Carbonaceous Microfossils Research Articles

Correlating trace element compositions, petrology, and Raman spectroscopy data in the ∼3.46 Ga Apex chert, Pilbara Craton, Australia

The Apex chert unit (∼3.46 Ga, Pilbara Craton, Australia) constitutes one of the oldest sedimentary units on Earth in which putative carbonaceous microfossils have been reported. The source of carbonaceous matter (CM) in this unit, however, is hotly debated. Hydrothermal fluids have circulated through the underlying crust and up into the bedded unit; these fluids could have remobilized sedimentary microbial biomass, generated abiological hydrocarbons, or harbored in situ chemolithoautotrophic microbial communities. Are there parts of the unit where microfossils might be best preserved?A potential fossil microbiota – if present – would probably be best preserved in the stratiform portion of the unit where hydrothermal influence seems to have been lowest. In order to shed light on the history of hydrothermal overprinting and the source of carbonaceous fractions in the Apex chert, we correlate here at a high spatial resolution petrographic observations and trace element analyses over a transect from the dyke where putative microfossils were found to the stratiform part where remnants of microbial mats were found. The layered, stratiform part of the unit has positive La anomalies up to 1.7, and Light Rare Earth Element depletions, indicating a seawater source. However, as far as 300 m from the dyke, the stratiform part also shows hydrothermal brecciation, high Eu anomalies (2–12; µ = 4.2) and chondritic Y/Ho ratios (24.3–30.3; µ = 27.0), indicating that hydrothermal fluids have laterally infiltrated over large distances. Overall, the pervasive influence of hydrothermal fluids throughout the entire unit and the presence of carbonaceous matter both in the sedimentary part and the hydrothermal dyke is consistent with a ‘hydrothermal pump’ model that was earlier proposed for the nearby Dresser Formation. In this model, organic matter from surface environments is circulated along with hydrothermal fluids and redistributed in the crust and overlying sediments, therefore complicating paleobiological interpretations. Raman measurements show that most of the CM experienced temperatures of ∼350 °C, while some samples contain CM with a variable, but markedly lower maturity (temperature ranging from 200 to 350 °C). Correlation to texture points out a potential mixing of pre-metamorphic CM with post-metamorphic CM during late hydrothermal events.

Ancient Oil as a Source of Carbonaceous Matter in 1.88-Billion-Year-Old Gunflint Stromatolites and Microfossils

The 1.88-billion-year-old Gunflint carbonaceous microfossils are renowned for their exceptional morphological and chemical preservation, attributed to early and rapid entombment in amorphous silica. The carbonaceous matter lining and partly filling filamentous and spherical structures is interpreted to be indigenous, representing thermally altered relicts of cellular material (i.e., kerogen). Here we show that stromatolitic black cherts from the Gunflint Formation, Schreiber Beach, Ontario, Canada, were saturated in syn-sedimentary oil. The thermally altered oil (pyrobitumen), which occurs in the stromatolites and intercolumn sediments, fills pores and fractures, and coats detrital and diagenetic grain surfaces. The occurrence of detrital bitumen grains in the stromatolites points to the proximity of shallow seafloor oil seeps and hence the possible existence of chemosynthetic microbes degrading hydrocarbons. We suggest that hydrocarbons that migrated through the silicifying stromatolites infiltrated semi-hollow microbial molds that formed following silica nucleation on the walls or sheaths of decayed cells. Upon heating, the hydrocarbons were transformed to nanoporous pyrobitumen, retarding silica recrystallization and enhancing detailed preservation of the carbon-rich microfossils. Hydrocarbon infiltration of silicified microbes offers a new explanation for the preservation of the Gunflint microfossils and may have played a role in the formation of some of Earth's oldest microfossils.

