Bylot Supergroup Research Articles

Organic matter sulfurization and organic carbon burial in the Mesoproterozoic

Throughout the Proterozoic Era, sedimentary organic carbon burial helped set the pace of global oxygenation and acted as a major modulator of atmospheric CO2 and climate. Although Proterozoic rocks generally contain low concentrations of organic matter (OM), there are key exceptions to this rule, including the relatively OM-rich Arctic Bay shales from Baffin Island, Canada (Bylot Supergroup, Borden Basin, ∼1.05 Ga). The mechanisms driving elevated OM concentrations in these and other Proterozoic shales remain poorly understood. In the Mesozoic and Cenozoic, organic matter sulfurization can be a major driver of enhanced OM burial across a range of redox conditions comparable to those inferred for many Proterozoic environments. Therefore, in this study, we evaluate the role of sulfurization in driving OM preservation in the Mesoproterozoic Borden Basin and discuss its relevance to Proterozoic systems in general. We present the first evidence for syngenetic-to-early-diagenetic OM sulfurization in a Proterozoic basin, which begins to fill a several-billion-year gap in our record of organic S across Earth history. We find that OM sulfurization was particularly extensive in shales from a relatively shallow-water section (Alpha River) but less extensive in shales deposited in deeper water (Shale Valley), which is consistent with models that infer sulfidic ‘wedges’ or O2-minimum-zone-type structures on shelf margins at least intermittently at this time. At the shallower site, organic S and pyrite are similarly 34S-depleted and thus likely formed at roughly the same time near the sediment–water interface under conditions previously interpreted to have been ferruginous to intermittently sulfidic. In contrast, at the deeper-water site, large S-isotope differences between pyrite and organic S along with low apparent OM sulfurization intensities indicate that pyrite formation was favored over OM sulfurization during early sedimentation under variable but primarily ferruginous conditions. Although Mesoproterozoic biomass can be substantially sulfurized, indicators of sulfurization intensity are not correlated with OM concentrations, and therefore sulfurization does not appear to have been the primary driver of enhanced OM concentrations in Arctic Bay Formation shales. The link between sulfurization and total OM preservation may have been modulated during the deposition of Arctic Bay Formation shales by differences in iron availability, nutrient cycling, and particle dynamics in the Mesoproterozoic.

Offshore bedrock geology of Eclipse Sound and Pond Inlet: connecting the structure and stratigraphy of Bylot and northern Baffin islands

Understanding the Mesoproterozoic and younger structural history of the Eclipse Sound/Pond Inlet area is essential for the interpretation of its Archean to Paleoproterozoic geological history and could have important implications for mineral and petroleum exploration models in the northern Baffin Bay area. The identification of potentially active faults is critical for understanding possible earthquake-related hazards in the area. The integrated interpretation of 1970s-vintage marine seismic data with hill-shaded bathymetry, aeromagnetic data, and onshore geology maps has facilitated the identification of probable Mesoproterozoic (Bylot Supergroup) to Holocene strata on and below the sea floor and a suite of episodically reactivated northwest-striking horst- and graben-bounding normal faults and fault zones. Fault displacement likely occurred during the development of the Mesoproterozoic Borden basin and the Cretaceous–Paleogene opening of Baffin Bay, and in some cases may continue today. Some faults become more west-trending toward the south, which requires parts of these faults to have intermittently accommodated transtensional and (or) transpressional motion, possibly explaining local folds and out-of-graben thrusting. Numerous previously unrecognised faults have been documented, with faults beneath Eclipse Sound (Eclipse Trough) spaced at 5 to 7 km intervals, and at least one fault zone (Cape Hay Fault Zone) that appears to be at least 250 km in length, suggesting faults of similar spacing and scale may be present under Baffin Bay. This study uses a multi-thematic office-based methodology that inexpensively, and with little environmental impact, facilitates the mapping of structures that intersect the sea floor in areas where glaciers have exposed bedrock.

