Eruption chronology of the Columbia River Basalt Group

45

The Geological Society of AmericaSpecial Paper 497

2013

Eruption chronology of the Columbia River Basalt Group

T.L. BarryDepartment of Geology, University of Leicester, University Road, Leicester, LE1 7RH, UK

S.P. KelleyDepartment of Physical Sciences, Centre for Earth, Planetary, Space and Astronomy Research,

Open University, Milton Keynes, MK7 6AA, UK

S.P. ReidelSchool of the Environment, Washington State University–Tri-Cities, 2710 Crimson Way, Richland, Washington 99354, USA

V.E. CampDepartment of Geological Sciences, San Diego State University, 5500 Campanile Drive, San Diego, California 92182, USA

S. SelfDepartment of Physical Sciences, Centre for Earth, Planetary, Space and Astronomy Research,

Open University, Milton Keynes, MK7 6AA, UK

N.A. JarboeDepartment of Earth & Planetary Science, University of California, 1156 High Street, Santa Cruz, California 95064, USA

R.A. DuncanCollege of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA

P.R. RenneBerkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA, and

Department of Earth & Planetary Science, University of California, Berkeley, California 94720, USA

ABSTRACT

The Columbia River fl ood basalt province, United States, is likely the most well-studied, radiometrically well-dated large igneous province on Earth. Compared with older, more-altered basalt in fl ood basalt provinces elsewhere, the Columbia River Basalt Group presents an opportunity for precise, accurate ages, and the opportunity to study relationships of volcanism with climatic excursions. We critically assess the available 40Ar/39Ar data for the Columbia River Basalt Group, along with K-Ar data, to establish an up-to-date picture of the timing of emplacement of the major forma-tions that compose the lava stratigraphy. Combining robust Ar-Ar data with fi eld

Barry, T.L., Kelley, S.P., Reidel, S.P., Camp, V.E., Self, S., Jarboe, N.A, Duncan, R.A., and Renne, P.R., 2013, Eruption chronology of the Columbia River Basalt Group, in Reidel, S.P., Camp, V.E., Ross, M.E., Wolff, J.A., Martin, B.S., Tolan, T.L., and Wells, R.E., eds., The Columbia River Flood Basalt Province: Geologi-cal Society of America Special Paper 497, p. 45–66, doi:10.1130/2013.2497(02). For permission to copy, contact [email protected]. © 2013 The Geological Society of America. All rights reserved.

46 Barry et al.

INTRODUCTION

The Columbia River continental fl ood basalt province (Fig. 1), with lavas stratigraphically defi ned as the Columbia River Basalt Group (Swanson et al., 1979, and references therein; Tolan et al., 1989), constitutes the mafi c part of Earth’s youngest large igneous province. Basalt volcanism is generally considered to have begun around 17 Ma in the latter part of the early Mio-cene, close in time to the earliest rhyolitic eruptions associated

with the Snake River Plain hotspot track at ca. 16.5 Ma (e.g., Rytuba and McKee, 1984; Swisher, 1992). Some workers have proposed a plate-tectonic origin for the province involving the passive rise of shallow mantle (e.g., Carlson and Hart, 1987; Smith, 1992; Hales et al., 2005; Tikoff et al., 2008), but others have embraced a deep-mantle origin associated with active rise of the Yellowstone mantle plume (e.g., Duncan, 1982; Brandon and Goles, 1988, 1995; Draper, 1991; Hooper and Hawkesworth, 1993; Geist and Richards, 1993; Camp, 1995; Dodson et al.,

constraints and paleomagnetic information leads to the following recommendations for the age of emplacement of the constituent formations: Steens Basalt, ca. 16.9 to ca. 16.6 Ma; Imnaha Basalt, ca. 16.7 to ca. 16 Ma; Grande Ronde Basalt, ca. 16 Ma to ca. 15.6 Ma; Wanapum Basalt, ca. 15.6 to ca. 15 Ma; and Saddle Mountains Basalt from ca. 15 Ma to ca. 6 Ma. The results underline the previously held observation that Columbia River Basalt activity was dominated by a brief, voluminous pulse of lava production during Grande Ronde Basalt emplacement. Under scrutiny, the data highlight areas of complexity and uncertainty in the timing of eruption phases, and demonstrate that even here in the youngest large igneous province, argon dating can-not resolve intervals and durations of eruptions.

Oregon

Nevada

Idaho

MontanaWashington

California

0 50

Kilometers

Miles

0 80Margin of CRBG

Columbia River

Snake River

Cas

cade

Ran

ge

Salmon River

Snake River

Pac

ific

Oce

an

Grande Ronde River

Imnaha River

River

Salem

Prineville

YakimaaPullman

Spokane

Wenatcheee

Columbia Gorge

Extent of GRBRB

Columbia Basin

OregonPlateau

Lowland

Blue Mountains

Wallowa

ClearwaterEmbayment

WeiserEmbayment

PascoPascoBasinn

John Day

Pasco

DeschutesRiver

KlickitatRiver

WillametteValley

Vantage

Lewiston Basin

Stee

ns M

ount

ain

Roc

kyR

ocky

Mou

ntai

ns M

ount

ains

Roc

ky M

ount

ains

Malheur Gorge

48°

47°

46°

45°

44°

43°

42°

123° 121° 119° 117°

Figure 1. Map of the Columbia River Basalt Group within the northwestern states of Washington, Oregon, Idaho and Nevada, USA, shows the main regions of basalt exposure, including Steens Mountain in the south, Malheur Gorge, and the Columbia and Snake Rivers to the north.

Eruption chronology of the Columbia River Basalt Group 47

1997; Hooper et al., 2002, 2007; Camp et al., 2003; Camp and Ross, 2004; Camp and Hanan, 2008; Wolff et al., 2008; Camp et al., this volume).

Large igneous provinces and their extrusive components, fl ood basalt lavas, represent exceptional volcanic events (Eldholm and Coffi n, 2000). It has been recognized for over 20 years that highly voluminous fl ood basalt provinces worldwide had peak periods of lava output (Rampino and Stothers, 1988) emplaced within geologically short periods of time (typically ~<1 m.y.; e.g., Courtillot et al., 1986; Duncan and Pyle, 1988; Chenet et al., 2008, 2009; Reichow et al., 2009). This signifi es a consid-erable potential for atmospheric loading by volcanic gases and for inundating landscapes with vast volumes of basalt lava. Such events have been linked to mass extinctions (e.g., Wignall, 2001; Courtillot and Renne, 2003; Saunders and Reichow, 2009) and near-surface arrival/emplacement of mantle plumes (e.g., Cour-tillot et al., 2003, and references therein).

The Columbia River Basalt Group is estimated to have a total volume of 210,000 km3 and an areal extent of 208,000 km2 (Reidel et al., this volume). Columbia River Basalt Group lavas are the product of the most voluminous outpouring of basalt lava within the Columbia River–Snake River–Yellowstone province. However, the basalt-dominated part of the province is an order of magnitude smaller in volume than many other large igneous provinces, such as the Deccan basalts (~1.3 × 106 km3; Jay and Widdowson, 2008). Conversely, it appears that volumes of lava emplaced during individual Columbia River Basalt Group erup-tions are as large as those reported from other provinces (Reidel et al., 1989; Reidel, 2005; Self et al., 2006; Reidel and Tolan, this volume). The environmental impact of Columbia River Basalt Group eruptions, or groups of eruptions, is uncertain. One motivation for detailed age constraints is to enable comparison between the timing of the eruptions, including periods of high time-averaged magma output, and any mid-Miocene climatic or biotic perturbations (e.g., Kender et al., 2009). In addition, it may be possible to obtain a better understanding of the absolute age of the Columbia River Basalt Group lavas and the relationship to rhyolitic “pre-Yellowstone” volcanism in the Oregon-Nevada-Idaho region.

This paper presents a compilation of K-Ar and Ar-Ar age determinations for the Columbia River Basalt Group and some related units. It discusses some of the past and present issues with defi ning the longevity and pulse-like nature of the basaltic vol-canism. It examines what is known about correlation with the paleomagnetic time scale for lava units with determined paleopo-larity, and proposes directions for future work in improving the understanding of Columbia River Basalt Group geochronology.

NOMENCLATURE AND PREVIOUS GEOCHRONOLOGICAL WORK ON THE COLUMBIA RIVER BASALT GROUP

In the previous Geological Society of America Special Paper 239 on Volcanism and Tectonism in the Columbia River

Basalt Province (Reidel and Hooper, 1989), the Columbia River Basalt Group was considered to consist of lavas from the Imnaha Basalt and the Grande Ronde Basalt, Wanapum Basalt, and Saddle Mountains Basalt of the Yakima Basalt Subgroup. A few outlying lava sequences that erupted synchronously with the Grande Ronde Basalt, e.g., Picture Gorge Basalt, were also included in the Columbia River Basalt Group. Since that time, developments in mapping and new geochronological studies have led to suggestions that the Steens (Mountain) Basalt and the Basalt of Malheur Gorge, lying to the south of the main Columbia River Basalt Group outcrop area, should be included in the Columbia River Basalt Group (Hooper et al., 2002, 2007; Camp et al., 2003; Camp and Ross, 2004). A new defi nition and the status for the Steens Basalt within the Columbia River Basalt Group are explained in Camp et al. (this volume). The basis for inclusion of the Steens Basalt within the Group are that there is a continuity of erupted magma types with shared isotopic end members (Camp and Hanan, 2008), conformable contacts between the major formations, an apparent northward-shifting locus of eruption sites over time, and contemporaneity within the majority of these lavas (see Camp et al., this volume). Reidel et al. (this volume) describe these updates and the current stratigraphic nomenclature of the Columbia River Basalt Group; Figure 2 provides a summary of the units for which we provide age dates.

The major subdivisions defi ned by previous workers have not changed with these latest revisions, except with the addi-tion of the Steens Basalt as the earliest Columbia River Basalt Group eruptions (Camp et al., this volume) and the Prineville Basalt, which, like the Picture Gorge Basalt, is interbedded with the Grande Ronde Basalt (Reidel et al., this volume). The princi-pal changes to the nomenclature are those to the subdivisions of the formations. Reidel et al. (this volume) present these changes and the current stratigraphic nomenclature and the best current estimates of the spatial extent and volume of these fl ows. More detailed descriptions of some of the units are provided in other chapters in this volume: e.g., Camp et al. (this volume) describe the Steens Basalt; Reidel and Tolan (this volume) describe revi-sions to the nomenclature of the Grande Ronde Basalt since 1989 (Reidel et al., 1989); and Martin et al. (this volume) and Vye (2009) describe the Frenchman Springs Member and Wanapum Basalt nomenclature since 1989. Proposed changes to the Saddle Mountains Basalt stratigraphy are provided in Reidel et al. (this volume).

Initial age determinations of Columbia River Basalt Group lavas, since the earliest radioisotopic dating by the 40K-40Ar tech-nique (e.g., Evernden and James, 1964; Holmgren, 1970, and ref-erences therein; McKee et al., 1977), placed the eruption of the whole province between ca. 17 and 6 Ma (for earlier compilations of age determinations, see also Swanson et al., 1979; Baksi, 1989; Tolan et al., 1989). The overall picture has not changed radically since then. These studies recognized early on that a major erup-tive pulse produced the Grande Ronde Basalt lavas, estimated to have been emplaced between ca. 16.0 and ca. 15.0 Ma (see also

48 Barry et al.

Long and Duncan, 1983; Baksi, 1989; Hooper et al., 2002). Tolan et al. (1989) plotted volume of erupted lava versus time, show-ing how the Grande Ronde Basalt lavas dominate volumetrically. These studies also proposed that the bulk of the Columbia River Basalt Group lavas could have been erupted in a short period of time (<1–2 m.y.; Holmgren, 1970; McKee et al., 1977), with the Grande Ronde Basalt pulse only occupying a brief span, conjec-tured to be ~400,000 yr (Barry et al., 2010), and unresolvable with standard geochronological approaches. Recent updates to volume estimates for the Columbia River Basalt Group do not alter this view, but they do clarify a single large peak (acme) of lava production, with slightly earlier peaks, or synchronous high-output periods in different geographic areas.

More recent attempts at determining ages of Columbia River Basalt Group lavas using the 40Ar/39Ar scheme have been aimed at reducing the errors on the absolute ages (usually ±1.5–0.5 m.y. with K/Ar), so that the temporal inter-relationships between the contributing lava piles (e.g., Steens, Grande Ronde Basalt) can be better understood, and the main pulses of lava effusion can be more precisely defi ned (e.g., Brueseke et al., 2007; Baksi, 2010; Jarboe et al., 2008, 2010; Barry et al., 2010). However, efforts at error reduction in ages of some of the Columbia River Basalt

Figure 2. Summary of the revised stra-tigraphy of the Columbia River Basalt Group, presented in Reidel et al. (this volume), showing the relationship of all the units presented here with radioiso-tope data.

1GSA Data Repository Item 2013234, Appendix DR 1—New 40Ar/39Ar data and Appendix DR2—Compilation of all available related K-Ar and 40Ar/39Ar data, is available at www.geosociety.org/pubs/ft2013.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boul-der, CO 80301-9140, USA.

Group lavas have met with limited success (Barry et al., 2010), even though the results of that study underline the rapidity of emplacement of the Grande Ronde Basalt.

A REVIEW OF RADIOMETRIC AGE DATA FOR THE COLUMBIA RIVER BASALT

To present a comprehensive data set of all known radiomet-ric age dates for the Columbia River Basalt units, we have com-piled new (Appendix DR11; see also Camp et al., this volume) and published 40Ar/39Ar data, and older K-Ar data (Appendix DR2 [see footnote 1]). All 40Ar/39Ar dates have been standardized to the normalization value for Fish Canyon sanidine (Jourdan and Renne, 2007) in order to enable accurate comparisons. Both 40Ar/39Ar data and older K-Ar data have been recalculated to the K decay constant of Renne et al. (2010).


Criteria for Selection of “Acceptable” Ages for Columbia River Basalt Units

Ages determined by either K-Ar or 40Ar/39Ar total fusion and incremental heating methods are susceptible to errors and large analytical uncertainties due to several factors, such as post-crystallization exchange of K and Ar with the local environment through low-temperature alteration; unsupported (“excess” or “inherited”) 40Ar that is trapped at the time of crystallization or through subsequent hydrothermal alteration; and reactor-induced 37Ar and 39Ar recoil from neutron irradiation. In order to evaluate all published and new age determinations for accuracy, and for the proposal of best estimates for the age and duration of each of the units within the Columbia River Basalt Group, we used the following selection criteria.

1. First, we assessed analytical reproducibility. There are a few instances where multiple ages have been determined for the same sample (e.g., PF7 from the Basalt of Sand Hollow, French-man Springs Member of the Wanapum Formation; see below), and the age has statistically good reproducibility. We view such reproducibility as strong evidence of a reliable age. Furthermore, we looked at individual sample analytical uncertainty (quoted as 1σ error in Tables 1–11), placing less signifi cance on analyses that have high analytical uncertainty.

2. Second, but of equal importance, we examined ages rela-tive to the stratigraphic position of the samples based on observed superposed sequences of lavas in exposed or cored sections.

