Late Miocene climate and life on land in Oregon within a ...€¦ · Clarendonian (12 Ma) fossil...

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Palaeogeography, Palaeoclimatology, P

Late Miocene climate and life on land in Oregon

within a context of Neogene global change

Gregory J. Retallack*

Department of Geological Sciences University of Oregon, Eugene, OR 97403-1272, United States

Received 13 November 2003; received in revised form 2 July 2004; accepted 20 July 2004

Abstract

Clarendonian (12 Ma) fossil soils, plants, molluscs, fish, and mammals of eastern Oregon allow reconstruction of late

Miocene paleogeography, paleoclimate, and paleoecology on land between the global thermal maximum of the middle Miocene

(16 Ma) and global cooling and drying of the late Miocene (7 Ma). Six different pedotypes of paleosols recognized near Unity

and Juntura allow reinterpretation of local mammalian paleoecology. Fossil beavers dominated gleyed Entisols of riparian

forest. Abundant camels and common hipparionine horses dominated Alfisols of wooded grassland and grassy woodland.

Diatomites overlying mammal-bearing beds have bullhead catfish [Ictalurus (Ameiurus) vespertinus], as well as fossil leaves

dominated by live oak (Quercus pollardiana). Fossil plants and soils of Unity and Juntura are most like those of grassy live oak

woodland and savanna on the western slopes of the Sierra Nevada in northern California today. Fossil plants and soils indicate

mean annual temperature of 12.9 (7.7–17.7) 8C and mean annual precipitation of 879 (604–1098) mm. Miocene paleoclimaticchanges in eastern Oregon show no relationship to changes in oxygen isotopic composition of marine foraminifera, usually

taken as an index of global paleoclimatic change. Mismatch between land and sea paleoclimatic records is most likely an

artefact of global ice volume perturbation of oxygen isotopic values. Instead, Miocene paleoclimatic change in eastern Oregon

parallels changes in carbon isotopic composition of marine foraminifera, presumably through fluctuations in greenhouse gases.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Miocene; Paleosols; Plants; Mammals; Oregon

1. Introduction

Because Miocene soils, plants, and animals were

close to our own time and modern in many respects,

the surprisingly large amplitude of Miocene paleo-

climatic change is of interest for understanding

0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.palaeo.2004.07.024

* Tel.: +1 541 3464558; fax: +1 541 3464692.

E-mail address: [email protected].

future global change. Evidence of global middle

Miocene (16 Ma) warmth includes lateritic paleosols

as far north and south as South Australia, Japan,

Oregon, and Germany (Schwarz, 1997). Middle

Miocene thermophilic trees such as sweetgum

(Liquidambar pachyphylla) grew as far north as

Alaska (Wolfe and Tanai, 1980), and coconuts

(Cocos zeylanica) dispersed as far south as New

Zealand (Fleming, 1975). Middle Miocene thermo-

alaeoecology 214 (2004) 97–123

G.J. Retallack / Palaeogeography, Palaeoclimatology, Palaeoecology 214 (2004) 97–12398

philic large foraminifera, corals, and molluscs

extended as far north as Oregon, Alaska, Japan,

and Kamchatkta (Moore, 1963; Itoigawa and Yama-

noi, 1990; Oleinik and Marincovich, 2001), and as

far south as South Australia and New Zealand (Li

and McGowran, 2000; Fleming, 1975). These

indicators of warmth disappeared from such high

latitudes by later middle Miocene (15 Ma), when

cold water minerals (glendonite pseudomorphs of

ikaite) formed in marine rocks of Oregon (Boggs,

1972). Benthic foraminiferal oxygen isotopic values

also indicate a global middle Miocene marine

thermal maximum, followed by late Miocene global

cooling (Zachos et al., 2001).

Fig. 1. Late Miocene fossil localities of Juntura, Ironside, and Unity, east

Unity (Thayer, 1957; Thayer and Brown, 1973; Reef, 1983; Walker and M

Tables 4–5; unnumbered localities have not yet yielded identifiable specim

These global trends also were felt in eastern

Oregon, where fossil leaves indicate marked cooling

and drying between about 16 and 15 Ma (Wolfe et al.,

1997; Graham, 1999), and a second cooling and

drying to a modern flora by latest Miocene (7 Ma;

Chaney, 1948). Paleosols of eastern Oregon show

similar changes between middle Miocene mesic

forests and late Miocene sagebrush steppe (Bestland

and Krull, 1997; Retallack et al., 2002a; Sheldon,

2003). Associated mammal faunas include a transition

from three-toed, mixed-feeding horses of the middle

Miocene (15 Ma, Barstovian) to modern monodactyl

grazing horses by late Miocene (7 Ma, Hemphillian;

Downs, 1956; Shotwell, 1970).

ern Oregon, with detailed geological map and cross-section around

acLeod, 1991). Numbered fossil localities and fossils are listed in

ens.

G.J. Retallack / Palaeogeography, Palaeoclimatology, Palaeoecology 214 (2004) 97–123 99

This study examines paleoclimate and paleo-

ecology of the intervening part of the late

Miocene within the context of Miocene global

paleoclimatic extremes, using evidence from fossil

soils, plants, molluscs, fish, and mammals in the

eastern Oregon. Previous study of mammalian

communities of this age from near Juntura

(Shotwell, 1963; Shotwell and Russell, 1963),

Ironside (Merriam, 1916), and Baker City (Downs,

1952) are here supplemented with additional

collections from near Unity, including reappraisal

of a locality for a previously collected gompho-

there skull and jaws (Orr and Orr, 1999). New

Fig. 2. Measured section of the Ironside Formation in Windlass Gulch, 3

and their development, calcareousness, and hue (using conventions of Re

collections of fossil plants and fish are also

reported, as well as new mapping of the area

around Unity (Fig. 1) and an assessment of the

paleoenvironmental significance of paleosols asso-

ciated with these fossils (Figs. 2 and 3). The

record of Neogene paleoclimatic fluctuation from

these and other paleosols in eastern Oregon and

Washington (Retallack, 1997b) does not correlate

with oxygen isotopic records from deep-sea

foraminifera but is in phase with foraminiferal

variations in carbon isotopic composition (Zachos

et al., 2001). This may indicate a closer relation-

ship with the carbon cycle and greenhouse gases,

miles northeast of Unity, Baker County, Oregon, showing paleosols

tallack, 1997a, 2001b).

Fig. 3. Geological sections of fossil quarries excavated by Shotwell

(1963) near Juntura, Oregon, showing paleosols and their develop-

ment, calcareousness, and hue (using conventions of Retallack,

1997a, 2001b).


such as CO2 and CH4 (Retallack, 2002), than with

deep-sea temperature or polar ice volume tracked

by oxygen isotopic composition.

2. Materials and methods

Mapping and measurement of two stratigraphic

sections near Unity, Baker County, Oregon (Figs.

2 and 4) involved collection of 95 rock specimens

and 62 fossils, which have been catalogued in

collections of John Day Fossil Beds National

Monument, Kimberly, Oregon (Scott Foss curator:

catalog on line at http://www.museum.nps.gov/).

Paleocurrents were sighted along trough cross bed

axes using a Brunton compass. Two stratigraphic

sections also were measured near Juntura, Mail-

heur County, Oregon (Fig. 3) at fossil quarries of

Shotwell (1963). Petrographic thin sections were

cut of all the collected rocks in order to support

field identifications of lithologies, and 36 thin

sections were point-counted to determine paleosol

textures and mineral composition (following the

methods of Retallack, 1997a, 2001b). Clays and

evaporite minerals were identified by X-ray

diffraction using a Rigaku computer-automated

goniometer.

Six distinct kinds of paleosols, or pedotypes, were

recognized (Table 1) throughout the mapped area

(Figs. 1 and 2) and also near Juntura (Fig. 3) and

Ironside. Each pedotype represents a distinct ancient

environment (Table 2). The pedotype names are from

the Sahaptin Native American language (Rigsby,

1965; Delancey et al., 1988) and are part of a wider

classification of Oregon Cenozoic paleosols (Retal-

lack et al., 2000). Descriptive and analytical data for

these paleosols are presented in Fig. 5 and Table 1.

Chemical analyses of 21 paleosol specimens by

Bondar Clegg of Vancouver, Canada, used X-ray

fluorescence of a borate-fused bead for major ele-

ments, titration for ferrous iron, and gravimetry for

loss on ignition. Bulk density for each specimen was

determined using the clod method. Of 109 paleosols

logged near Unity and 42 near Juntura, 45 had

pedogenic carbonate, and 5 were chemically analyzed

for paleoclimatic interpretation. These data are added

to 754 pedogenic carbonate measurements and 92

chemical analyses of other paleosols from Oregon and

Washington ranging back in age to 28.7 Ma (Gus-

tafson, 1978; Bestland and Krull, 1997; Tate, 1998;

Retallack, 2004), in order to assess local Neogene

paleoclimate and its correspondence with global

paleoclimatic change (Zachos et al., 2001).

