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Suggested citation:

Baitis, K.E., G.J. Retallack, and M.E. James. 2015. Gray Clay (Malpass Clay) in the West Eugene Wetlands 

and the Willamette Valley is Weathered and Redeposited Ash from Mount Mazama. U.S. Department of 

the Interior, Bureau of Land Management, National Operations Center, Denver, CO.

BLM/OC/ST-15/006+7100
GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

TECHNICAL NOTE 447 i

Gray Clay (Malpass Clay) in the 

West Eugene Wetlands and the 

Willamette Valley is Weathered 

and Redeposited Ash from 

Mount Mazama

Authors:

Karin E. Baitis, Bureau of Land Management (retired), Eugene District Office, Springfield, Oregon

Gregory J. Retallack, University of Oregon, Department of Geological Sciences, Eugene, Oregon

Michael E. James, James Geoenvironmental Services, Eugene, Oregon 

JUNE 2015
TECHNICAL NOTE 447

GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

ii

Acknowledgments

The outcome of this study results from a decade 

of support from many people and partners and 

funding from the Bureau of Land Management. 

Data were collected through cooperative 

efforts during the course of this project and 

in partnership with the Long Tom Watershed 

Council. Numerous people freely performed 

lab work and data interpretation. Professor Paul 

Engelking of the University of Oregon Chemistry 

Department conducted soil profile conductivity 

with ion chromatography. Professor Erwin 

Schutfort of the Oregon State University Radiation 

Center conducted C-14 radiometric, tephra, 

and pedological analyses and calculated trace 

element values by neutron activation. Countless 

others in the community spent time discussing 

and reviewing data and providing office space for 

storing samples and conducting work. In addition 

to those previously mentioned, the authors would 

like to personally thank William C. Sidle of the U.S. 

Environmental Protection Agency National Risk 

Management Research Laboratory; Mike Remboldt 

of KA Engineering; Mark Reed, Paul Wallace, Ewart 

Baldwin, Nate Sheldon, and John Donovan of the 

University of Oregon Department of Geological 

Sciences; and Herb Huddleston of the Oregon State 

University Crop and Soil Science Department.
GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

TECHNICAL NOTE 447 iii

Table of Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Geologic Setting of Malpass Clay and Dayton Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Stratigraphic Position of the Gray Clay in the West Eugene Wetlands . . . . . . . . . . . . 5

Chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Vertic Soil and Gilgai Relief . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

X-Ray Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Transmission Electron Microscope EDX Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Transmission Electron Microscope Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Mineralogy of Gray Clay Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Radiometric Ages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Palynology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
TECHNICAL NOTE 447

GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

iv

Abstract

The climactic eruption of Mount Mazama 

approximately 7,600 years ago distributed airfall 

tuff throughout what is now eastern and offshore 

Oregon. This distinctive tephrostratigraphic 

marker had been thought to be absent across the 

Willamette Valley. X-ray diffraction, examination of 

pyrogenic mineral suites, and new high-resolution 

radiometric dating (40AR/39AR) of Malpass clay 

from tuffaceous marker beds suggest that it is 

the missing ash from Mount Mazama. Over time 

and weathering, the ash has transformed into a 

smectitic gray imogolite and clay. Analysis of this 

widespread gray clay provided a radiocarbon date 

of 6,850 ± 70 years before present. Radiometric 

dating of the clay provided the ages of 7,987 ± 102, 

8,451 ± 57, and 8,540 ± 48 years before present. 

These dates are comparable with that of the 

climactic eruption of Mount Mazama.
GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

TECHNICAL NOTE 447 1

Introduction

Malpass clay is a widespread gray clay originating 

from the Holocene and found today in the 

Willamette Valley in Oregon (Figure 1). Theories of 

its origin vary from the idea that the clay came from 

weathering in place (Nikiforoff and Drosdoff 1943), 

from erosion from the Coast Range mountains 

(Parsons and Balster 1965, 1966; Balster and Parsons 

1968, 1969), from deposition of the Missoula floods 

roughly 12,500 years ago (Gelderman and Parsons 

1972; Roberts 1984), or from a separate windblown 

deposit (Glasmann et al. 1980; O’Connor et al. 2001). 

This technical note presents evidence that the 

unusual composition of Malpass clay and a suite of 

rhyodacitic minerals in other soils and sediments 

are derived from Mount Mazama volcanic ash, 

which has weathered in place and redeposited from 

surrounding hillslopes (Norgren 1962; Glenn 1965; 

James and Baitis 2003). 

The caldera-forming eruption of Mount Mazama 

approximately 7,600 years ago created Crater 

Lake, which is about 85 miles (136 km) southeast 

of Eugene and the southern end of the Willamette 

Valley (Figure 1). Most ash from Mount Mazama 

was borne eastward by prevailing winds from 

the Cascades onto desert soils and playa lakes of 

eastern Oregon, southeastern Washington, and 

northwestern Nevada (Crandell and Mullineaux 

1978; Kittleman 1973; Matz 1991; Mehringer et 

al. 1977; Sarna-Wojcicki et al. 1981). However, 

pumiceous Mazama ash also has been identified 

in lake deposits of western Oregon (Williams 1942; 

Kittleman 1973; O’Connor et al. 2001; Worona 

and Whitlock 1995) and in offshore marine cores 

(Goldfinger et al. 2003). Early studies of the volume 

of ash issued from Mount Mazama, calculated by 

Williams and Goles (1968), theorized that more 

magma erupted than had been mapped. The 

authors of these studies explained the discrepancy 

with subterranean withdrawal of magma. This tech 

note proposes that some of the missing tephra was 

deposited into the Willamette Valley and weathered 

into Malpass clay and the Greenback Member of 

the Willamette Formation.

An airfall ash deposit into the Willamette Valley 

would have required a wind from the south 

or southeast, sending the ash north and west. 

