Geosciences 2019, 9, 35; doi:10.3390/geosciences9010035 www.mdpi.com/journal/geosciences
Mineralogy of Eocene Fossil Wood from the
“Blue Forest” Locality, Southwestern Wyoming,
George E. Mustoe
*, Mike Viney
and Jim Mills
Geology Department, Western Washington University, Bellingham, WA 98225, USA
College of Natural Sciences Education and Outreach Center, Colorado State University,
Fort Collins, CO 80523, USA; email@example.com
Mills Geological, 4520 Coyote Creek Lane, Creston, CA 93432, USA; firstname.lastname@example.org
* Correspondence: email@example.com
Received: 7 December 2018; Accepted: 7 January 2019; Published: 10 January 2019
Abstract: Central Wyoming, USA, was the site of ancient Lake Gosiute during the Early Eocene.
Lake Gosiute was a large body of water surrounded by subtropical forest, the lake being part of a
lacustrine complex that occupied the Green River Basin. Lake level rises episodically drowned the
adjacent forests, causing standing trees and fallen branches to become growth sites for algae and
cyanobacteria, which encased submerged wood with thick calcareous stromatolitic coatings. The
subsequent regression resulted in a desiccation of the wood, causing volume reduction, radial
fractures, and localized decay. The subsequent burial of the wood in silty sediment led to a
silicification of the cellular tissue. Later, chalcedony was deposited in larger spaces, as well as in the
interstitial areas of the calcareous coatings. The final stage of mineralization was the precipitation
of crystalline calcite in spaces that had previously remained unmineralized. The result of this
multi-stage mineralization is fossil wood with striking beauty and a complex geologic origin.
Keywords: fossil wood; Blue Forest; Eden Valley; Lake Gosiute; chalcedony; quartz; calcite;
stromatolite; Green River Basin; Wyoming
This report describes the fossil wood preserved in Eocene lakebed sediments in southwestern
Wyoming, USA. The purpose of our research is to investigate the fossilization processes that
produced the complex mineralogy of Blue Forest wood. Taxonomic and paleoecology studies of
stratigraphically similar sites within the Green River Formation provide insights into the
paleogeography and paleoclimate associated with the Blue Forest. Fossil wood from the region is
sometimes referred to as coming from Eden Valley, a broad basin that does not have well-defined
boundaries. In this report, we use the locality name “Blue Forest” to describe an area that has long
been known to petrified wood aficionados as the Blue Forest. The name comes from the bluish-gray
color of the chalcedony that fills the open spaces in many specimens. The fossil wood has a complex
mineralogy, resulting from environmental changes in the ancient lake basin, and the subsequent
2. Geologic Setting
The beginning of the Green River basin can be traced to the Laramide Orogeny, a tectonic
episode that began in the Late Cretaceous and ended in the late Eocene. From the late Paleocene to
the middle Eocene (55–38 MA), the Green River Lake System covered large areas of western USA,
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having one of the longest durations of any known lake system . During the middle early Eocene,
three separate lakes coexisted—Fossil Lake, Lake Uinta, and Lake Gosiute—each lake occupying a
down-warped basin that accumulated sediment from the adjacent highlands (Figure 1). Lacustrine
deposition was mostly a continuous process, occurring on broad flood plains and complicated by the
transgressions and regressions caused by tectonic subsidence, climate change, and episodic volcanic
activity . By the late middle Eocene, only Uinta Lake remained, much reduced in size.
Figure 1. Green River Lake basin map modified from .
The fossil wood described in this report is associated with ancient Lake Gosiute. This lake
persisted for at least five million years, from approximately 53 to 49 MA. This history is recorded by
approximately 838 m of lacustrine deposits, predominately composed of limestone, sandstone, and
shale . Clastic sediments are rich in volcanic material transported by streams from the Absoroka
Volcanic Field in northwest Wyoming, USA. The basic structure of the region remained unchanged
during the life of Lake Gosiute, but the sediments record episodes of transgression, regression, and
climate change. During times of regression, the lake water commonly became saline, as evidenced
by the abundant evaporite minerals found in some of the stratigraphic members. From a regional
geologic perspective, the original Lake Gosiute lake basin was subsequently divided into two
structural basins that are now separated by the Continental Divide. Fossil wood is preserved in both
the Green River Basin and the Great Divide Basin. Five popular collecting localities (Figure 2) are
located in the Lake Gosiute lacustrine strata within the upper Green River Formation and the
overlying Bridger Formation (Figure 3). The detailed geology of this region can be found in a recent
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Figure 2. Fossil wood localities: (1) Big Sandy Reservoir; (2) Oregon Buttes; (3) Blue Forest; (4) Parnell
Draw; (5) Hay’s Ranch.
