Lithostratigraphy, Tephrochronology, and Rare Earth Element
Geochemistry of Fossils at the Classical Pleistocene
Fossil Lake Area, South Central Oregon
James E. Martin, Doreena Patrick,1 Allen J. Kihm,2
Franklin F. Foit Jr.,3 and D. E. Grandstaff4
Museum of Geology, South Dakota School of Mines and Technology, 501 East St. Joseph
Street, Rapid City, South Dakota 57701, U.S.A.
(e-mail: james.martin@sdsmt.edu)
ABSTRACT
One of the most famous fossiliferous Pleistocene sites in the Pacific Northwest is Fossil Lake, Oregon. Until recently,
fossil collections from the area were not stratigraphically controlled, owing to the lack of a detailed stratigraphic and
chronologic framework. Our field studies reveal at least nine exposed thin rhythmic fining-upward depositional
packages, most separated by disconformities. Analysis of interbedded tephras reveals that the Rye Patch Dam (∼646
ka), Dibekulewe (∼610 ka), Tulelake T64 (∼95 ka), Marble Bluff (47 ka), and Trego Hot Springs (23.2 ka) tephra layers
are present in the section, indicating deposition from more than ∼646 ka to less than 23 ka, which includes both the
late Irvingtonian and Rancholabrean North American land mammal ages, a much longer time span than previously
believed. Bones analyzed from eight of the defined units have distinctly different rare earth element (REE) signatures.
Fossils obtain REE during early diagenesis, and signatures are probably closely related to lake water compositions.
REE signatures in fossils from lower packages suggest uptake from neutral pH waters. In contrast, REE signatures
become increasingly heavy REE–enriched up-section, with positive Ce anomalies in the upper units. REE signatures
in fossils from the upper units are very similar to waters from modern alkaline lakes, such as Lake Abert, Oregon,
suggesting diagenetic uptake in increasingly alkaline and saline waters. These REE changes suggest increasing aridity
up-section, a contention reinforced by the habitat preferences of the terrestrial vertebrates preserved.
Introduction
brates. Although the region is well known, relatively little dating of the deposits has been undertaken. Most age speculations have been made on
the basis of specimens collected, with the exception of a radiometric date of 29,000 yr B.P. (Allison
1966, 1979) from snail shells near the base of the
stratigraphic section. This date and many fossils
suggest the Late Pleistocene (Rancholabrean North
American land mammal age; NALMA), which begins at 150 ka (Bell et al. 2004). However, no evidence of Bison, the primary index fossil for the Rancholabrean NALMA, had been published, and only
two possible specimens have been identified subsequently. The lack of diagnostic Bison remains
made the Rancholabrean designation somewhat
problematic. To refine the temporal relationships
and environmental changes during the Late Pleistocene, our investigations have concentrated on
careful stratigraphic collection of paleontological
The Fossil Lake area of Lake County, Oregon, may
represent the most important Quaternary vertebrate paleontological region in the Pacific Northwest. Fossil vertebrates and invertebrates have
been known from Fossil Lake since 1876, and since
1989, very large collections have been made from
these Pleistocene deposits, culminating in tens of
thousands of stratigraphically collected verteManuscript received May 4, 2004; accepted November 4,
2004.
1
Author for correspondence: Department of Earth and Environmental Science, University of Pennsylvania, 254-B Hayden
Hall, 240 South 33rd Street, Philadelphia, Pennsylvania 191046316, U.S.A.; e-mail: doreena@sas.upenn.edu.
2
Department of Geosciences, Minot State University, Minot,
North Dakota 58707, U.S.A.; e-mail: kihm@minotstateu.edu.
3
Department of Geology, Washington State University, Pullman, Washington 99164, U.S.A.; e-mail: foit@mail.wsu.edu.
4
Department of Geology, Temple University, Philadelphia,
Pennsylvania 19122, U.S.A.; e-mail: grand@temple.edu.
[The Journal of Geology, 2005, volume 113, p. 139–155] 䉷 2005 by The University of Chicago. All rights reserved. 0022-1376/2005/11302-0002$15.00
139
140
Table 1.
Oxide
SiO2
Al2O3
Fe2O3
TiO2
Na2O
K2O
MgO
CaO
Cl
Totala
No. tephra
samples
No. shards
analyzed
Similarity
coefficient
J. E. MARTIN ET AL.
Glass Chemistry of Composite Tephra Samples from Fossil Lake, Lake County, Oregon
Fossil Lake
tephra A
71.75
14.58
3.44
.47
4.24
3.52
.42
1.44
.11
100
(.36)
(.10)
(.12)
(.04)
(.18)
(.13)
(.05)
(.11)
(.02)
Rye Patch
Dam S28505.8
72.20
14.55
3.40
.44
3.70
3.68
.35
1.46
.13
100
Fossil Lake
tephra B
77.11
12.83
1.32
.07
3.58
4.28
.06
.59
.12
100
(.11)
(.03)
(.03)
(.01)
(.02)
(.09)
(.00)
(.01)
(.01)
Dibekulewe
Sample T64
ash bed
Fossil Lake
Tulelake
Fossil Lake Marble Bluff Fossil Lake
LD-19
tephra C
(17.01 m)
tephra D MSH set C
tephra E
76.7
13.2
1.30
.07
4.06
4.06
.05
.61
n.d.
100
70.19
15.09
3.16
.63
4.58
2.82
.81
2.55
.14
100
(.26)
(.06)
(.03)
(.01)
(.24)
(.01)
(.01)
(.01)
(.01)
70.6
14.9
3.15
.66
4.8
2.92
.70
2.29
n.d.
100
76.74
13.76
.95
.10
4.10
2.49
.25
1.55
.05
100
(.10)
(.07)
(.02)
(.01)
(.01)
(.02)
(.01)
(.00)
(.00)
76.5
14.0
.99
.10
4.1
2.4
.24
1.50
.07
100
75.93
13.44
1.64
.23
4.02
3.34
.24
1.05
.10
100
1
4
3
2
4
17
67
49
32
79
.97
.96
.96
.98
(.31)
(.06)
(.10)
(.01)
(.20)
(.04)
(.01)
(.03)
(.01)
Trego Hot
Springs
75.50
13.60
1.58
.23
4.50
3.32
.21
1.02
n.d.
(.20)
(.10)
(.04)
(.01)
(.10)
(.06)
(.01)
(.03)
100
.97
Note. Also shown are compositions of reference tephras, similarity coefficients, numbers of samples and shards analyzed, and the
analytical standard deviation (SD); 1 SD of the analyses given in parentheses. MSH p Mount St. Helens; n.d. p not determined.