3. Finally, we examined 40Ar/39Ar incremental heating experiments for evidence of excess 40Ar, and 37Ar and 39Ar recoil, sample alteration, and atmospheric argon (e.g., Koppers et al., 2000, and references therein). We avoided arbitrary “cutoff” val-ues as a means of discrimination for reasons partly discussed in Barry et al. (2012). All these factors may have variably caused a loss of precision (as demonstrated in the 1σ error data), but this procedure allows us to investigate potentially erroneous ages.

K-Ar ages can be evaluated using the fi rst two criteria only. In general, samples analyzed by 40Ar/39Ar incremental heating experiments produced high proportions of radiogenic 40Ar and analytical uncertainties (2σ) of <1%–5%. There is a weak inverse correlation between % K (as indicated by 37Ar/39Ar) and preci-sion, which refl ects variations in radiogenic 40Ar production. In addition to the previous criteria, we also considered fi eld rela-tions and paleomagnetic data when making our fi nal recommen-dations of currently preferred ages for individual units.

RECOMMENDATIONS FOR AGE RANGES OF COLUMBIA RIVER BASALT FORMATIONS

Steens Basalt

The Steens Basalt is the oldest lithostratigraphic unit of the Columbia River Basalt Group and the second largest unit, with an estimated volume of 31,800 km3 (Camp et al., this volume). Although partly covered by younger Tertiary volcanic rocks, it

is well exposed throughout southeastern Oregon, particularly along northerly trending fault scarps associated with Basin and Range extension that began in this region after ca. 12 Ma (Col-gan et al., 2004). The most complete section of Steens Basalt lies along the Steens Mountain escarpment, where an ~950 m section of the unit is exposed (Johnson et al., 1998) (Fig. 1). This thick sequence of lavas was originally defi ned as the “Steens Moun-tain Basalt” by Fuller (1930, 1931), but then redefi ned as “Steens Basalt” by Mankinen et al. (1985).

Paleomagnetic studies in the 1960s and 1970s described the lavas at Steens Mountain as a conformable lithostratigraphic unit with a paleomagnetic transition indicating a very short period of eruption, ranging from ~2000 to 50,000 yr (e.g., Baksi et al., 1967; Watkins, 1969; Gunn and Watkins, 1970). Other work-ers have redefi ned these lavas as a petrochemical type, where chemically similar Steens and “Steens-type” lavas appear to have erupted sporadically over several million years (Hart and Carl-son, 1985; Carlson and Hart, 1987; Brueseke et al., 2007). Camp et al. (this volume) discuss the rationale for defi ning “Steens Basalt” more succinctly, in part by restoring the stratigraphic designation to its original usage. Here, we follow the intent of Article 22 of the Stratigraphic Code of North America, which discourages the establishment of formal stratigraphic units that straddle known, identifi able regional disconformities (Anony-mous, 2005). We therefore retain the term “Steens Basalt” of Mankinen et al. (1985) but restrict its lithostratigraphic usage to the conformable sequence of lavas typifi ed in the Steens Moun-tain stratigraphic section. This criterion eliminates Ar-Ar ages for lavas lying above unconformities that are described as Steens Basalt elsewhere, including sample MB97-2 (15.17 ± 0.36 Ma) in Brueseke et al. (2007), and samples 00HS-6 and MB01-6A (15.97 ± 0.2 Ma and 14.35 ± 0.38 Ma, respectively) in Brueseke and Hart (2008).

Using chemical criteria, Camp et al. (this volume) subdi-vided the Steens Basalt into two informal members, the lower and upper Steens Basalt, with the former extending across the breadth of the Steens outcrop region and the latter being less extensive but thickest in the area of Steens Mountain. A detailed record of paleomagnetic data has also allowed workers to subdivide the Steens Basalt into three magnetostratigraphic units that are best exposed at Steens Mountain, where an ~160-m-thick middle sec-tion of transitional polarity (T) separates lower lavas of reverse polarity (R

0) from upper lavas of normal polarity (N

0) (Baksi et

al., 1967; Watkins, 1969; Gunn and Watkins, 1970; Mankinen et al., 1985; Jarboe et al., 2008, 2010).

The number of Ar-Ar ages for Steens Basalt (Tables 1 and 2; Appendix DR2) exceeds that of any other Columbia River Basalt Group unit. Despite this sizable analytical database, amassed from analyses made at different laboratories, the over-all range of acceptable ages is reasonably constrained between ca. 16.9 and ca. 16.4 Ma. (The older date of 17.29 ± 0.81 Ma for sample KL049 [Hooper et al., 2002] is imprecise, and its inclu-sion does not signifi cantly affect the mean or duration of Steens Basalt activity.) While several of the Steens Basalt lavas with

50 Barry et al.

TA

BLE

1. A

r-A

r D

AT

A F

OR

ST

EE

NS

BA

SA

LTS

For

mat

ion

Pal

eom

ag.

Sam

ple

ID

Typ

e of

m

ater

ial

%40

Ar*

37

Ar/

39A

r 40

Ar/

39A

r ag

e (p

ublis

hed)

(M

a)

Err

or

(1σ)

R

ef

Rec

alc.

ag

e (M

a)

Err

or

(1σ)

Ste

ens

Bas

alt

? F

ig. 1

.e

Grd

mas

s N

o in

fo

No

info

16

.06

0.01

1

16.3

9 0.

11

Ste

ens

Bas

alt

? F

ig. 1

.f G

rdm

ass

No

info

N

o in

fo

16.0

8 0.

02

1 16

.41

0.13

Ste

ens

Bas

alt

N

04C

P02

P

lag

sep

90.5

5 6.

16

16.7

2 0.

08

6 16

.78

0.08

Ste

ens

Bas

alt

N

03S

S02

P

lag

sep

62.0

1 20

.40

16.8

4 0.

20

5 16

.91

0.20

Ste

ens

Bas

alt

N

03S

S09

G-1

P

lag

sep

57.2

8 23

.50

17.1

4 0.

18

5 17

.20

0.18

Ste

ens

Bas

alt

N

03S

S09

G-2

P

lag

sep

56.5

5 23

.32

16.7

1 0.

19

5 16

.77

0.19

Ste

ens

Bas

alt

T

04C

P16

G

Pla

g se

p 49

.60

37.4

4 16

.74

0.36

6

16.8

1 0.

36

Ste

ens

Bas

alt

T

04P

J23

Pla

g se

p 72

.50

16.3

9 16

.53

0.12

6

16.5

9 0.

12

Ste

ens

Bas

alt

T

G-4

0 P

lag

sep

78.1

6 15

.54

16.6

7 0.

25

6 16

.74

0.25

Ste

ens

Bas

alt

T

G-4

1 P

lag

sep

62.6

1 19

.80

16.6

1 0.

28

6 16

.66

0.28

Ste

ens

Bas

alt

R

05C

P63

A

Pla

g se

p 43

.66

28.5

7 16

.46

0.47

6

16.5

3 0.

47

Ste

ens

Bas

alt

R

03P

M10

C

Pla

g se

p 16

.86

3.28

16

.80

0.24

5

16.8

6 0.

24

Ste

ens

Bas

alt

R

04P

J03A

S

anid

ine

54.3

3 30

.56

16.6

5 0.

17

6 16

.71

0.17

Ste

ens

Bas

alt

R

05S

B01

A

Pla

g se

p 39

.16

39.9

0 16

.17

0.24

6

16.2

3 0.

24

Ste

ens

Bas

alt

? M

B97

-32

Pla

g se

p 81

.91

19.1

4 16

.03

0.15

2

16.2

0 0.

16

Ste

ens

Bas

alt

? M

B97

-24

Grd

mas

s 53

.07

3.00

16

.06

0.18

2

16.2

3 0.

18

Ste

ens

Bas

alt

? M

B97

-25

Grd

mas

s 50

.40

12.5

7 16

.24

0.25

2

16.4

1 0.

25

Ste

ens

Bas

alt

? 86

-29

Grd

mas

s 56

.70

3.55

16

.31

0.10

2

16.4

8 0.

10

Ste

ens

Bas

alt

? M

B97

-87

Grd

mas

s 55

.04

4.47

16

.33

0.10

2

16.5

0 0.

10

Ste

ens

Bas

alt

? C

H82

-22B

G

rdm

ass

77.5

5 6.

64

16.5

5 0.

05

2 16

.72

0.05

Ste

ens

Bas

alt

? C

H82

-22G

G

rdm

ass

63.8

3 8.

27

16.5

9 0.

05

2 16

.76

0.05

Ste

ens

Bas

alt

? M

B97

-65

Grd

mas

s 61

.87

4.53

16

.40

0.09

2

16.5

7 0.

09

Ste

ens

Bas

alt

? M

B97

-47

Grd

mas

s 59

.57

5.32

16

.48

0.15

2

16.6

5 0.

15

Ste

ens

Bas

alt

? M

B97

-55

Grd

mas

s 61

.38

5.11

16

.58

0.09

2

16.7

5 0.

09

Ste

ens

Bas

alt

? M

B01

-12

Grd

mas

s 10

.53

13.1

0 16

.07

0.46

3

16.2

3 0.

46

Ste

ens

Bas

alt

? M

B01

-13

Pla

g se

p 72

.85

12.5

4 16

.45

0.17

3

16.6

1 0.

17

Ste

ens

Bas

alt

? M

B01

-24

Grd

mas

s 11

.36

12.0

7 16

.95

0.65

3

17.1

4 0.

66

Ste

ens

Bas

alt

? O

OH

S-X

N

o in

fo

No

info

No

info

16

.73

0.02

3

16.9

0 0.

02

Ste

ens

Bas

alt

? JC

02-P

F25

G

rdm

ass

48.7

6 8.

70

16.3

3 0.

48

4 16

.73

0.49

Ste

ens

Bas

alt

? JC

02-P

F23

G

rdm

ass

59.8

5 3.

44

16.0

7 0.

15

4 16

.47

0.15

Ste

ens

Bas

alt

? JC

02-P

F24

G

rdm

ass

46.3

5 6.

59

17.0

1 0.

90

4 17

.41

0.93

Ste

ens

Bas

alt

R

JS7-

5 P

lag

sep

44.9

1 36

.03

16.8

4 0.

22

7 16

.91

0.21

Ste

ens

Bas

alt

R

JS7-

1 P

lag

sep

43.9

0 27

.04

16.8

2 0.

11

7 16

.88

0.11

Ste

ens

Bas

alt

R

MF

94-6

3 P

lag

sep

40.1

2 26

.91

16.5

4 0.

14

7 16

.60

0.14

Ste

ens

Bas

alt

? M

D06

-142

P

lag

sep

55.6

2 25

.96

16.7

4 0.

11

7 16

.80

0.10

Ste

ens

Bas

alt

? C

R97

-1

Pla

g se

p 71

.04

13.7

3 16

.41

0.10

7

16.4

7 0.

10

(Con

tinue

d)

Eruption chronology of the Columbia River Basalt Group 51T

AB

LE 1

. Ar-

Ar

DA

TA

FO

R S

TE

EN

S B

AS

ALT

S (

Con

tinue

d)

For

mat

ion

Pal

eom

ag.

Sam

ple

ID

Typ

e of

m

ater

ial

%40

Ar*

37

Ar/

39A

r 40

Ar/

39A

r ag

e (p

ublis

hed)

(M

a)

Err

or

(1σ)

R

ef

Rec

alc.

ag

e (M

a)

Err

or

(1σ)

Ste

ens

Bas

alt

? C

R-3

58

Grd

mas

s 53

.22

6.78

16

.40

0.26

7

16.4

6 0.

25

Ste

ens

Bas

alt

? C

R-4

62

Grd

mas

s 8.

66

8.00

16

.51

0.41

7

16.5

7 0.

41

Ste

ens

Bas

alt

? K

CS

-05-

20

Pla

g se

p N

o in

fo

No

info

16

.22

0.15

8

16.4

0 0.

15

Ste

ens

Bas

alt

? C

H82

-22B

P

lag

sep

? ?

16.5

8 0.

05

9 16

.86

0.05

Ste

ens

Bas

alt

? C

H82

-22G

P

lag

sep

? ?

16.5

9 0.

02

9 16

.87

0.03

Ste

ens

Bas

alt

? H

85-1

0A

No

info

N

o in

fo

No

info

16

.30

0.17

10

16

.44

0.17

Ste

ens

Bas

alt

N

NL1

1-6

WR

Who

le-r

ock

No

info

N

o in

fo

16.5

0 0.

30

11

16.6

7 0.

30

Ste

ens

Bas

alt

N

SM

200

WR

Who

le-r

ock

No

info

N

o in

fo

16.2

0 0.

30

11

16.3

7 0.

30

Ste

ens

Bas

alt

N

SM

200

WR

Who

le-r

ock

No

info

N

o in

fo

16.9

0 0.

70

11

17.0

8 0.

71

Ste

ens

Bas

alt

R

SM

263

WR

Who

le-r

ock

No

info

N

o in

fo

16.3

0 0.

50

11

16.4

7 0.

51

Ste

ens

Bas

alt

R

SM

263

WR

Who

le-r

ock

No

info

N

o in

fo

15.8

0 0.

20

11

15.9

6 0.

20

Ste

ens

Bas

alt

R

SM

263

Pla

gP

lag

sep

No

info

N

o in

fo

16.6

0 0.

40

11

16.7

7 0.

40

Ste

ens

Bas

alt

tr

SM

421

WR

Who

le-r

ock

No

info

N

o in

fo

16.5

0 0.

60

11

16.6

7 0.

61

Ste

ens

Bas

alt

tr

SM

421

WR

Who

le-r

ock

No

info

N

o in

fo

17.1

0 0.

70

11

17.2

8 0.

71

Ste

ens

Bas

alt

tr

SM

441

WR

Who

le-r

ock

No

info

N

o in

fo

16.2

0 1.

70

11

16.3

7 1.

72

Ste

ens

Bas

alt

tr

SM

441

Pla

gP

lag

sep

No

info

N

o in

fo

16.4

0 1.

50

11

16.5

7 1.

52

Ste

ens

Bas

alt

tr

NL5

0 W

RW

hole

-roc

k N

o in

fo

No

info

16

.50

0.40

11

16

.67

0.40

Ste

ens

Bas

alt

tr

SM

129

WR

Who

le-r

ock

No

info

N

o in

fo

15.8

0 0.

40

11

15.9

6 0.

40

Ste

ens

Bas

alt

tr

SM

129

WR

Who

le-r

ock

No

info

N

o in

fo

15.7

0 0.

70

11

15.8

6 0.

71

Ste

ens

Bas

alt

tr

NL5

3-5

WR

Who

le-r

ock

No

info

N

o in

fo

16.1

0 0.

30

11

16.2

7 0.

30

Ste

ens

Bas

alt

tr

NL5

3-5

WR

Who

le-r

ock

No

info

N

o in

fo

16.4

0 0.

70

11

16.5

7 0.

