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Quaternary Research 6

Magnetostratigraphic evidence from the Cold Creek bar for onset

of ice-age cataclysmic floods in eastern Washington during

the Early Pleistocene

Christopher J. Pluhara,*, Bruce N. Bjornstadb, Stephen P. Reidelb,

Robert S. Coea, Paul B. Nelsona

aUniversity of California, Earth Science Department, 1156 High St. Santa Cruz, CA 95064-1077, USAbPacific Northwest National Laboratory, Applied Geology and Geochemistry (K6-81), P.O. Box 999, Richland, WA 99352, USA

Received 2 March 2004

Available online 9 September 2005

Abstract

This study provides a detailed magnetostratigraphy of sediments composing the Cold Creek cataclysmic flood bar in the Pasco Basin,

Washington. Our interpretation suggests onset of Missoula floods or similar events prior to 1.1 myr, later than previously suggested by

Bjornstad et al. [Bjornstad, B.N., Fecht, K.R., Pluhar, C.J., 2001. Long history of pre-Wisconsin, Ice Age cataclysmic floods: evidence from

southeastern Washington State. Journal of Geology 109 (6), 695–713]. Nonetheless these data suggest that Channeled Scabland features

formed over a much longer timespan than commonly cited, that continental ice sheets of the early Pleistocene reached as far south as those of

the late Pleistocene, and that similar physiography existed in eastern Washington and perhaps Montana to both generate and route Missoula-

flood-like events. This study adds paleomagnetic polarity results from 213 new samples of silts and sands derived from nine new drill cores

penetrating the Cold Creek cataclysmic flood bar to our previous database of 53 samples from four boreholes, resulting in a much more

robust and detailed magnetostratigraphy. Rock magnetic studies on these sediments show pure magnetite to be the predominant remanence-

carrying magnetic mineral, ruling out widespread remagnetization by secondary mineralization. The magnetostratigraphy at eastern Cold

Creek bar is characterized by a normal polarity interval bracketed by reversed polarities. Equating the normal zone with the Jaramillo

subchron (0.99–1.07 myr) affords the simplest correlation to the magnetic polarity timescale. Western Cold Creek bar was likely deposited

during the Brunhes chron (0–0.78 myr) since it exhibits mainly normal polarities with only two thin reversed-polarity horizons that we

interpret as magnetic excursions during the Brunhes.

D 2005 University of Washington. All rights reserved.

Keywords: Paleomagnetism; Magnetostratigraphy; Susceptibility; Hysteresis parameters; Hanford; Pasco Basin; Cold Creek bar; Missoula; Scabland;

Cataclysmic flood; Jokulhlaup

Introduction

Since acceptance of the Missoula flood hypothesis for

formation of eastern Washington’s Channeled Scabland

(Bretz, 1930), study has focussed mainly on the scores of

jokulhlaups now thought to have occurred during the last

glacial (e.g., Waitt, 1985; Atwater, 1986; Smith, 1993;

Steele, 1991; Clague et al., 2003; Benito and O’Connor,

0033-5894/$ - see front matter D 2005 University of Washington. All rights rese

doi:10.1016/j.yqres.2005.06.011

* Corresponding author. Fax: +1 831 459 3074.

E-mail address: cpluhar@es.ucsc.edu (C.J. Pluhar).

2003; O’Connor and Baker, 1992; McDonald and Busacca,

1992). Several workers have also inferred, using paleo-

magnetic polarity (e.g., McDonald and Busacca, 1988,

1992) and unpublished U/Th disequilibrium dating (sum-

marized in Baker et al., 1991) that cataclysmic floods have

occurred during the middle and early Pleistocene. These

studies of pre-late-Pleistocene deposits are valuable first

steps in working out the Quaternary chronology of

cataclysmic floods in eastern Washington, but are limited

by available surficial exposures and preservation of the

geologic record in an inherently erosive environment. In this

5 (2006) 123 – 135

rved.

C.J. Pluhar et al. / Quaternary Research 65 (2006) 123–135124

paper, we document the detailed paleomagnetic stratigraphy

of cataclysmic flood deposits in an area that contains one of

the most complete depositional records of Ice Age flooding,

the Cold Creek giant flood bar of the Pasco Basin. This

work builds upon our previous limited paleomagnetic

data (Bjornstad et al., 2001) as well as stratigraphic and

sedimentologic investigations of the last 50 yr at the

Hanford site (summarized in US DOE/RL 2002-39).

