Marinoan Glacial Deposits Research Articles

Marinoan glaciation in the Indian subcontinent - Anatomy and global implications

Detailed sedimentological analysis of the Pokaran Boulder Bed, representing the most basal unit of the Neoproterozoic-Lower Cambrian Marwar Supergroup, clearly indicates its glacial origin. The glacial sediments are interpreted as an ice-contact submarine fan deposit. Based on the detrital and inherited zircon population of the Marwar Supergroup sediments and interlayered pyroclastic deposits, a Marinoan cryochron of the Cryogenian Period is envisaged for the Pokaran Boulder Bed. The well-preserved Ediacaran elements in the post-glacial sedimentary succession also support a Cryogenian to Early Cambrian age of the Marwar Supergroup. The glacial deposition at the base of the Marwar Supergroup strengthens the regional correlation between the studied sediments with the Lesser Himalayan Blaini-Krol-Tal sediments and the Haqf Supergroup of Oman. The available zircon ages and paleomagnetic data of the Malani Igneous suite, along with the Marinoan glacial deposits, detrital zircon ages, and Ediacaran fossil elements of the Marwar Supergroup add significant information to the Neoproterozoic Earth history.

Sequence stratigraphy and paleoenvironmental significance of the Neoproterozoic Bambui Group, Central Brazil

The Bambui Group is a ca. 2,000 m-thick mixed biochemical-siliciclastic sedimentary succession that accumulated in the Sao Francisco Basin, central Brazil. Its age is contentious, but deposition during the Cryogenian-Ediacaran (ca. 635–550 Ma) is consistent with a new sequence stratigraphic model presented herein and previously published geochemical and structural data. Six stratigraphic sequences are recognized that record depositional evolution from an epeiric sea to foreland basin during the Marinoan snowball glaciation (ca. 635 Ma) to shortly after the Gaskiers (ca. 580 Ma) glaciation. Although this stratigraphic architecture differs from the classic lithostratigraphic and more recent chemostratigraphic and sequence stratigraphic interpretations of the Bambui Group, it is supported by correlation to other Cryogenian-Ediacaran successions globally. Collectively, these six sequences preserve a robust record of seawater redox structure leading up to the oxygenation threshold that triggered the evolution of multicellular animals in the Ediacaran.Sequence 1 is composed of Marinoan glacial deposits and an overlying cap carbonate with aragonite fan pseudomorphs and barite. Sequence 2 is composed of lime-mudstone that changes laterally into loess-rich peritidal phosphorites, suggesting oxygen stratification in shallow paleoenvironments. Sequence 3 is a carbonate succession with abundant stromatolites implying widespread oxygen production. The top of Sequence 3 is marked by a regional subaerial unconformity developed during uplift associated with the onset of the Brasiliano-Pan African Orogeny. Sequence 4 signals a return to loess derived sedimentation. Distal deposits contain abundant organic matter with framboidal pyrite reflecting bacterial sulfate reduction in low sulfate Ediacaran seawater. Sequence 5 is composed of well-developed aggradational parasequences composed of alternating deep subtidal organic-rich siltstones and shallow-water carbonate grainstones. Their well-defined cyclicity is interpreted to record extreme sea level fluctuations during the onset of the Gaskiers glaciation. Stromatolites suggest an oxygen-stratified water column. Sequence 6 marks the change from an epeiric sea to a foreland basin. Interbedded basinal glauconitic siltstone and shale loess-rich are correlative with shoreface sandstones. Authigenic glauconite requires suboxic bottom waters to precipitate and thus reflects oxygenation of distal settings. Most of the deposition ended by ca. 560–550 Ma when the entire Bambui Group was deformed during the assembly of West Gondwana.In sequences 1, 2, 3, 4, 5, and the lower portion of 6, the lateral continuity of lithofacies, predictable layer-cake stratigraphy, and lack of fluvial coarse siliciclastic deposits are characteristics of epeiric sea successions. The appearance of immature sandstone, poorly sorted conglomerate, braided fluvial deposits, and synsedimentary deformation in the upper part of Sequence 6 are interpreted as the onset of foreland basin development. Stacking relationships of redox sensitive, authigenic lithofacies in both the epeiric sea and foreland phases support recent research suggesting seawater redox conditions were in a state of flux during the apex of the purported Neoproterozoic Oxygenation Event. Organic matter and authigenic pyrite in Sequences 4 and 5 reflect increased primary productivity in surface waters fertilized by nutrients such as P and Fe adsorbed on loess particles. This mechanism could be expanded to a global scale and have contributed to the increase in atmospheric and oceanic oxygen levels. These findings redefine and highlight the significance of the Bambui Group for Neoproterozoic Earth history.

