Figure 4- - uploaded by Francisco R G Barroso
Content may be subject to copyright.
Ediacaran fauna. A, Charniodiscus arboreus (Scale 3 cm); B, Charniodiscus concentricus (Scale 4 cm); C, Cyclomedusa davidi (Scale 1 cm); D, Ediacaria flindersi (Scale 5 cm); E, Kimberella quadrata (Scale 2 cm); F, Medusinites asteroides (Scale 15 cm); G, Palaeophragmodictya reticulata (Scale 5 cm).
Source publication
This study reports the first known occurrence of the Ediacaran fauna in northeastern Brazil (at Pacujá Municipality, northwestern state of Ceará) and presents preliminary interpretations of its significance. Regional correlation indicates that the fossils originated in the Jaibaras Basin and that they may represent a new geological system. The depo...
Contexts in source publication
Context 1
... R.G. BARROSO et al. Genus Charniodiscus Ford, 1958Charniodiscus arboreus Glaessner, 1959 ( Figure 4A) Description: A frond-shaped impression preserved in epirelief, the petaloid sometimes in negative and sometimes in positive epirelief. Branches approxi- mately uniform, parallel, with the primary branches diverging from the central shaft at slightly oblique angles and the distal extremities connected to an exterior edge, preserved on only one side. ...
Context 2
... Charniodiscus Ford, 1958Species ? Charniodiscus concentricus Ford, 1958 ( Figure 4B) Description: Only the basal disc of this species was observed. The disc is a bipartite circular im- pression in positive epirelief, 6.9 cm in width and 13 cm in total diameter. ...
Context 3
... phylogenetic affinity of Ediacaria has also been attributed to cnidarians (McCall 2006). Glaessner andWade, 1966 Species Kimberella quadrata Glaessner andWade, 1966 ( Figure 4E) Description: An oval-shaped impression with the distal portions showing a relatively inflated outer edge and a distorted, raised internal zone. Anterior extremity with a neck-shaped structure. ...
Context 4
... Medusinites Glaessner, 1966 Species Medusinites asteroides Sprigg, 1949 ( Figure 4F) Description: A bipartite circular impression in positive epirelief. Outer raised disc separated from the inner disc by a deep surrounding depression. ...
Citations
... There are a series of small rift basins adjacent to the Parnaíba Basin, some of which can be shown to pass beneath the main basin. Their fill is ascribed to the Cambrian Jaibaras Formation (Vaz et al. 2007); recent work by Barroso et al. (2014) demonstrates that these basins may extend back to the Ediacaran (latest Neoproterozoic). Geophysical work and drilling results show that other such small basins occur below the main Parnaíba Basin (Soares et al. 2018), but these small rifts are not considered part of the Parnaíba Basin and are not considered further in this paper. ...
The processes that govern formation and development of cratonic basins are poorly understood, both individually and as a class. The cratonic Silurian-Jurassic Parnaíba Basin in NE Brazil is less well-studied than North American examples such as Williston, Illinois and Michigan but offers an opportunity to study stratal architecture both in outcrop and the subsurface. Published stratigraphic compilations emphasize basin-wide unconformities separating layer-cake, basin-wide packages; analysis of geological maps indicates this interpretation is over-simplified, with at least 12 local unconformities, concentrated along the SW basin margin. Comparison of basin-margin and intrabasinal well-correlation panels shows that unconformities are more common in marginal areas, so the current exposure margins may have some validity as near-original margins. Palaeocurrents suggest a consistent SW-NE transport direction for the Serra Grande and lower Canindé groups. Supporting heavy mineral analysis from Silurian-Carboniferous strata on the SW and NE basin margins show that assemblages are dominated by ultra-stable zircon, tourmaline and rutile with minor amounts of staurolite, especially in the SW and older units. Tourmaline varietal results are remarkably consistent across the basin. U-Pb age spectra from detrital zircons are dominated by Neoproterozoic grains, with subsidiary Meso-Paleoproterozoic provenance. We infer that the source terrane was a medium-grade regionally metamorphosed mica schist, probably the Araguaia Fold Belt. These results are indicative of a large sediment routing system feeding material across an evolving crustal sag, analogous to observations from North American cratonic basins; this suggests that cratonic basins may not have strongly shaped or controlled the routing system.
