Geological Society of America Bulletin · time scale is needed to correlate geologic and paleoclimatologic records that are being recov-ered at the temporal scale of astronomical

Geological Society of America Bulletin

doi: 10.1130/B30545.1 published online 24 February 2012;Geological Society of America Bulletin

Kaori Tsukui and William C. Clyde foreland basinstime scale: Paleomagnetism of radioisotopically dated tuffs from Laramide Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity

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Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale: Paleomagnetism of

radioisotopically dated tuffs from Laramide foreland basins

Kaori Tsukui† and William C. ClydeDepartment of Earth Sciences, University of New Hampshire, Durham, New Hampshire 03824, USA

ABSTRACT

Age calibration of the early to middle Eo-cene geomagnetic polarity time scale remains highly uncertain due to confl icting magneto-stratigraphic, radioisotopic, and astro chrono-logic results. In this study, new paleomagnetic polarity determinations of 29 ash-fall tuffs preserved in strata of fi ve Laramide foreland basins were used in conjunction with previ-ously published 40Ar/39Ar ages from the same tuffs to evaluate eight different calibration models for the early to middle Eocene part of the geomagnetic polarity time scale. Reliable paleomagnetic information was recovered from 23 tuffs, of which 17 showed normal po-larity and six showed reversed polarity. After comparison of the models with the paleomag-netic and radioisotopic data from the tuffs and an array of independent chronostrati-graphic observations, the new Willwood model is herein selected as the best alternative to the current geomagnetic polarity time scale calibration for the early to middle Eocene. Three important implications are apparent in our proposed model. First, the early Eocene is shortened by 0.6 m.y., and the middle Eocene is lengthened by 0.8 m.y. compared with the 2004 geomagnetic polarity time scale. Also, the early Eocene climatic optimum is esti-mated to have lasted from 52.9 to 50.7 Ma, ~1 m.y. longer than previously suggested, and overlapping in time with the inferred age of the Wasatchian-Bridgerian faunal transition. Our new model agrees with a previous astro-nomical model when it is tied to the oldest proposed age for the Paleocene-Eocene Ther-mal Maximum at 56.33 Ma.

INTRODUCTION

The geomagnetic polarity time scale plays an integral role in interpreting geologic records ranging from biologic evolution and climate change to seafl oor spreading, making the preci-sion of its calibration a matter of fundamental importance to our understanding of Earth his-tory (e.g., DeMets et al., 1994). However, for the pre-Neogene geomagnetic polarity time scale, where theoretical uncertainties in the orbital calculations do not permit reliable astronomi-cal calibration of magnetic polarity reversals, numerical calibration has been achieved mainly via interpolation among a limited number of radioisotopic age determinations that are cor-related to marine magnetic anomalies, and an assumption of smoothly varying seafl oor spreading (Cande and Kent, 1992; Ogg and Smith, 2004; Laskar et al., 2004). However, cal-ibration of intervening magnetic chrons using this interpolation method is highly sensitive to the number and accuracy of the tie points used, making the interpolated segments poorly de-fi ned in absolute time if insuffi cient numbers of tie points are used. Furthermore, the assumption about the rates of seafl oor spreading, on which the accuracy of the geomagnetic polarity time scale calibration relies, is not routinely tested by empirical data. A more precise and reliable time scale is needed to correlate geologic and paleoclimatologic records that are being recov-ered at the temporal scale of astronomical forc-ing (Lourens et al., 2005; Zachos et al., 2005; Sexton et al., 2011).

The early Eocene is a relatively poorly calibrated interval of the geomagnetic polar-ity time scale. In the most recent time scale (Ogg and Smith, 2004, hereafter GOS2004), the ~22-m.y.-long Eocene segment of the geo-magnetic polarity time scale is calibrated by applying a cubic spline function through only fi ve tie points that are spaced on average every ~4–5 m.y. In the previous two time scales by Cande and Kent (1995, hereafter CK95) and

Ogg and Smith (2004), radioisotopic ages of the tie points were obtained with different fl uence monitor standards, and thus the accuracy of the resultant time scale was compromised. Further-more, there is as much as a 1 m.y. difference in the age of the C20r-C21n chron boundary and ~1.2 m.y. difference in the duration of the early Eocene between the two time scales. Given that the geomagnetic polarity time scale serves as a global reference to which radioisotopic, mag-netostratigraphic, and biostratigraphic data are correlated, these types of uncertainties in the geomagnetic polarity time scale calibration can quickly propagate and affect studies that rely on it for chronostratigraphic purposes (Machlus et al., 2004).

Some of the most complete sedimentary rec-ords from the early and middle Eocene are pre-served in the Laramide basins of southwestern Wyoming, northeastern Utah, and northwestern Colorado (Bradley, 1964; Roehler, 1992a; Mur-phey, 2001). Within the last decade, a suite of ash-fall deposits of volcanic origin from these basins has been dated using high-precision 40Ar/39Ar geochronology, resulting in several different efforts to calibrate this interval of the geomagnetic polarity time scale (Murphey et al., 1999; Wing et al., 2000; Machlus et al., 2004; Murphey and Evanoff, 2007; Smith et al., 2003, 2006, 2008a). Additionally, Westerhold and Röhl (2009) developed the fi rst astronomically calibrated age model for the early to middle Eo-cene (base of C21r to base of C24n) based on marine sedimentary records from the western Atlantic (Demerara Rise, Ocean Drilling Pro-gram [ODP] Leg 207, Site 1258). Although the astronomical age model is not calibrated in ab-solute time because it is beyond 42 Ma, the limit of precise astronomical solutions, it provides an independent basis for determining the durations of chrons and forces reevaluation of the current calibration scheme for the Eocene part of the geomagnetic polarity time scale.

One way to assess the accuracy of the pro-posed calibration models is to determine the

For permission to copy, contact [email protected]© 2012 Geological Society of America

1

GSA Bulletin; Month/Month 2012; v. 1xx; no. X/X; p. 1–16; doi: 10.1130/B30545.1; 9 fi gures; 2 tables; Data Repository item 2012102.

†Present address: Division of Geochemistry, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA, and Richard Gilder Graduate School at the American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024, USA; e-mail: [email protected]


Tsukui and Clyde

2 Geological Society of America Bulletin, Month/Month 2012

paleomagnetic polarity of radioisotopically dated tuffs and compare the observed polarity to that predicted by each age model. McIntosh et al. (1992) used a similar approach on a se-quence of ignimbrites from the southwestern United States to provide four possible calibra-tions for the late Eocene to Oligocene. In this study, we determined the magnetic polarity of 29 tuffs from the Greater Green River, Wind River, Uinta, Fossil, and Piceance Creek Ba-

sins (Fig. 1; see Table DR1 for detailed de-scription of the sampled tuffs1). According to the 40Ar/39Ar geochronology of Smith et al. (2008a), these tuffs were sampled on average

every ~0.36 m.y., and thus they allow us to test the competing calibration models at a tempo-ral resolution much fi ner than that at which the Eocene geomagnetic polarity time scale is cur-rently calibrated.

An improved calibration of the early to middle Eocene time scale has important impli-cations for studies that rely on temporal syn-chroneity of geologic data for understanding causal mechanisms. For instance, a coupling

i

Rock Springs

Uplift

WY

CO

UT

CO

40°N

42°N

108°W110°W112°W

B

A'A

100 kmN

MT

ID

WY

COUT

38°N

Sample localityLines of cross sectionsX'X

Early Paleogene

Precambrian cored upliftsCambrian-Jurassic clasticCretaceous

Uinta Basin

Fossil Basin

Greater Green River Basin

Great Divide Basin

Washakie Basin

Sand Wash Basin

Wind River Basin

Piceance CreekBasin

Uinta Uplift

Granite Mtns Uplift

Wind River Uplift

Bridger Basin

C'CuW

Bl

St

OFa

Y

C’

C

B’B

A’A

LC

CB

TB

CP

HD

WL

G

S

C F ASB

B R

KSa

HF BESCM

6LM

Figure 1. Geologic map of the study area showing sampling locations of tuffs and lines of cross sections. See Table 1 for abbreviations of tuffs. The base map is after Burchfi el (1993).

1GSA Data Repository item 2012102, three addi-tional fi gures and fi ve additional tables that contain sample descriptions, measurements, and details of our method of age model evaluation, is available at http://www.geosociety.org/pubs/ft2012.htm or by re-quest to [email protected].


Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale

Geological Society of America Bulletin, Month/Month 2012 3

between the early Eocene climatic optimum, which represents the long-term Cenozoic peak in warmth (Zachos et al., 2001), and the Wasatchian-Bridgerian North American Land Mammal Age (NALMA; Wood et al., 1941) faunal turnover has been suggested based on their co-occurrences in chron C23r (Clyde et al., 2001; Woodburne et al., 2009). How-ever, other studies dispute the result (Smith et al., 2003, 2004; Clyde et al., 2004), and the underlying mechanism for this coupling re-mains elusive. A resolution to these questions will require an improved chronostratigraphy for the Greater Green River Basin and a more reliable geomagnetic polarity time scale to make precise correlations between deep-sea climatic proxies and terrestrial biotic records. Finally, a refi ned geomagnetic polarity time scale can help reconstruct the history of sea-fl oor spreading rates and thus test the assump-tion of smoothly and continuously varying seafl oor spreading on which the precision of the geomagnetic polarity time scale has tradi-tionally relied.