Mercury anomalies across the Cryogenian-Ediacaran boundary in South China

The Cryogenian-Ediacaran transition represents a pivotal geological period in the evolution of global climate, ocean chemistry, and early organisms. The transition is concurrent with the change from Marinoan glacial deposits to overlying cap carbonate/dolomite, which is followed by the appearance of novel animal and algae fossils. Unusual carbon cycling during the deposition of cap carbonate/dolomite is recorded by prominent negative carbonate carbon isotope (δ13Ccarb) anomalies. The mechanisms which drove melting of the Marinoan icesheets remain uncertain. To explore the cause of this dramatic climate warming and its effect on oceanic biogeochemical cycles, we measured Hg concentrations and isotopes, along with major and trace elements, of the sedimentary succession across the Cryogenian-Ediacaran boundary at the Jiulongwan and Huajipo sections, South China. Hg concentrations show spikes with a ∼ 2 times increase at the top of the Marinoan Nantuo Formation at both sections, which are likely associated with organic matter drawdown rather than enhanced volcanism as indicated by increased TOC contents and similar Hg isotopic signature as those of background Hg deposition. A conspicuous negative shift in δ202Hg along with a positive shift in Δ199Hg are observed in the cap dolomite of the Doushantuo Formation at both sections, which are ascribed to contribution of Hg from anoxic deep water and Hg associated with dissolved organic carbon (Hg-DOC), due to upwelling and oceanic oxygenation after deglaciation. Our Hg data argue against a sudden large igneous province (LIP) event causing Marinoan deglaciation. Results also indicate enhanced upwelling and oceanic oxygenation event during the Cryogenian to Ediacaran transition.

Geochronological and geochemical constraints on the origin of highly 13Ccarb-depleted calcite in basal Ediacaran cap carbonate

AbstractEdiacaran cap dolostone atop Marinoan glacial deposits contains complex sedimentary structures with extremely negative δ13Ccarb values in close association with oscillations in palaeoclimatic and oceanographic proxy records. However, the precise geological, geochronological and geochemical context of the cap dolostone is not clarified, which hampers us from correctly interpreting the extremely negative δ13Ccarb values and their causal relationships with the Snowball Earth hypothesis. In this study, we conducted detailed in situ geochronological and geochemical analyses on the calcite within the cap dolostone from the Ediacaran Doushantuo Formation in South China in order to define its formation and relationship to the Snowball Earth hypothesis. Petrographic observations show that formation of dolomite pre-dates precipitation of calcite and pyrite, which pre-dates quartz cementation in the basal cap carbonate. Calcite cement within the cap dolostone yielded a U–Pb age of 636.5 ± 7.4/17.8 Ma (2σ, MSWD = 1.6, n = 36/40), which is within uncertainty of a published dolomite U–Pb age of 632 ± 17 Ma (recalculated as 629.3 ± 16.7/22.9 Ma). These age constraints negate the possibility that the calcite cement was formed by late Ediacaran or Cambrian hydrothermal activity. The rare earth element distribution patterns suggest a dominant seawater origin overprinted by subsequent early Ediacaran hydrothermal activity. The combined age, petrographic and geochemical data suggest oxidization of methane clathrates in response to complicated interplay between eustasy and isostatic rebound and hydrothermal fluids.

Cryogenian Aspidella from northwestern Canada

The Ediacaran period saw the appearance of the first widespread ecosystems dominated by complex multicellular forms, with the earliest examples inhabiting deep waters below the photic zone. Contrary to expectation, the first appearance of large members of the Ediacara biota is sudden, and no definitive precursors have been described from earlier Neoproterozoic rocks; this sudden appearance has long been an enigma. Cryogenian fossils from deep-water deposits immediately pre-dating Marinoan glacial deposits within the Mackenzie Mountains, NW Canada, document the earliest stages of the growth of subphotic benthic communities. Among the previously described “Twitya discs” from this locality are at least two examples of the Ediacaran form-taxon Aspidella Billings, 1872. These fossils represent a nearly 80 Ma range extension of Aspidella and possible forerunners of the benthic communities of the Ediacara biota.