... Many models have been proposed for the tectono-magmatic evolution of the Jaibaras Basin (Oliveira, 2001) and according to the most accepted models, the Early Cambrian may have been the main period for both within-basin and plutonic magmatism, as well as the deposition of the Pacuj a Formation. Barroso et al. (2014), in a study that reports the first occurrence of Ediacaran fauna in northeastern Brazil, suggest the existence of a new geological event that would be placed between Pacuj a and Aprazível formations, in a depositional environment associated to a fluviomarine system. According to the authors, the inferred ages of the deposits are at least 560 Ma. ...
The Late Neoproterozoic to Early Paleozoic Jaibaras Basin is located between the southeastern margin of the Médio Coreaú Domain (MCD) and northwestern edge of the Ceará Central Domain (CCD) of the Borborema Province, Northeastern Brazil, which is also the location of the main axis of the Transbrasiliano Lineament. The lower volcano-sedimentary rocks in the Jaibaras Basin are composed of two lower Vendian-Cambrian sedimentary units, Massapê paraconglomerates and Pacujá reddish sandstones, and one volcanic unit, the Parapuí Formation. The whole set is partially crosscut by two post-tectonic granite suites, the Mucambo and Meruoca plutonic bodies. The upper Cambrian-Ordovician sequence in the basin is represented by the polymitic conglomerates of the Aprazível Formation. In order to gain information on the relationship between magmatic and sedimentary events, in this paper we present new U-Pb zircon and K-Ar geochronological analyses that have been carried out on both volcanic and plutonic magmatic lithotypes. According to our data, emplacement of the Meruoca Pluton took place over a large time interval (539 Ma to 523 Ma), and was synchronous with the emplacement of the Mucambo Pluton (532 Ma) and at lower part of the Parapuí Suite (535 Ma). The last ductile event related to the movement of the Sobral-Pedro II Shear Zone is dated at 530 Ma, and the subsequent emplacement of the Aroeira Suíte dikes occurred between 524 and 500 Ma (Middle Cambrian). Finally, at 492-484 Ma is the age of cooling of the magmatic system related to the Aroeiras Suite.
... No embasamento, esse lineamento corresponde a uma zona de cisalhamento plástica com direção NE-SW relacionada à orogênese Brasiliana. Uma etapa de movimentação tardia, em baixa temperatura, ocorreu no Ediacarano-Cambriano (idade baseada na fauna identificada por Barroso et al., 2014, no Graben de Jaibaras, NW do Ceará), controlou a estruturação de grabens no substrato da bacia (Cacama et al., 2015), também caracterizados em linhas sísmicas na porção central da bacia (Oliveira e Mohriak, 2003;Morais Neto et al., 2013;Antunes et al., 2015Antunes et al., , 2016 do LTB afetando as unidades pós-ordovicianas a eotriássicas, no interior da bacia, corresponde a falhas, juntas e bandas de deformação, sendo que Chamani (2011Chamani ( , 2015 detalhou a ocorrência de sismitos em várias unidades da BPar, interpretados como indícios de atividade tectônica ao longo do LTB e outras estruturas. Essas estruturas teriam se originado em resposta à impressão de novos campos de tensões, ainda no contexto paleotectônico do Gondwana ou relacionados à formação das margens continentais leste e equatorial sul-americanas. ...
... A interpretação das imagens dos sensores remotos permitiu reconhecer fotolineamentos tardios em relação à trama de alta temperatura das zonas de cisalhamento brasilianas do embasamento. Com base no registro estrutural observado das sequências molássicas brasilianas e nas idades referidas por Barroso et al. (2014) para o Grupo Jaibaras, esse evento dúctil-frágil D n pode ser posicionado no intervalo Ediacarano-Cambriano. Por outro lado, fotolineamentos associados a estruturas de caráter frágil, observados na cobertura sedimentar, foram associados a reativações póssilurianas de estruturas análogas ao LTB, que são também importantes ao longo dessa borda da bacia. ...