Extensive discussion of the latter is beyond the scope of

this paper, but the magnetostratigraphy presented here is

particularly useful because of the difficulties in stratigraphic

correlation and the seeming impossibility of providing age

constraints for the sands and gravels of the Cold Creek bar.

Geologic setting

During the last glaciation, the Okanogan and Purcell

Trench lobes of the Cordilleran ice sheet blocked drainage

of the Columbia and Clark Fork Rivers, impounding glacial

Lakes Columbia and Missoula, respectively (Fig. 1). Lake

Missoula drained periodically, as it filled enough to breach

(Walder and Costa, 1996; O’Connor and Baker, 1992) or

force an outlet beneath (Waitt, 1985) the ice dam to produce

the colossal floods of which Bretz found so much evidence

Figure 1. Principal elements of glacial cataclysmic floods. CCB identifies the location of Cold Creek bar.

in the Scabland of eastern Washington. Missoula Floods

dumped into glacial Lake Columbia, where varved sedi-

ments of that period are punctuated every 35 to 55 yr by

flood sediments (Atwater, 1984). This flood recurrence

timescale is corroborated by the approximate time interval

required to fill glacial Lake Missoula (Waitt, 1983, 1985).

Lake Columbia was not prone to emptying by periodic

jokulhlaups because a natural spillway down Grand Coulee

prevented that lake from attaining a depth sufficient to force

a way through its ice dam.

Missoula Floods entered the Channeled Scabland after

exiting Lake Columbia by overtopping drainage divides

along its southern edge. Floodwaters then scoured across the

Channeled Scabland in an anastamosing, distributary net-

work, incising into the surficial Palouse loess and in many

places through to underlying Columbia River basalt.

Floodwaters converged on the Pasco Basin and a con-

striction in the drainage network at Wallula Gap. This

hydraulic bottleneck resulted in temporary ponding of

floodwaters in the Pasco Basin, forming Lake Lewis (high

water mark shown in Fig. 2), which persisted for perhaps 5

days or less during each flood (O’Connor and Baker, 1992).

Floodwaters entering Lake Lewis dumped much of their

suspended load to form Cold Creek, Priest Rapids and

Gable Mountain flood bars (Fig. 2), while finer-grained

Figure 2. Digital elevation model map of the study area, a portion of the Pasco Basin. Extensive flood deposits accumulated in the Pasco Basin when

floodwaters temporarily filled it, forming Lake Lewis.

C.J. Pluhar et al. / Quaternary Research 65 (2006) 123–135 125

suspended load settled out to form slackwater (Touchet)

beds around the periphery of Lake Lewis and in back-

flooded areas such as the Yakima and Walla Walla basins.

Since the Channeled Scabland was eroded by Missoula

floods, while the Pasco Basin was the recipient of much of

this scoured material, one anticipates a more complete long-

term record of flooding to be preserved in the Pasco Basin

than in the Scabland.

The classic Missoula Floods were occasionally inter-

spersed with jokulhlaups resulting from terminal Lake

Columbia drainage, cataclysmic flooding from sudden

drainage of Lake Bonneville (not shown in Fig. 1) (e.g.,

O’Connor, 1993), and possible jokulhlaups derived from

beneath the Cordilleran ice sheet (Shaw et al., 1999). Since

varied events resulted in scouring of eastern Washington by

cataclysmic floods, we prefer the term Scabland Floods,

given that at most localities events from different sources

are indistinguishable in outcrop.

There is now abundant evidence (much of it not formally

published) that the late Pleistocene jokulhlaup phenomenon

described above occurred during earlier glaciations as well

(as summarized in Bjornstad et al., 2001). The most visible

indicator is the presence of well-developed paleosols

between some packages of cataclysmic flood sediments

C.J. Pluhar et al. / Quaternary Research 65 (2006) 123–135126

(e.g., Patton and Baker, 1978; McDonald and Busacca,

1988). Such strongly developed soils likely formed over

many thousands of years (Bjornstad et al., 2001) and

suggest that the flood deposits they cap stem from earlier

glaciations. U/Th disequilibrium dates on some of these

pedogenic carbonates yield ages of 0.2 to 0.4 myr (Baker et

al., 1991). In addition, the basal portions of many of these

localities exhibit reversed paleomagnetic polarity (McDo-

nald and Busacca, 1988, 1992; many unpublished works

summarized in Baker et al., 1991), indicating deposition

before 0.78 myr, the last major polarity (Brunhes-

Matuyama) reversal. The current study expands upon and

revises earlier paleomagnetic work (in Bjornstad et al.,

2001) on the Cold Creek bar that suggested that Scabland

Floods commenced prior to 1.77 myr, the end of the Olduvai

normal subchron.