Source. Rye Patch Dam, Williams (1994); Dibekulewe ash bed, Davis (1978); Sample T64 Tulelake and Trego Hot Springs, Rieck
et al. (1992); Marble Bluff MSH set C, Davis (1985).
a
Analyses normalized to 100% on a volatile-free basis.
samples from discrete lithologies. We identified
tephra layers interbedded within these lithological
units that aid in the temporal resolution. Herein,
we describe the detailed lithostratigraphy, tephrochronology, and rare earth element (REE) signatures
of the fossils and their implications for the history
and evolution of Fossil Lake.
Since its discovery in 1876, Fossil Lake has produced thousands of invertebrate and vertebrate fossils; many were described in the late nineteenth
and early twentieth centuries. Fish (Cope 1883),
birds (Shufeldt 1913; Howard 1946), and mammals
(Elftman 1931; Martin 1996) are particularly abundant. However, none of the early collections were
stratigraphically controlled. Therefore, the area has
been recollected over the last 15 yr, resulting in
thousands of specimens tied to thin lithostratigraphic units. A future goal is to analyze the new
collections in a temporal framework; therefore, a
detailed lithostratigraphic framework combined
with tephrochronology was required. The first portion of this contribution represents the results of
these undertakings. Based on the framework described herein, the second portion of the contribution represents an analysis of REE signatures in
fossils, which was to provide an independent assessment of paleoenvironments that could be compared with results derived from the fossil successions. REE signatures in fossils reflect the REE
content of the original pore waters during the fossilization process (Henderson et al. 1983; Trueman
1999) and may be used to infer original paleoenvironmental or early diagenetic conditions (Elderfield and Pagett 1986; Patrick et al. 2004). This ar-
ticle presents the first major study of REE in fossils
from a lake-dominated environment.
Analytical Methods
Electron Microprobe Analysis of Tephra. Mounts
for analysis of tephra samples consisted of four 8mm-diameter glass rings (cells) fixed to a standard
petrographic slide using epoxy, allowing four samples to be prepared simultaneously. A few hundred
milligrams of tephra followed by a few drops of
epoxy were added to each cell, thoroughly mixed,
and allowed to cure. The cells were trimmed using
a microdiamond saw, ground to thin section thickness, polished, and carbon coated.
The composition of the glass in the tephra was
determined using the Cameca Camebax electron
microprobe in the GeoAnalytical Laboratory located in the Department of Geology at Washington
State University. The analytical conditions are the
same as those described in Hallett et al. (2001), and
the tephra glass compositions are presented in table
1. The compositions of the glasses were compared
to one another and to the standard glasses in the
GeoAnalytical Laboratory’s database of more than
1500 Pacific Northwest tephra samples using the
similarity coefficient (SC) of Borchardt et al. (1972)
as a discriminator. The SC is the average of eight
oxide concentration ratios between the glasses in
the tephra sample and tephra standard. It was calculated using a weighting of 1.0 for the Si, Al, Ca,
Fe, and K oxide concentration ratios, 0.50 for the
Na oxide ratio, and 0.25 for the Mg and Ti oxide
ratios. Na was a given lower weighting because of
Journal of Geology
FOSSIL LAKE TEPHROCHRONOLOGY
well-recognized impediments to accurate measurement and susceptibility to post-depositional environmental modification. Mg and Ti were given
lower weighting because of their low concentrations and, consequently, their high relative error of
measurement. Similarity coefficients 10.94 (Rieck
et al. 1992) are generally considered permissive of
a correlation, provided they are consistent with
stratigraphic constraints.
REE Analysis. In this study, REE concentrations
were measured in vertebrate fossil materials from
each major stratigraphic unit at Fossil Lake to detect differences in REE signatures between the lithologic units, most of which were !0.5 m thick.
These REE signatures in fossils from the various
units were compared with REE signatures in natural waters to infer possible paleoenvironmental or
early diagenetic environments. The interpretations
from this study are compared with previous studies
of paleoenvironmental conditions at Fossil Lake
and neighboring pluvial Lake Chewaucan (Allison
1966, 1979, 1982; Cohen et al. 2000; Negrini et al.
2000).
Osteological elements from various species and
eggshells were sampled from all of the described
lithologic units (table 2). This allowed investigation
of changes in REE signature and paleoenvironment
with time, as well as possible influence of osteological material and species. Fossils were obtained
from two localities in the upper siltstone of the
fourth depositional package and from five sites in
the lower coarse unit of the third depositional package (table 2) to investigate possible lateral variations in signatures over the area of the Fossil Lake
Basin. The latter five sites are separated by up to
16 km.
Sample preparation techniques follow those by
Staron et al. (2001). Large intact bone specimens
were sampled by drilling using a Dremel variablespeed electric drill. Between samples, the drill was
cleaned with diluted (2%) trace-metal-grade nitric
acid or trace-metal-grade acetone and rinsed in distilled water. Cortical bone samples were immersed
in water and then in a diluted acetic acid solution
in an ultrasonic bath to remove matrix and secondary carbonate. Other matrix and secondary
minerals were removed by handpicking. Samples
were then rinsed with distilled water and dried.
Selected samples were analyzed using x-ray diffraction, revealing essentially pure apatite with
only minor amounts of other crystalline phases.
Cleaned bone fragments were mechanically
crushed in an acid-washed mortar and pestle. Approximately 0.2 g of powder was weighed for each
sample. Each sample was dissolved in about 2 mL
141
Ultrex-grade HNO3 and diluted to 100 mL. When
only smaller masses of bone were available, the
initial solution was diluted to smaller volumes to
preserve the mass/volume ratio. Samples were diluted to appropriate levels with 2% ultra-pure
HNO3. Initial samples were analyzed using a Finnigan MAT Element/1 high-resolution magnetic
sector inductively coupled plasma–mass spectrometer (ICP-MS) at Rutgers University. The Hawaiian
Basalt (BHVO) was used as a standard; precision and
accuracy were better than Ⳳ4% relative to certified
values. Analytical procedures followed those by
Field and Sherrell (1998). Subsequent analyses were
conducted using a VG Plasma-Quad 3 ICP-MS in
the Geology Department at Temple University. Results have been corrected for isobaric interferences
where necessary. The coefficient of variation for
most REEs is !Ⳳ5% of the analyzed value. Analytical results have been normalized relative to the
North American Shale Composite (NASC; Gromet
et al. 1984). Cerium anomalies (Ce∗) have been calculated from
Ce∗ p
2Ce N
(LaN ⫹ PrN)
⫺ 1,
where the subscript N indicates NASC-normalized
values.
Lithostratigraphy of Fossil Lake
Fossil Lake (fig. 1) is a small playa (∼16 km2) in the
Fort Rock Basin of south central Oregon, east of
Crater Lake (Mt. Mazama) and southeast of the
Newberry volcanic complex. These and other volcanoes have provided tephra and volcaniclastic debris to the region (Kuehn and Foit 2001). Fossil Lake
lies within a large fault-bounded basin in the northern portion of the Basin and Range Province. This
large basin trends nearly 65 km east-west and may
be considered to be composed of three smaller basins, which are connected at higher elevations:
westerly Fort Rock Basin, easterly Christmas
Valley Basin, and the Silver Lake Basin to the southwest. Fossil Lake lies within the Christmas Valley
Basin.
