Cap Dolomite Research Articles

Stratigraphic framework and sedimentary evolution during the Cryogenian-Ediacaran transition in northeastern Sichuan Basin, South China

The northeastern Sichuan area in the northern Yangtze margin has unique Ediacaran geological records, especially the Doushantuo Formation (DST), and become a hot research area in recent years. However, the Cryogenian-Ediacaran (C-E) boundary has not been precisely identified, which restricts the in-depth study of geological information during this crucial transitional period and is unfavorable for a systematic and complete understanding of the Yangtze Block and even the global paleogeographic pattern. This study conducted stratigraphy, sedimentology, and chronostratigraphy to establish the stratigraphic framework and sedimentary evolution of the C-E transition strata in northeastern Sichuan. The results showed that the Ediacaran sediments, without the cap dolomite, unconformably overlaid the Cryogenian sediments in the studied area. The Member II of the DST, characterized by 50–160 m of red-green sandstone (approximately equivalent to the original Chengkou “Guanyinya Formation”), directly overlaid the Cryogenian sediments and displayed a 623 ± 2.3 Ma maximum depositional age from the detrital zircon U–Pb dating. Regional stratigraphic correlations indicate that the C-E transition strata in northeastern Sichuan had a consistent lithological association and sedimentary sequence characteristics but differed from the Three Gorges. Typically, the upper Nantuo massive glacial diamictites transition to the icebergs rafted lonestone-bearing mudstones at the top, then change upward to DST barrier coast sandstones. The proposed DST of the northeastern Sichuan Basin was divided into three lithostratigraphic members without the regional Member I cap dolomite: (i) Member II purple-red, gray-green sandstone strata, (ii) Member III black mudstone strata, and (iii) Member IV P–Mn bearing strata. During the C-E transition, the study area experienced (i) the global deglaciation stage in the terminal Marinoan glaciation and (ii) the filling-leveling up stage with clastic rocks in the early Ediacaran. Overall, the early Ediacaran of northeastern Sichuan succeeded the paleogeographic features of the late Cryogenian.

Sedimentology, stratigraphy and glacial genetic model of the coromandel phosphate deposit, Vazante group, central Minas Gerais state, Brazil

The Coromandel sedimentary phosphate deposit is inserted at the external zone of the Brasília Fold Belt geological context, more specifically in the Retiro Formation, basal unit of the Vazante Group. Compared to the world-class Rocinha Formation deposits, widely studied due to high economic interest, the Coromandel Deposit is the one that most lacks studies focused on the main rocks description, faciological characterization and proposal of a plausible metallogenetic model according to the depositional environments. Thus, from the detailed field description of outcrops, geological cartography, microscopic petrography and X-ray diffraction and fluorescence analysis, it was possible to contribute to a better understanding of the deposit. The Retiro Formation is characterized by a glaciogenic facies association that comprises polymictic diamictite, conglomeratic sandstone, fine whitish arkosean, massive and laminated siltstone, besides phosphate facies, including phosphate siltstone, laminated phosphorite and brecciated phosphorite. The phosphatic facies occur in straight relation to the glaciogenic sediments and not rare directly over diamictite, which matrix can be phosphated as well. These rocks P2O5 contents range from 2% to 34% and the succession suggest an establishment of a glaciotransitional environment responsible to constrain favorable alkalinity, temperature and oxi-reduction conditions to directly chemical precipitation of fluorapatite in a mechanism analogue as the cap dolomites, supporting an idea of “cap phosphorites”. The phosphate source is related to the glacial sedimentary supply of continental origin (granite, carbonatite and other alkaline bearing rocks). In addition, the input of cold water, as result of ice melting episodes, became a limiting development factor for phosphorus metabolizing organism which improves the P2O5 system enrichment. For the unique genetic conditions, this phosphatic deposit has been called as the Coromandel Deposit Type.

Carbonate diagenesis: A celebration of the work of John Anthony Dawson (Tony) Dickson

Carbonate diagenesis: A celebration of the work of John Anthony Dawson (Tony) Dickson

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.