Mazama ash is 0.35-0.47 in (0.9-1.2 cm) thick in 

submarine canyons of offshore Oregon (Goldfinger 

et al. 2003); 0.39 in (1 cm) thick in Little Lake in the 

Oregon Coast Range (Worona and Whitlock 1995); 

Figure 1. Isopachs (cm) of Malpass clay in the Willamette Valley (solid lines, 

representing approximate thickness) (James and Baitis 2003) compared 

with isopachs of Mazama ash (broken lines) (Matz 1991). In addition, 0.35-

0.47 in (0.9-1.2 cm) of Mazama ash have been found at three locations in 

deep sea cores of the Oregon continental slope (Goldfinger et al. 2003), 

and 0.39 in (1 cm) of Mazama ash has been found in Little Lake in the 

Oregon Coast Range (Worona and Whitlock 1995).
TECHNICAL NOTE 447

GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

2

and 3.9 in (10 cm) or more thick west of the eastern 

Willamette Valley (Matz 1991). Malpass clay fills in 

the intervening country, where fresh Mazama ash 

has not been found. Malpass clay in the Willamette 

Valley is an order of magnitude thicker than the 

predicted 0.4-4 in (1-10 cm) between the Cascades 

and Coast Range and was likely redeposited by 

wind and water into local depressions. 

On May 18, 1980, the volcanic eruption of Mount 

St. Helens south of Seattle, Washington, showed 

similar ash distribution in the Pacific Northwest. 

The eruption began with a collapsing bulge of 

the volcano and a mudflow. Then a pyroclastic 

blast of mountain debris and hot volcanic gases 

shot out from the displaced bulge. Finally, a large 

Plinian column of gray ash shot 11 miles (18 km) 

into the atmosphere. Five subsequent vertical 

eruptions trailed off to the east at high elevation. 

Much of the column collapsed back on the volcano, 

distributing ash radially around the mountain; 

the ash redistributed later by landslides and rivers 

(Lipman and Mullineaux 1981). On May 25, 1980, 

winds sent airborne ash north and west to the 

coastal Washington towns of Grays Harbor, Oakville, 

and Hoquiam. Oakville received 0.75 in (1.9 cm) 

(Kliem et al. 2005). During this timeframe, Bureau 

of Land Management (BLM) (retired) soil scientist 

Karin Baitis observed 0.25-0.5 in (0.64-1.27 cm) of 

very fine, gray-colored ash at Hoquiam.

Geologic Setting of Malpass Clay 

and Dayton Soil

Stiff, gray and olive brown clays overlying Holocene 

alluvial deposits, paleosols, and Missoula flood 

silts in western Oregon’s Willamette Valley have 

been called Dayton soils or the Malpass Member 

of the Willamette Formation (O’Connor et al. 2001). 

Malpass clay is associated with the Senecal and 

Calapooyia geomorphic surface, suggesting a 

Missoula flood provenance, according to Parsons 

and Balster (1967) and Parsons et al. (1970). The 

Dayton soil has been identified as a Mollisol 

(Haploxeroll) by Patching (1987), with a dark crumbstructured surface (A) horizon. Its subsurface gray 

clay has been considered the argillic (IIB2t) horizon 

of a Planosol by Parsons and Balster (1967) or an 

Alfisol (Albaqualf) by Patching (1987). Analyses by 

the authors of this tech note suggest that both the 

surface soil (Dayton series) and paleosol (Malpass 

clay) with abundant noncrystalline colloids and 

a rhyodacitic suite of sand-size minerals is better 

classified as an Andisol or, more specifically, a vertic 

Epiaquand (Soil Survey Staff 1998).

The West Eugene Wetlands in the southern 

Willamette Valley rest above impervious gray 

Malpass clay, which is 59 in (150 cm) thick, with 

a clay-size fraction between 65% and 75% and a 

medium sand fraction between 3% and 7%. In the 

northern Willamette Valley in Woodburn along Mill 

Creek (Figure 2), the gray clay varies in thickness 

from 20 to 47 in (51 to 120 cm), with a clay-size 

fraction between 80% and 85% and a medium 

sand fraction of barely 1%. In both places, the 

slickensided clay is separated by a clear erosional 

truncation from overlying silty soils and sediments. 

Well-drained slopes show an orange to yellow 

silty unit with characteristic rhyodacitic minerals, 

such as bipyramidal quartz, rather than gray clay. 

This rhyodacitic silt thins to 3.9 in (10 cm) at Blue 

River east of Eugene and to 2.4 in (6 cm) at Walton 

west of Eugene. The rhyodacitic silt thins to 3.9 

in (10 cm) along the Middle Clackamas River east 

of Woodburn and to 3.9 in (10 cm) west of Forest 

Grove. The rhyodacitic silts on hillslopes are also 

known as the Greenback Member of the Willamette 
GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

TECHNICAL NOTE 447 3

Formation, and they extend well beyond the 

limits of Dayton soils and Missoula flood deposits 

(Parsons and Balster 1967). Well-drained Hazelair 

and Bellpine soils also include rhyodacitic minerals 

of Mazama ash, and they merge into Malpass 

clay on the valley floor. These other soils have 

only a dusting of rhyodacitic minerals and a bulk 

composition that reflects very different parent 

materials of bedrock hills. The Eocene-Oligocene 

Eugene and Fisher Formations (32-46 million years 

ago - Ma) to the south and east of Eugene are 

andesitic sandstones and conglomerates, with 

a few rhyodacitic ash-flow tuffs. The OligoceneMiocene Little Butte Volcanic Formation (17-35 Ma) 

to the east of Eugene are rhyolitic and andesitic 

flows of a precursor of Cascadean arc volcanism. 

The Eocene Lorane Siltstone and Spencer 

Formation (41-46 Ma) and Tyee Formation (46-51 

Ma) of the Coast Range to the west of Eugene are 

arkosic and micaceous from granitic source terranes 

(Retallack et al. 2004; McClaughry et al. 2010).

Figure 2. Mill Creek site in 

Woodburn, Oregon, where 

gray clay (stratum 2) lies 

between 1.5 ft (0.46 m) of 

alluvium on the surface 

and 3.8 ft (1.2 m) of organic 

peat below the surface. 