An early diagenetic deglacial origin for basal Ediacaran “cap dolostones”

The beginning of the Ediacaran Period (∼635 Ma) is marked by conspicuous dolostone units that cap Marinoan glacial deposits worldwide. The extent and sedimentary characteristics of the cap dolostones indicate that anomalous carbonate over-saturation coincided with deglacial sea-level rise and ocean warming. However, the geochemical variability within cap dolostones, both between continents, across single continental margins, and within individual stratigraphic sections has been difficult to reconcile with depositional models. Using a compilation of new calcium and magnesium isotope measurements in Marinoan cap dolostone successions worldwide, we show that the geochemical variability can be explained by early diagenetic dolomitization of aragonite along a spectrum of fluid- and sediment-buffered conditions. Dolostones from the outer platform formed under fluid-buffered conditions, whereas dolostones on the inner platform and foreslope environment formed under sediment-buffered conditions. This spatial pattern of dolomitizing conditions is consistent with buoyant recirculation of glacial seawater within carbonate platforms driven by the deglacial sea-level rise and development of a meltwater surface ocean. Using a numerical diagenetic model to evaluate the geochemical differences between sediment- and fluid-buffered cap dolostone units, we constrain the chemical and isotopic composition of both the dolomitizing fluid (glacial seawater [δ13C ∼ 0–2‰]), the meltwater lens (δ13C ∼−11‰), and the primary aragonite sediment (δ13C ∼−6 to −3‰). These model end-members do not imply that primary geochemical variability did not exist but demonstrates that it is not necessary to change the chemistry of seawater to explain the global stratigraphic variability in the geochemistry of basal Ediacaran cap dolostones. Our results provide a novel framework for understanding the geochemical variability of cap dolostone units, including large excursions in carbon isotopes, and how this variability is the product of local diagenetic processes expressed globally in continental margin environments following the last Snowball Earth.

Transient marine euxinia at the end of the terminal Cryogenian glaciation

Termination of the terminal Cryogenian Marinoan snowball Earth glaciation (~650–635 Ma) is associated with the worldwide deposition of a cap carbonate. Modeling studies suggest that, during and immediately following deglaciation, the ocean may have experienced a rapid rise in pH and physical stratification followed by oceanic overturn. Testing these predictions requires the establishment of a high-resolution sequence of events within sedimentary records. Here we report the conspicuous occurrence of pyrite concretions in the topmost Nantuo Formation (South China) that was deposited in the Marinoan glacial deposits. Sedimentary facies and sulfur isotope data indicate pyrite precipitation in the sediments with H2S diffusing from the overlying sulfidic/euxinic seawater and Fe (II) from diamictite sediments. These observations suggest a transient but widespread presence of marine euxinia in an ocean characterized by redox stratification, high bioproductivity, and high-fluxes of sulfate from chemical weathering before the deposition of the cap carbonate.

Mineralization age and metallogenic environment of Daotuo manganese deposits in Guizhou: Evidence from Re-Os isotopes

The Neoproterozoic is a critical interval in Earth’s history, having witnessed the breakup of supercontinent Rodinia, global glaciations (or the snowball Earth), and the evolution of multicellular organisms. It is proposed that the breakup of Rodinia might have triggered the snowball Earth, while the termination of the Neoproterozoic global glaciations might have triggered the evolution of life in the Ediacaran Period. Two snowball Earth events, the Sturtian (717–663 Ma) and the Marinoan (654–635 Ma) glaciations, were separated by an interglacial interval of ~10 million years (663–654 Ma). In South China, the Sturtian and Marinoan glacial deposits are represented by the Jiangkou and Nantuo Formations. The interglacial Datangpo Formation is characterized by widespread manganese (Mn) ore deposits, such the Datangpo and Xixibao in Guizhou, Minle in Hunan, and Xiushan in Chongqing, representing the most important metallogenic period of Mn in China. The Daotuo Mn ore, a superlarge-scale deposit (approximately 200 million tons), was first discovered in 2010 in northeastern Guizhou. It is hosted in a fault-bound depression system between the Yangtze block and the Jiangnan orogenic belt. The Mn ores were discovered in black shale of the Member I of the Datangpo Formation, and occur as rhodochrosite (MnCO3). In order to constrain the age of Mn ores, here we report Re-Os isochron age of the ore-bearing black shale from the Member I of the Datangpo Formation. The Re-Os isochron age of the Member I black shale is 660.6±7.5 Ma, consistent with the zircon U-Pb age of the tuff layer from the base of the Datangpo Formation and other Re-Os isochron ages from other localities (such as Canada, Australia, Scotland, and Mongolia). Furthermore, the initial value of 187Os/188Os from the Re-Os isochron age is 0.781, suggesting predominantly terrestrial input. Based on previous research, it is shown that during the Sturtian glaciation, under the influence of sea ice cover barrier and atmospheric material exchange, the hypoxic water led to the formation of iron and sulfide rich environments on the seafloor near hydrothermal vents, giving rise to a large number of rift basins rich in Mn2+. After the glaciation and water and atmospheric exchange of material recovery, the glacier meltwater carries the terrigenous material into the rift basin containing Mn2+, accompanied by the melting of the land glacier and the rapid increase of the atmospheric oxygen content. The surface water body of the rift basin is rapidly oxidized, and the oxidation degree is higher than the ocean area. Mn2+ are also oxidized to MnO2 and precipitate back into the environment, where they buried with organic matter, undergo diagenesis, and eventually form rhodochrosite.