During the Pleistocene, the entire basin was inundated and formed pluvial Fort Rock Lake (Allison 1940, 1979). At its maximum, the lake included
the Silver Lake Basin to the south and is estimated
to have been as much as 60 m (200 ft) deep, extending over 1200 km2 (500 square miles; Snyder
et al. 1964; Allison 1979). The lake appears to have
waxed and waned a number of times, probably related to the waxing and waning of Pleistocene gla-
Table 2.
Results of Rare Earth Element (REE) Analyses of Fossils from Fossil Lake, Oregon (ppm)
Specimen
Fish
Enamel/horse
Cortical/mammal
Fish
Fish
Cortical/mammal
Fish
Fish
Fish
Cortical/bird
Cortical/mammal
Cortical/bird
Fish
Trabecular/mammal
Fish
Cortical/mammal
Fish
Cortical/mammal
Cortical/bird
Cortical/mammal
Fish
Cortical/mammal
Trabecular/mammal
Fish
Fish
Rabbit
Note.
Package
No.
SDSMT
locality
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
1
1
1
1
2
2
3
3
3
3
3
3
3
4
5
5
6
6
6
7
7
7
7
8
8
8
10FL
12FL
13FL
16FL
18FL
19FL
23FL
24FL
25FL
26FL
28FL
30FL
41FL
43FL
44FL
45FL
32FL
34FL
49FL
36FL
50FL
47FL
48FL
38FL
39FL
40FL
V893
V893
V893
V893
V895
V895
V8914
V8934
V891
V891
V8914
V8914
V892
…
V891
V891
V8928
V8928
V8928
V8913
V8913
V8928
V8928
V8913
V8913
V8913
1939
158.1
529.1
313.9
1573
91.68
484.6
108.5
229.4
225.6
89.95
102.2
119.5
57.59
77.81
41.07
85.02
47.29
2092
94.50
462.7
15.27
15.51
94.23
40.92
30.13
4248
395.7
1124
685.3
3144
73.08
999.0
305.1
497.4
521.2
173.3
183.0
226.2
113.1
195.7
100.7
325.7
675.7
4608
382.3
812.1
115.7
38.8
1236
212.3
1030
522.2
33.96
134.4
74.71
417.1
6.43
113.77
14.58
53.37
47.01
18.99
19.88
24.92
13.65
22.27
12.15
17.71
15.42
605.1
25.58
118.0
4.10
4.54
31.85
13.81
12.96
2368
155.7
660.1
342.0
2099
28.94
520.4
71.08
261.2
225.3
89.54
93.34
114.5
65.68
99.70
52.32
87.79
64.94
2761
113.9
571.9
17.20
18.08
148.2
64.48
57.69
526.6
34.90
142.4
75.40
564.0
6.44
112.9
16.36
61.21
52.68
20.41
20.39
27.38
15.27
22.65
11.88
19.96
14.24
623.1
25.31
137.9
3.79
3.90
39.89
17.51
14.97
140.4
10.86
39.26
21.05
174.8
2.06
31.73
6.44
18.70
16.20
6.25
6.26
8.07
4.41
6.79
3.44
6.17
3.97
180.0
8.08
40.49
1.14
1.07
13.52
6.25
4.72
572.1
50.08
172.1
101.2
803.7
9.23
146.6
31.13
91.59
68.62
29.78
31.17
33.70
18.04
26.87
12.46
31.87
14.64
820.3
33.78
190.7
4.46
3.59
63.75
30.04
19.53
89.31
7.82
24.25
15.11
135.97
1.34
22.17
3.05
13.55
10.11
4.41
4.58
4.84
2.84
4.57
2.14
5.02
2.66
129.5
6.35
29.76
.84
.63
12.99
6.44
3.45
628.1
59.07
177.9
110.0
1100
9.98
157.3
21.24
102.0
74.00
32.87
34.07
35.47
21.22
34.40
15.90
39.77
19.44
868.9
50.50
231.6
6.13
3.86
112.09
57.44
27.33
137.1
14.69
43.25
25.88
287.3
2.46
36.91
4.67
25.07
18.14
8.17
8.72
8.53
5.47
8.52
3.95
10.93
4.82
192.0
13.54
61.14
1.56
.79
32.90
16.83
7.23
412.4
49.66
140.86
85.31
980.5
8.62
122.09
16.20
86.35
59.61
28.95
29.99
28.19
19.55
28.69
13.73
40.20
17.40
574.5
49.21
209.7
5.66
2.29
128.02
63.22
27.65
63.10
8.35
22.44
13.39
155.5
1.45
19.79
2.85
13.78
10.02
4.84
5.04
4.37
3.44
4.80
2.39
7.00
3.26
82.95
8.48
33.65
1.05
.35
23.17
10.56
5.26
416.9
61.93
155.5
94.73
1090
12.15
137.48
23.29
100.63
65.12
35.75
38.51
30.90
27.52
35.80
19.44
57.04
29.00
532.0
67.25
236.5
9.47
2.83
189.2
75.78
46.41
61.01
10.20
24.62
15.58
176.8
2.10
23.24
3.93
17.23
12.14
6.32
7.11
5.05
5.22
6.24
3.35
11.52
5.06
82.77
12.28
38.45
2.06
.45
35.64
12.73
8.30
Package numbers as in figure 2; REE concentrations in parts per million. No. p REE specimen number; SDSMT p South Dakota School of Mines and Technology.
Journal of Geology
FOSSIL LAKE TEPHROCHRONOLOGY
Figure 1. A, Outline map of Oregon. The shaded area
contains the Fossil Lake region in B. B, Map of the northern portion of Lake County, Oregon, showing location of
modern lakes and maximum extent of pluvial Fort Rock
Lake (dashed line; Allison 1982). The Fossil Lake area,
from which most stratigraphic sections and fossils were
obtained, is shown by the filled triangular area in the
Christmas Valley Basin (CV). FRB p Fort Rock Basin,
FR p town of Fort Rock, CA p California, ID p Idaho,
NV p Nevada, WA p Washington.
ciers. Following final desiccation, eolian processes
created extensive dune fields, particularly obvious
in the Fossil Lake area, where active dunes remain.
The fossils have been exposed due to wind and dune
migration, augmented by erosion from sheet wash.
The geology of Fossil Lake was discussed by Waring (1908), who agreed with Russell (1884) that the
region was flooded by large Pleistocene lakes.
Smith (1926) first used the Fossil Lake Formation
for 2.4–3 m of lake silts that contained numerous
Pleistocene fossils. Some of the first detailed geological investigations at Fossil Lake were those by
Ira Allison, who began investigations in 1939 and
published a series of articles culminating in his reviews of the geology and paleontology of Fossil
Lake (1966) and the regional geology (1979). When
Allison noted tephra layers at Fossil Lake, he used
them for correlation but not for absolute dating.