The upper members of the Green River Formation are overlain by and interfinger with mostly
the fluvial sediments of the Bridger Formation. The younger Bridger Formation beds are dominated
by limestone, but the oldest beds are rich in volcaniclastic sediment transported from the Absoroka
Volcanic Field, and resemble the composition of the youngest Lake Gosiute sediments. For this
reason, contacts between the Green River and Bridger Formations may not be readily
Figure 3. Generalized stratigraphy of the Eocene sediments in the Green River basin, showing the
relative position of the fossil wood localities. (1) Big Sandy Reservoir; (2) Oregon Buttes; (3) Blue
Forest; (4) Parnell Draw; (5) Hay’s Ranch. This diagram is a simplification; stratigraphic members
typically have interfingering contacts.
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Taken together, these three lake basins represent an important Fossil Lagerstätten that provides
a window into early Paleogene freshwater ecosystems (Figure 4). The paleontology of the Green
River Formation has been reviewed by Grande [1,7], who emphasized the Fossil Lake deposits. The
three Green River Lakes have distinctive fossil assemblages. Fossil fish were reported as early as
1856, and by the 1870s, commercial quarrying had begun at several Fossil Lake localities, an activity
that continues today. Fossil Lake was originally described as “unnamed Green River Lake west of
Gosiute Lake” . Despite its small size and brief duration, Fossil Lake is a prolific source of
vertebrate fossils. In 1972, Fossil Butte National Monument was established to protect a small part of
the fossil beds. Fossil fish are the best-known Green River Formation fossils, but other vertebrate
remains include birds, reptiles, and mammals. The Lake Uinta sediments include extensive shale
deposits that locally preserve abundant leaf fossils and pollen, as well as fossil insects and shorebird
Figure 4. Green River Formation fossils. (A) Palm frond, Sabalites powelli. Photo courtesy of Richard
Dillhoff. (B) Stingray, Heliobatis radians. (C) Fish, Priscacara serratus. (D) Sycamore leaf, Macginitea
The sediments from Lake Gosiute preserve leaf impressions and vertebrate remains, but the
most abundant fossils are silicified logs and limbs—the subject of this report. Lake Gosiute has been
described as a playa lake complex [19,20]. The sediments record large fluctuations in the position of
the shoreline. At times the broad, shallow lake became quite saline. Eutrophic conditions limited fish
habitats, favoring suckers and catfish , in marked contrast to the diverse fish populations that
flourished in the waters of Fossil Lake. Thick algal mats covered much of the lake bottom several
times during the lake’s history . As described later, the proliferation of algae and cyanophytes
was a factor in the fossilization process that preserved ancient wood.
Lake Gosiute was at its greatest areal extent during the time of the deposition of the Laney Shale
Member . The subsequent reductions in the size of Lake Gosiute may have been caused by an
influx of alluvial sediment and/or a reduction in precipitation. The final limnological evolution of
the lake has been interpreted as a transition from a saline, alkaline lake to a freshwater lake,
representing a change from a closed basin to an open basin . This change to a more fluvial
environment is evidenced by the lacustrine deposits of the Bridger Formation. The lake became
smaller and shallower, episodically filled by volcaniclastic deposition, then reappearing when the
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basin subsidence exceeded sedimentation [5,23]. Tectonic activity may have also played a role in the
sedimentary processes .
The final demise of Lake Gosiute occurred in the middle Eocene at ca. 44 Mya [1,7]. This
phenomenon was partly related to the delivery large volumes of volcaniclastic sediment caused by
eruptions in the Absoroka volcanic field, in what is now southwestern Montana and northwest
Wyoming, USA . In the source area, volcaniclastic mudflows preserved the extensive fossil
forests at Yellowstone National Park ; the mineralogy of these fossil woods is relatively simple,
where silicified trunks are preserved in an upright position. In contrast, the approximately
contemporaneous fossil forest of southwestern Wyoming shows multistage mineralization caused
by the rapid environmental changes that occurred along the shoreline of a shallow lake. Ultimately,
the demise of the Green River basin lakes was related to a drying trend. The early middle Eocene
Lake Gosiute fossil woods have subtropical affinities, but Late Eocene leaf and pollen fossils from
southern Wyoming indicate a warm temperate paleoclimate . Increased warming and decreased
precipitation continued through the late Cenozoic period, causing the original subtropical
environment to transition to a warm temperate climate regime, gradually progressing to the arid
desert conditions that presently exist in the region once covered by Lake Gosiute . By then, the
remains of the forests of the ancient lake shore had become petrified, protected by a blanket of
younger sediments, awaiting their eventual discovery in the modern era.
