Neoproterozoic-Cambrian Boundary Research Articles

Significance of a late neoproterozoic – Early Cambrian southern Baltica active margin in late-stage Rodinian and early Gondwanan reconstructions

New whole-rock geochemical data indicate changing provenance of sedimentary rocks deposited on the southern margin of Baltica in the late Neoproterozoic to early Paleozoic. The oldest part of the sequence is characterized by highly weathered detritus sourced from a mafic igneous source, inferred to be the nearby Volyn Large Igneous Province (L.I.P.), in an intracontinental rift (aulacogen) setting. Neoproterozoic–Cambrian boundary sequences are weakly weathered and sourced from intermediate to felsic rocks in an active margin setting whereas the youngest parts of the sequence exhibit significant quartz accumulation consistent with deposition of recycled material in a passive margin setting. These data support published U-Pb and Lu-Hf zircon data from the same sedimentary sequence where the Neoproterozoic-Cambrian boundary strata are characterized by anomalously high Ediacaran–Cambrian zircon contribution with juvenile isotopic composition. Taken together, these data indicate a cryptic late Neoproterozoic–early Cambrian active margin along the periphery of southern Baltica. Based on a correlation of similarly aged foreland basin and accretionary complexes, and links to paleomagnetically constrained plate reconstructions it is hypothesized that this detritus was sourced from the Avalonian micro-continent and a Cambrian arc terrane that is today preserved in high-grade metamorphic complexes of the European Variscides. These findings have potentially profound implications for our understanding of the supercontinent cycle that links the Rodinia to Pangea supercontinents, in particular the role played by Baltica-derived terranes in Paleozoic collisional orogeny.

The Neoproterozoic Gillen Formation, Amadeus Basin, central Australia: an intra-salt petroleum system and viable exploration target?

The Gillen Formation, the basal component of the lower Neoproterozoic Bitter Springs Group, occurs throughout the Amadeus Basin of central Australia and comprises a basal sequence of black shales and stromatolitic dolostones overlain by an evaporitic sequence of a thick halite sandwiched between two anhydrite/dolomite units. These Gillen lithologies include potential source, reservoir and seal units for petroleum, suggesting an analogy could be drawn with the prolific evaporitic Ara Group Petroleum System of South Oman. Although well data penetrating the Gillen Formation are very limited, organic-rich shales have been penetrated in both the north and south of the basin, while both well and seismic data indicate the ubiquitous presence of sealing evaporites across the basin. The Ara Group Petroleum System spans the Neoproterozoic–Cambrian boundary and is blessed with carbonate reservoirs composed of metazoan ‘reefs’ comprising Cloudina and Namacalathus, which provide an open framework porosity approaching 50%. By contrast, the Gillen Formation was deposited during the Tonian Period of the Neoproterozoic, some 300–350 million years older than the Ara Group, and has reservoir facies limited to stromatolitic carbonates with fenestral to vuggy porosities that are typically less than 15%. Nevertheless, seismic data reveal numerous intra-evaporite opportunities within the Gillen Formation that, seismically, are virtually indistinguishable from those being exploited in South Oman, suggesting that a Gillen Intra-Salt Petroleum System is a viable exploration target throughout a majority of the Amadeus Basin.

Copper, zinc, chromium and osmium isotopic compositions of the Teplá-Barrandian unit black shales and implications for the composition and oxygenation of the Neoproterozoic-Cambrian ocean