Sedimentary depocenters on Snowball Earth: Case studies from the Sturtian Chuos Formation in northern Namibia

Cryogenian synglacial deposits are regionally thin but locally thick, considering glacial duration, but the reasons for local thickening are poorly known. We studied three local thickenings of the Sturtian Chuos Formation in northern Namibia by measuring closely spaced columnar sections, not only of the synglacial deposits but also of the bounding pre- and post-glacial strata. This enabled incised paleovalleys filled by glacial debris to be distinguished from morainal buildups. In case 1, a U-shaped paleovalley, ∼450 m deep by ∼3.0 km wide, is incised into pre-glacial strata and 10% overfilled by ice-contact and subglacial meltwater deposits. In case 2, a wedge of glacial diamictite, ~220 m thick by 2.0 km wide, overlies a disconformity that is demonstrably not incised into underlying pre-glacial strata. The wedge, draped by a post-glacial cap carbonate and argillaceous strata, is erosionally truncated at its apex by Marinoan glacial deposits and their basal Ediacaran cap dolomite. The wedge was a positive topographic feature, either a terminal moraine or an erosional outlier of formerly more extensive glacial deposits. In case 3, a wedge of conglomerate, glacial diamictite, and subglacial lake deposits thickens to >2000 m where it abuts against granitoid basement rock uplifted along a border fault. Fault movement ceased before the Sturtian cap carbonate was deposited. The locus of maximum deposition shifted over time from proximal to distal with respect to the border fault, similar to Mesozoic half grabens developed above listric detachments imaged seismically on offshore North Atlantic margins.

Uranium isotope evidence for temporary ocean oxygenation in the aftermath of the Sturtian Snowball Earth

The appearance and radiation of animals are commonly attributed to Neoproterozoic oceanic oxygenation, yet independent geochemical evidence for such an event remains equivocal. Strata deposited between the Sturtian and Marinoan Snowball Earth glaciations (660 to 640 Ma) contain the earliest animal biomarkers and possible body fossils. To quantify the extent of seafloor oxygenation during this critical interval, we present uranium isotope ratios (238U/235U denoted as δ238U) from limestone of the Taishir Formation in Mongolia through two stratigraphic sections that are separated by ∼75 km within the same depositional basin. Above the Sturtian glacial deposits, through ∼150 m of stratigraphy, δ238U compositions have a mean value of −0.47‰. This interval is followed by a ∼0.3‰ decrease in δ238U, coincident with the Taishir negative carbon isotope excursion. Thereafter, δ238U values remain relatively low until the erosional unconformity at the base of the Marinoan glacial deposits. Using a box model, we show that the best explanation for the higher δ238U values of the post-Sturtian limestones is extensive—but temporary—oxygenation of the seafloor, and is inconsistent with a scenario involving only increased delivery of uranium to the oceans due to post-Snowball weathering. The decline in δ238U in overlying strata, coincident with the Taishir negative δ13C excursion, indicates a subsequent decrease in seafloor oxygenation. The U isotopic data, combined with modeling results, challenge the notion of a simple, unidirectional oxygenation of Neoproterozoic oceans.