Raman spectroscopy and structural heterogeneity of carbonaceous material in Proterozoic organic-walled microfossils in the North China Craton

Raman spectroscopy is a minimally intrusive and minimally destructive technique that has played an important role in analyzing Precambrian carbonaceous microfossils and microstructures. Previous studies have shown that heterogeneities in structural order of carbonaceous material (CM) as revealed by Raman spectroscopy can be preserved in Proterozoic silicified cyanobacterial fossils. However, such heterogeneities have not been documented in Proterozoic eukaryotic microfossils preserved as carbonaceous compressions. In this study, we used Raman spectroscopy to investigate the structural characteristics of organic-walled microfossils, including eukaryotes preserved as carbonaceous compressions, from the late Paleoproterozoic Ruyang Group and the early Neoproterozoic (Tonian) Liulaobei Formation in the North China Craton. Raman geothermometers indicate that CM experienced low-grade metamorphism with apparent peak metamorphic temperatures of ~200 °C for both the Ruyang and Liulaobei microfossils. Heterogeneities in structural characteristics of CM in eukaryotic microfossils were found among different subcellular structures of Shuiyousphaeridium macroreticulatum and among different taxa from the same stratigraphic horizon. We suggest that these heterogeneities can be attributed to differences in precursor organic compounds; thus, the importance of organic precursors should be considered in CM Raman geothermometry studies, especially in environments where heating events occurred quickly. The Raman data presented here are also consistent with hypotheses that the Ruyang microfossils Dictyosphaera delicata and S. macroreticulatum are biologically conspecific taxa with their vesicle walls having similar CM Raman features, and that the Liulaobei microfossils Leiosphaeridia jacutica and L. tenuissima are distinct taxa despite their similarities in morphologies other than vesicle wall thickness. Although more data are needed to verify the consistency of interspecies, intraspecies, and intraspecimen variability, this study indicates that Raman spectroscopy may have the potential to provide an independent test of early eukaryote taxonomy, which has been traditionally based only on morphological features, and to reveal heterogeneities of CM of early eukaryotic organisms.

Nanoscale 3D quantitative imaging of 1.88 Ga Gunflint microfossils reveals novel insights into taphonomic and biogenic characters

Precambrian cellular remains frequently have simple morphologies, micrometric dimensions and are poorly preserved, imposing severe analytical and interpretational challenges, especially for irrefutable attestations of biogenicity. The 1.88 Ga Gunflint biota is a Precambrian microfossil assemblage with different types and qualities of preservation across its numerous geological localities and provides important insights into the Proterozoic biosphere and taphonomic processes. Here we use synchrotron-based ptychographic X-ray computed tomography to investigate well-preserved carbonaceous microfossils from the Schreiber Beach locality as well as poorly-preserved, iron-replaced fossil filaments from the Mink Mountain locality, Gunflint Formation. 3D nanoscale imaging with contrast based on electron density allowed us to assess the morphology and carbonaceous composition of different specimens and identify the minerals associated with their preservation based on retrieved mass densities. In the Mink Mountain filaments, the identification of mature kerogen and maghemite rather than the ubiquitously described hematite indicates an influence from biogenic organics on the local maturation of iron oxides through diagenesis. This non-destructive 3D approach to microfossil composition at the nanoscale within their geological context represents a powerful approach to assess the taphonomy and biogenicity of challenging or poorly preserved traces of early microbial life, and may be applied effectively to extraterrestrial samples returned from upcoming space missions.

Doushantuo-Pertatataka–type acanthomorphs and Ediacaran ecosystem stability

Abstract Large (100 to ∼700 µm diameter) spheroidal carbonaceous microfossils ornamented with regularly arranged spinose or branched processes are globally distributed in the Ediacaran (635–542 Ma). These microfossils, collectively known as the Doushantuo-Pertatataka–type acanthomorphs, have been variously interpreted as a polyphyletic assortment of resting stages of eukaryotes, including animals. The stratigraphic range of the acanthomorphs has long been thought to be restricted to the interval between the uppermost Cryogenian glacial deposits and the largest-known carbon isotope excursion in Earth’s history, the Shuram event. The mid-Ediacaran disappearance of the acanthomorphs was puzzling until they were discovered in younger strata in south China, in northwestern Russia, and in Mongolia. Here, we report Doushantuo-Pertatataka–type acanthomorphs coeval with Cambrian-type small skeletal fossils. It appears that neither the Shuram event nor the emergence of macro-organisms, eumetazoans, and biologically controlled mineralization significantly affected the acanthomorphs, suggesting a marked stability of Ediacaran ecosystems up to the very beginning of the Cambrian.