Two Neoproterozoic-Cambrian shale suites that consist of normal (NBS) and metal-rich (HMBS) black shales from the Teplá-Barrandian Unit, Bohemian Massif (Czech Republic) are studied in terms of their Cu, Zn, Cr and Re–Os isotopic compositions to improve our knowledge of elemental and isotopic mass balance of these elements in Earth's oceans in the past and the extent of surface oxygenation at the Neoproterozoic–Cambrian boundary. The rock compositions indicate that the inputs of Cu and Zn have been controlled by variable proportions of metal-rich authigenic and low-metal terrigenous sources, the latter having exceptionally low δ65Cu and partially δ66Zn. In this respect, our data confirm previous studies conclusions that organic-rich sediments with high a terrigenous flux play an important role in the sinking of light Zn in the oceans and provide new evidence that these types of rocks also represent the currently unidentified burial of very light Cu in the ocean. The chromium isotopic composition reveals a complex history of Cr uptake and fractionation through the dominant Cr input from different terrigenous sources with variable δ53Cr followed by Cr fractionation that was controlled by redox states (NBS). On the other hand, mixing and/or continuous equilibrium isotopic exchange between seawater and hydrothermal Cr was the dominant process for the HMBS. The Re–Os contents of the NBS were controlled by hydrogenous components, which yield a poorly defined Re–Os age for the least disturbed samples of 555 ± 60 Ma, indicating an open-system behavior, while the HMBS Re–Os composition was largely influenced by hydrothermal fluids during deposition. The calculated authigenic δ65Cu, which is similar to that in the present-day ocean, and highly positive δ53Cr values of the HMBS suggest high levels of surface oxygenation during the Neoproterozoic–Cambrian transition. Furthermore, the authigenic δ66Zn, which is indistinguishable from that in the present-day ocean, may imply a constant Zn isotopic composition of the oceans from the Neoproterozoic because of the well-balanced cycle of phosphates, which were strongly adsorbed by Fe-hydroxides. The radiogenic nature of the initial 187Os/188Os (~0.6), although with a high associated error, seems to not confirm previous indications of an abrupt increase of seawater 187Os/188Os during the late Neoproterozoic due to the large-scale radiogenic Os flux into the ocean.

Bioturbation and directionality in Earth's carbon isotope record across the Neoproterozoic–Cambrian transition

Mixing of sediments by moving animals becomes apparent in the trace fossil record from about 550 million years ago (Ma), loosely overlapping with the tail end of the extreme carbonate carbon isotope δ13 Ccarbonate fluctuations that qualitatively distinguish the Proterozoic geochemical record from that of the Phanerozoic. These Precambrian-scale fluctuations in δ13 Ccarbonate (PSF-δ13 Ccarbonate ) remain enigmatic, due to their high amplitude and inclusion of global-scale negative δ13 Ccarbonate values, below anything attributable to mantle input. Here, we note that different biogeochemical-model scenarios plausibly explaining globally synchronous PSF-δ13 Ccarbonate converge: via mechanistic requirements for extensive anoxia in marine sediments to support sedimentary build-up of 13 C-depleted carbon. We hypothesize that bioturbation qualitatively reduced marine sediment anoxia by exposing sediments to oxygenated overlying waters, which ultimately contributed to decreasing the carbon cycle's subsequent susceptibility to PSF- δ13 Ccarbonate . Bioturbation may also have reduced the quantity of (isotopically light) organic-derived carbon available to contribute to PSF- δ13 Ccarbonate via ocean crust carbonatization at depth. We conduct a comparative modelling exercise in which we introduce bioturbation to existing model scenarios for PSF- δ13 Ccarbonate : expressing both the anoxic proportion of marine sediments, and the global organic carbon burial efficiency, as a decreasing function of bioturbation. We find that bioturbation's oxygenating impact on sediments has the capacity to prevent PSF- δ13 Ccarbonate caused by authigenic carbonate precipitation or methanogenesis. Bioturbation's impact on the f-ratio via remineralization is partially offset by liberation of organic phosphate, some of which feeds back into new production. We emphasize that this study is semiquantitative, exploratory and intended merely to provide a qualitative theoretical framework within which bioturbation's impact on long-term, first-order δ13 Ccarbonate can be assessed (and it is hoped quantified in more detail by future work). With this proviso, we conclude that it is entirely plausible that bioturbation made a decisive contribution to the enigmatic directionality in the δ13 Ccarbonate record, from the Neoproterozoic-Cambrian boundary onwards.

Ichnological evidence for the Cambrian explosion in the Ediacaran to Cambrian succession of Tanafjord, Finnmark, northern Norway