Radiocarbon dating of the 

gray clay (6,850 years before 

present) correlates directly to 

the Mount Mazama eruption 

(Stenger 2002).

A series of strata from different geologic events lie 

underneath the Willamette Valley floor. Malpass 

clay overlies the alluvial Willamette Formation 

deposits and grades into the Greenback Member 

extending laterally onto hillslopes both east into 

the Cascade Range and west into the Coast Range 

(Figure 3). The Malpass clay is thickest in the 

southern Willamette Valley and thins to the north 

(James and Baitis 2003). This contrasts with the 

Missoula flood silts that are thickest in the northern 

Willamette Valley and fine to the south (O’Connor 

et al. 2001). Previous researchers have considered 

the provenance of the Malpass clay as derived 

from the Willamette River system (McDowell 1991) 

based on descriptions of Willamette Formation 

characteristics (Balster and Parsons 1968; Glenn 

1965). In their study, the Irish Bend silts were found 

at approximately 370 ft (113 m) mean sea level 

(msl), at which point the Malpass clay thickens 

and is located in low lying, poorly drained areas. 

The clay is variable in thickness, with the deepest 

deposits found in what appear to be relict cut and 

fill stratigraphic units. In West Eugene beneath the 

Malpass clay, as many as three paleosols (fluvents) 

were found. The soil profiles are thin and red and 

appear well drained. The Wyatt Member is found 

primarily in the northern part of the valley (Parsons 

et al. 1973). In the Springfield-Eugene area, an 
TECHNICAL NOTE 447

GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

4

alluvial fan consisting of Linn gravels underlies 

the entire area. In West Eugene, the gravels are 

within 12 ft (3.7 m) of the soil surface, sometimes 

outcropping in sideslopes or ditchlines along 

channelized Amazon Creek.

Figure 3. Exaggerated (not to scale) block diagram of the Willamette Valley between Eugene and Woodburn, with the 

stratigraphic framework developed by Balster and Parsons (1968, 1969) and with the superposition and extent of the Malpass clay 

and Greenback Member (thickest in Eugene and fining toward Woodburn) developed from this study.

Materials and Methods

Hand-auger holes were located across the 

Willamette Valley by using Natural Resources 

Conservation Service soil maps. Two trenches were 

excavated to a depth of 10 ft (3 m) in West Eugene, 

and 50 auger holes ranging between 2.5 to 10 ft 

(0.76 to 3 m) in depth were aligned in two east-west 

transects from the Cascade foothills to the crest of 

the Coast Range and one north-south transect from 

Cottage Grove to Portland. Soil and mineral data 

were collected from each soil boring at intervals 

of 4-6 in (10-15 cm). More than 150 samples were 

described and analyzed for minerals. Soil and rock 

were described using standardized soil taxonomy 

and geologic descriptions. In addition, random 

cores were analyzed in the West Eugene Wetlands 

throughout a 10-year period, and samples were 

collected for further lab analysis.

Samples were sieved with size 50 mesh (0.297 mm 

openings), and both coarser and finer fractions 

were studied under binocular and petrographic 

microscopes for mineral composition and character. 

Crystals were easily segregated by water and 

handpicked. Hornblende crystals were selected 

from nine samples from the Eugene and Woodburn 

areas as representative of likely modified Mazama 

ash from the valley floor (Malpass clay) and 

adjacent hillslopes (Greenback Member).
GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

TECHNICAL NOTE 447 5

X-ray diffraction (XRD) analyses were submitted 

to two different labs at the University of Oregon: 

a Bruker instrument was used in the Center for 

Advanced Materials Characterization in Oregon 

(CAMCOR), and a Rigaku instrument was used in 

the Department of Geological Sciences. In addition, 

XRD was performed by J.R. Glasmann with the 

Willamette Geological Service using chemical 

treatments (James and Baitis 2003). Twelve polished 

thin sections were prepared in the University of 

Oregon thin section lab, with multiple grains of 

each mineral selected from representative sites. 

The thin section lab used a Cameca SX-50 electron 

microprobe at CAMCOR with four wavelength 

spectrometers with analytical conditions of 15 

kiloelectron volts (keV) and 20 nanoamperes (nA). 

Standards are generally pure metals, oxides, and 

some silicates and glasses. CAMCOR used an FEI 

Qanta scanning electron microscope to obtain 

images characterizing distinctive minerals and 

volcanic grains. CAMCOR also used the Titan 80-300 

instrument for transmission electron microscopy. 

Samples were prepared by diluting 100-150 g of 

each soil in 400 ml of distilled water. After agitation, 

the finest material in suspension after 2 or more 

days was extracted with a dropper. Each liquid 

sample was applied to a sample holder of copper 

wire mesh covered by a 5-nanometer (nm) layer 

of carbon. A single drop was placed with a sterile 

pipette on the holder, and slowly the liquid was 

dried off with the edge of adsorbent paper. The 

thin-layered sample was placed in the microscope 

under a vacuum where it was bombarded by highspeed electrons guided by strong electromagnetic 

rings. A high-resolution image was used to identify 

a mineral target, and X-rays generated from that 

area were collected by the detector and reported as 

TIFF files.

Radiometric dating (40Ar/39Ar) analyses were 

performed on hornblende mineral separates 

(0.50-0.75 mm), in order to determine crystal 

apparent ages. Mineral separates were cleaned 

in an ultrasonic bath of dilute nitric acid (HNO3). 

Irradiation at the Ohio State University Nuclear 

Reactor Lab included the IHLHb-1 standard. 

Crystals were spot heated with the Spectron 65W 

Nd:YAG continuous infrared 1,064-nm laser. Spot 

ablation was approximately 80 micrometers (μm), 

and analyses were replicated three times within the 

core of the crystal. Water and other condensable 

gases were trapped by an N2 cold finger. Evacuation 

of the sample chamber resulted in Ar volumes 

typically < 3 x 10-16 moles mV-1 40Ar. Under 

continuous laser beam mode, each sample was 

incrementally heated from 450° C to fusion (Lo Bello 

et al. 1987). 40Ar and 39Ar were measured in the VG 

3600 mass spectrometer with a typical 4.5-kilovolt 

(kV) acceleration potential and 400-milliamp 

(mA) trap current. Using the electron multiplier 

system, 40Ar/39Ar yields for atmospheric argon were 

approximately 310 ± 0.3%.

