Ediacaran in Uruguay: palaeoclimatic and palaeobiological implications

The Ediacaran to lowermost Cambrian successions of south‐eastern Uruguay preserve an unusual and significant record of deposits generated during the Gondwana assembly (ca 590 to 535 Ma). This study presents a review of data obtained through extensive field‐based mapping coupled with detailed sedimentology and stratigraphy of key formations. The geological units within the study area consist of the Maldonado Group (Playa Hermosa, Las Ventanas and San Carlos formations), the Arroyo del Soldado Group (Yerbal, Polanco Limestones, Barriga Negra and Cerro Espuelitas formations) and the Arroyo de la Pedrera Group (Piedras de Afilar and Cerro Victoria formations). The Maldonado Group is characterized by a glacially influenced volcanogenic‐sedimentary sequence with ice‐rafted debris and dropstones in the Playa Hermosa and Las Ventanas formations. The Arroyo del Soldado Group is a mixed siliciclastic‐carbonate succession, mainly represented by an intercalation of basal pink dolostones, banded siltstones, rhythmites of dolostone‐limestone, iron formations, cherts and conglomerates. Carbonates in the Polanco Limestones Formation are characterized by a negative δ¹³C excursion up to ?3·26‰ PeeDeeBelemnite. The Arroyo de la Pedrera Group consists of quartz arenites and stromatolitic/oolitic dolostones. Preliminary data indicate that the Precambrian–Cambrian could be contained within or at the base of this group. The entire succession is almost 6000 m thick, and contains a rich fossil assemblage composed of organic‐walled microfossils and small shelly fauna, including the index fossil Cloudina riemkeae. The stratigraphic and chemostratigraphic features are suggestive of a Gaskier age (ca 580 Ma) for the basal glacial‐related units. In this scenario, the results show the importance of lithostratigraphic, biostratigraphic and chemostratigraphic data of these Ediacaran units in the global correlation of terminal Proterozoic sedimentary rocks.     

Geological evolution of the Neoproterozoic Corumbágraben system (Brazil). Depositional context of the stratified Fe and Mn ores of the Jacadigo Group

The Corumbáregion is located near the junction of two young (545–480 Ma) Brasiliano provinces: the Chiquitos-Tucavaca aulacogen which cross cuts the Amazon craton-Rio Apa block, and the Paraguay fold belt. Formation of the aulacogen and the Paraguay basin, which is interpreted as a foreland basin, was roughly synchronous with the 590 Ma metamorphism in the neighbouring old Brasiliano Brasilia fold belt. Deformation and metamorphism in the Paraguay belt are bracketed between 545 and 500 Ma, whereas folding in the aulacogen occurred between 500 and 480 Ma ago.The NE–SW-trending, Corumbágraben system is located on the eastern part of the WNW–ESE-trending, Chiquitos-Tucavaca aulacogen. The extensional structures of the Corumbágraben system are attributed to flexure of the Amazon craton along the western border of the Paraguay basin.Stratigraphic successions in the Corumbágraben system resemble those of the adjacent cover rocks of the Amazon craton: a lower detrital unit (Jacadigo Group and Boqui, Puga, and Cadieus formations), partly of glacial origin, and an upper carbonate-rich unit, (CorumbáGroup and Pororó, Cerradinho, Bocaina, and Araras formations) with local preservation of an Ediacara-like fauna. The whole sequence is probably Vendian. In the cover sequence around Cuiabá, the carbonate unit is capped by siliciclastic sediments. In comparison to the cratonic cover rocks, the Jacadigo Group, which is filling the Corumbágraben system, differs on three points: it is thicker, contains chemical manganese and iron sedimentary intercalations and was affected by a diagenetic to epimetamorphic event during which temperatures reached 250–280°C. These features are in good agreement with sedimentation of the Jacadigo Group in an extensional environment where Fe and Mn, of probable hydrothermal origin, would be generated by leaching of hypothetical, hidden intrusive mafic plutons associated with graben formation.     