Methods

As part of research into the nature and extent of

subsurface contamination at the U.S. Department of

Energy’s Hanford site, many drill cores have been recently

collected from the Cold Creek bar. We have drawn our

paleomagnetic samples from some of these drill cores,

which were collected using a ‘‘split spoon’’ sampling device

deployed either on a wireline or threaded rods. In either case

neither absolute azimuthal core orientation, nor relative

azimuth between core segments was preserved. Thus,

magnetic polarity for these cores is based solely on

paleomagnetic inclination. The split spoons consisted of

10-cm-diameter, 0.6- to 1.8-m-long hollow tubes containing

three Lexani liners. Intact sediment core was collected by

driving the split spoon ahead of the drill bit into undisturbed

sediment. The split spoon was then pulled up, opened, and

the liners were capped and labeled with core number, depth

interval, and up direction.

In the laboratory, drill core segments were split in half

lengthwise. To collect paleomagnetic samples, five faces of

a sample cube were carved from the center line of a core

segment, using stainless-steel instruments. An 8-cm3, plastic

box, open on one end and labeled with core number, depth,

and ‘‘up’’ arrow, was placed over each sample before the

rear, sixth face of the sample cube was carved away from

the drill core. Finally, a few drops of sodium silicate

impregnating solution were added to the sediment and

allowed to dry before securing the plastic cover.

We collected 213 new paleomagnetic samples from nine

drill cores to add to the 53 samples from four cores already

described in Bjornstad et al. (2001). The new and old data

are hereafter treated together and integrated. Samples fall

into two geographic groups (Fig. 2). Coreholes on the

eastern side of the Cold Creek flood bar penetrated sands

and gravels while those on the western side of the flood bar

in a minor flood channel penetrated mainly silty sands and

sands.

Stratigraphically, samples span the entire Hanford

formation, an informal name given to Pleistocene cataclys-

mic flood sediments of the Pasco Basin (see US DOE/RL-

2002-39 for details), as well as the upper parts of the units

that underlie it: either the Cold Creek unit, a fine-grained,

pedogenically altered unit of eolian to fluvial origin thought

to be Pliocene in age (US DOE/RL-2002-39), or the fluvial

(in this vicinity), Mio-Pliocene (Smith et al., 2000; Packer

and Johnston, 1979) Ringold Formation. Samples were

always collected from the most fine-grained horizons

available. Nonetheless, this often meant sampling medium-

to coarse-grained sands.

Paleomagnetic remanence

The magnetic moments of samples were measured in a

3-axis DC SQUID cryogenic magnetometer housed in a

magnetically shielded room that cancels more than 99% of

Earth’s ambient magnetic field. In order to reveal whether

the samples carry stable magnetization directions, we

performed progressive stepwise demagnetization experi-

ments, thereby preferentially removing low-stability, secon-

dary magnetic overprint components. Demagnetization

continued at increasing levels until the sample’s moment

direction became random or the maximum demagnetization

level had been achieved. Both alternating-field (AF) and

thermal demagnetization techniques were employed to a

maximum of 200 mT or 700-C, respectively. Thermal

demagnetization, conducted on samples after sawing them

out of their plastic boxes, appears to yield slightly better

results than AF, as suggested by the higher likelihood in

some cores of revealing a reversed polarity using the former.

Typically, 18 demagnetization/measurement steps were

conducted for each sample. Representative demagnetization

data are shown in Figure 3 and are discussed in the next

section.

Most samples required mean destructive fields (MDF),

the AF demagnetization level necessary to remove 50% of

the initial natural remanent magnetization (NRM), of only

10 mT or less. When thermally demagnetized, similarly low

demagnetization of 200-C was usually sufficient to elimi-

nate 50% of NRM. Often this low-stability component is a

subvertical downwards-inclined component parallel to the

induced magnetization associated with the drill string.

Figure 4 shows this overprint component, determined by

principal component analysis (PCA) (Kirschvink, 1980;

Cogne, 2003), separated from the characteristic remanent

magnetization (ChRM—derived from the demagnetization

steps that decay unidirectionally towards the origin) for

samples from core W22-48. Declinations are random since

each drill core segment is azimuthally unoriented. The

normal polarity ChRMs exhibit a mean inclination of 53.7- T16.9- (method of McFadden and Reid, 1982), statistically

indistinguishable from the mean geocentric axial dipole

(GAD) inclination for this site of around 65-. In this core,

the very steep, downward-inclined overprint is easily

Figure 3. Judging Polarity. (A) A typical normal-polarity sample from E24-21, 213.0V. (B) A typical indeterminate-polarity sample from E24-21, 198.2V. (C) A typical reversed-polarity sample from E24-21, 233.0V.All of these samples exhibit drilling-induced secondary magnetic components.