Radiocarbon analyses on soil carbon were 

performed commercially by β-scintillation counting 

via benzene synthesis. Three samples were 

collected from a trench in West Eugene at 15 in 

(38 cm), 42 in (107 cm), and 65 in (165 cm) and sent 

to Beta Analytic for analysis. Two samples were 

collected in the West Eugene Wetlands in a soil core.

Stratigraphic Position of the Gray Clay 

in the West Eugene Wetlands

The inception of this study began with an 

assessment of soils on public lands managed by the 

BLM in the West Eugene Wetlands in 2001, using 

stratigraphic observations to identify depositional 
TECHNICAL NOTE 447

GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

6

units, particularly Missoula flood silts. No local 

archaeologic, engineering, or geologic reports 

were found documenting the presence of Missoula 

flood silts in the surrounding Eugene area; this 

study was designed to test whether they were 

present. Observations of trenches and cores taken 

during the course of this study concluded that 

the Missoula flood silts did not extend south of 

Eugene’s Mahlon Sweet Field airport at an elevation 

of approximately 374 ft (114 m) msl. However, very 

massive amounts of gray clay were found in West 

Eugene, frequently overlain by thin veneers of 

Willamette River overbank flood deposits, which 

weathered into silty soils.

In West Eugene, the gray clay is always found in 

saturated conditions on the valley floor (Figure 4); 

it becomes impervious in winter, creating wetlands; 

it is gray and olive brown in color (2.5Y, according 

to standard soil color charts); and it varies in depth. 

The clay is massive with evidence of slickensides; 

possesses few clay skins; and has no soil particles, 

root channels, or bedding planes. It is found 

beneath an alluvial soil approximately 18 in (46 cm) 

thick. The clay has been found as thick as 5.8 ft (1.8 

m). The clay-size fraction is typically between 65% 

and 75%, with a medium sand fraction between 

3% and 7%, including a variety of rhyodacitic ash 

minerals (James and Baitis 2003, 2014).

Figure 4. The gray clay is 

massive, varies in depth, and 

is always found in saturated 

conditions on the Willamette 

Valley floor.

The gray clay is not found in well-drained hillslope 

positions. The same rhyodacitic suite of minerals 

is found on well-drained grayish red and brown 

soils of hillslopes (outside of saturated conditions), 

but without the gray clay (Figure 5). The silty 

surface soils on hillslopes (Greenback Member of 

the Willamette Formation) extend well beyond 

the limits of Dayton soils and Missoula flood 

deposits (Parsons and Balster 1967). Well-drained 

Bellpine, Cornelius, Jory, Hazelair, Melbourne, and 

Willakenzie soil series include some minerals of 

Mazama ash in the soils’ upper inches (centimeters), 

which merges into the Malpass clay on the valley 

floor. The deposits are thickest in the southern 

Willamette Valley, thinning northward (James and 

Baitis 2003). The stratigraphic profile in Figure 

5 displays the position of the gray clay from the 

hillslope to the valley floor. A paleosol was found 

at the base of each soil auger hole (James and 

Baitis 2003, 2014), directly beneath the gray clay. 

The paleosols are well-drained and red and brown, 

indicating that a well-oxygenated soil surface 

existed before the Mount Mazama eruption, back to 

about 28,000 years ago.
GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

TECHNICAL NOTE 447 7

Figure 5. Transect locations of soil borings across the West Eugene Wetlands (to the left). The stratigraphic cores illustrate the gray 

clay found on the Willamette Valley floor under saturated conditions, while mineralogy reflects the ash found on the hillslope.

Table 1 provides an example of variation in soil 

textures from two sample sites (hillslope and 

valley bottom) in the West Eugene Wetlands. Rock 

fragment grains weather quickly in the moist 

valley floor sites and more slowly on well-drained 

hillslopes. Their demise results in the formation of 

gels and clays in the bottomland samples, which 

are evident in the 16.5% higher value in the 

fine texture category of sample BLM 9-4 (Table 1) 

(James and Baitis 2003). Sample BLM 9-4 has 10.8% 

less material in the “> 100 μm fine sand” category 

and 1.7% less material in the “> 50 μm coarse sand” 

category. In terms of visual inspection, the hillslope 

fraction is mainly volcanic glass and pumice, 

whereas the bottomland sample lacks both. Both 

samples include a unique suite of rhyodacitic 

minerals of sand size related to the Mazama 

eruption. The glassy tephra weathers easily to 

colloids in saturated valley bottom sites, yet it 

maintains a silty texture on better drained hillslopes.

Table 1. Textural comparison of hillslope reddish clayey silt (Sample BLM 12-3) and bottomland gray clay soils (Sample BLM 9-4), 

from James and Baitis (2003).

Sample # Textural Class Weight % Sample # Textural Class Weight %

BLM 12-3 Clay 5 + 15 min 23.7 BLM 9-4 Clay 5 + 15 min 40.2

BLM 12-3 < 100 μm silt 51.6 BLM 9-4 < 100 μm silt 47.3

BLM 12-3 > 100 μm fine sand 19.7 BLM 9-4 > 100 μm fine sand 8.9

BLM 12-3 > 50 μm coarse sand 5.0 BLM 9-4 > 50 μm coarse sand 3.3

Chromatography

Anion content of groundwater and soil slurries was 

analyzed by chromatography at the University of 

Oregon (James and Baitis 2003) and by Apex Labs 

(James and Baitis 2014). High values of chloride 

and sulfate were found in the gray clay slurries at a 

sample site near Walmart in Willow Creek, but even 

higher values were found in nearby groundwater 

(from piezometers PZ-2i and PZ-4i) (Table 2).
TECHNICAL NOTE 447

GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

8

Table 2. Chromatography results in parts per million from Apex Labs.