O limite sudeste da Bacia do Parnaíba (BPar), região fronteiriça aos Estados do Piauí, da Bahia e de Pernambuco (Nordeste do Brasil), é cartografado como uma não conformidade entre o Grupo Serra Grande (GSG, Siluriano) e o embasamento pré-cambriano. Todavia, essa borda é caracterizada por uma expressiva orientação NE, paralela ao Lineamento Transbrasiliano,bem como pelo estreitamento das faixas aflorantes das formações siluro-devonianas, em comparação ao observado mais ao norte, na borda leste. Um enxame de fotolineamentos NE, interpretados a partir de imagens de sensores remotos, ocorrem tanto ao longo do limite da bacia com o embasamento cristalino, como nas unidades sedimentares eopaleozoicas. Com o suporte de dados de campo, três padrões estruturais foram reconhecidos e correlacionados a eventos deformacionais distintos. A Deformação Dn, de idade ediacarana-cambriana, é reconhecida apenas no embasamento cristalino pré-cambriano, sendo relacionada a uma etapa tardia, retrometamórfica e de caráter plástico-frágil das zonas miloníticas brasilianas. A assinatura desse evento inclui zonas de cisalhamentos transcorrentes destrais orientadas NE-SW e veios de quartzo preenchendo juntas de distensão E-W. A Deformação Dn+1 corresponde a uma nova etapa de movimentação transcorrente destral nas estruturas NE-SW, acompanhada por falhas normais e juntas de distensão com orientação WNW a E-W, neste caso também afetando as unidades siluro-devonianas. A Deformação Dn+2 é caracterizada por falhas normais, diques básicos correlacionados à Suíte Sardinha, eocretácea), veios silicosos e cristas de arenitos silicificados, todos com orientação NE-SW. Essa última fase resulta de uma distensão com orientação NW-SE, análoga àquela reconhecida mais ao leste, nas Bacias Interiores do Nordeste do Brasil, associada ao rifteamento do Atlântico Sul.
... There, limestone-marl rhythmites occur at the base, which pass into dark gray to black, finely laminated, organic-rich limestones, at the top. Barroso et al. (2014) reported the occurrence of probable soft-bodied Ediacaran fossils in the overlying Jaibaras Group, similar to the White Sea assemblage, which could assure an Ediacaran age to this unit (560 -542 Ma, Narbonne 2005; et al. 2007). These biostratigraphic relationships can provide a minimum late Ediacaran age for the Frecheirinha Formation, but it needs more detailed study to prove the paleontological affinity of the fossils. ...
This report reviews and incorporates new elemental and isotope chemostratigraphic data for correlation of Neoproterozoic carbonate-dominated successions in South America (Argentina, Bolivia, Brazil, Paraguay and Uruguay). These thick mixed carbonate/siliciclastic successions were largely deposited in epicontinental basins or accumulated on passive margins on the edges of cratons (e.g. São Francisco, Amazonia, Rio Apa Block, Pampia and Río de la Plata paleocontinents) during extensional events related to the rifting of the Rodinia Supercontinent. From the stratigraphic point of view, these successions occur as three mega-sequences: glaciogenic, marine carbonate platform (above glaciomarine diamictites or rift successions), and dominantly continental to transitional siliciclastics. In the orogenic belts surrounding cratons, carbonate-dominated successions with important volcanoclastic/siliciclastic contribution have been, in most cases, strongly deformed. The precise ages of these successions remain a matter of debate, but recently new paleontological and geochronological data have considerably constrained depositional intervals. Here, we report high-resolution C, O, Sr, and S isotope trends measured in well-preserved sample sets and mainly use Sr and C isotopes in concert with lithostratigraphic/biostratigraphic observations to provide detailed correlations of these successions. The establishing of a high-level and definite chemostratigraphic correlation between Neoproterozoic basins in South America is the main goal of this work.
... Apart of skeletal records of acritarchs 16,18 , calcified or organic-walled macroscopic Cloudina lucianoi 14 , Corumbella werneri 13 and extremely rare conulariids 17 recorded in the Ediacaran Corumbá Group, Mato Grosso do Sul, Brazil, impressions of macroscopically complex, soft-bodied organisms are scarce in South American deposits of this age [22][23][24] . Siliciclastic sedimentary strata preserved in the Neoproterozoic Itajaí Basin, southern Brazil, record poorly preserved impressions assigned to Parvancorina sp., Charniodiscus? sp. and Cyclomedusa sp 23 . ...
The origin, affinity and paleoecology of macrofossils of soft-bodied organisms of the terminal Ediacaran
Period have been highly debated. Previous discoveries in South America are restricted to small shelly
metazoans of the Nama Assemblage. Here we report for the first time the occurrence of discoidal
structures from the Upper Ediacaran Cerro Negro Formation, La Providencia Group, Argentina.
Specimens are preserved in tabular sandstones with microbially-induced sedimentary structures. Flute
marks and linear scours at the base of the sandstone layers indicate deposition under high energy,
episodic flows. Stratigraphic, sedimentologic, petrographic and taphonomic analyses indicate that the
origin of these structures is not related to abiotic process. Preservational and morphological features,
as invagination and the presence of radial grooves, indicate that they resemble typical morphs of the
Aspidella plexus. The large number of small-sized individuals and the wide range of size classes with
skewed distribution suggest that they lived in high-density communities. The presence of Aspidella in
the Cerro Negro Formation would represent the first reliable record of Ediacaran soft-bodied organisms
in South America. It also supports the paleogeographic scenario of the Clymene Ocean, in which a
shallow sea covered part of the southwest Gondwana at the end of the Ediacaran.