3. Geological background

Only rocks of the Clarendonian North Ameri-

can Land Mammal bAgeQ around Unity aredescribed and mapped in detail here (Figs. 1

and 2). Few fossils have been recovered from

poor exposures of the Ironside Formation near

Ironside (Merriam, 1916) and Baker City, Oregon

(Downs, 1952). The Unity, Ironside, and Baker

City sequences and local faunas are similar to

http://www.museum.nps.gov/

Fig. 4. Badlands of the late Miocene Ironside Formation in central Windlass Gulch (95–135 m of measured section Fig. 2), northeast of Unity. A

gray airfall tuff is visible to the lower right and two thin white tuffs in the middle of these badlands.


better-known sequences and faunas of the Juntura

Formation near Juntura, Malheur County (Fig. 3;

Bowen et al., 1963; Greene, 1973; Fiebelkorn et

al., 1982; Johnson et al., 1996).

3.1. John Day Formation (late Eocene–early

Miocene)

The oldest rocks exposed near Unity are 300 m of

rhyodacitic tuff breccias, interpreted as lahars by Reef

(1983). He considered them Eocene in age, based on

fossil leaves (Rhamnidium chaneyi) and permineral-

Table 1

Paleosols of the Ironside and Juntura Formations and their classification

Pedotype Sahaptin

meaning

Diagnosis

Abiaxi Bitterroot Near-mollic surface (A) horizon over

weakly weathered subsurface (Bw)

Cmti New Shale and siltstone with root traces

Monana Underneath Thin impure lignite (O) on hale and

siltstone (A) with shallow root traces

Skaw Scare Thin dark gray to black spotted siltstone

with root traces (A) over bedded siltstone

Tutanik Hair Near-mollic brown thick surface (A) with

deep calcareous and siliceous

rhizoconcretions and nodules (Bk)

Xaus Root Sandstone with root traces (A) often

ferruginized over bedded sandstone

ized palm wood (Palmoxylon sp. indet.). However,

Walker (1990) reports K–Ar ages from hornblende and

plagioclase of 19.6F0.8 and 19.5F0.6 Ma, respec-tively (early Miocene), and regarded these rocks as

outliers of the late Eocene to early Miocene John Day

Formation. The laharic tuff breccia is overlain by a 100-

m-thick rhyodacitic flow of similar petrographic and

geochemical composition to the tuff breccia, additional

tuff breccia, and local basalt flows (Reef, 1983;Walker,

1990). These volcanic rocks are overlain by about 20 m

of red claystone, including numerous paleosols of the

Luca pedotype, which are late middle Eocene to early

Australian

classification

(Stace et al., 1968)

F.A.O.

World Map

(F.A.O., 1974)

U.S. Soil

Taxonomy

(Soil Survey

Staff, 1999)

Brown clay Eutric Cambisol Haplosalid

Alluvial soil Eutric Fluvisol Fluvent

Humic gley Eutric Histosol Fibrist

Wiesenboden Eutric Gleysol Mollic

Endoaquent

Brown earth Chromic Luvisol Typic

Natrixeralf

Alluvial soil Eutric Fluvisol Psamment

Table 2

Paleosols of the Ironside and Juntura Formations and their interpretation

Pedotype Paleoclimate Past vegetation Past animal life Paleotopographic

setting

Parent

material

Time of

formation

Abiaxi No indication Early successional

wooded grassland

cf. Cormohipparion

sphenodus, Prosthennops,

Gomphotherium osborni

Riparian fringe

and low terraces

of floodplain

Volcaniclastic

silt

1000–2000

years

Cmti No indication Riparian early

successional

woodland

Proboscidea indet.,

Merycodus

Streamside levee

deposits

Volcaniclastic

silt

5–100

years

Monana No indication Swamp woodland None found Lake margin and

floodplain swamp

Volcaniclastic

silt and

diatomite

100–1000

years

Skaw No indication Riparian floodway

woodland:

Gramineae,

Equisetum,

Populus, Salix

Meterix, Scapanus,

Hypolagus, Spermophilus

junturensis, Diprionomys,

Eucastor malheurensis,

Peromyscus, Vulpes,

Mammut furlongi,

Prosthennops, Merychyus

Seasonally

waterlogged

streamside swale

Volcaniclastic

silt

50–1000

years

Tutanik Subhumid

(800–1000 mm

mean annual

precipitation),

summer-dry

Grassy woodland Mylagaulus, Mammut

furlongi, cf.

Cormohipparion

sphenodus, Aphelops,

Procamelus grandis,

Megatylopus

Well-drained

floodplain

Volcaniclastic

silt

2000–5000

years

Xaus No indication Riparian early

successional

woodland

Merycodus Streamside sand

bars

Volcaniclastic

sand

5–100

years


Oligocene (Uintan to Orellan) in age near Clarno and

the Painted Hills, Oregon (Bestland et al., 1999;

Retallack et al., 2000). Comparable red paleosols also

are found in the upper John Day Formation east of

Kimberly, Oregon, where they formed on well-drained

uplands (Retallack et al., 2000).

3.2. Strawberry and Dooley Volcanics (middle

Miocene)

Platy, 2-pyroxene, basaltic andesites are character-

istic of middle Miocene (radiometric ages 15–10 Ma),

Strawberry Volcanics, with smaller amounts of basalt,

rhyolite, rhyolitic tuff, vent breccia, and micronorite

(Robyn, 1977). The Strawberry Volcanics are calcalka-

line and were not comagmatic with voluminous

tholeiitic basalts of the Columbia River Basalt Group,

which also are middle Miocene (radiometric age 17–15

Ma; Sheldon, 2003). Calcalkaline volcanism is pro-

duced by melting above subduction zones, but Straw-

berry Volcanics and other coeval andesitic volcanoes

(Walker, 1990) were far inland from subduction of the

Farallon Plate during the middle Miocene (Orr et al.,

1992). Middle Miocene Strawberry stratovolcanoes

rose at least 1000 m above valley floors of the

Paleozoic and Mesozoic basement (Robyn, 1977).

Dooley Volcanics near Unity are 150 m thick and

include three distinct units of rhyolitic, welded, ash

flow tuffs (Reef, 1983). The ash flow tuffs are light

gray with abundant pumice, rock fragments, and

phenocrysts of plagioclase, biotite, and quartz.

Dooley Volcanics reach a thickness of 2400 m

around volcanic centers only a few kilometers to the

north and northwest of the mapped area. One of the

last eruptive events of the Dooley Volcanics was a

rhyolite dome dated by K–Ar at 14.7F0.4 Ma(Evans, 1992).

3.3. Ironside Formation (late Miocene)

The Ironside Formation is at least 250 m of

conglomerates, siltstones, diatomites and lignites

Fig. 5. Petrographic and chemical data for newly described paleosols of Tutanik and Xaus paleosols from 4 to 6 m, Tutanik, Cmti, Xaus, Skaw,

and Abiaxi paleosols from 41 to 46 m, and Abiaxi, Cmti and Skaw paleosols from 229 to 232 m in reference section of Ironside Formation in

Windlass Gulch (Fig. 2).



named for the small village of Ironside, 15 km east

on U.S. highway 26 from Unity (Thayer, 1957;

Thayer and Brown, 1973). Sediments and paleosols

along the highway east of Ironside are identical with

those near Unity and particularly with the lower part

of a measured section in Windlass Gulch (0–7 m in

Fig. 2). Diatomites like those in the upper part of

the Ironside Formation near Unity, also crop out in

the hills north of Ironside. A fossil locality southeast

of Baker City also exposes tuffs and conglomerates

like those of the lower Ironside Formation (95–100

m in Fig. 2) in Windlass Gulch (Downs, 1952;

Evans, 1992).

These three areas are linked by late Miocene

fossil mammals of the Clarendonian North American

Land Mammal "Age" (8.4–12.3 Ma of Prothero,

Fig. 6. Late Miocene (Clarendonian) fossil mammals from Unity: (A)–(B

and 9017, respectively); (C)–(D) Gomphotherium osborni, skull and mandi

indet., molar with chipped crown (JODA9000). Scale bars as indicated.

1998), although nomenclatural difficulties remain.

For example, a fossil proboscidean skull from Unity

on display at the Oregon Museum of Science and

Industry in Portland (Fig. 6C–D) has remained

undescribed beyond a photograph and account of

its collection (Baldwin, 1964; Orr and Orr, 1999).