Allison did obtain a radiocarbon date of 29,000 yr
B.P. from fossil snails (Allison 1966, 1979) lying
below a tephra layer at a site where prolific bird
remains occur. We have reinvestigated the site,
known as the Bird Site, and sampled the associated
tephra (tephra layer B, Dibekulewe; table 1; fig. 2)
and snails.
Our efforts in understanding the lithostratigra-
143
phy of the area came in 1974 when one of us (J. E.
Martin) studied environmental impacts at the site
for the Bureau of Land Management. Lithological
units were differentiated initially by weathering
profile, which mirrors composition. All of the units
at Fossil Lake are thin, and although Allison (1966)
recognized only three tephra layers interbedded
within the detrital units, we determined that five
separate tephra layers exist. These tephra layers are
normally associated with a rhythmic depositional
pattern of fining-upward sequences. Normally,
each sequence is about a meter and a half thick and
composed of a basal basalt conglomerate grading
upward into a tuffaceous, friable, basaltic sandstone
(Bertog et al. 1997) overlain by a silty claystone,
representing lacustrine deposition. We recognize at
least nine fining-upward depositional packages exposed in the Fossil Lake area. Erosional processes,
providing the key for recognition, differentially preserve the upper claystone of each depositional
package. These claystones were noted by Allison
(1966), but he miscorrelated some, did not discern
the subtle differences between them, and thus did
not recognize the repeated depositional pattern.
Once the pattern was recognized, it was tested by
extensive trenching and correlation of the tephra
layers.
These efforts resulted in the following composite
section (fig. 2). The base of the exposed section is
represented by a yellow claystone, which is more
than a meter thick; its base was not located. This
claystone, which hosts tephra layer A (Rye Patch
Dam; table 1; fig. 2), may represent the uppermost
interval of a depositional package (package 1; fig.
2). This claystone is overlain by basaltic gray-brown
unconsolidated sandstone, which contains an interbedded, discontinuous tephra layer (tephra layer
B, Dibekulewe; table 1), representing the base of
the second depositional package. Scattered basaltic
pebbles lie at the base of the sand, and weathering
has resulted in ferruginous staining. Eolian processes result in depressions within this unit
throughout the Fossil Lake area, and these are characterized by concentrations of snails. From this
level, Allison (1966) obtained a 14C date on snails
of 29,000 yr B.P. This sand grades upward into a
transitional brownish siltstone with some sand interbeds grading into a white claystone, totaling 90
cm. The white claystone is composed principally
of reworked tephra and represents the uppermost
unit of the second depositional package. The base
of the third package represents a major disconformity and contains a pebble to small cobble basal basalt gravel, only a few centimeters thick, which is
laterally discontinuous. The conglomerate is over-
144
J. E. MARTIN ET AL.
Figure 2. Composite stratigraphic sections, analyzed tephra layers (table 1), and tephra ages at Fossil Lake, Oregon.
Fossil Lake sediments have been subdivided into numbered depositional packages comprising lithostratigraphic units.
Grain sizes for individual units are indicated by lengths of the bars (scale at base of section). Major unconformities
occur between packages 3 and 4 and at the base of package 5.
lain by a basaltic, loosely consolidated sandstone
(approximately 80 cm), which contains a distinctive 8-cm pumice bed (tephra layer C, Tulelake
T64), also noted by Allison (1966). The sand is in
turn overlain by a brown, silty claystone, up to 60
cm thick, capping the third rhythmite. Another
sandstone (30 cm thick) with scattered pebbles occurs at the base of the fourth package. The uppermost layer of the fourth package, a distinctive, tuffaceous tan siltstone ranging from 24 to 46 cm thick,
overlies this sand. Another tephra layer (tephra
layer D, Marble Bluff; table 1; fig. 2) is associated
with the siltstone, resulting in the distinctive color.
Above this package, lateral variation exists. To the
east, at least three additional packages lie between
the tan siltstone and the base of the uppermost
package. To the west, a disconformity exists, and
the base of the suprajacent package consists of distinctively cross-bedded, ferruginous sands. The additional packages to the east (5–7) are each com-
posed of poorly consolidated basaltic sandstone
with scattered pebbles from 12 to 70 cm thick overlain by a brown siltstone/claystone, ranging from
12 to 24 cm. Within the siltstone/claystone of the
seventh package is a bright white, powdery tephra
layer (tephra layer E, Trego Hot Springs), the youngest tephra within the succession. The widespread
upper package of ferruginous sand (up to 145 cm
thick) grades upward into a brown siltstone (12 cm),
which is in turn overlain by a white claystone (55
cm) characterized by salmonid remains. In most
areas, the white claystone represents the uppermost unit; however, one area exhibited remnant
pockets of sand a few centimeters thick, indicating
the succession was thicker, but erosion has reduced
the section essentially to the top of the white claystone. Overall, nine fining-upward depositional
packages have been distinguished, which appear to
have been formed during repeated waxing and waning of the Pleistocene lakes.
Journal of Geology
FOSSIL LAKE TEPHROCHRONOLOGY
Tephrochronology and Radiocarbon Dates
Although fossils from Fossil Lake are useful for age
constraints, only a few independent absolute age
dates have been obtained (Allison 1966, 1979). Recognition of the interbedded tephra layers within the
depositional packages indicated that a chronologic
framework could be developed to complement
the paleontological and stratigraphic frameworks.
Once the composite section (fig. 2) was proven and
tested through widespread outcrop and trench correlation, the tephras were analyzed, and geochemical comparisons were made to known regional
tephra layers. The results were previously summarized by Martin and Kihm (2003), and more recent unpublished dates of some tephras are included in the following discussion.
The lowest tephra layer, tephra layer A (fig. 2),
lies at the top of the lowermost exposed package
at the top of a yellow claystone. The glass in this
tephra is a very good match (SC p 0.97; table 1) to
that in the Rye Patch Dam tephra believed to be
equivalent to the Desert Springs ash flow near
Bend, Oregon, estimated to be 630 ka (Rieck et al.
1992) and dated by laser fusion 40Ar/39Ar to
686 Ⳳ 50 ka (R. J. Fleck, pers. comm. 2004). However, based on the age and position of the overlying
Lava Creek B tephra (639 Ⳳ 0.2 ka; Lanphere et al.
2004) and the Bruhes/Matuyama Chron boundary
in the Tulelake core (Rieck et al. 1992), SarnaWojcicki (pers. comm. 2004) estimates the Rye
Patch Dam ash bed to be ∼646 ka. The second package contains tephra layer B in the basal sand unit
and compares well (SC p 0.96) with the Dibekulewe ash bed of the western United States, similarly
estimated to be ∼610 Ⳳ 20 ka (A. Sarna-Wojcicki,
pers. comm. 2004). Fossil gastropods described and
14
C dated by Allison (1966) come from this level.