Geologic Setting

The ash-fall tuffs collected for this study were deposited in fl uvial and lacustrine environments after being transported from the Absaroka and Challis volcanic fi elds of northwestern Wyo-ming and Idaho via winds or rivers (Surdam and Stanley, 1980; Chetel et al., 2011). In general, the tuff-bearing formations record geomorphic and hydrologic changes resulting from the inter-play between regional tectonics and climate (Fig. 2; Pietras et al., 2003). The Wasatch and Wind River Formations contain mostly fl uvial facies (e.g., red paleosol mudstones and channel sandstones), whereas the Green River Forma-tion is dominated by lacustrine facies (e.g., lami-nated shales interbedded with thin sandstones and limestones). The Bridger Formation and its lateral correlatives, the Fowkes Formation in Fossil Basin and the Washakie Formation in Washakie Basin, are composed of fi ne-grained volcaniclastic deposits with laterally extensive limestone and sandstone marker beds. On fresh surfaces, these water-laid tuffs are usually no

more than 30 cm thick and vary in color from white to light gray to orange in outcrop. Grain size of the tuffs varies from very fi ne sand to clay, and textural differences correlate with different depositional settings (see GSA Data Repository Table DR1 [see footnote 1]). For instance, the lacustrine tuffs (e.g., K-spar and Sixth tuffs) are relatively fi ne grained and are more distinctive in outcrop, having sharp upper and lower contacts with the surrounding mud-stone or limestone units. They also typically lack the evidence of postdepositional rework-ing that is otherwise common in the fl uvial tuffs (e.g., Basal Bridger E tuff), which are relatively coarser grained, have more diffuse contacts with the facies above and below, and often contain pumice clasts or fragments. Signs of postdepo-sitional zeolitization caused by saline and alka-line lake water are common in the matrix of the lacustrine tuffs (Ratterman and Surdam, 1981). A grading-upward pattern of the biotite crystals, which were abundant in many tuffs, indicates that the tuffs were deposited in single events, thus ensuring that the paleomagnetic data from

Figure 2. Representative fence diagrams for Fossil Basin (A-A′), Greater Green River Basin (B-B′), and Uinta Basin (C-C′) along lines of cross sections shown in Figure 1. Approxi-mate stratigraphic positions of sampled tuffs are shown rela-tive to major lithologic bound-aries (solid line) and marker beds (broken line, b—Buff marker bed, m—Mahogany marker bed, h—Horse Bench sandstone). Dotted lines show general interbasinal correlation. For global positioning system (GPS) coordinates and abbre-viations of the tuffs, see Table 1. Lithologic units are colored ac-cording to lake type assign ment of Smith et al. (2008a). Abbre-viations of lithologic units are as follows: Tfbh—Bulldog Hollow Member of the Fowkes Forma-tion; Tfs—Sillem Member of the Fowkes Formation; Twb—Bullpen Member of the Wasatch Formation; Tga—Angelo Member of the Green River Formation; Tgf—Fossil Butte Member of the Green River Formation; Twm—main body of the Wasatch Formation; Twk—Washakie Formation; Tb—Bridger Formation; Tgl—Laney Member of the Green River Formation; Tgw—Wilkins Peak Member of the Green River Formation; Tgt—Tipton Member of the Green River Forma-tion; Twc—Cathedral Bluffs Tongue of the Wasatch Formation; Tu—Uinta Formation; Tgsl—sandstone and limestone facies of the Green River Formation; Tgs—saline facies of the Green River Formation; Tgu—upper member of the Green River Formation; Tgtr—transitional interval of the Green River Formation; Tg—main body of the Green River Formation. Lithostratigraphic data are adapted from Buchheim (1994); Oriel and Tracey (1970; profi le A); Culbertson (1961); Roehler (1992a, 1992b); Murphey and Evanoff (2007; profi le B); Remy (1992); Dane (1954); Bryant et al. (1989); Prothero (1996; profi le C).

BridgerBasin

RockSprings

UpliftWashakie

Basin

0

30

60

90

120

0

600

300

1200

IndianCanyon

GateCanyon

200

300

100

0A B CA′ B′ C′

Fossil Basin

Volcaniclastic

Alluvial

Fluvial-lacustrine

Fluctuating profundal

Interbedded evaporites

Evaporative

Tgw

Tgt

Tb

Twk

Tu

Tgu

Tgtr

Tg

Twm

Twb

Tfs

Tfbh

Twm

Twcm

h

Tga

Twm

150

Twm

Stra

tigra

phic

thic

knes

s (m

)

CuW

Bl

StO

Fa

LCCB

TB

CP

G

S

CF

SB

BR

Sa

HF

BE SCM

6LM

Tgl

b A

K

Tgf

900

Tgsl

Tgs


Tsukui and Clyde


these tuffs record an instantaneous polarity at the time of deposition. Previous studies (e.g., Reynolds, 1979; Hayashida et al., 1996; Iwaki and Hayashida, 2003) have shown that water-laid ash-fall tuffs typically yield early acquired detrital remanent magnetizations, probably of detrital origin, that record paleomagnetic fi eld directions acquired in a relatively short time period.

METHODS

Paleomagnetic Sampling

Paleomagnetic samples were collected from 29 previously dated ash-fall tuffs in fi ve Laramide Basins (Figs. 1 and 2; Smith et al., 2003, 2006, 2008a, 2010; Murphey et al., 1999; Machlus et al., 2004; Murphey and Evanoff, 2007). In the fi eld, tuffs were located with the help of colleagues (see Acknowledgments) or by global positioning system (GPS) coordinates and rock descriptions provided in the literature (Table DR1 [see footnote 1]). Effort was made to sample from the exact same locations that had been sampled for 40Ar/39Ar analysis in the previous studies (Smith et al., 2003, 2008a). At each site, weathered materials were removed to expose a fresh surface for sampling. Five to ten separately oriented samples were collected as 2.5-cm-diameter cylinders with a portable gas-powered core drill (85 samples total) or as oriented hand samples (43 samples total), which were later cut into 8 cm3 samples with a saw. Whenever possible, the fi nest-grained parts of a tuff were sampled for paleomagnetic analy-sis because they may contain single-domain grains, which are more likely to preserve reli-able characteristic remanent magnetizations (ChRM; Butler, 1992). However, Smith et al. (2003, 2008a) sampled the coarsest fraction of each deposit (i.e., base) in order to collect the largest possible crystals for radioisotopic dat-ing. In one case (Boar tuff), the tuff bed was too friable for paleomagnetic sampling, so samples were collected from a siliciclastic layer immedi-ately (~20 cm) above. Also, where stratigraphic integrity of the tuffs was unclear (e.g., due to suspected slumping as in the case of the White Lignitic and Wavy tuffs), samples were col-lected from multiple locations within the same bed to average out the possible effect of post-depo si tional disturbance.

Laboratory and Data Selection Procedures

The paleomagnetic samples were demagne-tized by alternating fi eld (AF) and/or thermal methods between 2.5 mT and 100 mT and/or 25 °C and 690 °C, respectively. Natural rema-

nent magnetization (NRM) was measured on an HSM 2 SQUID-based spinner magnetom-eter inside a three-dimensional, direct current (DC) coil, low-fi eld cage at the Paleomagnetism Laboratory at the University of New Hamp-shire. Characteristic components of NRM were calculated by least squares analysis through the high-coercivity or high-unblocking-temperature demagnetization paths that trended toward the origin (Kirschvink, 1980). The maximum angular deviation (MAD) of the ChRM was calculated for each sample, and at least three samples with MADs less than 20° were used to determine a statistically robust site mean polar-ity. Those samples with MADs above 20° were rejected from further analyses. Sites that passed Watson’s test for randomness at the 95% signifi -cance level at N = 4 with directional precision parameter (k) of >10 (<10) were classifi ed as al-pha (beta) sites (Watson, 1956). Alpha 95 (α

95)

expresses within-site scatter as the 95% cone of confi dence around the estimated mean direc-tion. The relatively liberal data quality criteria on MAD and precision parameter were chosen because the objective of the study is to deter-mine polarity for specifi c tuff horizons, rather than a precise paleopole. For the samples col-lected from beds with measurable dips, tectonic correction was performed to the measured dec-linations and inclinations. Virtual geomagnetic poles (VGP) were calculated for the alpha and beta sites, and VGP latitude was used to infer magnetic polarity of sites, assuming that the ChRM is primary in origin.

To investigate the magnetic mineralogy, we conducted isothermal remanent magnetiza-tion (IRM) acquisition experiments on a suite of samples cut into 1 cm3 cubes. Using a ASC IM10 impulse magnet, samples were sub-jected to a magnetic fi eld of different intensities (0.12 T, 0.4 T, and 1.1 T) along three orthogo-nal axes, isolating low-, medium-, and high-coercivity fractions in the x, y, and z directions, respectively (Lowrie, 1990). Subsequently, the acquired IRM was thermally demagnetized at steps of 25, 50, 75, 100, 125, 150, 200, 250, 300, 400, 500, 540, and 580 °C. The three-axis IRM experiment shows the ferromagnetic min-eral content of a sample using the characteristic coercivities and temperature-dependent proper-ties of different magnetic minerals. Bulk sus-ceptibility was measured on selected samples to further facilitate interpretation of the magnetic mineralogy of the tuffs.

