Geochronology of the upper Alturas Formation, northern ... · Fossil vertebrates from the Alturas Formation in northern California have previ-ously been considered important for defining

Palaeontologia Electronica palaeo-electronica.org

Geochronology of the upper Alturas Formation, northern California: Implications for the Hemphillian-Blancan North

American Land Mammal Age boundary

Steven R. May, Andrei M. Sarna-Wojcicki, Everett H. Lindsay, Michael O. Woodburne, Neil D. Opdyke, Elmira Wan,

David B. Wahl, and Holly Olson

ABSTRACT

Fossil vertebrates from the Alturas Formation in northern California have previ-ously been considered important for defining the age of the boundary between theHemphillian and Blancan North American Land Mammal Ages. Diatomaceous mud-stone of the upper Alturas Formation contain fossil mammals including the arvicolinerodent Mimomys (Ogmodontomys) sawrockensis that is diagnostic of Blancan faunas.New paleomagnetic and geochemical data from the upper Alturas Formation constrainthe age of the first stratigraphic occurrence of M. (O.) sawrockensis at Crowder FlatRoad to between 4.5 and 4.6 Ma. This age is approximately 0.2-0.4 Ma younger thanpreviously reported such that the oldest record of Mimomys in North America, south of55oN, is from Panaca, Nevada, and is constrained geochronologically to be approxi-mately 4.9 Ma. The Hemphillian – Blancan North American Land Mammal Age bound-ary probably occurs within magnetic polarity Chron C3n.3r at approximately 4.9 Ma.

Steven R. May. University of Texas, Jackson School of Geosciences, Vertebrate Paleontology Laboratory, R7600, Austin, Texas 78758, USA. [email protected] M. Sarna-Wojcicki. United States Geological Survey, 345 Middlefield Road MS-973, Menlo Park, California 94025, USA. [email protected] H. Lindsay. University of Arizona, Department of Geosciences, Gould-Simpson Building #77, 1040 E 4th St., Tucson, California 85721, USA. [email protected] O. Woodburne. Museum of Northern Arizona, 3101 N. Ft. Valley Rd., Flagstaff, California 86001, USA. [email protected] D. Opdyke. University of Florida, Dept. of Geological Sciences, Gainesville, FL 32611. [email protected] Wan. United States Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA. [email protected] B. Wahl. United States Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA. [email protected] Olson. United States Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA. [email protected]

PE Article Number: 17.3.43ACopyright: Palaeontological Association November 2014Submission: 14 May 2014. Acceptance: 12 November 2014

May, Steven R., Sarna-Wojcicki, Andrei M., Lindsay, Everett H., Woodburne, Michael O., Opdyke, Neil D., Wan, Elmira, Wahl, David B., and Olson, Holly. 2014. Geochronology of the upper Alturas Formation, northern California: Implications for the Hemphillian-Blancan North American Land Mammal Age boundary. Palaeontologia Electronica 17.3.43A: 1-13.palaeo-electronica.org/content/2014/985-alturas-geochronology

MAY ET AL.: ALTURAS GEOCHRONOLOGY

Keywords: geochronology; Hemphillian; Blancan; arvicoline

INTRODUCTION

North American land mammal ages (Wood etal., 1941) have been classically viewed as biochro-nologic units based on the evolution and dispersalof fossil mammals. Their utility for correlation isdependent on the ability to define unambigousboundaries and to characterize the interveningtime intervals by the joint occurrence of multipletaxa (Woodburne, 2004). Widespread immigrationevents of single or multiple taxa are commonlyregarded as potential criteria for boundary defini-tion (Repenning, 1967; Woodburne, 1996). Histori-cally, fossil mammals from the Alturas Formation inCalifornia have played a key role in the definition ofthe base of the Blancan mammal age based on thefirst occurrence of the arvicoline rodent Mimomysand interpretation of associated geochronologicdata.

The Alturas Formation is exposed in valleys ofthe Modoc Plateau in Northeastern California andincludes approximately 200-300 m of volcaniclasticand diatomaceous sedimentary rocks that weredeposited during the Late Miocene and Pliocene(Figure 1). Strata of the Alturas Formation are gen-erally flat lying and are commonly overlain by oliv-ine basalts. Collins (1999) measured ~118 m ofAlturas Formation sediments at Rattlesnake Butteand ~170 meters of Alturas Formation at CrowderFlat Road (Figure 2). She recognized six different

depositional facies represented by pyroclastic/debris flows, fluvial, lacustrine, and pyroclastic /hydrovolcanic strata and interpreted these strata torecord deposition in a continental back-arc setting.Especially common in the upper 50 meters of theCrowder Flat Road section, Collins also docu-mented the presence of diatomite and diatoma-ceous mudstone representing a well-developedlacustrine environment. Krebs et al. (1987)described a Pliocene diatom assemblage from theAlturas Formation, presumably from the CrowderFlats Road section based on reference to an “inpress” paper by C.A. Repenning.

The Alturas Formation is overlain by Plioceneolivine basalts that have been described by a num-ber of authors including McKee et al. (1983) andCarmichael et al. (2006). Previously referred to asthe Warner basalt, Carmichael et al. (2006)described the basalt at the top of the Barnes Gradealong the Crowder Flat Road section as the Devil’sGarden Basalt and published an 40Ar/39Ar plateauage of 4.31 +/- 0.18 Ma.

