches using available LiDAR for spot elevation control. John Olson performed the deep trench excavations and assisted with digital pole-cameral scans of trench walls. Vanessa Van Der Borg assisted with trench wall paleoliquefaction sketches and sampling for radiocarbon analysis.