A eukaryote assemblage intercalated with Marinoan glacial deposits in South Australia

Composite hematite–silica structures recovered from a siltstone bed in the Elatina Formation of South Australia include (1) sub-circular to whorl-shaped forms, (2) elongate to half-moon-shaped forms and (3) and lozenge-shaped forms locally linked into chains. They range from 200 to 500 µm in diameter and are interpreted as eukaryote tests. Evidence for internal etching of a calcite core of some tests indicates that at least some of the hematite–silica fabrics were acquired through replacement. Carbon isotope values of −20‰ δ 13 C are suggestive of precipitation by microbial activity, and imply a change in ambient fluid chemistry associated with a pH reduction. The tests occur within sandstone beds that were deposited on a tidally modulated braidplain during the Marinoan glaciation at the end of the Cryogenian. The quartz grains in the sandstone sample lack the typical textures (surface striae, internal fractures or irregular grain boundaries) expected for glacially transported material. Thus, on textural grounds we argue that the eukaryote tests represent a proglacial ecosystem during a late Cryogenian snowball Earth event. Supplementary material: Video files of digital X-ray tomographs (μCT) in the longitudinal and transverse planes are available at: https://doi.org/10.6084/m9.figshare.c.2209723 .

Sedimentological perspectives on climatic, atmospheric and environmental change in the Neoproterozoic Era

AbstractSedimentological insights underpin many of the important recent advances in understanding of Earth system behaviour in the Neoproterozoic Era. This article reviews three main areas: (i) chemical proxies and their preservation, with emphasis on carbonate facies; (ii) glacial and post‐glacial facies, including their age constraints; (iii) sedimentary evidence for biotic innovations and responses. Chemostratigraphy plays an important role in ordering Neoproterozoic events and defining disturbances to the carbon cycle. There is increasing attention being paid to assessing the role of diagenetic origination or modification of chemostratigraphic signals. Alongside this, new criteria for identifying primary dolomite and precursor metastable phases such as ikaite have been developed. In respect of oxygenation, geochemical proxies substantiate the concept of a Neoproterozoic Oxygenation Event as a very gradual transition, the ocean being at any one time a heterogeneous assemblage of ferruginous, sulphidic and oxic conditions, with some evidence of increasing deep‐sea oxidation through the Ediacaran Period. Techniques such as Fe‐speciation need to be supplemented with proxies sensitive to suboxic conditions. More generally, it is predicted that petrographically constrained microanalytical studies will also become more important in reconstructing palaeoenvironmental conditions. The global distribution of Neoproterozoic glacial deposits combined with palaeomagnetic evidence supports the concept of panglaciations in which ice sheets reached sea‐level in the tropics. Advances in radiometric dating have demonstrated the synchronous onset of global (Sturtian) glaciation at 717 Ma and the demise of a second (Marinoan) glaciation at 635 Ma, and plausibly indicate long durations for each (55 Myr for Sturtian and 5 to 15 Myr for Marinoan). However, a compilation of radiometric dates indicates ambiguities indicating the need for further improvements to the radiometric and Sr‐isotope database to understand events within the Sturtian time frame, the timing of onset of Marinoan glaciations, and the age and synchroneity of individual negative δ13C anomalies. Sturtian deposits are typically thick, rift‐related successions containing a range of environments influenced or dominated by dynamic glaciers, as well as ice‐free marine intervals. Marinoan glacial deposits, by contrast, tend to be thin and continental. During the latter interval, oxygen isotope systematics of sulphate demonstrate that atmospheric CO2was high, as predicted by Snowball Earth theory, and that sedimentation was influenced by orbital forcing. The Sturtian record, by comparison, needs to be searched for evidence of cold‐climate hiatuses on the one hand and orbital forcing on the other. Cap carbonate formation appears to have coincided with rising sea‐levels following panglaciations. Snowball theory considers that they formed rapidly in the postglacial greenhouse, but an alternative model of slower formation with clastic sediment starvation during transgression may prove to be consistent with new data and models showing extensive glacier terminations on land. For all facies, but especially caps, the use of microanalytical techniques and holistic studies of petrogenesis are future priorities. There are important discrepancies between molecular clock predictions of early metazoan origination and the hard evidence from sedi‐mentary records which largely depend on local exceptional preservation by early diagenesis. A variety of life survived panglaciation, and there is little evidence that glaciations directly caused oxygenation or stimulated evolution.