Although the original collection of fossil mam-mals from the Crowder Flats Road section wasmade by J.A. Shotwell in 1958 (UO 2424), plantand animal fossils from the Alturas Formation hadbeen known for many decades. Dorf (1933) wasthe first to use the name Alturas Formation in hisdiscussion of fossil floras from California and con-sidered it to be “upper Pliocene” on the basis ofstratigraphic relations and vertebrate fossils whoseage assignment were referenced to Chester Stock,oral communication, 1929. LaMotte (1936) contin-ued the use of the name Alturas Formation anddescribed it as consisting of isolated exposures ofconglomerate, sandstone, ash, and shale and ref-erenced localities at Rattlesnake Butte, DavisCreek, Tuledad Canyon, and Warner Valley. R.A.Stirton contributed a short section of LaMotte’s1936 paper wherein he identified Vulpes sp., Rhi-noceratidae, Tayassuidae, Neohipparion sp., Alti-camelus sp., Mastodontinae, Camelidae, Felis sp.,and Carnivora from six localities in “the vicinity ofAlturas,” presumably from the Alturas formation atRattlesnake Butte according to LaMotte. Stirtonconcluded “there is not sufficient identifiable mate-rial in our collection from Alturas for an accuratedetermination of the age of the stratum from whichthey were obtained.” The general character of thisfauna and a radiometric date from near Rattle-

80 km

California

Alturas

Reno

I-80

US-395

299

I-5

Redding

Nevada

Oregon

Susanville

Modoc Plateau

Basinand

Range

Cascade Range

SierraNevada

KlamathMountains

42.0 0 N

N

FIGURE 1. Location map showing Alturas, Californiaand major physiographic provinces. I-5, I-80, US-395,and 299 are all road identifications.

2

PALAEO-ELECTRONICA.ORG

snake Butte (Evernden et al., 1964) suggested aHemphillian age.

Axelrod (1944) provided further discussion ofthe fossil flora from the Alturas Formation based onre-examination of previous materials and fromadditional material obtained from the “Alturas for-mation near Rattlesnake Butte.” His fossil locality(UCMP 117) is described as being 1/2 mile SW ofRattlesnake Butte along the north bank of the PitRiver (Figure 2). He describes the Alturas Forma-tion as including a few hundred feet of relativelyhorizontal strata including tuffaceous sandstones,conglomerates, tuffaceous shales, and clays. Axel-rod (1944) concluded a Pliocene age for the Altu-ras flora. Evernden et al. (1964) published a K/Ardate from plagioclase in a tuff near the base of theAlturas Formation of 8.1 Ma.

The first stratigraphic occurrence of the micro-tine rodent Mimomys (Ogmodontomys) sawrock-ensis in the Alturas Formation was interpreted byRepenning (1987, 2003) to be dated at ~ 4.8 Maand was considered by him to be the oldest occur-rence in North America and therefore to constrainthe base of the Blancan Land Mammal Age atapproximately 4.8 Ma. In their review of the Blan-can Land Mammal Age, Bell et al. (2004) statedthat “the Blancan is currently defined by the firstappearance in North America south of 55o N lati-

tude of arvicoline rodents in the genera Mimomys,Ogmodontomys, and Ophiomys.” They refered toRepenning’s (1987) discussion of Mimomys(Ogmodontomys) sawrockensis from Alturas asthe earliest occurrence of these taxa, but alsonoted a potentially earlier occurrence of Mimomysfrom Panaca, Nevada (Lindsay et al., 2002).

The objective of this study is to provide newdata and reevaluate existing data concerning theage of the upper Alturas Fauna and to review thesignificance of this age with respect to theHemphillian-Blancan boundary.

Terminology

We utilize the notation of “m” and “M” for lowerand upper molars, respectively. Abbreviationsassociated with fossil localities include: UCMP –University of California Museum of Paleontology,UO – University of Oregon, and USGS – U.S. Geo-logical Survey.

METHODS

Tephrochronology

Volcanic glasses separated from all tephrasamples were analyzed by electron-microprobe tomeasure the abundances of Na, Si, Al, Fe, Mg, Mn,Ca, Ti, and K. Detailed laboratory procedures andmethods of chemical analysis are described inSarna-Wojcicki et al. (2005, 2011). Chemical anal-ysis of the volcanic glasses was performed by elec-tron microprobe. We used GSC and An40 asstandards, and RLS 132, a homogenous obsidianfrom La Puebla, Mexico, as an internal standard(Myers et al., 1976). The ZAF data reduction pro-gram was used to obtain oxide concentrations.Internal, polished surfaces of 15 to 20 individualglass shards were analyzed for each sample. Themeans of the individual shard analyses are aver-aged for the oxides to obtain an overall composi-tion for each sample.

Results of analyses of the volcanic glass fromthe samples collected in this study were comparedwith ~5800 analyses of tephra samples containedin the U.S. Geological Survey’s database of UpperNeogene tuffs collected from the western U.S.,Alaska, and Mexico. These data are stored at theU.S. Geological Survey's Tephrochronology Labo-ratory in Menlo Park, California. Analytical data onthe chemical composition of volcanic glasses oftephra layers that we obtained in this study werefirst evaluated using the similarity coefficient ofBorchardt et al. (1972), and Borchardt (1974). Thesimilarity coefficient is used as a guide to select a

x

x

x

CrowderFlat Road

CrowderFlat Road

RattlesnakeButte

SignalButte

Canby

Alturas

I-395

5 km

299

299

BarnesGrade

Approximate Edge ofPlateau Capped by

Devil’s Garden BasaltPit River

Pit River

Measured section

N

FIGURE 2. Locations of Crowder Flat Road, SignalButte, and Rattlesnake Butte localities near Alturas, Cal-ifornia. Expanded map of Barnes Grade area alongCrowder Flat Road.

3


pool of candidate samples that are further evalu-ated in terms of the chemical and geological crite-ria for the closeness of a match. The means ofoxide analyses for the individual shards are aver-aged to obtain an overall composition for eachsample. We used a computer program that com-pares any single sample with all previously ana-lyzed samples, for those elements that areconsidered the most reliable in chemical identifica-tion of tephra layers, and that allow us to distin-guish most clearly between tephra layers of similarcomposition (Sarna-Wojcicki et al., 1984; Sarna-Wojcicki and Davis, 1991; Sarna-Wojcicki, 2000).This approach lists the tephra samples that matchmost closely to the sample that is being evaluatedfor a specified set of elements, and these samplesare ranked in order of the value of the similaritycoefficient. The highest ranking samples representa pool of candidates for further evaluation for cor-relation.