PALEOMAGNETIC RESULTS

Statistics for all paleomagnetic sites exam-ined in this study are summarized in Table 1. NRM intensities are highly variable, ranging

over three orders of magnitude between 0.092 mA/m and 21.912 mA/m before demagneti-zation (Table DR2 [see footnote 1]). Neither intensity nor grain size appears to control de-magnetization behavior or the quality of the isolation of a ChRM. Tuffs deposited in fl uvial environments (Wind River, Bridger, and Fowkes Formations) generally have higher NRM inten-sities than those deposited in lacustrine settings (Green River Formation). The fl uvial tuffs also have higher magnetic susceptibility compared with lacustrine tuffs, likely due to higher con-centrations of volcanigenic ferro/ferrimagnetic minerals (Dunlop and Özdemir, 1997; Table DR3 [see footnote 1]). After initial pilot studies were carried out to determine the most effective demagnetization protocol for each tuff, 73 out of 128 samples were demagnetized using thermal demagnetization, and the remaining 55 samples were demagnetized with AF demagnetization. In some cases, both methods were combined to resolve a ChRM.

Demagnetization revealed either one or two NRM components in every sample. Most re-versed polarity samples exhibited overprint components, which were unblocked by 400 °C or randomized by 40 mT (Fig. 3). The overprint components show highly variable directions, making it diffi cult to infer the cause or timing of the secondary magnetization acquisition. After removal of the low-temperature or low-coercivity component, ChRM was progressively demagnetized up to 690 °C or 100 mT (Table DR2 [see footnote 1]). Five samples that had best-fi t lines to the ChRM with MADs above 20° were rejected from further analyses. The av-erage MAD of the ChRM for the remaining 123 samples was 8.16°, and the ChRM from those samples was used to estimate site mean direc-tions. Of the 23 sites that passed Watson’s test for randomness at the 95% signifi cance level, six sites were characterized by reversed polar-ity with ChRM showing S-SE declinations and moderate to steep negative inclinations (mean of Dectec = 159°, Inc

tec = −62° [Dectec—declina-

tion in tectonic coordinates, Inctec—inclination in tectonic coordinates], α

95 = 25.3°). The re-

maining 17 sites showed normal polarity with N-NW directions and moderate to steep posi-tive inclinations (mean of Dectec = 1.6°, Inctec = 56°, α

95 = 7.8). Eighteen sites with k > 10 are

referred to as alpha sites; fi ve sites with k < 10 are referred to as beta sites. The beta sites are characterized by anomalous site mean declina-tions between 7° and 59° for normal polar-ity sites and 56.9° for the one reversed polarity site. Inclinations of their ChRM, however, are largely consistent between samples within a site and therefore are included in the calculation of site VGPs, but we caution that those polarity




TAB

LE 1

. SU

MM

AR

Y S

ITE

STA

TIS

TIC

S

In g

eogr

aphi

c co

ordi

nate

sIn

tect

onic

coo

rdin

ates

Site

*F

orm

atio

nN

ame

Abb

revi

atio

nLa

t(°

)Lo

ng (°)

Str

ike†

(°)

Dip

†

(°)

N§

Dec

linat

ion

(°)

Incl

inat

ion

(°)

Dec

linat

ion

(°)

Incl

inat

ion

(°)

R#

k**

α 95††

(°)

Lat vg

p§§

(°)

Long

vgp

§§

(°)

BR

0501

αB

ridge

rH

enry

s F

ork

HF

N41

°72′

5.40

″W

110°

9′27

.59″

00

417

5−4

5.9

175

−45.

93.

815

.79

23.9

−75.

626

8B

R05

02β

Gre

en R

iver

Gre

yG

N41

°39′

25.0

0″W

109°

17′1

7.79

″23

27

444

3738

.935

.73.

55.

4343

.550

.52.

3

BR

0503

αG

reen

Riv

erS

and

But

teS

BN

41°2

0′44

.11″

W10

8°40

′11.

20″

00

434

6.1

58.8

346.

158

.83.

945

.913

.779

.315

6.2

BR

0504

Gre

en R

iver

Ana

lcite

AN

41°2

0′43

.13″

W10

8°40

′ 12.

31″

337

104

53.5

74.1

58.4

64.2

10.

9918

048

.431

4.5

BR

0505

αG

reen

Riv

erS

ixth

6N

41°3

2 ′35

.12″

W10

9°28

′53.

59″

227

64

1.4

58.1

355.

553

.64

88.1

9.8

81.8

97.6

BR

0506

αG

reen

Riv

erLa

yere

dL

N41

°32′

37.5

9″W

109°

28′5

5.89

″22

76

517

.361

.18.

857

.75

82.6

28.

582

.53

BR

0507

αG

reen

Riv

erM

ain

MN

41°3

2′32

.75″

W10

9°28

′53.

01″

227

64

3.7

48.7

359.

244

.44

198.

76.

574

.573

.2

BR

0508

αB

ridge

rC

hurc

h B

utte

CB

N41

°28′

34.7

2″W

110°

8′4.

29″

00

435

3.5

58.4

353.

558

.44

128.

38.

184

.513

6.4

BR

0509

Gre

en R

iver

Sch

eggs

SN

41°3

1′9.

89″

W10

9°19

′ 30.

23″

211

63

315.

7−4

2.2

317.

3−4

81.

61.

4118

09.

210

7.6

BR

0510

βG

reen

Riv

erF

ireho

leF

N41

°21′

3.67

″W

109°

22′5

7.31

″27

09

349

.8−4

1.9

56.9

−47.

32.

76.

7651

.72.

623

.1

BR

0511

αG

reen

Riv

erC

Bed

CN

41°2

1′1.

84″

W10

9°24

′0.5

9″18

96

44

61.4

353.

460

.34

404.

74.

685

161.

5

BR

0512

αB

ridge

rB

asal

Brid

ger

EB

EN

41°7

′34.

41″

W11

0°8′

15.8

1 ″12

23

435

3.5

39.8

351.

842

.14

73.5

210

.871

.894

.5

BR

0513

αB

ridge

rLe

avitt

Cre

ekLC

N41

°14′

12.6

7″W

110°

12′4

1.45

″12

74

168.

2−6

8.4

186.

3−7

0.2

3.8

16.1

323

.6−7

6.3

85.5

CP

0646

αB

ridge

rC

ontin

enta

l P

eak

CP

N42

°16′

6.02

″W

108°

43′ 7

.40″

00

416

0.7

−65.

116

0.7

−65.

13.

947

.03

13.5

−75.

57.

4

KT

0701

Wag

on B

edW

hite

Lig

nitic

WL

N42

°42′

54.3

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535

496

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1234

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ger

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0704

αG

reen

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413

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932

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110°

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04

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716

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7

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110°

57′5

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04

351.

354

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.780

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ridge

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ek M

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SC

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41°7

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9

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39°5

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′ 6.6

9″0

05

7.3

53.3

7.3

53.3

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58.2

81.5

23.5

KT

0710

βG

reen

Riv

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ilyO

N40

°2′5

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110°

31′ 4

2.10

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06

59.4

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6

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yon

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40°1

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423

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0716

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9

*α a

nd β

ref

er to

alp

ha a

nd b

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site

s, r

espe

ctiv

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(see

text

). S

ites

with

out α

or

β d

esig

natio

n di

d no

t sat

isfy

the

Wat

son’

s te

st (

Wat

son,

195

6) a

nd w

ere

excl

uded

from

pol

arity

det

erm

inat

ion.

† Str

ike

and

dip

are

mea

sure

d us

ing

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-han

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le (

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to th

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ht o

f str

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.§ N

is n

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r of

sam

ples

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lyze

d fo

r pa

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agne

tic p

olar

ity d

eter

min

atio

n.# R

is r

esul

tant

vec

tor.

**k

is p

reci

sion

par

amet

er.

††α 9

5 is

con

e of

95%

con

fi den

ce a

bout

est

imat

ed m

ean

dire

ctio

n.§§

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p an

d Lo

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p re

fer

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titud

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eom

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re p

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nor

th a

nd e

ast.


Tsukui and Clyde


determinations are of lesser quality than those for the alpha sites. These sites could record tran-sitional polarities or may have been affected by vertical-axis rota tions. The 18 alpha sites are of both normal and reversed polarity and pass a re-versal test at the 95% confi dence level (Tauxe,

1998). The mean direction (dec/inc) when all reversed sites are inverted is 353.6°/57° (α

95 =

5.5, k = 40.23, N = 18), which is statistically in-distinguishable from the expected early Eocene direction for southwestern Wyoming (349°/61°; Fig. 4; Diehl et al., 1983).