The composition of the Blue Forest fossil woods was evaluated using a variety of laboratory
methods. SEM photomicrographs were made using a Tescan Vega SEM at Western Washington
University, using fractured specimens mounted on 1 cm diameter aluminum mounts using an epoxy
adhesive, and sputter-coated with Pd. The mineral composition of the fossil woods was confirmed
using the major element analysis obtained from the SEM samples using an EDAX energy-dispersive
spectrometer. Optical photomicrographs were made using a Zeiss petrographic microscope
equipped with a 5-megapixel digital camera. The density of the fossil wood and percent relict
organic matter were calculated using a previously described method . This technique determines
the relict organic matter based on the weight loss of the powdered fossil wood following heating at
450 °C. The percentage of preserved organic matter is calculated relative to the estimated density of
the wood prior to mineralization, based on the densities of the modern wood genera. The calcium
carbonate content of the fossil wood and stromatolite were determined by measuring the amount of
carbon dioxide released when the powdered specimens react with HCl in a sealed chamber .
Trace elements were measured using an Agilent 7500ce ICP-MS spectrometer with a New Wave
UP213 laser ablation system at the Western Washington University Advanced Materials Science and
Engineering Center. The following elements and mass numbers were analyzed: Ti (48), V (51), Cr
(52), Mn (55), Fe (56), Co (59), Ni (60), and U (238). These elements were selected because they are the
most common pigments for determining mineral color. Laser beam diameter was 55 micrometers,
with the light output set at 55% of the full power. Six 2-mm long parallel lines were laser-etched on
the sample surface. For each laser line, the data were collected as 15 replicate measurements for each
element. The data were averaged, applying background corrections on the data from the blank
samples. The PPM concentrations were calculated using the U.S. Institute of Standards and
Technology glass reference samples, NIST 610 and NIST 614, for calibration. The results were
calculated using Microsoft Excel.
5. Previous Studies
Because of the interfingering nature of stratigraphic members, lack of continuous outcrop
exposure, and scarcity of stratigraphic marker beds, the correlation between various fossil wood
localities is somewhat uncertain. However, at some localities that have a clearly evident
stratigraphy, the silicified wood is preserved at multiple levels. Examples include Bridger Formation
exposures at Oregon Buttes, representing a 203 m stratigraphic section. Tree stumps are preserved in
an upright position in a 1.5 m-thick claystone bed that lies just above the contact with the underlying
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Green River Formation Laney Shale Member. The younger Bridger Formation strata include a 32-m
thick siltstone/claystone sequence that preserves the in situ stumps, and a 0.6 m stratum that
contains stromatolite-coated fossil wood . This stratigraphic distribution of fossil wood was
recognized as evidence that the preservation of ancient wood resulted from repeated episodes of
transgression and regression of the ancient lake, not a single event.
Green River Formation fossils have been known to geoscientists from as early as the late 1800s.
By the 1930s, petrified wood enthusiasts were searching Lake Gosiute localities, but the first serious
study dates to 1954, when Kruse  described 11 new species of angiosperm wood from specimens
collected at the Hays Ranch locality, in northern Eden Valley (Figures 2 and 3, location 5).
Subsequent investigations have also focused on the fossil woods from the northern part of the Green
River basin, near the contact of the upper Green River and lower Bridger Formations. The
most-studied localities are near Big Sandy Reservoir (Figures 2 and 3, location 1). Silicified palm
trunk (Palmoxylon) is a common floral element, comprising three species [32,33]. More recent studies
of the Big Sandy Reservoir revealed four angiosperm taxa in addition to a single species of
Palmoxylon . A greater floral diversity was observed at a slightly older site at Parnell Draw
(Figures 2 and 3, location 4), where 17 taxa were recognized, including both conifers and
angiosperms . These studies provide insights for understanding the middle Eocene plant
communities that bordered Lake Gosiute. The abundance of palm trunk tissue, and the presence of
dicotyledonous woods with subtropical affinities are evidence of a warm paleoclimate in central
Wyoming during the late Early Eocene . The deposition of the Green River basin Eocene rocks
has long been recognized as occurring during warm temperate, subtropical, and tropical climate
conditions . The palynology of early Eocene samples suggests a humid subtropical to warm
temperate climate, with summer rainfall and only mild frost, with an estimated mean temperature of
12.8 °C (55·F) . Alternately, the climate of the basin ﬂoor during this time may have been
The paleoflora of the Blue Forest deposit has not been studied in detail, but the general
characteristics suggest that the habitat conditions were somewhat different from the
above-mentioned locations. Palms appear to have been absent, conifers were rare, and angiosperms
had a low diversity. Many specimens were Schinoxylon or Edenoxylon. Both genera are considered
members of the Anacardaceae (Cashew) Family. Other taxa include various unidentified dicots and
a conifer. The taxa reported from various sites in the Eden Valley area are listed in Table 1. These
data are included to provide a regional paleobotanical overview, because of the scarcity of
taxonomic information for Blue Forest specimens.
Table 1. Known plant taxa from southwestern Wyoming localities.