Vendian of the Fore-Yenisei sedimentary basin (southeastern West Siberia)

Fossiliferous Upper Vendian strata are discovered in the Upper Proterozoic to Lower Paleozoic Fore-Yenisei sedimentary basin under a thick Mesozoic-Cenozoic cover in southeastern West Siberia. Two depositional systems are recognized based on sedimentological features: (1) wave- and current-agitated shoreface-forereef-biohermal reef system (Vostok-3 Borehole section) and (2) tidal flat-evaporite basin (Averinskaya-150 Borehole section). The forereef facies yielded fossilized tubular calcareous skeletons of reef-building metazoans Cloudina riemkeae, Cloudina hartmannae and Cloudina carinata, phosphatized Namacalathus-like fossils, and a diversity of tubular phosphatized and agglutinated tubular fossils. The fossil assemblage can be interpreted as the evidence of ecological complexity of the reef system. Paleontological characteristics suggest correlation of the Vendian strata with the lowermost Purella antiqua Assemblage Zone and the boundary interval with the underlying Anabarites trisulcatus Assemblage Zone of the Siberian Platform. Therefore, at least in the late Proterozoic, the Fore-Yenisei sedimentary basin was part of a larger pericratonic depositional system on the western margin of the Siberian Craton.     

The Vendian passive continental margin in the southern Siberian Craton: geochemical and isotopic (Sr,Sm–Nd) evidence and U–Pb dating of detrital zircons by the LA-ICP-MS method

Geochemical and isotopic (Sm–Nd and Sr) studies of deposits of the Baikal and Oselok Groups in the southern Siberian Craton and LA-ICP-MS U–Pb dating of detrital zircons show that they accumulated in passive continental-margin settings in the Vendian. The time limits of sedimentation were assessed on the basis of Sr chemostratigraphy of carbonate deposits of the Baikal Group and LA-ICP-MS U–Pb dating of detrital zircons in first-cycle terrigenous deposits of the Oselok Group. The main provenances for rocks of these groups were constant. These were rocks of the cover and basement of the Siberian Craton. Tuffite horizons in upper portions of the groups are the only sign of Late Vendian activation of this block, which is reflected in changes of geochemical indices of terrigenous rocks and their younger Sm–Nd model ages.     

Acritarchs of Las Ventanas Formation (Ediacaran, Uruguay): Implications for the timing of coeval rifting and glacial events in western Gondwana

Acritarchs and other organic-walled microfossils occurring in siltstones of the Las Ventanas Formation (Quebrada de Viera and El Perdido members) are systematically described and illustrated. The assemblage includes the following species: Leiosphaeridia tenuissima, Leiosphaeridia minutissima, Lophosphaeridium sp., Soldadophycus bossii, Soldadophycus major, Soldadophycus sp. and Vendotaenia antiqua. The microflora is characterized by low diversity (six species), dominance of L. tenuissima, absence of acanthomorphic acritarchs, and relatively large size of sphaeromorphs, reaching 400 μm in diameter. A number of species are shared with acritarch assemblages preserved in the overlying Arroyo del Soldado Group. Differences between assemblages include the occurrence of abundant Bavlinella faveolata and small size of spheroids in the Arroyo del Soldado Group. The assemblage occurring in Las Ventanas Formation is assigned to the Ediacaran Leiosphere Palynoflora, which spans the interval between the base of the Ediacaran (end of the Marinoan Glaciation, 635 Ma) and the termination of the Gaskiers Glaciation (582 Ma). An early Ediacaran age between 615 and 579 Ma is also supported by available radiometric ages. An extensional setting for Las Ventanas basin is suggested on the basis of the bimodal, synsedimentary volcanism, strong palaeorelief, great thickness of alluvial fan conglomerates and the evolution from continental to open marine environments. Diamictites occurring in the Quebrada de Viera Member are described for the first time, including associated dropstones which suggest a glacial origin. If confirmed, this would be one more example of the association between rifting and glaciation in the Neoproterozoic, coeval with a low-diversity, high-abundance acritarch microflora. A causal relationship between these tectonic, climatic and biologic events is discussed.     

Some Specific Features of Early Vendian Sedimentation in the Southern and Middle Urals

The existence of the pre-Early Vendian hiatus in the Bashkir Meganticlinorium is supported by large incised valleys filled with Early Vendian sediments both on the western and eastern limbs of the meganticlinorium. It is suggested that a great glacioeustatic lowering of the sealevel occurred in the sedimentation basin. Taking into account the reconstruction of similar Early Vendian events for many provinces elsewhere, we can suppose that Early Vendian sedimentation basins in the southern and middle Urals were connected with the World Ocean. The comparative study of Lower Vendian sedimentary sequences in the Urals and Norway demonstrates similarities in rock associations. The correlation of Lower Vendian sections in the Urals with modern facies models of glacial sedimentation shows that glacimarine sequences of the southern Urals fit the shelf sedimentary associations proper. The character of section at the Serebryanka level in the middle Urals and the spatial distribution of mixtites therein correspond to sedimentation on the basin shelf and slope.     