Tephra layer C, which lies in the lower sandstone
of the third depositional package, consists of pumice fragments, the composition of which is a very
good match (SC p 0.97) to sample T64 from a
depth of 17.01 m in Tulelake, estimated to be between 72 and 119 ka or ∼95 Ⳳ 23 ka (A. SarnaWojcicki, pers. comm. 2004). A basal basalt lag suggests a significant basal unconformity compared
with those of most other depositional packages.
Tephra layer D is associated with the claystone of
the fourth depositional package. The composition
of the glass in this tephra is an excellent match
(SC p 0.98) to that in the Marble Bluff bed (Mount
St. Helens set C) of Davis (1978, 1985), which has
been dated at 47 Ⳳ 2 ka (Berger and Busacca 1995;
Negrini et al. 2000). The uppermost tephra (tephra
layer E), which occurs in the siltstone unit of the
145
seventh depositional package, is a very good match
(SC p 0.97) to the 23.2 Ⳳ 0.3 ka Trego Hot Springs
tephra (Benson et al. 1997).
These dates are not consistent with the 14C date
of 29,000 Ⳳ 2000 yr B.P. (Isotopes I-1641) taken
from gastropod shells collected by Allison (1966)
from the tephra layer near the base of the second
exposed depositional package. The ages of all but
the uppermost Trego Hot Springs tephra layer are
significantly older than this 14C age. Studies (Tanners 1970; Goslar and Paxlut 1985; Yates 1986) have
suggested that 14C dates from snails may not be
reliable because of recrystallization of the carbonate or uptake of isotopically old carbonate carbon.
In an attempt to resolve this inconsistency, new
mollusk shells with original shell coloration were
collected from Allison’s (1966) locality. Scanning
electron microscopic (SEM) examination did not reveal any recrystallization of the newly collected
shell material. However, an even younger 14C date
of 13,420 Ⳳ 70 yr B.P. (Beta no. 173726) was obtained for one of these shells. Extensive excavation
within Allison’s sampling site revealed a probable
explanation for this date incongruence: every gastropod found in situ was found in a rodent burrow,
indicating intrusion by bioturbation. Therefore, it
appears that the buried gastropods at this level are
not the result of primary deposition. Moreover,
fish, bird, and mammal remains were seldom found
in the burrows but were ubiquitous in the surrounding bedded sand unit of the second depositional package. The variability in the gastropod
dates and the high degree of fossilization of the
mammalian bones from this low in the section
make the radiocarbon dates suspect. As noted previously, although they might be environmentally
or geographically excluded from this area, the absence of the Rancholabrean index fossil Bison also
casts some doubt on the younger ages proposed by
Allison (1966, 1979).
Moreover, Fossil Lake contains the same tephra
sequence, encompassing roughly the same time interval as neighboring Summer Lake (Davis 1985;
Negrini et al. 2000; Kuehn and Foit 2001). Although
all of the tephra layers at Fossil Lake appear to be
slightly reworked and therefore hosted in potentially younger sediments, the significant disconformity observed at the base of sedimentary package 3, which hosts the Tulelake T64 tephra layer
(tephra layer C), is stratigraphically consistent with
the unconformity reported at Summer Lake (Davis
1985; Negrini et al. 2000). This disconformity not
only explains the absence at Fossil Lake of the
many tephra layers in the age range of 130–250 ka
reported in the Ana River section at Summer Lake
146
J. E. MARTIN ET AL.
(Davis 1985) but also suggests that the tephrochronological age of the sediments above the disconformity is accurate. A deep core taken close to and extending approximately 50 m below the Ana River
outcrop revealed the Dibekulewe ash bed (SarnaWojcicki et al. 2001), which is consistent with its
stratigraphic position at Fossil Lake. To date, the
Rye Patch Dam tephra has not been observed in
this core.
Thus, it appears that the Fossil Lake section is
significantly older than suggested by Allison (1966,
1979), that the lower two sedimentary packages
(fig. 2) were deposited during the Irvingtonian
NALMA, and that packages 3–8 were deposited
during the Rancholabrean NALMA as previously
suggested by Martin and Kihm (2003). A major disconformity below package 3 represents much of
the later Irvingtonian and early Rancholabrean
NALMA (∼600–150 ka) based on tephrochronology.
Therefore, the later part of Irvingtonian II, Irvingtonian III, and early Rancholabrean of Bell et al.
(2004) are degraded.
REE Signatures in Fossils
Factors Influencing REE Signatures in Fossils. Extensive discussions concern sources of REE, mechanisms controlling the REE content of fossils, and
the amount of fractionation of REE between fluids
and osteological materials (Bernat 1975; Wright et
al. 1984, 1987; Elderfield and Pagett 1986; Reynard
et al. 1999; Trueman 1999; Armstrong et al. 2001;
Trueman and Tuross 2002; Kemp and Trueman
2003). Biogenic apatite crystals in living organisms
are very small (Lowenstam and Weiner 1989; Weiner and Traub 1992); their structures have low degrees of crystallinity (Person et al. 1995) and contain relatively high concentrations of carbonate,
sodium, and other species. Therefore, biogenic apatite crystallites are relatively soluble and reactive.
During fossilization, the crystallites of biogenic apatite recrystallize (Brophy and Hatch 1962; Person
et al. 1995) and grow. REE and most other trace
elements (TEs) are adsorbed from solutions onto
the biogenic apatite crystal surfaces (Tuross et al.
1989a, 1989b) and then incorporated into the fossilizing material during apatite crystal growth
(Trueman 1999; Armstrong et al. 2001). Henderson
et al. (1983) suggested that the values of distribution coefficients for all of the REE, except perhaps
Ce, are similar (see also Trueman 1999; Trueman
and Tuross 2002). Therefore, signatures and ratios
of REE in fossils should be very similar to the average composition of the fluids from which they
were obtained, particularly if the adsorption coef-
ficients are also large, resulting in near-quantitative
removal of REE from solution (Kemp and Trueman
2003; see however, Reynard et al. 1999).
REE incorporation into the fossil may be largely
accomplished within a few thousand years (Trueman 1999; Patrick et al. 2001; Trueman and Tuross
2002). If fossilization is rapid relative to the sediment deposition rate, the REE signature in the fossil will primarily reflect the surface water chemistry; however, if deposition is rapid, REE may be
taken up primarily from sediment pore waters (Elderfield and Pagett 1986; Kemp and Trueman 2003).
Incorporation of REE, fluorine, and other TEs, and
growth of larger, more crystalline apatite decreases
the solubility, specific surface area, and reactivity
of the resulting fossilized material. Most studies
(see discussion in Trueman 1999; Armstrong et al.
2001) have suggested that once REEs are incorporated in the bone, they are retained and provide a
stable signal reflecting the depositional or early diagenetic environment. Exceptions occur only when
the fossil material is dissolved and reprecipitated
or recrystallized during high-grade metamorphism,
processes that have not taken place in the Fossil
Lake sediments.
