International Geology Review, Vol. 48, 2006, p. 78–88.
Copyright © 2006 by V. H. Winston & Son, Inc. All rights reserved.

Coccoid-Like Microstructures in a 3.0 Ga Chert
from Western Australia
YUICHIRO UENO,1
Research Center for the Evolving Earth and Planets, Department of Environmental Science and Technology,
Tokyo Institute of Technology, Post No. S2-17, Midori-ku, Yokohama 226-8503, JapanYUKIO ISOZAKI,
Department of Earth Science and Astronomy, University of Tokyo, Meguro, Tokyo 153-8902, JapanAND KENNETH J. MCNAMARA
Department of Earth and Planetary Sciences, Western Australia Museum, Perth, WA 6000, Australia

Abstract
Organic-walled spheroidal microstructures were discovered in a 3.0 Ga chert of the Cleaverville
Formation, Western Australia. The spheroids are composed of solitary or paired cell-like units
enclosed by an outer envelope, which are apparently similar to cyanobacterial microfossils. However, some of the spheroidal structures appear to be related to the arrangement of the surrounding
minerals, and some to overprint diagenetic fabrics. Despite several cell-like characteristics, such asorganic composition, paired nature, and multi-layered envelopes, at least some spheroids wereformed during diagenesis, and thus are of diagenetic origin rather than being fossilized cells. It has
been generally difficult to distinguish Archean microfossils from abiotic carbonaceous structures,
however, and such microscopic observations could demonstrate the abiotic origin of some superficially fossil-like structures produced during diagenesis.

Introduction

guish real microfossils from these abiologically
produced fossil-like structures (Cady et al., 2003;
Hofmann, 2004).
Here, we report new types of spheroidal carbonaceous structures in the 3.0 Ga chert from theCleaverville Formation in the Pilbara craton, Western Australia (Fig. 1), which resemble coccoidal
prokaryote. The purpose of this study is to document
their mode of occurrence and morphological variations. Both biological and abiological interpretations are plausible for the origin of the structures.
We further discuss the two possibilities in order tosearch for the criteria distinguishing authentic
microfossils.

MORPHOLOGICALLY PRESERVED ARCHEAN microfossils are direct evidence for the existence of life on
the early Earth (e.g., Altermann and Kazmierczak,
2003). The putative microfossils so far have beenreported from Archean strata, such as the 3.5–3.0
Ga Pilbara Supergroup, Australia (Awramik et al.,
1983; Schopf, 1993; Rasmussen, 2000; Ueno et al.,2001) and the penecontemporaneous Swaziland
Supergroup, South Africa (Knoll and Barghoorn,
1977; Walsh and Lowe, 1985; Westall et al., 2001).
They show filamentous and coccoidal morphologies,
some of which resemble cyanobacteria (Awramik etal., 1983; Schopf, 1993). In contrast to well-preserved microfossils in younger Proterozoic strata,
the biological origins of some Archean microfossils
are still a matter of debate (e.g., Brasier et al., 2002;
Schopf et al., 2002). Inorganic minerals coated bycarbonaceous materials can possibly show prokaryote-like morphologies (Schopf and Oehler, 1976;
Buick, 1990; Brasier et al., 2002; Garcia-Ruiz et al.,2003). It is not yet well established how to distin-

Cleaverville Formation
The Cleaverville Formation is composed of bedded chert, banded iron formation, siltstone, sandstone, and minor volcaniclastic rocks (Hickman etal., 2001). The unit overlies pillowed basaltic greenstone of the Regal Formation, and is unconformably
overlain by the 2775 ± 10 Ma Mt. Roe Basalt (Hickman et al., 2001). The minimum depositional age of
the Cleaverville Formation is constrained by a 3014

1Corresponding author: email: ichiro@depe.titech.ac.jp

0020-6814/06/854/78-11 $25.00

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COCCOID-LIKE MICROSTRUCTURES

79

FIG. 1. A. Geological map of the Pilbara craton, Western Australia. B. Geological map of Cleaverville Beach, showing
the sample locality (Locality A).