Ediacaran cap dolomite of Shennongjia, northern Yangtze Craton, South China

Ediacaran cap dolomites are strong evidence for the glaciation during the Neoproterozoic. Stratigraphic-sedimentological studies combined with δ13C and δ18O isotope analyses are used for defining the processes of post-glacier environmental changes and sea-level rise caused by glacier melting and the reconstruction of the depositional environments for the Doushantuo cap dolomites in Shennongjia, northern Yangtze Craton (YC). Like many other cap dolomites in the YC and worldwide, those in Shennongjia exhibit strongly negative δ13Ccarb excursions, ranging from −7.3 to −2.5 ‰ (averagely from −5.4 to −4.1 ‰). A laminated clast-free calcareous siltstone commonly occurs between the cap dolomite and the underlying diamictites, indicating a low-energy, shallow-water depositional environment, wherein alkaline conditions developed before the deposition of the cap dolomite. The most prominent features of the cap dolomites in Shennongjia are the very thin thickness and monotonous sedimentary structures. The maximum thickness of the cap dolomites from the six sections in Shennongjia is 1.8 m, and the thinnest cap dolomite is 0.4 m with simple laminations. Sedimentary structures such as low-angle cross-bedding, giant wave ripples, sheet-crack cement, and tepee breccias are rare in Shennongjia except in the area near the Three Gorges. These evidences, therefore, point to the deposition of the Shennongjia cap dolomite in restricted shallow-water tidal environments.The space–time distribution of sedimentary facies in Shennongjia and the southeastern areas to Shennongjia indicate that the Ediacaran cap dolomite was formed on a ramp. Accordingly, we propose a sedimentary facies model for the formation of the cap dolomite, covering the inner and middle ramp facies. This model explains the thickness variations and distributions of the cap dolomites at both the local and regional scale, including their sedimentary structures, compositions, and barite occurrences. The model successfully illustrates why the cap dolomites in the Shennongjia area share a unique set of sedimentary features. Finally, the cap dolomite is interpreted as a diachronous deposit (bottom to top), tracking glacioeustatic flooding and recording ocean-wide changes over the time following deglaciation, and a correlation and cause of negative δ13Ccarb excursions in cap dolomites worldwide, particularly in those in the Yangtze Carton, are further discussed.

Effect of dolomitization on isotopic records from Neoproterozoic carbonates in southwestern Mongolia

Carbon isotope values from shallow-marine carbonate rocks, including those from many dolomitized successions, are the primary lens through which we interpret the ancient carbon cycle. Carbon isotopes are typically regarded as being robust to alteration during dolomitization due to the high carbon content of the rock compared to the fluid. However, chemostratigraphic studies of the Neoproterozoic Tsagaan-Olom Group in southwestern Mongolia exhibit 3–12‰ differences in carbon isotopes between stratigraphically equivalent limestone and dolomitized successions. To understand the origins of this geochemical variation, we conducted detailed geological mapping, petrographic, isotopic (δ13C, δ18O, δ44/40Ca, and δ26Mg), and fluid inclusion analyses of carbonates in the Taishir, Ol, and Shuurgat formations of the Tsagaan-Olom Group. Stratigraphic and textural constraints distinguish fabric retentive early dolomitization (Dolomitization Event 1) that occurred in the Taishir and Ol formations during and soon after deposition of the ca. 635 Ma Marinoan cap dolomite of the Ol Formation and fabric destructive dolomitization that occurred after the deposition of the younger Shuurgat Formation (Dolomitization Event 2), either prior to or during early Cambrian foreland basin formation. The dolomitizing fluids moved through porous stratigraphic units bound by impermeable shale of the Khongor and Zuun-Arts formations. Dolomitization homogenized the isotopic records of all Neoproterozoic Tsagaan-Olom Group dolomites; undolomitized successions show more extreme negative excursions from more positive background values. The salinity and isotopic values of seawater, the dolomitizing fluids, and the original platform carbonate were estimated from fluid inclusion data and a numerical model of diagenetic dolomitization. These results demonstrate that both early and late dolomitization can have a profound effect on carbon isotopic records of carbonates; however, with a multi-isotope approach the original carbon isotopic composition of platform-top carbonate can be distinguished from that of seawater and other dolomitizing fluids.