HAY’S RANCH: SE of Big Sandy Reservoir
BIG SANDY RESERVOIR UF327: NE of Big Sandy Reservoir
Arecaceae (Palmae) 
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PARNELL DRAW: 42 km east of Farson, WY
Cf. Laurinoxylon stickai
Cf. Mastixia sp.
spp. (7 unknown taxa) Unknown 
Our studies focus on the fossil wood from the Blue Forest beds that lie within the Laney Shale
Member (locality 3, as shown in Figures 2 and 3). This site, which is the most popular collecting
locality for amateur collectors, has received little attention from paleobotanists. The scarcity of
research is the result of the following two factors: the low-angle host rocks are covered by desert soil,
and the era when fossil wood could be collected from the surface is long past; specimen discovery
now usually requires laborious excavation (Figure 5). An additional disadvantage is that the
stratigraphic relationship of individual excavation pits is seldom known with certainty. The rewards
of studying the Blue Forest site are that wood specimens display a complex mineralogy, great
beauty, and have taxonomic compositions that may be dissimilar to other Lake Gosiute fossil wood
Figure 5. Blue Forest site. (A,B) Trenches excavated during 2018 field work. (C) Partially exposed
horizontal log, showing thick stromatolitic coating. Photos by MikeViney.
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In the early era of specimen collecting, upright tree trunks were observed at the southwestern
Wyoming sites. These fossils were particularly common in areas of pronounced topographic relief,
which provide access to extended stratigraphic sequences. For example, in a 203 m stratigraphic
section of the Bridger Formation at Oregon Buttes (Figures 2 and 3), upright stumps are preserved in
the lowest depth of 1.5 m, and in a 32-m thick section that begins at 108 m. At 148 m above the basal
contact, a 0.7 m sandy stratum contains algal-covered wood . The Palmoxylon specimens found
near Big Sandy Reservoir all came from upright stems . At Blue Forest, a gentle topography and
shallow inclination of bedding reduce opportunities for observing specimens in a stratigraphic
context. Anecdotal reports indicate that upright trunks were discovered by early collectors at Blue
Forest, but at present, intact specimens only occur in the subsurface. These fossils typically have
horizontal orientations, suggesting that they represent woody debris that was submerged on the
lake bottom. The common preservation of bark suggests that the wood has not been transported a
long distance, as evidenced by the lack of abrasion. This taphonomy has important implications; the
presence of stramatolite-coated upright trunks suggests that fossilization was related to changes in
lake level that caused the inundation of a standing forest. The abundance of sub-horizontal trunks
and limbs may have resulted from the toppling of dead trees, as well as the delivery of botanical
debris from the adjacent watershed.
6.2. Stromatolitic Coatings
A striking characteristic of wood from southwestern Wyoming, particularly at Blue Forest, is
the thick casing of biogenic calcium carbonate that surrounds silicified wood (Figures 5B, 6 and 7).
These coatings commonly show stromatolitic layering, appearing to have originated when living
microorganisms were subject to precipitation of calcium carbonate. Later, open spaces in the
carbonate were filled with chalcedony (Figure 8). Quantitative analyses show the microbial coatings
on fossil limbs consist of ~60 wt. % CaCO
; no CaCO
was detected in the surrounding sandy
sediment, convincing evidence that the calcareous material was of a biogenic origin, and not from
later inorganic precipitation.
Stromatolites are abundant in the Lake Gosiute deposits, including individual units that have a
regional extent (Surdam and Wray 1976). A 1928 report  described “algal reefs” in the Green
River Formation, noting their resemblance to the deposits made by modern cyanophytes. One
unicellular microbe was identified as Chlorellopsis coloniata, first named for a morphologically similar
fossil organism from the Miocene lakebed deposits in the Rhine graben, Germany . A
filamentous Green River microbe was named Confervites mantiensis) . Bradley  included a
photograph of wood surrounded by C. coloniata, but modern SEM images show that both unicellular
and filamentous organisms are preserved in the carbonate layers that enclose silicified wood (Figure
9). Ostracod shells are commonly preserved in Lake Gosiute sediments, sometimes occurring in the
interstices of the stromatolitic carbonate layers (Figure 10). The significance of these algal coatings is
discussed later in this report.
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Figure 6. Freshly-excavated Blue Forest specimens. (A) Log coated with silicified biogenic calcareous
material. (B) Limb surrounded by a stromatolitic layer. (C) Slender limb, fractured during
excavation. Photos by Mike Viney, 2018.
Figure 7. (A–D) Fossil limbs encased in stromatolitic coatings.
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Figure 8. Calcareous stromatolites with chalcedony fillings. (A) general view. (B,C)
Chalcedony-filled zones in are visible in magnified images.