Sample Site Date Sample Number Element Result

Piezometer-4i 10/17/13

A3J0036-01 Chloride 1,130 ppm

A3J0096-01RE1 Sulfate 8.17 ppm

Piezometer-2i 4/9/13

A12C521-01 Chloride 1,890 ppm

A12C521-01 Sulfate 439 ppm

Willow Creek Walmart 11/7/12

A12J404-01 Chloride 64.7 ppm

A12J404-01RE1 Sulfate 25.3 ppm

Willow Creek W Fork 2/25/13

A13Ba45-01 Chloride 4.42 ppm

A13Ba45-01 Sulfate 161 ppm

High concentrations of chloride in the groundwater 

may be due to evaporation over the impermeable 

clayey substrate. The presence of volatiles indicates 

that these elements are weathering out of the 

Malpass clay. This points to a naturally high 

background level and is corroborated by regional 

conductivity studies. Elevated conductivities are 

consistently found throughout the Amazon Basin as 

compared to rivers in the Coast Range and Cascade 

Range (James and Baitis 2003).

Vertic Soil and Gilgai Relief

Gray clay on the Willamette Valley floor commonly 

shows gilgai microrelief (Figure 6, Photo A). The two 

kinds of gilgai relief in the West Eugene Wetlands 

include circular gilgai (vernal pools) and linear 

gilgai (microtopographic hummocks). Vernal pools 

are more than 35 ft (11 m) in circumference and less 

than 1 ft (0.3 m) in depth. Some of the cracks in the 

gray clay can reach in excess of 3 ft (0.9 m) in depth 

along Willow Creek (BLM Speedway parcel).

Figure 6. In photo A, the shrink/swell of the gray clay exhibits itself in linear gilgai relief in the form of hummocky microrelief. As 

part of the soil mass is forced upward to release pressure from compression, the formation of a mound occurs. Slickensides are a 

soil characteristic within the gray clay. The sides of the peds become shiny as they press against each other during the squeezing 

of the swelling clays. In photo B, taken on the BLM Balboa parcel, the Dayton IIBt begins approximately 12 in (30 cm) below the soil 

surface. Cracks become clearly evident in the gray clay hidden beneath a surface soil. 

Susanna Hamilton, WREN
GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

TECHNICAL NOTE 447 9

Soils are classified from the top down, using 

diagnostics of the surface horizon. However, the 

properties of the subsurface horizons may be used 

when measurements fall within a buried genetic 

horizon (Figure 6, Photo B). The IIB2t of the Dayton 

soil series (typically mapped where the gray clay is 

found) is approximately 16 in (41 cm) from the soil 

surface and typed as a fine, montmorillonitic, mesic 

Typic Albaqualf (Patching 1987). Taxonomically, 

based on the soil surface, the soil is classified as 

an Alfisol. The Dayton soil series are alluvial, and 

a lithologic discontinuity occurs at the B horizon 

(IIBt). It is here that the gray clay beneath the 

surface soil has vertic characteristics. 

The formation of gilgai microtopography occurs 

from the shrinking and swelling of smectite clays 

that form deep cracks when dry (Kishne et al. 2009; 

Kodikara et al. 1999; Dinka and Lascano 2012). With 

rain, the clay swells and heaves the underlying 

material upward in polygonal patterns because the 

soil expands as its volume increases with moisture. 

In addition, surficial material that falls into the 

cracks during the dry season forces the soil mass 

upwards, which forms mounds. Small mounds 

and hollows grow in amplitude as the landscape 

buckles, creating microtopographic hummocks and 

intervening swales, which during the rainy season 

create vernal pools on the impermeable wet clays 

below. In Australia, circular gilgai are found on level 

ground and linear gilgai on modest slopes (Paton 

1974), and this is true for Eugene as well.

X-Ray Diffraction

The University of Oregon’s CAMCOR lab performed 

clay mineralogy studies using XRD on untreated 

clay samples from both a bottomland site from the 

BLM Nielsen parcel and a hillslope site from the 

BLM Oak Hill parcel in West Eugene. After sieving, 

samples were left to settle for 15 minutes, and 

floating clay particles were extracted. The samples 

were heated to 120o C and 600o C. The Bruker 

instrument was used with no chemical treatments.

The valley floor sample from the BLM Nielsen parcel 

displayed a diffuse peak from 4 o to 7o, indicating 

noncrystalline colloids with minor smectite clay 

(Figure 7). Mica, kaolinite, and halloysite, expected 

from surrounding soils, were not present, since 

there were no peaks at 9o, 12o, or 14.5o 2θ.

Figure 7. X-ray diffractogram of the gray clay sampled from the 

valley bottom in West Eugene at the BLM Nielsen parcel.
TECHNICAL NOTE 447

GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

10

Quartz was indicated by peaks near 20.8 o, 27 o, 36.8 o, 

39.3 o, 40.3 o, 42.4 o, 46 o, 50 o, and 55 o 2θ, and large 

bipyramidal grains were observed in this sample. 

Andesine plagioclase is also indicated by peaks at 

5.2 o, 19.5 o, 25 o, 35 o, and 37.5 o 2θ.

A single pulverized pumice grain from the fine sand 

fraction of a hillslope soil also lacked clay peaks and 

was evidently volcanic glass, with small crystals of 

quartz and plagioclase (Figure 8).

Figure 8. X-ray diffractogram of 

white lithic fine sand grains from a 

hillslope soil sample.

A third untreated XRD sample of the gray clay 

(Malpass clay), taken from a trench in the West 

Eugene Wetlands, was conducted in 2002 on the 

Rigaku instrument in the University of Oregon’s 

Department of Geological Sciences. The results 

also supported a soil rich in amorphous material 

evolving slowly toward a 2:1 clay ratio.

XRD samples of the gray clay that were chemically 

treated by J.R. Glasmann (James and Baitis 2003) 

defined a smectite peak (Figure 9). Crystalline peaks 

were found to be quartz and plagioclase, similar to 

the untreated samples in Figures 7 and 8.