The excavation is still visible within an Abiaxi

paleosol on a low knoll southeast of a prominent

bluff, east of Unity (locality 8 of Table 4; Walker,

1990, figs. 5 and 9). George Gaylord Simpson

examined the specimen in 1953 and suggested it was

bMiomastodon merriamiQ (Orr and Orr, 1999). A moreappropriate name is bGomphotherium cingulatumQof Downs (1952), based on a lower jaw discovered

by Albert Werner at a locality 15 km east of Baker

City and about 80 km northeast of Unity. This and

) Hipparionini gen. indet., molar fragments (JODA specimens 9004

ble (Oregon Museum of Science and Industry); (E) Prosthennops sp.


other validly established taxa were not recognized

by Lambert and Shoshani (1998), who considered

the genus oversplit. Tobien (1972) went so far as to

include all North American Gomphotherium in

bGomphotherium productumQ. Here, I follow Mad-den and Storer (1985) in using their broadly defined

Gomphotherium osborni (Barbour) for the Unity

skull. Another gomphothere tooth, probably con-

specific, was found by Elmer Molthan in the

Ironside Formation 400 m south of Ironside Post

Office, which is only 15 km east of Unity

(bTetrabelodonQ sp. of Merriam, 1916). Gomphothe-

Fig. 7. Fossil plants and fish from the Ironside Formation near Unity, Orego

box elder, Acer negundoides, winged seed (JODA9409); (C) white o

pollardiana leaf (JODA9058a); (E) shagbark hickory, Carya bendirei, l

(JODA9050a); (G) bullhead catfish, Ictalurus (Ameiurus) vespertinus, sku

rium and two additional proboscidean species are

recognized from the Clarendonian Juntura Formation

to the south in Malheur County (Shotwell and

Russell, 1963): a two-tusker bMammut furlongiQ (aspecies not recognized by Lambert and Shoshani,

1998), and a shovel-tusker similar to Platybelodon

barnumbrowni (but regarded as a new genus by

Lambert and Shoshani, 1998).

There are also nomenclatural problems with

Clarendonian horse teeth from Unity and Juntura,

because among the numerous teeth and jaws, there is

no skull with facial fossae or other diagnostic

n: (A) sycamore, Platanus dissecta, leaf (specimen JODA9408); (B)

ak, Quercus prelobata leaf (JODA9053); (D) live oak, Quercus

eaf (JODA9055); (F) swamp ash, Fraxinus dayana, winged seed

ll and partial skeleton (JODA9043a). Scale bars all 1 cm.


features. The names given to these teeth (bHipparionanthonyiQ of Merriam, 1916 and bHipparion con-doniQ of Shotwell and Russell, 1963) are consideredby MacFadden (1984) likely junior synonyms of

Cormohipparion occidentale or Cormohipparion

sphenodus, the latter most likely on the basis of

size. Shotwell and Russell (1963) considered that

the large collection of teeth and jaws from

Juntura represented only one species.

Late Miocene geological age is also indicated by

fossil plants from near Unity and Juntura. A new

fossil plant assemblage in Juniper Gulch near Unity

(Fig. 7; locality 6 of Table 4, at 160 m in Fig. 2) is

here designated the Unity flora. Entire to weakly

dentate oak leaves like those dominating this

assemblage have been identified as bQuercushannibaliQ, but this name has been included withinQuercus pollardiana (Knowlton) by Axelrod (1995)

or identified with the similar, but more dentate,

living Quercus chrysolepis (Wolfe for Leopold and

Wright, 1985). The Unity flora is taxonomically

most like the late Miocene Stinkingwater flora in the

Juntura Formation near Juntura (Chaney and Axel-

rod, 1959) and the Weiser flora from the Poison

Creek Formation of Idaho (Dorf, 1936; Leopold and

Wright, 1985).

The thick gray volcanic ash of the Ironside

Formation at 171 m in Windlass Gulch is minera-

logically most similar to the 11.31 Ma CPTXI Ash of

the Great Basin (Perkins et al., 1998), and another

gray tuff at 101 m is most like the 11.59 Ma CPTIX

Ash (Figs. 2 and 3). A geological age of 11–12 Ma is

similar to the age of the Juntura Formation, 78 km to

the south in Malheur County. A whole-rock K–Ar

date for 12-m-thick basalt 152 m stratigraphically

below a fossil mammal quarry (OU locality 2337 of

Fig. 3) and 85 m above the Stinkingwater flora

(Chaney and Axelrod, 1959) near Juntura is 12.4 Ma

(Evernden and James, 1964; corrected using Dal-

rymple, 1979). A tuff 76 m stratigraphically above

the mammal quarry has been dated by K–Ar on

shards at 11.5F0.6 Ma (Fiebelkorn et al., 1982), andthe Devine Canyon ashflow tuff at the base of the

Drewsey Formation, 107 m above the quarry, has

been dated by 39Ar/40Ar of sanidine as 9.7 Ma

(Johnson et al., 1996; five similar K–Ar dates are

tabulated by Greene, 1973). Also comparable in age

and lithology are volcanic ashes, conglomerates, and

diatomites of the Coal Valley Formation of Nevada

dated by the K–Ar technique at 11.4–9.4 Ma (Golia

and Stewart, 1984).

4. Late Miocene paleogeography of eastern Oregon

4.1. Sedimentological evidence

The depositional basin of the Ironside Forma-

tion was bounded by volcanic terranes in the

Strawberry and Dooley Volcanics to the north and

west, and fault blocks of Paleozoic and Mesozoic

basement to the north and southwest. There was

erosional paleorelief on the underlying Dooley

Volcanics and Clarno Formation, as indicated by

several inliers cropping out south of Juniper

Gulch. The stepped system of northeast-extensional

faults (Fig. 1) did not exist during deposition,

when this broad alluvial and lacustrine basin

extended southeast from highlands to the north

and west (Fig. 8).

Basin-marginal boulder breccias and conglomer-

ates interfinger with diatomites and tuffaceous

siltstones of the Ironside Formation in road cuts

over Dooley Mountain and north of Whited

Reservoir (Fig. 1). Paleocurrent directions measured

from trough cross bedding in conglomeratic paleo-

channels in the Windlass Gulch section (Fig. 9)

indicate that streams flowed southeast. This early

phase of river channels and floodplains was

followed by extensive oligotrophic lakes, which

eventually accumulated diatomites out to the hilly

margins of the depositional basin. A final deposi-

tional phase of diminished lake area is recorded by

fluvial gravels and lignites at the top of the exposed

sequence near Unity (Fig. 2).

4.2. Paleosol evidence

Various paleosols represent different sedimentary

environments and communities within these river

valleys and lake shores. Weakly developed paleosols

(Abiaxi, Cmti, Xaus) represent ephemeral commun-

ities on the landscape, such as vegetation colonizing

stream and lake margins after flooding. Weakly

developed carbonaceous paleosols (Skaw) represent

riparian or lake-margin marsh, and moderately

Fig. 8. Reconstructed late Miocene (Clarendonian) landscape ecology of the area near Unity, eastern Oregon.

Fig. 9. Paleocurrents measured from trough cross bedding in

conglomerates at 123–131 m (left) and 33–39 m (right) in the

measured section in Windlass Gulch (Fig. 2).


developed peaty paleosols (Monana) represent

swamp woodlands, where decay of organic debris

was suppressed by seasonally high water table.

In contrast, Tutanik paleosols represent stable

floodplains and alluvial terraces. Tutanik paleosols

are thick, brown, and have little relict bedding due to

bioturbation (including siliceous rhizoconcretions;

Fig. 10A–B) and to development of soil structure

(fine subangular blocky peds) and microfabric (skel-

masepic porphyroskelic plasmic fabric of Brewer,

1976). They are also more aluminous and less rich in

Fig. 10. Chalcedony nodules and rhizoconcretions from Tutanik paleosols of the Ironside Formation near Unity, Oregon: (A) petrographic thin

section under crossed nicols of three rhizoconcretions (black holes with bright banded haloes) in oblique (lower left) and cross-sections

(specimen JODA9225); (B) rhizoconcretion (JODA9311); (C) near-spherical nodule (JODA9310a); (D) lobed nodule (JODA9310b); (E) cigar-

shaped nodule (JODA9309a); (F) tear-shaped nodule (JODA9310c); (G) cross-section of septarian nodule (JODA9310d); (H)–(I) petrographic

thin sections under crossed nicols of dendritic chalcedony and void-filling dolomite (JODA9310d). Scale bars as indicated.


alkalis and alkaline earths than associated paleosols

(Fig. 5). Although volcanic ash was an important part

of Tutanik paleosol parent material, few recognizable

shards have escaped weathering. This degree of

weathering is found after 8–27 ka in tropical highland

soils of New Guinea (Ruxton, 1968). Tutanik

paleosols have common clay skins in thin section

and a higher proportion of fine pedogenic clay than

associated paleosols (Fig. 5). The degree of enrich-

ment of clay is intermediate between that seen in

upper Modesto soils dated at 10 Ka and lower

Modesto soils dated at 40 Ka in alluvium of the

Merced River, San Joaquin Valley, California

(Harden, 1982, 1990).