Ediacaran in Uruguay: Facts and controversies

The Ediacaran of Uruguay has been regarded as containing a significant geological and paleontological record, which would make these successions critical to unraveling diverse aspects regarding the assembly of southwestern Gondwana and to understanding the conditions surrounding the rise of animal life in a period punctuated by drastic paleoenvironmental changes. However, a review of currently available data leads to the conclusion that, although variable, the stratigraphy, distribution and age of these units remain ambiguous. The same is true for existing basin models and tectonic evolution, which show different and sometimes contradicting supporting evidence. Here, we propose that the Ediacaran record consists of the Maldonado Group (Playa Hermosa, Las Ventanas and San Carlos formations), and the Tacuarí, Barriga Negra, Rocha and Sierra de Aguirre formations. The Arroyo del Soldado Group (Yerbal, Polanco Limestones and Cerro Espuelitas formations) and the Arroyo de la Pedrera Group (Piedras de Afilar and Cerro Victoria formations) were likely deposited between 700 and 1000 Ma. The best available radiometric age constraints indicate intense magmatic–tectonic activity occurred between 600 and 560 Ma, incompatible with previous models suggesting a stable, Atlantic-type passive margin on this portion of southwestern Gondwana. Further research is needed in order to firmly establish a consistent litho- and chronostratigraphic framework; particularly, before attempting any regional or global correlation, and inferences on global paleoenvironmental and paleobiological events.     

Lithogeochemistry of the fine-grained siliciclastic rocks of the Vendian Serebryanka Group of the Central Urals

Sedimentation environments were reconstructed for the Early Vendian successions of the western slope of the Central Urals, which comprises one of the most complete sections of the terminal Precambrian system in northern Eurasia. It was shown that, despite the presence of several diamictite levels in the sections of the Serebryanka Group, mature and multiply recycled fine-grained siliciclastic materials (CIA = 65–77) were delivered into the sedimentation basin over the whole Early Vendian. Based on the lithochemical characteristics of shales, the climate of Serebryanka time can be estimated as semiarid-semihumid, similar to that dominating in Late Vendian paleocatchments. Based on relatively high Mo/Mn values (0.011–0.024), it was suggested that anoxic or similar conditions existed in the basin of Buton time, whereas other sedimentary complexes of the Serebryanka Group were formed in well aerated environments. The systematics of Sr, Ba, Zr, Cu, and V in fine-grained siliciclastic rocks and Sr isotopic data for carbonate rocks indicate that the sediments were accumulated in a fresh-water basin. The values of trace-element indicator ratios, e.g., Th/Sc, La/Sc, Th/Cr and others, in the shales of the Serebryanka Group and Nd model age estimates indicate that a variety of mainly Early Proterozoic complexes, ranging from granitoids to basic rocks, occurred in the Early Vendian paleocatchments. The basic rocks were eroded most extensively probably in the end of Serebryanka time. Based on the Ce/Ce* values of shales, it was concluded that submarine volcanism had no significant influence on sedimentation processes in the Early Vendian. An exception is Koiva and Kernos time, when hematite-bearing shales were accumulated in association with pillow basalts in some zones of the basin. The distribution of the compositions of shales from various formations of the Serebryanka Group in discrimination diagrams suggests that the Early Vendian sedimentary sequences were formed in passive geodynamic settings.     

Age and sources of late precambrian sedimentary sequences of the Southern Baikal Region: Results of the U-Pb LA-ICP-MS dating of detrital zircons

The first data on the age of detrital zircons are given for Late Precambrian terrigenous rocks of the Baikal Group and Ushakovka Formation of the southern flank of the Siberian Craton. The ages obtained for 348 zircons cover the Paleoarchean to Late Ediacaran period, demonstrate the dynamics of change of sources of the clastic material in the sedimentation basin, and mark the changes of the Late Precambrian tectonic regimes. The age of the youngest group of detrital zircons extracted from the rocks of the Kachergat Formation allows us to restrict the upper age limit of accumulation of the rocks of the Baikal Group to the Late Ediacaran (Late Vendian).     