Radiogenic isotope chemostratigraphy reveals marine and nonmarine depositional environments in the late Mesoproterozoic Borden Basin, Arctic Canada

Abstract The ca. 1050 Ma Bylot Supergroup in Arctic Canada is one of the best-preserved archives of late Mesoproterozoic geochemistry and biology and offers evidence that this period of Earth history may have been more biogeochemically dynamic than previously appreciated. The Bylot Supergroup was deposited in the Borden Basin and is the most thoroughly studied stratigraphic succession from a series of broadly contemporaneous late Mesoproterozoic intracratonic basins known as the Bylot basins. This ∼6-km-thick mixed carbonate-siliciclastic succession has undergone minimal postdepositional deformation and is now exposed on Baffin and Bylot Islands, Nunavut, Canada. Deep-water and tidally influenced carbonate facies, traditionally interpreted as marine, have yielded important insights into the evolution of Proterozoic seawater chemistry; however, more recent studies indicate that the Borden Basin was restricted marine or lacustrine for portions of its depositional history. Here, we present new multiproxy radiogenic isotope chemostratigraphic data spanning the Bylot Supergroup. A comparison of stratigraphic trends in radiogenic isotope data from hydrogenous (black shale 187Os/188Os and limestone 87Sr/86Sr) and detrital (whole-rock mudstone 143Nd/144Nd) sedimentary phases elucidates the complex hydrologic history of the Borden Basin and reconciles these disparate interpretations. Episodic coupling and decoupling between the composition of basin waters (from Os and Sr isotopes) and the local weathering input to the basin (from Nd isotopes) indicate that depositional environments within the Borden Basin fluctuated between marine and nonmarine (i.e., lacustrine). Variations in basin hydrology controlled secular sedimentation patterns through changes in basin water chemistry. These interpretations help to characterize the environment in which the early red algal fossil Bangiomorpha pubescens evolved. Episodically restricted epeiric seaways, such as within the Borden Basin, were relatively widespread within Rodinia and may have exerted unique selective pressures on eukaryotic evolution in the Mesoproterozoic Era. Hydrogenous and detrital radiogenic isotope chemostratigraphy, as implemented in this study, may provide a useful paleoenvironmental framework for future paleontological studies aimed at testing the role of freshwater environments in eukaryotic evolution. In addition, 87Sr/86Sr compositions from 81 new middle Bylot Supergroup marine limestone samples, calibrated by recent Re-Os geochronology, contribute to the terminal Mesoproterozoic marine 87Sr/86Sr curve. These data display a rise from ∼0.705 to 0.706 that reflects weathering of the active Grenville orogenic belt and demonstrates a global increase in chemical weathering during the amalgamation of Rodinia.

The Taphonomy of Proterozoic Microbial Mats and Implications for Early Diagenetic Silicification

The complex nature of growth and decomposition in microbial mats results in a broad range of microbial preservation. Such taphonomic variability complicates both the description of microbial elements preserved within geologic materials and the potential interpretation of microbial biomarkers. This study uses a taphonomic assessment to explore the preservation of different microbial components within silicified microbial mats of the late Mesoproterozoic (~1.0 Ga) Angmaat Formation, Bylot Supergroup, Baffin Island. The Angmaat Formation consists of unmetamorphosed and essentially undeformed strata that represent intertidal to supratidal deposition within an evaporative microbial flat. Early diagenetic silicification preserved microbial communities across a range of environments, from those episodically exposed to persistently submerged. Here, we present the development of a new methodology involving the use of high-resolution image mosaics to investigate the taphonomy of microfossils preserved in these mats. A taphonomic grade is assigned using a modified classification that accounts for both the taphonomic preservation state (good, fair, poor) of individual microfossils, as well as the degree of compaction of the overall mat. We show that although various taphonomic states occur within each of the silicified mats, the overall taphonomic assessment differentiates between well-preserved mats that are interpreted to have been silicified during active growth, to highly degraded and compacted mats that are interpreted to represent preservation during later stages of biological decomposition. These data indicate that even small changes in the timing of silicification may have substantial implications on our identification of microbial biomarkers and, therefore, our interpretation of early Earth ecosystems.