4.3. Paleontological evidence

Diatomites near Unity have not been studied in

detail, but of 108 diatom taxa described from coeval

diatomites of Juntura, only two are found in estuaries

and the sea: bCoscinodiscus miocenicusQ andbCoscinodiscus subtilisQ of Van Landingham (1967;perhaps better referred to Actinocyclus according to

Bradbury et al., 1985). Melosira granulata, Melosira

distans, and Fragilaria construens dominate individ-

ual collections (58–88%) and indicate fresh water of

low alkalinity, low in dissolved solids, neutral to

slightly acidic in pH, and low in nutrient levels

(Bradbury et al., 1985). Significant (8%) amounts of

C. subtilis in one sample (Van Landingham, 1967)

may indicate connection to the sea by way of low-

gradient streams (Bradbury et al., 1985). Fossil

aquatic molluscs from Juntura and Unity (Tables 4–

5; Taylor, 1985) are evidence for a Miocene course of

the Snake River southwest into the Pit River of

northern California, before Basin and Range faulting

and northward capture by the Columbia River (Van

TassellL et al., 2001). Lakes were also needed by

many of the fossil birds (Brodkorb, 1961): cormor-

ants, coots, mergansers, teals, and extinct straight-

billed flamingos (Table 5).

5. Late Miocene paleoclimate of eastern Oregon

5.1. Evidence from paleosols

Only Tutanik paleosols are useful paleoclimatic

proxies, because other pedotypes were too weakly

developed to show a paleoclimatic signature. Differ-

ences between Tutanik and other comparably devel-

oped Cenozoic paleosols from Oregon may reflect

changing Neogene paleoclimate. Tutanik paleosols

are generally similar in their brown color and clayey

subsurface (argillic horizons) to Skwiskwi paleosols

of the Oligocene John Day and latest Miocene


Rattlesnake Formations (Retallack et al., 2000,

2002a), and to early Miocene Tima paleosols of

the John Day Formation (Retallack, 2004). Tutanik

paleosols lack thick silcretes of Tima paleosols and

have silica–micrite rhizoconcretions (Fig. 8A–B) and

chalcedony septarian nodules (Fig. 8C–I) unknown

in Skwiskwi paleosols. The large amount of silt

(never less than 27% by volume) and sodium (soda/

potash ratiosN1) also ally Tutanik paleosols with

Tima rather than Skwiskwi paleosols. Tutanik

paleosols also differ from paleosols of the late

Miocene (7 Ma) Rattlesnake Formation, such as

the Tatas pedotype, which was crumb-structured,

moderately leached, and calcareous (Retallack et al.,

2002a). Tutanik paleosols thus show greater affin-

ities with interpreted summer-dry (xeric) paleosols,

such as Tima, than summer-wet (ustic) paleosols,

such as Swkiskwi and Tatas.

Modern soils like Tutanik paleosols are found

near Redding, northern California (map unit Lc3-2a

with duric phase of F.A.O., 1975), where mean

annual temperature is 17.7 8C, and mean annualprecipitation is 1040 mm, January and July temper-

atures are 8 and 29 8C, respectively, and precip-itation is 216 mm in January, but only 5 mm in July

(Ruffner, 1985). In contrast, for Baker City near

Unity, mean annual temperature is 7.6 8C, meanannual rainfall is 270 mm, temperature is more

seasonal (�4 8C in January, 19 8C in July), butprecipitation is less seasonal (35 mm in January, 12

mm in July; Ruffner, 1985).

Independent estimates of former precipitation can

be gained from chemical composition of clayey

horizons and from carbonate in rhizoconcretions of

Tutanik paleosols at depths of 79–91 cm. This

depth (D in cm) can be used to estimate mean

annual rainfall (P in mmF156) using a transferfunction derived from study of modern soils

(Retallack, 1994), once allowance is made for burial

compaction (Sheldon and Retallack, 2001) due to at

least 60 m of overlying Pleistocene fanglomerate

(Evans, 1992). A large database of chemical

analyses of North American soil B horizons has

been used to derive yet other transfer functions for

mean annual precipitation (P in mmF182) andmean annual temperature (T in 8CF4.4) from twochemical measures: S, the molar ratio Na2O+K2O/

Al2O3; and C, molar Al2O3/(Al2O3+CaO+Na2O)

times 100 (following Sheldon et al., 2002), as

follows:

P ¼ 139:6þ 6:388D� 0:01303D2

P ¼ 221e0:02C

T ¼ � 18:516 Sð Þ þ 17:298:

Paleoclimatic results of these calculations (Table 3)

are 450–800 mm mean annual precipitation from

depth to carbonate and 600–1100 mm from chemical

composition of clayey horizons. Because the carbo-

nate-depth transfer-function was designed for use with

nodules (Retallack, 1994, 2000), rather than rhizo-

concretions, these estimates are less likely to be

reliable than the subhumid regime estimated from

chemical composition of B horizons. Geochemical

composition of paleosol parent rhyodacitic and

andesitic airfall ash shows insignificant change

through time compared with paleosol modification

(Perkins et al., 1998; Retallack et al., 2000).

A highly seasonal climate is indicated by concen-

trically banded rhizoconcretions with silica and

micrite in Tutanik paleosols (Fig. 10A–B). Season-

ality is also indicated by the unusually abundant silt

and soda in Tutanik paleosols, especially considering

their likely mean annual precipitation (Table 3). In

such rainy climates, there would not be so much salt

or dust unless summers were very dry, and most rain

and snow were during winter and spring. Furthermore,

the scarce carbonate and distinctive silica–micritic

rhizoconcretions (Fig. 10A–B) are like those found in

modern soils of Mediterranean (summer-dry) climates

(Chadwick et al., 1987, 1995). Siliceous rhizoconcre-

tions from a few Tutanik paleosols (45, 59, and 69 m

in Fig. 2) grade insensibly through cigar- and tear-

shaped forms into ellipsoidal septarian nodules of a

form not previously reported from paleosols (Fig.

10C–F). Some of these contain fossil wood or lumpy

root traces morphologically comparable to rhizobially

nodulated roots. Others are hollow and cracked, with

void-filling dolomite rhombs (Fig. 10G,I). Micro-

fabric of the nodules is radially oriented, dendritic

chalcedony, creating an external fabric reminiscent of

cone-in-cone structure. The ellipsoidal nodules are

superficially similar to chalcedony nodules of the

bbutton bedsQ, a lacustrine facies of the middle

Table 3

Mean annual temperature and precipitation for Tutanik paleosols of the Ironside Formation

Estimated geological

age (Ma in Fig. 5)

Stratigraphic level

(m in Fig. 3)

Mean annual precipitation

(mm) from Bk

Mean annual precipitation

(mm) from chemical

composition

Mean annual temperature

(8C) from chemicalcomposition

11.59 100.5 644F156 (503–785)11.60 Unity Reservoir 657F156 (516–798)11.69 83 588F156 (447–729) 916F182 (734–1098) 13.3F4.4 (8.9–17.7)11.83 45.1 601F156 (460–742) 934F182 (752–1116) 13.2F4.4 (8.8–17.6)11.95 16.6 643F156 (502–784)11.96 14 617F156 (476–758)11.97 12.5 626F156 (485–767)11.00 Ironside 640F156 (499–781)12.00 5.2 627F156 (486–768) 786F182 (604–968) 12.1F4.4 (7.7–16.5)12.01 3.3 636F156 (495–777)12.02 1.7 649F156 (508–790)12.10 Denny Flat 571F156 (430–712)12.11 Denny Flat 594F156 (453–735)Means All listed 623F156 879F182 12.9F4.4


Miocene Barstow Formation of California (Palmer,

1957). My own examination of the Barstow nodules

found conspicuous relict bedding and microcrystalline

textures, unlike the septarian and radial-dendritic

crystal form of the Unity nodules. Nevertheless,

highly alkaline late-summer aridity could have played

a role in the formation of both kinds of siliceous

nodules.

5.2. Evidence from fossil plants

Close floristic composition of Unity (Table 5)

and Stinkingwater floras (Chaney and Axelrod,

1959) with the modern flora of California supports

evidence from associated paleosols for extension of

Mediterranean climate into eastern Oregon during

the late Miocene. The paleoclimatic affinities of

these floras are quite distinct from more humid and

warm climates evident during the middle Miocene

thermal maximum, represented locally by the

Austin, Tipton, Baker, and Mascall floras (Oliver,

1934; Brown for Gilluly, 1937; Chaney and

Axelrod, 1959), and before the advent of modern

sagebrush and riparian communities during the

Pliocene (Chaney, 1948; Ashwill, 1983, Retallack

et al., 2002a). Mean annual precipitation of around

900–1000 mm and mean annual temperature of 13

(8–18) 8C have been estimated from other lateMiocene fossil floras of the Great Basin (Wolfe,

1994; Wolfe et al., 1997; Graham, 1999). A

summer-dry paleoclimatic regime for the Unity flora

is also compatible with frequent wildfires indicated

by charcoal associated with middle to late Miocene

fossil floras of southeastern Oregon (Taggart et al.,

1982; Taggart and Cross, 1990).