U–Pb geochronology of the southern Brasília belt (SE-Brazil): sedimentary provenance, Neoproterozoic orogeny and assembly of West Gondwana

The Brasília belt borders the western margin of the São Francisco Craton and records the history of ocean opening and closing related to the formation of West Gondwana. This study reports new U–Pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the São Francisco Craton, and date the orogenic events leading to the amalgamation of West Gondwana.Ages of detrital zircons (by ID–TIMS and LA-MC-ICPMS) were obtained from metasedimentary units of the passive margin of the São Francisco Craton from the main tectonic domains of the belt: the internal allochthons (Araxá Group in the Áraxá and Passos Nappes), the external allochthons (Canastra Group, Serra da Boa Esperança Metasedimentary Sequence and Andrelândia Group) and the autochthonous or Cratonic Domain (Andrelândia Group). The patterns of provenance ages for these units are uniform and are characterised as follows: Archean–Paleoproterozoic ages (3.4–3.3, 3.1–2.7, and 2.5–2.4 Ga); Paleoproterozoic ages attributed to the Transamazonian event (2.3–1.9 Ga, with a peak at ca. 2.15 Ga) and to the ca. 1.75 Ga Espinhaço rifting of the São Francisco Craton; ages between 1.6 and 1.2 Ga, with a peak at 1.3 Ga, revealing an unexpected variety of Mesoproterozoic sources, still undetected in the São Francisco Craton; and ages between 0.9 and 1.0 Ga related to the rifting event that led to the individualisation of the São Francisco paleo-continent and formation of its passive margins. An amphibolite intercalation in the Araxá Group yields a rutile age of ca. 0.9 Ga and documents the occurrence of mafic magmatism coeval with sedimentation in the marginal basin.Detrital zircons from the autochthonous and parautochthonous Andrelândia Group, deposited on the southern margin of the São Francisco Craton, yielded a provenance pattern similar to that of the allochthonous units. This result implies that 1.6–1.2 Ga source rocks must be present in the São Francisco Craton. They could be located either in the cratonic area, which is mostly covered by the Neoproterozoic epicontinental deposits of the Bambuí Group, or in the outer paleo-continental margin, buried under the allochthonous units of the Brasília belt.Crustal melting and generation of syntectonic crustal granites and migmatisation at ca. 630 Ma mark the orogenic event that started with westward subduction of the São Francisco plate and ended with continental collision against the Paraná block (and Goiás terrane). Continuing collision led to the exhumation and cooling of the Araxá and Passos metamorphic nappes, as indicated by monazite ages of ca. 605 Ma and mark the final stages of tectonometamorphic activity in the southern Brasília belt.Whilst continent–continent collision was proceeding on the western margin of the São Francisco Craton along the southern Brasília belt, eastward subduction in the East was generating the 634–599 Ma Rio Negro magmatic arc which collided with the eastern São Francisco margin at 595–560 Ma, much later than in the Brasília belt. Thus, the tectonic effects of the Ribeira belt reached the southernmost sector of the Brasília belt creating a zone of superposition. The thermal front of this event affected the proximal Andrelândia Group at ca. 588 Ma, as indicated by monazite age.The participation of the Amazonian craton in the assembly of western Gondwana occurred at 545–500 Ma in the Paraguay belt and ca. 500 Ma in the Araguaia belt. This, together with the results presented in this work lead to the conclusion that the collision between the Paraná block and Goiás terrane with the São Francisco Craton along the Brasília belt preceded the accretion of the Amazonian craton by 50–100 million years.     

Paleoclimatic Environments of the Formation of Upper Vendian Rocks on the Belomorian-Kuloi Plateau,Southeastern White Sea Region

Reconstruction of Vendian climatic conditions on the East European Craton is of principle importance for elucidation of the Ediacarian biota habitat. However, paleogeographic reconstructions for this time are largely based on fragmentary and controversial paleomagnetic data. The degree of rock maturity deduced from lithochemical indicators allowed us to identify two stages of Late Vendian sedimentation on the Belomorian-Kuloi Plateau. The first (Lyamitsa-Verkhov) stage was characterized by the delivery of immature clastic material to the basin from a provenance with arid climate. The second (Erga-Padun) stage was marked by the input of relatively mature aluminosilicate clastics from a provenance with mild humid climate. The sedimentation stages approximately coincide with replacement of the shallow-water marine environment by the fluvioalluvial environment marked by steady and intense perennial river drainage from a highland in the northeast. In the Late Vendian (since 555 Ma ago), the northeastern area of the East European Craton was influenced by humid climate.__________Translated from Litologiya i Poleznye Iskopaemye, No. 3, 2005, pp. 267–280.Original Russian Text Copyright © 2005 by Grazhdankin, Podkovyrov, Maslov.     