AbstractThe Ediacaran to Cambrian transition of the Digermul Peninsula consists of an ichnologically rich, thick, conformable, shallow marine siliciclastic succession that crosses the Neoproterozoic–Cambrian boundary. The Tanafjord section has been independently dated by published palynological and new body fossil discoveries. As is also observed at the current Global Stratotype Section and Point (GSSP) for the Ediacaran–Cambrian boundary at Fortune Head in Newfoundland, Canada, there is a marked increase in burrow size and inferred behavioural diversity around the inferred boundary level at a surface without a significant hiatus. The diversity of this trace fossil assemblage presents an opportunity to compare the ichnological first appearance datums between the GSSP and another sedimentologically similar, but palaeogeographically remote, succession. We found that the first appearance datums of taxa in Finnmark broadly support the definition and stratigraphic extents of the Harlaniella podolica, Treptichnus pedum, Rusophycus avalonensis and Cruziana tenella zones. Our work shows that there is a marked increase in ichnodiversity in the lowermost Fortunian, mostly in the form of shallow tier traces. The main post-Fortunian ichnological innovation is the evolution of bulk sediment deposit feeding that is first evidenced by spreiten burrows at around the base of Cambrian Stage 2 in both the GSSP and in the Tanafjord section.Supplementary material: Additional images of trace fossils from the studied section are available at https://doi.org/10.6084/m9.figshare.c.3462561

Ediacara-type fossils from Lower Cambrian strata in Qingzhen County, Guizhou Province, South China

INTRODUCTION The Ediacara fauna, generally believed to contain the oldest metazoan fossils, has been described from about 25 localities around the world (Hofmann et al., 1990). It was commonly considered that this fauna was restricted to Late Neoproterozoic (Glaessner, 1984). These were suggested to be mostly nonlocomotory quasi-autotrophs that were wiped out during the mass extinction at the Neoproterozoic– Cambrian boundary (Seilacher, 1984). The oldest Ediacara-type body fossils were discovered below Proterozoic glacial deposits (Hofmann et al., 1990), but mainly restricted to Late Neoproterozoic (Glaessner, 1984). However, some elements of this fauna were continually discovered in Cambrian strata (Hagadom and Waggoner, 2000; Hagadom et al., 2000; Jensen et al., 1998; Crimes et al., 1995; Narbonne et al., 1991). In Qingzhen County, Guizhou Province, South China, there is a well exposed stratigraphic succession spanning the Neoproterozoic–Cambrian boundary. Here, an Ediacara fauna, including Tirasiana

Late Neoproterozoic carbonate productivity in a rifting context: the Adoudou Formation and its associated bimodal volcanism onlapping the western Saghro inlier, Morocco

AbstractAn interval of episodic carbonate productivity, lithostratigraphically recognized as the ‘Calcaires inférieurs’ (upper member of the Adoudou Formation), took place across the Neoproterozoic–Cambrian transition onlapping the western Saghro inlier, Morocco. Sedimentation of the ‘Calcaires inférieurs’ was highly variable: in relatively stable substrates, a peritidal-dominated mixed platform is recorded where deposition was primarily controlled by autocyclic processes and accommodation space availability, whereas, in unstable substrates, the tectonic activity associated with the inherited block-faulting basement led to deposition of complex slide sheets composed of penecontemporaneous isoclinal folds and disrupted strata. The uppermost part of the ‘Calcaires inférieurs’ displays a negative δ13C shift reaching values of −6.5‰. This shift may represent the δ13C excursion to −6‰ that marks the Neoproterozoic–Cambrian boundary in the western Anti-Atlas. Two volcanic episodes bracketed the carbonate productivity. They consist of lower basaltic flows and an upper rhyolitic ignimbrite, with a SiO2 gap between 52 and 74 wt%. The basic rocks resemble those of tholeiitic magmas in continental rifts. The felsic rocks show high light to heavy rare earth element abundances and negative Nb, Ta, P and Ti anomalies, and were probably generated as a result of either fractional crystallization coupled with relative crustal contamination, or from a different magmatic source. The lower basic flows of tholeiitic affinity predated and geochemically differ from the alkaline magmatism of the Alougoum volcanic complex (Boho jbel) that surrounds the neighbouring Bou-Azzer inlier.

Was there a conflict at the Neoproterozoic-Cambrian boundary: Evidence from sulfur isotope composition?

Based on evaporite sequences of the Irkutsk amphitheater, it is shown that sulfur in the Vendian-Lower Cambrian sedimentary sulfates displays a very wide scatter of sulfur isotope ratios and enrichment in heavy sulfur up to average values of δ34S ≈ +(27–30)‰. These features are related to sulfate reduction, which is distinctly expressed like other secondary alterations in the studied sections. Average δ34S values reflect the dynamics of isotopic effects rather than the initial sulfur composition of the oceanic water. The Irkutsk amphitheater can be considered natural model of sulfur isotopic variations in ancient evaporites. Existing concepts of the sulfur isotopic composition of Cambrian oceans need to be revised.