5.3. Evidence from fossil animals

Abundance of bullhead catfish in siltstones with

the Unity flora (Fig. 7; Table 4) can be contrasted

with the overlying diatomites, which have a sparse

fish fauna of other kinds of fish. The leaves and

catfish probably represent a nearshore lacustrine

habitat distinct from that of the open lake. Bullhead

catfish lived in western North America from the

Eocene to Pliocene, but after that time became

restricted to eastern North America. They have been

artificially reintroduced to Oregon and Idaho within

the last 120 years. Today, they live in low-gradient

streams and lakes at elevations less than 1000 m in

warm temperate climates with at least 230 frost-free

days and at least 400 mm mean annual precipitation

(Van Tassell et al., 2001).

Brodkorb (1961) invokes Bergmann’s rule (Brown

and Lomolino, 1998) to argue for a warmer than

present climate from the small size of Juntura

cormorants, coots, and teals compared with living

relatives. Oxygen isotopic studies of fossil mammal

teeth from Juntura, Oregon, indicate mean annual

temperatures of about 18 8C, and a mean annual range

Table 4

Late Miocene (Clarendonian) megafossils of the Ironside Formation, near Unity, Oregon

Fossil taxon Similar living taxon Common name Loc.

Equisetum sp. indet. Equisetum arvense Common horsetail 1–2

Gramineae sp. indet. Gramineae Grass 1–2

Sagittaria sp. indet. Sagittaria cuneata Arum-leaved arrowhead 3

Salix sp. indet. Salix scouleriana Scouler’s willow 3

Populus sp. indet. Populus balsamifera Black cottonwood 3

Trapa americana Knowlton (1898) Trapa natans Water chestnut 3–5

Platanus dissecta Lesquereux (1878) Platanus occidentalis Sycamore 6

Quercus pollardiana (Knowlton) Axelrod (1995) Quercus chrysolepis Maul oak 6

Quercus prelobata Condit (1944) Quercus garryana Oregon white oak 6

Quercus simulata Knowlton (1898) Quercus myrsinaefolia Chinese oak 6

Cercocarpus ovatifolius Axelrod (1985) Cercocarpus betuloides

var. blancheae

Birchleaf mountain

mahogany

6

Acer negundoides MacGinitie (1934) Acer negundo Box elder 6

Fraxinus dayana Chaney and Axelrod (1959) Fraxinus caroliniana Swamp ash 6

Carya bendirei (Lesquereux) Chaney and Axelrod (1959) Carya ovata Shagbark hickory 6

Ptelea miocenica Berry (1931) Ptelea trifoliata Carolina hop tree 7

Radix junturae Taylor (1963) Radix auriculata Aquatic big-ear snail 7

Ictalurus (Ameiurus) vespertinus Miller and Smith (1967) Ictalurus melas Black bullhead catfish 6

Gomphotherium osborni (Barbour) Madden and Storer (1985) None Extinct four-tusker 8

Prosthennops sp. indet. None Extinct large peccary 8

Rhinocerotidae sp. indet. None Large extinct rhino 9

cf. Cormohipparion sphenodus (Cope)

MacFadden and Skinner (1977)

None Extinct cursorial

three-toed horse

10

cf. Megatylopus gigas Matthew and Cook (1909) None Extinct large camel 11

cf. Merycodus sp. Antilocapra americana Pronghorn 11–12

Note: these are identifcations of newly collected material in collections of John Day Fossil Beds National Monument. Legal and NAD27 UTM

coordinates to localities are 1—Windlass Gulch section 60 m NWH NEH SEH SEH S33 T12S R38E 11T 0416253 4925216; 2—Windlass

Gulch section 44 m NWH NEH SEH SEH S33 T12S R38E 11T 0416262 4925216; 3—Windlass Gulch section 61 m NWH NEH SEH SEH

S33 T12S R38E 11T 0416262 4925216; 4—Denny Flat southeast SEH SEH SEH SWH S2 T13S R37E 11T 0409723 492339; 5—Denny flat

southeast NEH NEH NEH NEH S2 T13S R37E 11T 0409761 492353; 6—Windlass Gulch section 158 m NWH NWH SWH SWH S34 T12S

R38E 11T 0416526 495228; 7—Windlass Gulch section 225 m NEH NWH SWH S34 T12S R38E 11T 0416665 4925404; 8—east of Unity

SEH NEH SEH NWH S11 T13S R37E 11T 0408804 4922516; 9—Windlass Gulch section 100 m NEH SWH SWH SWH S34 T12S R38E

11T 0416596 4925185; 10—Windlass Gulch section 5 m SEH NWH SEH SEH S33 T12S R38E 11T 0416227 4925153; 11—Denny Flat

northwest NWH SWH SWH SWH S35 T12S R37E 11T 0408416 4925224; 12—Windlass Gulch section 89 m NEH NEH SEH SEH S33 T12S

R38M 11T 0416315 4925249.


of temperature of some 25 8C (Kohn et al., 2002),compared with 7.6 and 23 8C, respectively, for BakerCity today (Ruffner, 1985).

Indications of a late Miocene climate at Juntura,

wetter than at present, come from the sewellel

(Tardontia in Table 5). Similar aplodontids are

now restricted to Oregon and Washington, west of

the Cascades (Shotwell, 1958). The western Amer-

ican mole (Scapanus) is also now found well west

of Juntura and Unity (Hutchison, 1968). On the

other hand, subhumid to semiarid rangeland con-

ditions are indicated by ground squirrels, pocket

gophers, and extinct mylagaulines (Shotwell, 1958,

1970).

6. Late Miocene palaeoecology of eastern Oregon

6.1. Evidence from paleosols

A detailed local mosaic of vegetation (Fig. 8)

can be inferred from the relationship of paleosols to

local sedimentary facies and from plant-induced

features of the paleosols (Figs. 2 and 3; Table 3).

Tutanik paleosols are finely structured (granular

peds about 1 cm across) with abundant fine root

traces as well as scattered rhizoconcretions, as in

soils supporting grass with scattered woody shrubs

or trees (Retallack, 1997b, 2001b). The paleosols

lack fine crumb peds found in sod-forming grass-

Table 5

Late Miocene (Clarendonian) megafossils of the Juntura Formation

near Juntura

Fossil taxon Common name

Sphaerium sp. cf. S. lavernense

Herrington (Herrington and

Taylor, 1958)

Aquatic lake orb

mussel

Pisidium sp. cf. P. clessini

Neumayr and Paul (1875)

Aquatic pea clam

Pisidium sp. indet. Aquatic pea clam

Fluminicola junturae Taylor, 1963 Aquatic pebble snail

Hydrobiidae indet. Small aquatic snail

Viviparus turneri Hannibal, 1912 River snail

Radix junturae Taylor, 1963 Aquatic big-ear snail

Carinifex shotwelli Taylor, 1963 Keeled ramshorn snail

Promenetus sp. indet. Aquatic ramshorn snail

Ictalurus peregrinus Lundberg, 1975 Catfish

Cyprinidae gen. indet. Squawfish

Catastomidae gen. indet. Sucker

Phalacrocorax leptopus

Brodkorb, 1961

Small cormorant

Ciconiidae gen. indet. Extinct stork

Megapaloelodus opsigonus

Brodkorb, 1961

Straight-billed

early flamingo

Eremochen russelli

Brodkorb, 1961

Small goose

Querquedula pullulans

Brodkorb, 1961

Small teal

Ocyplonessa shotwelli

Brodkorb, 1961

Extinct merganser

Neophrontops dakotensis

Compton, 1935

Extinct Old

World vulture

Fulica infelix Brodkorb, 1961 Small coot

Meterix sp. indet. Extinct large shrew

Anouroneomys minimus Hutchison

& Bown (Bown, 1980)

Extinct small shrew

Mystipterus sp indet. Extinct nonburrowing

mole

Scalopoides sp. indet. A Extinct weakly

burrowing mole

Scalopoides sp. indet. A Extinct weakly

burrowing mole

Scapanus sp. cf. S. shultzi

Tedford, 1961

Western American

mole

Chiroptera gen et sp. indet. Extinct bat, like big

brown bat

Hesperolagomys sp. cf. H. galbreathi

Dawson (Clark et al., 1964)

Extinct pika

Hypolagus sp. cf. H. fontinalis

Dawson, 1958

Extinct rabbit

Tardontia sp. cf. T occidentale

(Macdonald) Shotwell, 1958

Extinct sewellel

Mylagaulus sp. indet. Extinct horned rodent

Epigaulus minor Hibbard and

Phillis (1945)

Extinct horned rodent

Fossil taxon Common name

Ammospermophilus junturensis

(Shotwell & Russell) Korth, 1994

Ground squirrel

Ammospermophilus sp. indet. Ground squirrel

Eutamias sp. indet. Extinct chipmunk

Hystricops sp. indet. Extinct large beaver

Eucastor malheurensis Shotwell

and Russell (1963)

Extinct beaver

Microtoscoptes sp indet. Extinct vole

Copemys dentalis (Hall)