Fluid-chemical evidence for one billion years of fluid flow through Mesoproterozoic deep-water carbonate mounds (Nanisivik zinc district, Nunavut)

Ancient carbonate rocks commonly contain numerous post-depositional phases (carbonate minerals; quartz) recording successive diagenetic events that can be deciphered and tied to known or inferred geological events using a multi-pronged in situ analytical protocol. The framework voids of large, deep-water microbial carbonate seep-mounds in Arctic Canada (Mesoproterozoic Ikpiarjuk Formation) contain multiple generations of synsedimentary and late cement. An in situ analytical study of the post-seafloor cements used optical and cathodoluminescence petrography, SEM-EDS analysis, fluid inclusion (FI) microthermometry and evaporate mound analysis, LA-ICP-MS analysis, and SIMS δ18O to decipher the mounds’ long-term diagenetic history. The six void-filling late cements include, in paragenetic order: inclusion-rich euhedral dolomite (ED), finely crystalline clear dolomite (FCD), hematite-bearing dolomite (HD), coarsely crystalline clear dolomite (CCD), quartz (Q), replacive calcite (RC) and late calcite (LC). Based on the combined analytical results, the following fluid-flow history is defined: (1) ED precipitation by autocementation during shallow burial (fluid 1; Mesoproterozoic); (2) progressive mixing of Ca-rich hydrothermal fluid with the connate fluid, resulting in precipitation of FCD followed by HD (fluid 2; also Mesoproterozoic); (3) precipitation of hydrothermal dolomite (CCD) from high-Ca and K-rich fluids (fluid 3; possibly Mesoproterozoic, but timing unclear); (4) hydrothermal Q precipitation (fluid 4; timing unclear), and (5) RC and LC precipitation from a meteoric-derived water (fluid 5) in or since the Mesozoic. Fluids associated with FCD, HD, and CCD may have been mobilised during deposition of the upper Bylot Supergroup; this time interval was the most tectonically active episode in the region’s Mesoproterozoic to Recent history. The entire history of intermittent fluid migration and cement precipitation recorded in seemingly unimportant void-filling mineral phases spans over 1 billion years, and was decipherable only because of the in situ protocol used. The multiple-method in situ analytical protocol employed in this study substantially augments the knowledge of an area’s geological history, parts of which cannot be discerned by means other than meticulous study of diagenetic phases, and should become routine in similar studies.

Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis

Although the geological record indicates that eukaryotes evolved by 1.9–1.4 Ga, their early evolution is poorly resolved taxonomically and chronologically. The fossil red alga Bangiomorpha pubescens is the only recognized crown-group eukaryote older than ca. 0.8 Ga and marks the earliest known expression of extant forms of multicellularity and eukaryotic photosynthesis. Because it postdates the divergence between the red and green algae and the prior endosymbiotic event that gave rise to the chloroplast, B. pubescens is uniquely important for calibrating eukaryotic evolution. However, molecular clock estimates for the divergence between the red and green algae are highly variable, and some analyses estimate this split to be younger than the widely inferred but poorly constrained first appearance age of 1.2 Ga for B. pubescens. As a result, many molecular clock studies reject this fossil ex post facto. Here we present new Re-Os isotopic ages from sedimentary rocks that stratigraphically bracket the occurrence of B. pubescens in the Bylot Supergroup of Baffin Island and revise its first appearance to 1.047 +0.013/–0.017 Ga. This date is 150 m.y. younger than commonly held interpretations and permits more precise estimates of early eukaryotic evolution. Using cross-calibrated molecular clock analyses with the new fossil age, we calculate that photosynthesis within the Eukarya emerged ca. 1.25 Ga. This date for primary plastid endosymbiosis serves as a benchmark for interpreting the fossil record of early eukaryotes and evaluating their role in the Proterozoic biosphere.

Proterozoic microbial mats and their constraints on environments of silicification.