± 6 Ma intrusive granophyre, and a maximum age by
the presence of 3018 ± 3 Ma detrital zircons in the
volcaniclastic rock (Hickman et al., 2001). The
metamorphic grade of the Cleaverville Formation
including the rocks studied here is generally low(prehnite-pumpelleyite to lowermost greenschist
facies; Ohta et al., 1996; Hickman et al., 2001).
The depositional environment of the Cleaverville
Formation is poorly understood and somewhat
controversial. Three different tectonic settings havebeen proposed for the deposition of the chert andunderlying pillow basalt, including: (1) mid-oceanic
ridge (Ohta et al., 1996; Kato et al., 1998); (2)oceanic island arc (Kiyokawa and Taira, 1998); and(3) continental margin (Hickman et al., 2001). The
chert is mainly composed of fine-grained silica andshows massive or parallel lamination, although lacking both coarse-grained terrigenous clastics andwave-influenced sedimentary structures such as
cross-lamination (Ohta et al., 1996; Kato et al.,1998). Their absence may suggest that chemical
precipitation of silica was responsible for the formation of the chert. A positive Eu anomaly of the chertindicates that it was precipitated from silica-richhydrothermal water (Kato et al., 1998). An anomalously low Al/Ti ratio of some Cleaverville cherts
possibly resulted from either hydrothermal alteration or chemical weathering (Sugitani et al., 1996).

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Samples and Methods
The spheroid-bearing black chert was collectedfrom Cleaverville Beach (named Locality-A; UTM
grid 2356-008149; Fig. 1). At this locality, a ~20 mthick black laminated chert bed overlies pillowedbasaltic greenstone and is overlain in turn by siltstone. The greenstone/sedimentary rock sequencestrikes NE-SW and dip steeply at ~70° to the north.
Continuous outcrop of the chert can be traced laterally for ~500 m. In this outcrop, the black laminatedchert is generally disturbed by secondary faults,
often stained by Fe oxide, and partly folded,
although the lowermost part of about 80 cm thickness is less deformed, showing continuous originallamination. We collected a relatively unaltered chert
specimen (~2 kg) from the lowermost part.
The hand specimen of the black chert was first
cut perpendicular to the bedding surface. Seven
doubly polished thin sections were made to cover
the entire cut surface. Sedimentological and micropaleontological observations were conducted underthe microscope in transmitted and reflected light. In
addition, a laser Raman microspectrophotometer
was used for characterizing carbonaceous materials
and inorganic minerals. Details for the Raman
analysis are given elsewhere (Ueno et al., 2004).
More than 15,000 carbonaceous spheroids wereidentified in the seven thin sections. We focused on
about 500 spheroids, which are relatively clear inshape and away from cracks and veins. The 3D
morphologies of the 500 spheroids were describedin detail, and then photographed individually formeasuring their dimensions.

Lithology
The spheroid-bearing black laminated chert ismainly composed of an interlocking mosaic of
microcrystalline quartz (<~10 µm) with disseminated black carbonaceous matter (kerogen) andminor amounts of fine-grained pyrite and carbonate(typically <5 µm). The chert contains no coarsegrained clastics or their remnants (Fig. 2). Detailed
microscopic observations identified five distinctive
sedimentary fabrics (i.e., F1 to F5; Fig. 2). Eachfabric typically constitutes a 0.1–10 mm thick layer(Fig. 2A), although one fabric sometimes changes
laterally into another (Fig. 2C).
F1 represents homogeneous fabric (Figs. 2A and
2B). The relatively clear color of the F1 texture isdue to a low amount of disseminated kerogen. The

carbonaceous spheroids are sparsely distributedin F1.
F2 is composed of densely packed carbonaceous
spheroids, which are aligned along the bedding
surface and generally constitute a thin (~100 µm)
layer (Figs. 2G and 2H). Some spheroids are elongated parallel to the bedding, indicating post-depositional compaction (Fig. 2G). The F2 layer istypically overlain by F3 (Fig. 2G), locally boundedby stylolites (Fig. 2H), where the aggregate of thespheroids is disturbed and broken. On the other
hand, the bottom part of the F2 layer grades into theF1 in many cases.
F3 is characterized by a cloudy texture (Fig. 2F)with lesser amounts of the spheroids than in F2 andmore common than in F1. Contrasts in kerogen concentrations are responsible for the grey to black
cloudy appearance. The spheroids are randomly
distributed, and some are attached to each other.
The F3 layer typically overlies F2, whereas the topof the F3 layer is invariably bounded by secondary
infiltrated silica (F4) with irregular boundaries.
F4 is a sill or lens, which apparently infilled preexisting open space (Figs. 2A and 2D). F4 consistsof chalcedony, now composed of microcrystalline
quartz (Fig. 2D). The symmetric crystal growth
pattern clearly indicates a secondary origin for F4.
Irregular shaped, diffuse boundaries between the F4
and the other textures suggests that silica-rich fluid
infiltrated before consolidation, probably during
early diagenesis. No carbonaceous spheroids occur
in F4.
F5 is a lenticular intralaminae structure (Figs.
2A, 2C, and 2E), which constitute a >1 cm thicklayer. Billowy kerogen-rich thin layers surroundkerogen-poor lenticular masses. Both the lenses andsmall kerogenous grains in F5 are generally elongated parallel to the bedding surface, suggesting
compaction (Fig. 2E). Spheroids are rare in F5.
These fabrics are further overprinted by randomly oriented quartz veins (Figs. 2A, 2C, 2E, and
2F). In contrast to F4, they have sharp boundaries,
and the quartz is coarse grained (>100 µm). Kerogenis absent from the veins.
In summary, silica-rich fluid infiltrated in morethan two stages (i.e., the chalcedonic sill of F4 andlater-stage quartz veins), and this may be responsible for silicification of the black chert. The latestage quartz veins would have formed after compaction because they cut the compaction-related
structures of F2 and F5. On the other hand, thechalcedonic sill of F4 may have formed during early