71

Ste

ens

Bas

alt

tr

NL5

3 P

lag

Pla

g se

p N

o in

fo

No

info

16

.90

0.40

11

17

.08

0.40

Ste

ens

Bas

alt

tr

SM

166

WR

Who

le-r

ock

No

info

N

o in

fo

16.1

0 0.

30

11

16.2

7 0.

30

Ste

ens

Bas

alt

tr

SM

166

WR

Who

le-r

ock

No

info

N

o in

fo

16.1

0 0.

40

11

16.2

7 0.

40

Ste

ens

Bas

alt

tr

SM

224

WR

Who

le-r

ock

No

info

N

o in

fo

16.4

0 0.

30

11

16.5

7 0.

30

Ste

ens

Bas

alt

tr

SM

224

WR

Who

le-r

ock

No

info

N

o in

fo

16.3

0 0.

70

11

16.4

7 0.

71

Ste

ens

Bas

alt

tr

SM

224

Pla

gP

lag

sep

No

info

N

o in

fo

15.2

0 0.

70

11

15.3

6 0.

71

Ste

ens

Bas

alt

tr

NL6

9 W

RW

hole

-roc

k N

o in

fo

No

info

16

.90

0.90

11

17

.08

0.91

Bas

. of M

alhe

ur G

orge

?

KL0

49

Grd

mas

s N

o in

fo

No

info

17

.00

0.80

12

17

.29

0.81

Bas

. of M

alhe

ur G

orge

?

KL0

33

No

info

N

o in

fo

No

info

15

.80

2.80

12

16

.13

2.81

N

ote:

%40

Ar*

= 1

00 ×

[40A

r Tot

al –

(36

Ar T

otal ×

295

.5)]

/40A

r Tot

al. P

aleo

mag

netic

info

rmat

ion

(tak

en o

r in

ferr

ed fr

om s

ourc

e pa

pers

): N

—no

rmal

pol

arity

; R—

reve

rse

pola

rity;

tr—

tran

sitio

nal p

olar

ity. D

escr

iptio

n of

type

of m

ate

rial:

Grd

mas

s—gr

ound

mas

s (a

lso

impl

ies

that

it is

the

grou

ndm

ass

mat

eria

l dev

oid

of a

ny la

rge

phen

ocry

sts)

; Pla

g se

p—pl

agio

clas

e se

para

te;

San

idin

e—sa

nidi

ne s

epar

ate;

Who

le-r

ock

(tak

en fr

om th

e te

rmin

olog

y pr

esen

ted

in th

e so

urce

pap

ers)

impl

ies

that

ther

e ha

s be

en n

o se

para

tion

of p

heno

crys

t mat

eria

l. R

ecal

c. a

ge—

all d

ata

has

been

rec

alcu

late

d w

ith a

dec

ay c

onst

ant o

f Ren

ne e

t al.

(201

0) a

nd

stan

dard

ized

to th

e no

rmal

izat

ion

valu

e fo

r F

ish

Can

yon

sani

dine

(Jo

urda

n an

d R

enne

, 200

7). D

ata

sour

ces:

1—

Bak

si a

nd F

arra

r (1

990)

; 2—

Bru

esek

e et

al.

(200

7); 3

—B

rues

eke

and

Har

t (20

08);

4—

Col

gan

et a

l. (2

006)

; 5—

Jarb

oe e

t al.

(200

8); 6

—Ja

rboe

et a

l. (2

010)

; 7—

Cam

p et

al.

(thi

s vo

lum

e); 8

—S

carb

erry

et

al. (

2009

); 9

—S

wis

her

et a

l. (1

990)

; 10—

Sho

emak

er a

nd H

art (

2002

); 1

1—B

aksi

et a

l. (1

991)

; 12—

Hoo

per

et a

l. (2

002)

. Num

bers

in g

ray

italic

are

thos

e ag

es,

plus

err

or, t

hat f

all o

utsi

de c

riter

ia o

f dat

a ac

cept

abili

ty.

52 Barry et al.

ages younger than 16.5 Ma are analytically acceptable, others underlie either well-dated sanidine-bearing tuffs or other Steens Basalt lavas with ages ≥16.5 Ma (Henry et al., 2006; Jarboe et al., 2008, 2010).

A precise estimate of the age and duration of Steens vol-canism comes from integrating the large number of acceptable ages with paleomagnetic data. Jarboe et al. (2008, 2010) used such combined data to correlate the age of the Steens reversal to the 16.72 Ma transition separating the C5Cr and C5C.3n chrons on the geomagnetic polarity time scale of Gradstein et al. (2004) (see below). The acceptable ages for Steens Basalt can be fur-ther constrained by the consistency of the Steens paleomagnetic and chemical stratigraphy across the Steens outcrop area (Camp et al., this volume), which leads us to suggest that the most accurate age range for the entire Steens succession is between ca. 16.9 and 16.6 Ma.

Imnaha Basalt

The stratigraphic relationship between the Imnaha and Steens Basalts is best revealed in the Malheur Gorge area of east-central Oregon (Fig. 1). Here, the Basalt of Malheur Gorge was subdivided by Hooper et al. (2002) into three stratigraphic

and compositionally distinct tholeiitic units, with the follow-ing weighted mean 40Ar/39Ar ages: the lower Pole Creek basalt (16.9 ± 0.8 Ma), upper Pole Creek basalt (16.5 ± 0.3 Ma), and Birch Creek basalt (15.7 ± 0.1 Ma). Lying above this succes-sion, there are the ca. 15.3 Ma Hunter Creek basalt and Din-ner Creek tuff. Hooper et al. (2002) described the lower Pole Creek basalt as being chemically equivalent to lower Steens Basalt, the upper Pole Creek basalt as chemically equivalent to Imnaha Basalt, and the Birch Creek and Hunter Creek basalts as chemically equivalent to Grande Ronde Basalt, in correct strati-graphic order (Fig. 3).

The upper Pole Creek basalt (Imnaha) appears to lie con-formably above lower Pole Creek basalt (lower Steens Basalt) in the Malheur Gorge region. Thus, Imnaha Basalt is inferred to lie at the same stratigraphic level as upper Steens Basalt. Although lava fl ows from these units may well be interbedded south of the Malheur Gorge, the outcrops are too poor to verify this relationship.

The timing of Imnaha volcanism is diffi cult to constrain pre-cisely. Many of the available age determinations have insuffi cient data reported to make a full assessment of reliability (Tables 3 and 4). The 40Ar/39Ar ages for Imnaha Basalt range from 17.82 to 15.35 Ma; we exclude sample ages for Columbia River Basalt

TABLE 2. K-Ar DATA FOR STEENS BASALTS

Formation Sample ID K (wt%)

40*Ar/total 40Ar

K-Ar age (published)

(Ma)

Error (1σ)

Ref Recalc. age (Ma)

Error (1σ)

Steens Basalt 11-3 1.53 Av. 21.50 Av. 15.2 Av. 0.3 1 15.7 0.3


Steens Basalt 11-7 1.50 15.60 15.1 0.4 1 15.6 0.4

Steens Basalt 17-4(i) 0.84 Av. 45.65 Av. 15.1 Av. 0.4 1 15.6 0.4

Steens Basalt 17-4(ii) 0.83 41.50 14.9 0.3 1 15.4 0.3

Steens Basalt 17-6 0.84 47.10 14.8 0.4 1 15.3 0.4

Steens Basalt 51-1 1.31 60.60 14.8 0.5 1 15.3 0.5

Steens Basalt 51-2 1.34 63.70 15.4 0.4 1 15.9 0.4







Steens Basalt 68-4 1.00 90.70 15.2 0.9 1 15.7 0.9

Steens Basalt 68-5 0.92 93.60 15.3 1.4 1 15.8 1.4

Steens Basalt 68-6 1.00 70.60 15.5 0.4 1 16.0 0.4

Steens Basalt 70-3 0.95 74.50 14.7 0.5 1 15.2 0.5

Steens Basalt 70-5 0.83 65.50 15.0 0.4 1 15.5 0.4

Steens Basalt RCG-185-67 0.26 No info 15.3 1.0 2 15.8 1.0

Steens Basalt KA1251 0.37 No info 14.7 – 3 15.2 –

Steens Basalt KA1165 1.64 No info 14.5 – 3 15.0 –

Note: Recalculated ages make the assumption that publications up to and including 1977 used the decay constant of Dalrymple and Lanphere (1969), and that publications after that time used the decay constant of Steiger and Jäger (1977). Data from pre-1969 publications were similarly recalculated using Dalrymple and Lanphere (1969) values, as they based their calculations on decay constant information presented in Aldrich and Wetherill (1958). All data have been recalculated with a decay constant of Renne et al. (2010). Data sources: 1—Baksi et al. (1967); 2—Fiebelkorn et al. (1982), quoting Greene et al. (1972); 3—Fiebelkorn et al. (1982), quoting Evernden and James (1964).


DB-25 (17.82 Ma; Duncan, 1983), sample identifi ed as top of N

0 unit, Imnaha Basalt of Fig. 1a (17.61 Ma; Baksi and Far-

rar, 1990), MLC-2 (15.35 Ma; Barry et al., 2010), and KL164 (17.07 Ma; Hooper et al., 2002) based on our understanding of Imnaha Basalt and its inferred stratigraphic positions (Fig. 4).

The inferred conformable stratigraphic relationship between the lower Steens and Imnaha successions suggests that lower-most Imnaha Basalt is ca. 16.7 Ma in age. Discarding the ages that appear to be out of stratigraphic order, and the imprecise ages for KL319 and KL248 (Hooper et al., 2002), we fi nd that the remaining data appear to be divided into an older group in the south and a younger group in the north. For example, an age of 16.92 ± 0.21 (Jarboe et al., 2010) was obtained from a lava at Squaw Butte in one of the southernmost outcrops of Imnaha Basalt in Idaho, and an age of 15.95 ± 0.31 Ma (Barry et al., 2010) was obtained from a sample collected at the confl uence of the Grande Ronde and Snake Rivers, Washington, directly below the oldest Grande Ronde Basalt lavas (Table 3). Such a geographic progression is consistent with an overall northward migration of Imnaha dike intrusion and volcanism with time (see also Camp and Ross, 2004).

The available 40Ar/39Ar ages in the Malheur Gorge and the apparent stratigraphic equivalency of the Imnaha and upper Steens lavas in that region (Fig. 3) suggest that the oldest Imnaha lavas erupted at ca. 16.7 Ma. The combined data lead us to conclude that the duration of Imnaha Basalt volcanism was ~700,000 yr, from ca. 16.7 Ma to 16 Ma. A ca. 16 Ma age for the last Imnaha fl ows would place the lavas in the C5Cn.1n (normal polarity) paleomagnetic chron (Fig. 4). Although the Imnaha lavas in northeastern Oregon and southeastern Washington appear to have erupted during a single geomagnetic chron of normal polarity, the older ages for the more southern outcrops suggest that the entire 700,000 yr duration may correspond with as many as fi ve geomagnetic chrons on the geomagnetic time scale of Gradstein

Figure 3. A north-south cross section from Steens Mountain in the south to the Columbia River Plateau in the north, summarizing the stratigraphic relationship of the Steens and Imnaha Basalts with con-straints from the Malheur Gorge area. The diagram shows how Im-naha Basalt is thought to correlate with upper Steens Basalt, and that lower Steens Basalt would be older than Imnaha Basalt, and Grande Ronde Basalt would be younger than upper Steens Basalt.

et al. (2004) (see Fig. 4). In this case, two short reverse polarity chrons (C5Cn.2r and C5Cn.1r) have yet to be discovered in the Imnaha Basalt lavas.

Grande Ronde Basalt and Picture Gorge Basalt

Grande Ronde Basalt lavas are the products of the greatest volume of lava production in the Columbia River Basalt Group (Tolan et al., 1989; Reidel et al., 1989, this volume). The Imnaha and Grande Ronde Basalt lavas are not known to be interbedded, but in the absence of any evidence for a signifi cant hiatus between the Imnaha Basalt and the Grande Ronde Basalt, it seems likely that Grande Ronde Basalt volcanism followed soon after the last Imnaha lavas.

The Grande Ronde Basalt is recognized to span at least four paleomagnetic zones and, on this basis, has been strati-graphically subdivided into R

1, N

1, R

2, and N

2 units (where

R indicates reversed and N indicates normal; Swanson et al., 1979; Reidel et al., 1989, this volume). The earliest lavas have a transitional paleomagnetic signature (Hooper et al., 1979), indicating that the fi rst Grande Ronde Basalt lavas were erupted during a polarity transition.

Published radiometric ages for Grande Ronde Basalt lavas are presented in Tables 5 and 6. These data indicate that Grande Ronde Basalt volcanism began ca. 16 Ma. Although many samples have been analyzed from the R

1 succession, several

analyses produce ages that appear to be stratigraphically out of order, either too old (KL-92-231 and KL-91-93; Lees, 1994) or too young (KL034; Hooper et al., 2002), and they are therefore omitted from our estimates of unit ages and durations. Sample CRB05-030 (Barry et al., 2010), which produced an age of 15.59 ± 0.10 Ma, is from a dike of the Teepee Butte Member, Basalt of Joseph Creek. Field exposures indicate that the dike intrudes older R

1 lavas of Buckhorn Springs (Reidel and Tolan,

1992). From this evidence, correlation with the Gradstein et al. (2004) polarity time scale, and the lack of evidence for clear stratigraphic relation between Grande Ronde Basalt and Imnaha units, we conclude that the onset of Grande Ronde Basalt activity occurred before 15.6 Ma at ca. 16 Ma. On the basis of the Ar dat-ing evidence from Grande Ronde Basalts alone, we cannot rule out that volcanism may have started as early as 16.3 Ma. This appears to be at odds, however, with the known fi eld relations and dating evidence for the Imnaha Basalts. Assuming the younger date, the R

1 section of Grande Ronde Basalt would correlate with

the long C5Br chron. The older onset would match the R1 sec-

tion with either of the short C5Cn.2r or C5Cn.1r chrons, and this would strongly support the likelihood that the base of the Grande Ronde Basalt could be diachronous, resulting in slightly different ages for Grande Ronde Basalt lavas sitting on Imnaha Basalt.

The cessation of Grande Ronde Basalt volcanism is equally diffi cult to date precisely. Ages for the N

2 Sentinel Bluffs Mem-

ber, Basalt of Museum, range from 15.27 Ma to 15.89 ± 0.1 Ma (Duncan, 1982; Long and Duncan, 1983, respectively). Although these are both from lavas at the top of the pile, there may still be

https://www.researchgate.net/publication/247408605_Revisions_to_the_estimates_of_areal_extent_and_volume_of_the_Grande_Ronde_Basalt_Group?el=1_x_8&enrichId=rgreq-b590b475-54e3-4b83-942b-1bb5ed3e29b1&enrichSource=Y292ZXJQYWdlOzI2MzE4OTYzNTtBUzoxMDk0NzY4NDU3MjM2NDhAMTQwMzExMjcyMjM4OQ==

54 Barry et al.T

AB

LE 3

. Ar-

Ar

DA

TA

FO

R IM

NA

HA

BA

SA

LTS

For

mat

ion

Mem

ber

Pal

eom

ag.

sign

al

Sam

ple

ID

Typ

e of

m

ater

ial

%40

Ar*

37

Ar/

39A

r 40

Ar/

39A

r ag

e (p

ublis

hed)

(M

a)

Err

or

(1σ)

R

ef

Rec

alc.

ag

e (M

a)

Err

or

(1σ)

Imna

ha B

asal

t

? C

RB

DB

-25

Grd

mas

s 84

.12

0.61

17

.50

0.30

1

17.8

2 0.