Recurrent anoxia recorded in shallow marine facies at Zhangcunping (western Hubei, China) throughout the Ediacaran to earliest Cambrian

In order to reconstruct the redox history of surface seawater for Ediacaran shallow marine facies on the Yangtze Platform, as well as to understand its implications for geological events (oxidation and phosphatization) and early evolution of life, we have conducted a systematic high-resolution investigation on microfacies analyses, pyrite morphology as well as sulfur and carbon contents in a continuous carbonate Ediacaran–Cambrian succession (drilled 407-core) in the Zhangcunping area of western Hubei (South China). The framboidal pyrites and SNAV/TOC (NAV = non-acid volatile sulfur) based redox reconstruction show a dynamic history of recurrent shallow marine anoxia. The early aftermath of the Marinoan Glaciation (cap dolomite to phosphorites) was characterized by either absence of pyrite or dominated by large framboids, low TOC contents and phosphorite deposition as well as well-preserved microfossils, indicating a minor oxygenated (dysoxic to oxic) photic shallow marine ecosystem. Meanwhile, the uppermost Doushantuo and Dengying formations were potentially fully oxygenated due to the lack of pyrites as well as low SNAV and TOC contents. Correlating with macrofossil assemblages (Miaohe biota and Ediacara biota) found in western Hubei, a major oxygenated marine ecosystem can be outlined for the late Ediacaran. In addition, a complicated diagenetic microfabric overprints the fine microstructure of microfossils preserved in phosphorites, leaving remarkable taphonomic puzzles for future study.

Neoproterozoic marine dolomite hardgrounds and their relationship to cap dolomites

Neoproterozoic cap dolomites are unusual and distinctive marker units that occur after large Cryogenian glaciations. Several thin dolomite units that have features resembling cap dolomites occur within the Cryogenian-Ediacaran succession of the Adelaide Geosyncline. While each of these dolomite units has distinctive features, they also share many similarities with cap dolomites including: stratigraphic thickness ranging from several centimetres to several metres; impure dolomicrite composition; graded beds with a sharp top that become more dolomite-rich upwards; and evidence of volume expansion in the form of sheet cracks, tepees or other soft- sediment deformation structures. The carbon isotopic compositions for these thin dolomite horizons have values ranging from ∼−5 to +2 per mil VPDB, which overlap those commonly found in both Sturtian and Marinoan cap dolomites. The generally negative carbon isotope compositions of these dolomites and cap dolomites is most easily explained by mixing of marine and authigenic carbonate.Sedimentological evidence indicates the graded beds within cap dolomites and other dolomite units are a form of submarine hardground or firmground, where dolomicrite is precipitated authigenically at or near the seafloor during periods of little or no sedimentation. Dolomite replacement of clay and other silicates is an important process in the formation of these dolomicrite firmgrounds. Volume expansion structures are a product of this near-surface dolomicrite precipitation, brought about by the high degrees of dolomite supersaturation with a consequent large force of crystallization and unlimited (marine) carbonate source for the precipitating dolomite.We suggest that this deepwater authigenic dolomicrite precipitation is a result of the dominantly anoxic nature of Cryogenian- early Ediacaran oceans and caused by alkalinity-producing anoxic bacterial reactions. When sedimentation rates slow sufficiently (e.g. during transgressions), thin dolomicrite horizons are produced. In this hypothesis, cap dolomites may largely be the result of sediment starvation, combined with a possible increase in carbonate saturation during and following the post-glacial transgressions associated with the Sturtian and Marinoan glacial events. This scenario explains most of the sedimentological features of these unusual carbonates. The widespread development of such authigenic carbonate would have profound implications for the interpretation of Precambrian marine carbon isotopic compositions.

Discussion of “Depositional ages and provenance of the Neoproterozoic Damara Supergroup (northwest Namibia): Implications for the Angola–Congo and Kalahari cratons connection” by Débora B. Nascimento, Renata S. Schmitt, André Ribeiro, Rudolph A. J. Trouw, Cees W. Passchier, and Miguel A. S. Basei

Discussion of “Depositional ages and provenance of the Neoproterozoic Damara Supergroup (northwest Namibia): Implications for the Angola–Congo and Kalahari cratons connection” by Débora B. Nascimento, Renata S. Schmitt, André Ribeiro, Rudolph A. J. Trouw, Cees W. Passchier, and Miguel A. S. Basei

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.

Redeposited Neoproterozoic (?) glacial deposits on the winter coast (Winter Mountains, White Sea)

Large dolomite fragments, part of which contain lonestones, while others look like cap dolomites, were found on the Winter coast of the White Sea. It is assumed that these rocks were plowed by the Late Quaternary glacier from Neoproterozoic glacial deposits lying probably on the White Sea bottom.