For a more complete and detailed discussionof field criteria, petrographic characteristics, miner-alogy, chemical analysis of glass, and data evalua-tion methods used here, see Sarna-Wojcicki(1971), Sarna-Wojcicki et al. (1984), Sarna-Wojcicki and Davis (1991), Sarna-Wojcicki (2000),and Sarna-Wojcicki et al. (2005, 2011).

Paleomagnetism

Six to eight oriented hand samples were col-lected by May and Repenning in 1984 from each of10 initial sites along Crowder Flat Road. A site con-sisted of a single sedimentary bed, and sampleswere separated laterally by no more than 2 meters.This initial sampling spanned approximately 35meters of the Alturas Formation below the contactwith the Devil’s Garden Basalt. A single orientedblock of the basalt was also collected, and multiple1-inch diameter cores were drilled from this block.Bulk sedimentary rock samples were also collectedat four of the sites for analysis of magnetic mineral-ogy.

Oriented hand samples were cut to fit 1 in3

plastic boxes and all paleomagnetic and rock mag-netic measurements from this initial sample setwere made at the University of Arizona Departmentof Geosciences paleomagnetic laboratory. Boththermal and alternating field demagnetization wasapplied to individual samples to determine themost effective procedure for defining the primaryremanant magnetization.

Demagnetization experiments for the initialset of samples included stepwise alternating fielddemagnetization at 5 mT intervals up to 30 mT on

all specimens. Higher demagnetization steps up to70 mT were performed on selected specimens. AFdemagnetization up to 30 mT generally resulted ina loss of intensity from 30% to 95% of the NRM.Because of a viscous normal component present inmost of the primary reversed samples, an initialincrease in intensity was often observed. Stepwisethermal demagnetization was applied to severalsamples in steps from 90 to 600°C using a modi-fied ASC TD-48 thermal specimen demagnetizer.Stable magnetizations were defined using both AFand thermal demagnetization (Figure 3).

Jsat-T experiments on magnetic extracts frombulk sedimentary rock samples at sites AC003,AC004, AC006, and AC007 were performed tohelp identify the primary magnetic phase. A strongfield thermomagnetic balance was used to mea-sure the apparent weight change of the sample inan applied field as the temperature was increasedthen decreased. All experiments were carried outin < 1 atmosphere of argon with a heating rate of15°C/minute. Jsat-T curves observed in the ther-momagnetic experiments are illustrated in Figure4. These experiments demonstrated a general lossin magnetic intensity as a function of heating to610oC with all remanance removed by 580oC. Thissuggests that low Ti-magnetite is the primary mag-netic mineral consistent with the results of the AFand thermal demagnetization experiments. Weinterpret the magnetization in the Alturas Forma-tion sediments to be a primary detrital remanantmagnetization acquired at the time of deposition.

Nine additional sites (011- 019) were collectedin 2008 to extend the magnetic polarity stratigraphyfarther down section along the Crowder Flat Roadand to provide additional resolution elsewhere inthe section. One site (001) was collected from thelight gray ash in a roadcut along US 395 at SignalButte, south of Alturas (Figure 2). During this fieldwork, five samples were collected at each site, andsamples were again placed in small plastic cubes.These samples were processed at the University ofFlorida in 2008 using AF demagnetization in pro-gressive steps from 0 – 80 mT. Interpretation ofmagnetic polarity for this second set of sampleswas unambiguous and the results are consistentwith the initial samples collected in 1984.

Progressive demagnetization data from allspecimens were analyzed with vector demagneti-zation diagrams. Within each site, all samplesrecord the same polarity, but the dispersion wasoccasionally quite high. Within-site alpha95 valuesranged from a few degrees to >20o. Polarity deter-minations for all sites were unambiguous based on

4


the position and trend of the horizontal and verticalcomponents as seen in equal area stereo plots ofthe data (Figure 5). The site mean directions forsites with alpha95 < 20o barely pass a reversalstest at the 95% confidence interval (Figure 6). Thisis likely due to a present field secondary magneti-zation that was not completely removed from thereversed polarity sites. No fold test was availableas all strata within the Crowder Flat Road sectionare essentially flat lying.

RESULTS

Tephrochronology

Results of chemical analysis of volcanic glassfrom the tephra layers are presented in Table 1.Most of the 13 layers from this section do notmatch other, previously analyzed tephra layers inthe USGS data base, and we suspect that the for-mer were derived from the nearby Cascade Rangeto the west or from other local sources just east of

UP, N

Down, S

Inclination Declination

1.0 E-3 A/m 1.0 E-3 A/m 1.0 E-3 A/m

1.0 E-3 A/m 1.0 E-3 A/m 1.0 E-3 A/m

AC001D

AC003E AC003A AC004D

AC006B AC007E

Horiz, E

15 mT

60 mT

525o

525o

350o 350o

10 mT

20 mT

40 mT

10 mT50 mT

50 mT

NRM

NRM NRMNRM

NRM

NRM

W

UP, N

Down, S

Horiz, EW

FIGURE 3. Paleomagnetic results from selected samples illustrating behavior during alternating field and thermaldemagnetization.

0

1.0

200 400

Temperature oC

Stro

ng F

ield

Mag

netiz

atio

n (J

s(T)

/Js(

20 o C

))

600

AC006

0

1.0

200 400 600

AC007

0

1.0

200 400 600

AC003

0

1.0

200 400 600

AC004

FIGURE 4. Results of strong field thermomagneticexperiments on magnetic extracts from bulk sedimentsamples illustrating general loss in magnetic intensity asa function of heating with all remanance removed by580oC.

5


6

TAB

LE

1.

Che

mic

al c

omp

ositi

on o

f vo

lcan

ic g

lass

sha

rds

from

tep

hra

laye

rs a

nal

yzed

in t

his

stu

dy,

and

com

para

tive

com

pos

ition

s of

cor

rela

tive

tep

hra

laye

rs f

rom

oth

er lo

calit

ies.