Figure 9. This x-ray diffraction 

pattern represents the silt and 

clay fraction of soil material 

from a sample found between 

10 and 21 in (25 and 53 cm), 

using the glycol saturation 

method. The shifting peaks 

demonstrate that the clay is 

smectitic.
GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

TECHNICAL NOTE 447 11

Volcanic glass and noncrystalline colloids are 

characteristic of Andisols (Soil Survey Staff 1998), 

in which devitrification of glass to amorphous 

weathering products, such as imogolite and 

allophane, is the primary weathering process. 

Andisols are generally young soils on fresh volcanic 

ash; with time, they gain clay, soil structure, and the 

features of other soil orders (Lowe 1986; Shoji et al. 

1993). Such a model works well for the gray clay, 

which in the field has a sharp top like a discrete 

paleosol and has no clay skins and less structural 

development. Smectite clay is beginning to form 

and imparting gilgai structure even under a mollic 

surface and grassy cover. 

Based on the analyses, the authors of this study 

recommend that the buried Malpass clay and its 

exposed Dayton soil series be reclassified as an 

Andisol and, more specifically, as a vertic Epiaquand 

(Soil Survey Staff 1998). The Andisol soil order 

was not available in the U.S. taxonomy at the 

time of previous soil surveys (Parsons and Balster 

1967; Patching 1987), but judging from the high 

proportion of noncrystalline materials present 

in Malpass clay, this soil order is appropriate. 

Previously, Malpass clay was considered a buried 

soil, separated from the overlying Mollisol 

(Haploxeroll) (Patching 1987), or as a surface soil 

(Dayton series) (Balster and Parsons 1967). However, 

during this study, we found no genetic relationship 

between the surface soil and the Malpass clay, as 

implied by identification as a Planosol (Parsons and 

Balster 1967) or Albaqualf (Patching 1987). Also, there 

are no connecting clay skins or gradational contacts.

Transmission Electron Microscope EDX Analyses

Electron microprobe analyses confirm differences 

between the Malpass clay and the reddish gray 

silty hillslope soils. Samples of Malpass clay and 

underlying alluvium were collected from a valley 

bottom soil profile on the BLM Nielsen parcel 

to a depth of 8 ft (2.4 m). The sampling intervals 

were 1.8-2.7 ft (0.55-0.82 m), 3-3.7 ft (0.91-1.1 

m), and 4.2-4.9 ft (1.3-1.5 m) within the gray clay. 

The sampling intervals were 6.2-7.0 ft (1.9-2.1 

m) and 7.0-8.0 ft (2.1-2.4 m) from alluvial layers 

beneath the gray clay. Samples from weathered 

andesitic sandstones of the Eugene Formation 

were collected from a hillside located on the BLM 

Speedway parcel about 15-18 ft (4.6-5.5 m) above 

the valley floor and depths from the surface of 6 in 

(15 cm), 24 in (61 cm), 30 in (76 cm), and 36 in (91 

cm). Weathered arkosic-micaceous samples of the 

Spencer Formation were collected from the BLM 

Greenhill parcel at depths of 5 in (13 cm), 16 in (41 

cm), and 24 in (61 cm). Arkosic-micaceous samples 

of the Tyee Formation were collected from Panther 

Creek Road in upper Wolf Creek and also from IP 

Deeded Road west of Noti at a depth of 18 in (46 

cm). Samples of basaltic andesites of the Little Butte 

Volcanic Formation were collected at Hills Creek 

east of Jasper from a depth of 12 in (30 cm) in a 

wetland and 12 in (30 cm) in a terrace.

The Malpass clay (Figure 10 and Table 3) can be 

contrasted with the underlying paleosols (Figure 

11 and Table 4). Aluminum (Al) and silicon (Si) 

were common components of the gray clay, and 

frequently, silica alone was found. Rhyodacitic 

magmas are enriched in silica. The strong carbon 

(C) peak and small copper (Cu) peak was related 

to interaction between the electron beam and 

the sample platform. Few bases were represented 

in the paleosol, and no volatile elements were 

detected, compared with the Malpass clay. 

Chromatography analyses of the Malpass clay and 

Amazon Creek also indicated the presence of sulfur 

(S), chlorine (Cl), and fluorine (F). There is a direct 

relationship between the chemistry of the surface 

water and soil mineralogy in West Eugene.
TECHNICAL NOTE 447

GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

12

Figure 10. Spectral representation of gray clay collected from the BLM Nielsen parcel between 1.8 and 2.7 ft (0.55 and 0.82 m).

Table 3. Elemental composition of gray clay collected from the BLM Nielsen parcel between 1.8 and 2.7 ft (0.55 and 0.82 m).

Element Weight % Atomic % Uncertainty % Correction k-Factor

Carbon (C) 98.95 99.57 1.44 0.26 4.032

Fluorine (F) 0.28 0.18 0.20 0.62 1.623

Sodium (Na) 0.11 0.05 0.03 0.81 1.181

Silicon (Si) 0.12 0.05 0.03 0.92 1.000

Sulfur (S) 0.07 0.02 0.02 0.93 1.015

Chlorine (Cl) 0.11 0.03 0.03 0.95 1.055

Iron (Fe) 0.00 0.00 100.00 0.99 1.359

Copper (Cu) 0.33 0.06 0.06 0.99 1.601

Figure 11. Spectral representation of a paleosol collected from the BLM Nielsen parcel between 7.0 and 8.0 ft (2.1 and 2.4 m).

Table 4. Elemental composition of a paleosol collected from the BLM Nielsen parcel between 7.0 and 8.0 ft (2.1 and 2.4 m).

Element Weight % Atomic % Uncertainty % Correction k-Factor

Carbon (C) 93.79 95.98 1.46 0.26 4.032

Oxygen (O) 3.90 3.00 0.21 0.49 2.008

Aluminum (Al) 0.25 0.11 0.04 0.92 1.030

Silicon (Si) 2.04 0.89 0.10 0.92 1.000
GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

TECHNICAL NOTE 447 13

Transmission Electron Microscope Images

The difference between an amorphous colloid 

of the Malpass clay and more crystalline bedrock 

was observed from examination at the nanometer 

scale in transmission electron microscope images 

(Figure 12). Samples of the gray clay appeared 

noncrystalline and wavy to dark without structure. 