FIG. 2. Photomicrographs of the spheroid-bearing black laminated chert. All the figures show vertical cross-sections of the chert. Tops of the figures are stratigraphic tops. A.
Photograph of a thin section. F1 to F5 indicate sedimentary fabrics. Remaining figures show magnified views of F1 (B and C), F2 (G and H), F3 (D, F, G, and H), F4 (D), and F5(C and E). See text for details.

COCCOID-LIKE MICROSTRUCTURES

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UENO ET AL.

FIG. 3. Photomicrographs showing individual carbonaceous spheroids. A–C, P–Q. Small-sized spheroids. D–N, R–
W, AA–BB. Large-sized populations. X–Z. Exceptionally large specimens. The scale at the bottom left represents 80 µm
for all the figures.

diagenesis, possibly before compaction, because of
the irregular-shaped boundary of F4 and others(Figs. 2A and 2D). The absence of coarse-grained
clastic materials suggests that the chert was originally deposited as carbonaceous mud before silicification. However, direct precipitation of silica cannot
be fully dismissed.

Carbonaceous Spheroids
The carbonaceous spheroids are coccoid-likemicrostructures that are composed of thin (<1 µm)
organic walls (Fig. 3). Figure 4 shows their Raman

spectra, indicating carbonaceous compositions.
Their three-dimensional structures are preserved ina microcrystalline quartz matrix (Fig. 5). Very finecarbonate particles (< 1 µm) occur along some spheroids (Figs. 4 and 5). Some spheroids are further
replaced and/or coated by pyrite (Fig. 6E), and subsequently by Fe oxide (Fig. 6F). In these alteredspecimens, their thin organic walls are typically
obscure.
The spheroids consist of solitary, or most commonly paired, cell-like units, which are generally
enclosed by an outer envelope (Fig. 3). Among the
500 specimens observed, ~75% are paired, and theCOCCOID-LIKE MICROSTRUCTURES

FIG. 4. Representative Raman spectra for the carbonaceous spheroids. O-peak (near 1600 cm–1) and D-peak (near1350 cm–1) are from carbonaceous material. The microscope objective used is 100x; thus the analytical spot size is about
1 µm. Details of the micro-Raman analytical procedure are given in Ueno et al. (2004).

FIG. 5. Photomicrographs in transmitted white light (A1 and B1) and plane-polarized light (A2 and B2), showing
relationships between the carbonaceous spheroids and the surrounding matrix minerals (cryptocrystalline quartz). Dark
and light particles are carbonaceous materials and carbonates, respectively. See also Figure 4 for their Raman spectra.
Scale bar in A1 represents 20 µm for all the figures.

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UENO ET AL.

FIG. 6. Photomicrographs showing relationships between the carbonaceous spheroids and sedimentary or diagenetic
structures. A, C, and D. Non-compressed spheroids in F5. Arrow indicates the spheroids cutting billowy laminations. Box
in C represents area of Figure D. B. Spheroids and rounded thin layers of carbonaceous materials in the F3. E. Alteredspheroid partly replaced by pyrite (arrow). F. Altered spheroids replaced by Fe oxide (arrow).

remaining ~25% are single. Exceptionally, several
spheroids contain three cell-like units in the outerenvelope (Figs. 3AA and 3BB). Some spheroids are
squeezed and elongated parallel to the bedding surface (Figs. 2G and 3O). The deformed spheroids
exhibit irregular morphologies such as S shapes(Fig. 3O).
Individual cell-like units are spheroidal or hemispheroidal, some of which are crumpled (Figs. 3Tand 3V). Paired two cell-like units in most cases
have similar diameters. The pair are apparently
compressed at attachment (Figs. 3D–3H, 3R, and3S), although some appear to terminate into anothercell-like unit (Fig. 3BB). Some relatively small celllike units are centrally or excentrally located in theouter envelope (Figs. 3D and 3F). Internal structures of the cell-like units are generally divided into
three types: (1) those containing an irregularly
shaped, crumpled organic envelope, which typicallysurrounds a smaller kerogenous mass (Figs. 3H, 3J,