If REE signatures in fossil bones are similar to
those of the early diagenetic fluids from which the
REE were obtained, as suggested by the previous
discussion, an understanding of REE signatures in
modern natural waters and how they are affected
by environmental properties and geological variables is required to infer paleoenvironmental conditions. REE concentrations have been measured in
seawater, rivers, lakes, and continental pore and
ground waters (Elderfield et al. 1990; Johannesson
et al. 1993, 1996, 2000; Möller and Bau 1993; Johannesson and Zhou 1999; Dia et al. 2000; Biddau
et al. 2002; and review articles by Byrne and Sholkovitz 1996; Johannesson and Xiaoping 1997). Factors controlling REE concentrations, speciation,
and signatures in natural waters are still a matter
of discussion (Byrne and Kim 1993; Johannesson
and Xiaoping 1997; Johannesson et al. 1995; Tang
and Johannesson 2003). Byrne and Kim (1993) and
Johannesson et al. (1995) suggested that many oceanic and continental waters are approximately saturated with respect to REE phosphate coprecipitates. REE concentrations and signatures in natural
waters appear to be largely controlled by solution
pH and redox; concentrations of complex-forming
ligands, particularly carbonate and possibly organics; sorption on hydrous ferric oxides (HFOs) and
manganese oxides, as well as by the composition
of source rocks or minerals providing REE to fluids
Journal of Geology
FOSSIL LAKE TEPHROCHRONOLOGY
(Johannesson and Xiaoping 1997; Hannigan and
Sholkovitz 2001; Tang and Johannesson 2003).
Some representative NASC-normalized REE signatures are shown in figure 3. Signatures from
acidic environments tend to be middle REE (MREE)
to light REE (LREE) enriched; those from neutral
environments may be MREE-, LREE-, or high REE
(HREE)-enriched or have flat patterns (fig. 3A)
showing no specific enrichment (Johannesson and
Xiaoping 1997). Seawater and alkaline lakes (fig. 3B)
tend to be HREE enriched (Johannesson et al. 1993;
Möller and Bau 1993). The strength of different
REE-ligand complexes varies along the REE series.
For example, HREEs form stronger complexes with
carbonate than do the LREEs (Wood 1990; Haas et
al. 1995) and may thus be more soluble and less
adsorbed than LREE in higher pH alkaline solutions
(Johannesson et al. 1993; Möller and Bau 1993;
Byrne and Sholkovitz 1996). Alkaline lakes are often relatively more HREE enriched than seawater
(fig. 3B) as a result of greater carbonate complexing
(Johannesson et al. 1993). In areas dominated by
silicate rocks, major features of dissolved REE signatures are not strongly affected by rock type. However, river and ground water in limestone terrains
may have LREE-depleted signatures and negative
Ce anomalies that are inherited from the marine
limestones and seawater REE values (Johannesson
and Xiaoping 1997). Dissolution of HFOs or phosphates may also provide an LREE-enriched or
MREE-enriched source for some river and lake waters (Johannesson and Zhou 1999; Hannigan and
Sholkovitz 2001).
Variations of REE signatures of natural waters
and fossils may be represented on a ternary diagram
(fig. 4) with NASC-normalized Yb, Gd, and Nd
(YbN, GdN, and NdN) at the vertices (Patrick et al.
2002b, 2004). Yb is a representative HREE, Gd an
MREE, and Nd an LREE. These elements have been
chosen to facilitate comparisons with previous research on aqueous REE (see Patrick et al. 2002b,
2004 for further discussion). The ternary diagram
allows the basic shape of the REE pattern to be
represented. NASC-normalized samples that plot
in the middle of the diagram (33% YbN, 33% GdN,
and 33% NdN) have equal amounts of these elements and will have flat, shalelike REE patterns.
Patterns plotting near the vertices will be enriched
in that and neighboring REE; for example, patterns
plotting near Yb will be HREE-enriched.
REE compositions of some representative natural
waters are plotted in figure 4. Representative acid
mine drainage (Worrall and Pearson 2001), river,
spring, and circumneutral pH ground waters (Kreamer et al. 1996; Dia et al. 2000; Johannesson et al.
147
Figure 3. North American Shale Composite (NASC)–
normalized rare earth element (REE) signatures for some
representative natural waters from various environments. A, Acidic to circumneutral pH environments. B,
Alkaline lakes and seawater. Reported pH values have
been truncated to one significant figure. References:
Summer, Mono, Abert Lakes (Johannesson et al. 1993);
PZ-264, PJ1-335 (Dia et al. 2002); Army, Grapevine, J-13,
Tippipah (Johannesson et al. 2000); Quaking 4.1, Mullica
(Elderfield et al. 1990); Lake Van (Möller and Bau 1993);
Sardinia sample 5 (Biddau et al. 2002); Seawater (Pacific
average near-surface seawater; Piepgras and Jacobsen
1992).
2000; Stetzenbach et al. 2001; Biddau et al. 2002)
tend to plot in the upper left part to middle of the
ternary diagram as they tend to be LREE enriched
and MREE enriched or have flat patterns. In contrast, seawater (Piepgras and Jacobson 1992) and the
alkaline lakes (Möller and Bau 1993; Stetzenbach
et al. 2001), which are HREE-enriched, plot toward
the lower right (Yb-enriched) portion of the dia-
148
J. E. MARTIN ET AL.
Figure 4. Ternary diagram showing variations of rare
earth elements in terrestrial waters from various relevant
environments. Circumneutral pH (Johannesson et al.
1996, 2000; Kreamer et al. 1996; Stetzenbach et al. 2001),
Alkaline lakes (Johannesson et al. 1993; Möller and Bau
1993), Pacific surface seawater (Piepgras and Jacobsen
1992; most analyses of REE in open ocean waters plot in
this area), acid mine drainage (Worrall and Pearson 2001),
shallow ground water (Dia et al. 2000), Sardinia (Biddau
et al. 2002).
gram. The GdN/YbN ratio in various selected natural waters as a function of pH is shown in figure
5. Although significant scatter occurs in the data,
which may result from differences in concentration
of complex-forming ligands (particularly carbonate), the GdN/YbN ratio generally decreases with increasing pH. Alkalinity also increases, particularly
in waters having pH values greater than about 7;
thus, the decrease in GdN/YbN ratio at higher pH
also reflects increased carbonate concentration and
complexing of the HREE.
Some terrestrial waters have pronounced positive
or negative cerium anomalies (see examples in fig.
3). Negative cerium anomalies, such as that in seawater (fig. 3B), in which the Ce data point lies below the line connecting the La and Pr or Nd points,
are commonly interpreted as resulting from oxidizing conditions (Wright et al. 1984, 1987; German
and Elderfield 1990; Sholkovitz and Schneider
1991). Under oxidizing conditions, cerium may be
oxidized from Ce3⫹ to Ce4⫹ and adsorbed or coprecipitated with iron or manganese oxides or other
phases. Positive anomalies are usually interpreted
as indicating reducing conditions, under which
Ce4⫹ is reduced and released from solid phases into
solution. However, oxidizing highly alkaline lake
waters, such as those of lakes Van and Abert (fig.