C.J.

Pluharet

al./Quatern

ary

Resea

rch65(2006)123–135

127

Figure 4. Core W22-48 ChRM and drilling-induced overprints. These stereonets depict PCA directions with associated MAD error elipses. The ChRM mean

inclination of 53.7- T 16.9- is indistinguishable from the expected mean inclination at this latitude for a GAD field.

C.J. Pluhar et al. / Quaternary Research 65 (2006) 123–135128

distinguishable in six out of fifteen samples. We interpret

this to be a drilling-induced overprint parallel to the drill

string. The overprint directions identified here display

inclinations greater than 70-.

Determination of primary magnetization polarity

We stress that because all paleomagnetic samples

discussed here derive from sampling methods in which no

absolute azimuthal sample orientation is preserved, only

sample inclination can be used to reveal polarity. Further-

more, the sediments composing the Hanford formation are

not ideally suited to paleomagnetic analysis due to their

coarse and unlithified nature. Thus, in this study, we

interpret the data to the best of our ability, recognizing the

limitations of the geological material available.

The stereonet and Zijderveld diagrams of Figure 3 depict

three classes of data: normal, reversed and indeterminate

polarity. PCA was applied to some sample demagnetization

paths to determine best-fit ChRMs. We take up-inclination

ChRMs to be reversed polarity and down-inclination to be

normal polarity, with the following exception: samples that

exhibit very shallow ChRM inclinations, between �20- and+20-, we term indeterminate. The shallowness of this

inclination cutoff is justified because sample collection by

driving the split-spoon sampler can cause significant

inclination shallowing through compaction.

Many samples never stabilize on one direction and thus

were not amenable to PCA. This presents no problem to our

main goal, the determination of paleomagnetic polarity,

since a sample exhibiting up-inclined directions at higher

level demagnetization steps is almost certainly reversed-

polarity even if the remanence direction wanders around on

the up-inclination hemisphere of a stereonet (e.g., Fig. 3C).

For sample data not amenable to PCA, we determine

polarity as follows. If a sample exhibits dominantly up (or

dominantly down) inclinations steeper than �20- (or 20-) at

‘‘higher demagnetization levels’’ then it is considered

reversed polarity (or normal polarity). If the sample wanders

between up- and down-inclinations during the higher

demagnetization steps, or if the sample exhibits evidence

for a reversed component as the middle of three (or more)

components, then the sample is considered of indeterminate

polarity. Where samples exhibit very scattered directions

that define their polarity, they are classed as ‘‘low quality.’’

This designation is also assigned to a few samples that

exhibit only a very steep, very tightly defined ChRM that is

reminiscent of a drilling overprint, (e.g., drill core W11-39,

sample 49.6V exhibiting an 85.0- inclination ChRM and a

maximum angular deviation (MAD) of 0.7-).

Rock magnetic experiments

We measured weak-field magnetic susceptibility as a

function of temperature to determine the magnetic minera-

logy of a random subset of samples. This indicates

whether magnetic minerals are present that could have

resulted from secondary growth after deposition. In another

set of experiments, we measured magnetic hysteresis

parameters on sub-samples or individual sediment grains

to test for the presence of large (multi-domain) magnetite

grains, which would be easily remagnetized by applied

magnetic fields such as the Earth’s, or that resulting from

the drill string. A third dataset comes from the stepwise

demagnetization experiments already described, which

provide additional information about stability of remanence

from the coercivity and blocking temperature spectra of the

samples.

The main goal of the susceptibility experiments was to

assess whether significant, potentially secondary, magnetic

minerals such as hematite were present. The data, as shown

in Figure 5, demonstrate that the vast majority of the

susceptibility signal is due to magnetite, a detrital and

therefore primary mineral in this type of sediment. Both the

Figure 5. Thermomagnetic curves for selected samples. These weak-field magnetic susceptibility versus temperature data clearly show that the dominant

magnetic mineral is pure magnetite, as indicated by the strong Verwey transition near �155-C and a Curie point near 580-C. Arrows distinguish the heating

and cooling segments of these curves.