Geochemistry of C, O, and Sr isotopes in the Neoproterozoic carbonates from the southwestern Patom paleobasin, southern Middle Siberia

Scatter of δ13C (from −11.6 to 8.4‰) and sequence of carbon isotope events in the Vendian carbonates from the southwestern Patom paleobasin (Vitim, Bol’shaya Chuya, and Chaya rivers) are similar to those previously established in the reference northeastern sections. It was found that cap dolomites crowning the glaciogenic diamictites of the Dzhemkukan Formation are characterized by moderately low δ13C (from −7.4 to −5.1‰), while the limestones, marls, and calcareous aleurolites of the Zhuya Group (Nikol’skoe and Chencha formations) correlable with the Shuram-Wonoka event (580–650 Ma) have extremely low δ13C values (up to −11.9‰). Intervals with negative δ13C are separated by positive excursion of δ13C (most part of the Barakun, Valyukhta, and Uluntui formations) with extreme up to 8.4‰. One more positive excursion (δ13C up to 6.1‰) was identified in the dolomitic unit sandwiched between the limestones of the Chencha Formation and the sandstones of the Zherba Formation on the Vitim and Bol’shaya Chuya rivers; in the northeastern sections of the Patom Complex, the positive excursion corresponds to the hiatus, but occurs at similar level (∼550 Ma) in many sections of Oman, China, Namibia, Australia, and other regions. Ultrahigh δ13C (from 11.7 to 15.2‰) values were found in ∼150-m-thick succession of “coaly” carbonates in the Lokatyk Block (Bol’shaya Chuya River), which were presumably erroneously ascribed to the Barakun Formation. These carbonates likely marking the Neoproterozoic anoxic peak are not known among the Vendian (Ediakaran) successions, but have previously been described in the sediments with an inferred age of 720–650 Ma in the Bambui Group (eastern Brazil) and Keele Formation of the Neoproterozoic Windermere Group (northwestern Canada). Scatter of δ18O in the studied carbonates (from 6.3 to 28.5‰) was mainly caused by postsedimentary transformations. General correlation between δ18O and δ13C is absent. Both positive and negative correlations can be observed within limited intervals, indicating that the behavior of O and C isotope systems was not controlled by a common mechanism. The lowest 87Sr/86Sr = 0.7084 in the main chemostratigraphic marker (Chencha Formation) in the extreme west (Chaya River) does not differ much from the 87Sr/86Sr value in the Chencha Formation in the northeastern sections, which confirms the hydrological homogeneity of the Patom paleobasin. The lowest 87Sr/86Sr (0.70815) in the carbonates of the Uluntui Formation of the Chaya section, however, is much lower than in the carbonates from the stratotype area (0.70842) of the Uluntui Formation (Kuznetsov et al., 2013). This casts some doubts on the correlation of these stratigraphic units.

Synsedimentary deformation and the paleoseismic record in Marinoan cap carbonate of the southern Amazon Craton, Brazil

Event Layers in Neoproterozoic cap carbonates of Brazil's southwestern Amazon Craton record post-Marinoan synsedimentary seismicity. The 35 m-thick cap carbonates overlie glaciogenic sediments related to the Marinoan glaciation (635 Ma) and are comprised of two units: the lower cap consists of dolomite (∼15 m thick) and the upper cap is limestone (∼25 m thick). The cap dolomite includes pinkish crystalline dolostone with even parallel lamination, stratiform stromatolites, eventual tube structures and megaripple bedded peloidal dolostone interpreted as shallow (euphotic) platform deposits. The cap limestone onlaps the cap dolomite and consists of red marl, gray to black bituminous lime mudstone, bituminous shale with abundant calcite crystal fans (pseudomorphs after aragonite) and even parallel lamination interpreted as moderately deep to deep platform deposits. Five successive events of synsedimentary deformation were recognized in the cap carbonates exposed at Mirassol d'Oeste and Tangará da Serra, in Central Brazil: Event 1 – large to small-scale load cast structures in the contact between dolostones and glaciogenic sediments; Event 2 – stromatolitic lamination truncated by tube structures; Event 3 – vertical to subvertical fractures and faults, and large-scale synclines and anticlines with chevron folds; Event 4 – conglomerate and breccia filling neptunian dykes limited by undeformed beds; and Event 5 – slump and sliding deposits found only in the upper part of the cap limestone. Event 1 was produced by hydroplastic dynamics likely induced by isostatic rebound during ice cap melting in the final stages of the Marinoan glaciation. Events 2 and 5 are autocyclic in nature, and related to depositional processes. Event 2 is linked to fluid and methane escape from organic degradation of microbial mats and domes that formed tubestones; Event 5 is associated to collapse and sliding/slumping in the platform and slope. The reliable orientations of synsedimentary faults, and fractures and folds of events 3 and 4 are consistent with regional extensional tectonics associated with earthquakes that triggered sediment deformation. The 200 km that separate the occurrences of cap carbonates suggest that important seismic events took place during the early Ediacaran in the southern Amazon Craton.