Co

ncen

trat

ion

s of

ma

jor

and

min

or o

xide

s gi

ven

belo

w a

re in

oxi

de w

eig

ht p

erce

nt,

reca

lcul

ated

to a

100

% fl

uid

-fre

e b

asis

. Orig

inal

tota

ls o

n an

alys

is,

and

num

ber

s of

sha

rds

ana

lyze

d fo

r ea

ch s

am

ple

, a

re g

iven

on

the

right

sid

e of

the

tab

le.

Not

e th

at s

ever

al s

am

ple

s ha

ve m

ultip

le c

om

pos

ition

al m

odes

. Te

phra

lay-

ers

pre

sent

in

the

Cro

wde

r F

lat

Roa

d s

ect

ion

wes

t o

f A

ltura

s ar

e lis

ted

in

str

atig

rap

hic

orde

r, fr

om y

oun

gest

(to

p o

f ta

ble

) to

old

est.

The

str

atig

raph

ic s

epa

ratio

nbe

twee

n th

e b

ase

of e

ach

teph

ra la

yer

and

the

De

vil's

Gar

den

basa

lt at

the

top

of

the

sect

ion

are

also

giv

en.

Ana

lyse

s w

ere

mad

e o

n a

JEO

L 89

00

ele

ctro

n m

icro

-pr

obe

. Ana

lyse

s w

ere

con

duct

ed d

urin

g 1

979

– 2

009

by C

harle

s M

eye

r, Ja

me

s W

alke

r, D

avi

d W

ahl,

and

Elm

ira W

an, U

.S. G

eolo

gica

l Sur

vey

Teph

roch

ron

olog

y La

b-

orat

ory,

Men

lo P

ark

, Cal

iforn

ia.

Fie

ld S

amp

le N

um

ber

Dat

e o

f an

alys

isS

iO2

Al2

O3

Fe2

O3

Mg

OM

nO

CaO

TiO

2N

a2O

K2

OTo

tal

(re

ca

lcu

late

d)

Tota

l (o

n

an

aly

sis

)N

o. s

ha

rds

a

na

lyze

d

ALT

-1(1

), T

42

-311

/23

/19

82

76

.57

12.3

91.

580

.07

0.0

10

.48

0.1

83

.43

5.2

910

0.0

094

.52

*

ALT

-1(2

), T

7-2

, pd

1/2

3/1

97

97

6.3

512

.49

1.55

0.0

60.

03

0.5

30.

20

3.3

45

.45

100

.00

94.0

5*

ALT

-16

T3

04-6

3/2

5/1

99

47

6.3

812

.45

1.55

0.0

70.

04

0.5

50.

19

2.9

45

.83

100

.00

95.5

01

9

ALT

-2A

-N T

567

-1(P

op

1)

1/8/

200

97

6.2

012

.48

1.62

0.0

80.

03

0.5

20.

18

2.6

36

.28

100

.02

94.6

611

ALT

-2A

-N T

567

-1(P

op

2)

1/8/

200

97

4.2

412

.90

2.68

0.0

80.

07

0.6

60.

22

2.9

26

.23

100

.00

94.6

92

ALT

-21

T3

17-5

RE

DO

4/9

54/

11/1

995

76

.36

12.5

81.

580

.07

0.0

30

.51

0.1

93

.43

5.2

610

0.0

193

.62

17

AS

W-6

1185

-28

T1

09

-29

/24

/19

85

76

.94

12.1

31.

530

.07

0.0

20

.49

0.1

83

.41

5.2

410

0.0

192

.41

12

Kilg

ore

Tu

ff (m

ean

)7

6.7

012

.33

1.56

0.0

70.

03

0.5

20.

19

3.2

75

.33

100

.00

~2

40

Sta

nd

ard

de

via

tion

(1

sig

ma

)0

.31

0.1

60.

040

.01

0.0

10

.04

0.0

10

.27

0.3

90

.01

ALT

-12

T2

99-2

4/1

4/1

99

47

3.6

714

.79

1.73

0.1

60.

08

0.6

60.

20

4.7

14

.01

100

.01

93.6

21

8

ALT

-26

T5

74-1

8/4/

200

97

5.9

215

.06

1.69

0.1

60.

07

0.6

40.

19

3.4

12

.86

100

.00

94.6

32

0

ALT

-28A

T57

4-3

8/4/

200

97

6.0

515

.10

1.71

0.1

60.

08

0.6

60.

21

3.2

72

.76

100

.00

93.6

31

9

ALT

-28B

T57

4-4

8/4/

200

97

6.1

115

.01

1.69

0.1

60.

07

0.6

30.

20

3.2

82

.85

100

.00

93.8

92

0

ALT

-11

A T

567

-21/

8/2

009

69

.02

15.3

23.

850

.88

0.0

92

.38

0.7

24

.55

3.1

910

0.0

094

.82

15

ALT

-11B

T56

7-3

(Po

p2)

1/8/

200

96

5.2

115

.57

5.79

1.5

10.

12

3.6

70.

97

4.5

42

.62

100

.00

95.1

77

ALT

-11B

T56

7-3

(Po

p1)

1/8/

200

97

1.9

215

.64

2.34

0.2

70.

10

0.9

40.

34

5.1

43

.32

100

.01


the Cascades. One of these layers, however,matches well with widespread units that have beenpreviously identified and dated elsewhere in thewestern conterminous U.S.