C.J. Pluhar et al. / Quaternary Research 65 (2006) 123–135 129

Curie temperatures near 580-C and the Verwey (phase

change from monoclinic to cubic magnetite) transitions

around �155-C exhibited by all 24 samples tested are

highly diagnostic. A small hump (e.g., Fig. 5, E24-21,

171.5V) in most of the heating curves near 275-C probably

indicates minor titanomaghemite, a metastable oxidation

product of titanomagnetite. Titanomaghemite is common in

lavas, and is likely also detrital, derived from the Columbia

River basalt. The disappearance of the small hump after

heating beyond 275-, and the often substantial irreversible

increase in susceptibility upon cooling, suggests inversion

of titanomaghemite to magnetite plus ilmenite (e.g.,

Ozdemir, 1987). Thus, thermal demagnetization experi-

ments may be swamped at high temperature steps by

magnetite production if not properly shielded during cool-

ing, and caution should be exercised in interpreting such

high temperature demagnetization data. Minor hematite is

also indicated in some samples by a continued coherent

directional signal after demagnetization above 580-C,though its presence is not evident in the thermomagnetic

data (Fig. 5). The hematite signal is always overwhelmed by

magnetite and is not considered significant in the context of

polarity and remanence.

Figure 6 shows sample hysteresis data, a proxy measure-

ment for magnetic mineral grain size (Day et al., 1977) and

thus stability of sample magnetization. Samples plotting in

the multi-domain region of the diagram are generally less

stable paleomagnetically than samples plotting in the single-

or pseudo-single-domain grain regions. The samples ana-

lyzed span the entire pseudo-single domain range and most

likely represent mixtures of single, pseudo-single and

multidomain grains (Dunlop, 2002a,b), and fall on a mixing

line between them. These data suggest that the samples

probably contain some very stable primary magnetization

and some easily changeable (secondary) magnetization, the

latter being due to multidomain magnetite. This multi-

domain magnetite explains the low MDF values described

earlier.

We have observed no difference in the average magnetic

mineralogy or domain state between samples of the eastern

and western Cold Creek bar. Therefore, any differences in

polarity between the eastern and western Cold Creek bar are

Figure 6. Day plot of hysteresis parameters (Day et al., 1977) showing Hanford formation magnetic granulometry. The samples measured, actually sand-grain-

sized subsamples of paleomagnetic samples, fall on a likely mixing line between single and multi-domain magnetite grains. The three samples specifically

identified were chosen to demonstrate the hysteresis behavior of pure-composition end members of the sediment components. All other samples are multi-

grained and of mixed composition. There is no indication that samples from one core are more stably magnetized than those from another.

C.J. Pluhar et al. / Quaternary Research 65 (2006) 123–135130

likely real and not due to magneto-mineralogic differences

in samples between the two areas. Furthermore, the

existence of reversed-polarity directions and the stability

of this signal throughout demagnetization support the

hypothesis that the polarity information carried by these

samples is primary and was established during or soon after

deposition.

Magnetostratigraphic interpretation

In interpreting the magnetostratigraphy, we have two

quality control checks. First, to guard against upside-down

core sections, we know that drilling-induced magnetization

is downwards inclined and near vertical. Of the approx-

imately 50% of samples that exhibited a subvertical

component, only one had been mis-oriented, as indicated

by its drilling component’s upward instead of downward

inclination. In addition, where we have multiple samples

from one coherent length of core, the azimuths of their

magnetizations (declination) should be oriented relative to

one another. Figure 7 shows data from three groups of such

samples. In the core segment where ChRMs are statistically

indistinguishable (Fig. 7A), we have confidence that the

results are robust. Where ChRMs are within the same

quadrant (Fig. 7B), results are slightly less certain, though

still satisfactory. Poor reproducibility within a core segment

(Fig. 7C) is obvious when samples are grouped in this way,

and permits rejection of these data. The poor reproducibility

of this core segment appears to coincide with shallowed

ChRMs. Note also that despite some grouping of ChRMs in

Figure 7C, the two groups, one with up inclination and the

other with down inclination and with relative declinations

about 90- apart, are not stratigraphically coherent. Rema-

nence fluctuates between the two groups down-section and

thus is not considered trustworthy.

Using the criteria previously described, we assigned a

polarity to each sample and noted which samples were of

poor quality. These poor quality and indeterminate polarity

samples were ignored when interpreting magnetostratigra-

phy. Used in this way, the remaining sample polarities for

the most part fall into stratigraphically bounded polarity

zones that laterally correlate to adjacent drill holes.