Our paleomagnetic results show that ash-fall deposits in general can reliably record the an-cient geomagnetic fi eld via the acquisition of a detrital remanent magnetization and can typi-cally provide paleomagnetic polarity data even when the surrounding carbonate and siliciclas-tic rocks are not well suited for paleomagnetic analysis (Strangway and McMahon , 1973; Reynolds, 1979; Sheriff and Shive, 1982; Hayashida et al., 1996; Iwaki and Hayashida , 2003). The sites for which results could not be statistically distinguished from random may have been handicapped by possible postmag-netization slumping (e.g., Wavy and White Lignitic tuffs) or severe postdepositional chemical alteration (likely in the Analcite , Curly, and Wavy tuffs). Step-wise thermal de-magnetization of three-component IRM indi-cates a dominance of magnetite in some tuffs (e.g., Henrys Fork and Grey tuffs) and a domi-nance of hematite in others (e.g., Basal Bridger E and Leavitt Creek tuffs; Fig. 5). These in-terpretations of the IRM results are consistent with the laboratory unblocking temperatures of the ChRM of the respective tuffs (Table DR2 [see footnote 1]).

EVALUATION OF AGE MODELS

Evaluation of the Input Data

Paleomagnetic Data of the TuffsThe reliability of our paleomagnetic results is

demonstrated by the passage of a reversal test based on 23 sites, the correspondence between the mean direction for all 18 alpha sites and the expected direction for the early Eocene, and the lack of complications in magnetic mineralogy, as indicated by IRM experiments. However, postdepositional diagenesis, which has been shown to have affected carbonate and siliciclas-tic deposits of the lacustrine Green River For-mation, could potentially have obscured the primary ChRM in some of the tuffs (Strangway and McMahon, 1973; Sheriff and Shive, 1982). In addition, some of the beta sites with simi-lar inclinations but variable declinations may record short-term fi eld behavior. Although the possibility of unrecognized overprints cannot be refuted due to a lack of opportunity to perform a fold test in the subhorizontal strata that charac-terize the study area, the criteria described here attest to the reliability of the polarity determina-tion of the tuffs.

40Ar/39Ar Data of the Tuffs from Previous Studies

The 40Ar/39Ar geochronologic data reported by Smith et al. (2008a, 2010), together with our paleomagnetic results, comprise the input

KT0703D

S

W, UpB

KT0706D

W

N, UpC

300 °C660 °C

130 °C

200 °C

525 °C

MAD = 7.7°

MAD = 1.9°

BR0506F

W

N, UpA

10 mT

100 mT

MAD = 1.6°

10 mT50 mT

50 mT

560 °C

560 °C

400 °C

400 °C

Figure 3. Vector end-point diagrams (Zijderveld, 1967) for three representative samples with different textures and from different depositional environments. Open (fi lled) squares show vector end points in the vertical (horizontal) plane. (A) BR0506F (Layered tuff) is a laminated tuff deposited in a lacustrine setting. Alternating fi eld (AF) demagnetization isolated a single component for which characteristic remanent magnetization (ChRM) was demagnetized by 100 mT with a maximum angular deviation (MAD) of 1.6°. It is of normal polarity. (B) KT0703D (Tabernacle Butte tuff) was deposited along with pumice clasts in a fl uvial setting. Thermal demagnetization isolated a single component for which ChRM was demagnetized by 660 °C with a MAD of 1.9°. It is of reversed polarity. (C) KT0706D (K-spar tuff) is a homogeneous crystalline tuff deposited in a lacustrine setting. Thermal demagneti-zation revealed two components. An overprint component was removed by 400 °C, leaving the characteristic component that was demagnetized by 525 °C with a MAD of 7.7°. The ChRM is of reversed polarity. Final demagnetization step and two intermediate steps are shown. Tics represent intensity increments of 0.5 mA/M.




data used to evaluate competing age models for the geomagnetic polarity time scale. Smith et al. (2008a) determined 40Ar/39Ar ages of 29 tuff beds (22 ash beds and 3 volcaniclastic sand beds) based on 2234 analyses of sanidine and biotite crystals using laser incremental heating experiments and/or laser fusion experiments on single- and multiple-crystal aliquots. The types of 40Ar/39Ar analyses used include (in order of accu racy): (1) sanidine single-crystal laser fusion , (2) biotite single-crystal step heat-ing, (3) sanidine multiple-crystal laser fusion , (4) biotite single-crystal laser fusion, and (5) biotite multiple-crystal analyses. These age determinations are consistent with the strati-graphic order of the tuff beds and thus are deemed reliable to the fi rst degree.

Those ages with no distinct outliers due to contamination by xenocrysts or without indi-cations of 40Ar* loss are most preferred. Con-tamination by xenocrysts is readily identifi able using single-crystal fusion analyses because of their signifi cantly older ages. In the case of the Sixth tuff, in which <10% contamination by xenocrystic or detrital grains was found, those analyses derived via only concordant incre-mental heating of individual biotite crystals

North North

Dec = 354°, Inc = 57°k = 44, α95 = 6°

Dec = 173°, Inc = –56°k = 28, α95 = 15°

Figure 4. Equal-area projections of mean characteristic remanent magnetization defi ned by progressive demagnetization directions of 18 alpha sites in tectonic coordinates. Circles show a 95% cone of confi dence around the estimated mean direction. Filled (open) symbols lie on the lower (upper) hemisphere of the projection. Sta-tistical parameters (directional precision parameter [k] and α95) are also shown. When the reversed sites are inverted, the mean direction (dec/inc) for all the alpha sites is 353.6°/57°, which is very close to the expected direction for the early Eocene based on the Eocene ref-erence pole for North America (349°/61°; Diehl et al., 1983).

0

20

40

60

80

100

120

140

160

180

0

50

100

150

200

250

300

0

100

200

300

400

500

600

700

800

0 100 200 300 400 500 600 700

0

50

100

150

200

250

300

350

400

BR0512B

Mag

netiz

atio

n (m

A/m

)M

agne

tizat

ion

(mA

/m)

CA

0 100 200 300 400 500 600

0 100 200 300 400 500 600 7000 100 200 300 400 500 600

Temperature (°C) Temperature (°C)

DB

BR0501A

BR0513IBR0502A

Figure 5. Step-wise thermal demagnetization of acquired isothermal remanent magneti-zation for selected samples. The samples were demagnetized in three orthogonal axes after fi elds of 0.12 T, 0.4 T, and 1.1 T were applied to the x (diamonds), y (squares), and z (triangles) axes, respectively. Observed unblocking temperatures in-dicate the presence of magne-tite in (A) BR0501A (Henrys Fork tuff) and (B) BR0502A (Grey tuffs) and hematite in (C) BR0512B (Basal Bridger E tuff) and (D) BR0513I (Leavitt Creek tuff).


Tsukui and Clyde


were used to infer the depositional age (Min et al., 2001; Smith et al., 2006, 2008a). Incre-mental heating experiments on multiple-crystal sanidine aliquots yielded internally concordant plateau ages that were consistent with fusion ages, thus demonstrating the absence of 40Ar* loss in these samples. With the exception of the Yellow and Strawberry tuffs, only 3.7% of all sanidine analyses were excluded from all age estimates.

In the absence of sanidine, biotite was ana-lyzed instead. Laser incremental heating ex-periments and electron probe microanalyses were performed on euhedral biotite crystals to determine the presence of alteration-related 40Ar* loss and 39Ar

K recoil induced during irra-

diation. Discordant age spectra were correlated with the presence of K-depleted alteration phases and resulted in considerable age scat-ter (Smith et al., 2008b). For high-precision age determination, age spectra that were con-cordant and reproducible were used with the weighted mean as the best estimate of the eruptive age.

All 40Ar/39Ar ages were calculated relative to the Taylor Creek rhyolite (28.34 ± 0.28 Ma; Renne et al., 1998) in Smith et al. (2008a) but were recalculated to the astronomically cali-brated age of 28.201 Ma for the Fish Canyon sanidine standard (FCs

K08; Kuiper et al., 2008)

in Smith et al. (2010). By intercalibrating all of the 40Ar/39Ar ages to FCsK08, it is possible to make precise comparison with astrochronol-ogy and ages derived by other methods such as U-Pb chronometer, because the FCsK08 reduces the absolute uncertainty from ~2.5% to less than 0.25% and it eliminates the ~1% discrep-ancy between radioisotopic and astronomical dating. This is an important point because the age models under consideration in this study are based on astrochronology as well as calibration points that were obtained under different cali-brations. In the following discussion, to allow intercomparison of ages obtained by 40Ar/39Ar dating using different fl uence monitor standards and astronomically determined ages, we used the FCs

K08-calibrated 40Ar/39Ar ages as reported

in Smith et al. (2010; see their Supplement table DR2) with a 2σ fully propagated uncer-tainty in all cases. A summary of all of the input data used to evaluate the age models is available in Table DR4 (see footnote 1).