The occurrence of microfossiliferous, early diagenetic chert in Proterozoic successions is broadly restricted to peritidal marine environments. Such coastal environments are amongst the most environmentally variable of marine environments, experiencing both enhanced evaporation and potential influx of terrestrial freshwaters. To better understand potential conditions under which silicification occurs, we focus on microfossiliferous early diagenetic chert from the Mesoproterozoic Bylot Supergroup, northern Baffin Island. Spectacular preservation of silicified microbial mats, their associated mineral phases, and the petrographic fabrics of the chert itself require that silicification occurred at the sediment-water interface, penecontemporaneously with mat growth. In some cases, silica is the primary precipitated mineral phase and is not associated with replacement of precursor mineral phases. In other cases, silica deposition includes the mimetic replacement of carbonate, gypsum, and halite mineral phases. These petrographic constraints suggest that silicification potentially occurred under a range of fluid chemistries associated with environmental variability in nearshore peritidal environments. Here we provide the first direct thermodynamic modeling of hypothetical Proterozoic seawater solutions, seawater-derived brines, and mixed seawater-freshwater solutions, and demonstrate that peritidal environments are capable of providing a wide range of fluid chemistries under which early diagenetic silica can both precipitate and replace primary mineralogical phases. Despite the thermodynamic potential for silica deposition under a wide range of fluid compositions, chert is not ubiquitous in Proterozoic nearshore environments, suggesting that the kinetics of silica polymerization exert a primary control over deposition.

Re–Os age constraints and new observations of Proterozoic glacial deposits in the Vazante Group, Brazil

A new Re–Os radiometric age date for an organic-rich shale horizon from the Vazante Group in Brazil, coupled with geological observations, provide evidence for late Mesoproterozoic glacial episodes, conflicting with the general view of greenhouse conditions marked by a eustatic high stand at this time. Field observations of a reverse fault juxtaposing older Mesoproterozoic sedimentary rocks above younger Neoproterozoic strata provide a new stratigraphic framework and reconcile the apparent inversion of U–Pb detrital zircon ages through the succession. Combined, the geochronological, geochemical and stratigraphic evidence suggest that the Vazante Group sediments accumulated along a passive margin of the São Francisco craton and are correlative with the neighboring Paranoá Group. Biomarker, sulfur isotope and iron speciation analyses support the interpretation of a strongly stratified water column during post-glacial transgression and deposition of one of the bituminous shale horizons. The relationship of the glaciogenic Vazante Group to other late Mesoproterozoic successions, such as the non-glacial Atar Group in West Africa and Bylot Supergroup in arctic Canada, however, remains enigmatic.

Geologic and Geochronologic Constraints on the Timing of Mineralization at the Nanisivik Zinc-Lead Mississippi Valley-Type Deposit, Northern Baffin Island, Nunavut, Canada

The Nanisivik mine is located on Borden Peninsula, northern Baffin Island, about 750 km north of the Arctic Circle. The deposit is hosted by Mesoproterozoic sedimentary rocks of the Bylot Supergroup that rest unconformably on the Mary River Group, an Archean to Proterozoic basement complex of granites and gneisses. The Mesoproterozoic strata have been folded into broad, open folds with north-south–trending axes. The sulfides at Nanisivik are hosted in petroliferous dolostones of the Society Cliffs Formation. A series of long-lived, east-west–trending, normal faults divide the Nanisivik area into a series of horst and grabens. All known sulfide bodies are associated with normal faults, particularly where they cut the fold axes of antiforms. The geometry of the main sulfide body indicates that sulfides were emplaced after tilting and folding of the host strata and that a horizontal feature such as a gas-water interface controlled the precipitation of the sulfides. The sulfides show no displacement across the Keystone fault, although the fault has displaced the host stratigraphy by about 150 m and has displaced the Mine dike, a Franklin-aged intrusion (~723 Ma), by about 40 m with steep oblique-slip movement. A progressive potassium feldspar alteration selvage, about 3 m wide, on the margin of the Mine dike has been recognized. Textural evidence and immobile element ratios indicate that this is an alteration product of the dike, interpreted to have formed by the hydrothermal event that emplaced the sulfides. Similar alteration assemblages are present throughout the mine and are interpreted to be altered siliciclastic sedimentary rocks. The 40 Ar- 39 Ar ages of orthoclase from these alteration assemblages indicate a Middle Ordovician (461 Ma) age of mineralization. Although the regional process that resulted in the formation of the Nanisivik deposit remains unclear, it is possible that topographically driven fluid may have resulted from localized Middle Ordovician uplift in the region, as seen at the Navy Board High. Basinal brines may have migrated along the basement contact within the overlying Mesoproterozoic siliciclastic sedimentary rocks, and local basement topography, such as the horst and graben structures, may have focused fluid flow into the Society Cliffs Formation at Nanisivik. Local structural and stratigraphic traps would ultimately have localized and controlled the precipitation of the sulfides.