The Itaja�� foreland basin: a tectono-sedimentary record of the Ediacaran period, Southern Brazil

The Itajaí Basin located in the southern border of the Luís Alves Microplate is considered as a peripheral foreland basin related to the Dom Feliciano Belt. It presents an excellent record of the Ediacaran period, and its upper parts display the best Brazilian example of Precambrian turbiditic deposits. The basal succession of Itajaí Group is represented by sandstones and conglomerates (Baú Formation) deposited in alluvial and deltaic-fan systems. The marine upper sequences correspond to the Ribeir?o Carvalho (channelized and non-channelized proximal silty-argillaceous rhythmic turbidites), Ribeir?o Neisse (arkosic sandstones and siltites), and Ribeir?o do Bode (distal silty turbidites) formations. The Apiúna Formation felsic volcanic rocks crosscut the sedimentary succession. The Cambrian Subida leucosyenogranite represents the last felsic magmatic activity to affect the Itajaí Basin. The Brusque Group and the Florianópolis Batholith are proposed as source areas for the sediments of the upper sequence. For the lower continental units the source areas are the Santa Catarina, S?o Miguel and Camboriú complexes. The lack of any oceanic crust in the Itajaí Basin suggests that the marine units were deposited in a restricted, internal sea. The sedimentation started around 600?Ma and ended before 560?Ma as indicated by the emplacement of rhyolitic domes. The Itajaí Basin is temporally and tectonically correlated with the Camaqu? Basin in Rio Grande do Sul and the Arroyo del Soldado/Piriápolis Basin in Uruguay. It also has several tectono-sedimentary characteristics in common with the African-equivalent Nama Basin.     

Neoproterozoic events in the Tcholliré area: Pan-African crustal growth and geodynamics in central-northern Cameroon (Adamawa and North Provinces)

The Rei Bouba Group is a sedimentary and volcanic sequence (750–?650 Ma), regarded as a remnant of a Pan-African (back-arc?) orogenic basin that separated a remobilized Paleoproterozoic crust from an accretionary area (Poli Complex: 800-650 Ma). The latter was subjected to early deformation (D1) and intruded by calc-alkalic plutons (670 Ma). Transpressive tectonics and major thrusting, with emplacement of synkinematic granite, occurred at ca 630 Ma (D2-3) and a late compression (D4), with emplacement of calc-alkalic granite, occurred at ca 570 Ma. The Tcholliré Shear Zone is regarded as part of the major boundary between a recently accreted crust and the remobilized margin of the Congo Craton.     

Carbon Isotopic Profile Across the Bilara Group Rocks of Trans-Aravalli Marwar Supergroup in Western India: Implications for Neoproterozoic — Cambrian Transition

The rocks of Marwar Supergroup in the trans-Aravalli sector in western India are presumed to span the time interval between Neoproterozoic and early Cambrian. This, predominantly unfossiliferous, marine sedimentary sequence is characterized by a lower arenaceous facies (Jodhpur Group), middle carbonate facies (Bilara Group) and upper argillaceous— arenaceous facies (Nagaur Group) rocks. The sedimentation has been essentially in a shallow basin, described either as the fore-land slope of the rising Aravalli mountains or a sag-basin which developed and evolved due to subsidence of the updomed crust during Neoproterozoic Malani magmatism that failed to open rifts. The carbon isotopic profile for the Bilara Group carbonate rocks in the lower part shows marked oscillations and broadly negative δ¹³C character with negative anomalies as low as <−4.3‰_PDB, observed near the base of Dhanapa Formation (lower unit) and <−6.5‰_PDB in the overlying Gotan Formation (middle unit). The upper part of the profile shows a gradual positive shift. The carbon isotopic signatures of the Bilara Group rocks can be correlated with the end-Neoproterozoic — early Cambrian (Vendian — Tommotian) carbon isotopic evolution curve. Extremely low δ¹³C values indicate the glaciation related cold climatic postulates of the end-Neoproterozoic, followed by the warmer climatic conditions as indicated by the positive shift. The carbon isotopic data for Gotan Formation carbonates, at variance with the globally observed δ¹³C trends for early Tertiary, do not support the recently proposed Tertiary age for the Bilara Group.     