31

Imna

ha B

asal

t

? F

ig.1

.a

Grd

mas

s N

o in

fo

No

info

17

.26

0.16

2

17.6

1 0.

17

Imna

ha B

asal

t

? N

o in

fo

Grd

mas

s N

o in

fo

No

info

15

.40

0.20

3

15.6

5 0.

20

Imna

ha B

asal

t

? N

o in

fo

Grd

mas

s N

o in

fo

No

info

15

.50

0.30

3

15.7

5 0.

31

Imna

ha B

asal

t

? C

JC-1

G

rdm

ass

79.5

0 0.

99

16.0

8 0.

34

4 16

.15

0.35

Imna

ha B

asal

t

? C

JC-1

(rpt

) G

rdm

ass

65.7

1 5.

68

16.0

6 0.

15

4 16

.12

0.15

Imna

ha B

asal

t

? C

JC-3

(1)

Grd

mas

s 52

.19

0.23

15

.89

0.30

4

15.9

5 0.

31

Imna

ha B

asal

t

? C

JC-5

G

rdm

ass

66.1

7 6.

08

16.1

1 0.

27

4 16

.17

0.27

Imna

ha B

asal

t

? M

LC-2

G

rdm

ass

71.0

9 1.

69

15.2

9 0.

11

4 15

.35

0.11

Imna

ha B

asal

t

? M

LC-2

(rpt

) G

rdm

ass

73.8

8 3.

74

15.7

0 0.

25

4 15

.76

0.25

Imna

ha B

asal

t

? 04

WB

01A

P

lag

sep

52.0

2 37

.53

16.8

5 0.

21

5 16

.92

0.21

B. o

f Mal

heur

Gor

ge

U. P

ole

Cre

ek

? K

L319

G

rdm

ass

No

info

N

o in

fo

15.2

0 2.

80

3 15

.40

2.85

B. o

f Mal

heur

Gor

ge

U. P

ole

Cre

ek

? K

L001

G

rdm

ass

No

info

N

o in

fo

15.5

0 0.

60

3 15

.76

0.61

B. o

f Mal

heur

Gor

ge

U. P

ole

Cre

ek

? K

L248

G

rdm

ass

No

info

N

o in

fo

16.6

0 0.

80

3 16

.90

0.82

B. o

f Mal

heur

Gor

ge

U. P

ole

Cre

ek

? K

L164

G

rdm

ass

No

info

N

o in

fo

16.8

0 0.

30

3 17

.07

0.31

B. o

f Mal

heur

Gor

ge

Pol

e C

reek

N

04

PC

03

Pla

g se

p 68

.03

20.1

3 16

.45

0.11

5

16.5

2 0.

11

N

ote :

%40

Ar*

= 1

00 ×

[40A

r Tot

al –

(36

Ar T

otal ×

295

.5)]

/40A

r Tot

al. D

escr

iptio

n of

type

of m

ater

ial:

Grd

mas

s—gr

ound

mas

s (a

lso

impl

ies

that

it is

the

grou

ndm

ass

mat

eria

l dev

oid

of

any

larg

e ph

enoc

ryst

s); P

lag

sep—

plag

iocl

ase

sepa

rate

. Rec

alc.

age

—al

l dat

a ha

ve b

een

reca

lcul

ated

with

a d

ecay

con

stan

t of R

enne

et a

l. (2

010)

and

sta

ndar

dize

d to

the

norm

aliz

atio

n va

lue

for

Fis

h C

anyo

n sa

nidi

ne (

Jour

dan

and

Ren

ne, 2

007)

. Dat

a so

urce

s: 1

—D

unca

n (1

983)

; 2—

Bak

si a

nd F

arra

r (1

990

), in

clud

ed in

Lon

g an

d D

unca

n (1

983)

; 3—

Hoo

per

et a

l. (2

002)

; 4—

Bar

ry e

t al.

(201

0); 5

—Ja

rboe

et a

l. (2

010)

. Num

bers

in g

ray

italic

s ar

e th

ose

ages

, plu

s er

ror,

that

fall

outs

ide

crite

ria o

f dat

a ac

cept

abili

ty.

TA

BLE

4. K

-Ar

DA

TA

FO

R IM

NA

HA

BA

SA

LTS

For

mat

ion

Sam

ple

ID

K (

wt%

) 40

*Ar/

tota

l 40

Ar

K-A

r ag

e (p

ublis

hed)

(M

a)

Err

or

(1σ)

R

ef

Rec

alc.

age

(M

a)

Err

or

(1σ)

Imna

ha B

asal

t D

B-1

2 0.

70

0.29

16

.8

0.9

1 16

.9

0.9

Imna

ha B

asal

t D

B-1

5 0.

81

0.23

16

.5

1.1

1 16

.6

1.1

Imna

ha B

asal

t D

B-1

9 0.

83

0.56

17

.4

0.3

1 17

.5

0.3

Imna

ha B

asal

t D

B-2

3 0.

75

0.37

17

.0

0.2

1 17

.1

0.2

Imna

ha B

asal

t I-

74

1.74

N

o in

fo

17.3

0.

5 2

17.4

0.

5

Imna

ha B

asal

t D

B-6

0.

74

No

info

16

.0

1.0

2 16

.1

1.0

Imna

ha B

asal

t D

B-7

0.

69

No

info

13

.9

2.5

2 14

.0

2.5

Imna

ha B

asal

t D

B-2

0 0.

97

No

info

12

.6

1.5

2 12

.7

1.5

Imna

ha B

asal

t D

B-2

0.

47

No

info

5.

3 4.

5 2

5.4

4.5

N

ote:

Rec

alcu

late

d ag

es m

ake

the

assu

mpt

ion

that

pub

licat

ions

afte

r 19

77 u

sed

the

deca

y co

nsta

nt o

f Ste

iger

and

Jäg

er (

1977

). A

ll d

ata

have

bee

n re

calc

ulat

ed w

ith a

dec

ay c

onst

ant o

f Ren

ne e

t al.

(201

0). D

ata

sour

ces:

1—

McK

ee e

t al.

(198

1); 2

—F

iebe

lkor

n et

al.

(198

2),

quot

ing

D.A

. Sw

anso

n an

d E

.H. M

cKee

(19

82, p

erso

nal c

omm

un.)

.


Figu

re 4

. A p

lot o

f av

aila

ble

40A

r/39

Ar

data

for

the

Col

umbi

a R

iver

Bas

alt G

roup

(ex

clud

ing

Sadd

le M

ount

ains

Bas

alt)

vs.

geo

mag

netic

pol

arity

tim

e sc

ale.

All

40A

r/39

Ar

erro

rs a

re 1

σ. S

ymbo

ls in

bla

ck c

orre

spon

d to

dat

a in

the

tabl

es th

at fu

lfi ll

our a

ccep

tabi

lity

crite

ria,

whe

reas

thos

e th

at fa

ll sh

ort o

f our

cri

teri

a ar

e sh

own

with

whi

te s

ym-

bols

. Dat

a so

urce

s: A

—B

aksi

and

Far

rar (

1990

); B

—B

rues

eke

et a

l. (2

007)

; C—

Bru

esek

e an

d H

art (

2008

); D

—C

olga

n et

al.

(200

6); E

—Ja

rboe

et a

l. (2

008)

; F—

Jarb

oe

et a

l. (2

010)

; G—

Cam

p et

al.

(thi

s vo

lum

e); H

—Sc

arbe

rry

et a

l. (2

009)

; I—

Swis

her e

t al.

(199

0); J

—Sh

oem

aker

and

Har

t (20

02);

K—

Hoo

per e

t al.

(200

2); L

—D

unca

n (1

983)

; M—

Bar

ry e

t al.

(201

0); N

—L

ees

(199

4); O

—D

unca

n (1

982)

; P—

Lon

g an

d D

unca

n (1

983)

; Q—

Bak

si e

t al.

(199

1); Z

—ne

w d

ata,

this

con

trib

utio

n. G

eom

ag-

netic

tim

e sc

ale

is f

rom

Gra

dste

in e

t al.

(200

4). T

he d

iagr

am d

emon

stra

tes

how

dif

fi cul

t it i

s to

inte

rpre

t arg

on a

ges

for

a la

rge

igne

ous

prov

ince

, eve

n w

hen

ther

e is

as

muc

h da

ta a

s av

aila

ble

for

the

Col

umbi

a R

iver

Bas

alt G

roup

. Sha

ded

area

s sh

ow o

ur r

ecom

men

ded

age

rang

es f

or th

e m

ajor

for

mat

ions

of

the

Col

umbi

a R

iver

Bas

alt

Gro

up; s

ome

confi

den

ce c

an b

e gi

ven

to a

ge o

f th

e ba

se o

f un

its, b

ut th

e ag

es o

f th

e to

ps o

f un

its a

re le

ss c

erta

in.

56 Barry et al.TA

BLE

5. A

r-A

r D

ATA

FO

R G

RA

ND

E R

ON

DE

BA

SA

LTS

Mem

ber

Uni

tP

aleo

mag

. si

gnal

Sam

ple

IDTy

pe o

f m

ater

ial

%40

Ar*

37A

r/39

Ar

40A

r/39

Ar

age

(pub

lishe

d(M

a)E

rror

(1σ)

Ref

Rec

alc.

ag

e (M

a)E

rror

(1σ)

(Uni

ts s

ittin

g on

R

hyol

ite o

f Cot

tonw

ood

Mtn

?K

L046

Grd

mas

sN

o in

foN

o in

fo15

.50

0.70

115

.76

0.72

Gra

nd R

onde

Bas

alt)

Rhy

olite

of C

otto

nwoo

d M

tn?

Kl0

46G

rdm

ass

No

info

No

info

15.7

00.

201

15.9

60.

21

Rhy

olite

of C

otto

nwoo

d M

tn?

KL2

30G

rdm

ass

42.5

8†0.

90†

14.6

01.

001

14.8

41.

02

Din

ner

Crk

tuff

?K

L246

Grd

mas

s89

.33†

0.73

†15

.10

2.00

115

.28

2.09

Din

ner

Crk

tuff

?K

L206

Grd

mas

s95

.64†

0.04

†15

.40

0.60

115

.66

0.61

Din

ner

Crk

tuff

?D

C27

0G

rdm

ass

No

info

No

info

15.3

00.

401

15.5

50.

10

Sen

tinel

Blu

ffsM

cCoy

Can

yon

N2

DC

-12-

3202

Grd

mas

s49

.90

0.87

15.1

00.

302

15.2

80.

30

Sen

tinel

Blu

ffsN

o in

foN

2D

C-1

2-22

68G

rdm

ass

36.7

71.

0215

.60

0.20

215

.86

0.20

Sen

tinel

Blu

ffsM

useu

mN

2W

H1a

-1G

rdm

ass

80.8

52.

5915

.48

0.11

415

.54

0.11

Sen

tinel

Blu

ffsM

useu

mN

2W

H1b

-1G

rdm

asssv

86.8

10.

5115

.71

0.13

415

.77

0.13

Sen

tinel

Blu

ffsM

useu

mN

2W

H1b

-2G

rdm

assa-

sv94

.61

3.40

15.5

50.

164

15.6

10.

16

Sen

tinel

Blu

ffsM

useu

mN

2C

RB

05-1

06G

rdm

assa

43.7

43.

0515

.94

0.10

416

.00

0.10

Ort

ley

Flo

w 2

abo

ve N

2-R

2N

2D

C-1

2-39

26G

rdm

ass

65.1

30.

4915

.20

0.30

215

.38

0.31

Win

ter W

ater

Um

tanu

mN

2D

C-1

2-34

34G

rdm

ass

46.9

61.

0316

.00

0.20

316

.20

0.21

Win

ter W

ater

Um

tanu

mN

2D

C-1

2-33

62G

rdm

ass

28.9

40.

1916

.00

0.20

216

.20

0.21

Gro

use

Cre

ekU

pper

mos

t R2

R2

DC

-12-

2990

Grd

mas

s51

.29

0.70

15.6

00.

302

15.8

00.

30

Wap

shill

a R

idge

Wap

shill

a R

idge

R2

CR

B05

-033

Grd

mas

s92

.60

1.08

15.4

60.

114

15.5

20.

12

Wap

shill

a R

idge

Wap

shill

a R

idge

R2

Fig

.1.d

Grd

mas

sN

o in

foN

o in

fo15

.70

0.14

516

.02

0.15

From

Mal

heur

Gor

ge

B. o

f Hun

ter

Crk

R2

No

info

No

info

No

info

No

info

15.3

00.

101

15.5

50.

10

From

Mal

heur

Gor

ge

B. o

f Hun

ter

Crk

R2

HO

R09

Grd

mas

s50

.24†

1.67

†15

.80

0.60

§1

16.0

50.

61

From

Mal

heur

Gor

ge

B. o

f Hun

ter

Crk

R2

KL-

91-1

02P

lag

sep

64.2

5†1.

47†

16.5

01.

206

16.8

11.

22

From

Mal

heur

Gor

ge

B. o

f Hun

ter

Crk

R2

KL-

92-2

69G

rdm

ass

62.7

3†1.

17†

15.0

00.

736

15.2

80.

74

From

Mal

heur

Gor

ge

B. o

f Hun

ter

Crk

R2

KL-

92-2

78G

rdm

ass

75.8

7†2.

53†

15.9

00.

266

16.2

00.

26

Dow

ny G

ulch

Dow

ny G

ulch

N1

Fig

.1.c

Grd

mas

sN

o in

foN

o in

fo15

.94

0.10

516

.26

0.11

Teep

ee B

utte

B. o

f Jos

eph

Crk

R1

CR

B05

-030

Grd

mas

s93

.33

1.61

15.5

30.

104

15.5

90.

10

Buc

khor

n S

prin

gsN

o in

foR

1C

JC-7

Grd

mas

s45

.34

4.20

16.2

50.

144

16.3

10.

14

Buc

khor

n S

prin

gsN

o in

foR

1F

ig. 1

bG

rdm

ass

No

info

No

info

16.0

30.

115

16.3

60.