Lindsay, 1972

Extinct small field

mouse

Copemys sp. cf. C. esmeraldensis

(Wood) Lindsay, 1972

Extinct large field

mouse

Copemys sp indet. Extinct large field

mouse

Leptodontomys sp. indet. Extinct pocket mouse

Perognathus sp. indet. Pocket mouse

Diprionomys sp. indet. Extinct pocket mouse

Pliosaccomys sp. indet. Extinct pocket gopher

Macrognathomys nanus Hall, 1930 Extinct birch mouse

Borophagus sp. indet. Short-face bone-

crushing dog

Aelurodon sp. indet. Long-face bone-

crushing dog

Vulpes sp. indet. Fox

Leptarctus sp. indet. Extinct badgerlike

mustelid

Hoplictis sp. indet. Extinct like honey

badger

Sthenictis junturensis Shotwell and

Russell (1963)

Extinct otterlike

musteline

Pseudailurus sp. indet. Extinct small cat

Gomphotherium sp. indet. Extinct four-tusk

proboscidean

Mammut furlongi Shotwell and

Russell (1963)

Extinct two-tusk

mastodon

cf. Platybelodon barnumbrowni

(Barbour) Barbour, 1932

Extinct shovel-tusker

cf. Cormohipparion sphenodus (Cope)

MacFadden and Skinner (1977)

Extinct three-toed

horse

Tapiridae gen. indet. Tapir

Aphelops sp. indet. Extinct hornless

rhinoceros

Prosthennops sp. indet. Extinct large peccary

Merychyus major Leidy, 1858 Extinct piglike oreodon

Procamelus grandis Gregory, 1942 Extinct camel

Megatylopus sp. indet. Extinct large camel

Note: this is a compilation of the Black Butte local fauna of

Shotwell (1963, 1967, 1970), Hutchison (1968), and Taylor (1963)

as revised by Honey et al. (1998), Lambert and Shoshani (1998),

Baskin (1998), Martin (1998), Lander (1998), and Wright (1998).

Table 5 (continued)


land soils (Mollisols, Phaeozems, Chernozems;

Retallack et al., 2002a), so that grasses were most

likely bunch grasses.


Other paleosols with prominent relict bedding

and large woody root traces supported plant

formations early in ecological succession after

disturbance. For example, Xaus paleosols of in-

channel sandy bars probably supported riparian

woodland, and Cmti and Abiaxi paleosols of levee

siltstones probably supported riparian gallery wood-

land. Cmti paleosols are thinner, less sodic, and

have more relict bedding than Abiaxi paleosols (Fig.

7), indicating a shorter period of vegetation growth

and successional development. Early successional

marsh ecosystems are represented by carbonaceous

clayey paleosols with small root traces and relict

bedding (Skaw pedotype). Swamp ecosystems are

represented by lignitic paleosols with large woody

root traces (Monana pedotype). The swamp paleo-

sols have impure coals and charcoal and were

probably seasonally dry, but there is no comparable

evidence of seasonal exposure of marsh paleosols,

which may have been associated with perennial

watercourses.

6.2. Evidence from fossil plants

Only one kind of paleosol contained preserved

fossil plants: weakly developed paleosols of the

Skaw pedotype (localities 1–3 of Table 4). The best

preserved fossil plants were found within laminated

diatomites and siltstones (localities 4–7 of Table 4).

These are interpreted as leaf litter of marsh soils

(Skaw pedotype) and leaves blown into an oligo-

trophic lake, respectively. In all cases, there is a bias

toward aquatic plants, such as water chestnut,

arrowhead, and horsetail.

Despite these taphonomic limitations, the Stin-

kingwater, Weiser, and Unity floras include locally

abundant fossil grasses and live oak comparable to

those of summer-dry parts of California today. The

Unity flora is most similar to interior live oak

woodland of Barbour and Major (1988) and canyon

live oak woodland of Sawyer and Keeler-Wolf

(1995). These oak woodlands are at moderate

elevation below the zone of abundant conifers on

the southwestern slopes of the Sierra Nevada

flanking the northern California Great Valley. The

oak trees are largely evergreen and up to 15 m high

in both closed and open formation. Their grassy

understorey is brown and dry for most of the

summer. Mesic elements in the Unity flora (Carya,

Ptelea) are not found in modern Californian live oak

woodlands. These middle Miocene holdover taxa are

evidence that late Miocene dry woodlands of Unity

were not exactly like Californian vegetation today.

No taxodiaceous conifers were found near Unity,

where they would be expected considering charcoal

and woody root traces in Monana paleosols. These

swamp conifers are common in the coeval Stinking-

water flora of Juntura (Chaney and Axelrod, 1959)

and the Weiser flora of Idaho (Dorf, 1936).

Fossil grasses are represented by sheathing leaves

near Unity (Table 4) and by abundant pollen in coeval

rocks of Idaho (Leopold and Wright, 1985). Miocene

grasses of the Great Plains and Oregon are unlikely to

have been floristically similar, given differences

between fossil dicot floras of Oregon and the Great

Plains (Thomasson, 1979; Graham, 1999; Strfmberg,2002) and differences between sodic–silicic Miocene

paleosols of Oregon (Retallack, 2004) and calcareous

Miocene paleosols of the Great Plains (Retallack,

1997b, 2001b). A comparable modern vegetation

distinction is the western wheat grass (Agropyron

spicatum) rangeland province of northern California

and the Great Basin, and blue grama (Bouteloua

gracilis) grasslands of the western Great Plains

(Leopold and Denton, 1987).

6.3. Evidence from fossil animals

Quarry collections from Juntura formed the basis

for Shotwell’s (1963) pioneering study of Clarendo-

nian mammalian paleocommunities (Fig. 11), and

these quarries were revisited in order to examine

their paleosols (Fig. 3). Shotwell’s (1963) bpondbank communityQ came from Skaw paleosols (theonly pedotype found), whereas his bsavannacommunityQ came largely from sandstones immedi-ately overlying a Tutanik paleosol. His bsavannacommunityQ was mainly camels, hipparioninehorses, two-tusker mastodon, and rhinoceros. The

bpond bank communityQ, which from paleosol andsedimentological evidence appears more likely to

have been riparian woodland, was mainly fish and

turtles, but also had many beavers, as well as rabbits

and a variety of rodents and insectivores (Table 5).

The weak development and carbonaceous and

manganiferous composition of Skaw paleosols are

Fig. 11. Specimen numbers of fossil mammals of Clarendonian

bsavannaQ and bpond bankQ communities by Shotwell (1963), fromquarries of Fig. 3. Numerous fish and turtle bones from locality

2337 are not included in these figures.


evidence of seasonally inundated streamside swales

(Table 2), where fish were trapped and devoured by

predators.

Contrary to Shotwell (1963), Bernor et al. (1988)

concluded that coeval late Miocene (11 Ma)

hipparionine horses of Austria were forest browsers,

because forest leaves were found in associated lake

beds. This view is not supported by the abundance

of hipparionines in Tutanik paleosols and rarity in

Skaw paleosols of Juntura and Unity (Fig. 11).

Later studies of hipparionine tooth wear (Hayek et

al., 1992) and associated mammalian fauna (Webb,

1983) also support Shotwell’s (1963) conclusion that

hipparionines were grassland and grassy woodland

mixed feeders. Open grassy terrane was also

required by an extinct species of Old World vulture

from Juntura (Table 5).

Proboscideans are now associated with mosaics of

grassy and woody vegetation, which they maintain

by destructive feeding (Owen-Smith, 1988). A

diversity of proboscidean habitats may be expected

from the high diversity of Clarendonian probosci-

deans, including eight genera in North America and

three in the Unity and Juntura faunas (Tables 4–5).

Shovel-tuskers (Platybelodon) are commonly por-

trayed as aquatic shovelers, although tusk wear

patterns indicate scraping against bark, twigs, peb-

bles, or other hard debris (Lambert, 1992). Four-

tuskers (Gomphotherium) also show tusk wear from

bark stripping and digging, and two-tusk mastodons

(Mammut) are known from Pleistocene stomach

contents to have eaten conifer needles, cones, and

grass (Lambert and Shoshani, 1998). Associated

paleosols are Abiaxi for Gomphotherium, Tutanik

for Mammut, and probably Skaw for cf. Platybelo-

don (uncertainty comes from poor exposure and

inexact location). The inferred paleoenvironments of

these paleosols (Table 3) cannot be claimed to

represent habitat preferences based on only one or

two specimens of each species.

Paleosols also support the idea that late Miocene

rangelands of the Pacific Northwest were distinct from

those of the Great Plains (Leopold and Denton, 1987).