Trends in oolite dolomitization across the Neoproterozoic–Cambrian boundary: A case study from Death Valley, California

Changes in dolomite fabric within the Neoproterozoic–Cambrian stratigraphic succession in Death Valley, California preserve evidence for a first-order biogeochemical change in early diagenetic environments through the Neoproterozoic–Cambrian transition. Oolite forms the primary focus of the study so that the general paleoenvironmental setting and presumed susceptibility to early diagenesis and fluid flow can be kept relatively constant across geologic time. Two key generalizations can be made: (1) late Neoproterozoic dolomitization in Death Valley occurred very early in the paragenetic sequence, was volumetrically significant, and preserved fine detail of the original aragonite or calcite precursor; and (2) Early Cambrian dolomitization in Death Valley occurred late in the paragenetic sequence with burial, was commonly incomplete, and obliterated details of the calcite or aragonite precursor; early diagenetic dolomitization of any kind was absent. The dolomitization fabrics in Death Valley parallel the presumed pattern of seawater Mg/Ca with respect to mimetic dolomitization in the Paleozoic (low Mg/Ca, little/no mimetic dolomitization) and Cenozoic (higher Mg/Ca, more common mimetic dolomitization). However, the volume of Neoproterozoic mimetic dolomitization is immense compared to the Cenozoic examples. We speculate that a fundamental shift in the early diagenetic environment occurred in late Neoproterozoic time, perhaps in association with secular variation in the sulfate concentration of seawater and the advent and proliferation of penetrative bioturbation that destroyed sediment-capping microbial mats. An oscillatory trend in the fabric and volume of dolomite through the Phanerozoic has been suggested, but the termination of volumetrically significant mimetic dolomitization across the Neoproterozoic–Cambrian transition, a critical time in Earth history for many reasons, dwarfs the oscillatory trends noted in the Phanerozoic.

Facies distribution of the Lower Cambrian cryptic microbial and epibenthic archaeocyathan‐microbial communities, western Anti‐Atlas, Morocco

AbstractMarine microbial communities recorded in the Moroccan Anti‐Atlas were unaffected across the Neoproterozoic–Cambrian transition. A stromatolite‐dominated consortium was replaced at the beginning of the Atdabanian (ca 20 Myr after the Neoproterozoic–Cambrian boundary) by shelly metazoan and thromboid consortia, which contain the oldest biostratigraphically significant fossils of the Moroccan Cambrian. The associated collapse of microbial mat (stromatolitic) growth appears to coincide with a change from pre‐Atdabanian shallow‐water restricted conditions into Atdabanian deeper, open‐sea conditions. It is postulated that this environmental change led to an episode of improved water circulation over carbonate platform interiors, promoting shelly metazoan immigration into the region. The Tiout/Amouslek lithostratigraphic contact in the early Atdabanian marks the end of an episodically unstable seafloor as suggested by the abundance of slumping and sliding structures, and synsedimentary microfaults and cracks recorded in the underlying Tiout Member. Concurrent with the transition is the occurrence of a network of cryptic fissures and cavities that provided habitats for a coelobiontic chemosynthetic–heterotrophic microbial community composed of stromatolitic crusts, Renalcis–Epiphyton–Girvanella intergrowths, and Kundatia thalli. In the overlying Amouslek Formation, archaeocyathan–thromboid reefs were constrained by substrate stability, water depth and subsidence rate. Four reef geometries are distinguished: (i) patch reefs surrounded by shales, (ii) bioherms in which flank beds intercalate laterally with carbonate and shale inter‐reef sediments, (iii) biostromes or low‐relief structures formed as a result of lateral accretion of patch reefs, and (iv) kalyptrate complexes that nucleated because of a marked tendency for aggregation, and in which patch reefs and bioherms occur stacked together bounded by clay–marl–silt seams.