Microfossils and biofacies of the Vendian fossil biota in the southern Siberian Platform

Based on the study of the litho- and biofacies of the Vendian Nepa Horizon in the central area of the Siberian Platform inland, a paleoecological model for the Vendian microbiota has been developed. The sedimentation environments of the Katanga saddle have been reconstructed, and three sedimentary systems have been recognized: (1) lower continental, formed by the deposits of proluvial fans and riverbeds of temporary streams; (2) middle transgressive, made up of littoral sand facies in the lower part and of fine-clastic shelf strata in the upper part; and (3) upper, of sea highstand, composed of alternating sand bank facies and fine-clastic lagoon deposits. Four biofacies have been recognized in the fine-terrigenous deposits of the Nepa Horizon: (1) Appendisphaera, represented by a Doushantuo-Pertatataka acanthomorph assemblage; (2) Transitional, with a great diversity of plankton and benthic (including complex) taxa; (3) Vanavarataenia, dominated by Vanavarataenia complex benthic algae; and (4) Oscillatoriopsis, represented by taxonomically poor biotas with morphologically simple (mainly prokaryotic) remains. These biofacies are confined to the following sedimentation environments: Appendisphaera is widespread in the distal open-sea areas; the Transitional biofacies is localized in the distal environments of the semi-isolated inner basin; Vanavarataenia occurs in the proximal areas; and Oscillatoriopsis is typical of the shallow-water environments, both extended (corresponding to the highstand period) and local.     

Three Palaeoproterozoic basins—Yerrida,Bryah and Padbury—Capricorn Orogen,Western Australia

The Palaeoproterozoic Yerrida, Bryah and Padbury Basins record periods of sedimentation and magmatism along the northern margin of the Archaean Yilgarn Craton. Each basin is characterised by distinct stratigraphy, igneous activity, structural and metamorphic history and mineral deposit types. The oldest of these basins, the Yerrida Basin (ca 2200 Ma) is floored by rocks of the Archaean Yilgarn Craton. Important features of this basin are the presence of evaporites and continental flood basalts. The ca 2000 Ma Bryah Basin developed on the northern margin of the Yilgarn Craton during backarc sea‐floor spreading and rifting, the result of which was the emplacement of voluminous mafic and ultramafic volcanic rocks. During the waning stages of the Bryah Basin this mafic to ultramafic volcanism gave way to deposition of clastic and chemical sedimentary rocks. At a later stage, the Padbury Basin developed as a retroarc foreland basin on top of the Bryah Basin in a fold‐and‐thrust belt. This resulted from either the collision of the Pilbara and Yilgarn Cratons (Capricorn Orogeny) or the ca 2000 Ma westward collision of the southern part of the Gascoyne Complex and the Yilgarn Craton (Glenburgh Orogeny). During the Capricorn Orogeny the Bryah Group was thrust to the southeast, over the Yerrida Group. Important mineral deposits are contained in the Yerrida, Bryah and Padbury Basins. In the Yerrida Basin a large Pb–carbonate deposit (Magellan) and black shale‐hosted gossans containing anomalous abundances of Ba, Cu, Zn and Pd are present. The Pb–carbonate deposit is hosted by the upper units of the Juderina Formation, and the lower unit of the unconformably overlying Earaheedy Group. The Bryah and Padbury Basins contain orogenic gold, copper‐gold volcanogenic massive sulfides, manganese and iron ore. The origin of the gold mineralisation is probably related to tectonothermal activity during the Capricorn Orogeny at ca 1800 Ma.     

A section of Vendian in the east of West Siberian Plate (based on data from the Borehole Vostok 3)

The Borehole Vostok 3 drilled in the east of the West Siberian Plate (Tomsk Region) revealed a Vendian section in the depth range 5002–3870 m, which was subdivided into the Poiga, Kotodzha, and Raiga Formations based on geological, geophysical, and paleontological data. In the Kotodzha and Raiga Formations, typical Upper Vendian fossils of Cloudina hartmanae and Namacalathus sp. were found along with diverse Platysolenites, which are commonly considered to be of zonal significance in Lower Cambrian strata. Hence, the stratigraphic interval with abundant diverse Platysolenites has a wider stratigraphic range than it was believed earlier and seems to cover the Upper Vendian and Lower Cambrian deposits. The Borehole Vostok 3 is the first Siberian occurrence of the fossils Namacalathus, the world's fourth occurrence of the Cloudina-Namacalathus association, and the first site where coexisting Platysolenites and typical Vendian organisms have been found. Therefore, the borehole provides one of the most informative (in paleontological context) Upper Vendian sections.     