Organic-rich microfossils produced by oil infiltration of hollow silicified bacteria: Evidence from the ca. 340 Ma Red Dog Zn-Pb deposit, Alaska

Abstract The remarkable preservation of carbonaceous microfossils in Precambrian chert has been attributed to rapid silica precipitation enclosing original cellular carbon. However, while silicification of bacteria can preserve cellular morphology, in modern silicifying environments, organic molecules are rapidly destroyed after death, raising questions about the origin of carbon in ancient microfossils. Here, we show that carbonaceous matter in filamentous microfossils in chert from the Carboniferous Red Dog Zn-Pb deposit, northern Alaska, represents relicts of migrated oil. Black, carbonaceous microfossils are associated with chert stained brown by solidified oil surrounding hairline fractures. The black filaments contain thin carbon films (<50 nm) lining minute quartz crystals in intracellular cavities. Our observations indicate that silica nucleation on bacterial cell walls was followed by the decomposition of cellular carbon, producing silica-encased, bacterial molds that were later infiltrated by oil. This process represents a new mode of fossilization that raises questions about the origin of carbonaceous matter in some of Earth’s oldest microfossils.

A morphogram for silica-witherite biomorphs and its application to microfossil identification in the early earth rock record.

Archean hydrothermal environments formed a likely site for the origin and early evolution of life. These are also the settings, however, were complex abiologic structures can form. Low‐temperature serpentinization of ultramafic crust can generate alkaline, silica‐saturated fluids in which carbonate–silica crystalline aggregates with life‐like morphologies can self‐assemble. These “biomorphs” could have adsorbed hydrocarbons from Fischer–Tropsch type synthesis processes, leading to metamorphosed structures that resemble carbonaceous microfossils. Although this abiogenic process has been extensively cited in the literature and has generated important controversy, so far only one specific biomorph type with a filamentous shape has been discussed for the interpretation of Archean microfossils. It is therefore critical to precisely determine the full distribution in morphology and size of these biomorphs, and to study the range of plausible geochemical conditions under which these microstructures can form. Here, a set of witherite‐silica biomorph synthesis experiments in silica‐saturated solutions is presented, for a range of pH values (from 9 to 11.5) and barium ion concentrations (from 0.6 to 40 mmol/L BaCl2). Under these varying conditions, a wide range of life‐like structures is found, from fractal dendrites to complex shapes with continuous curvature. The size, spatial concentration, and morphology of the biomorphs are strongly controlled by environmental parameters, among which pH is the most important. This potentially limits the diversity of environments in which the growth of biomorphs could have occurred on Early Earth. Given the variety of the observed biomorph morphologies, our results show that the morphology of an individual microstructure is a poor criterion for biogenicity. However, biomorphs may be distinguished from actual populations of cellular microfossils by their wide, unimodal size distribution. Biomorphs grown by diffusion in silica gel can be differentiated by their continuous gradient in size, spatial density, and morphology along the direction of diffusion.

Iron mineralization and taphonomy of microfossils of the 2.45–2.21 Ga Turee Creek Group, Western Australia

We report a new assemblage of carbonaceous microfossils intimately associated with siderite and Fe-silicates, from a black chert nodule included in iron formation of the ca. 2.45–2.21Ga Turee Creek Group, Western Australia. This chert comprises microbial fabrics dominated by filaments preserved in matrix of nano- to micrometric quartz. Filaments occur in clumps and in a cobweb-like fabric interspersed with coarse crystalline, void filling quartz granules. We studied this chert with optical microscopy combined with Scanning Transmission Electron Microscope observations of Focused Ion Beam sections of microfossils. This distinguished three types of well-preserved fossil interpreted as polysaccharide sheaths that usually do not preserve chains of cells (trichomes): Type 1 comprises narrow filaments with thin continuous kerogen sheaths, Type 2 comprises narrow filaments with thick granular sheaths, and Type 3 comprises broad filaments with thin sheaths. Type 4 filaments are poorly preserved as granular kerogen. Organic ultrastructures of Type 2–4 microfossils are variably replaced by siderite crystals, associated with minor Fe-silicates. Iron isotope analyses on bulk powder and reactive iron fraction show indistinguishable and highly positive δ56Fe values (+1.45‰ relative to the reference IRMM-014), indicating that the bulk of siderite derives from reduction of Fe(III)-oxides. This provides indirect evidence that the microbial community was originally associated with Fe(III)-oxides. Siderite and Fe-silicates are found with Type 2–4 but not Type 1 filaments, suggesting that only the former were encrusted by Fe(III)-oxides, which may have been reduced in situ. Siderite and Fe-silicates could result from oxidation of organic matter in filaments coupled with microbial and/or thermal reduction of Fe(III)-bio(?)minerals. The increasing abundance of siderite correlated with decreasing organic matter preservation in filaments supports that this reaction occurred to variable extents, in situ on each microfossil. Type 2–4 microfossils may thus represent iron-oxidizing bacteria. These microbial mats display strong similarities with those associated with immediately overlying carbonate rocks of the Turee Creek Group, where filaments were interpreted as sulfur-oxidizing bacteria. Some filamentous bacteria can oxidize both iron and sulfur. Such metabolic versatility could have enabled benthic microbial mats to thrive in the drastically changing chemical conditions of the Great Oxidation Event.