In contrast, residual soil and bedrock appeared 

as brighter, thicker, angular crystalline materials. 

The glass of volcanic ash is itself amorphous 

and noncrystalline, and it weathers initially to 

noncrystalline colloids such as allophane or 

imogolite in Andisols (Lowe 1986; Shoji et al. 1983).

Figure 12. Transmission 

electron microscope images 

of gray clay. Noncrystalline 

gel (left). Bedrock crystalline 

structure (right).

Mineralogy of Gray Clay Unit

A distinctive suite of rhyodacitic minerals (Figure 13) 

was identified in the Malpass clay and Greenback 

Member of the Willamette Formation: (1) euhedral, 

tabular crystals of clear plagioclase (andesineoligoclase); (2) euhedral, magnetic, light green 

orthopyroxene (hypersthene) with opaque 

inclusions of ilmenite and magnetite; (3) coarsegrained, dark brown to black hornblende; (4) 

coarse-grained quartz, both bipyramidal and 

singly pyramidal; (5) euhedral spinel; (6) flat 

hexagonal ilmenite with a dark green glass nucleus; 

(7) chromite; and (8) zircon. Other components 

included fine-grained granodioritic or rhyodacitic 

rock fragments and white to brown pumice 

and shards.
TECHNICAL NOTE 447

GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

14

Figure 13. Volcanic minerals 

from gray clay collected 

in Eugene and Woodburn, 

Oregon: (A) bipyramidal 

quartz, (B) white to tan 

pumice with tiny tubules, 

(C) hornblende, (D) twinned 

hypersthene, and (E) 

hexagonal ilmenite. Note 

how well formed the minerals 

appear, with few signs of wear 

from fluvial transport.

Some of the mineral grains of the gray clay 

(Malpass clay and Dayton soils) are similar to 

those well documented in Mount Mazama ash 

(Kittleman 1973; Druitt and Bacon 1989; Bacon 

and Druitt 1988; Williams 1942). The dominant 

pyroxene mineral is hypersthene, which dominates 

clinopyroxene in Mazama ash (Kittleman 1973). 

Furthermore, the low-Calcium (Ca) (1.1% weight) 

pyroxene had abundant intergrowths of ilmenite 

and magnetite with nearly equal iron oxide (FeO) 

(19.7%) and magnesium oxide (MgO) (22.9%), 

which is very similar to Mazama hypersthene (Druitt 

and Bacon 1989). Hornblende grains were larger 

than the pyroxenes and nonmagnetic. Their total 

iron content was low (12%); their alumina content 

was high (10%); and their titania content was high 

(3.2%). Again, the match with Mazama ash is almost 

identical (Druitt and Bacon 1989). 

In Eugene, a few glass shards were found floating 

freely in the clay matrix (Figure 14). The clay at 

Woodburn had more abundant glass shards and 

fine pumiceous material. Glass is readily weathered, 

and this may explain its rarity in the gray clay.

Figure 14. Glass shards found in clay samples collected in Eugene, shown under a scanning electron microscope and 

in thin section.
GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

TECHNICAL NOTE 447 15

Other constituents common to the gray clay were 

not diagnostic of, but do not exclude, a Mazama 

ash source. Euhedral plagioclase grains showed 

zoning and twinning and a few with intergrowth 

of potassium feldspar (antiperthite). Plagioclase 

compositions were oligoclase-andesine (An30-

36), with relatively equal calcium oxide:sodium 

oxide (CaO:Na2O) values (6.7%:7.5%). Quartz was 

found in the gray clay but was not common in the 

rhyodacitic Mazama ash and was less common 

in other Cascade basaltic-andesite tephra. Lithic 

fragments with metallic inclusions appeared to 

have been hydrothermally altered and were of 

uncertain origin; ilmenite is common as a shiny 

black hexagonal platy mineral, sometimes with 

an angular depression or glass-filled core. Other 

accessory minerals included magnetite, titaniferous 

magnetite, ulvospinel, chromite, and hafniumrich zircon.

Radiometric Ages

Radiocarbon

In Eugene, radiocarbon dates via scintillation 

counting were obtained from bulk soil carbon. The 

radiocarbon dates included 1,480 ± 60 yr before 

present (BP) at 7.9 in (20 cm) below the surface 

horizon of modern Mollisol; 16,000 ± 50 yr BP from 

the middle of the Malpass clay; and 6,280 ± 50 yr 

BP from the base of the Malpass clay. In addition, a 

date near the surface, but within the Malpass clay, 

was 2,930 ± 30 yr BP. In Woodburn, Oregon, Stenger 

(2002) reported a radiocarbon date of 6,850 ± 70 yr 

BP from charcoal in the Malpass clay.

Below the Malpass clay, a paleosol at 7 ft (2.1 m) 

was dated 28,810 ± 180 yr BP, which falls within 

the Wisconsin Glacial Stage. The sample had 

many carbon fragments. In a regional review, 

O ́Connor et al. (2001) commented that the age of 

surfaces beneath the Missoula flood silt deposits, 

throughout the valley, ranges between 22,000 and 

28,000 years ago, but no Missoula flood silts have 

yet been found in West Eugene.

Radiocarbon dating of the Malpass clay is thus 

comparable with that of the climactic Mount 

Mazama eruption of 6,845 ± 50 radiocarbon yr 

(Bacon and Druitt 1988), calibrated to 7,627 ± 150 

yr BP (Zdanowicz et al. 1999). The anomalous 16 

kiloannus (ka) 14C date in Eugene may represent 

fossil plant material carried in the climactic eruption 

of Mount Mazama, while a younger 2,930 14C date 

can be explained by a high water table, plant 

rooting, or debris falling down soils cracks.