and 3K); (2) those containing a smaller centrally or
excentrally located kerogenous mass (Figs. 3L–3M);
and (3) those without any kerogenous structure(Figs. 3E, 3G, and 3R).
Size distributions of the measured 460 spheroids
are shown in Figure 7. The diameters of the cell-likeunits range from 4 to 89 µm, and show a bimodal
size distribution. Cell-like units of the smaller population are 4 to 22 µm (average 12.6 µm: n = 210),
whereas those of larger population are 23 to 43 µm(average 31.4 µm; n = 253), excluding seven exceptionally large specimens (Figs. 3X and 3Y; 47 to 89µm). The morphologies of the two size populations
are almost identical.

Discussion
Two possibilities are considered here for theorigin of the carbonaceous spheroids. One is that thespheroids are cellularly preserved microfossils. The

COCCOID-LIKE MICROSTRUCTURES

85

FIG. 7. Size distributions of the carbonaceous spheroids. They are separated into three size populations by theirdiameters of cell-like units: (1) 4–22 µm (oblique lines); (2) 23–43 µm (grey); and (3) >47 µm (black). Illustration shows
the measured dimensions.

morphologies of the spheroids are apparently similar to those of fossilized prokaryotes, especiallyensheathed, colonial, coccoid cyanobacteria. Morphologically similar microfossils have been reported
from various Proterozoic strata (e.g., Schopf andBlacic, 1971; Hofmann, 1976; Knoll and Golubic,
1979; Nyberg and Schopf, 1984), although not fromthe Archean so far. The alternative possibility for
producing the coccoid-like spheroids is the episodic
crystal growth of an inorganic mineral (potentiallycarbonate and/or silica) during diagenesis. Concentric silica and Fe-oxide microspheres and sphericalcarbonate micronodules, which are sometimes
coated by kerogen, have been reported from several
Precambrian cherts (e.g., Klein and Fink, 1976;
Awramik et al., 1983; Nyberg and Schopf, 1984).
However, criteria distinguishing fossilized coccoidfrom abiotic spheroids have not been well established. Here, we examine the two contrasting possibilities from the following viewpoints: (1)predominance of paired spheroids; (2) regularity orirregularity of the structures; (3) relationshipbetween organic wall and matrix minerals; (4) relationship between the spheroids and sedimentaryfabrics; (5) size of the cell-like unit; and (6) carbonaceous composition. Some of these characteristics of
the Cleaverville spheroids may reflect a microfossil
origin, whereas others rather suggest an inorganic
origin.
Paired morphologies of the spheroids possibly
could have been produced by cell division. On theother hand, nucleation of two nodules in proximity
to one another could also produce such paired
morphologies inorganically. However, about 75% of
the specimens examined are paired rather thansingle, even in sparsely distributed domains (F1). It
is unlikely that the two nucleation sites of mineral
growths would so charactistically occur adjacent to
one another. If the hypothetical initial precipitates
were dumbbell-shaped, the paired spheroids might
be preferentially produced inorganically. Recent
experiments of crystal growth by Knorre and Krumbein (2000) produced dumbbell-shaped precipitatesand paired spherical carbonate nodules. Hence, thepaired morphology does not necessarily indicate
cell division of fossilized microbes.
Regular concentric morphologies would be
easily produced by mineral growth. In contrast,

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UENO ET AL.