On the basis of the chemical composition ofits volcanic glass, we identify a light-gray vitric tuff,situated 15.6 m below the Devil’s Garden Basalt,as the Kilgore Tuff, dated 4.45 ± 0.05 Ma (Morganand McIntosh, 2005) (Table 1, ALT-1, ALT-16) (Fig-ure 7). The type locality of the Kilgore Tuff, justsouth of the Snake River Plain in Idaho, exhibitsreversed magnetic polarity and represents theyoungest, large-volume ignimbrite and plinianpumice fall erupted from the Heise caldera (Figure8). This caldera and its associated deposits are theimmediate predecessor of the Yellowstone calderasituated to the east of the Heise volcanic field.Geochemical analyses of the volcanic glass fromthis tuff, and its correlative samples in the Alturasarea (ALT-1, ALT-16, ALT-2A-N) match well (Table1).

In the area of this study, the Kilgore Tuff isalso present in a road cut along U.S. Highway 395at Signal Butte, about 7 miles south of Alturas, Cal-ifornia (Figure 2). Chemical analysis of the tuff indi-cate that most of the shards present in the tephra

layer at Signal Butte match well with the KilgoreTuff, but a small number of them are chemically dif-ferent, representing a minor mode, possibly owingto detrital contamination with a different ash. Paleo-magnetic sites from the Kilgore Tuff at CrowderFlat Road (AC006) and Signal Buttes (001) bothexhibit reversed polarity as reported in the typeregion (Morgan and McIntosh, 2005).

In addition to the Alturas area, the Kilgore Tuffhas been identified at several other locations in thewestern conterminous U.S., interbedded with non-marine sediments, as well as in marine sedimentsof the northeastern Pacific Ocean (Figure 8; Table1). The presence of the Kilgore Tuff in marine sedi-ments in deep-ocean cores of the northeast PacificOcean presents the possibility of comparingclosely contemporaneous marine and non-marinefauna and flora, and interpreting changes in non-marine faunas such as those discussed here withina broader paleoclimatic context.

Magnetic Polarity Stratigraphy

The resulting magnetic polarity stratigraphy isshown in Figure 9. The upper part of the sectionincluding the Devil’s Garden Basalt is reversedpolarity down though site AC005 (R2). Thisreversed polarity interval includes site AC006,which is the Kilgore Tuff. A normal polarity intervalis then observed from site AC004 through 019(N2). This normal polarity interval includes the pri-mary fossil mammal locality (UO 2424 = VF 155)that records the first stratigraphic occurrence ofMimomys (Ogmodontomys) sawrockensis.Another reversed polarity interval is recorded insites AC002 through 016 (R1). Normal polarity is

Lower HemisphereUpper Hemisphere

1 2

B

N N

FIGURE 6. Site mean directions (1) and reversals test(2) for sites from both sample sets with site meanalpha95 < 20o. Reversed polarity site “B” is from theDevil’s Garden Basalt.

AC001

Lower HemisphereUpper Hemisphere

NRM

30 mT

FIGURE 5. Stereonet displays of sample directions fromsite AC001 before (NRM) and after (30 mT) demagneti-zation. A normal polarity overprint in the NRM directionsis removed by alternating field demagnetization.

7


then observed again at site 015 just above the firstnormal fault below the basalt (fault a) (N1). Areversed to normal transition is recorded againbelow the second fault (fault b) between sites 014and 018. Based on our analysis of the throw onthese two faults, we interpret this to be the samepolarity transition as observed between sites 016and 015. This correlation implies that 015 isroughly equivalent to 018. Although the equivalenttuffs sampled at ALT-11B and ALT-11A are geo-chemically distinct, they are so similar that we

believe they represent slightly different age erup-tions from a single evolving magma chamber. Nor-mal polarity continues down section through site011. A fault may be present between sites 012 and011, but the displacement on this fault is uncertain.

A silver-gray ash is correlated geochemicallyacross the two faults (samples ALT-12, ALT-26;ALT-28A; and ALT-28B are all geochemically indis-tinguishable) (Figure 9). This correlation is consis-tent with a throw of ~2.4 m across fault a and 0.5 macross fault b. Correlation across faults a and b

MagneticPolarity Sites

60

BasaltOlivine basalt flow = “Devi’s Garden Basalt”

Red baked contactBrown paleosolGray-brown diatomaceous mudstone

Gray diatomaceous mudstone




Gray diatomaceous mudstone (locally sandy)

Interbedded white / gray diatomite, diatomaceousmudstone, tuffaceous mudstones and thinsandstones

Interbedded gray diatomaceous mudstone,tuffaceous mudstone, and thin sandstone


Gray diatomaceous ash (ALT 11, 11B)

White / light gray diatomaceous mudstone,tuffaceous mudstone

White diatomaceous mudstone, locally sandyBlue-gray basaltic sandstoneGray - brown mudstoneDark - brown claystone

Light gray ash (ALT 11A)

fault a

Fault ??

fault b

Dark gray basaltic sandstone

Dark grey basaltic sandstone

Dark gray basaltic sandstone

Covered

Covered

Gray mudstone

Gray tuffaceous mudstone

Gray tuffaceous sandstoneWhite to brown mudstone

Light gray vitric tuff (ALT 16 = ALT 1) = Kilgore Tuff

Gray-brown sandstone

AC010

AC009AC008

AC007AC006AC005

AC004AC003019

017016015

014

018013

012

011

b smc

AC002

AC001

50

40

30

Met

ers

20

10

0

Lithology

FIGURE 7. Measured stratigraphic section along Crowder Flat Road. Open symbols represent reversed polaritypaleomagnetic sites and closed (black) represent normal polarity. Bone identifies stratigraphic location of UO 2424with Mimomys (Ogmodontomys) sawrockensis fossils. ALT XX designate sample locations for tephrochronology)Grain size/lithology: c=claystone, m=mudstone, s=sandstone, b=basalt.

8


results in the magnetic polarity stratigraphy asshown in Figure 10.