Microfossils from cap dolomites of the Lower Vendian Churochnaya Formation in the Polyudov Range (North Urals): Paleoecological approach to interpretation of Late Proterozoic glaciations

Microfossils from silicified varieties of cap dolomites crowning the section of the tillite-bearing Lower Vendian (Ediacaran) Churochnaya Formation in the Polyudov Range (North Urals) are characterized. These microfossils are the first to be found from Vendian sections of the region and from all the terminal post-glacial sediments, one of the most significant global glaciation Marinoan of the African Glacial Era, when glaciers reached the equator. They are represented by remains of hormogonian and chrococcacean cyanobacteria as well as possible green filamentous algae. This microbiotic assemblage is of relatively low diversity, being composed of taxa with wide stratigraphic ranges characteristic of Proterozoic conservative microbiotas developed in shallow-water siliceous-carbonate facies. The lack of phytoplanktonic microfossils in this biota including Pertatataka-type acanthomorphic acritarchs or Ediacaran Complex Acanthomorph Palynoflora (ECAP) is also consistent with the conclusion on shallow-water deposition of the Churochnaya Formation. Moreover, most cyanobacteria representatives occurring in the latter are characteristic of shallow-water arid environments that confirm a theory of significant temperature increase during accumulation of the cap dolomites after termination of the above-mentioned glaciation. In addition to these microfossils, the cap dolomite member of the Churochnaya Formation contains filamentous and coccoidal pseudofossils formed under influence of post-sedimentary fluids. In their morphometric parameters, they resemble structures described from Archean sections as microfossil remains, which may be a key to interpreting their nature.

Sr isotope geochemistry and Pb–Pb geochronology of the Neoproterozoic cap carbonates, Tangará da Serra, Brazil

The Mirassol d'Oeste Formation (cap dolomites) and the base of the Guia Formation (cap limestones) represent the cap carbonates of the Neoproterozoic Araras Group, located along the southern border of Amazonian Craton, central Brazil. Petrographical and microfacies descriptions together with geochemical and mineralogical data of the top of the Mirassol d'Oeste and the base of the Guia formations in the Tangará da Serra area allowed us to identify the main diagenetic features (neomorphism, chemical compaction, and fractures) and to evaluate the terrigenous contribution. Dolomitization has been documented in a limestone horizon of the lower part of Guia Formation through petrography, X-ray diffractometry, and geochemistry. Despite diagenetic alteration, sedimentary structures of the Tangará da Serra cap carbonate are well preserved (laminar bedding and crystal fans). Pb–Pb dating of the Guia limestones yielded an age of 622 ± 33 million years. This early Ediacaran age is considered as the deposition age and constitutes further evidence that the cap carbonates of the Araras Group are related to the Marinoan glaciation, which took place at the end of the Cryogenian. The assessment of the primary 87Sr/86Sr signature of the Guia limestones has been improved by sequential leaching of diluted acetic acid, which allowed the elimination of highly radiogenic Sr contributions due to terrigenous grains. The 87Sr/86Sr ratios of 0.70709–0.70729 are regarded as the primary Sr isotopic signature when the carbonates precipitated. The terrigenous contribution can explain the discrepancy with the more radiogenic 87Sr/86Sr ratios previously obtained on the cap carbonate. At a global scale, the Sr isotopic signature around 0.7071–0.7073 of the cap carbonates of the Araras Group is compatible with Sr marine evolution curves at the end of the Marinoan glaciation. However, such low Sr values suggest that the abrupt rise could have happened asynchronously and heterogeneously in the early Ediacaran oceans.