well-preserved authentic coccoid microfossils, forexample those of ~830 Ma Bitter Springs fossil biota(Schopf, 1968; Schopf and Blacic, 1971; Knoll andGolubic, 1979), tend to show various irregularshapes reflecting cell divisional pattern, mutualcompression, post-mortem degradation, anddeformation during diagenesis. The Cleavervillespheroids are typically regular and concentric, inwhich the space between the inner and the outerwalls is constant (Figs. 3A–3E, 3G, 3M, 3R–Y,
3AA, and 3BB). These regularities are expected ifthey are precipitates (Klein and Fink, 1976). However, some Cleaverville spheroids exhibit more
irregular morphologies, including a crumpled internal envelope and excentrally located kerogenous
mass in the relatively rounded cell-like units (Figs.
3H, 3J–L, 3N, 3P, 3Q, and 3Z), eccentrically locatedcell-like units in the common envelope (Fig. 3F),
and gently deformed outlines such as S-shaped (Fig.
3O), ellipsoidal (Figs. 3J–3K), and irregularly
deformed (Fig. 3I) structures. Spherical mineral
growth could not simply explain these irregular morphologies. Especially, crumpled internal envelopes
in some cell-like units (Figs. 3E, 3G, and 3H) arerather similar to degraded cells of fossilized coccoids (e.g., Golubic and Hofmann, 1976; Knoll andGolubic, 1979). Even though most spheroids show aregular shape expected for inorganic precipitates,
some co-existing ones exhibit more irregularshapes, rather indicating an origin as fossilized
cells. In such a case, where these two types occur
together, regularity or irregularity in morphology is
not critical to assess the origin of the spheroids.
Crystal structures and distributions of surrounding minerals are also helpful to distinguish microfossils from inorganic structures. In the concentric
regular-shaped specimens, microcrystalline quartz
grains apparently align along the spheroidal morphology, and very fine-grained carbonates occur
along organic walls (Fig. 5A). This may suggest thatcrystal growth produced the structure. On the other
hand, non-concentric irregular-shaped specimens
are not correlated with the arrangement of the matrix
quartz (Fig. 5B). These morphological and mineralogical differences indicate that the two types (i.e.,
regular-shaped mineralogically controlled spheroid
and irregular-shaped ones) are different in origin.
However, we still cannot rule out the possibility thatthe two were the same in origin (either biological or
abiological), and partial alteration and/or deformation produced the difference after deposition.

Correlation between the spheroids and sedimentary fabrics is also helpful to distinguish microfossils from diagenetic structures. Some F2
spheroids are compressed parallel to the bedding
surface (Fig. 2G) and some are disturbed by stylolites (Fig. 2F). These relations indicate that the
deformed spheroids were present before compaction. However, other spheroids appear to have grownover the sedimentary fabric F5 (Figss 6A, 6C, and
6D). In F5, siliceous lenses and small kerogen clots
are elongated, and thus are interpreted to have been
compressed. Nevertheless, some spheroids in thesame F5 are not compressed (Fig. 6C), suggesting
growth after compaction. Further, some spheroids
cut the billowy lamination of F5 (Fig. 6A). Therefore, at least some spheroids are not microfossils but
represent inorganic structures produced during
diagenesis.
The diameters of the cell-like units (4 to 89 µm)
are significantly greater than those of typicalprokaryotic cells (mostly ~1 µm; Holt et al., 1994),
and are rather similar to those of some cyanobacterial cells. Modern chroococcalean cyanobacteria,such as Chroococcus turgidus, are usually 3 to 30 µm
in diameter (Holt et al., 1994) and possibly range up
to 50 µm (Desikachary, 1959). From this aspect, theCleaverville spheroids are unusually larger thanordinary prokaryotes, although their large size
cannot fully dismiss a microfossil origin.
The carbonaceous compositions of the spheroids
could be easily explained by a microfossil origin. On
the other hand, spherically grown inorganic minerals (nodules) are not necessarily associated withcarbonaceous material. However, organic matter
may have coated episodically grown nodule surfaces
or be incorporated within nodules (e.g., Buick,1990). Actually, non-coccoid-like thin layers ofkerogen rarely occur in the F3, which apparently
coat botoryoidal silica (Fig. 6B), suggesting mobilityof the carbonaceous material. This indicates thatmineral growth could have produced carbonaceous
multi-layers in the chert studied here. Hence, thecarbonaceous compositions of the spheroids do not
simply indicate a microfossil origin.

Summary
The carbonaceous spheroids from the 3.0 Ga
Cleaverville chert morphologically resemble coccoidal prokaryotes, especially cyanobacteria. Some
irregular-shaped specimens appear strikingly similar to degraded prokaryotic cells. Nevertheless,COCCOID-LIKE MICROSTRUCTURES

co-existing regular-shaped concentric spheroids areapparently controlled by mineral growth, some ofwhich have overprinted diagenetic structures.
Therefore, at least some spheroids would have been
produced by mineral growth during diagenesis.
Although biological origin of all the spheroids is not
fully dismissed, these observations demonstrateexamples of some inorganically produced coccoidlike structures.

Acknowledgments
We thank S. Maruyama, A Yamagishi, and B. F.
Windley for helpful discussions and suggestions. We
also thank M. Terabayashi, Y. Kato, H. Ohta, T.
Kabashima, and K. Kitajima for great assistance inthe field work. Field collaboration with A. Thorne
and A. H. Hickman was helpful and much appreciated. Critical comments by S. Awramik and B.
Runnegar greatly improved the manuscript. This
research was supported by grants from the Ministryof Education, Culture, Sports, Science, and Technology of Japan. YU is grateful for the Research
Fellowships of the Japan Society for the Promotionof Science for Young Scientists.

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