Correlation of the magnetic polarity stratigra-phy with the magnetic polarity time scale of Grad-stein et al. (2012) is straightforward given thegeochronologic control from the Devil’s GardenBasalt and the Kilgore Tuff. We interpret the strati-graphically lowest normal polarity magnetozone(N1) as C3n.3n. The next younger normal magne-tozone (N2) is interpreted as C3n.2n. R2 includesthe Kilgore Tuff dated at 4.45 +/- 0.05 Ma (Morganand McIntosh, 2005) and the Devils Garden Basaltdated at 4.31 +/- 0.18 Ma (Carmichael et al., 2006).This polarity zonation and the associated radiomet-ric dates are consistent with the time scale of Grad-stein et al. (2012).

DISCUSSION AND CONCLUSIONS

In Repenning’s (2003) review of Mimomys inNorth America, he discussed M. sawrockensisfrom the Upper Alturas fauna and assigned it anage of 4.8 Ma. At the time, he interpreted this to bethe earliest occurrence of Mimomys in North Amer-ica and defined the beginning of the Blancan NorthAmerican Land Mammal Age. Repenning stated:“The immigration of Mimomys to North Americamarks the beginning of the Blancan Mammal Ageby original definition. The earliest dated NorthAmerican record is the Upper Alturas Fauna of Cal-ifornia, which is well dated by paleomagnetic stra-tigraphy, tephra “fingerprinting”, and potassium–argon dating at 4.8 Ma.” This reflected an interpre-

tation based on a previous K-Ar date from theDevil’s Garden Basalt of 4.7 +/ 0.5 Ma (cited byRepenning, 2003 as “Silberman pers. comm.”) andan associated interpretation that the normal polar-ity zone that included the first stratigraphic occur-rence of Mimomys was Chron C3n.4n.

Repenning (2003) described the primary fossillocality for M. sawrockensis in the Alturas Forma-tion as:

“Upper Alturas fauna: Barnes Grade, ModocCounty, California; USGS, VPR M-1505(transferred to the Denver office in 1985) (=CAS locality 36805 collected for diatoms byG.D. Hanna, C. Chesterman, and C. Jenningsin Sept. 1959; = UO locality 26916 collectedby J.A. Shotwell in the late 1950’s). AlongBarnes Grade of Crowder Flat road leadingup to Big Sage Reservoir, north of US 299and 4 mi west of Alturas, California about 72 ftbelow a capping basalt (dated 4.7 +/- 0.5 Maby M.L. Silberman, personal commun. …, 6 ftbelow prominent white, diatomaceous tephradeposit dated 4.8 Ma by “fingerprint” correla-tion to a marine biochronology core off theCalifornia Coast (A.M. Sarna-Wojcicki, per-sonal commun., 1985), and 5 ft above theThvera event of the Gilbert Chron in a paleo-magnetic section measured up Barnes Gradeto the capping basalt by S.R. May and myselfin 1984… This was the original discoverylocality of the Upper Alturas fauna.”

In addition to the new geochronological datapresented here, it should be noted that “UO 26916”

HeiseVolcanic Field

Great SaltLake

SevierBasin

PuebloValleyAlturas

Tule Lake

Eel RiverBasin

DSDP-36-10-2

DSDP-34-5-29-153

ALT-21ALT-16

OregonIdaho

Utah

NevadaCalifornia

ALT-1EL-75-PV 99-BL-53B

01BL30A

M89SRP-8

M88-TH-08

Areal Distribution of theKilgore Tuff (4.45 +/- 0.05 Ma)

ASW-61185-28Bear lake

FIGURE 8. Areal distribution of the Kilgore Tuff (4.45 ± 0.05 Ma; Morgan and McIntosh, 2005). This tuff was eruptedfrom the Heise Volcanic Field, much of which is now covered by the Snake River Plain basalts.

9


is a specimen number rather than a University ofOregon locality number. The actual locality is UO2424 (= VF 155) (T. Fremd, personal commun.,2009) and is presumed to be equivalent to USGS,VPR M-1505. The collection is still housed at theUniversity of Oregon and includes 6 specimens ofMimomys (T. Fremd, personal commun., 2009).

The new data and interpretations presented inthis paper bear directly on the age of this localitydescribed in Repenning (2003). The Ar/Ar datapublished by Carmichael et al. (2006) for the basaltat the top of the Crowder Flat road section indicatean age of 4.31 +/- 0.18 Ma rather that 4.7 Ma. The

ash that occurs above the fossil mammal locality isnow known to be the Kilgore Tuff that has beenradiometrically dated at 4.45 +/- 0.05 Ma (Morganand McIntosh, 2005). The normal polarity intervalassociated with the fossil mammal locality is nowinterpreted to be C3n.2n (4.49-4.63 Ma, Gradsteinet al., 2012). The fossil mammal locality alongCrowder Flat road that contains the first strati-graphic occurrence of M. sawrockensis is nowinterpreted to be approximately 4.5 - 4.6 Ma andtherefore not the first occurrence of Mimomys inNorth America.

fault a

fault b

? fault

ALT-11AALT-11B

Devil’s Garden Basalt4.31 +/- 0.18 Ma

Kilgore Tuff (ALT 16 = ALT-1)

014

018013

012

011

b smc

MagneticPolarity Sites

Interpreted MagneticStartigraphy

60

Basalt

AC010

AC009AC008

AC007AC006AC005

AC004AC003019

017016015

AC002

AC001

50

40

30

Met

ers

20

10

0

b smc

N1

N2

R1

R2

ALT-12

ALT-26ALT-28A

ALT-28B

FIGURE 9. Alturas Formation stratigraphic section with magnetic polarity stratigraphy, key tuffs, and correlationacross faults. The fault displacements are constrained by detailed correlation of multiple tuffs. 2.5 m down to the southon fault a and 0.5 m up to the south on fault b. Correlation of tuffs across these faults allows the normal to reversedpolarity transition from 015 – 016 to be correlated with the same transition from 018 – 014. The bone symbol rep-resents the stratigraphic position of fossil locality UO 2424. Grain size scale same as in Figure 7.