Stratigraphy and environmental conditions of the terminal Neoproterozoic–Cambrian Period in Oman: evidence from sulphur isotopes

Terminal Neoproterozoic and Cambrian evaporite rocks worldwide record a marked sulphur isotope excursion to very heavy δ 34 S isotopes that reflect a period of significant burial of reduced sulphur. New δ 34 S data from anhydrite and pyrite of the Ara Group in Oman extend this record to Arabia. Depositional history of the Ara Group spans c . 8 Ma across the terminal Neoproterozoic–Cambrian boundary. Therefore, these rocks provide a detailed record of sulphur isotopic evolution in this important time interval. Ara Group sulphur isotope values include a mean δ 34 S anhydrite of 39.4‰ ± 2.6‰ (1σ; range 32.4–46.4‰; n =61), and a mean δ 34 S pyrite of 1.4‰ ± 7.3‰ (1σ; range −10.8‰ to 11.4‰; n =12). The isotope signatures were not altered significantly during diagenesis and are representative of the isotopic composition of parent waters in the basin. Marine bromine concentrations in halite and marine fossils in associated carbonate rocks indicate that these waters had a marine origin. Bacterial sulphate reduction was the dominant process leading to the segregation between isotopically heavy sulphur incorporated in evaporite rocks and isotopically light sulphur buried in pyrite and organic matter. Strong sulphate reduction operated in a hypersaline and partly anoxic basin; these environmental conditions were also conducive to the preservation of organic material and make the Ara Group a prolific hydrocarbon province. Significant variation of the observed values is attributed to the inherent variability of bacterial sulphate reduction, including partially closed-system behaviour, disproportionation processes, limited diagenesis, and possibly secular variation of sulphur isotopes within the basin. Ara Group δ 34 S anhydrite values overlap with data from other terminal Neoproterozoic to Cambrian basins, but the mean δ 34 S anhydrite is highest in Oman. This observation cannot be attributed solely to the specific anoxic conditions and strong bacterial sulphate reduction in Oman, because such conditions were widespread in most evaporite basins of the terminal Neoproterozoic–Cambrian Periods. Instead, slight age differences between these basins can introduce a bias in the sulphur isotope curve. Excellent preservation of Ara Group deposits and in particular the precise age constraints make the Ara Group representative for the seawater sulphur isotopic composition at the terminal Neoproterozoic–Cambrian boundary. As a result of the inherent variability of bacterial sulphate reduction, δ 34 S values at the lower end of the spectrum should be most representative for the global signal. Based on results of this study, the global signal is estimated to be approximately 35–40‰.

The Proterozoic and Earliest Cambrian Trace Fossil Record; Patterns, Problems and Perspectives

The increase in trace fossil diversity across the Neoproterozoic-Cambrian boundary often is presented in terms of tabulations of ichnogenera. However, a clearer picture of the increase in diversity and complexity can be reached by combining trace fossils into broad groups defined both on morphology and interpretation. This also focuses attention on looking for similarites between Neoproterozoic and Cambrian trace fossils. Siliciclastic sediments of the Neoproterozoic preserve elongate tubular organisms and structures of probable algal origin, many of which are very similar to trace fossils. Such enigmatic structures include Palaeopascichnus and Yelovichnus, previously thought to be trace fossils in the form of tight meanders.A preliminary two or tripartite terminal Neoproterozoic trace fossil zonation can be be recognized. Possibly the earliest trace fossils are short unbranched forms, probably younger than about 560 Ma. Typical Neoproterozoic trace fossils are unbranched and essentially horizontal forms found associated with diverse assemblages of Ediacaran organisms. In sections younger than about 550 Ma a modest increase in trace fossil diversity occurs, including the appearance of rare three-dimensional burrow systems (treptichnids), and traces with a three-lobed lower surfaces.

Stratigraphic and structural framework of the Neoproterozoic Paracuellos Group, Iberian Chains, NE Spain