Evidence for reversal of basin polarity during carbonate ramp development in the Mesoproterozoic Borden Basin, Baffin Island

Distribution of facies in the lower half of the Bylot Supergroup suggests overall westward deepening of the Mesoproterozoic Borden Basin. In marked contrast, the upper half of the succession records a reversal in the overall bathymetric trend, such that the eastern portion underwent relative deepening as the west experienced relative shallowing. Strata deposited during this reversal belong to the Victor Bay Formation, a ramp composed predominantly of limestone. Karsting of carbonate strata and development of an angular unconformity in the west contrast with back-stepping and drowning of the ramp in the east, followed by mantling by deep-water limestone, carbonaceous carbonate, and turbidites. Increased accommodation space during this time, via both tectonic subsidence and eustatic sea-level rise, led to a profusion of stromatolite pinnacle reefs and large biostromes. The reversal of basin polarity is best reconciled with development of a distal foreland basin superimposed on the Borden aulacogen. Crustal rethickening and uplift occurred along reactivated basement faults during an eastward-directed compressional event and could be related to thrusting of similar age and vergence in the Coppermine River Group of northwestern Canada.

Geochemistry of a 1.2 Ga carbonate-evaporite succession, northern Baffin and Bylot Islands: implications for Mesoproterozoic marine evolution

A 4‰ positive shift in the carbon isotopic composition of the oceans, recorded globally in marine carbonate rocks at ∼1.3 Ga, suggests a significant change in Mesoproterozoic carbon cycling. Enhanced burial fluxes of organic carbon, relative to inorganic carbon, implied by this isotopic shift may have resulted in increased oxygenation of the Earth's biosphere, as has been suggested for similar Paleoproterozoic and Neoproterozoic carbon isotope events. This hypothesized Mesoproterozoic oxygenation event may be recorded in the geologic record by the appearance of the oldest preserved, laterally extensive, bedded marine CaSO 4 evaporites in the ∼1.2 Ga Grenville and Bylot supergroups. Speculation that the appearance of extensively preserved marine gypsum and/or anhydrite reflects increased biospheric oxygenation has been challenged, however, by the hypothesis that CaSO 4 precipitation prior to the Mesoproterozoic may have been inhibited by significantly higher marine carbonate saturation, which would have facilitated carbonate precipitation and effectively limited Ca 2+ availability during seawater evaporation (Grotzinger, J.P., 1989. Controls on Carbonate Platform and Basin Development, vol. 44, SEPM, Tulsa, OK, pp. 79–106), regardless of O 2 levels. The 1.2 Ga Society Cliffs Formation (Bylot Supergroup, northern Baffin Island) consists of ∼720 m of peritidal carbonates, evaporites, and minor siliciclastic rocks. Evaporites occur predominantly in the lowermost 300 m of the Society Cliffs Formation, where gypsum beds (1–250 cm thick) constitute up to 15% of the exposed strata. Stratigraphic and sedimentologic constraints, as well as isotopic (C, O, Sr) and elemental (Ca, Sr, Na, K, Ba) compositions of evaporites and associated carbonates, indicate a marine origin for Society Cliffs gypsum. An upsection increase in δ 34S of Society Cliffs gypsum (from +22‰ to +32‰ VCDT) is therefore interpreted to reflect primary variation in Mesoproterozoic marine sulfate compositions, although the inferred rapidity of isotopic change requires a marine sulfate reservoir significantly smaller than that of the modern ocean. Examination of the maximum fractionation between coeval sulfide and sulfate reservoirs, however, indicates that Mesoproterozoic oceans were not sulfate-limited with respect to bacterial sulfate reduction either before or after the hypothesized 1.3 Ga oxygenation event. Although increased ocean-atmosphere oxygenation may have increased marine sulfate concentrations at this time, the exact role of a Mesoproterozoic oxygenation event cannot be ascertained. Furthermore, high Mg/Ca ratios measured in Society Cliffs gypsum suggest that elevated Mg 2+ concentrations in Proterozoic marine systems may have helped sustain carbonate hypersaturation, and that Ca 2+-limitation may have played a significant role in the Proterozoic record of evaporite deposition.