10


The geochronology presented here applies toonly the upper 55 meters of the Alturas Formationwhile Collins (1999) measured approximately 170meters of total section along Crowder Flat road.Older strata within this section are almost certainlyHemphillian in age. Repenning (2003) suggestedthere were two distinct faunas from the Alturas For-mation with his use of “Upper Alturas Fauna” asopposed to the Hemphillian age fauna that herefers to through citations of Axelrod (1944),Evernden et al. (1964) and Wood et al. (1941).Therefore, the Hemphillian – Blancan boundary islikely captured within the Alturas Formationalthough the exact location of the boundary is notyet defined in a single, continuous, fossiliferoussection.

In their review of the Hemphillian-Blancanboundary, Bell et al. (2004) state that “the BlancanNorth American Land Mammal Age is defined bythe first appearance in North America, south of 55o

N, of arvicoline rodents in the genera Mimomys,Ogmodontomys, and Ophiomys.” Given the newage constraints presented in this paper for the firstoccurrence of Mimomys in the Alturas Formation,the oldest occurrence of Mimomys in North Amer-ica is interpreted to be at Panaca, Nevada (Lindsayet al., 2002). Mou (1997) named Mimomys pana-caensis from the Panaca Formation as a newmicrotine species that is similar to M. mcknightiand M. sawrockensis and occurs first within stratacorrelated with chron C3n.3r. Mou (1998) furtherargued that the enamel microstructure of M. pana-caensis differs from Ogmodontomys and Ophio-

Magnetic PolarityTime Scale

Kilgore Tuff 4.45 +/- 0.05 MaMorgan and McIntosh (2005)

Gradstein et. al. (2012)

Devil’s Garden Basalt

4.300

C3n.1r

C3n.2n

C3n.2r

C3n.3n

C3n.3r

4.493

4.631

4.799

4.896

Lowest startigraphicoccurrence of Mimomys

Observed magnetostratigraphyand associated radiometric dates

4.31 +/- 0.18 MaCarmichael et. al. (2006)

4.3Ma

4.4

4.5

4.6

4.7

4.8

4.9

N1

N2

R1

R2

FIGURE 10. Proposed chronostratigraphy illustrating correlation to the magnetic polarity time scale of Gradstein et al.(2012) and radiometric ages for the Devil’s Garden Basalt and the Kilgore Tuff.

11


mys, but is similar to Eurasian Mimomys. Mou(2011) named a new genus of arvicoline rodentfrom Panaca as Nevadomys, but considered thatwhereas M. panacaensis was an immigrant, Neva-domys was endemic to North America.

The Upper Alturas fauna is significantlyyounger than the Panaca fauna and only con-strains the Hempillian-Blancan boundary to beolder than ~4.6 Ma. Lindsay et al. (2002) con-cluded that paleontologic, radioisotopic, and mag-netostratigraphic data from Panaca wereconsistent with placement of the Hemphillian/Blan-can boundary near the boundary between chronsC3n.3r and C3n.4n. Magneostratigraphic and teph-rochronologic data from the Horned Toad Forma-tion in the northern Mojave Desert, California,indicate that the latest Hemphillian Warren fauna isalso correlative with chron C3n.3r (May et al.,2011). This suggests that the Hemphillian – Blan-can North American Land Mammal Age boundaryoccurs within C3n.3r or approximately 4.9 Ma asalso concluded by Gradstein et al. (2012).

ACKNOWLEDGMENTS

We would like to acknowledge C. Repenningfor introducing us to the Crowder Flat Road sectionof the Alturas Formation and for pointing out itspotential significance for the Hemphillian-Blancanboundary through numerous discussions. T. Fremdkindly provided help with the University of Oregonmammalian paleontology collection. The manu-script was improved by comments from Dr. L.Jacobs and one anonymous reviewer.

REFERENCES

Axelrod, D.I. 1944. The Alturas floras. Carnegie Institu-tion of Washington Publication, 553:263-284.

Bell, C.J., Lundelius Jr., E.L., Barnosky, A.D., Graham,R.W., Lindsay, E.H., Ruez Jr., D.R., Semken Jr., A.,Webb, S.D., and Zakrzewski, R.J. 2004. The Blan-can, Irvingtonian, and Rancholabrean Mammal Ages,p. 232-314. In Woodburne, M.O. (ed.), Late Creta-ceous and Cenozoic Mammals of North America.Columbia University Press, New York.

Borchardt, G.A. 1974. The SIMAN coefficient for similar-ity analysis. Classification Society Bulletin, 3:1-8.

Borchardt, G.A., Aruscavage, P.J., and Millard, H.T., Jr.1972. Correlation of Bishop Ash, a Pleistocenemarker bed, using instrumental neutron activationanalysis. Journal of Sedimentary Petrology, 42:301-306.

Carmichael, E.S.E., Lange, R., Hall, C., and Renne, P.2006. Faulted and tilted Pliocene olivine-tholeiitelavas near Alturas, NE California, and their bearingon the uplift of the Warner Range. Geological Societyof America Bulletin, 118:1196-1211.

Collins, N.E. 1999. Stratigraphy and geochemistry of theAlturas Formation, Modoc Plateau, Northeastern Cal-ifornia. Unpublished M.S. Thesis, Humboldt StateUniversity, CA.

Dorf, E. 1933. Pliocene floras of California. CarnegieInstitution of Washington Publication, 412:1-112.

Evernden, J.E., Savage, D.E., Curtis, G.H., and James,G.T. 1964. Potassium-argon dates and the Cenozoicmammalian chronology of North America. AmericanJournal of Science, 262:145-198.

Gradstein, F.M., Ogg, J.O., Schmitz, M., and Ogg, G.2012. The Geologic Time Scale 2012. Elsevier B.V.,Oxford, England.

Krebs, W.N., Bradburry, J.P., and Theriot, E. 1987. Neo-gene and Quaternary lacustrine diatom biochronol-ogy, western USA. Palaios, 2:505-513.