Argon/Argon (40Ar/39Ar)

Hornblende crystals from the Malpass clay and 

Greenback Members collected in West Eugene 

and Woodburn yielded apparent ages of 7,987 ± 

102, 8,451 ± 57, and 8,540 ± 48 yr BP. Laser probe 

step-heating plateau ages (Figure 15) represented 

weighted means of the 39Ar release spectra. The 

stepwise Ar release patterns were well developed 

and consistent among the three samples with 2σ 

less than 102 yr. These hornblende crystals did not 

exhibit recognizable excess 40Ar.
TECHNICAL NOTE 447

GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

16

Figure 15. 40Ar/39Ar laser fusion dating yielded 

apparent ages of 7,987 ± 102, 8,451 ± 57, and 

8,540 ± 48 yr BP.

Mount Mazama erupted about 7,600 years ago, 

according to radiocarbon dating (Zdanowicz et 

al. 1999). Given known variation in magmatic 

crystal ages (Simon et al. 2008), the 40Ar/39Ar dates 

presented here are compatible with origin from 

Mount Mazama.

Palynology

Mantling of the Willamette Valley with a 0.39- to 

3.9-inch (1- to 10-cm) layer of fine textured volcanic 

ash from Mount Mazama would have changed the 

species composition of vegetation in the Willamette 

Valley, particularly where airfall ash fell and then 

washed into swales with depth. Paleoecologic data 

(pollen studies) from Beaver Lake near Corvallis, 

Oregon, in the mid-Willamette Valley (Pearl 1999) 

indicate that the Willamette Valley experienced 

a pronounced change in drainage after 7,500 

calibrated yr BP, coincident with the Mount Mazama 

eruption (Figure 16). Xeric species, such as oak, fir, 

and alder, adjacent to riparian gallery forests (Pearl 

1999) were suddenly replaced with grass, Oregon 

ash, willow, and other wetland-tolerant plants. 

These data demonstrate that after the Mazama 

ash fell, wetlands were created throughout the 

Willamette Valley. 
GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

TECHNICAL NOTE 447 17

Figure 16. Display of how the vegetative species composition around Beaver Lake in the central Willamette Valley shifted 

dramatically to wetland species after the Mount Mazama eruption some 7,600 years ago (Pearl 1999).

Conclusions

This study contradicts previously held ideas that 

Malpass clay and the Greenback Member of the 

Willamette Formation are of an alluvial or lacustrine 

origin or related to flooding of the Willamette Valley 

by the Missoula floods. Instead, Malpass clay and 

the Greenback Member are considered weathered 

remnants of volcanic ash from the eruption of 

Mount Mazama some 7,600 years ago.

Paleoecologic data from Beaver Lake in the midWillamette Valley indicate that the Willamette 

Valley experienced a pronounced change in 

drainage after Mount Mazama erupted (Pearl 1999). 

Suddenly, wetland habitats appear, replacing more 

xeric species, such as oak, fir, and alder, adjacent to 

riparian gallery forests.

A unique suite of minerals related to Mount 

Mazama are found in the gray clay: (1) euhedral, 

tabular crystals of clear plagioclase (andesineoligoclase); (2) euhedral, magnetic, light green 

orthopyroxene (hypersthene) with opaque 

inclusions of ilmenite and magnetite; (3) coarsegrained, dark brown to black hornblende; (4) 

coarse-grained quartz, both bipyramidal and singly 

pyramidal; (5) euhedral spinel; (6) flat hexagonal 

ilmenite; (7) chromite; and (8) zircon. 

Hornblende crystals analyzed by 40Ar/39Ar dating 

yielded apparent ages of 7,987 ± 102, 8,451 ± 57, 

and 8,540 ± 48 yr BP. These dates fall within the 

bounds of the Mount Mazama eruption, given 

known variation in magmatic crystal ages (Simon 

et al. 2008). These dates also fall within the bounds 

of Mazama ash with a radiocarbon date of 6,850 

± 70 yr BP from Woodburn (Stenger 2002). Below 

the gray clay of the West Eugene Wetlands, the 

paleosols are well drained and date back to 
TECHNICAL NOTE 447

GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

18

28,810 yr BP. No Missoula silts, which are 12,500 

years old, were found in the West Eugene area.

The two types of gilgai relief in the West Eugene 

Wetlands include circular gilgai (vernal pools) and 

linear gilgai (microtopographic hummocks). These 

features were created by the shrink/swell and vertic 

properties, which are characteristic of the gray clay 

unit found throughout the wetlands (Paton 1974). 

The gray clay has a weak signature of disordered 

smectite clay (montmorillonite); has characteristics 

of a colloid with little crystallinity; and has a 

chemical relationship of silica to aluminium 

represented as 2:1 or 3:1. The clay has little to no 

soil structure and no clay skin development.

This mineralogically distinctive soil layer derived 

from the climactic 7,600-year-old Mazama eruption 

serves as a geochronological time marker in the 

Willamette Valley, presented by surface soils of the 

Dayton series and a buried soil of the Malpass clay. 

Although these have been regarded in the past as 

Alfisols and Mollisols (Parsons and Balster 1967; 

Patching 1987), the demonstration of noncrystalline 

colloids indicates that the Dayton soil series and 

Malpass clay are better classified as Andisols and, 

more specifically, as vertic Epiaquands (Soil Survey 

Staff 1998). The Malpass clay represents a paleosol 

developed between about 4,000 and 7,000 years 

ago, before cover by silts and Mollisols. Both the 

Malpass clay paleosol and the surface Dayton soil 

series have been arrested in their development 

by poor drainage in lowland locations and 

exacerbated by their fine grain size. Other soils of 

the Willamette Valley were rejuvenated by Mazama 

ash fall but lost most of their volcanic glass and 

imogolite to retain only a dusting of rhyodacitic 

minerals of the Greenback Member. These Hazelair 

and Bellpine soil series are Haploxerolls and 

Halpoxerults (respectively), which have progressed 

beyond the Andisol stage of weathering. 
GRAY CLAY (MALPASS CLAY) IN THE WEST EUGENE WETLANDS AND THE WILLAMETTE VALLEY IS WEATHERED AND REDEPOSITED ASH FROM MOUNT MAZAMA

TECHNICAL NOTE 447 19

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TECHNICAL NOTE 447

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20

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