3B), may also have positive Ce anomalies, which
may result from the stabilization of Ce4⫹ pentacarbonate complexes in the highly alkaline, high
ionic strength waters (Möller and Bau 1993; Johannesson et al. 1996).
The differences in REE signatures in natural waters, which are a function of environmental pH,
redox, and alkalinity, may be compared with variations of REE signatures in fossils. Because the
fossil REE signatures reflect the original early diagenetic water composition from which the REEs
were obtained, some aspects of the ancient water
chemistry and early diagenetic paleoenvironment
may be inferred.
REE in Vertebrate Fossils. Results of REE analyses
of fossils from the various lithostratigraphic units
at Fossil Lake are given in table 2. Various available
osteological materials were analyzed from each depositional package. REE signatures in cortical bone
materials from fish, mammals, birds, and avian eggshell from sedimentary package 3 are shown in figure 6. Although the concentrations differ, REE signatures from bones of this unit show similar
patterns. This confirms that REE signatures in fossils are not affected by species or diet. Samples were
obtained from three different localities separated by
up to 16 km. This suggests that REE signatures may
be somewhat laterally extensive in lacustrine sediments. Sample locations are not near the probable
margins of pluvial Fort Rock Lake (Allison 1966,
1979), and signatures in fossils from marginal lake
sediments could possibly differ. The REE signature
of a fossil bird eggshell (⫹) is also shown in figure
Figure 5. Variation of GdN/YbN with pH in some representative natural waters. References as in figure 4.
Journal of Geology
FOSSIL LAKE TEPHROCHRONOLOGY
Figure 6. Rare earth element (REE) signatures of various osteological materials from package 3 (unit 5) sampled at sites separated by up to 16 km (see table 2). The
similarity in these REE signatures suggests that they are
laterally similar within stratigraphic units of the Fossil
Lake and that species and osteological material type has
little effect on REE ratios.
6. This sample has REE concentrations similar to
the fossil bones (table 2), consistent with previous
analyses of dinosaur eggshell and bone by Samoilov
and Benjamini (1996). However, this signature differs slightly from the others, having less HREE enrichment. This difference is not surprising, given
the difference in mineralogy (calcite; Lowenstam
and Weiner 1989), but suggests that REE signatures
in eggshells may not provide results consistent
with bone for taphonomic studies or paleoenvironmental studies. Therefore, results from eggshell
have not been used in this study. In a few cases,
some variation in LREE and MREE was also observed in REE signatures from trabecular material.
This seems to be due to difficulty in cleaning matrix from the porous trabecular bone. Therefore, for
this study, only cortical bone, enamel, and dentine,
which produce consistent patterns, were used for
stratigraphic comparisons.
Although REE signatures were similar within depositional packages, they differed between units
(fig. 7). The lower units have generally flat patterns,
with no significant relative REE enrichment,
whereas the upper units have much higher slopes
to their signatures and relatively greater HREE enrichment. Lower units lack significant Ce anomalies, whereas upper units have positive Ce anomalies. The unique signatures in the fossil material
act as fingerprints for each of the defined lithostratigraphic units. Discriminant analysis (Patrick et al.
149
2002a) of this data shows that these REE signatures
from the various units are significantly different,
indicating that these signatures can be used to determine or constrain fossil provenance.
Figure 8 shows an increase in YbN/GdN (HREE/
MREE) ratios up the lithostratigraphic section. Results for different depositional packages are also
shown in a NdN-GdN-YbN ternary diagram (fig. 9).
Samples from the lowest unit plot toward the center of the diagram, indicating a relatively flat signature with equal NASC-normalized concentrations of Nd, Gd, and Yb, as shown in figure 7.
Samples from higher depositional packages plot toward the YbN vertex, indicating progressive HREEenrichment in the fossils. The relatively linear
trend in the data suggests that the variety of signatures in the fossils results from a mixing or evolutionary relationship between two end members,
one near package 1, having a relatively flat REE
signature, and the other near package 8, having a
strongly HREE-enriched signature. As shown in figure 7, this trend is also accompanied by an increase
in the Ce anomaly. Because REEs in fossil bone
Figure 7. Representative rare earth element (REE) signatures in fossils from various units at Fossil Lake,
Oregon. A, Upper units (sedimentary packages 5–8); B,
lower units (packages 1–4).
150
J. E. MARTIN ET AL.
Discussion and Paleoenvironmental
Interpretation
waters in which units 5–8 were deposited were
highly saline.
Therefore, increased HREE enrichment in fossils
coupled with positive Ce anomalies through the
section indicate that waters of pluvial Fort Rock
Lake evolved from circumneutral pH with relatively low salinity and, with minor fluctuations,
became progressively more alkaline, basic, and saline over time. Fossils from the upper units have
REE signatures similar to those of waters from
some modern alkaline lakes, such as Lake Abert,
Oregon, or Lake Van, Turkey (fig. 3B). Such changes
in diagenetic water chemistry might result from
increasing aridity due to progressive decrease in effective precipitation in this area during the later
Pleistocene. These suggestions are consistent with
the faunal record, which exhibits a decrease in taxonomic diversity in the upper units, as well as the
presence of species, such as jackrabbits and sage
grouse, adapted to more arid conditions.
This interpretation may be compared with those
of Allison and others for Fort Rock Lake, as well
as those from other pluvial lakes in southern
Oregon. Allison (1966, 1979, 1982) observed a number of wave-cut lake beach terraces at different levels, which he attributed to stadial-interstadial lake
level fluctuations; however, he was not able to determine any relative or absolute dates for these terraces. After deposition of the uppermost salmonidbearing layer, the lake became isolated and underwent progressive desiccation (Allison and Bond
Circumneutral pH waters tend to have LREEenriched or MREE-enriched or flat patterns, whereas basic and alkaline waters have HREE-enriched
patterns (Johannesson et al. 1993; Kreamer et al.
1996; Johannesson and Xiaoping 1997). Thus, comparison of REE signatures in fossils with those of
modern waters suggests that Fort Rock Lake waters
became progressively more basic and alkaline with
time.
REE signatures in fossils from the lower part of
the section, packages 1–4, have no major Ce anomaly, whereas fossils from the upper part of the section may have large positive anomalies (fig. 8). Positive Ce anomalies may arise from reducing redox
conditions, due to the reduction of Ce4⫹ to Ce3⫹
and release from sediments or to stabilization of
the soluble Ce-pentacarbonate complex in highly
alkaline waters (Johannesson et al. 1993; Möller
and Bau 1993). There is no sedimentary evidence,
such as the occurrence of organic-rich layers, pyrite, or other sulfides, that lake waters from the
upper units were strongly reducing; therefore, the
occurrence of positive Ce anomalies suggests that
Figure 9. Ternary diagram showing variations of rare
earth elements in lithologic units at Fossil Lake, Oregon.