Searching for an oxygenation event in the fossiliferous Ediacaran of northwestern Canada

Late Neoproterozoic (Ediacaran) strata from northwestern Canada provide a thick and rich sedimentological record, preserving intercalated carbonates and shale extending from the ~635million year old Marinoan glacial deposits up through the ~541million year old Precambrian–Cambrian boundary. This region also holds one of the classic localities for the study of early animal life, with the ensuing suggestion that this temporal interval captures a gross change in the O2 content of the Earth's atmosphere. To test this hypothesis and bring records of northwestern Canada into line with other Ediacaran, fossil-bearing basins, we provide a detailed geochemical reconstruction from the Wernecke Mountains of the Yukon. Where possible, we also extend these records to the Ogilvie Mountains to the west and previously published data from the Mackenzie Mountains to the east.Our work in the Wernecke Mountains is set against a composite δ13C record for carbonate that preserves three distinct Ediacaran isotope excursions, the lowermost of which (preserved in the Gametrail Formation) is a putative Shuram excursion equivalent. What emerges from a multi-proxy (Fe speciation, sulfur isotopes, major and trace element analyses) reconstruction is a picture of a persistently anoxic and ferruginous Ediacaran ocean. Notably absent is geochemical evidence for a prominent oxygenation event, an expectation given the appearance of animals and large swings in δ13C. The new insight gained through these data challenges the idea of an Ediacaran jump in atmospheric oxygen, which in turn muddles the link between animal evolution and local geochemical environments.

Regional recognition of the Neoproterozoic Sturtian–Marinoan boundary, Northern and Central Adelaide Geosyncline, South Australia

Stratigraphic and sedimentological investigation of the interglacial succession within the Cryogenian-aged Umberatana Group of the Northern and Central Flinders Ranges reveals a complex array of sedimentary successions lying between the Sturtian and Marinoan glacial deposits. The Sturtian–Marinoan Series boundary was first defined from the Adelaide area at the uppermost contact of the Brighton Limestone. In the Northern Flinders Ranges the Sturtian–Marinoan Series boundary has been positioned at the uppermost contact of the Balcanoona Formation, which is thought to correlate with the Brighton Limestone. In the Northern Flinders Ranges a major unconformity separates the Sturtian and Marinoan-aged sedimentary successions (Nepouie–Upalinna Subgroups). In moderately deep marine depositional settings, this submarine unconformity is located at the base of the Yankaninna Formation where erosion has deeply incised (up to 300 m) into the underlying Tapley Hill Formation. In shallower marine settings the unconformity is found at the base of the Weetootla Dolomite. In very deep water depositional settings this unconformity is not recognised, and the Yankaninna Formation appears to conformably overlie the Tapley Hill Formation suggesting that this erosional feature is restricted to shallow and moderately deep depositional settings. This unconformity presents a regionally persistent chronostratigraphic marker horizon, which reliably marks the Sturtian–Marinoan Series boundary at the base of the Yankaninna Formation from shallow shelfal to deep-water basinal settings throughout the Northern Flinders Ranges. In the Central Flinders Ranges the post-Sturtian glacial stratigraphy records a very similar depositional record to that observed in the Northern Flinders Ranges. In the Central regions the Tapley Hill Formation is overlain by deep-marine carbonates and calcareous shales of the Wockerawirra Dolomite and Sunderland Formations, respectively. The base of the Wockerawirra Dolomite is found to be in erosional contact with the underlying Tapley Hill Formation. This stratigraphic relationship, together with lithological similarities, indicates the Wockerawirra Dolomite and Sunderland Formation of the Central Flinders Ranges are lateral correlatives of the Yankaninna Formation of the Northern Flinders Ranges. The regional nature of the Sturtian–Marinoan unconformity in the Adelaide Geosyncline suggest the possible existence of a glacio-eustatic event that may correlate with glacials/glaciation elsewhere on the Earth during the Cryogenian.

Nature and significance of the Neoproterozoic Sturtian–Marinoan Boundary, Northern Adelaide Geosyncline, South Australia

The Cryogenian succession of the Northern Flinders Ranges reveals a complex sedimentary record between the Sturtian and Marinoan glacial deposits. A major unconformity separates the Sturtian and Marinoan-aged sedimentary successions in the area. This forms a subaerial erosion surface with terrestrial and marginal marine infill directly above the Angepena and Balcanoona Formations in their respective localities. This exposure surface is here correlated with the previously documented submarine unconformity between the Yankaninna Formation and the underlying deep marine Tapley Hill Formation. This erosional event provides a chronostratigraphic marker horizon that coincides approximately with thepreviously defined Sturtian–Marinoan Time Series boundary in the Northern Flinders Ranges. These stratigraphic relationships also constrain lateral facies relationships between the Oodnaminta ReefComplex (Balcanoona Formation) and the Angepena Formation. Similarly, the shallow-water Weetootla Dolomite is correlated with the deeper water carbonates of the Yankaninna Formation.