Abstract The Neoproterozoic Paracuellos Group of the Iberian Chains constitutes the core of two disconnected faulted blocks, named the Paracuellos and Codos antiforms. Precise lithostratigraphic correlations between both areas are not possible due to the structural complexity and because marker beds do not persist laterally. This paper presents a crustal cross-section of the Neoproterozoic axial core (the Paracuellos antiform) based on surface geology, boreholes and seismic reflection profiles. Seismic reflection data reveal that the basement was directly involved by a major Hercynian structure, named here the Paracuellos fault, which splits longitudinally the Paracuellos axial core. In seismic profiles this fault occurs as a northeasterly-dipping reflector (60–70° steep), evidencing a bivergent geometry of the lateral crustal elements. The sedimentary evolution of the Neoproterozoic Iberian platform ranges from transgressive, non-cyclic, offshore to hemipelagic, black and green shales (Sestrica Formation) to progradational trends recording shoaling during episodes of rapid sediment influx (Saviñán Formation), presumably in response to a low standing sea-level. The siliciclastic succession is punctuated in the inner platform by deposition of phosphatic limestones (Codos Bed), representing a major shoaling event and demarcating a sharp regional change of sedimentation separating two similar siliciclastic tendencies. A diagenetically induced bedded chert (Frasno Bed) occurs in the outer platform, and is interpreted as being the product of at least two silicification episodes. Both the Codos and Frasno Beds are overlain by the Aluenda Formation, which exhibits nearshore to offshore features. An important sedimentary discontinuity appears across the Neoproterozoic-Cambrian transition. The Cambrian(?) Bámbola Formation is paraconformable with the Paracuellos Group displaying a gradual transition in inner platform areas, whereas an erosive unconformity occurs in outer areas. The horizon of the Neoproterozoic-Cambrian boundary is not identified in the Iberian Chains, where neither Cadomian deformation nor discordances are recognisable.

Palaeobiology and evolution of the earliest agglutinated Foraminifera:Platysolenites, Spirosolenitesand related forms

Tubular agglutinated fossils of Platysolenites antiquissimus Eichwald, 1860, P. cooperi n.sp. and Spirosolenites spiralis Glaessner, 1979 are examined from selected lower Cambrian successions in Avalonia (England; Wales; Newfoundland) and Baltica (Russia and Estonia). Platysolenites cooperi n.sp. is shown to extend below the first appearance of P. antiquissimus in the Neoproterozoic-Cambrian boundary stratotype region, SE Newfoundland, and to occur at higher stratigraphic levels in Wales and Finnmark. Taphonomic, teleological and ultrastructural studies on well-preserved material are consistent with a similar grade of organization and mode of life for P. antiquissimus and the living astrorhizacean foraminiferid Bathysiphon. However, agglutinated proloculi are here described from both rectilinear P. antiquissimus and coiled S. spiralis, which suggests that neither were typical astrorhizaceans.

Neoproterozoic slope deposits, Mackenzie Mountains, northwestern Canada: implications for passive-margin development and Ediacaran faunal ecology

The youngest formations of the Neoproterozoic Windermere Supergroup in northwestern Canada (Gametrail, Blueflower, and Risky formations) record the transition from slope to shelf deposition on a prograding passive margin. Eleven facies associations are recognized, representing environments ranging from carbonate- and siliciclastic-dominated continental slope to open carbonate shelf and siliciclastic shoreface. Seven simple sequences are recognized, which can be grouped into three composite sequences. Combination of the data presented here with previous work on underlying and overlying formations indicates that the sequence-stratigraphic record is least detailed in the deepest-water facies and most detailed in shelf facies, reflecting the relative inability of high-frequency relative sea-level oscillations to affect deposition in deep-water settings. Falling-stage deposits are especially common in the upper slope region. Several major sequence boundaries (unconformities) are clustered in the interval a short distance below the Precambrian-Cambrian boundary. The most significant of these occurs high in the Blueflower Formation, not at the top of the Risky Formation as commonly inferred. This interval containing several surfaces may reflect thermal uplift related to the rifting recorded in rocks of this age in the southern Canadian Cordillera. Renewed subsidence (thermal relaxation) commenced just prior to the Neoproterozoic-Cambrian boundary, giving rise to a thick succession of shelf to nonmarine basal-Cambrian deposits. Ediacaran body fossils previously reported from the studied units occur in a range of slope to shoreface environments, including some facies that were deposited below the photic zone. The most common taxa occur across a spectrum of facies and were apparently ecological generalists.