Previous Age Models

Since the early compilation of the seafl oor magnetic anomaly pattern by Cande and Kent (1992), eight calibration models have been pro-posed for the early to middle Eocene part of the geomagnetic polarity time scale. These efforts

not only refl ect growing interest in Eocene time (e.g., Jaramillo et al., 2010; Sexton et al., 2011) but also underscore uncertainties in this part of the time scale. Although one of the goals of the geomagnetic polarity time scale is to provide an age estimate for magnetic chron bound-aries, the radioisotopic age determinations of the tie points and width estimates of the ma-rine magnetic anomalies, which are two major pillars in the construction of the geomagnetic polarity time scale, have inherent analytical and measure ment uncertainties. The resultant calibration of the chrons thus has built-in un-certainties. Such uncertainties are not always quantifi ed or acknowledged, but it is crucial to recognize them and try to further improve the accuracy and precision of the time scale using additional empirical data from deposits that yield both paleomagnetic polarity and radioiso-topic data. The age models we evaluate in this study include the two most recent geomagnetic polarity time scales (CK95, GOS2004; Cande and Kent, 1995; Ogg and Smith, 2004), models B, C, and D of Machlus et al. (2004), models by Smith et al. (2008a, 2010), an extended version by Wing et al. (2000), and the astronomical models of Westerhold and Röhl (2009) (Table DR5; Fig. DR1 [see footnote 1]).

Geomagnetic Polarity Time ScalesCK95 is a revised version of the time scale

by Cande and Kent (1992), and it incorporates changes that arose as a result of a new age esti-mate for the Cretaceous-Paleogene (K-Pg) boundary (65 Ma rather than 66 Ma) and an astronomical age estimate for the base of chron C3n.4n (5.23 Ma). Its Eocene part is cali-brated by fi tting a cubic spline through three tie points: 33.7 Ma at the Eocene-Oligocene (E-O) boundary, 46.8 Ma in the middle Eocene, and 55.0 Ma at the Paleocene-Eocene (P-E) bound-ary. However, as pointed out by Machlus et al. (2004), there are important uncertainties on the age estimates of the two older tie points. Con-fl icting K-Ar ages (45.8 ± 0.5 Ma and 46.8 ± 0.5 Ma) have been proposed for the middle Eo-cene tie point (Bryan and Duncan, 1983), and the age estimate for the early Eocene tie point is questionable because of an unconformity of unknown duration that separates the dated tuff and the tie point (Aubry et al., 1996). GOS2004 uses the same method of interpolation as CK95 but is based on fi ve calibration points, three of which are new additions since CK95 (however, note that a new 40Ar/39Ar age was used for the tie point at C21n.33; sensu Ogg and Smith, 2004).

Models by Machlus et al. (2004)Machlus et al. (2004) proposed three alter-

native models (models B, C, and D) for the

interval between chrons C15 and C29 based on different calibration points, but in all cases, they used a natural cubic spline fi t. In model B, the two older tie points of CK95 were replaced with an unpublished age (45.6 Ma) of Swisher and Montanari (in Berggren et al., 1995) and an interpolated age for the Paleocene-Eocene boundary (55.3 Ma; Wing et al., 2000). In model C, the two older tie points of CK95 were replaced with a single-crystal biotite 40Ar/39Ar age for the Sixth tuff (48.8 Ma; Machlus et al., 2004) and the aforementioned Paleocene-Eocene boundary age. In model D, the two older tie points of CK95 were replaced by the age for the Sixth tuff only. In these models, the Cretaceous-Paleogene and Eocene-Oligocene boundaries were left unchanged as in CK95, resulting in a time scale where the early Eocene was lengthened at the expense of the Paleocene and middle Eocene.

Model by Smith et al. (2008a, 2010)Smith et al. (2008a, 2010) recalibrated the

interval, C24 through C20 based on 40Ar/39Ar ages of ash beds, existing magnetostratigraphic data, NALMA biostratigraphy from the Big-horn Basin, Wind River Basin, Greater Green River Basin, Uinta Basin, Devil’s Graveyard Formation in Texas, and Absaroka volcanic province, as well as marine biostratigraphy from San Diego region (see table 4 in Smith et al., 2008a). However, uncertainties remain in the correlation of some of the tuffs to local magnetostratigraphic records (e.g., Layered tuff, Sixth tuff, and Continental Peak tuff). This model implies the presence of several short-duration polarity chrons that are not shown in the original marine magnetic anomaly records of Cande and Kent (1992). Smith et al. (2008a) attributed those to tiny wiggles described in Cande and Kent (1992).

Model by Wing et al. (2000)In Wing et al. (2000), a 40Ar/39Ar sani-

dine age from the bentonitic tuff at the base of chron C24n.1n (also referred to as “Will-wood Ash”) was used in place of the calibra-tion point at the Paleocene-Eocene boundary of CK95 (Wing et al., 1991; Tauxe et al., 1994). Chrons between C29 and C22 were calibrated by linear interpolation between the bentonitic tuff and two calibration points of CK95 at the Cretaceous-Paleogene bound-ary and in the middle Eocene. We have ex-trapolated the original calibration to chron C20n to accommodate some of the younger tuffs collected from the Uinta Basin in this study. This new version, which includes the extrapolated segment, is referred to as the Willwood model in the following discussion.




Models by Westerhold and Röhl (2009)Westerhold and Röhl (2009) used a funda-

mentally different approach than those previ-ously discussed, which relied on radioisotopic ages of discrete tuff horizons. Instead, the Westerhold and Röhl (2009) models are based on orbital tuning and cycle counting of Fe in-tensity data from ODP Leg 207, Site 1258, and they provide estimates of chron durations for the interval between chrons C20 and C24. In the tuned model, the cycles are tuned to the stable 405-k.y.-long eccentricity cycle, whereas the cycle-counted model is based on cycle counting assuming 21 k.y. and 95 k.y. for the precession and short eccentricity cycles, respectively. The accuracy of an astronomical time scale based on orbital cyclicities depends on the accuracy of the orbital solution used (although it is presently not available for the Paleogene), the stratigraphic completeness of climatic proxy records, proper correlation between the proxy records and the orbital solution, the accuracy of magnetostrati-graphic data for the section in which the proxy records are identifi ed and correlated, and fi nally, the assumed value of sediment accumulation rates between two astronomically tuned cali-bration points. Uncertainties in the Westerhold and Röhl (2009) geochemical data set include: potential ambiguities in magnetostratigraphy of ODP 1258 due to near-equatorial paleolatitudes and weak magnetization; potential unrecog-nized faults in Hole 1258A; and ambiguities in construction of the composite section (Wester-hold and Röhl, 2009).

In contrast to the models based on 40Ar/39Ar age determinations, testing the Westerhold and Röhl (2009) models requires us to fi rst anchor them to a reference point, as they are fl oat-ing age models due to the uncertain age of the Paleocene-Eocene Thermal Maximum and un-certainties in orbital solutions for this interval (Laskar et al., 2004; Westerhold et al., 2007, 2008). To calibrate their models in absolute time, we used three astronomically proposed ages for the top of chron C24r as anchor points (53.53 Ma, 53.93 Ma, and 54.33 Ma according to option 1, option 2, and option 3, respectively; Westerhold et al., 2007). Because Westerhold and Röhl (2009) used two methods to calibrate the observed cycles (tuning and cycle counting), and each of them is anchored to three proposed age estimates for the top of chron C24r, in total, six variations are considered for evaluation in this study (these models will be referred to as T-1 to T-3 and CC-1 to CC-3, where T and CC stand for tuning and cycle counting, respectively, and the number following the hyphen corresponds to either option 1, 2, or 3). Tuffs that are younger than the younger limit of a particular age model are not considered in such cases. Figure DR1

summarizes all of the calibration models con-sidered in this study, showing different ways in which chrons are scaled according to different tie points and interpolation methods used (see footnote 1).

Method of Age Model Evaluation

The paleomagnetic polarity results from the 23 alpha and beta sites were considered in conjunction with the most recently published 40Ar/39Ar ages of the tuffs (Smith et al., 2008a, 2010) to evaluate geomagnetic polarity time scale age models for the interval between ca. 53 and 44 Ma. For the purpose of evaluating these age models, the 40Ar/39Ar and paleomagnetic polarity data are assumed reliable (see previous section on “Evaluation of the Input Data” for uncertainties of the polarity and 40Ar/39Ar data). Because every age model predicts a polarity for the 40Ar/39Ar age range of each tuff defi ned by a 2σ fully propagated uncertainty, we deter-mined the number of tuffs that have concor-dant predicted and measured polarities within a framework of a particular age model. The tuffs for which 40Ar/39Ar age ranges fall within a single chron in a particular age model are re-ferred to as category I tuffs and are included in the calculation of the “index of agreement” (see following). However, 40Ar/39Ar age ranges of some tuffs in any given age model are likely to span chron boundaries by chance, preventing an unequivocal assessment of the congruence between the predicted and observed polarity. These tuffs are classifi ed as category II and are excluded from the evaluation of the particular age model because they are equivocal. Note that the category assignment is age model spe-cifi c, and thus any given tuff may be classifi ed

as category I according to one age model but as category II according to another. The tuffs with relatively large uncertainties in 40Ar/39Ar age determinations are weighted less in this method of evaluation because they are more likely to be classifi ed as category II (e.g., Yellow and Straw-berry tuffs).

An index of agreement (IA = number of cat-egory I tuffs with concordant measured and pre-dicted polarity divided by the total number of category I tuffs within a particular age model) was calculated for each model and used to defi ne the model with maximum congruence between expected and measured polarity. Because the number of category I tuffs (i.e., denominator) varies among the models, the index of agreement provides a semiquantitative means to measure the effective explanatory power of each model with an expectation that a perfectly congruent calibration model would have an IA of 1.