... Apart of skeletal records of acritarchs 16,18 , calcified or organic-walled macroscopic Cloudina lucianoi 14 , Corumbella werneri 13 and extremely rare conulariids 17 recorded in the Ediacaran Corumbá Group, Mato Grosso do Sul, Brazil, impressions of macroscopically complex, soft-bodied organisms are scarce in South American deposits of this age [22][23][24] . Siliciclastic sedimentary strata preserved in the Neoproterozoic Itajaí Basin, southern Brazil, record poorly preserved impressions assigned to Parvancorina sp., Charniodiscus? sp. and Cyclomedusa sp 23 . ...
The origin, afnity and paleoecology of macrofossils of soft-bodied organisms of the terminal Ediacaran
Period have been highly debated. Previous discoveries in South America are restricted to small shelly
metazoans of the Nama Assemblage. Here we report for the frst time the occurrence of discoidal
structures from the Upper Ediacaran Cerro Negro Formation, La Providencia Group, Argentina.
Specimens are preserved in tabular sandstones with microbially-induced sedimentary structures. Flute
marks and linear scours at the base of the sandstone layers indicate deposition under high energy,
episodic fows. Stratigraphic, sedimentologic, petrographic and taphonomic analyses indicate that the
origin of these structures is not related to abiotic process. Preservational and morphological features,
as invagination and the presence of radial grooves, indicate that they resemble typical morphs of the
Aspidella plexus. The large number of small-sized individuals and the wide range of size classes with
skewed distribution suggest that they lived in high-density communities. The presence of Aspidella in
the Cerro Negro Formation would represent the frst reliable record of Ediacaran soft-bodied organisms
in South America. It also supports the paleogeographic scenario of the Clymene Ocean, in which a
shallow sea covered part of the southwest Gondwana at the end of the Ediacaran.
... Grant, 1990; Gaucher & Germs, 2010). Essa hipótese abre maiores possibilidades para a descoberta de 'vendobiontes' no Grupo Bambuí, como os recentemente reportados no " Arenito Contra Fogo " da Bacia Jaibaras, noroeste do Ceará (Barroso et al., 2014). ...
O registro fóssil dos depósitos sedimentares proterozoicos, paleozoicos e mesozoicos do Cráton do São Francisco inclui estruturas de origem microbiana (microbialitos, oncólitos), palinomorfos terrestres e marinhos, microfósseis silicosos e carbonáticos de origem marinha, metazoários neoproterozoicos, artrópodes, uma variedade de bioturbações, vertebrados actinopterígios, sarcopterígios e dinossauros. Com exceção dos microbialitos, que são amplamente distribuídos, os fósseis são relativamente escassos se comparados com a extensão geográfica dos depósitos e concentrados em alguns horizontes estratigráficos excepcionais. A raridade dos achados pode ser explicada pela falta de prospecção nas áreas mais distantes das localidades tradicionalmente conhecidas, e também ao baixo potencial de preservação dos fósseis em ambientes deposicionais do Meso e Neoproterozoico, ou de regimes climáticos extremos, i.e. glacial para o Paleozoico e desértico para as porções mesozoicas da Bacia Sanfranciscana. Trabalhos recentes de prospecção, no entanto, têm revelado um potencial significativo para novas descobertas, especialmente na região norte de Minas Gerais. Parte dos registros cenozoicos está distribuída em bacias restritas incluindo as de Gandarela e Fonseca (Eoceno-Mioceno), onde basicamente se preservaram folhas de angiospermas e gimnospermas, palinomorfos e insetos, e a Bacia de Juatuba, com registro predominante de folhas de angiospermas O Quaternário constitui uma exceção do ponto de vista da diversidade e abundância de fósseis, especialmente devido à ampla ocorrência de mamíferos em sistemas cársticos. A descontinuidade dos depósitos sedimentares no Cráton do São Francisco mostram hiatos de cerca de 250 milhões de anos entre as seções fossilíferas do Neoproterozoico ao Paleozoico e de cerca de 125 milhões de anos (Eocretáceo ao Pleistoceno) para a fauna de vertebrados.