Indeterminate, i.e., unknown, polarity is assigned to the

Figure 7. ChRMs for selected E17-21 core segments. In a few cases, the occurrence of multiple samples from a given drill core segment allows assessment of

data quality. Sample ChRMs for a given core should be azimuthally oriented relative to one another unless disturbed post-depositionally or during drilling. (A)

Three samples from this core segment exhibit excellent correspondence of azimuth and inclination and thus are of excellent paleomagnetic quality. Their Fisher

mean direction is D = 201.5-, I = �61.5-, kappa = 39.0, a95 = 20.0, indistinguishable from the mean GAD inclination, �65-, for reversed polarity at this site.

(B) Three samples from this segment are all up-inclination and within the same quadrant, suggesting good paleomagnetic quality. Their mean inclination,

�58.6- T 26.1- (method of McFadden and Reid, 1982), is also indistinguishable from a GAD inclination. (C) Five samples from this core segment exhibit both

up and down inclinations intermixed and azimuths that are not interpretable as opposite polarities. The shallowed mean inclination (�29.6- T 21.0-, with all

directions flipped to the upper hemisphere) of these ChRMs suggests compaction shallowing and may be a good indicator of paleomagnetic signal disturbance

either post-depositionally or during drilling.

C.J. Pluhar et al. / Quaternary Research 65 (2006) 123–135 131

depth zones where normal and reversed polarities appear to

be intimately mixed.

Eastern Cold Creek bar

Figure 8 depicts a cross-section through the eastern side

of the Cold Creek bar (cross-section location in Fig. 2).

Interpreted sample polarities are shown along with the

proposed magnetostratigraphic interpretation. Cores E24-21

and E17-22 show evidence of a normal polarity zone in the

lower part of the stratigraphy with reversed polarities above

and below. Due to the admixture of reversed and normal

samples, we assign indeterminate polarity zones near the

tops of both of these cores. E33-335 and E33-338 exhibits a

normal polarity at the base with indeterminate polarity

above that. These drill cores come from an area more

proximal to the highest energy flood routes, and the

indeterminate polarity may reflect this in poor paleomag-

netic recording. The sampled intervals in E17-21 and E17-

23 both show only reversed polarity, although the sampling

was sparse. If interpreted in this way, each polarity zones

contains (1) indeterminate polarity samples, (2) low quality

samples of both polarities, (3) good quality samples of the

appropriate polarity for that zone. The sole exception to this

is one normal polarity sample within the upper reversed

zone in drill core E17-22 near 655V elevation.Note that in E24-21 the lower magnetic reversal of the

normal polarity zone coincides with what has been

interpreted as a weak paleosol (for data on Cold Creek bar

paleosols see Reidel and Ho, 2002). The upper boundary of

the normal zone probably also coincides with a paleosol

since reversed polarity is found above the paleosol horizon

in E17-23, while in E24-21 normal polarity is found below

the paleosol. The paleosols are described as carbonate-

cemented and/or iron stained horizons a few cm thick. The

correspondence between reversals and these alleged paleo-

sols is the basis of correlation between E17-21/23 and

E24-21/E17-22. A putative paleosol not coinciding with a

polarity reversal is encountered in five out of six of these

cores at about 21.3 (70 ft.) below ground surface. This

paleosol has been used to correlate between drill cores in the

southern part of this cross-section (Reidel and Ho, 2002) as

indicated in Figure 8. We see little evidence for a reversed-

to-normal polarity change, corresponding to the transition to

the present normal field, near the tops of these drill cores.

However, such a transition is expected since Cold Creek

bar’s surface appears depositional rather than erosional.

The simplest interpretation of eastern Cold Creek bar

magnetostratigraphy is that it records a time interval of

predominantly reversed polarity before the last major

polarity reversal (Brunhes-Matuyama, 0.78 myr). Given

this, the normal zone in this stratigraphy is best explained as

the next major normal subchron, the Jaramillo (0.99–

1.07 myr). However, there are alternative explanations that

cannot be eliminated, such as (1) the normal polarity zone

could be the Olduvai (1.77–1.95 myr) or some other normal

Figure 8. Magnetostratigraphy at eastern Cold Creek bar. We interpret magnetostratigraphy by excluding low quality and indeterminate-polarity samples.

Reversed and normal polarity zones result where only samples of that polarity are included. Where polarities are mixed, the zone is termed indeterminate. We

interpret the normal zone to be the Jaramillo subchron (0.99–1.07 myr) and relate the paleosols to interglacial episodes.