12

B. o

f Mal

heur

Gor

geB

irch

Cre

ekR

1K

L034

Grd

mas

s64

.88†

0.76

†14

.40

0.40

114

.64

0.41

B. o

f Mal

heur

Gor

geB

irch

Cre

ekR

1K

L-91

-93

Grd

mas

s73

.04†

1.24

†17

.00

0.53

617

.32

0.54

B. o

f Mal

heur

Gor

geB

irch

Cre

ekR

1K

L220

Grd

mas

s45

.65†

1.69

†15

.10

0.30

115

.35

0.30

B. o

f Mal

heur

Gor

geB

irch

Cre

ekR

1K

L-92

-225

Grd

mas

s79

.47†

0.87

†16

.00

0.39

616

.30

0.40

B. o

f Mal

heur

Gor

geB

irch

Cre

ekR

1K

L-92

-231

Grd

mas

s58

.64†

1.33

†16

.90

0.67

617

.21

0.68

B. o

f Mal

heur

Gor

geB

irch

Cre

ekR

1K

L278

Grd

mas

s75

.87†

2.53

†15

.90

0.10

116

.20

0.10

Not

e: %

40A

r* =

100

× [40

Ar To

tal –

(36

Ar To

tal ×

295

.5)]

/40A

r Tota

l. D

escr

iptio

n of

type

of m

ater

ial:

Grd

mas

s—gr

ound

mas

s (a

lso

impl

ies

that

it is

the

grou

ndm

ass

mat

eria

l dev

oid

of a

ny la

rge

phen

ocry

sts)

; Pla

g se

p—pl

agio

clas

e se

para

te; G

rdm

asssv

—gr

ound

mas

s m

ater

ial (

fres

h) th

at h

as b

een

sele

cted

from

a s

egre

gatio

n ve

sicl

e cy

linde

r; G

rdm

assa-

sv—

alte

red

grou

ndm

ass

mat

eria

l fro

m a

seg

rega

tion

vesi

cle

cylin

der;

Grd

mas

sa —al

tere

d gr

ound

mas

s. R

ecal

c. a

ge—

all d

ata

have

bee

n re

calc

ulat

ed w

ith a

dec

ay c

onst

ant o

f Ren

ne e

t al.

(201

0) a

nd s

tand

ardi

zed

to th

e no

rmal

izat

ion

valu

e fo

r F

ish

Can

yon

sani

dine

(Jo

urda

n an

d R

enne

, 200

7). D

ata

sour

ces:

1—

Hoo

per

et a

l. (2

002)

; 2—

Dun

can

(198

2), i

nclu

ded

in L

ong

and

Dun

can

(198

3); 3

—Lo

ng a

nd D

unca

n (1

983)

; 4—

Bar

ry e

t al.

(201

0); 5

—B

aksi

and

Far

rar

(199

0); 6

—Le

es (

1994

). † A

vera

ged

valu

es c

alcu

late

d fr

om d

ata

pres

ente

d in

Lee

s (1

994)

, ass

umin

g th

at d

ata

are

quot

ed a

s cc

ST

P ×

10–1

0 (c

ubic

cen

tilite

rs a

t sta

ndar

d te

mpe

ratu

re p

ress

ure

x 10

–10 )

.§ E

rror

val

ue a

men

ded

from

pub

lishe

d va

lue

to v

alue

orig

inal

ly q

uote

d in

Lee

s (1

994)

, whi

ch s

tate

d th

at e

rror

s ar

e gi

ven

as 1

σ.


questions about diachronism along this boundary. The younger of these ages is in good agreement with the youngest date of 15.28 ± 0.74 Ma (Lees, 1994) from the Basalt of Hunter Creek, Wapshilla Ridge Member, at the top of the Grande Ronde Basalt succession in the Malheur Gorge area. However, the Basalt of Hunter Creek is R

2, rather than N

2, and both dates are at odds

with data for the overlying Wanapum Basalt (see following). Wanapum Basalt data suggest that Grande Ronde Basalt volca-nism must have fi nished by ca. 15.6 Ma, if not before.

If we assume Grande Ronde Basalt volcanism started at ca. 16.0 Ma and ended at ca. 15.6 Ma, then the total duration for Grande Ronde Basalt volcanism would be ~0.4 m.y., con-sistent with the ~420,000 yr duration estimated by Barry et al. (2010). Such an age range for the Grande Ronde Basalt presents an inconsistency with the current geomagnetic time scale. The available data suggest that Grande Ronde Basalt lavas most likely erupted within the geomagnetic chron C5Br (15.974–15.160 Ma; Gradstein et al., 2004). However, this poses a problem for the signifi cance of the geomagnetic reversals recorded within the Grande Ronde Basalt; there appears to be a lack of short mag-netic reversals during C5Br (e.g., Gradstein et al., 2004). We have no solution to this dilemma at present, though we note that there has been a history of moving geomagnetic transitions if neces-sary to fi t with chronologic constraints (e.g., Steens reversal once thought to be 15.5 Ma when dated by K-Ar technique [Baksi et al., 1967] has since moved to 16.7 Ma following modern studies, with further calls to shift the geomagnetic reversals constraining the Grande Ronde Basalt succession [Baksi et al., this volume]). Potential solutions to this mismatch may be (1) that the reversals measured in the Grande Ronde Basalt are merely short excur-sions within the long interval of reverse polarity; or (2) that rapid reversals C5Cn.2r to C5Cn.1n (currently dated from 16.453 Ma

to 15.974 Ma; Gradstein et al., 2004) should all be younger than 16 Ma; or (3) that the groundmass ages determined for the Grande Ronde Basalt and Imnaha are too young by ~0.5 m.y.

The Picture Gorge Basalt of northern Oregon (Fig. 2) is a coeval unit with the middle of the Grande Ronde Basalt vol-canism. Originally dated by K-Ar (Watkins and Baksi, 1974), these lavas have received no modern treatment by 40Ar/39Ar dat-ing methods (Table 7). Ages regarded as generally acceptable by Baksi (1989) range from 16.4 Ma to 15.2 ± 0.4 Ma, thus overlap-ping and agreeing with Grande Ronde Basalt 40Ar/39Ar data.

Wanapum Basalt

The Wanapum Basalt is distinctive within the Columbia River Basalt Group stratigraphy in that it follows an appar-ently signifi cant time break, expressed in the fi eld in the western part of the province by the Vantage interbeds (Smith, 1988). Although the Vantage sediments are not found every-where between the underlying Grande Ronde Basalt and the Wanapum Basalt, they are extensive and locally up to 20 m in thickness. Also, several of the Wanapum lavas are coarsely feldspar-phyric, which helps with their recognition and correla-tion in the fi eld (Beeson et al., 1985).

The basal Wanapum Basalt lavas are represented by the fairly locally distributed Eckler Mountain Member, confi ned to the central part of the province (Tolan et al., 1989; Reidel and Tolan, this volume). Eckler Mountain Member lavas outcrop around the Blue Mountains of southeastern Washington, for which we have but one 40Ar/39Ar plateau age (15.76 ± 0.17 Ma; Barry et al., 2010; Table 8). Additional information about the beginning of Wanapum Basalt volcanism comes from data for the Frenchman Springs Member. The lowest unit dated within

TABLE 6. K-Ar DATA FOR THE GRANDE RONDE BASALTS (GRB)


40*Ar/total 40Ar


(Ma)

Error (1σ)


Error (1σ)

6.0 4.51 1 6.0 3.51 .vA 13.0 .vA 54.1 .vA 4777 BRG

4.0 9.41 1 4.0 8.41 24.0 51.1 5677 BRG

4.0 3.51 1 4.0 2.51 45.0 15.1 94PS87 BRG

5.0 2.51 1 5.0 1.51 14.0 93.1 WA7977 BRG

4.0 1.51 1 4.0 0.51 84.0 60.1 5977 BRG

4.0 0.51 1 4.0 9.41 16.0 41.1 4977 BRG

8.0 7.51 1 8.0 6.51 34.0 80.1 2277 BRG

2.0 6.51 1 2.0 5.51 25.0 03.1 7277 BRG

4.0 1.51 1 4.0 0.51 25.0 61.1 9077 BRG

4.0 6.41 1 4.0 5.41 84.0 30.1 9277 BRG

4.0 8.51 1 4.0 7.51 94.0 05.1 7577 BRG

2.0 3.51 1 2.0 2.51 36.0 54.1 5577 BRG

GRB–Buckhorn Springs No info No info No info 16.5 – 2 16.6 –

Note: Recalculated ages make the assumption that publications up to and including 1977 used the decay constant of Dalrymple and Lanphere (1969), and that publications after that time used the decay constant of Steiger and Jäger (1977). All data have been recalculated with a decay constant of Renne et al. (2010). Data sources: 1—Fiebelkorn et al. (1982), and references therein; 2—Reidel et al. (1989).


58 Barry et al.

TABLE 7. K-Ar DATA FOR THE PICTURE GORGE BASALTS


Atm Ar K-Ar age (published)

(Ma)

Error (1σ)


Error (1σ)

Picture Gorge Basalt 14 0.63 38.6 14.9 0.3 1 15.4 0.3







Note: Recalculated ages make the assumption that publications up to and including 1977 used the decay constant of Dalrymple and Lanphere (1969). All data have been recalculated with the decay constant of Renne et al. (2010). Data source: 1—Watkins and Baksi (1974). Atm Ar (atmospheric argon) is presented instead of 40Ar*/total Ar for the Picture Gorge data because only atmospheric argon data are available.

TABLE 8. Ar-Ar DATA FOR THE WANAPUM FORMATION

Member Unit Sample ID Type of material

%40Ar* 37Ar/39Ar 40Ar/39Ar age (published)

(Ma)

Error (1σ)


Error (1σ)

Priest Rapids B. of Rosalia Ds1-(1) Grdmass 90.87 1.21 This study – 0 15.07 0.07

yduts sihT 55.0 01.68 ssamdrG pud )2(-1sD – 0 15.25 0.08 yduts sihT 42.4 23.77 ssamdrG )3(-1sD – 0 14.80 0.17

Roza DF1-(1)* Grdmass 86.11 2.79 16.34 0.24 1 16.41 0.25

ssamdrG )1(-3LC 35.32 0.31 This study – 0 12.87 0.37 351-60BRC

dup Grdmass 79.37 0.67 This study – 0 14.98 0.06

Frenchman Springs

B. of Sand Hollow

No information No info No info No info 15.30 – 3 15.59 –

yduts sihT 51.0 72.87 ssamdrG pud 7FP – 0 15.27 0.18 91.0 81.51 1 91.0 21.51 85.0 29.07 ssamdrG )tpr( 7FP 11.0 67.41 1 11.0 07.41 51.0 72.87 ssamdrG 7FP

ssamdrG 4LC –7.08 0.47 This study – 0 16.42 0.38 ssamdrG 800-50BRC 17.43 1.25 This study – 0 13.54 0.16

yduts sihT 10.3 81.57 ssamdrG 940-50BRC – 0 14.71 0.08

Frenchman Springs

B. of Palouse Falls

DC-12-2236 Grdmass 45.93 1.57 22.80 0.30 2 23.21 0.31

Frenchman Springs

B. of Ginkgo DC-12-2212 Grdmass 69.17 1.38 15.50 0.20 2 15.78 0.20

yduts sihT 10.1 43.66 ssamdrG 110-50BRC – 0 12.48 0.14 yduts sihT 39.1 28.19 ssamdrG )1(-8CsF – 0 16.20 0.10 yduts sihT 05.1 98.35 ssamdrG pud )2(-8CsF – 0 15.97 0.28

Eckler Mountain B. of Dodge CRB05-081 Grdmass 67.67 7.96 15.70 0.17 1 15.76 0.17

Note: Sample DF1-(1) is marked with *. It appeared in the online electronic appendix of Barry et al. (2010) as part of a preliminary test of suitable material for dating the Grande Ronde Basalts. As such, it is here presented as a recalculated age, but it has never been discussed in any previous manuscript. Description of type of material: Grdmass—groundmass (also implies that it is the groundmass material devoid of any large phenocrysts). Recalc. age—all data have been recalculated with a decay constant of Renne et al. (2010) and standardized to the normalization value for Fish Canyon sanidine (Jourdan and Renne, 2007). Data sources: 0—this study; 1—Barry et al. (2010); 2—Duncan (1982); 3—Reidel et al. (1989).

https://www.researchgate.net/publication/223338888_Age_calibration_of_the_Fish_Canyon_sanidine_40Ar39Ar_dating_standard_using_primary_K-Ar_standard?el=1_x_8&enrichId=rgreq-b590b475-54e3-4b83-942b-1bb5ed3e29b1&enrichSource=Y292ZXJQYWdlOzI2MzE4OTYzNTtBUzoxMDk0NzY4NDU3MjM2NDhAMTQwMzExMjcyMjM4OQ==


the Frenchman Springs Member (Basalt of Ginkgo) provided ages between 16.20 Ma and 15.78 ± 0.05 Ma (Appendix DR1 and Duncan [1982], respectively; excluding the single date of 12.48 ± 0.14 Ma [Appendix DR1], on grounds of it being strati-graphically out of place). Interestingly, though, this places the timing of Wanapum Basalt volcanism in confl ict with the end of Grande Ronde Basalt volcanism, unless both Grande Ronde Basalt and Wanapum volcanism were active at the same time, within potentially different parts of the large igneous province. This is not easy to reconcile with the presence of the interven-ing Vantage sediments and would require detailed mapping and dating along the contact to resolve. It is evident, though, that the hiatus between the end of Grande Ronde Basalt activity and the onset of Wanapum Basalt volcanism may have been brief, despite the presence of the Vantage sediments. We propose that the age of the base of the Wanapum Basalt is ca. 15.6 Ma, allow-ing time for the Grande Ronde Basalt to erupt after a start date of ca. 16 Ma, but within error of the youngest acceptable age for the lowest part of the Frenchman Springs Member, Basalt of Ginkgo (Tables 8 and 9).

The best estimate for the top of the Frenchman Springs Mem-ber comes from the dates of the Basalt of Sand Hollow below (14.76–15.27 ± 0.04 Ma; Barry et al. [2010] and Appendix DR1, respectively), and the Roza Member above (14.98 ± 0.06 Ma; Appendix DR1). These ages are unresolvable from each other and thus are coincidental with the best estimate for the age of the top of the Wanapum Basalt, at ca. 15 Ma. This is consistent with the age of the overlying Basalt of Rosalia, Priest Rapids Member, dated at 14.80–15.25 ± 0.01 Ma (Table 8).

In summary, the age range for the Wanapum Basalt is from ca. 15.6 Ma to ca. <15 Ma, suggesting that the ~10 eruptions of the Wanapum Basalt occurred spasmodically for a little more than half a million years. The lava fl ow fi elds often have thin sediments between them (Beeson et al., 1985; Jolley et al., 2008) and weathering of fl ow tops beneath a contact, indicative of rea-sonably long average hiatuses between eruptions. Clearly, the

TABLE 9. K-Ar DATA FOR THE WANAPUM FORMATION

Member Unit Sample ID K (wt%) 40*Ar/total 40Ar


(Ma)

Error (1σ)


Error (1σ)

Priest Rapids B. of Lolo No info No info No info 13.1–16.2 (probable 14.9)

No info 1 13.5–16.7 (15.4)

–

Priest Rapids B. of Rosalia No info No info No info 11.9–15.5 No info 1 12.3–16.0 –

Priest Rapids B. of Rosalia KA1236 1.16 0.51 14.5 No info 2 15.0 –

ofni oN ofni oN ofni oN azoR 12.5–16.3 No info 1 12.9–16.9 –

Frenchman Springs B. of Ginkgo No info No info No info 13.4 No info 1 13.9 –

Eckler Mountain Icicle Flat Basalt

VC79-282 Av. 0.73

Av. 0.38 16.4 1.2 3 16.5 1.2

Note: Recalculated ages make the assumption that publications up to and including 1977 used the decay constant of Dalrymple and Lanphere (1969), and that publications after that time used the decay constant of Steiger and Jäger (1977). Data from pre-1969 publications are similarly recalculated using Dalrymple and Lanphere (1969) values, as they based their calculations on decay constant information presented in Aldrich and Wetherill (1958). All data have been recalculated with a decay constant of Renne et al. (2010). Data sources: 1—dates given as reported, including the notation of “probable” age, as in U.S. Energy Research and Development Administration (1976); 2—Evernden and James (1964); 3—Krueger Enterprises Inc., Geochron Laboratory Division (1984, personal commun.).

overlap with the Grande Ronde Basalt is problematic, and under-standing may be advanced by more targeted dating programs in the future.