Late Miocene summer-dry siliceous alfisol and aridisol

paleosols of Oregon (Retallack et al., 2002a) can be

contrasted with summer-wet Mollisols of Nebraska

and Montana (Retallack, 1997b, 2001b). The hippar-

ionine province of the Pacific Northwest was distinct

from a Pliohippus province of southern California and

the Great Plains during the Clarendonian (Shotwell,

1963). Late Miocene (7 Ma) grazing Pliohippus in

Oregon appeared in calcareous Mollisol paleosols

reflecting an incursion of summer-wet Gulf Coast air

masses into eastern Oregon at that time (7.3 Ma;

Retallack et al., 2002a). The tridactyl mixed-feeding

hipparionines may reflect rigors of summer-dry range-

land vegetation botanically and structurally distinct

from summer-wet rangeland of monodactyl grazing

Pliohippus. Comparable differences in vegetation and

seasonality may also explain the numerical dominance

of foregut-fermenting ruminant camels over hindgut-

fermenting horses at Juntura (Fig. 11) and other

western Clarendonian faunas (Tedford and Barghoorn,

1993), whereas horses dominated Clarendonian

assemblages of Nebraska and Texas (Webb, 1983).

Muzzle morphology of the camels Procamelus and

Megatylopus has been taken to indicate that they were

browsers, perhaps mixed feeders, but not grazers

(Dompierre and Churcher, 1996). Similar dominance

of ruminants over perissodactyls is found in Kenyan

(Retallack et al., 2002b) and Greek (Solounias, 1981)

Miocene faunas. By reprocessing their cud, ruminants

are sustained by less-nutritious forage than perisso-

dactyls of comparable body sizes (Janis et al., 1994).

Summer-dry seasons in Kenya, Greece, and western


North America selected for ruminant-dominated fau-

nas able to cope with seasonal shortages of forage,

whereas equid-dominated faunas of the Great Plains

and Gulf Coast were supported by year-round forage.

7. Implications for global change

7.1. Neogene paleoclimate in eastern Oregon

A detailed Neogene paleoclimatic time series (Fig.

12A–C) can be constructed by extending the results

from paleosols presented here to other paleosols,

including paleosols from the early Miocene upper

John Day Formation near Kimberly, Oregon (Retal-

lack, 2004) dated by extrapolation from 40Ar/39Ar

radiometry (Fremd et al., 1994), the middle Miocene

Mascall Formation and Columbia River Basalts near

Dayville, Oregon (Bestland and Krull, 1997; Sheldon,

2003) dated by magnetostratigraphy and radiometry

(Draus and Prothero, 2002), the latest Miocene

Rattlesnake Formation near Dayville, Oregon (Retal-

lack et al., 2002a) dated by magnetostratigraphy

(Hoffman and Prothero, 2002), the Mio–Pliocene

Ringold Formation near Pasco and Taunton, Wash-

ington (Gustafson, 1978; Smith et al., 2000) dated by

magnetostratigraphy (Gustafson, 1985; Morgan and

Morgan, 1995), and the Palouse Loess near Helix,

Oregon, and Washtucna, Washington dated by mag-

netostratigraphy, tephrochronology, and thermolumi-

nescence dating (Busacca, 1989, 1991; Tate, 1998;

Blinnikov et al., 2002).

Fig. 12. Miocene paleoclimate of eastern Oregon follows the

oceanic foraminiferal carbon isotopic record but not the oceanic

oxygen isotopic record nor the record of volcanism in Oregon: (A)

mean annual precipitation from depth to Bk horizon of 799

paleosols (heavy line and 1r error envelope): (B) mean annualprecipitation from chemical composition of 97 paleosol Bt horizons

(heavy line and 1r error envelope); (C) mean annual temperaturefrom chemical composition of paleosols (heavy line and 1j errorenvelope): (D) frequency of lavas erupted from Cascades volcanoes

(from McBirney et al., 1974); (E) oxygen isotopic data (y18Oapatite)from fossil horse teeth in Oregon and Nebraska (Kohn et al., 2002;

Passy et al., 2002); (F) oxygen isotopic (y18Ocarb) and carbonisotopic values (y13Ccarb) of 8959 samples of marine foraminiferafrom deep-sea cores (from Zachos et al., 2001). Abbreviations are

Geringian (G.), Monroecreekian (MO.), Harrisonian (HA.), Hemi-

ngfordian (HEMI.), Barstovian (BAR.), Clarendonian (CLAR.),

Hemphillian (HEM.), and Blancan (BLA.). Most recent Irvingto-

nian and Rancholabrean are unlabelled.

The resulting curve shows great variation in both

precipitation (Fig. 12A–B) and temperature (Fig. 12C)

through time and is comparable with variation in

carbon isotopic composition of benthic marine fora-

minifera (Fig. 12F). Also comparable are time series of

North American mammalian diversity, which was


higher during warmer and wetter times than during

cooler and drier times (Alroy et al., 2000; Janis, 2002).

7.2. Mismatch of Neogene terrestrial and marine

oxygen isotopic records

Neither paleoclimatic fluctuations from paleosols

(Fig. 12A–C) nor oxygen isotopic fluctuations from

mammal teeth (Fig. 12E) correspond with fluctua-

tions in oxygen isotopic composition of benthic

marine foraminifera (Fig. 12F). Marine and terrestrial

carbonate oxygen isotopic records do not correspond

during the late Paleocene (Koch et al., 2003) or late

Permian (MacLeod et al., 2000) either. Although

marine oxygen isotopic records have been regarded

as a global paleoclimatic standard (Zachos et al.,

2001), their mismatch with continental records is a

fundamental problem for global change studies.

The mismatch between North American mamma-

lian evolutionary dynamics and marine oxygen iso-

topic records has been discussed at length by Alroy et

al. (2000), who attribute the mismatch to biotic

insensitivity to climatic forcing. This is contrary to

the observation of iterative evolution of mammal

ecomorphs within North American land mammal

bagesQ which average 2.3 Myr in duration (Martin,1994; Meehan and Martin, 1994; Meehan, 1996,

1999). The sediments of each mammal age in the

Great Plains are bounded at the base by erosional

downcutting, interpreted as evidence of a wet climatic

phase, and terminated by shallow calcic paleosols or a

caprock caliche, interpreted as evidence of a dry

climatic phase (Schultz and Stout, 1980). I have made

comparable observations in Montana and Oregon

(Retallack, 1997b, 2001a; Retallack et al., 2002a).

Late Oligocene mammalian communities of Oregon

also show alternation between sagebrush steppe with

Hypertragulus and wooded grassland communities

with Nanotragulus on Milankovitch time scales (41–

100 ka) within paleosols with differing depth to calcic

horizons reflecting alternating semiarid and subhumid

paleoclimates (Retallack, 2004; Retallack et al., 2004).

Finally, there is annual variation in oxygen isotopic

composition of growth bands within Miocene mam-

malian tooth enamel (Fox, 2001; Kohn et al., 2002).

North American mammals show clear sensitivity to

climate on time scales ranging from evolutionary to

ecological contrary to Alroy et al. (2000).

A possible reason for mismatch of marine oxygen

and continental paleoclimatic records is the Cenozoic

evolution of C4 photosynthetic pathways in land

plants. The C4 (Hatch–Slack) pathways of tropical

grasses result in isotopically heavier plant carbon than

C3 (Calvin–Benson) pathways (Cerling et al., 1997),

and this pathway increases oxygen isotopic values of

plant as well because operating on CO2 (Farquhar et

al., 1993). The antiquity of the C4 pathway has been

estimated from phylogenetic analyses at 25 Ma

(Kellogg, 1999), from isotopic analyses of paleosols

at 15 Ma (Kingston et al., 1994; Morgan et al., 1994),

and from isotopic and anatomical studies of permin-

eralized fossil grasses at 12.5 Ma (Nambudiri et al.,

1978; age revised by Whistler and Burbank, 1992).

Isotopic studies of paleosol carbonate and mammalian

teeth show that C4 grasses became widespread in the

southern Great Plains after 6.6 Ma but were never

common in Oregon or the northern Great Plains

(Cerling et al., 1997; Passy et al., 2002; Fox and

Koch, 2003). C4 grasses avoid cool and summer-dry

climates today (Sage et al., 1999). The spread of C4grasses after 6.6 Ma is too late to explain the earlier

divergences of marine carbon and oxygen isotopic

records (Fig. 12F), mismatch of marine and terrestrial

oxygen isotopic records (Fig. 12A–C,F), and the

generally upward trend of marine oxygen isotopic

values (Fig. 12F).

A more likely explanation for mismatch of the

oceanic oxygen isotopic and continental paleocli-

matic proxies is isotopic depletion as rain shadows

spread and intensified with uplift of the Cascade

volcanic and other western mountain ranges (Kohn

et al., 2002). The idea of Cenozoic drying by means

of an orographic rain shadow has been a traditional

explanation for pronounced Neogene aridity evident

from fossil plants (Chaney, 1948; Ashwill, 1983).

The growth of the Oregon Cascades can be inferred

from numerous radiometrically dated eruptions (Fig.

12D; McBirney et al., 1974; Priest, 1990), but this

does not match either marine or continental records

(Fig. 12A–F). The Cascade and Klamath Mountain

rain shadow has been important in creating generally

dry long-term climate in eastern Oregon since the

late Oligocene (30 Ma; Retallack et al., 2000).