Figure 9. SEM images of silicified stromatolites, showing the presence of (A) filamentous and (B)
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Figure 10. Ostracods in stromatolitic calcium carbonate from Lake Gosiute. (A,B) Stromatolite with
ostracod valves in spaces between algal masses. (C,D) High-magnification views of individual
6.3. Wood Silicification
At the Blue Forest site, fossil wood is commonly found buried at depths of about 1 m, covered
by a combination of surface alluvium and silty shale. In areas where the shale is weathered, large
specimens are sometimes detected using metal rods as probes; more commonly, discoveries are
made by random excavations that may or may not be productive.
Fossil wood specimens range in size, from small limbs to logs with diameters of up to
approximately 0.5 m. Regardless of size, fossil wood is typically enclosed within a thick rind of
calcareous stromatolites. Cellular tissues are mineralized with chalcedony (Figure 11). Some
specimens show visible evidence of partial degradation of the wood prior to mineralization (Figure
12). In a few instances, the wood has been almost entirely destroyed, leaving only a thin bark layer
surrounding a cast of the interior wood (Figure 13). As described later, the secondary mineralization
includes zones of banded chalcedony and sparry calcite.
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Figure 11. Thin section showing well-preserved dicotyledenous angiosperm wood. (A) Transverse
section showing annual rings and abundant vessels. (B) Ordinary transmitted light illumination. (C)
Polarized light view, showing vessels filled with chalcedony.
Figure 12. Fossil wood showing evidence of extensive decay prior to mineralization. (A) Transverse
view showing silicified stromatolite coating, and a large chalcedony-filled fracture. (B) Transverse
thin section, showing extensive chalcedony enclosing wood fragments. (C) Higher magnification
shows the degradation of wood prior to fossilization.
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Figure 13. Eocene limb casts from southwestern Wyoming, USA. (A) Dark silicified bark surrounds
banded chalcedony cast; Big Sandy reservoir locality. (B) Dark peripheral zone, inferred to be relict
bark, encloses a limb cast, consisting of a thick chalcedony layer and a central area of crystalline
quartz; Blue Forest locality.
SEM images show cells mineralized with chalcedony. However, a high magnification shows the
material to have a sub-spherical microtexture reminiscent of opal-A lepispheres (Figure 14). This
architecture suggests that chalcedony may have formed from the transformation of an opal
precursor. The solid-state transformation of opal to chalcedony during wood petrifaction has
previously been reported from various other sites [40–45]. Mineralization was facilitated by the
porosity of the enclosing clastic sediment (Figure 15), which allowed for the flow of mineral-bearing
groundwater. Abundant volcaniclastic material provided a source of soluble silica.
Figure 14. SEM images. (A, B) Longitudinal view showing silicified cells. (C) Chalcedony, showing
relict opal lepispheres. (D) Tangential view, showing ray cells that are partially open (upper arrow),
but with some cells containing crystalline silica (lower arrow) that precipitated during a later
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Figure 15. SEM images of siliceous silty shale. (A) Poorly sorted fine-grained sediment. (B) Note the
porosity caused by interstitial spaces, and the encrustation of clast surfaces as a result of diagenesis.
6.4. Relict Organic Matter
Eocene wood from southwestern Wyoming, USA, contains relatively large amounts of relict
organic matter compared with the small amounts present in most silicified wood . This organic
material causes a reduction in density, causing the fossil wood to retain permeability. The dark
brown fossil wood readily bleaches to a lighter color, a phenomenon observable in many specimens
(Figure 16). Under laboratory conditions, the bleaching can be caused by a 450 °C heating or
treatment with 5% sodium hypochlorite laundry bleach. The degree of preservation of the original
organic matter can be quantitatively estimated based on the weight loss after 450 °C heating,
comparing the weight loss to the presumed density of the wood prior to mineralization, based on the
comparative density values for modern woods. The data can be understood this way: for silicified
wood that preserves 10% relict organic matter, 90% of the original wood has been destroyed during
the fossilization process. Table 2 listss the data for 56 angiosperm specimens: five dicots from Blue
Forest and a monocot (palm) from the northern part of the Green River Basin. In comparison, the
calculated percentage of relict organic matter in the chalcedony-mineralized woods from 11 other
Cenozoic sites in North America ranged from 0.65% to 9.11%, with a mean of 3.70% .
Table 2. Relict organic matter.
Sample Type Calculated
% of Original
2.42 2.94% 0.60 7.1%
2.39 6.19% 0.60 14.8%
2.31 5.96% 0.60 13.8%
2.52 0.97% 0.60 2.4%
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Figure 16. Bleached zones. (A–E) The dark color of the Blue Forest fossilized wood is caused by relict
organic matter. In many specimens, partial bleaching was caused by the permeation of groundwater
into the porous fossil wood.
