Robert J. Horodyski's research while affiliated with Tulane University and other places

An assemblage of filamentous microfossils is preserved in black shales of the approximately 1.2 Ga old Dismal Lakes Group from a locality about 110 km west-southwest of Coppermine, Northwest Territories, Canada. These microfossils occur as empty, 1–13 μm wide, tubular structures which are compressed parallel to lamination. The wider filaments probably represent the empty sheaths of Lyngbya-like oscillatoriacean cyanophytes, whereas the narrower filaments could represent either oscillatoriacean filaments or bacterial filaments. Filaments 3.0–7.0 μm wide are comparable to Siphonophycus crassiusculum and rare filaments 10.5–13.0 μm wide are comparable to Siphonophycus beltensis, both from shales of the Middle Proterozoic Belt Supergroup, Montana. Sphaeromorphs are rare and lack distinctive surface features which would permit their assignment to particular taxa.This shale-facies microbiota appears to consist entirely of organisms that had relatively thick sheaths which were highly resistant to degradation and diagenetic alteration. This microfossil assemblage, as well as other Proterozoic shale-facies assemblages, probably are biased toward degradation-resistant taxa and may not be representative of the original microbial communities from which they were derived, nor of the Proterozoic biosphere. Nevertheless, such microfossils offer promise for intercontinental biostratigraphic correlation and provide data useful in understanding the evolution of Precambrian life.

Irregular, low relief bedding-plane markings which probably represent impressions of thin, wrinkled algal mats have recently been detected in mudstones from the 1100 m.y. old Snowslip Formation, Belt Supergroup, Glacier National Park, Montana. Algal-mat impressions are rarely reported from ancient strata, yet algal mats are common in certain modern environments. Originating primarily in moist settings and in relatively quiescent shallow-water settings, such structures may be of some value in interpreting ancient depositional environments.

Microfossils have been discovered in cavity-fill and replacement silica that occurs between chert-breccia clasts in 1200-million-year-old paleokarst at the top of the Mescal Limestone, central Arizona, and in ∼800-million-year-old paleokarst at the top of the Beck Spring Dolomite, southeastern California. Microbial communities on Precambrian (>550 million years ago) land may have been extensive enough to affect weathering, erosion, sedimentation, and geochemical processes.

Proterozoic black shales and other fine-grained siliciclastic rocks contain a variety of fossils. These fossils are illustrated by examples from three Proterozoic units in the western USA—the Belt Supergroup (Middle Proterozoic), the Chuar Group (Late Proterozoic), and the upper Pahrump Group (Late Proterozoic).A variety of microfossils and megafossils occur in the Belt Supergroup, western Montana. Acritarchs and filamentous microfossils are moderately well preserved in sub-greenschist facies black shale in the Little Belt Mountains. The biologic affinity of the acritarchs is uncertain; they could represent the vegetative or encystment stage of eukaryotic algae or the outer envelope of colonial coccoid cyanobacteria. Most of the filaments in these shales are of a size and morphology that is consistent with prokaryotic affinities; however, rare 20- and 40-μm-wide filaments could be either prokaryotic or eukaryotic. Higher-grade sub-greenschist facies dark-gray to black mudstones in Glacier National Park contain poorly preserved acritarchs and spheroidal and filamentous pseudomicrofossils. They are of interest because they provide information useful for distinguishing authentic microfossils from pseudofossils in mudstones and shales that have undergone substantial burial metamorphism.Several authentic megafossils occur in lower Belt shales in the Little Belt and Big Belt Mountains, Montana. One of these is the carbonaceous compression Beltina danai, which could represent either fragments of a microbial mat or fragments of a sheet-like megascopic alga. The megafossil Grypania spiralis has the form of a coiled ribbon and tentatively is interpreted as a megascopic alga. Green to purple mudstones and muddy sandstones in the lower Belt Supergroup in Glacier National Park contain markings that resemble a string of beads; they are interpreted as megafossils that may have eukaryotic algal affinities.Black shales in the Late Proterozoic Kwagunt Formation, Chuar Group, Grand Canyon, Arizona, contain a variety of acritarchs and filamentous microfossils. Of special interest is the vase-shaped microfossil Melanocyrillium, which is interpreted as the sporangium of a eukaryotic and probably megascopic alga. Melanocyrillium also occurs in black mudstone in the Late Proterozoic Pahrump Group, southeastern California.

Abundant extractable hydrocarbons, in association with well preserved kerogen, have been recognised in a dolostone from the Chuar Group (approx. 850 Ma). The biomarkers present include C15-C30, acyclic isoprenoids, C26-C28 steranes, C27-C35 hopanes and extended tricyclic terpanes. Among the striking characteristics of the distributions are the dominance of steranes and methyl steranes lacking side-chain alkylation and the presence of putative C29-C35 neohopanes and gammacerane. The high abundance of steranes is consistent with the presumed eukaryotic affinities of several types of microfossils, including Chuaria, present in associated mudstones. This stratum is the oldest in which gammacerane has been found, and suggests contributions from protozoa to the depositional environment which appears to have been hypersaline. There is a strong correlation between the carbon isotopic composition of the kerogen and that of the various polarity fractions of the bitumens. Chemical degradation and pyrolysis of the kerogens yielded n-alkanes with similar characteristics to the bitumens, but there are differences in the patterns of distribution of the acyclic isoprenoid, sterane and triterpane biomarkers.