Lobopodians

Lobopodians

A Test of the Biogenicity Criteria Established for Microfossils and Stromatolites on Quaternary Tufa and Speleothem Materials Formed in the “Twilight Zone” at Caerwys, UK

The ability to distinguish the features of a chemical sedimentary rock that can only be attributed to biology is a challenge relevant to both geobiology and astrobiology. This study aimed to test criteria for recognizing petrographically the biogenicity of microbially influenced fabrics and fossil microbes in complex Quaternary stalactitic carbonate rocks from Caerwys, UK. We found that the presence of carbonaceous microfossils, fabrics produced by the calcification of microbial filaments, and the asymmetrical development of tufa fabrics due to the more rapid growth of microbially influenced laminations could be recognized as biogenic features. Petrographic evidence also indicates that the development of "speleothem-like" laminae was related to episodes of growth interrupted by intervals of nondeposition and erosion. The lack of any biogenic characteristics in these laminae is consistent with their development as a result of variation in the physicochemical parameters that drive calcite precipitation from meteoric waters in such environmental settings.

3.46 Ga Apex chert ‘microfossils’ reinterpreted as mineral artefacts produced during phyllosilicate exfoliation

Filamentous microstructures from the 3.46billionyear (Ga)-old Apex chert of Western Australia have been interpreted as remnants of Earth's oldest cellular life, but their purported biological nature has been robustly questioned on numerous occasions. Despite recent claims to the contrary, the controversy surrounding these famous microstructures remains unresolved.Here we interrogate new material from the original ‘microfossil site’ using high spatial resolution electron microscopy to decode the detailed morphology and chemistry of the Apex filaments. Light microscopy shows that our newly discovered filaments are identical to the previously described ‘microfossil’ holotypes and paratypes. Scanning and transmission electron microscopy data show that the filaments comprise chains of potassium- and barium-rich phyllosilicates, interleaved with carbon, minor quartz and iron oxides. Morphological features previously cited as evidence for cell compartments and dividing cells are shown to be carbon-coated stacks of phyllosilicate crystals. Three-dimensional filament reconstructions reveal non-rounded cross sections and examples of branching incompatible with a filamentous prokaryotic origin for these structures.When examined at the nano-scale, the Apex filaments exhibit no biological morphology nor bear any resemblance to younger bona fide carbonaceous microfossils. Instead, available evidence indicates that the microstructures formed during fluid-flow events that facilitated the hydration, heating and exfoliation of potassium mica flakes, plus the redistribution and adsorption of barium, iron and carbon within an active hydrothermal system.

Revealing the biotic origin of silicified Precambrian carbonaceous microstructures using Raman spectroscopic mapping, a potential method for the detection of microfossils on Mars

Demonstrating the biogenicity of carbonaceous microfossils can be relatively difficult because of their small size and simple shape, and to the degradation of the associated organic molecules with time. For Precambrian fossils, it generally requires the use of several techniques to study the shape and the composition of the structure itself, as well as its mineral environment. The ability to identify both organic matter and minerals using Raman spectroscopy makes it a key technique in the field of micropaleontology. Raman instruments are also being developed for the upcoming missions to Mars, ExoMars and Mars 2020, both dedicated to the search for past or present traces of life. However, demonstrating the biotic origin of carbonaceous matter in geological materials using this technique is controversial. Here, we show that Raman mapping instead of single spot analysis can detect variations in the composition of carbonaceous matter associated with fossilized microbes in the 800‐Ma‐old microfossils from the Draken Formation, Svalbard. This discovery is of great interest because it permits assessment of the biotic origin of a fossilized carbonaceous structure. Raman mapping could thus be of crucial importance in the near future for detecting potential fossilized microbial remains in Martian rocks. Copyright © 2015 John Wiley & Sons, Ltd.