6.5. Secondary Chalcedony
Blue Forest fossil wood originated as silicified stem tissue enclosed within a thick biogenic
calcium carbonate rind, but the specimens commonly show evidence of other stages of mineral
precipitation. These include spaces that contain layered or botryoidal chalcedony. This chalcedony
occurs as coatings around limbs, where the outermost layers of the stem tissue (bark) adhered to the
algal coating, while the inner region shrank to a smaller diameter. The resulting void is commonly
filled with chalcedony (Figure 17).
Figure 17. Secondary chalcedony. (A,B) Chalcedony filling peripheral zone caused by wood
shrinkage. (C,D). Polarized light optical photomicrographs show separation of wood as a result of
desiccation. Arrows show the bark layer, which remained attached to rock matrix.
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Chalcedony commonly occurs in the open spaces in voids created by shrinkage prior to
silicification, and in angular zones created by the brittle fracture of silicified wood. Polarized light
photomicrographs show that this chalcedony was typically precipitated in multiple episodes,
producing multi-layered structures (Figure 18).
Figure 18. Polarized light optical photomicrographs of chalcedony in fractures. (A) Two layers of
chalcedony fill an angular fracture. (B) In this shrinkage zone, the outer chalcedony layer shows a
botryoidal form (arrows), the inner zone consist of polygonal sectors with a radiating fibrous
Chalcedony also commonly fills large fractures and decayed areas (Figure 19).
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Figure 19. Banded chalcedony filling shrinkage cracks and decayed areas in a Blue Forest limb.
6.6. Crystalline Quartz
Chalcedony is the most common material filling fractures and voids, but crystalline quartz is
present in some specimens. These quartz crystals were typically deposited on cavities that are lined
with a layer of chalcedony (Figure 20). This morphology may be evidence of multiple silica
precipitation events; the rapid deposition of chalcedony from groundwater that contained relatively
high levels of dissolved silica, and a later precipitation event when low levels of dissolved silica
allowed for well-ordered crystals to develop at a slow rate. This phenomenon has been described for
fossil wood from other locations .
Figure 20. Coarsely-crystalline quartz fills large voids in some Blue Forest specimens. (A) Quartz
filling interior spaces in decayed wood. (B) Quartz was also precipitated in the peripheral spaces,
where thin layers of chalcedony provide a substrate.
6.7. Crystalline Calcite
The presence of coarsely-crystalline calcite is a striking characteristic of Blue Forest fossil wood
(Figure 21). The mineral occurs as a filling material for large void spaces. Calcite commonly occurs as
an over-coating on chalcedony layers, but chalcedony has not been deposited on calcite. These
characteristics suggest that calcite precipitation represents the final stage of mineralization. Evidence
supporting this multistage mineralization includes specimens that have been extensively silicified,
but still contain open regions that could later be sites for the precipitation of calcite or quartz crystals
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(Figure 22). The only requirement would be the existence of a crack or porosity that would allow for
an influx of mineral-bearing groundwater.
Calcite is often dissolved by amateur collectors, because they seek to show the maximum
amount of the blue chalcedony in their finished polished display specimens. Experience has taught
them that the elimination of the yellow sparry calcite will often reveal the underlying botryoidal
chalcedony, a feature felt by many collectors to be superior to the color contrast provided by the
original calcite infilling of the cavity. Thus, yellow calcite is not seen in collections as often as it may
have occurred in the original specimens.
Figure 21. Yellow calcite crystals occur in void spaces within the Blue Forest fossilized limbs, and in
the perimeter zones. (A,B) Calcite overlays thin chalcedony (blue arrows). (C) Unidentified dicot
wood with large area of banded chalcedony enclosing calcite (red arrow).
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Figure 22. Blue Forest limb cast containing a large central void, a potential site for the future
precipitation of calcite or quartz crystals.
6.8. Trace Element Geochemistry
Trace elements determinations were performed for Blue Forest materials that included calcite,
chalcedony, and two fossil woods Table 3. The significance of these data is considered in the
Table 3. Trace elements in Blue Forest geologic materials (PPM).
Sample Ti V Cr Mn Fe Co Ni Cu U
calcite 4 2 1 7545 458 3 2 4 0
chalcedony 47 28 34 186 1087 3 6 54 1
wood #2 133 699 22 171 675 2 1 25 4
The complex mineralization of the Lake Gosiute fossil wood specimens is as a result of the
environmental changes in the ancient lake basin, combined with subsequent diagenetic processes.
The abundance of fossil wood specimens is evidence of the lakeside forests that flourished during
the early Eocene. As noted above, taxonomic compositions vary among the various collecting
localities, indicating that the ancient plant communities were controlled by local habitat conditions.
These variations may be related to small differences in geologic age, because Lake Gosiute was
experiencing rapid rates of change as a result of climate, volcanic activity, and tectonism. However,
several generalizations can be made. The Green River paleofloras are typically subtropical in
character, as evidenced by the abundant palm fossils, as well as the affinities of many
dicotyledenous taxa with plants that currently inhabit warm, wet environments.