Neoproterozoic rifting in the southern Georgina Basin, central Australia: Implications for reconstructing Australia in Rodinia

[1] A system of northwest striking Neoproterozoic rift basins underlies Paleozoic strata in the southern Georgina Basin of central Australia. Normal faults bounding these rift basins were selectively reactivated during the mid-Paleozoic Alice Springs Orogeny and are now expressed as high-angle reverse faults that invert the preexisting rift basins. Exhumed and eroded rift basin remnants are present in the hanging wall of the Oomoolmilla, Lucy Creek, Tarlton, and Toomba reverse faults, and rift basins may be preserved in the subsurface beneath the Toko Syncline and Burke River Structural Belt. Rift basin fill indicates two periods of extension: a major rift-forming episode between approximately 700 and 650 Ma (coeval with Sturtian glacial deposits) and a second episode of extension at approximately 600 Ma (coeval with Marinoan glacial deposits). This northwest striking rift system in central Australia supports results from other regions, indicating that the Neoproterozoic continental margin of Australia consisted of northwest striking rift segments offset by northeast striking transform faults. Such a configuration is geometrically incompatible with a Laurentian continental margin consisting of northeast striking rift segments and conflicts with reconstructions such as SWEAT and AUSWUS that match Australia with western Laurentia in the Rodinia supercontinent.

Th–U–total Pb geochronology of authigenic monazite in the Adelaide rift complex, South Australia, and implications for the age of the type Sturtian and Marinoan glacial deposits

The Adelaide rift complex in South Australia contains the type sections for Sturtian and Marinoan glacial deposits. The litho- and chemo-stratigraphy of these deposits play a central role in evaluating global Neoproterozoic ice age hypotheses and Rodinia supercontinent reconstructions, but reliable depositional age constraints have been extremely limited. We report results of in situ Th–U–total Pb (electron microprobe) dating of detrital and authigenic monazite in two samples from the Umberatana Group (Sturtian Holowilena Ironstone and pre-Marinoan Enorama Shale) in the Central Flinders Ranges. Several texturally and chemically distinct detrital and authigenic populations are recognized. Detrital dates range from 1600 Ma to 760 Ma and most relate to well-known orogenic or igneous events in surrounding cratonic regions. Authigenic monazite grew in three or more pulses ranging from 680 Ma to 500 Ma. The date of 680 ± 23 Ma (2 σ) for the earliest generation of authigenic monazite in sandstone from the Enorama Shale (1) provides an estimate for the age of the base of the Trezona carbon isotopic anomaly just beneath the Marinoan glacial deposits, (2) provides an absolute minimum age constraint on the underlying Sturtian glacial deposits, and (3) supports proposed correlations between type Marinoan deposits and precisely dated glacial deposits in Namibia and China, which bracket the presumed Marinoan equivalents between 655 and 635 Ma. This age is inconsistent with a Re–Os isochron age of 643 ± 2.4 Ma (2 σ) on shales near the bottom of the Sturtian–Marinoan interglacial succession, stratigraphically > 3000 m below the Enorama Shale sample, and militate against the hypothesis that the type Marinoan is correlative with the 580 Ma Gaskiers glaciation. Monazite growth near 600 Ma and again at about 500 Ma probably represent hydrothermal fluid-flow events, the latter of which also corresponds to the well-known Delamerian Orogeny during which the Adelaide sediments were folded into their present structural pattern.

The effect of rising atmospheric oxygen on carbon and sulfur isotope anomalies in the Neoproterozoic Johnnie Formation, Death Valley, USA