Microbial mats implicated in the generation of intrastratal shrinkage (‘synaeresis’) cracks

AbstractIntrastratal shrinkage (often termed ‘synaeresis’) cracks are commonly employed as diagnostic environmental indicators for ancient salinity‐stressed, transitional fluvial‐marine or marginal‐marine depositional environments. Despite their abundance and use in facies interpretations, the mechanism of synaeresis crack formation remains controversial, and widely accepted explanations for their formation have hitherto been lacking. Sedimentological, ichnological, petrographic and geochemical study of shallow marine mudstone beds from the Ordovician Beach Formation of Bell Island, Newfoundland, has revealed that crack development (cf. synaeresis cracks) on the upper surface of mudstone beds is correlated with specific organic, geochemical and sedimentological parameters. Contorted, sinuous, sand‐filled cracks are common at contacts between unbioturbated mudstone and overlying sandstone beds. Cracks are absent in highly bioturbated mudstone, and are considered to pre‐date firmground assemblages of trace fossils that include Planolites and Trichophycus. The tops of cracked mudstone beds contain up to 2·1 wt% total organic carbon, relative to underlying mudstone beds that contain around 0·5 wt% total organic carbon. High‐resolution carbon isotope analyses reveal low δ13Corg values (−27·6‰) on bed tops compared with sandy intervals lacking cracks (−24·4 to −24·9‰). Cracked mudstone facies show evidence for microbial matgrounds, including microbially induced sedimentary structures on bedding planes and carbonaceous laminae and tubular carbonaceous microfossils in thin section. Non‐cracked mudstone lacks evidence for development of microbial mats. Microbial mat development is proposed as an important prerequisite for intrastratal shrinkage crack formation. Both microbial mats and intrastratal shrinkage cracks have broad palaeoenvironmental distributions in the Precambrian and early Phanerozoic. In later Phanerozoic strata, matgrounds are restricted to depositional environments that are inhospitable to burrowing and surface‐grazing macrofauna. Unless evidence of synaeresis (i.e. contraction of clay mineral lattices in response to salinity change) can be independently demonstrated, the general term ‘intrastratal shrinkage crack’ is proposed to describe sinuous and tapering cracks in mudstone beds.

Raman Imaging of Metastable Opal in Carbonaceous Microfossils of the 700–800 Ma Old Draken Formation

Opaline silica was detected, with Raman spectroscopy, in carbonaceous microfossils (especially Myxococcoides) in silicified filamentous microbial mats within dolomitized conglomerates of the Draken Formation (-800 to -700 Ma). High-resolution electron microscopy (HRTEM) and microprobe analyses were used to confirm the nature of this phase in the quartz matrix of the microbial mats. The silica likely precipitated in a microcrystalline form onto the organic macromolecules around, and within, the degrading microorganisms and preserved them by inhibiting the natural phase change to quartz. The Raman signal of opaline silica associated with carbonaceous matter and other biosignatures could be a potential indicator of biogenicity. This kind of association could be very useful during the future ExoMars mission (ESA/Roscosmos, 2018) that will search for traces of past life on Mars.

Burgess Shale-Type Microfossils from the Middle Cambrian Kaili Formation, Guizhou Province, China

Diverse carbonaceous microfossils, including exceptionally preserved remains of non-biomineralizing metazoans, are reported from a basal middle Cambrian interval of the Kaili Formation (Guizhou Province, China). The application of a gentle acid maceration technique complements previous palynological studies by revealing a larger size-class of acritarchs, a richer assemblage of filamentous microfossils, and a variety of previously unrecovered forms. Metazoan fossils include Wiwaxia sclerites and elements derived from biomineralizing taxa, including chancelloriids, brachiopods and hyolithids, in common with previously studied assemblages from the early and middle Cambrian of Canada. In addition, the Kaili Formation has yielded pterobranch remains and an assemblage of cuticle fragments representing “soft-bodied” worms, including a priapulid-like scalidophoran. Our results demonstrate the wide distribution and palaeobiological importance of microscopic “Burgess Shale-type” fossils, and provide insights into t...