Rocky Mountain barriers isolated eastern Oregon

from Gulf Coast cyclonic circulation since at least

early Miocene (19 Ma), as indicated by the


appearance of sodic–silicic rather than calcic paleo-

sols (Retallack, 2004) and very different oxygen

isotopic composition of fossil horse teeth in Oregon

and Nebraska (Fig. 12E). Fossil floras from the

northern Great Basin indicate high altitudes since at

least the early Miocene, for example, 2100 m for the

middle Miocene 49 Camp flora of northwestern

Nevada (Wolfe et al., 1997). Eastern Oregon was

elevated during Miocene initial eruptions of the

Yellowstone hot spot (Humphreys et al., 2000).

The most important reason for a mismatch of

oceanic and continental records is isotopic enrichment

of oceanic oxygen by growth of isotopically depleted

polar ice caps, which has long been recognized to

compromise direct paleotemperature interpretation of

marine oxygen isotopic values (Zachos et al., 2001).

This factor is not unrelated to the growth of mountains

with their expanding rain shadows, because montane

ice caps enrich the isotopic composition of atmos-

pheric oxygen as well. Ice cap fluctuation in volume

introduces a highly variable bias into paleotemper-

ature interpreted from the oxygen isotopic record of

marine benthic foraminifera. This bias can be cor-

rected using independent estimates of paleotemper-

ature from Mg/Ca ratios of foraminifera (Lear et al.,

2000), but this is compromised by dissolution and

other forms of diagenesis (de Villiers, 2003), and by

crustal recycling and other long-term influences on

ocean chemistry (Veizer et al., 2000).

7.3. Neogene greenhouse–icehouse fluctuation and its

causes

Greenhouse mechanisms for Neogene paleocli-

matic variation are suggested by correspondence of

the Oregon paleosol sequence (Fig. 12A–C) with the

marine signal of carbon sequestration inferred from

benthic foraminiferal y13C decrease and carbon oxi-dation inferred from benthic foraminiferal y13Cincrease (Fig. 12F). High carbon isotopic values in

the middle Miocene ocean correspond to warm-wet

conditions in Oregon, whereas low carbon isotopic

values in the Plio–Pleistocene ocean corresponded to

cool-dry conditions in Oregon. High carbon isotopic

values in marine foraminifera reflect diminished burial

of chemically reduced carbon and higher atmospheric

CO2 levels due to some combination of oceanic

respiration, volcanic emission, or impact destruction

of biomass. A middle Miocene high in atmospheric

CO2 has been confirmed by stomatal index estimates

of atmospheric CO2 (Kürschner et al., 1996; Retallack,

2001c, 2002). Oceanic proxies of atmospheric CO2 do

not show such variation (Pagani et al., 1999; Pearson

and Palmer, 2000) but may be compromised by

volatility of isotopic composition and runoff (Retal-

lack, 2002). Despite these problems, a useful working

hypothesis is control of paleoclimate by a varying

greenhouse effect from changing atmospheric con-

centrations of CO2.

Plausible mechanisms for transient climatic warm-

ing include impact, volcanism (Coutillot, 2002) and

methane clathrate release (Kennett et al., 2002). There

are numerous large (N10 km diameter) Neogene

craters: Haughton, Canada (24 km, 23F1 Ma), Ries,Germany (24 km, 15F0.1 Ma), Karla, Russia (10 km,5F1 Ma), ElTgygytygn, Russia (18 km, 3.5F0.5 Ma),Bosumtwi, Ghana (10.5 km, 1.03F0.02 Ma), andZhamanshin, Kazachstan (14 km, 0.9F0.1 Ma;Dressler and Reimold, 2001; website http://

www.unb.ca/passc/impactdatabase accessed Jan. 10,

2003). Time series of volcanic eruptions are also

strongly episodic (McBirney et al., 1974), with some

exceptionally large eruptions, such as the middle

Miocene Columbia River flood basalts of Oregon

(Wignall, 2001). Columbia River Basalts and Ries

Crater impact may have been involved in the middle

Miocene peak of global warmth, but why was there

not a comparable warm spike after the 23-Ma

Haughton Crater or a better correlation between

North American paleoclimate and Cordilleran volcan-

ism (Fig. 12D). Because of methane’s distinctively

low carbon isotopic values, such methane outbursts

are suspected when carbon isotopic compositions fall

by more than 4x (Jahren et al., 2001). There are nonegative excursions of this magnitude in Neogene

carbon isotopic records (Fig. 12F), thus methane is

unlikely to be the whole explanation either.

A counterpoint to transient carbon-oxidizing events

was progressive long-term Neogene global cooling,

perhaps related to changing oceanic current config-

uration (Broecker, 1997), mountain uplift (Raymo and

Ruddiman, 1992), or biotic intensification of weath-

ering with sod–grassland coevolution (Retallack,

2001a). Both mountain uplift and increased oceanic

thermohaline circulation create oligotrophic alpine and

polar marine communities, with less carbon sequestra-

http://www.unb.ca/passc/impactdatabase


tion potential, but grasslands provided more organic

and nutrient-rich runoff to oceanic phytoplankton

(Retallack, 2001b). Grasslands in addition had greater

amounts of soil carbon, higher albedo, and lower rates

of transpiration than the woodlands and shrublands

they replaced in subhumid to semiarid regions of the

world (Nepstad et al., 1994; Jackson et al., 2002), and

this also could have contributed to global drying and

cooling (Retallack, 2001b). This mechanism of global

change is biological, because it is driven by coevolu-

tion of grasses and grazers, with crumb-textured,

organic, fertile soils nourishing hypsodont ungulates

capable of withstanding abrasion from dusty, siliceous

grasses (Retallack, 2001a). The progressive evolution

and spread of crumb-textured soils, siliceous grasses,

and hypsodont ungulates proceeded against a backdrop

of volatile climatic change, and both short-sod and tall-

sod grassland paleosols appearing at times of relatively

high temperature and humidity (Fig. 12A). Grasslands

did not merely fill in dry times and places (Kohn et al.,

2002) but were a biological force for climatic cooling

and drying in their own right (Retallack, 2001b).

Paleoclimatic warm spikes due to volcanism, bolide

impact, and methane clathrate release may also have

been exacerbated by newly coevolved Cenozoic

grasslands, because ruminants liberate CO2 and CH4.

The middle Miocene thermal optimum was a time of

unusually high diversity of North American plants and

mammals, in part due to evolution within high-

productivity grassland–woodland mosaics in a warm-

wet CO2 greenhouse (Janis, 2002), and in part due to

immigration from Asia through high-latitude land

bridges (Wolfe and Tanai, 1980; Kohno, 1997;

Lambert and Shoshani, 1998; Webb, 1998). There

were more grazing mammal species then ever before,

and also more browsers than before or after (Janis et

al., 2000). The middle Miocene greenhouse peak was

preceded and followed by several other episodes of

greenhouse paleoclimate of lesser magnitude, includ-

ing early Clarendonian communities described here.

Warm-wet conditions of greenhouse transients, even if

initiated by tectonic or impact forcing, would be

enhanced by increased mammalian diversity and

cropping of vegetation. Cold-dry conditions follow

carbon and water sequestration by grassland soils, their

high albedo and burial of carbon-rich grassland soil

crumbs. Paleoclimatic cycles on million-year time

scales could reflect alternating evolutionary advances

by plants and animals. Such reciprocating coevolution

is apparent from current estimates of the times for

evolution of bunch grassland paleosols (33 Ma), short-

sod grassland paleosols (19 Ma), and tall-sod grass-

land paleosols (7.3 Ma; Retallack, 1997b, 2001b,

2004; Retallack et al., 2002a), which alternate with

evolution of horse tridactly (36 Ma), cursoriality (25

Ma), hypsodonty and springing gait (17 Ma), mono-

dactyly, wide muzzles, and knee-locking (12 Ma), and

large size and passive-stay shoulder (5 Ma; MacFad-

den, 1992; Janis and Wilheim, 1993; Hermanson and

MacFadden, 1992, 1996). This schedule is compatible

with a coevolutionary process long envisioned for

grassland ecosystems (Jacobs et al., 1999). Global

change involves more than just impact or volcanic

forcing, but a complex interplay of factors for which

detailed proxies are now becoming available (Fig. 12).

These various records will have to become even more

detailed before answers become clear.

Acknowledgements

For introduction to the fossils of this area, I

thank Ted Fremd and his collecting crew, Scott

Foss, Lia Vella, Delda Findieson, Alois Pajak,

Daniella Gould, Matthew, and La Rue Rogers. Kent

Nelson allowed access to his land near Hereford.

Mary Oman of the Baker City Bureau of Land

Management helped with geological reconnaissance,

funding, and research clearance. Megan Smith, Ben

Cook, and Russell Hunt helped with fieldwork and

fossil collection. Alicia Duncan prepared numerous

difficult petrographic thin sections, and Nathan

Sheldon provided bulk density measurements.

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Honey,

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Late Miocene climate and life on land in Oregon within a ...€¦ · Clarendonian (12 Ma) fossil...

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