Neoproterozoic sulfur-isotope variation in Australia and global implications

Seven sulfur-isotope stages are apparent in Australian Neoproterozoic sections. (1) From 840 Ma to the Sturtian glaciation (700 Ma) δ 34S sulfate varied little (+19 to +17.5‰) and δ 34S sulfide ranged from −20 to +23‰. (2) The Sturtian glaciation was followed by (3) a rise in δ 34S sulfide to an average of +30‰ (seen also in China, Namibia and Canada) and in δ 34S sulfate at least to +26‰ and possibly to +45‰. The sharp rise in δ 34S of all sulfur fractions (sulfate, sulfide, and organic) leads us to speculate that sulfide depleted in 34S was deposited on the abyssal plain and that residual sulfate enriched in 34S remained beneath a stagnant, ice-covered ocean during the Sturtian glaciation and was brought to the shallow continental margin during the post-glacial transgression. (4) δ 34S sulfide and δ 34S sulfate fall from 640 to 607 Ma ( δ 34S sulfate down to +17‰) probably from oxygen excess (through massive organic carbon burial), which causes weathering of sulfide depleted in 34S continental sulfide and previously deposited sulfide on the abyssal plain. (5) The oxic environment of the Marinoan (605–595 Ma) glaciation minimises sulfate-reduction (unlike the Sturtian) so that δ 34S sulfate is unvarying. (6) The post-glacial environment has sulfide depleted in 34S (average δ 34S sulfide<−30‰). Burial of a substantial quantity of organic-carbon causes extensive bacterial reduction of sulfate and a rise in δ 34S sulfate to a peak of at least +32‰ (7) around the Neoproterozoic–Cambrian boundary and a corresponding rise in δ 34S sulfide, explained by release of methane gas from clathrate and a subsequent heatwave causing ocean stagnation and oxygen depletion.

The Neoproterozoic Supercontinent: Rodinia or Palaeopangaea?

The Rodinia reconstruction of the Neoproterozoic Supercontinent has dominated discussion of the late Precambrian Earth for the past decade and originated from correlation of sedimentary successions between western North America and eastern Australia. Subsequent developments have sited other blocks according to a distribution of ∼1100 Ma orogenic belts with break-up involving a putative breakout of Laurentia and rapid reassembly of continent crust to produce Gondwana by early Phanerozoic times. The Rodinia reconstruction poses several serious difficulties, including: (a) absence of palaeomagnetic correlation after ∼730 Ma which requires early fragmentation of continental crust although geological evidence for this event is concentrated more than 150 Ma later near the Cambrian boundary, and (b) the familiar reconstruction of Gondwana is only achieved by exceptional continental motions largely unsupported by evidence for ocean consumption. Since the geological evidence used to derive Rodinia is non-unique, palaeomagnetic data must be used to evaluate its geometrical predictions. Data for the interval ∼1150–500 Ma are used here to test the Rodinia model and compare it with an alternative model yielding a symmetrical crescent-shaped analogue of Pangaea (Palaeopangaea). Rodinia critically fails the test by requiring Antarctica to occupy the location of a quasi-integral Africa, whilst Australia and South America were much closer to their Gondwana configurations around Africa than implied by Rodinia. Palaeopangaea appears to satisfy palaeomagnetic constraints whilst surmounting geological difficulties posed by Rodinia. The relative motions needed to produce Gondwana are then relatively small, achieved largely by sinistral transpression, and consistent with features of Pan-African orogenesis; continental dispersal did not occur until the Neoproterozoic–Cambrian boundary. Analogies between Palaeopangaea and (Neo)pangaea imply that supercontinents are not chaotic agglomerations of continental crust but form by episodic coupling of upper and lower mantle convection leading to conformity with the geoid.

Evolution and Ecology of Neoproterozoic-Lower Cambrian Trace Fossils, NW Canada

Neoproterozoic to subtrilobite Cambrian strata in the Mackenzie Mountains, Canada, provide a test case for the current paradigm of evolutionary stages in the development of the early infauna. Three predominantly siliciclastic formations (Ingta, Backbone Ranges, and Vampire formations) collectively exhibit 13 facies associations representing nonmarine to distal shelf environments, and can be subdivided into 18 simple sequences, each of 800,000-1,000,000 years duration. Trace fossil occurrences were digitized at the level of simple sequences and plotted on a matrix with facies associations (environment) on the x-axis and simple sequences (time) on the y-axis. This analysis shows that evolution was a first-order control on trace fossil distribution, and that environmental factors exerted an important second-order control. Trace fossils were most abundant in normal marine environments and less so in marginal-marine settings with fluctuating or brackish salinity. Eolian and fresh-water deposits were barren, presumably because animals able to tolerate these conditions had not evolved. Turbulence excluded organisms from shoreface environments. Several biostratigraphically important ichnotaxa (e.g., Treptichnus and Rusophycus), however, show broad environmental tolerance across the marine shelf Taphonomic factors led to fine preservation in interbedded sandstone-mudstone facies, but to diagenetic obliteration of trace fossils in most carbonate facies. Trace fossils provide a consistent indicator of the Neoproterozoic-Cambrian boundary. Four evolutionary zones can be recognized: (I) Simple burrows, (II) Treptichnus pedum Zone, (III) Rusophycus avalonensis Zone, and (IV) Cruziana tenella Zone. Global biostratigraphic and chemostratigraphic correlations imply that Zone I is terminal Neoproterozoic and the overlying three zones subtrilobite Cambrian in age. These zones are present in the Newfoundland Neoproterozoic-Cambrian boundary GSSP, the East European Platform, and elsewhere.