Chapter 28 Late Cryogenian (Vendian) glaciogenic deposits in the Marnya Formation, Oselok Group, in the foothills of the East Sayan Range, southwestern Siberian Craton

Abstract The glaciogenic deposits of the Ulyakha Member (Mb.) referred to as Ulyakha diamictite (tillite), and the associated Late Cryogenian (Early Vendian) glaciofluvial deposits, are the lower strata of the Marnya Formation (Fm.) of the Oselok Group (Gr.) in the southwestern Siberian craton, which rest erosionally over the Cryogenian (Late Riphean) Karagassy Group. The Oselok Group deposits fill the Peri-Sayan foredeep and their stratigraphic equivalents immediately overlie the basement of the Siberian Platform. The glaciogenic deposits, including diamictite, breccia, boulder conglomerate and sandstone, had multistage sedimentation, were widespread, and underlie a cap dolomite of the lower Ozerki Mb. The Late Cryogenian (Early Vendian) age of the Ulyakha Mb. is defined by traces of shallow-marine soft-bodied animals in the middle part of the Ozerki Mb., and by findings of Ediacaran-like Metazoa in the overlying Bolshaya Aisa Mb. of the Marnya Fm. Deposition of the Ulyakha tillites and associated rocks has been correlated with the Late Cryogenian (Marinoan) glaciation based on the δ 13 C patterns in carbonate deposits in the Marnya and Uda formations and their stratigraphic position much below the Pre-Manykai and Zhuya δ 13 C negative signatures, which have been equated to the Wonoka anomaly.

Geochemical properties of Neoproterozoic “cap dolomites” in the Patom paleobasin and problem of their genesis

The characteristic feature of many Upper Neoproterozoic glacial sequences is their “cap carbonates” (CC) resting without visible unconformity upon glaciogenic diamictites. Such an unusual association, peculiar structures and textures, and negative δ13C values (approximately −4 ± 2‰) that are atypical of marine carbonates provoked long debates about the nature of these carbonates, which play an important role in the Snowball Earth hypothesis. According to this hypothesis, the Earth was entirely covered by ice during large-scale glaciations, and CC accumulation was related to the global change in geochemical processes. In this work, we discuss data on the chemical and isotopic (C, O, Sr) compositions of CCs, which overlie glacial sediments of the Nichatka and Bol’shoi Patom formations accumulated in different parts of the Neoproterozoic Patom paleobasin (Central Siberia). High concentrations of Fe (up to 6400 ppm), Mn (2320 ppm), and radiogenic Sr (87Sr/86Sr0 up to 0.7172) established in CCs indicates a strong influence of the continental flow. Extraordinary Snowball Earth conditions are not necessary for the accumulation of these rocks, geochemical and sedimentological properties of which may be explained by the discharge of thawing waters into partly or completely isolated near-glacier basin, their intermittent freezing, and/or washout of “frozen” carbonates from the surface of thawing glaciers. The peculiar thin-laminated texture of CC may be related to seasonal processes of climatic cycles. They were accumulated in the course of general (relatively long-term) depletion of the atmosphere and hydrosphere in 13C, which has nothing to do with the CC formation as a specific type of carbonate sediments. Amplitude and duration of the negative δ13C excursion in carbonates associated with the Lower Vendian glacial sediments (665–635 Ma) are appreciably lower than the negative anomaly in rocks of the Zhuya Group that likely correspond to the Shuram-Vonoka Event (∼560−580 Ma ago), which probably marks the crucial point in the Precambrian deglaciation: mass destabilization of methane hydrates and degradation of the Early Vendian psychrosphere in oceans.

Depósitos carbonáticos de Tangará da Serra (MT): uma nova ocorrência de capa carbonática neoproterozóica no sul do Cráton Amazônico

Cap carbonates are exceptional deposits related to the end of Neoproterozoic low-latitude glaciations. In the Tangará da Serra region, south Amazon craton, was described a Neoproterozoic carbonatic succession with approximately 20 m thick that includes the upper part of the Mirassol d'Oeste Formation and the base of the Guia Formation, respectively the dolomitic and calcareous caps of basal Araras Group. The cap dolomite is composed of pinkish peloidal dolograinstones with planar to low angle truncated laminations interpreted as deposits of a shallow to moderately deep platform. The cap limestone consists in massive to laminated siltstone and megaripple bedded crystalline limestone interpreted as deposits of moderately deep wave dominated mixed platform. Rippled crystalline limestone with crystals fans (pseudomorphosed aragonite) interbedded with shales were interpreted as oversaturated-CaCO3 deep platform deposits. Limestone with slump structures, convolute bedding and sinsedimentary faults characterize deposits of slope and neptunians dykes, filled by calcareous breccias, and isolated deformed limestone beds suggest seismic activity during the sedimentation. The succession of Tangará da Serra extends the occurrence of cap carbonates in the southern Amazon craton and corroborate with the presence of a large carbonate to mixed platform formed during the transgression after the glaciation Puga, correlate to Marinoan event