Organic-walled microfossils from the Proterozoic Thule Supergroup, Northwest Greenland

The Proterozoic rocks of the intracratonic Thule Basin on the northern margin of the Canadian–Greenlandic shield have recently been referred to as the Thule Supergroup. The succession comprises unmetamorphosed, unfolded sedimentary and magmatic rocks, which are, with few exceptions, not deformed. Five groups have been defined and fine-grained siliciclastic samples from the uppermost three (Baffin Bay, Dundas and Narssârssuk groups), yielded moderately well-preserved acanthomorphic and non-acanthomorphic acritarchs, prasinophytes and filamentous microfossils. The age of the supergroup is quite poorly constrained, at the base by a 1268 Ma basaltic sill, and at the top by a basic dyke swarm with a K/Ar age range between 630 and 725 Ma, that cuts the entire succession. Biostratigraphical studies further constrain the age of the Thule Supergroup and suggest that the succession is of a late Mesoproterozoic/earliest Neoproterozoic age. This age assignment is corroborated by independent stable isotope data and correlation with the Bylot Supergroup to the south. This leads us to propose that the Thule Supergroup was deposited between c. 1300 and c. 1200 Ma. Microfossil taxa previously interpreted as exclusively occurring in pre-Varangerian, Neoproterozoic rocks (i.e. Simia annulare Jankauskas, Tasmanites rifejicus Jankauskas and possibly Vandalosphaeridium varangeri Vidal) are consequently re-evaluated. Process-bearing acritarchs, herein attributed under open nomenclature to ? Comasphaeridium sp., ` Trachyhystrichosphaera truncata' and ? Vandalosphaeridium sp., are of special palaeobiological importance since acanthomorphic acritarchs have rarely been reported from pre-Neoproterozoic successions.

δ13C stratigraphy of the Proterozoic Bylot Supergroup, Baffin Island, Canada: implications for regional lithostratigraphic correlations

The Bylot Supergroup, northern Baffin Island, contains >1500 m of platform, shelf, and slope carbonates deposited between ~ 1270 and ~ 723 Ma. Limited chronostratigraphic data have led to the broad correlation of the Bylot Supergroup with predominantly Neoproterozoic successions in northern and western Laurentia; yet, detailed correlation has been impossible given biostratigraphic and lithostratigraphic limitations. Carbon-isotope chemostratigraphy represents a potential dataset to constrain such interregional correlations. Carbon isotopic data from the Bylot Supergroup and broadly coeval successions from Somerset Island and northwest Greenland reveal distinct stratigraphic trends in δ13C, with intervals of moderate 13C enrichment (+3.5 ± 1‰) punctuated by excursions to slightly negative values (-1.0 ± 1‰). Although the scale of the observed variation is muted relative to Neoproterozoic standards, the dissimilarity of values to those recorded in northwestern Laurentia suggests that these strata delineate a discrete depositional interval. Comparison of isotopic values with published data indicates that δ13C values between approximately -1.0 and +4.0‰ are characteristic of the interval between ~ 1300 and ~ 800 Ma. This pattern is distinct from that of younger Neoproterozoic successions, which typically record values >+5‰, and older Mesoproterozoic successions, which record values near 0‰, and suggests that these moderately positive values may be useful for broad time correlation. Compilation of new and published data permits the tentative reconstruction of a global Mesoproterozoic carbon isotopic curve.