Understanding the Application of Raman Spectroscopy to the Detection of Traces of Life

Investigating carbonaceous microstructures and material in Earth's oldest sedimentary rocks is an essential part of tracing the origins of life on our planet; furthermore, it is important for developing techniques to search for traces of life on other planets, for example, Mars. NASA and ESA are considering the adoption of miniaturized Raman spectrometers for inclusion in suites of analytical instrumentation to be placed on robotic landers on Mars in the near future to search for fossil or extant biomolecules. Recently, Raman spectroscopy has been used to infer a biological origin of putative carbonaceous microfossils in Early Archean rocks. However, it has been demonstrated that the spectral signature obtained from kerogen (of known biological origin) is similar to spectra obtained from many poorly ordered carbonaceous materials that arise through abiotic processes. Yet there is still confusion in the literature as to whether the Raman spectroscopy of carbonaceous materials can indeed delineate a signature of ancient life. Despite the similar nature in spectra, rigorous structural interrogation between the thermal alteration products of biological and nonbiological organic materials has not been undertaken. Therefore, we propose a new way forward by investigating the second derivative, deconvolution, and chemometrics of the carbon first-order spectra to build a database of structural parameters that may yield distinguishable characteristics between biogenic and abiogenic carbonaceous material. To place Raman spectroscopy as a technique to delineate a biological origin for samples in context, we will discuss what is currently accepted as a spectral signature for life; review Raman spectroscopy of carbonaceous material; and provide a historical overview of Raman spectroscopy applied to Archean carbonaceous materials, interpretations of the origin of the ancient carbonaceous material, and a future way forward for Raman spectroscopy.

Testing the survival of microfossils in artificial martian sedimentary meteorites during entry into Earth’s atmosphere: The STONE 6 experiment

If life ever appeared on Mars, could we find traces of primitive life embedded in sedimentary meteorites? To answer this question, a 3.5-byr-old volcanic sediment containing microfossils was embedded in the heat shield of a space capsule in order to test survival of the rock and the microfossils during entry into the Earth’s atmosphere (the STONE 6 experiment). The silicified volcanic sediment from the Kitty’s Gap Chert (Pilbara, Australia) is considered to be an excellent analogue for Noachian-age volcanic sediments. The microfossils in the chert are also analogues for potential martian life. An additional goal was to investigate the survival of living microorganisms ( Chroococcidiopsis) protected by a 2-cm thick layer of rock in order to test whether living endolithic organisms could survive atmospheric entry when protected by a rocky coating. Mineralogical alteration of the sediment due to shock heating was manifested by the formation of a fusion crust, cracks in the chert due to prograde and retrograde changes of α quartz to β quartz, increase in the size of the fluid inclusions, and dewatering of the hydromuscovite-replaced volcanic protoliths. The carbonaceous microfossils embedded in the chert matrix survived in the rock away from the fusion crust but there was an increase in the maturity index of the kerogen towards the crust. We conclude that this kind of sediment can survive atmospheric entry and, if it contains microfossils, they could also survive. The living microorganisms were, however, completely carbonised by flame leakage to the back of the sample and therefore non-viable. However, using an analytical model to estimate the temperature reached within the sample thickness, we conclude that, even without flame leakage, the living organisms probably need to be protected by at least 5 cm of rock in order to be shielded from the intense heat of entry.

Microfossils in stromatolitic cherts from the proterozoic allamoore formation of west texas

A diverse, well-preserved microbiota has been discovered in petrographic thin sections of stromatolitic black chert of the ca. 1000–1100 Ma-old Allamoore Formation from the Van Horn region of west Texas. Geologic evidence indicates that the mat-building microbial community inhabited a shallow-water environment, probably intermittently exposed to subaerial desiccation. The microfossils occur in two interlaminated sedimentary fabrics: a flat-lying to wavy stromatolitic fabric, formed during relatively quiescent periods, and an intraclastic fabric that reflects deposition in agitated water, perhaps during storms. The carbonaceous microfossils are three-dimensionally preserved, permineralized in fine-grained chert probably of early diagenetic origin. The assemblage includes solitary and colonial unicellular cyanobacteria (Entophysalidaceae or Chroococcaceae), the tubular sheaths of at least three microbial taxa (apparently of oscillatoriacean affinities), and cellular prokaryotic filaments. The common occurrence of large spheroidal unicells, some in excess of 100 μm in diameter, suggests that fossil eukaryotes (green or red algae) may also be represented. The Allamoore cherts comprise one of only a few microfossiliferous Precambrian deposits now known from western North America.