Saddle Mountains Basalt

Lavas of the Saddle Mountains Basalt are characteristi-cally constrained by topography and form thick, channelized or dammed lavas with well-developed columnar jointing. Unlike other parts of the Columbia River Basalt Group stratigraphy, there is a bias in the age dating data set for the Saddle Mountains Basalt (Appendix DR2); there was an abundance of early K-Ar analyses on this late stage of the Columbia River Basalt Group stratigraphy (e.g., McKee et al., 1977), but there has since been a paucity of modern 40Ar/39Ar studies (Tables 10 and 11; Appendix DR2). We have thus constrained the stratigraphy, where possible, on the basis of the available 40Ar/39Ar data, but many of the units have recommended ages given by corrected K-Ar dates.2

The Saddle Mountains Basalt volcanism began shortly after the cessation of the underlying Wanapum Basalt. The Mabton sedimentary interbed occurs between the Wanapum and Sad-dle Mountain Basalts, but there is no evidence for an erosional unconformity. Given that Wanapum Basalt activity appears to have ended close to 15 Ma, we can be fairly confi dent that the Saddle Mountains lavas are younger than 15 Ma. The oldest date, a K-Ar age of 14.57 Ma (U.S. Energy Research and Develop-ment Administration, 1976), comes from the Umatilla Member, supporting a short hiatus between the end of the Wanapum and the beginning of the Saddle Mountains volcanism. Basaltic activ-ity seems to have persisted through to ca. 6 Ma (on the basis of

2K-Ar values have been corrected for changes to the K decay constant. Publica-tions prior to, and including, 1977 are assumed to have used the decay constant of Dalrymple and Lanphere (1969). Publications with a date after 1977 are assumed to have used the decay constant of Steiger and Jäger (1977). All K-Ar values have been calculated to a newer value proposed by Renne et al. (2010).

60 Barry et al.T

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alcu

late

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es m

ake

the

assu

mpt

ion

that

pub

licat

ions

up

to a

nd in

clud

ing

1977

use

d th

e de

cay

cons

tant

of D

alry

mpl

e an

d La

nph

ere

(196

9), a

nd th

at

publ

icat

ions

afte

r th

at ti

me

used

the

deca

y co

nsta

nt o

f Ste

iger

and

Jäg

er (

1977

). D

ata

from

pre

-196

9 pu

blic

atio

ns a

re s

imila

rly r

ecal

cula

ted

usin

g D

alry

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e an

d La

nphe

re (

1969

) va

lues

, as

they

bas

ed th

eir

calc

ulat

ions

on

deca

y co

nsta

nt in

form

atio

n pr

esen

ted

in A

ldric

h an

d W

ethe

rill (

1958

). A

ll da

ta h

ave

been

rec

alcu

late

d w

ith a

de

cay

cons

tant

of R

enne

et a

l. (2

010)

. Av—

aver

age,

not

atio

n fo

r av

erag

e of

mor

e th

an o

ne v

alue

. Dat

a so

urce

s: 1

—M

cKee

et a

l. (1

977)

; 2—

date

s gi

ven

as r

epor

ted,

in

clud

ing

the

nota

tion

of “

prob

able

” ag

e, in

U.S

. Ene

rgy

Res

earc

h an

d D

evel

opm

ent A

dmin

istr

atio

n (1

976)

; 3—

Kru

eger

Ent

erpr

ises

Inc

, Geo

chro

n La

b. D

ivis

ion

(198

4,

pers

onal

com

mun

.); 4

—E

vern

den

and

Jam

es (

1964

).


an unverifi able K-Ar age of 6.2 Ma for the lower Monumental Member at the top of the Saddle Mountains Basalt), although frequency of eruptions tailed off at ca. 12 Ma.

Final Recommendations

The recommendations are based on a nonbiased approach to examining, where possible, the raw isotope data, coupled with detailed knowledge of fi eld constraints and paleomagnetic data. This does not always lead to the most obvious suggestions, and in particular there are clear shortfalls in our recommenda-tions, for example, the apparent overlap in timing between the end of the Grande Ronde Basalt volcanism and the start of the Wanapum Basalt, without supporting evidence for synchronicity of these two formations. Furthermore, individual dates are not always within stratigraphic order. For example, Basalt of Rosalia, Priest Rapids Member (15.07–15.25 ± 0.11 Ma) clearly strati-graphically overlies the Roza Member (14.98 ± 0.06 Ma). At fi rst glance, the age data might appear at odds with that relation-ship, but the age data are within the margins of analytical error and therefore valid (Table 8). However, this has been a rigorous exercise to examine all the available radioisotope age data for the Columbia River Basalt Group, and our fi nal recommendations are presented in Figure 4.

DISCUSSION

Volume versus Timing of Eruptions and Effusion Rates for the Columbia River Basalt Group

The thick succession of Steens Basalt (31,800 km3 estimated total volume) erupted between ca. 16.9 Ma and ca. 16.6 Ma, with activity concentrated around 16.7 Ma (Table 1). Using an alterna-tive method to examining effusion rates, Camp et al. (this volume) were able to constrain the eruption of the bulk of the Steens Basalt to <50,000 yr, giving an average effusion rate of 0.67 km3/yr. They acknowledge the possibility that lingering eruptions of much smaller volume could extend the overall duration beyond this interval, but that it is unlikely to exceed ~300,000 yr.

The Imnaha Basalt appears to have erupted between 16.7 Ma and 16.0 Ma (Tables 3 and 4). Current estimates for the volume of the Imnaha Basalt are 11,000 km3 (Reidel et al., this volume). Over a time span of 700,000 yr, this would suggest a time-averaged magma eruption rate of 0.016 km3/yr, although clearly mean output rates for individual eruptions or eruptive epi-sodes are likely to have been higher than this for specifi c peri-ods of time, on account of volcanism being more sporadic, with variable-length hiatuses between eruptions.

With an estimated eruptive volume of 150,000 km3, the Grande Ronde Basalt phase of effusion represents an enormous outpouring of lava (Fig. 5). The effusion rates for the Grande Ronde Basalt give a time-averaged magma eruption rate of 0.375 km3/yr, i.e., somewhat lower than estimates for the Steens lava sequences. Individual Grande Ronde Basalt eruptions varied

considerably in volume, but many were considerably greater than 1000 km3 in size. As with all other parts of the stratigraphy, erup-tive events would not have been equally spaced in time; some eruptions may have followed short periods of volcanic repose (possibly as low as months to several years), whereas others could have been considerably longer (many thousands to >104 yr).

The volumes of the Wanapum and Saddle Mountains Basalts lava dramatically decline with time (Fig. 5, inset). After the main pulse of the Grande Ronde Basalt, volcanism rapidly waned. Furthermore, volcanism shifted geographically toward the Snake River Plain and centralized silicic systems (e.g., Pierce et al., 2002, and references therein).

The amount of lava erupted over the life span of the large igneous province has long been investigated using the best avail-able age data versus volume estimates (Baksi, 1989; Hooper et al., 2002; Tolan et al., 1989). A revised version of Figure 5 of Tolan et al. (1989) is shown as Figure 5 here, with the latest revi-sions adding detail to earlier attempts (shown with a log scale for volume). Little has changed from earlier attempts at estimating volume-time relationships, but additional details of mean magma eruption rates show how the initial start-up of the Steens Basalt and the highly voluminous Grande Ronde Basalt dominate the formation of the Columbia River Basalt Group.

Suggestions for Future Work

In compiling the available radiometric age data for the Columbia River Basalt Group, there were surprisingly few 40Ar/39Ar dates on the Saddle Mountains Basalt. It appears that much of the early interest (1970s and 1980s), when the most appropriate dating technique was K-Ar, concentrated on the episodic eruptions of the Saddle Mountain Basalt. Despite the continuing interest in the periodicity of large-scale eruptions, there has been little work on the youngest part of the Columbia River Basalt Group. As such, this waning phase of volcanism is poorly constrained, and much of the available data cannot be critically assessed for its quality (Table 8). It would be inter-esting, for example, to examine the younger Saddle Mountain Basalt eruptions that occurred at a time of change from a domi-nantly basaltic system (the Columbia River Basalt Group) to a bimodal one (which presently manifests itself as the Yellow-stone system).

Although modern 40Ar/39Ar analyses can date to within 1% precision, there are few high-precision dates from the Columbia River Basalt Group units. A handful of analyses (e.g., Henry et al., 2006) have produced highly precise results, but on sanidines from intercalated tuffaceous units. One study examined different types of material from the Wanapum and Grande Ronde Basalts in an attempt to fi nd the type of material that gave the best ana-lytical results for these crystal-poor basalts (Barry et al., 2010). Surprisingly, the study found that large basalt-hosted feldspar crystals yielded meaningless results. Yet, in the Ar dating of the Steens Basalt, good results have been obtained from analyzing feldspar crystals (e.g., Jarboe et al., 2008). We speculate that


62 Barry et al.

complex feldspar antecrysts are more abundant in later lavas, and bring with them multiple phases of fl uid inclusions. These inclu-sions carry a budget of volatile concentrations, which appears from the data to complicate analyses of not just the host crys-tals but also nearby groundmass. To avoid such issues, it seems that material from intercalated silicic ash deposits, which may contain sanidine, would be a profi table target for future dating. Additionally, it seems worthwhile to undertake further detailed studies of the large antecrysts (e.g., Ramos et al., 2005, 2009) to better understand the origin and nature of these crystals. They

clearly have much to tell us about the plumbing, accumulation, and storage of material prior to a fl ood basalt eruption.

There are a number of K-Ar ages for Picture Gorge Basalt (Baksi et al., 1991), but there is an absence of Ar-Ar ages. Pic-ture Gorge Basalt appears to be derived from the same magma source as Steens Basalt, both having identical mixing arrays with the same isotopic end members (Camp and Hanan, 2008). Many of the Picture Gorge lavas are strongly plagioclase-phyric with large phenocrysts similar in appearance to those found in Steens Basalt. The implication of these relationships suggests that the

Lower Monumental

Ice Harbor

Pomona

Asotin-Wilbur Cr

Imnaha

Roza

Swamp CrFeary CrCraigmont

Frenchman Springs

Steens

Eckler Mt.

Grande Ronde

Esquatzel

Priest Rapids

Umatilla

WeisenfelsRidge

Elephant Mt.

Buford

Picture Gorge

0

18 17 16 15 14 13 12 11 10 9 8 7 6 5

Million Years Before Present

Vol

ume

(km

3 )

1,000

10,000

100,000

1,000,000

100V

olum

e (k

m3 )

Age (Ma)

Age (Ma)

Figure 5. Volume (km3) (logarithmic scale) against time (Ma) (linear scale) for the dated units of the Columbia River Basalt Group. The plot shows how volcanism waned signifi cantly after the Basalt of Priest Rapids, Wanapum Basalt, around 15 Ma. Inset: The same plot but with both scales linear, to illustrate the huge volumetric outpouring of the Grande Ronde Basalt in comparison with other Columbia River Basalt units. Revised from Tolan et al. (1989). Inset fi gure: S—Steens Basalt; IB—Imnaha Basalt; GRB—Grande Ronde Basalt; FS— Frenchman Springs; R—Roza; PR—Priest Rapids; U—Umatilla; A-WC—Asotin-Wilbur Creek; ESQ—Esquatzel; P-W—Pomona; EM—Elephant Mountain; IH—Ice Harbor; LM—Lower Monumental.



very good Ar-dating results obtained from the Steens feldspars (e.g., Jarboe et al., 2008) could be replicated with Picture Gorge feldspars. Since the Picture Gorge lavas are interbedded with Grande Ronde Basalt, a focused Ar-dating study on Picture Gorge Basalt could provide better age constraints on the Grande Ronde lavas.

Well-constrained geochronologic data coupled with the geomagnetic time scale should place constraints on the timing of reversals. There is clearly scope for improved studies of the Imnaha and Grande Ronde Basalt relating to the ways in which these units correlate with the geomagnetic time scale. Age con-straints for Imnaha Basalt currently place it within a period of rapid reversals, where no reversals are currently recognized in the fi eld (Fig. 4). In contrast, the dates for the Grande Ronde Basalt currently fall within a long reversed period, when the rock record shows four reversals (Fig. 4). As has been done in the Steens succession (Jarboe et al., 2008), the Imnaha and Grande Ronde Basalt need a focused, combined paleomagnetic and argon dating study to examine (1) whether reversals occur within the Imnaha and (2) whether the reversals noted in the Grande Ronde Basalt are merely excursions.

Given the complex geometry of eruption units, with abut-ting internal lobes and sheets, and scope for variable activity across a lava fi eld (e.g., Vye, 2009; Vye-Brown et al., 2013), it seems highly likely that different units could be emplaced at the same time. The chances of diachroneity of units also seem highly plausible and could help explain some of the Ar dates; e.g., the uppermost part of the Imnaha could be much older in one place than another, and could have continued while Grande Ronde Basalt volcanism started elsewhere. To verify this, detailed fi eld and chronological studies would be needed, but they may help to elucidate some of the complexities in the relationships among the Steens Basalt, Imnaha Basalt, and Grande Ronde Basalt.

SUMMARY

From a detailed assessment of the available radioisotope data for the Columbia River Basalt Group, we compiled the most accurate and precise ages and propose the most valid dates for each of the formations, and some key units. Our recommenda-tions are based on a critical analysis of all the available raw data. Using our stated criteria, we recommend that the overall age range for Steens Basalt is ca. 16.9–16.6 Ma, but that the greatest bulk of Steens Basalt may have erupted in <50,000 yr, concen-trated at ca. 16.7 Ma, as demonstrated by paleomagnetic evidence on the estimated eruption rate of the Steens reversal (Camp et al., this volume). Steens activity was possibly synchronous with the onset of Imnaha Basalt volcanism, which began at ca. 16.7 Ma and ended ca. 16.0 Ma. Shortly after Imnaha volcanism fi nished, Grande Ronde Basalt volcanism started at ca. 16.0 Ma, but ended by ca. 15.6 Ma, and possibly considerably earlier. Dates for the Grande Ronde Basalt are currently at odds with data for the over-lying Wanapum Basalt activity, which appears to have started by 15.6 Ma and continued until ca. 15.0 Ma. The data support

evidence that suggests the Vantage sediments were only a short-lived feature (Reidel and Tolan, this volume) and do not repre-sent a long hiatus between the voluminous Grande Ronde Basalt and the sporadic volcanism of the Wanapum Basalt. During the period of time in which the Wanapum Basalt erupted, the locus of activity shifted to the SE with the onset of Snake River Plain bimodal volcanism. Following the Wanapum Basalt volcanism, activity declined during the emplacement of the Saddle Moun-tains Basalt. Activity associated with the Columbia River Basalt Group ceased by 6 Ma.