C.J. Pluhar et al. / Quaternary Research 65 (2006) 123–135132

subchron, or (2) the normal polarity zone represents a time

period between magnetic excursions during the Brunhes

chron. The first alternative was the favored interpretation of

the much more limited paleomagnetic data presented in

Bjornstad et al. (2001). With samples now spanning the full

thickness of the Hanford formation at eastern Cold Creek

bar, this interpretation requires that a large part of geologic

time is missing from the top of the eastern Cold Creek bar

cores. The latter explanation seems unlikely due to the

relative thickness of normal and reversed zones compared to

the very short duration of excursions (e.g., Liddicoat, 1992)

and the long durations of normal polarity time in between.

However, the episodic nature of Cold Creek bar deposition

could produce either result, or others not discussed.

Western Cold Creek bar

Western Cold Creek bar magnetostratigraphy records a

substantially different situation and probably spans a

different time-period than eastern Cold Creek bar.

C.J. Pluhar et al. / Quaternary Research 65 (2006) 123–135 133

Figure 9 shows interpreted sample polarities, magneto-

stratigraphy and stratigraphy (from Serne et al., 2002,

2004a,b) of the western cross-section. Most significant is

the preponderance of normal-polarity samples and the

dearth of both indeterminate and reversed polarities. Given

this, we interpret the sampled interval of the western Cold

Creek bar as recording a period during the Brunhes normal

chron (0–0.78 myr).

While the western Cold Creek bar is largely of normal

polarity, there are a handful of reversed and indeterminate

polarity samples that correlate along two stratigraphic

horizons, roughly mimicking subsurface stratigraphic struc-

ture. These horizons are thin—usually recorded by only one

sample each per core. The upper reversed horizon must be

less than 5.2 m (17 ft.) thick from the normal samples

bracketing it, while the lower is less than 3.0 m (10 ft.) thick.

Thus they probably, at most, record one multi-jokulhlaup

glacial stage or one interval of colluvial reworking/paleosol

formation each. The reversed polarity of these samples

appears to reflect the geomagnetic field because (1) the

upper one is particularly stratigraphically coherent from one

core to another, and (2) they do not result from sample

misorientation, as evidenced by the properly oriented

Figure 9. Magnetostratigraphy and stratigraphic cross-section of western Cold Cr

horizons. Our best hypothesis is that these sediments record part of the Brunhes no

period. Timescale is adapted from Singer et al. (2002).

drilling overprint in most of them. The consistency of the

upper reversed horizon from one core to another bolsters the

case that the western Cold Creek bar records primary

polarity. Since the reversed horizons are thin and are not

associated with well-developed soil horizons they likely

represent two of the brief magnetic excursions of roughly

104 yr duration Cold Creek bar during the Brunhes chron.

Many short polarity excursions are known or suspected to

have occurred during the Brunhes chron (e.g., Singer et al.,

2002) (see Fig. 9), but our data do not permit a

determination as to which ones may have been recorded.

Paleoclimatic interpretation of eastern Cold Creek bar

Hanford formation sedimentation represents cataclysmic

flood events (US DOE/RL 2002-39) resulting from a

Cordilleran ice sheet extensive enough to form large ice-

dammed or subglacial lakes in the upper Columbia River

drainage basin. Previous studies show that Missoula flood

sedimentation in eastern Washington occurred during a

period of only a few thousand years around the last

glacial maximum (Waitt, 1985), while the much longer

eek bar. Polarity is almost entirely normal except for two reversed-polarity

rmal chron (0–0.78 myr) and capture two magnetic excursions during that

C.J. Pluhar et al. / Quaternary Research 65 (2006) 123–135134

interglacial intervals before and after are characterized by

depositional hiatuses and pedogenesis at the study area. If

this can be generalized to the entire Hanford formation,

the majority of the time spanned by the section is

represented by depositional hiatuses and the candidate

paleosols of Reidel and Ho (2002), while the majority of

the section was deposited during very short periods with

high deposition rates. The correspondence of the polarity

boundaries with the putative paleosols is thus not

surprising.

Based on this logic, the basal jokulhlaup sediments were

deposited during a glacial interval and reversed polarity. A

hiatus then occurred with attendant pedogenesis. During this

hiatus the Earth’s magnetic field reversed to normal polarity.

The reversed to normal boundary and the putative paleosol

probably represents a hiatus of at least the few thousand years

necessary for a magnetic reversal (Singer and Pringle, 1996;

Clement, 2004). A return to glacial conditions then occurred,

as evidenced by renewed Hanford formation deposition.