New data on biostratigraphy of the Vendian Nemakit-Daldynian stage in the southern Siberian platform

New paleontological finds in sediments of the Upper Vendian Nemakit-Dadynian Stage from different areas of the southern Siberian Platforms (Yenisei Range, Sayany region, central areas, Patom Highland) are discussed. The base of the Lower Cambrian Tommotian Stage (∼540 Ma) is established at the first appearance level of characteristic small-shelled fossils in the western part of the Patom Highland. This boundary coincides with the episode of a brief sea-level fall and enhanced terrigenous sedimentation in the basin with dominant carbonate sediment deposition. The base of the Purella antiqua Zone (544 Ma), which is registered by paleontological data in the Yenisei Range sections, is marked by the replacement of terrigenous sedimentation by the carbonate one. In the northwestern Yenisei Range, this boundary coincides, in addition, with a major hiatus and subsequent paleobasin extension. The base of the Nemakit-Daldynian Stage (base of the Anabarites trisulcatus Zone, ∼550 Ma) in most sections corresponds to the onset of sea transgression. In addition to small-shelled remains, sediments of this zone contain characteristic ichnofossils and Vendotaenia flora. It is shown that defined boundaries are traceable through the entire southern part of the Siberian Platform. This makes them useful for wider stratigraphic interpretations.     

Provenance of late Paleoproterozoic cover sequences in the central Gawler Craton: exploring stratigraphic correlations in eastern Proterozoic Australia using detrital zircon ages,Hf and Nd isotopic data

Provenance data from Paleoproterozoic and possible Archean sedimentary units in the central eastern Gawler Craton in southern Australia form part of a growing dataset suggesting that the Gawler Craton shares important basin formation and tectonic time lines with the adjacent Curnamona Province and the Isan Inlier in northern Australia. U–Pb dating of detrital zircons from the Eba Formation, previously mapped as the Paleoproterozoic Tarcoola Formation, yields exclusively Archean ages (ca 3300–2530 Ma), which are consistent with evolved whole-rock Nd and zircon Hf isotopic data. The absence of Paleoproterozoic detrital grains in a number of sequences (including the Eba Formation), despite the proximity of voluminous Paleoproterozoic rock units, suggests that the Eba Formation may be part of a Neoarchean or early Paleoproterozoic cover sequence derived from erosion of a multi-aged Archean source region. The ca 1715 Ma Labyrinth Formation, unconformably overlying the Eba Formation, shares similar depositional timing with other basin systems in the Gawler Craton and the adjacent Curnamona Province. Detrital zircon ages in the Labyrinth Formation range from Neoarchean to Paleoproterozoic, and are consistent with derivation from >1715 Ma components of the Gawler Craton. Zircon Hf and whole-rock Nd isotopic data also suggest a source region with a mixed crustal evolution (ε_Nd –6 to –4.5), consistent with what is known about the Gawler Craton. Compared with the lower Willyama Supergroup in the adjacent Curnamona Province, the Labyrinth Formation has a source more obviously reconcilable with the Gawler Craton. Stratigraphically overlying the Eba and Labyrinth Formations is the 1656 Ma Tarcoola Formation. Zircon Hf and whole-rock Nd isotopic data indicate that the Tarcoola Formation was sourced from comparatively juvenile rocks (ε_Nd –4.1 to + 0.5). The timing of Tarcoola Formation deposition is similar to the juvenile upper Willyama Supergroup, further strengthening the stratigraphic links between the Gawler and Curnamona domains. Additionally, the Tarcoola Formation is similar in age to extensive units in the Mt Isa and Georgetown regions in northern Australia, also shown to be isotopically juvenile. These juvenile sedimentary rocks contrast with the evolved underlying sequences and hint at the existence of a large-scale ca 1650 Ma juvenile basin system in eastern Proterozoic Australia.