Spheroidal and filamentous organic-walled microfossils have been detected in ca. 1.7 Ga old cherts of the Hornby Bay Group, Northwest Territories, Canada. The majority of the spheroidal forms range from 1 to 4 mu m in diameter, are referable to the genus Sphaerophycus, and probably represent the preserved sheaths of chroococcacean cyanophytes. A single, robustwalled, 27 mu m diameter, spheroidal microfossil of undetermined affinity is also present. The filamentous forms are tubular, unbranched, and range from 1 to 8 mu m in diameter. They appear to represent the preserved sheaths of nostocalean cyanophytes or filamentous bacteria. The filaments illustrate the relationship between matrix mineralogy and the fidelity of preservation of organic-walled microfossils. Where they occur in silica the filaments are preserved as three-dimensional tubular microstructures, which are readily recognized as microfossils. In contrast, where they extend from silica into adjacent dolomite they are highly compressed and not readily discernable as microfossils. This compression appears to have been caused by aggrading neomorphism and pressure dissolution of the carbonate minerals, and it illustrates the contribution of diagenesis, in addition to decomposition of organic material, in causing the paucity of microfossils in Precambrian carbonate rocks. -Authors

The Belt Supergroup is a thick sequence of Middle Proterozoic strata occurring in western Montana, northern Idaho, and adjacent parts of Washington state, Alberta, and British Columbia (Ross 1963, Harrison 1972). In Glacier National Park the Belt strata are well exposed, relatively unmetamorphosed, and in many places structurally simple. In the central and northeastern part of the park the exposed Belt section is about 2900 m thick and consists of argillite, sandstone, and sandy and muddy limestone and dolostone. Most of these strata were deposited in shallow subtidal, intertidal, and alluvial settings, with offshore deposits occurring only in portions of the Appekunny Argillite (for a summary of the sedimentary geology of the Belt Supergroup in Glacier National Park see Horodyski 1983).

The Belt Supergroup is a thick, dominantly fine-grained sequence of Middle Proterozoic strata occurring in western Montana, northern Idaho, and parts of Washington state, Alberta, and British Columbia. The sequence in Glacier National Park is located along the northeastern part of present exposures of the Belt Supergroup; it is ∼2.9 km thick, extremely well exposed, and for the most part structurally simple. Although it was subjected to lowermost greenschist-facies metamorphism, primary sedimentary structures are exceptionally well preserved.

Pseudomicrofossils and authentic microfossils occur in the approximately 1.3 Ga-old Appekunny Argillite, lower Belt Supergroup, Glacier National Park, Montana. These microstructures occur in finely-laminated, dark gray and black mudstones that were deposited in an offshore environment and were subsequently subjected to moderate burial metamorphism. The pseudomicrofossils consist of 0.1–0.3 μm thick organic envelopes that encapsulate 1–20 μm diameter authigenic crystals. These crystals, petrographically identified as apatite (Ca5(PO4)3OH, F, Cl) occur isolated, in clusters, and in filiform aggregates up to 200 μm long. The organic envelopes apparently originated through the accretion, around these crystals, of organic matter which was finely disseminated throughout the sedimentary matrix.The authentic microfossils are poorly preserved sphaeromorphs which, unlike the pseudomicrofossils, are highly compressed parallel to lamination. These microfossils are several tenths of a micrometer to ca. 1 μm thick and range from 22–82 μm long and 17–71 μm wide. They commonly exhibit tears and folds that evidently were produced during compaction of the enclosing argillaceous matrix. Due to poor preservation, these specimens cannot be assigned to known Proterozoic sphaeromorph genera.Inasmuch as the pseudofossils could be misinterpreted as microfossils, they well illustrate the need to exercise caution when interpreting organic microstructures preserved in Precambrian strata. Not being compressed parallel to lamination, the pseudomicrofossils are readily distinguished from the authentic microfossils which are strongly compressed parallel to lamination. The authentic microfossils demonstrate that organic-walled microfossils can be preserved in strata subjected to moderate burial metamorphism, and they provide information useful for recognizing and interpreting microfossils in similarly metamorphosed argillaceous strata.

A 1200 Ma old microbiota is well preserved in black cherts of the Dismal Lakes Group about 130 km southwest of Coppermine, District of Mackenzie, Northwest Territories, Canada. These cherts appear to have formed by a combination of early diagenetic silicification of calcareous layers and permineralization of noncalcareous layers deposited in a very shallow subtidal or lower intertidal setting on a carbonate platform.The preserved assemblage is largely or entirely of cyanophytic affinity with coccoid forms dominating over filamentous forms. Many elements of this microbiota are morphologically similar to the 1900 Ma old Belcher Islands assemblage and 850 Ma old Bitter Springs assemblage.The following taxa are present: Chroococcaceae: Sphaerophycus parvum Schopf; Sphaerophycus medium n. sp.; Sphaerophycus? sp.; Myxococcoides grandis n. sp.; Myxococcoides? sp.Entophysalidaceae: Eoentophysalis dismallakesensis n. sp.Oscillatoriaceae: Oscillatoriopsis curta n. sp.; Oscillatoriopsis robusta n. sp.; Oscillatoriopsis sp.; Filiconstrictosus sp.; Eomicrocoleus crassus n. gen., n. sp.Incertae sedis: Archaeoellipsoides grandis n. gen., n. sp.; Biocatenoides? sp.