ACKNOWLEDGMENTS

New 40Ar/39Ar data presented in Appendix DR1 were funded by Natural Environment Research Council grant NER/A/S/2003/00444 to S. Self, S.P. Kelley, and T.L. Barry. Thanks are extended to C.L. Vye-Brown for use of samples she had col-lected. M. Widdowson and M.-N. Guilbaud are also thanked for assistance with collection of samples used to provide new age dates in this study. We are also very grateful to reviewers Robert Fleck and Ingrid Ukstins-Peate for their thoughtful and useful comments, which helped to improve this paper.

REFERENCES CITED

Aldrich, L.T., and Wetherill, G.W., 1958, Geochronology by radioactive decay: Annual Review of Nuclear Science, v. 8, p. 257–298, doi:10.1146/annurev.ns.08.120158.001353.

Anonymous, 2005, North American Stratigraphic Code: American Asso-ciation of Petroleum Geologists Bulletin, v. 89, no. 11, p. 1547–1591, doi:10.1306/07050504129.

Baksi, A.K., 1989, Reevaluation of the timing and duration of extrusion of the Imnaha, Picture Gorge, and Grande Ronde Basalts, Columbia River Basalt Group, in Reidel, S.P., and Hooper, P.R., eds., Volcanism and Tec-tonism in the Columbia River Flood-Basalt Province: Geological Society of America Special Paper 239, p. 105–112.

Baksi, A.K., 2010, Comment to “Distribution and geochronology of the Oregon Plateau (U.S.A.) fl ood basalt volcanism: The Steens Basalt revisited” by M.E. Brueseke et al.: Journal of Volcanology and Geothermal Research, v. 196, p. 134–138, doi:10.1016/j.jvolgeores.2010.04.007.

Baksi, A.K., 2013, this volume, Timing and duration of volcanism in the Columbia River Basalt Group: A review of existing radiometric data and new constraints on the age of the Steens through Wanapum Basalt extrusion, in Reidel, S.P., Camp, V.E., Ross, M.E., Wolff, J.A., Mar-tin, B.S., Tolan, T.L., and Wells, R.E., eds., The Columbia River Flood Basalt Province: Geological Society of America Special Paper 497, doi:10.1130/2013.2497(03).

Baksi, A.K., and Farrar, E., 1990, Evidence for errors in the geomagnetic polar-ity time-scale at 17–15 Ma: 40Ar/39Ar dating of basalt from the Pacifi c Northwest, USA: Geophysical Research Letters, v. 17, p. 1117–1120, doi:10.1029/GL017i008p01117.

Baksi, A.K., York, D., and Watkins, N.D., 1967, Age of the Steens Mountain geomagnetic polarity transition: Journal of Geophysical Research, v. 72, p. 6299–6308, doi:10.1029/JZ072i024p06299.

Baksi, A.K., Hall, C.H., and York, D., 1991, Laser probe 40Ar/39Ar dating stud-ies on sub-milligram whole-rock basalt samples: The age of the Steens Mountain geomagnetic polarity transition (revisited): Earth and Planetary Science Letters, v. 104, p. 292–298, doi:10.1016/0012-821X(91)90210-9.

Barry, T.L., Self, S., Kelley, S.P., Reidel, S., Hooper, P., and Widdowson, M., 2010, New 40Ar/39Ar dating of the Grande Ronde lavas, Columbia River Basalts, USA: Implications for duration of fl ood basalt eruption episodes: Lithos, v. 118, p. 213–222, doi:10.1016/j.lithos.2010.03.014.

Barry, T.L., Self, S., Kelley, S.P., Reidel, S., Hooper, P., and Widdowson, M., 2012, Response to Baksi, A., 2012, “New 40Ar/39Ar dating of the Grande

64 Barry et al.

Ronde lavas, Columbia River Basalts, USA: Implications for duration of fl ood basalt eruption episodes” by Barry et al., 2010—Discussion: Lithos, v. 146–147, p. 300–303.

Beeson, M.H., Fecht, K.R., Reidel, S.P., and Tolan, T.L., 1985, Regional cor-relations within the Frenchman Springs Member of the Columbia River Basalt Group: New insights into the middle Miocene tectonics of north-western Oregon: Oregon Geology, v. 47, p. 87–96.

Brandon, A.D., and Goles, G.G., 1988, A Miocene subcontinental plume in the Pacifi c Northwest: Geochemical evidence: Earth and Planetary Science Letters, v. 88, p. 273–283, doi:10.1016/0012-821X(88)90084-2.

Brandon, A.D., and Goles, G.G., 1995, Assessing subcontinental lithospheric mantle sources for basalts: Neogene volcanism in the Pacifi c Northwest, USA, as a test case: Contributions to Mineralogy and Petrology, v. 121, p. 364–379, doi:10.1007/s004100050102.

Brueseke, M.E., and Hart, W.K., 2008, Geology and Petrology of the Mid-Miocene Santa Rosa–Calico Volcanic Field, Northern Nevada: Nevada Bureau of Mines and Geology Bulletin 113, 44 p.

Brueseke, M.E., Heizler, M.T., Hart, W.K., and Mertzman, S.A., 2007, Distri-bution and geochronology of Oregon Plateau (U.S.A.) fl ood basalt volca-nism: The Steens basalt revisited: Journal of Volcanology and Geothermal Research, v. 161, p. 187–214, doi:10.1016/j.jvolgeores.2006.12.004.

Camp, V.E., 1995, Mid-Miocene propagation of the Yellowstone mantle plume head beneath the Columbia River Basalt source region: Geology, v. 23, p. 435–438, doi:10.1130/0091-7613(1995)023<0435:MMPOTY>2.3.CO;2.

Camp, V.E., and Hanan, B.B., 2008, A plume-triggered delamination origin for the Columbia River Basalt Group: Geosphere, v. 4, no. 3, p. 480–495, doi:10.1130/GES00175.1.

Camp, V.E., and Ross, M.E., 2004, Mantle dynamics and genesis of mafi c mag-matism in the intermontane Pacifi c Northwest: Journal of Geophysical Research, v. 109, no. B8, doi:10.1029/2003JB002838.

Camp, V.E., Ross, M.E., and Hanson, W.E., 2003, Genesis of fl ood basalts and Basin and Range volcanic rocks from Steens Mountain to the Malheur River Gorge, Oregon: Geological Society of America Bulletin, v. 115, p. 105–128, doi:10.1130/0016-7606(2003)115<0105:GOFBAB>2.0.CO;2.

Camp, V.E., Ross, M.E., Duncan, R.A., Jarboe, N.A., Coe, R.S., Hanan, B.B., and Johnson, J.A., 2013, this volume, The Steens Basalt: Earliest lavas of the Columbia River Basalt Group, in Reidel, S.P., Camp, V.E., Ross, M.E., Wolff, J.A., Martin, B.S., Tolan, T.L., and Wells, R.E., eds., The Colum-bia River Flood Basalt Province: Geological Society of America Special Paper 497, doi:10.1130/2013.2497(04).

Carlson, R.W., and Hart, W.K., 1987, Crustal genesis on the Oregon Plateau: Journal of Geophysical Research, v. 92, p. 6191–6206, doi:10.1029/JB092iB07p06191.

Chenet, A.-L., Fluteau, F., Courtillot, V., Gerard, M., and Subbarao, K.V., 2008, Determination of rapid Deccan eruptions across the Cretaceous-Tertiary boundary using paleomagnetic secular variation: Results from a 1200-m-thick section in the Mahabaleshwar escarpment: Journal of Geo-physical Research, v. 113, B04101, doi:10.1029/2006JB004635.

Chenet, A.-L., Courtillot, V., Fluteau, F., Gerard, M., Quidelleur, X., Khadri, S.F.R., Subbarao, K.V., and Thordarson, T., 2009, Determination of rapid Deccan eruptions across the Cretaceous-Tertiary boundary using paleo-magnetic secular variation: 2. Constraints from analysis of eight new sections and synthesis for a 3500-m-thick composite section: Journal of Geophysical Research, v. 114, B06103, doi:10.1029/2008JB005644.

Colgan, J.P., Dumitru, T.A., and Miller, E.L., 2004, Diachroneity of Basin and Range extension and Yellowstone hotspot volcanism in northwestern Nevada: Geology, v. 32, no. 2, p. 121–124, doi:10:1130/G20037.1.

Colgan, J.P., Dumitru, T.A., McWilliams, M., and Miller, E.L., 2006, Timing of Cenozoic volcanism and Basin and Range extension in northwestern Nevada: New constraints from the northern Pine Forest Range: Geologi-cal Society of America Bulletin, v. 118, no. 1/2, p. 126–139, doi:10.1130/B25681.1.

Courtillot, V.E., and Renne, P.R., 2003, On the ages of fl ood basalt events: Comptes Rendus Geoscience, v. 335, p. 113–140, doi:10.1016/S1631-0713(03)00006-3.

Courtillot, V., Besse, J., Vandamme, D., Montigny, R., Jaeger, J.J., and Cappetta, H., 1986, Deccan fl ood basalts at the Cretaceous/Tertiary boundary?: Earth and Planetary Science Letters, v. 80, p. 361–374, doi:10.1016/0012-821X(86)90118-4.

Courtillot, V., Davaille, A., Besse, J., and Stock, J., 2003, Three distinct types of hotspots in the Earth’s mantle: Earth and Planetary Science Letters, v. 205, p. 295–308, doi:10.1016/S0012-821X(02)01048-8.

Dalrymple, G.B., and Lanphere, M.A., 1969, Potassium-Argon Dating: San Francisco, W.H. Freeman & Co., 258 p.

Dodson, A., Kennedy, B.M., and DePaolo, D.J., 1997, Helium and neon iso-topes in the Imnaha Basalt, Columbia River Basalt Group: Evidence for a Yellowstone plume source: Earth and Planetary Science Letters, v. 150, p. 443–451, doi:10.1016/S0012-821X(97)00090-3.

Draper, D.S., 1991, Late Cenozoic bimodal magmatism in the northern Basin and Range Province of southeastern Oregon: Journal of Volcanology and Geother-mal Research, v. 47, p. 299–328, doi:10.1016/0377-0273(91)90006-L.

Duncan, 1982, 40Ar/39Ar Dating of Columbia River Basalts: Rockwell/Hanford Report M29-SBB-258874, 10 p.

Duncan, 1983, 40Ar/39Ar Radiometric Age Determinations Final Report of Completed Work: Rockwell/Hanford Report M30-SBB-294449, 6 p.

Duncan, R.A., and Pyle, D.G., 1988, Rapid eruption of the Deccan fl ood basalts at the Cretaceous/Tertiary boundary: Nature, v. 333, p. 841–843, doi:10.1038/333841a0.

Eldholm and Coffi n, M., 2000, Large igneous provinces and plate tectonics, in Richards, M.A., Gordon, R.G., and van der Hilst, R.D., eds., The His-tory and Dynamics of Global Plate Motion: American Geophysical Union Geophysical Monograph 121, p. 309–326.

Evernden, J.F., and James, G.T., 1964, Potassium-argon dates and the Tertiary fl oras of North America: American Journal of Science, v. 262, p. 945–974, doi:10.2475/ajs.262.8.945.

Fiebelkorn, R.B., Walker, G.W., MacLeod, N.S., McKee, E.H., and Smith, J.G., 1982, Index to K-Ar Age Determinations for the State of Oregon: U.S. Geo-logical Survey Open-File Report 82-596, 40 p., index map, scale 1:1,000,000.

Fuller, R.E., 1930, The Petrology and Structural Relationship of the Steens Mountain Volcanic Series of Southeastern Oregon [Ph.D. dissertation]: Seattle, Washington, University of Washington, 282 p.

Fuller, R.E., 1931, The geomorphology and volcanic sequence of Steens Moun-tain in southeastern Oregon: Washington University Publications in Geol-ogy, v. 3, p. 1–30.

Geist, D., and Richards, M.A., 1993, Origin of the Columbia Plateau and Snake River Plain: Defl ection of the Yellowstone plume: Geology, v. 21, p. 789–792, doi:10.1130/0091-7613(1993)021<0789:OOTCPA>2.3.CO;2.

Gradstein, F., Ogg, J., and Smith, A., 2004, A Geologic Time Scale: Cambridge, UK, Cambridge University Press, 589 p.

Greene, R.C., Walker, G.W., and Corcoran, R.E., 1972, Geologic Map of the Burns Quadrangle: U.S. Geological Survey Open-File Map I-680, scale 1:250,000.

Gunn, B.M., and Watkins, N.D., 1970, Geochemistry of the Steens Mountain Basalts, Oregon: Geological Society of America Bulletin, v. 81, p. 1497–1516, doi:10.1130/0016-7606(1970)81[1497:GOTSMB]2.0.CO;2.

Hales, T.C., Abt, D.L., Humphreys, E.D., and Roering, J.J., 2005, A lithospheric instability origin for Columbia River fl ood basalts and Wallowa Moun-tains uplift in northeast Oregon: Nature, v. 438, p. 842–845, doi:10.1038/nature04313.

Hart, W.K., and Carlson, R.W., 1985, Distribution and geochronology of Steens Mountain–type basalts from the northwestern Great Basin: Isochron-West, v. 43, p. 5–10.

Henry, C.D., Castor, S.B., McIntosh, W.C., Heizler, M.T., Cuney, M., and Chemillac, R., 2006, Timing of oldest Steens Basalt magmatism from precise dating of silicic volcanic rocks, McDermitt caldera and northwest Nevada volcanic fi eld: Eos (Transactions, American Geophysical Union), Fall Meeting Supplement, abstract V44C-08.

Holmgren, D., 1970, K-Ar dates and paleomagnetics of the type Yakima Basalt, central Washington, in Gilmour, E.H., and Stradling, D.F., eds., Proceed-ings of the Second Columbia River Basalt Symposium: Cheney, Washing-ton, Eastern Washington State College Press, p. 189–199.

Hooper, P.R., and Hawkesworth, C.J., 1993, Isotopic and geochemical con-straints on the origin and evolution of the Columbia River basalt: Journal of Petrology, v. 34, p. 1203–1246, doi:10.1093/petrology/34.6.1203.

Hooper, P.R., Knowles, C.R., and Watkins, N.D., 1979, Magnetostratigraphy of the Imnaha and Grande Ronde Basalts in the southeast part of the Colum-bia Plateau: American Journal of Science, v. 279, p. 737–745.

Hooper, P.R., Binger, G.B., and Lees, K.R., 2002, Ages of the Steens and Columbia River fl ood basalts and their relationship to extension-related calc-alkaline volcanism in eastern Oregon: Geological Society of Amer-ica Bulletin, v. 114, p. 43–50, doi:10.1130/0016-7606(2002)114<0043:AOTSAC>2.0.CO;2.