Deposition was probably followed by another interruption

with coincident paleosol formation and a transition back to

reversed polarity, again suggesting a hiatus for a period

sufficient for magnetic polarity reversal. Return to full glacial

conditions is indicated by further Hanford formation depo-

sition, succeeded by a hiatus with pedogenesis, followed by

another glaciation and renewed deposition. Hence, four

glacial maxima are recorded by these sediments with three

intercalated hiatuses, two of which correspond to magnetic

reversals. More conservatively, if the interpreted presence of

weak paleosols is rejected, and only the magnetic reversals

are interpreted as representing hiatuses, then three glacial

maxima are recorded.

Discussion and conclusions

Investigations of the Cold Creek flood bar stratigraphy

in the Pasco Basin of eastern Washington yield a longer,

more complete record of pre-late-Pleistocene cataclysmic

Scabland flood events than any surface exposures studied

to date. Our preferred interpretation of the magnetostrati-

graphic results is that the 76-m (250-ft.) thick section of

largely reversed Hanford formation sediments at the

eastern Cold Creek bar records a period starting before

1.07 myr (before the Jaramillo subchron) and ending

between 0.78 and 0.99 myr (between the Jaramillo

subchron and the Brunhes-Matuyama reversal), with the

uppermost portions of the section being of indeterminate

polarity. The two magnetic reversals within this stratig-

raphy coincide with weak paleosols and thus fall during

interglacial depositional hiatuses. Therefore, it appears that

at least three glacial maxima during the early Pleistocene

are recorded at the eastern Cold Creek bar. We interpret

the 27-m (90-ft.) thick (largely normal polarity) section

sampled at the western Cold Creek bar as spanning some

part of the Brunhes normal chron (0–0.78 myr).

The forgoing interpretations have several implications.

First, the onset of cataclysmic flooding in eastern Wash-

ington predated 1.07 myr, so that physiographic elements

conducive to periodic jokulhlaups had already developed by

this time somewhere in the greater Columbia River drainage

system, and formation of the Channeled Scabland had

already begun. Hence the Cordilleran ice sheet during the

early to mid-Pleistocene must have reached latitudes south-

erly enough to affect the Columbia River drainage basin.

This is in agreement with recent results (Balco et al., 2005)

that date two Laurantide icesheet tills in Missouri as early

Pleistocene and late Pliocene. Second, the markedly

different magnetostratigraphy of the eastern and western

Cold Creek bars implies that flood routes and loci of

deposition have evolved with time. Third, if the western

Cold Creek bar records some portion of the Brunhes chron

then the reversed horizons within that magnetostratigraphy

record magnetic excursions—the first suggestion of such an

occurrence in Scabland Flood deposits. Dating these

horizons and correlating them to the magnetic polarity time-

scale could further refine the eastern Washington jokulhlaup

stratigraphy.

Given the inherent episodicity of jokulhlaup deposition

and the possibility of initial erosion during each event,

followed by deposition, there remains the possibility of

undetected hiatuses throughout our magnetostratigraphy.

Hence competing hypotheses cannot be fully rejected at

present. Among these are the possibility, suggested by

Bjornstad et al. (2001), that the eastern Cold Creek bar

records the Olduvai normal subchron (1.77–1.95 myr)

rather than the Jaramillo (0.99–1.07 myr). However, with

the current dataset and selecting the most conservative

interpretation, we arrive at the stated conclusion because

(1) unless we choose the first pre-Brunhes normal

subchron, there is no basis for favoring any particular

antecedent subchrons, and (2) correlating with the

Jaramillo requires the minimum missing section within the

eastern and/or between the eastern and the western Cold

Creek bar. Another possibility that cannot be rejected is that

cores in both the eastern and the western Cold Creek bar

record parts of the Brunhes chron, and that the reversed

zones at both areas record magnetic excursions. This

hypothesis obviates a missing section at the top of the

eastern Cold Creek bar cores, but requires that the majority

of section at this area be deposited during two separate

excursions—a chain of events that would seem to be

extremely unlikely.

Acknowledgments

Many thanks to Fred Mann, Karl Fecht, Robert S.

Anderson and Eli Silver. Helpful peer review was provided

by Jim O’Connor, Randy Enkin, and Vic Baker. Battelle

Memorial Institute funded this research through contracts

4584 and 403148-A-B3.

C.J. Pluhar et al. / Quaternary Research 65 (2006) 123–135 135

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