7.1 Fossilization Processes
The occurrence of fossil wood at multiple stratigraphic levels in the well-exposed strata at
Oregon Buttes  is evidence that the conditions required for wood petrifaction existed multiple
times during the history of Lake Gosiute. This interpretation is supported by the differing
stratigraphic positions of other Eden Valley fossil wood occurrences (Figure 3). For these sites, a
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preliminary step in the fossilization process was the development of lakeside forests. The existence
of abundant plant life is easy to explain, given the subtropical climate and fertile volcaniclastic-rich
The lack of surface bedrock exposure at Blue Forest limits stratigraphic observations. However,
Eocene fossil wood occurrences at other southwestern Wyoming locations include in situ stumps
enclosed within lacustrine sediments, clear evidence that rising lake waters produced drowned
forests. This interpretation is supported by the thick coatings of calcareous stromatolite that occur on
almost every specimen. Modern lakes provide analogs for this phenomenon. In his pioneering
studies on Green River Formation algal reefs, Bradley [38,45] compared the deposits to the modern
biogenic calcium carbonate deposition at Green Lake, New York, USA, where coatings form on
fallen trees along the lake shore. This mineral precipitation occurs in association with cyanophytes
(formerly known as “blue-green algae”), which grow in lake water saturated with calcium carbonate
[46,47]. Ancient examples include Miocene and Pleistocene microbial travertines in southern
Germany , and an Upper Jurassic fossil forest at Lulworth Cove in southern England. The latter
example consists of upright tree trunks that were entombed in calcareous stromatolites in a shallow
hypersaline marine basin . The general aspects of the calcium carbonate deposition in aqueous
environments were reviewed in 1996 . At Gosiute Lake, regionally significant stromatolitic
carbonate deposits have thicknesses that range from 1.5 to 15 m, including nine to twelve units
within the Laney Shale Member, the stratigraphic host for the Blue Forest. The stromatolite
deposition has been interpreted as resulting from a rise in the lake level along a shoreline where the
topographic gradient was only 16–33 cm/km . At a low gradient, a small transgressive event
could potentially produce an extensive area of drowned forest, where trees are killed by having their
roots immersed in alkaline water.
The thick microbial coatings are evidence that the trees and fallen branches became entombed
in a protective calcareous layer prior to wood petrifaction. However, this mineralization appears to
have occurred following the regression of the lake level, exposing the organic material to subaerial
conditions. This interpretation is based on the desiccation features observed in many Blue Forest
specimens. These features include tissue shrinkage, which produced extensive radial fractures, and
empty spaces along the peripheries of many specimens (Figures 17–19). Areas of tissue decay also
suggest subaerial exposure, which facilitates fungal degradation.
7.2. Sequential Mineralization
7.2.1. Step 1:Wood Silicification
Mineralization began after the desiccated algal-coated wood was buried in clastic sediment. The
permeability of the algal carbonate allowed for the penetration of silica-bearing groundwater. The
silty sediment was likely a product of fluvial transport; the abundance of volcaniclastic material
provided a ready source of soluble silica. As noted previously (Figure 9), the initial silica
precipitation may have been in the form amorphous opal (opal-A); the present chalcedony
composition is presumably the result of a diagentic transformation.
7.2.2. Step 2: Secondary Chalcedony
Botryoidal or banded chalcedony that occupies large internal zones, or in the form of peripheral
layers, represents a later stage of mineralization. This chalcedony, which occurs as banded layers
and as botryoidal coatings, appears to have originated as a primary precipitate that formed after the
cellular tissue was already silicified. This episodic style of mineralization may have been caused by
fluctuations in the water table, which, in a shallow basin, would have been primarily controlled by
fluctuations in the lake level. The silicification of interstitial spaces in the calcareous stromatolites
(Figure 9) may have occurred at this time.
7.2.3. Step 3: Calcite Crystallization
Geosciences 2019, 9, 35 21 of 26
The final stage in wood petrifaction occurred when crystalline calcite was deposited in the
remaining open spaces. The formation of this calcite presumable represents a slow rate of
precipitation, allowing time for the development of relatively large crystals. Evaporative carbonate
precipitation was particularly likely in the mudflats bordering the lake, where the sediments were
saturated with saline/alkaline lake water . The switch from silicification to calcite precipitation in
the wood may have been as a result of changing geochemical gradients; the conditions favoring the
precipitation of the two elements are related to pH (Figure 23). The chemical conditions may have
been highly localized, related to the unique environment of the wood as it simultaneously
experienced degradation and preservation. The loss of >85% of the original organic matter (Table 2)
is evidence of degradation that may have produced pH and Eh gradients during the process of
mineralization; the detailed preservation of cellular morphology is evidence that this tissue loss
occurred at a gradual rate, which allowed for the anatomical features to be replicated by silica. The
absence of secondary calcite or chalcedony in the silty matrix demonstrates that the precipitation of
both minerals was limited to the entombed wood.