Hooper, P.R., Camp, V.E., Reidel, S.P., and Ross, M.E., 2007, The origin of the Columbia River fl ood basalt province: Plume versus nonplume models,


in Foulger, G.R., and Jurdy, D.M., eds., Plates, Plumes, and Planetary Processes: Geological Society of America Special Paper 430, p. 635–668, doi:10.1130/2007.2430(30).

Jarboe, N.A., Coe, R.S., Renne, P.R., Glen, J.M.G., and Mankinen, E.A., 2008, Quickly erupted volcanic sections of the Steens Basalt, Columbia River Basalt Group: Secular variation, tectonic rotation, and the Steens Mountain reversal: Geochemistry Geophysics Geosystems, v. 9, no. 11, doi:10.1029/2008GC002067.

Jarboe, N.A., Coe, R.S., Renne, P.R., and Glen, J.M.G., 2010, The age of the Steens reversal and the Columbia River Basalt Group: Chemical Geology, v. 274, p. 158–168, doi:10.1016/j.chemgeo.2010.04.001.

Jay, A.E., and Widdowson, M., 2008, Stratigraphy, structure, and volcanology of the south-east Deccan continental fl ood basalt province: Implications for eruptive extent and volumes: Journal of the Geological Society of London, v. 165, p. 177–188, doi:10.1144/0016-76492006-062.

Johnson, J.A., Hawkesworth, C.J., Hooper, P.R., and Binger, G.B., 1998, Major and Trace Element Analyses of Steens Basalt, Southeastern Oregon: U.S. Geological Survey Open-File Report 98-482, 26 p.

Jolley, D.W., Widdowson, M., and Self, S., 2008, Volcanogenic nutrient fl uxes and plant ecosystems in large igneous provinces: An example from the Columbia River Basalt Group: Journal of the Geological Society of Lon-don, v. 165, p. 955–966, doi:10.1144/0016-76492006-199.

Jourdan, F., and Renne, P.R., 2007, Age calibration of the Fish Canyon sanidine 40Ar/39Ar dating standard using primary K-Ar standards: Geochimica et Cosmochimica Acta, v. 71, p. 387–402, doi:10.1016/j.gca.2006.09.002.

Kender, S., Peck, V.L., Jones, R.W., and Kaminski, M.A., 2009, Middle Miocene oxygen minimum zone expansion offshore West Africa: Evidence of global cooling precursor events: Geology, v. 37, p. 699–702, doi:10.1130/G30070A.1.

Koppers, A.A.P., Staudigel, H., and Wijbrans, J.R., 2000, Dating crystal-line groundmass separates of altered Cretaceous seamount basalts by the 40Ar/39Ar incremental heating technique: Chemical Geology, v. 166, p. 139–158, doi:10.1016/S0009-2541(99)00188-6.

Krueger Enterprises Inc., Geochron Laboratory Division, 1984, unpublished data available as data fi le from author S. Reidel.

Lees, K.R., 1994, Magmatic and Tectonic Changes through Time in the Neo-gene Volcanic Rocks of the Vale Area, Oregon, North Western USA [Ph.D. thesis]: Milton Keynes, UK, The Open University, 283 p.

Long, P.E., and Duncan, R.A., 1983, 40Ar/39Ar ages of Columbia River Basalt from deep boreholes in south-central Washington: Eos (Transactions, American Geophysical Union), v. 64, p. 90.

Ludwig, K.R., 2003, Isoplot 3.13: A Geochronological Toolkit for Microsoft Excel: Berkeley Geochronology Center Special Publication 4.

Mankinen, E.A., Prevot, M., Gromme, C.S., and Coe, R.S., 1985, The Steens Mountain (Oregon) geomagnetic polarity transition: 1. Directional his-tory, duration of episodes, and rock magmatism: Journal of Geophysical Research, v. 90, p. 10,393–10,416, doi:10.1029/JB090iB12p10393.

Martin, B.S., 1989, The Roza Member, Columbia River Basalt Group; chemical stratigraphy and fl ow distribution, in Reidel, S.P., and Hooper, P.R., eds., Volcanism and Tectonism in the Columbia River Flood-Basalt Province: Geological Society of America Special Paper 239, p. 85–104.

Martin, B.S., Tolan, T.L., and Reidel, S.P., 2013, this volume, Revisions to the stratigraphy and distribution of the Frenchman Springs Member, Wanapum Basalt, in Reidel, S.P., Camp, V.E., Ross, M.E., Wolff, J.A., Martin, B.S., Tolan, T.L., and Wells, R.E., eds., The Columbia River Flood Basalt Province: Geological Society of America Special Paper 497, doi:10.1130/2013.2497(06).

McKee, E.H., Swanson, D.A., and Wright, T.L., 1977, Duration and volume of Columbia River basalt volcanism, Washington, Oregon, and Idaho: Geo-logical Society of America Abstracts with Programs, v. 9, p. 463.

McKee, E.H., Hooper, P.R., and Kleck, W.D., 1981, Age of Imnaha Basalt—Oldest basalt fl ows of the Columbia River Basalt Group, northwest United States: Isochron-West, v. 31, p. 31–33.

Pierce, K.L., Morgan, L.A., and Saltus, R.W., 2002, Yellowstone plume head: Postulated tectonic relations to the Vancouver slab, continental boundar-ies, and climate, in Bonnichsen, B., White, C.M., and McCurry, M., eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Prov-ince: Idaho Geological Survey Bulletin, v. 30, p. 5–33.

Ramos, F.C., Wolff, J.A., and Tollstrup, D.L., 2005, Sr isotope disequilibrium in Columbia River fl ood basalts: Evidence for rapid shallow-level, open-system processes: Geology, v. 33, p. 457–460, doi:10.1130/G21512.1.

Ramos, F.C., Wolff, J.A., and Tollstrup, D.L., 2009, Evaluating the origin of Columbia River Basalt: Insights from mineral scale isotope variations

[abs.]: Geological Society of America Abstracts with Programs, v. 41, no. 7, p. 225.

Rampino, M.R., and Stothers, R.B., 1988, Flood basalt volcanism during the past 250 million years: Science, v. 241, p. 663–668, doi:10.1126/science.241.4866.663.

Reichow, M.K., Pringle, M.S., Al’Mukhamedov, A.I., Allen, M.B., Andreichev, V.L., Buslov, M.M., Davies, C., Fedoseev, G.S., Fitton, J.G., Inger, S., Medvedev, A.Ya., Mitchell, C., Puchkov, V.N., Safonova, I.Yu., Scott, R.A., and Saunders, A.D., 2009. The timing and extent of the eruption of the Siberian Traps large igneous province: Implications for the end-Permian environmental crisis: Earth and Planetary Sciences Letters, v. 277, p. 9–20, doi:10.1016/j.epsl.2008.09.030.

Reidel, S.P., 2005, A lava fl ow without a source: The Cohassett Flow and its com-positional components, Sentinel Bluffs Member, Columbia River Basalt Group: The Journal of Geology, v. 113, p. 1–21, doi:10.1086/425966.

Reidel, S.P., and Hooper, P.R., eds., 1989, Volcanism and Tectonism in the Columbia River Flood-Basalt Province: Geological Society of America Special Paper 239, 386 p.

Reidel, S.P., and Tolan, T.L., 1992, Eruption and emplacement of fl ood basalt: An example from the large-volume Teepee Butte Member, Columbia River Basalt Group: Geological Society of America Bulletin, v. 104, p. 1650–1671, doi:10.1130/0016-7606(1992)104<1650:EAEOFB>2.3.CO;2.

Reidel, S.P., and Tolan, T.L., 2013, this volume, The Grande Ronde Basalt, in Reidel, S.P., Camp, V.E., Ross, M.E., Wolff, J.A., Martin, B.S., Tolan, T.L., and Wells, R.E., eds., The Columbia River Flood Basalt Province: Geolog-ical Society of America Special Paper 497, doi:10.1130/2013.2497(05).

Reidel, S.P., Tolan, T.L., Hooper, P.R., Beeson, M.H., Fecht, K.R., Bentley, R.D., and Anderson, J.L., 1989, The Grande Ronde Basalt, Columbia River Basalt Group; stratigraphic descriptions and correlations in Wash-ington, Oregon, and Idaho, in Reidel, S.P., and Hooper, P.R., eds., Volca-nism and Tectonism in the Columbia River Flood-Basalt Province: Geo-logical Society of America Special Paper 239, p. 21–53.

Reidel, S.P., Camp, V.E., Tolan, T.L., and Martin, B.S., 2013, this volume, The Columbia River fl ood basalt province: Stratigraphy, areal extent, volume, and physical volcanology, in Reidel, S.P., Camp, V.E., Ross, M.E., Wolff, J.A., Martin, B.S., Tolan, T.L, and Wells, R.E., eds., The Columbia River Flood Basalt Province: Geological Society of America Special Paper 497, doi:10.1130/2013.2497(01).

Renne, P.R., Mundil, R., Balco, G., Min, K., and Ludwig, K.R., 2010, Joint determination of 40K decay constants and 40Ar*/40K for the Fish Canyon sanidine standard, and improved accuracy for 40Ar/39Ar geochronology: Geochimica et Cosmochimica Acta, v. 74, p. 5349–5367, doi:10.1016/j.gca.2010.06.017.

Rytuba, J.J., and McKee, E.H., 1984, Peralkaline ash fl ow tuffs and calderas of the McDermitt volcanic fi eld, southeast Oregon and north central Nevada: Journal of Geophysical Research, v. 89, p. 8616–8628.

Saunders A.D., and Reichow, M.K., 2009, The Siberian Traps and the End-Permian mass extinction: A critical review: Chinese Science Bulletin, v. 54, p. 20–37.

Scarberry, K.C., Meigs, A.J., and Grunder, A.J., 2009, Faulting in a propagat-ing continental rift: Insight from the late Miocene structural development of the Abert Rim fault, southern Oregon, USA: Tectonophysics, v. 488, no. 1–4, p. 71–86.

Self, S., Widdowson, M., Thordarson, T., and Jay, A.E., 2006, Volatile fl uxes during fl ood basalt eruptions and potential effects on the global environ-ment: A Deccan perspective: Earth and Planetary Science Letters, v. 248, p. 518–532, doi:10.1016/j.epsl.2006.05.041.

Shoemaker, K.A., and Hart, W.K., 2002, Temporal controls on basalt genesis and evolution on the Owyhee Plateau, Idaho and Oregon, in Bonnichsen, B., White, C.M., and McCurry, M., eds., Tectonic and Magmatic Evolu-tion of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin, v. 30, p. 313–328.

Smith, A.D., 1992, Back-arc convection model for Columbia River Basalt genesis: Tectonophysics, v. 207, p. 269–285, doi:10.1016/0040-1951(92)90390-R.

Smith, G.A., 1988, Sedimentology of proximal to distal volcaniclas-tics dispersed across an active foldbelt: Ellensburg Formation (late Miocene), central Washington: Sedimentology, v. 35, p. 953–977, doi:10.1111/j.1365-3091.1988.tb01740.x.

Steiger, R.H., and Jäger, E., 1977, Subcommission on Geochronology: Con-vention on the use of decay constants in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, p. 359–362, doi:10.1016/0012-821X(77)90060-7.

66 Barry et al.

Sutter, J.F., 1978, K-Ar ages of Cenozoic volcanic rocks from the Oregon Cas-cades west of 121° 30′: Isochron-West, v. 21, p. 15–21.

Swanson, D.A., Wright, T.L., Hooper, P.R., and Bentley, R.D., 1979, Revisions in Stratigraphic Nomenclature of the Columbia River Basalt Group: U.S. Geological Survey Bulletin 1457-G, 59 p.

Swisher, C.C., III, 1992, 40Ar/39Ar Dating and Its Application to the Calibra-tion of the North American Land Mammal Ages [Ph.D. thesis]: Berkeley, California, University of California, 239 p.

Swisher, C.C., III, Ach, J.A., and Hart, W.K., 1990, Laser fusion 40Ar-39Ar dat-ing of the type Steens Mountain Basalt, southeastern Oregon, and the age of the Steens geomagnetic polarity transition: Eos (Transactions, Ameri-can Geophysical Union), v. 71, p. 1296.

Reidel, S.P., and Tolan, T.L., 2013, this volume, The late Cenozoic evolu-tion of the Columbia River system in the Columbia River fl ood basalt province, in Reidel, S.P., Camp, V.E., Ross, M.E., Wolff, J.A., Mar-tin, B.S., Tolan, T.L., and Wells, R.E., eds., The Columbia River Flood Basalt Province: Geological Society of America Special Paper 497, doi:10.1130/2013.2497(08).

Tikoff, B., Benford, B., and Giorgis, S., 2008, Lithospheric control on the ini-tiation of the Yellowstone hotspot: Chronic reactivation of lithospheric scars: International Geology Review, v. 50, p. 305–324, doi:10.2747/0020-6814.50.3.305.

Tolan, T.L., Reidel, S.P., Hooper, P.R., Beeson, M.H., Fecht, K.R., Bentley, R.D., and Anderson, J.L., 1989, Revisions to the estimates of the areal extent and volume of the Columbia River Basalt Group, in Reidel, S.P., and Hooper, P.R., eds., Volcanism and Tectonism in the Columbia River Flood-Basalt Province: Geological Society of America Special Paper 239, p. 1–20.

U.S. Energy Research and Development Administration, 1976, Preliminary Feasibility Study on Storage of Radioactive Wastes in Columbia River Basalts: U.S. Energy Research and Engineering Division Report ARH-ST-137 under contract E(45–1)-2130, 183 p.

Vye, C.L., 2009, Flow Field Formation and Compositional Variations of Flood Basalt Eruptions [Ph.D. thesis]: Milton Keynes, UK, Open University, 320 p.

Vye-Brown, C., Self, S., and Barry, T.L., 2013. Architecture and emplacement of fl ood basalt fl ow fi elds: Case studies from the Columbia River Basalt Group, NW USA: Bulletin of Volcanology, v. 75, doi:10.1007/s00445-013-0697-2.

Watkins, N.D., 1969, Non-dipole behavior during an Upper Miocene geomag-netic polarity transition in Oregon: Geophysical Journal of the Royal Astronomical Society, v. 17, p. 121–149, doi:10.1111/j.1365-246X.1969.tb02316.x.

Watkins, N.D., and Baksi, A.K., 1974, Magnetostratigraphy and oroclinal folding of the Columbia River, Steens, and Owyhee Basalts in Oregon, Washington, and Idaho: American Journal of Science, v. 274, p. 148–189, doi:10.2475/ajs.274.2.148.

Wignall, P.B., 2001, Large igneous provinces and mass extinctions: Earth-Science Reviews, v. 53, p. 1–33, doi:10.1016/S0012-8252(00)00037-4.

Wolff, J.A., Ramos, F.C., Hart, G.L., Patterson, J.D., and Brandon, A.D., 2008, Columbia River fl ood basalts from a centralized crustal magmatic system: Nature Geoscience, v. 1, no. 2, doi:10.1038.ngeo124.

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