Late Palaeoproterozoic (∼1800–1600 Ma) stromatolites, Cuddapah Basin, southern India: cyanobacterial or other bacterial microfabrics?

Late Palaeoproterozoic stromatolites from the Vempalle Formation of the Cuddapah Supergroup, south-eastern India, contain well-preserved microfabrics. These exhibit a range of complex clotted to bush-like micritic fabrics set in microsparite and sparite cements. No calcified microfossils confidently referable to calcified cyanobacteria have been recognized, although micritic masses crudely resembling the Phanerozoic calcified cyanobacterium Angusticellularia are present. It is suggested that these Vempalle stromatolitic microfabrics compare more closely with modern calcified non-cyanobacterial biofilms. This interpretation questions the widespread assumption that cyanobacteria were important components of stromatolite-building microbiotas during the Proterozoic and instead emphasizes the possibility that examples similar to those studied were dominated by non-cyanobacterial bacteria. It is generally thought likely that cyanobacteria were important components of stromatolite-building microbiotas during the Proterozoic, and that early lithification by carbonate precipitation was widespread and intense. If these assumptions are correct then the apparent absence of diverse calcified cyanobacteria suggests that these organisms, which are conspicuous in the Early Palaeozoic, underwent evolutionary radiation close to the Neoproterozoic–Cambrian boundary.

Neoproterozoic of the Mackenzie Mountains, northwestern Canada

Neoproterozoic strata of the Mackenzie Mountains are more than 10 km thick, and comprise two supergroups of differing style. The Mackenzie Mountains Supergroup (4–6 km thick), an epicratonic succession of mainly shallow-water siliciclastic and carbonate strata, contains detrital zircons dated at 1080 Ma and is cut by dykes and plugs dated at 780-778 Ma. The overlying Windermere Supergroup (5–7 km thick) comprises basal rift deposits and glacial diamictites that are overlain by three, kilometre-scale, siliciclastic to carbonate “Grand Cycles” deposited on a passive margin. “Grand Cycles” in the eastern Mackenzie Mountains were deposited in a shallow-water setting, whereas contemporaneous cycles in the western Mackenzies were deposited on a continental slope; mapping of successive shelf-margin positions reveals southwestward progradation. The Windermere Supergroup is unconformably overlain by the Ingta Formation, which contains the Proterozoic-Cambrian boundary. Subtrilobite Cambrian strata are approximately 1.5 km thick, and are overlain by trilobite-bearing Lower Cambrian carbonates of the Sekwi Formation. Fossils are abundant and biostratigraphically useful in these Neoproterozoic strata. Acritarchs, carbonaceous megafossils, and microbial structures occur throughout the entire succession, but are especially well preserved in the Little Dal Group of the Mackenzie Mountains Supergroup. Edicara-type fossils first appear below the second Windermere glacial unit (Ice Brook diamictite), and range upwards through 2.5 km of strata to near the top of the Windermere; three distinctive assemblages corresponding to the Twitya, Sheepbed, and Blueflower formations can be recognized. The Neoproterozoic-Cambrian boundary occurs within the Ingta Formation, and is marked by an abrupt change from simple, subhorizontal burrows ( Planolites-Torrowangea Assemblage) to complex feeding burrows of the Phycodes Assemblage. Small shelly fossils of Nemakit-Daldyn aspect occur in the upper part of the Ingta Formation, slightly above the basal Cambrian boundary. The presence of two glacial diamictites in the Windermere and numerous sequence boundaries and flooding surfaces throughout the succession provides opportunities for regional and global event correlation. The palaeomagnetic record shows three hair-pin curves that should be recognizable in continents attached to Laurentia. Carbonate interbeds throughout the succession typically yield relatively unaltered carbon isotope ratios; C and Sr chemostratigraphy shows marked excursions that appear to be globally correlatable.