δ<SUP>13</SUP>C stratigraphy of the Proterozoic Bylot Supergroup, Baffin Island, Canada: implications for regional lithostratigraphic correlations

δ<SUP>13</SUP>C stratigraphy of the Proterozoic Bylot Supergroup, Baffin Island, Canada: implications for regional lithostratigraphic correlations

Shale-Facies Microfossils from the Proterozoic Bylot Supergroup, Baffin Island, Canada

Organic-walled microfossils are reported from the Borden Rift Basin of northern Baffin Island and Bylot Island. The diverse assemblage, obtained by palynological acid maceration, comprises 39 taxa and occurs in fine-grained siliciclastic and carbonate units from all but the two lowermost formations of the Bylot Supergroup; their age is bracketed between 1,270 and 750 Ma. Of 307 samples analyzed from all parts of the basin, 133 yielded identifiable remains. The microbiota comprises the following taxa:Siphonophycus septatum, S. robustum, S. rugosumn. comb.,S. kestron, S. capitaneum, Pellicularia tenera, Polythrichoides lineatus, Obruchevella valdaica, Karamia? sp., opaque filaments,Oscillatoriopsisspp.,Palaeolyngbyasp.,Arctacellularia tetragonalan. comb.,A. ellipsoidea, Chlorogloeaopsis contextan. comb.,C. kanshiensisn. comb.,Navifusa bacillarisn. comb.,N. majensis, N. actinomorphan. comb.,Leiosphaeridia minutissima, L. tenuissima, L. crassa, L. jacutica, L. ternata, Lophosphaeridium granulatum, Valeria lophostriata, Pterospermopsimorpha insolita, Eomicrocystis elegans, E. malgica, Symplassosphaeridiumspp.,Synsphaeridiumspp.,Satka squamifera, Satkaspp.,Coneosphaera arctican. sp.,Coneosphaerasp.,Spumosina rubiginosa, andFabiformis baffinensisn. sp. The taxonPalaeocalothrix divaricatus, present in some macerates, is possibly a modern fungal contaminant. Abundance and taxonomic diversity varies considerably between formations and environments, with the Arctic Bay Formation exhibiting the greatest productivity and species diversity of the seven formations studied, and the transgressive muddy shelf deposits being the most productive of the 11 environments recognized. This microfossil assemblage of predominantly planktonic species has distinctive taxa in common with Middle to Late Riphean sequences in Siberia (Lakhanda and Miroedikha Groups, ca. 950 to 850 Ma), the lower part of the Thule Group (< 1,270 Ma) in northwest Greenland, the Fury and Hecla Group in western Baffin Island, and taxa reported from the upper part of the Bushimay Supergroup in Zaire. On the other hand, some elements in the Bylot Supergroup are more characteristic of Vendian beds on the Russian Platform, indicating longer stratigraphic ranges for such taxa than previously thought. The compositional similarity between the Borden Basin and Siberian sequences supports palinspastic reconstructions juxtaposing Siberia and Laurentia during the middle to late Proterozoic, and the opening of the Poseidon Ocean.

Shelf-facies microfossils from the Uluksan Group (Proterozoic Bylot Supergroup), Baffin Island, Canada

Coccoid and filamentous microfossils in the Borden Rift Basin on northwestern Baffin Island and western Bylot Island are reported from 14 localities at six general stratigraphic levels in the Society Cliffs Formation, and one in the Victor Bay Formation. At least seven of 12 fossiliferous Society Cliffs samples, and one of two Victor Bay samples, are from cherty dolostones formed in shallow restricted, peritidal evaporitic environments. The remaining six fossiliferous samples accumulated in somewhat deeper, less restricted waters. The assemblage is preserved in black chert nodules and layers and comprises 30 taxa, including the filamentous formsArchaeotrichion? sp.,Tenuofilum septatum, Eomycetopsis robusta, Siphonophycus inornatum, S. kestron, Rhicnonema antiquum, Brachypleganonsp.,Eomicrocoleussp.,Uluksanella baffinensisn. gen. and sp.,Talakania? sp., the coccoid taxaEosynechococcus medius, E. grandis, Archaeoellipsoides obesus, Sphaerophycus parvum, S. medium, Myxococcoides minor, M. grandis, Melasmatosphaeraspp.,Phanerosphaerops capitaneus, Palaeoanacystissp.,Tetraphycus hebeiensis, Eogloeocapsa bella, Gloeodiniopsis magna, G.sp. 1,G. mikros, Polybessurus bipartitus, Cymatiosphaera? sp., the fusiformEupoikilofusa? sp., and rare specimens of two unidentified and problematic taxa.