LaMotte, R.S. 1936. The Upper Cedarville flora of north-western Nevada and adjacent California. CarnegieInstitution of Washington Publication, 455:57-142.

Lindsay, E., Mou, Y., Downs, W., Pederson, J., Kelly, T.,Henry, C., and Trexler, J. 2002. Recognition of theHempillian/Blancan Boundary in Nevada. Journal ofVertebrate Paleontology, 22:429-442.

May, S.R., Woodburne, M.O., Lindsay, E.H., Albright,L.B., Sarna-Wojcicki, A., Wan, E., and Wahl, D.B.2011. Geology and mammalian paleontology of theHorned Toad Hills, Mojave Desert, California. Palae-ontologia Electronica, 14.3.28A:63pp, http://palaeo-electronica.org/2011_3/11_may/index.html

McKee, E.H., Duffield, W., and Stern, R. 1983. Late Mio-cene and early Pliocene basaltic rocks and theirimplications for crustal structure, northeastern Cali-fornia and south-central Oregon. Geological Societyof America Bulletin, 94:292-304.

Morgan, L. and McIntosh, W. 2005. Timing and develop-ment of the Heise volcanic field, Snake River PlainIdaho, western USA. Geological Society of AmericaBulletin, 117:288-306.

Mou, Y. 1997. A new arvicoline species (Rodentia: Crieti-dae) from the Pliocene Panaca Formation, SoutheastNevada. Journal of Vertebrate Paleontology, 17:376-383.

Mou, Y. 1998. Schmelzmuster of Mimomys panacaensis.p. 79-90. In Tomida, Y.Y., Flynn, L.J., and Jacobs,L.L. (eds.), Advances in Vertebrate Paleontology andGeochronology. National Science Museum Mono-graphs, No. 14, Tokyo, Japan.

Mou, Y. 2011. Cricetid rodents from the Pliocene PanacaFormation, southeastern Nevada, USA. Palaeontolo-gia Electronica, 14.3.31A:53 pp, palaeo-electron-ica.org/2011_3/19_mou/index.html

12


Myers, A.T., Raymond, G.H., Connor, J.J., Conklin, N.M.,and Rose, H.J., Jr. 1976. Glass reference standardsfor the trace-element analysis of geological materials- comparison of inter-laboratory data. U.S. GeologicalSurvey Professional Paper, 1013:1-29.

Repenning, C.A. 1967. Paleartic-Nearctic mammaliandispersal in the late Cenozoic, p. 288-311. In Hop-kins, D.M. (ed.), The Bering Land Bridge. StanfordUniversity Press, Stanford, California.

Repenning, C.A. 1987. Biochronology of the microtinerodents of the United States, p. 236-268. In Wood-burne, M.O. (ed.), Cenozoic Mammals of NorthAmerica-Geochronology and Biostratigraphy. Univer-sity of California Press, Berkeley, California.

Repenning, C.A. 2003. Chapter 17. Mimomys in NorthAmerica. Bulletin American Museum of Natural His-tory, 279:469-512.

Sarna-Wojcicki, A.M. 1971. Correlation of late Cenozoicpyroclastic deposits in the central Coast Ranges ofCalifornia. Unpublished Ph. D. Dissertation, Univer-sity of California, Berkeley.

Sarna-Wojcicki, A.M. 2000. Tephrochronology, p. 357-377. In Noller, J.S., Sowers, J.M., and Lettis, W.R.(eds.), Quaternary Geochronology: Methods andApplications. American Geophysical Union Refer-ence Shelf 4.

Sarna-Wojcicki, A.M. and Davis, J.O. 1991. QuaternaryTephrochronology, p. 93-116. In Morrison, R.B. (ed.),Quaternary Nonglacial Geology: ConterminousUnited States. Geological Society of America,Decade of North American Geology Series, K-2,Boulder, Colorado.

Sarna-Wojcicki, A.M., Deino, A.L., Fleck, R.J., McLaugh-lin, R.J., Wagner, D., Wan, E., Wahl, D., Hillhouse,J.W., and Perkins, M. 2011. Age, composition, andareal distribution of the Pliocene Lawlor Tuff, andthree younger Pliocene tuffs, California and Nevada.Geosphere, 7:599-628.

Sarna-Wojcicki, A.M., Bowman, H.R., Meyer, C.E., Rus-sell, P.C., Woodward, M.J., McCoy, G., Rowe, J.J.,Jr., Baedecker, P.A., Asaro, F., and Michael, H. 1984.Chemical analyses, correlations, and ages of upperPliocene and Pleistocene ash layers of east-centraland southern California. U.S. Geological Survey Pro-fessional Paper, 1293:40.

Sarna-Wojcicki, A.M., Reheis, M.C., Pringle, M.S., Fleck,R.J., Burbank, D., Meyer, C.E., Slate, J.L., Wan, E.,Budahn, J.R., Troxel, B., and Walker, J.P. 2005.Tephra layers of Blind Spring Valley and relatedupper Pliocene and Pleistocene tephra layers, Cali-fornia, Nevada and Utah: Isotopic ages, correlation,and magnetostratigraphy. U.S. Geological SurveyProfessional Paper, 1701:63.

Wood, H.E. II, Chaney, R.W., Clark, J., Colbert, E.H.,Jepsen, G.L., Reeside Jr., J.B., and Stock, C. 1941.Nomenclature and correlation of the North Americancontinental Tertiary. Bulletin of the Geological Societyof America, 52:1-48.

Woodburne, M.O. 1996. Precision and resolution inmammalian chronostratigraphy: Principles, practices,examples. Journal of Vertebrate Paleontology,16:531-555.

Woodburne, M.O. 2004. Principles and Procedures, p. 1-20. In Woodburne, M.O. (ed.), Late Cretaceous andCenozoic Mammals of North America. Columbia Uni-versity Press, New York.

13

Documents

Geochronology of the upper Alturas Formation, northern ... · Fossil vertebrates from the Alturas Formation in northern California have previ-ously been considered important for defining