Results of Age Model Evaluation

Calculated IAs range from 0.29 to 0.60, with the highest value recorded for the Willwood model, followed by model C and T-3 (IA = 0.56; Fig. 6; Table DR5 [see footnote 1]). No age model has an IA close to 1, and we interpret this result to indicate that further modifi cations to the models are necessary to reconcile the paleomagnetic polarity data with the radioiso-topic data. Because the use of IA was not sat-isfactory in defi ning one best model, the three models with the highest IA values (Willwood, model C, and T-3) are further discussed in the following. It is worth noting that the same test was performed using the 40Ar/39Ar age estimates considering only the 2σ analytical uncertainty (as opposed to fully propagated uncertainty),

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

CK95

GO

S20

04

Mod

el B

Mod

el C

Mod

el D

Will

woo

d

Sm

ith e

t al

.

T-1

CC-

1

T-2

CC-

2

T-3

CC-

3

0.50

0.31

0.44

0.56

0.50

0.60

0.40 0.

44

0.50

0.38

0.29

0.56

0.50

Inde

x of

agr

eem

ent

(Westerhold and Röhl, 2009)(Machlus et al., 2004)

mod

el

(20

10)

Figure 6. Index of agreement (IA) for all the calibration models evaluated in this study. See text for derivation of IA and abbrevia-tions of the model names.


Tsukui and Clyde


but the IA values did not change signifi cantly, implying that the match between the model and the input data is not controlled by the size of uncertainty associated with each 40Ar/39Ar age.

Further adjustments were made to the three models by shifting the chron ages in order to force at least a minimum overlap between ex-pected and measured polarity for all category I tuffs. Chron boundaries nearest to discordant category I tuffs (those with unmatched expected and measured polarities) were shifted to either the top or bottom of the 40Ar/39Ar age range of the tuffs so that predicted and measured polari-ties matched at least minimally within the age range determined for the tuffs. The ages of either the top or base of the discordant category I tuffs toward which the chron bound aries were shifted were then tentatively used as best approximate age estimates for the polarity boundaries. For example, four chron boundaries (C20r-C20n, C21n-C20r, C22n-C21r, and C22r-C22n) in the Willwood model were incre mentally shifted to the top of the Strawberry tuff age range at 43.34 Ma, base of the Oily tuff age range at 45.57 Ma, base of the Church Butte tuff age range at 49.21 Ma, and top of the Main tuff age range at 50.11 Ma, respectively (Fig. 7). Hereafter, the three adjusted models will be re-ferred to as new Willwood, new model C, and new T-3 to differentiate them from the original versions (Figs. DR2 and DR3; Table DR5, see the last three columns [see footnote 1]).

DISCUSSION

Comparison with Chronostratigraphic Data

The three modifi ed models (new Willwood, new model C, and new T-3) were evaluated in a regional chronostratigraphic context to de-termine which one integrates existing chrono-stratigraphic constraints in the most coherent way. To do so, we used bio- and magnetostrati-graphic data from sections in the Bighorn Ba-sin (Clyde et al., 1994), Greater Green River Basin (Clyde et al., 1997, 2001), and Uinta Basin (Prothero, 1996). These bio- and magneto-stratigraphic rec ords were placed within each of the three calibration frameworks in exactly the same manner as they were correlated to the geomagnetic polarity time scale in the origi-nal studies (Fig. 7; Figs. DR2 and DR3 [see footnote 1]). These magnetostratigraphic sec-tions were chosen because they had been cor-related to the NALMA biostratigraphy, which is important for evaluating the veracity of the age models because the biostratigraphy is es-tablished with respect to magnetostratigraphy for a given area, independent of geomagnetic

polarity time scale calibration in absolute time. Comparison of the three models with these in-dependent chronostratigraphic data favors the new Willwood model over the new model C and the new T-3 model, although the new T-3 model is very similar to the older half of the new Willwood model. The chronostratigraphic data used to support the new Willwood model are summarized below and in Table 2:

(1) Walsh (1996) and Prothero (1996) showed that the Bridgerian-Uintan NALMA bound-ary should lie in chron C21n. The Sage Creek Mountain pumice from the Bridger E lithol-ogy approximates the position of the boundary because the Bridger E faunal assemblage that contains a mix of Bridgerian and Uintan fauna is found ~7 m above the base of the Bridger E lithology (Evanoff et al., 1994; Robinson et al., 2004; Murphey and Walsh, 2007). The new Willwood model places the Sage Creek Moun-tain pumice in chron C21n.

(2) The Tabernacle Butte locality yields a Br-3 fauna (the youngest subage of the Bridg-erian NALMA) according to West (1973). The new Willwood and new T-3 models place the Tabernacle Butte tuff in chron C21r. This place-ment is consistent with the bio- and magneto-stratigraphic records of Clyde et al. (2001), which place Br-1b in C22n, and records from Prothero (1996), which place the Bridgerian-Uintan boundary in chron C21n.

(3) The Church Butte tuff is exposed at the fossiliferous Grizzly Buttes locality, which has been associated with a Br-2 fauna (upper Blacksforkian subage; Alexander and Burger, 2001). The Br-2 fauna was found in chron C21r in the Delmar Formation in California (Rob-inson et al., 2004). The Delmar Formation has been correlated directly to marine biostratigra-phy and thus has a reliable chron assignment. The new Willwood and new T-3 models place the Church Butte tuff in chron C22n. This cor-relation is also consistent with that of Clyde et al. (2001), who placed the underlying Br-1b in chron C22n.

(4) Prothero (1996) proposed a polarity stra-tigraphy from Indian Canyon of the Uinta Ba-sin that spans from the Horse Bench Sandstone marker bed to the limestone and sandstone facies of the Green River Formation (Fig. 2). The Fat and Oily tuffs can be correlated to his section based on their meter levels with respect to the Horse Bench Sandstone and the saline facies–limestone-sandstone facies transition. The new Willwood model places the Fat tuff within chron C21n, and this correlation is in agreement with Prothero (1996). His magneto-stratigraphic correlation places the Oily tuff in chron C19r, which confl icts with the polarity determination of this study.

(5) The Continental Peak tuff is exposed in the South Pass section in a stratigraphic interval that is interpreted to be Bridger B (or uppermost Bridger A; Zeller and Stephens, 1969; Gunnell, 2006, personal commun.). The new Willwood and new T-3 models place the tuff in chron C21r; this is consistent with the magneto stratigraphy of Clyde et al. (2001) from the same South Pass section, which correlates the lower part of the Bridger Formation with chron C22n. This con-trasts with the new model C because it correlates the Continental Peak tuff with chron C23n.1r, which has been associated with the older Wasatch Formation in the magneto stratigraphy of Clyde et al. (2001).

(6) The Willwood Ash is a bed of particular interest for calibration of the geomagnetic polar-ity time scale because it is located at the base of chron C24n.1n and is used as one of the fi ve cali-bration points in GOS2004 and in the original age model by Wing et al. (2000) (Tauxe et al., 1994; Ogg and Smith, 2004). It was dated at ca. 52.8 Ma by Wing et al. (1991) but has been recently redated and recalibrated to 52.9 ± 0.18 Ma (2σ fully propagated uncertainty) rela-tive to the FCs

K08 by Smith et al. (2004, 2010).

The new Willwood model is expected to meet these magnetostratigraphic and radioisotopic criteria, since these data were used in building the calibration model by Wing et al. (2000) from which the Willwood model was derived. How-ever, the new model C and new T-3 model place the base of chron C24n.1n outside the error of the Willwood Ash.

(7) The tuff just below the Alamo Creek basalt in the Lower Member of the Devil’s Graveyard Formation lies just below rocks of reversed po-larity and above the Junction faunas , which con-tain a mixture of the Bridgerian-Uintan faunas (Prothero and Emry, 1996; Walton, 1992). Smith et al. (2010) recalibrated the age of this tuff to 46.85 ± 0.14 Ma using the FCs

K08. Based on its

40Ar/39Ar age, the new Willwood model places the tuff in chron C21n, i.e., the same chron the Bridgerian-Uintan boundary is predicted to lie by Prothero (1996). In contrary, the new model C places the tuff in chron C22n, which is correlated to Br-1b by Clyde et al. (2001).

(8) The Mission Valley Ash bed has been correlated to Ui-3 and chron C20n (Prothero and Emry, 1996; Walsh et al., 1996; Robinson et al., 2004). Smith et al. (2010) recalibrated the age of this tuff to 43.35 ± 0.49 Ma using the FCs

K08. The new Willwood model places the tuff

in chron C20n based on its 40Ar/39Ar age, con-sistent with the bio- and magnetostratigraphic records of Prothero (1996). The new model C places the tuff in chron C21n instead.