Variações dos isótopos de C e Sr em carbonatos pré e pós-glaciação Jequitaí (Esturtiano) na região de Bezerra-Formosa, Goiás

Carbonate rocks of the upper Paranoá Group and lower Bambuí Group (Sete Lagoas Formation) have been founded at the occidental border of the São Francisco craton. These units have been separated by the Sturtian glaciation, identified by Jequitaí Formation, but in some localities the glacial record is absent and is difficult to separate these two carbonates units. In this paper two sections with a good stratigraphic control have been chosen to find C, O and Sr isotopic values to have tools for the correlations. Carbonates of the Paranoá Group present a trend of positive δ 13 C PDB varying between +0.8 e +2.7‰, of δ 18 O PDB varying between -9.0 e -4.7‰ and 87 Sr/ 86 Sr ranging from 0.7063 to 0.7068. The diamictite sample of Jequitaí Formation has δ 13 C PDB values of -1.8‰ and δ 18 O PDB values of -5.1‰. The cap dolomites of the Sete Lagoas Formation are associated with an extremely negative δ 13 C PDB values between -6.0 e -3.2‰, δ 18 O PDB values between -9.1 e -5.3‰ and high 87 Sr/ 86 Sr ratios suggesting a radiogenic Sr isotope anomaly. Limestone and muddy-limestone above the cap dolomite still starts with extremely negative δ 13 C PDB values (-6.0‰) and are followed by an upward trend of increasing carbon isotope values up to +9.2‰. In the same interval the δ 18 O PDB values ranging from -10.1‰ at the base to -5.5‰ in direction to the top of formation and primary 87 Sr/ 86 Sr ratios between 0.70745 and 0.70758. The Carbon and Sr isotope data presented here reveal significant differences between carbonates from the Paranoá Group and the Bambuí Group. These data developed here provides the framework for a new regional isotopic correlation model to the carbonates units of the Paranoá and Bambuí groups.

Variações dos isótopos de C e Sr em carbonatos pré e pós-glaciação Jequitaí (Esturtiano) na região de Bezerra-Formosa, Goiás

Carbonate rocks of the upper Paranoa Group and lower Bambui Group (Sete Lagoas Formation) have been founded at the occidental border of the Sao Francisco craton. These units have been separated by the Sturtian glaciation, identified by Jequitai Formation, but in some localities the glacial record is absent and is difficult to separate these two carbonates units. In this paper two sections with a good stratigraphic control have been chosen to find C, O and Sr isotopic values to have tools for the correlations. Carbonates of the Paranoa Group present a trend of positive δ 13 C PDB varying between +0.8 e +2.7‰, of δ 18 O PDB varying between -9.0 e -4.7‰ and 87 Sr/ 86 Sr ranging from 0.7063 to 0.7068. The diamictite sample of Jequitai Formation has δ 13 C PDB values of -1.8‰ and δ 18 O PDB values of -5.1‰. The cap dolomites of the Sete Lagoas Formation are associated with an extremely negative δ 13 C PDB values between -6.0 e -3.2‰, δ 18 O PDB values between -9.1 e -5.3‰ and high 87 Sr/ 86 Sr ratios suggesting a radiogenic Sr isotope anomaly. Limestone and muddy-limestone above the cap dolomite still starts with extremely negative δ 13 C PDB values (-6.0‰) and are followed by an upward trend of increasing carbon isotope values up to +9.2‰. In the same interval the δ 18 O PDB values ranging from -10.1‰ at the base to -5.5‰ in direction to the top of formation and primary 87 Sr/ 86 Sr ratios between 0.70745 and 0.70758. The Carbon and Sr isotope data presented here reveal signific ant differences between carbonates from the Paranoa Group and the Bambui Group. These data developed here provides the framework for a new regional isotopic correlation model to the carbonates units of the Paranoa and Bambui groups.