Figure 23. Solubility of calcite and silica at 25 °C. Data adapted from [52,53]. Reprinted from .
Sawn specimens show channels that were conduits for the entry of silica-bearing solutions from
an external source, resulting in the precipitation of chalcedony in peripheral spaces and interior
zones. Crystalline calcite occupies chalcedony-lined voids (Figure 24).
Figure 24. Transverse section of fossil wood, showing sinuous chalcedony-filled channels (arrows),
and sparry calcite filling a crescent-shape peripheral zone.
Geosciences 2019, 9, 35 22 of 26
7.2.4. Step 4: Quartz Crystallization
Like calcite, zones of clear quartz crystals developed in the spaces that remained open during
the earlier stages of mineralization (Figures 21 and 23). The formation of quartz rather than
chalcedony likely occurred as a result of lower levels of dissolved Si in groundwater, resulting in
slower precipitation rates, which provided time for the silica to develop a well-ordered lattice
structure. Similar mineral transitions are common in geodes, where crystal-lined central cavities
have developed on layered chalcedony substrates .
Not every specimen exhibits the full mineralization sequence. Secondary chalcedony is
ubiquitous, calcite crystals are common, and zones of quartz crystals are observed less frequently.
When present, the combination of all of the mineralization phases may produce specimens of a great
complexity (Figure 25).
Figure 25. Blue Forest slab containing multiple mineral phases. A = algal coating; CA = calcite; CL =
chalcedony; E = empty space; W = silicified wood; Q = quartz.
7.3 Evidence from Trace Elements
Trace element abundances provide evidence for the origin of color of the mineral phases, and
for the multiple-stage mineralization processes. Crystalline calcite in the Blue Forest specimens
ranges from yellow to yellowish orange in color. Analyses of a typical sample reveal that the color is
likely to be caused by elevated levels of manganese, with iron also playing a role (Table 3).
Geosciences 2019, 9, 35 23 of 26
Manganese commonly occurs in association with calcite (e.g., manganoan calcite). These specimens
are typically light pink , suggesting that the yellowish color of the Blue Forest calcite is
influenced by the presence of Fe. Translucent blue-gray chalcedony perhaps owes its color to optical
effects, but elevated levels of Fe suggest that this element may be a pigment. The colors produced by
iron are determined by the oxidation state and elemental abundance, but a wide range of colors may
be produced . As noted above, relict organic matter may be a cause of the dark brown color of the
silicified wood. However, elevated levels of V, Mn, Fe, and Ti (for sample 2) suggest that these trace
elements may contribute to the color. However, the susceptibility of bleaching of the fossil wood
suggests that relict organic matter is the most important pigment.
The trace element variations support multiple episodes of mineralization. The initial
silicification of wood tissue involved silica-bearing groundwater that also contained Ti, V, Mn, and
Fe. In contrast, the V levels were low in the solutions that later produced chalcedony. This element is
likewise low in the groundwater associated with calcite precipitation, but the levels of dissolved Mn
and Fe were high. The presence of trace elements in fossil wood has received relatively little
attention, but modern instrumental methods greatly facilitate a rapid and accurate determination of
these constituents. Our data suggest the value of trace elements for interpreting mineralization
The Eocene lacustrine strata of southwestern Wyoming have preserved the abundant vertebrate
fossils that have been the subject of much research. In comparison, fossil wood is highly prized by
fossil collectors, but the geologic processes responsible for the preservation of these fossils have
received little attention. The same can be said for many fossil wood localities, where research has
commonly been focused on taxonomy and paleoecology. Blue Forest, Wyoming, USA, is an example
of a site where fossil wood has a complex mineralogy that resulted from multiple geologic events,
ranging from changes in the lake levels that affected lakeside forests, to multiple stages of mineral
precipitation during diagenesis. Many questions remain to be answered, particularly in regard to the
paleobotanical aspects of the fossil wood. The significance of our work is the demonstration that
complex fossilization processes can be interpreted using geologic information in combination with
the standard methods of microcopy and geochemistry.
Author Contributions: M.V. initiated this project, performed the field work, and provided most of the research
specimens. J.M. contributed information based on his earlier visits to the site. Both coauthors provided
photographs and participated in the writing and editing of the manuscript. G.E.M. contributed analytical data
and wrote the first draft.
Funding: This research received no external funding
Acknowledgments: We thank Nareerat Boonchai and Steven Manchester, from the Florida Museum of Natural
History, for sharing their knowledge of Eocene woods from Wyoming, USA. Kyle Mikkelsen provided
technical assistance for trace element analysis. Mary Klass participated in field work.
Conflicts of Interest: The authors declare no conflict of interest
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