(9) The Blue Point Ash (Hiza, 1999) lies near the base of chron C21r (Sundell et al., 1984;




42

43

44

45

46

47

48

49

50

51

52

53

54

Age

(M

a)

C20r

C21r

C22r

C23r

C21n

C22n

C23n.2n

C24n.1n

C24n.3n

St O Fa Bl SCMSa TB HF LC CP 6 L M G B Y F R KCB

Wa-6

Tuffs (alpha and beta sites only)

C20n

C24r

Bio-magnetostratigraphyfrom previous studies

Willwood Ash

~Wa-7

Wa-5 Clyde et al. (1994)

Br-0Br-1a

Wa-7

Br-1b

Clyde et al. (1997,2001)

Br-3

Ui-1

Ui-2

Ui-3

Prothero(1996)

Wa-4

Figure 7. Paleomagnetic polarity and 40Ar/39Ar ages of alpha and beta sites shown within the calibration framework of the proposed new Willwood model. Bio- and magnetostratigraphic data from the relevant sections are shown to the right. Filled (open) boxes indicate sites of normal (reversed) polarity, whereas height of each box indicates 2σ fully propagated uncertainty of a 40Ar/39Ar age as determined by Smith et al. (2010). Ovals show the younger or older end of discordant category I tuffs toward which some chron boundaries were shifted. The position of the Willwood Ash is also shown with its associated 2σ fully propagated uncertainty. Wa—Wasatchian North American Land Mammal Age (NALMA), Br—Bridgerian NALMA, Ui—Uintan NALMA. Biostratigraphic boundaries are shown with broken lines because their stratigraphic positions are not precisely known. See Table 1 for abbreviations of the tuffs.


Tsukui and Clyde


Eaton, 1985). Its age has been recalibrated by Smith et al. (2010) to 48.41 ± 0.17 Ma using the FCs

K08. The new Willwood and new T-3

models place the ash in chron C21r, which falls between Br-1b and Br-3 according to bio- and magnetostratigraphy of Clyde et al. (2001) and Prothero (1996). In contrary, the new model C places it in chron C22r through C23n.1n, which were correlated to the early Bridgerian NALMA by Clyde et al. (2001). Sundell et al. (1984) as-signed the late Bridgerian age for the part of their measured section in which the Blue Point Ash lies, so the new Willwood and new T-3 models are both consistent with their observa-tions regarding biostratigraphy.

Implications of New Calibration

The new Willwood model provides a calibra-tion framework in which the regional bio- and magnetostratigraphic data can be integrated most coherently (Table 2; Fig. 7), and thus it is tentatively considered to be the best alternative for calibrating the early to middle Eocene geo-magnetic polarity time scale (Fig. 8). In addi-tion, the similarity between the new Willwood and new T-3 models provides independent sup-port for the oldest age for the Paleocene-Eocene Thermal Maximum (56.33 Ma) as proposed by Westerhold et al. (2008). In light of new infor-mation regarding the cyclostratigraphically de-termined position of the ash −17 in Site 550, Westerhold et al. (2009) also concluded that the option 3 age estimate is most consistent with the published 40Ar/39Ar sanidine age of ash −17 if it is calculated relative to FCs

K08 (Storey et al.,

2007). Using the astronomically determined durations for the Paleocene (Westerhold et al., 2008), the option 3 age for the Paleocene-Eocene Thermal Maximum implies an age of 66.05 Ma for the Cretaceous-Paleogene boundary, which is very close to the estimate by Kuiper et al. (2008) (Westerhold et al., 2009). The agreement between the new Willwood model derived by paleomagnetic polarity and radioisotopic age determinations of terrestrial tuffs with the new T-3 marine astronomical age model represents an important step toward integrating astronomi-cal, radioisotopic, and paleomagnetic data from both marine and terrestrial records to construct a more robust calibration for the geomagnetic polarity time scale for this time interval. Addi-tional progress in this regard could come from a well-resolved and reproducible land-based cyclostratigraphy in the Green River Formation (Machlus et al., 2008; Meyers, 2008).

For the most part, our proposed new Will-wood model makes the study interval older than previously calibrated in GOS2004 (chrons C20n through C24n), with appreciable length-

TAB

LE 2

. CH

RO

NO

ST

RA

TIG

RA

PH

IC C

RIT

ER

IA U

SE

D T

O A

SS

ES

S A

GE

MO

DE

LS

Crit

eria

nu

mbe

r*O

bser

ved

data

(O

bs)

from

pre

viou

s bi

o- a

nd m

agne

tost

ratig

raph

ic

data

and

exp

ecte

d (E

xp)

outc

ome

acco

rdin

g to

age

mod

els

Wha

t new

mod

el C

pre

dict

sW

hat n

ew W

illw

ood

mod

el p

redi

cts

Wha

t new

T-3

mod

el

pred

icts

1(O

bs)

Brid

geria

n/U

inta

n N

ALM

A b

ound

ary

in C

21n

(n12

Cni

MC

S)

X(n22

Cni

MC

S√

.D.

N)

n12C

niM

CS

)pxE(

2(O

bs)

TB

loca

lity

prod

uces

Br-

3 fa

una

(r12

Cni

BT

)X(

r22C

niB

T√

(r12

Cni

BT

)√

)(E

xp)

TB

in C

21n

or C

21r

3(O

bs)

CB

ass

ocia

ted

with

Br-

2 fa

una

(n22

Cni

BC

)X(

n32C

niB

C√

(n22

Cni

BC

)√

)(E

xp)

CB

in C

21n,

C21

r, or

C22

n

4n12

Cni

aF

)sbO(

(n12

Cni

aF

)X(

n22C

n ia

F√

* .D.

N)

n1 2C

nia

F)px

E(

5(O

bs)

CP

from

Brid

ger

B li

thol

ogy

(r12

Cni

PC

)X(

r1. n32C

niP

C√

(r12

Cni

PC

)√

)(E

xp)

CP

in C

22n

or y

oung

er

6(O

bs)

Will

woo

d A

sh a

t the

bas

e of

C24

n.1n

and

dat

ed a

t 52

.9±0

.18

Ma

C24

n.1n

at 5

1.85

Ma

(X)

(√)

but c

orre

spon

denc

e is

exp

ecte

dC

24n.

1n a

t 53.

42 M

a (X

)(E

xp)

C24

n.1n

(ba

se)

lies

betw

een

53.0

8 an

d 52

.72

Ma

7(O

bs)

Tuff

just

bel

ow th

e A

lam

o C

reek

bas

alt l

ies

abov

e th

e B

ridge

rian/

Uin

tan

NA

LMA

bou

ndar

yTu

ff ju

st a

bove

the

Ala

mo

Cre

ek b

asal

t in

C22

n (X

)Tu

ff ju

st a

bove

the

Ala

mo

Cre

ek b

asal

t in

C21

n ( √

)N

.D.†

(Exp

)Tu

ff ju

st b

elow

the

Ala

mo

Cre

ek b

asal

t in

C21

n or

abo

ve

8(O

bs)

Mis

sion

Val

ley

Ash

bed

is c

orre

late

d to

C20

n an

d U

i-3 fa

una

Mis

sion

Val

ley

Ash

bed

in C

21n

(X)

The

Mis

sion

Val

ley

Ash

bed

in

C20

n an

d C

20r

(√)

N.D

.†

(Exp

)M

issi

on V

alle

y A

sh b

ed in

C20

n

9(O

bs)

Blu

e P

oint

ash

in C

21r

Blu

e P

oint

ash

in C

22r

and

C23

n.1n

(X

)B

lue

Poi

nt a

sh in

C21

r (√

)B

lue

Poi

nt a

sh in

C21

r (√

)(E

xp)

Blu

e P

oint

ash

in C

21r

Not

e: M

ore

deta

iled

disc

ussi

on a

nd r

efer

ence

s ca

n be

foun

d un

der

the

corr

espo

ndin

g cr

iteria

num

ber

in te

xt. A

bbre

viat

ions

of t

uffs

are

exp

lain

ed in

Tab

le 1

. √ (

X)

indi

cate

s ag

reem

ent (

disa

gree

men

t)

betw

een

the

obse

rved

dat

a an

d pr

edic

tion.

*Crit

eria

num

ber

corr

espo

nds

to th

e nu

mbe

r in

the

text

in “

Com

paris

on w

ith C

hron

ostr

atig

raph

ic D

ata.

”† N

.D.—

no d

ata

and

indi

cate

s th

at th

e tu

ffs o

f int

eres

t fal

l out

side

of t

he a

ge r

ange

of t

he p

artic

ular

mod

el.

†




ening of four contiguous normal chrons (C19n, C20n, C21n, and C22n) at the expense of their corresponding reversed chrons (except for chron C19r, which was lengthened; Fig. 8). The chrons in the older part of the study in-terval (between chrons C22r and C24n) are all shortened as a result. The proposed calibra-tion model also changes the age calibration of major biological and stage boundaries. Within the new calibration framework, the Wasatchian-Bridgerian NALMA boundary in chron C23r is bracketed between 51.96 and 52.53 Ma, and the Bridgerian-Uintan NALMA in chron C21n is bracketed between 48.03 and 45.57 Ma. The new Willwood model places the Ypresian-Lutetian stage boundary at ca. 49.21 Ma, out-side the 2σ error quoted for the stage boundary in GOS2004, making the Ypresian stage (early Eocene) ~0.61 m.y. shorter in duration, whereas the Lutetian is lengthened by ~0.84 m.y. in dura-tion compared to GOS2004 (Fig. 8). Finally, the new calibration implies that the early Eocene climatic optimum is ~2.2 m.y. in duration from 52.9 to 50.7 Ma. This is ~1 m.y. longer than has been previously proposed (Zachos et al., 2001) and could have important implications for under-

standing climate dynamics at the height of the Cenozoic greenhouse climate and its effect on mammalian evolution on land. The new esti-mates on the timing of the early Eocene climatic optimum and Wasatchian-Bridgerian faunal transition overlap signifi cantly and are thus con-sistent with hypotheses linking biotic change to climatic forcing at this time (Clyde et al., 2001; Woodburne et al., 2009).