Carbonates within the Rainstorm Member in the terminal Neoproterozoic Johnnie Formation of Death Valley, California record a remarkable negative carbon isotope anomaly – to a nadir of near − 11‰ – that accompanies a dramatic rise in trace sulfate abundance and fall of carbonate associated sulfate δ 34S values. The carbonates, including the laterally extensive Johnnie Oolite, were deposited during marine flooding atop a sequence boundary best observed in cratonward sections. The 12C-rich alkalinity delivered to the seawater pool was likely associated with a dramatic rise in atmospheric O 2, which resulted in 1) the oxidation of exposed continental shelf sediments rich in fossil organic matter and sulfides, and 2) the ventilation of the oceans. Similar carbon isotope anomalies are recorded in broadly equivalent successions that post-date known Marinoan glacial deposits and pre-date the Precambrian–Cambrian boundary in Oman, India, China, Australia, and Namibia, supporting the view that the oxygenation event was global. Thus, the biogeochemical anomaly post-dates the last widely recognized Neoproterozoic ice age, but is nonetheless associated with sea level drawdown that is likely to be eustatic in origin. The radiation of a high diversity microbiota and the first appearance of large metazoan fossils (Ediacaran animals) follow this carbon isotope anomaly, suggesting that a critical threshold with respect to atmospheric O 2 had been crossed by this time. A negative excursion of similar magnitude occurs in overlying strata at the Precambrian–Cambrian boundary, which likely reflects similar processes.

C–O–Sr isotopic stratigraphy of cap carbonates overlying Marinoan-age glacial diamictites in the Paraguay Belt, Brazil

Abstract The Neoproterozoic Paraguay Belt, located along the southeastern border of the Amazon Craton and eastern border of the Rio Apa Block, comprises a thick passive margin succession of glaciomarine turbidites, carbonates and siliciclastic sedimentary rocks that were deformed during the Brasiliano/Pan-African Orogeny. The carbonate sequence comprises a 1300 m thick succession of platformal carbonate rocks (Araras Formation), which directly overlie Marinoan glacial diamictites (Puga Formation). The late Neoproterozoic age of this carbonate sequence is indicated by the presence of Cloudina lucianoi and Corumbela werneri found in the upper part of the Corumba Group (Southern Paraguay Belt). Carbon and Sr isotope data were obtained on marine carbonate samples across three different sections of the Araras Formation. δ 13 C PDB values of the carbonate from the border of the basin range between −10.5 and −2.7‰, including a short stratigraphic interval of cap dolomite (22 m) directly overlying diamictites. The isotopic profile across the basal 200 m of deep shelf laminated microcrystalline limestone and clay-limestone (cap-carbonate), that overlie glacial diamictites, also show negative δ 13 C PDB values (−5.3 and +0.6‰). In contrast with the lower portion of the carbonate sequence, a 800 m thick unit of shallow-water dolostones forming the upper part of the carbonate sequence present uniform positive δ 13 C PDB values ranging from +1.9 to +2.4‰. Dolostone and arenaceous dolostone of the upper parts of the Araras Formation have high δ 13 C PDB values (+4.1 to +9.6‰) which are followed by an abrupt decrease in δ 13 C values, down to −1.0‰. The carbon isotope profile of the Paraguay Belt is quite different from the Bambui Group an other important Neoproterozoic sequences in Central Brazil, which also overlie glacial sediments. Negative δ 13 C PDB values are found only within the lowermost meters of these sequence and are followed by an important and extensive positive carbon isotope excursion (values ranging from +6.9 to +16.0‰). On the other hand, the δ 13 C PDB profile observed across the Araras Formation exhibits an approximately 200 m thick section of carbonates with low δ 13 C PDB values. Carbonates of the Araras Formation have 87 Sr / 86 Sr ratios ranging from 0.70753 to 0.70803, comparable to cap carbonates that overly Marinoan glacial deposits from other neoproterozoic sequences.

New constraints on the ages of Neoproterozoic glaciations in south China

The most complete Neoproterozoic successions in south China contain three diamictite intervals in the Changan, Tiesiao, and Nantuo Formations. The youngest and most widespread Nantuo glacial deposit overlies the Datangpo Formation and underlies the fossil- rich Doushantuo Formation. Previous authors have correlated the Nantuo diamictite to either Sturtian or Marinoan glacial deposits elsewhere. Here we report sedimentary and δ13C chemostratigraphic data of the Doushantuo cap dolostone, which overlies the Nantuo Formation. Facies-dependent variation in δ13C is interpreted as evidence for spatial heterogeneity in δ13C and/or temporal diachroneity in the initiation of cap carbonate sedimentation. Sedimentary and chemostratigraphic data are indicative of a Marinoan age for the Nantuo glaciation. This inference is supported by a new U-Pb zircon age of 663 ± 4 Ma from a tuffaceous bed in the Datangpo Formation. The new date and other isotopic ages from south China constrain the age of the Changan and Tiesiao glaciation(s) as between 761 ± 8 Ma and 663 ± 4 Ma, and the Nantuo glaciation as between 663 ± 4 Ma and 599 ± 4 Ma.