Figure 8. YbN/GdN ratios and cerium anomalies in fossils from depositional packages at Fossil Lake, Oregon.
Lines connect average values for each package or unit.
reflect the composition of lake bottom or pore waters during fossilization, the patterns observed may
be related to changes in water composition and paleoenvironment during deposition of the Fossil
Lake sediments.
Journal of Geology
FOSSIL LAKE TEPHROCHRONOLOGY
1983). Greenspan (1984) suggested that lake waters
had essentially completely disappeared in the Fossil
Lake Basin by about 10,000 yr ago. Lake waters may
have then reappeared, at least ephemerally, during
Neoglacial or other higher rainfall periods (Smith
1926; Allison and Bond 1983). These interpretations could be consistent with a long-term shift
from neutral to alkaline waters with time but provide little additional detail. Palacios-Fest et al.
(1993), Negrini et al. (2000), and Cohen et al. (2000)
have produced detailed paleoenvironmental records for Summer Lake, Oregon, the remnant (with
Lake Abert) of pluvial Lake Chewaucan (Allison
1982; Davis 1985), which covered an area just to
the south of the pluvial Fort Rock Lake, of which
the Fossil Lake Basin forms the easternmost part.
The exposed stratigraphic interval at Summer Lake
(Lake Chewaucan) covers the time period from
about 250,000 yr ago to the present (Negrini et al.
2000; Kuehn and Foit 2001), generally equivalent
to the middle and upper portions (above depositional package 3; fig. 2) of the Fossil Lake sequence.
Paleosalinity estimates for Summer Lake based
on ostracode assemblages suggest moderate to high
salinities from 230 to 190 ka, decreasing salinities
between 190 and 165 ka, and then increasing salinities between 165 and 162 ka, below the base of
a major unconformity (Cohen et al. 2000). Generally very highly saline waters are indicated between
ca. 102 and 80 ka and between 50 and 30 ka, separated by a period of low salinity (Cohen et al. 2000,
their figs. 8, 9, and 17). Late Pleistocene and Holocene waters were apparently highly variable in
salinity, with Holocene waters being highly saline
(Cohen et al. 2000). At present, Summer and Abert
Lakes are highly saline and have basic pH and alkaline waters (Van Denburgh 1975; Johannesson et
al. 1993; Palacios-Fest et al. 1993; Cohen et al.
2000). Johannesson et al. (1993) measured REE signatures of waters from both of these modern lakes
(fig. 3B). It is likely that, given the similarity in
geologic features and stream and spring inputs, waters of pluvial Fort Rock Lake would have followed
a similar evaporitic evolutionary trend (Hardie and
Eugster 1970). Therefore, it seems likely that highsalinity periods in Fort Rock Lake would also produce highly alkaline and high pH waters.
Ce anomalies in fossils from the various lithologic units (fig. 8) may record salinity variations,
such as those found at Summer Lake (Cohen et al.
2000). If the positive Ce anomalies result from high
salinities and alkalinities rather than redox variations, as discussed previously, lake waters may
have had low salinity during deposition of packages
1–5. Thereafter, salinities fluctuated, becoming
151
more highly saline during deposition of package 6
and the lower part of package 7 (unit 13), less saline
during deposition of the upper part of package 7
(unit 14), and returning to highly saline conditions
in package 8, before final desiccation of the lake.
Such salinity variations are broadly similar to those
seen by Cohen et al. (2000) in Lake Chewaucan;
however, given the low resolution of REE sampling
from this study, it is difficult to determine precise
stratigraphic correlations between the REE variations at Fossil Lake and the reported salinity variations at Summer Lake. High-resolution paleoclimate studies, similar to those of Negrini et al.
(2000) and Cohen et al. (2000), should be undertaken at Fossil Lake to determine possible correlations between these two basins. Also, further lateral studies need to be conducted to determine
whether distinguishable variations in the REE patterns from west to east can be seen as a result of
the variations due to proximity of the fossil material to the margins of the lake.
Conclusions
Fossil Lake, Oregon, has been studied extensively
over the last century and is well known for its abundance of Pleistocene fossils; however, reliable dating and stratigraphic correlations were lacking. The
original dates did not agree with the preservation
of the fossil assemblages, and the repeated packages
of sands and gravels grading into clays had not been
previously recognized. This study establishes a lithostratigraphic framework consisting of at least nine
exposed rhythmic depositional packages with approximate ages based on five interbedded tephras.
The tephra layers, which include the Rye Patch
Dam (∼646 ka), Dibekulewe (∼610 ka), Tulelake
T64 (∼95 ka), Marble Bluff (47 ka), and the Trego
Hot Springs (23.2 ka), indicate that the deposits at
Fossil Lake are much older than previously proposed and reveal the ephemeral nature of the Late
Pleistocene lakes in Oregon during the Irvingtonian
and Rancholabrean NALMAs.
The REE signatures of fossil bones are consistent
within the defined units but vary vertically between different units. Therefore, REE signatures
may be used to determine or constrain fossil provenance. Consistency of REE signatures from various fossil types indicates that the REE signature is
not dependent on diet or taxon. REE signatures in
fossils from the lower units were relatively flat;
comparisons with REE patterns in modern waters
suggest near-neutral pH low alkalinity waters. REE
signatures in fossils from the upper units of Fossil
Lake are greatly HREE-enriched and have positive
152
J. E. MARTIN ET AL.
Ce anomalies (fig. 7), similar to signatures from
extant, highly saline alkaline lakes, such as Lake
Abert or Van (fig. 3B). These signatures can be explained by increasing aridity through the Pleistocene, leading to an increase in alkalinity of the lake.
This shift in water chemistry in the upper units is
coincident with a general decrease in diversity of
fossil taxa and those adapted to more xeric conditions, further corroborating the geochemical data.
ACKNOWLEDGMENTS
This research was supported through financial assistance from the United States Bureau of Land
Management, Temple University, and the Desert
Research Institute. W. Cannon, District Archaeol-
ogist, Lakeview District, Bureau of Land Management, has been particularly instrumental in overseeing the long-term project to completion. R.
Hanes of the Oregon State Office, Bureau of Land
Management, who is in overall charge of the paleontology of the area, has guided our project over
the years. Many students and colleagues have cooperated in field studies during the past 15 years,
particularly W. and B. Harrold, who have participated in nearly every field expedition. We thank P.
Field and L. Bolge for aid with the ICP-MS analyses.
The manuscript was improved by reviews by R.
Feranec, A. Sarna-Wojcicki, and C. Trueman. We
particularly thank A. Sarna-Wojcicki for sharing
unpublished age estimates of certain tephras.
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