Implications for Seafl oor Spreading Rates

Our proposed model implies more frequent variations in South Atlantic seafl oor spread-ing rates, thus challenging the assumption of smoothly varying seafl oor spreading employed in the development of the geomagnetic polar-ity time scale (Fig. 9; Cande and Kent, 1992; Ogg and Smith, 2004). However, it is important to note that our estimates of seafl oor spreading rates are highly dependent on the estimates of the anomaly widths, which themselves are un-certain (see following). Our estimates of ~15–25 mm/yr for the absolute rates are within the range found for the last 16 m.y. in the northern South Atlantic, although relatively low (Brozena,

1986), and in agreement with calculated rates for the Eocene by Mosar et al. (2002). It is pos-sible that a very slow spreading ridge like the Atlantic may be characterized by sporadic mag-matic supply, resulting in nonuniform rates in seafl oor spreading over these relatively short time scales (e.g., Allerton et al., 2000).

An important but commonly overlooked addi-tional source of error in calibrating the geomag-netic polarity time scale and calculating rates of seafl oor spreading is the uncertainty of the esti-mated positions of seafl oor magnetic anomaly boundaries with respect to the spreading ridge (see table 4 in Cande and Kent, 1992; Wester-hold et al., 2008). For the South Atlantic, the uncertainties in chron spacings between chrons C19 and C24 amount to 27.082 km, with chron C23 having the largest uncertainty (17.3% of the reported width; Fig. 9). These uncertainties are expressed in distance but can be converted to time using an age model. Thus, temporal un-certainties inherent in the geomagnetic polarity time scale calibration are derived not only from the commonly acknowledged problems asso-ciated with the quality of radioisotopic age estimates of the tie points and the assumption of smoothly varying seafl oor spreading rates, but also from uncertain positions of magnetic anomalies, making it diffi cult to identify domi-nant sources of uncertainty in the nonastronomi-cally calibrated part of the time scale. Because of the large uncertainties in the underpinnings of the geomagnetic polarity time scale, this study suggests that calibration of the pre-Neogene geomagnetic polarity time scale is best achieved through directly integrating radioisotopic dat-ing with magnetostratigraphy (e.g., McIntosh et al., 1992), although the seafl oor magnetic anomaly record will remain the only continuous blueprint of magnetic reversals. The departure from the traditional paradigm of relying heavily on the seafl oor magnetic anomaly profi le as a standard for scaling magnetic polarity chrons in the development of a time scale is demonstrated by the recent efforts to calibrate the entire Ceno-zoic geomagnetic polarity time scale using astronomical tuning (e.g., Pälike et al., 2001; Lourens et al., 2004; Westerhold et al., 2008).

CONCLUSIONS

This study combines new paleomagnetic po-larity data from volcanic tuffs with previously published radioisotopic ages to systematically assess eight proposed calibration models for the early to middle Eocene part of the geomagnetic polarity time scale. The ash-fall deposits were sampled on average every ~0.36 m.y. and thus allow for precise calibration of the interpolated segments of the time scale. The new Willwood

GOS2004model

New Willwood

C19nC19rC20nC20rC21nC21rC22nC22rC23n.1nC23n.1rC23n.2nC23rC24n.1nC24n.1rC24n.2nC24n.2rC24n.3n

% differenceAge calibration (Ma)Chron Distance (km)

South Atlantic spreading center

947.96954.12977.651006.061060.241094.711117.551130.781150.831153.901155.751168.201178.961184.031185.611186.341188.051195.35

40.43940.67141.59042.77445.34647.23548.59949.42750.73050.93251.05751.90152.64853.00453.11653.16753.28653.808

40.2140.5341.7943.3445.5748.0349.2150.1151.0351.1951.2951.9652.5352.8052.9152.9653.0753.57

0.570.35-0.48-1.32-0.49-1.68-1.26-1.38-0.59-0.51-0.46-0.100.230.380.390.400.400.45

40

42

44

46

48Age

(M

a)

50

52

54

GO

S20

04

Will

woo

d m

odel

C19r

C20n

C20r

C21n

C21r

C22n

C22r

C23r

C23n

C24n

C18r

from

New

Figure 8. Comparison of GOS2004 (Ogg and Smith, 2004) and proposed (new Willwood model) calibration of the early to middle Eocene geomagnetic polarity time scale shown schematically and in table format. The broken line indicates the Ypresian-Lutetian stage boundary. Distance data are from Cande and Kent (1992). Underlined ages are linearly ex-trapolated ages from Wing et al. (2000), whereas bold ages are new in this study derived via incremental adjustment of chron boundaries near discordant category I tuffs (see text for discussion). Ages in italics are the same as originally proposed in Wing et al. (2000).


Tsukui and Clyde


model is considered the most robust calibra-tion scheme because it is most consistent with presently existing chronostratigraphic data. Our proposed calibration model makes chrons be-tween C24n.1r and C20n older than proposed in GOS2004, caused in part by lengthening of several normal chrons (C19n, C20n, C21n, and C22n), and results in seafl oor spreading rates that are more variable than implied by GOS2004. The largest change in the calibration is made for the older age of chron C21n, with as much as a 0.79 m.y. difference between the two models, resulting in a shortening of the early Eocene by 0.61 m.y. and lengthening of the middle Eo-cene by 0.84 m.y. The new model supports the temporal overlap of the Wasatchian-Bridgerian faunal turnover in North America with the early Eocene climatic optimum, suggesting a possible relationship between climatic and biotic change at this time. Our proposed new Willwood model agrees with the orbitally tuned model of Wester-

hold and Röhl (2009) if the oldest proposed age for the Paleocene-Eocene Thermal Maxi-mum (56.33 Ma) is used, and this implies that the Paleocene-Eocene Thermal Maximum and Cretaceous-Paleogene boundary are older than generally accepted.

ACKNOWLEDGMENTS

We thank Mike Smith, Paul Murphey, Gregg Gun-nell, and Matt Jones for assistance with fi eld work, and the University of Minnesota Institute of Rock Magnetism for a travel grant and access to the facility. Thomas Westerhold generously provided unpublished data. Julie Bryce and Joel Johnson provided useful comments on an earlier draft. We also thank our re-viewers, Brad Singer, John Geissman, Mike Smith, and an anonymous reviewer, for their extensive re-views and comments, which improved the manuscript signifi cantly. This project was funded by the National Science Foundation grant EAR-0642291 to W. C. Clyde and a Geological Society of America Research Grant and an American Association of Petroleum Geol ogists Grant-in-Aid to K. Tsukui.

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Allerton, S., Escartín, J., and Searle, R.C., 2000, Extremely asymmetric magmatic accretion of oceanic crust at the ends of slow-spreading ridge segments: Geology, v. 28, p. 179–182, doi:10.1130/0091-7613(2000)28<179:EAMAOO>2.0.CO;2.

Aubry, M.-P., Berggren, W.A., Stott, L., and Sinha, A., 1996, The Upper Paleocene–Lower Eocene stratigraphic record and the Paleocene-Eocene boundary carbon isotope excursion: Implications for geochronology, in Knox, O.B., Corfi eld, R.M., and Dunay, R.E., eds., Correlation of the Early Paleogene in Northwest Eu-rope: Geological Society of London Special Publica-tion 101, p. 353–380.

Berggren, W.A., Kent, D.V., Swisher, C.C., III, and Aubry, M.-P., 1995, A revised Cenozoic geochronology and chronostratigraphy, in Berggren, W.A., Kent, D.V., Aubry, M.-P., and Hardenbol, J., eds., Geochronol-ogy, Time Scales and Global Stratigraphic Correlation: Tulsa, Oklahoma, SEPM (Society for Sedimentary Geol ogy) Special Publication 54, p. 129–212.

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New Willwood model

Figure 9. Age-distance chart for the study interval based on GOS2004 (solid line) and new Willwood model (broken line). Locations of magnetic chrons are shown by black vertical lines, the positions of which are relative to the mid-ocean-ridge crest in the South Atlantic (Cande and Kent, 1992). Shaded areas represent ranges of possible positions for reversal boundaries at the 95% confi dence level (table 4 in Cande and Kent, 1992). Plot at the bot-tom shows spreading rates inferred by the calibration of the two models. Notice that the changes in spreading rate according to GOS2004 (solid line) are smoothed out due to the interpolation effect.




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SCIENCE EDITOR: CHRISTIAN KOEBERL

ASSOCIATE EDITOR: BRADLEY S. SINGER

MANUSCRIPT RECEIVED 16 MAY 2011REVISED MANUSCRIPT RECEIVED 16 SEPTEMBER 2011MANUSCRIPT ACCEPTED 25 SEPTEMBER 2011

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Geological Society of America Bulletin · time scale is needed to correlate geologic and paleoclimatologic records that are being recov-ered at the temporal scale of astronomical