... Costa et al. (1975) subdividiram esse grupo em quatro unidades, da base para o topo: a Formação Massapê, onde predominam rochas conglomeráticas e brechoides com intercalações de arenitos arcoseanos; segue, em contato transicional, a Formação Pacujá, composta de arenitos arcoseanos, folhelhos e siltitos; outra unidade, posteriormente redefinida como a Suíte Parapuí, é composta de rochas extrusivas e intrusivas encaixadas nas demais unidades citadas, incluindo basaltos , andesitos, traquitos e riolitos; por fim, e também em contato gradacional com a Formação Pacujá (Jardim de Sá et al., 1979), ocorre a Formação Aprazível, composta de conglomerados e brechas polimíticas, com intercalações de arenitos. Dados paleontológicos obtidos por Barroso et al. (2014) permitem estimar uma idade ediacarana-cambriana para as rochas pertencentes a esse grupo. A borda NW do GJ corresponde à " Zona de Cisalhamento Café-Ipueiras " , adjacente à qual ocorre o Granito Meruoca (Figura 3 ), cuja idade média ponderada obtida com a técnica de U-Pb em zircão é de 523 ± 10 Ma (Archanjo et al., 2009). ...
O Lineamento Transbrasiliano é uma importante zona de cisalhamento com direção NE-SW, relacionada à orogênese Brasiliana e que evoluiu em estágios de alta até baixa temperatura. No presente trabalho, investigou-se a assinatura estrutural e geofísica da porção norte do Lineamento Transbrasiliano (fronteira Ceará-Piauí). Na literatura é comum a referência à reativação fanerozoica dessa estrutura, a qual teria originado diversos grabens tardi-brasilianos precedentes à sinéclise paleozoica do Parnaíba, a exemplo do Graben de Jaibaras. Em etapas evolutivas mais jovens, as falhas que seccionam as unidades da Bacia do Parnaíba, ao longo de toda a extensão do Lineamento Transbrasiliano, exprimem a sua reativação em eventos sucessivos. O mapa magnético da região exibe anomalias com direção NE, interpretadas como a assinatura do Lineamento Transbrasiliano (e das estruturas brasilianas da Província Borborema) na sua expressão de alta temperatura. O Graben de Jaibaras é marcado por uma faixa anômala retilínea com alta suscetibilidade magnética, aparentemente sem continuidade expressiva no substrato da Bacia do Parnaíba. As análises geométrica e cinemática das estruturas da área, utilizando dados de sensor remoto e de campo, permitiram caracterizar quatro fases de deformação. A fase deformacional Dn ocorre de modo exclusivo no Graben de Jaibaras (o que permite inferir sua idade, ediacarana-cambriana), com desenvolvimento de estruturas de temperatura mais elevada, comparativamente aos eventos subsequentes. As fases deformacionais D1 , D2 e D3 ocorrem afetando tanto o Graben de Jaibaras como as sequências paleozoicas da borda NE da Bacia do Parnaíba, com geração de estruturas em temperatura baixa, essencialmente rúpteis/cataclásticas, que são tentativamente correlacionadas com os estágios de abertura do Atlântico ou mesmo com eventos orogênicos distais de idade Paleozoica
... Apart of skeletal records of acritarchs 16,18 , calcified or organic-walled macroscopic Cloudina lucianoi 14 , Corumbella werneri 13 and extremely rare conulariids 17 recorded in the Ediacaran Corumbá Group, Mato Grosso do Sul, Brazil, impressions of macroscopically complex, soft-bodied organisms are scarce in South American deposits of this age [22][23][24] . Siliciclastic sedimentary strata preserved in the Neoproterozoic Itajaí Basin, southern Brazil, record poorly preserved impressions assigned to Parvancorina sp., Charniodiscus? sp. and Cyclomedusa sp 23 . ...
The Neoproterozoic sedimentary cover of the Río de La Plata Craton in the Sierras Bayas - Olavarría region, located in the northwestern sector of the Tandilia System, is represented by the Sierras Bayas Group (Villa Mónica, Cerro Largo, Olavarría and Loma Negra formations; Fig. 1), which is unconformably overlain by the Cerro Negro Formation (Poiré and Spalletti, 2005). The Sierras Bayas Group is composed of a variety of different marine sedimentary rocks which are characterized by the absence of strong deformation and metamorphism (Gómez Peral et al., 2007). Compared to the Sierras Bayas Group, the Cerro Negro Formation received less attention over the last years. In the Sierras Bayas region (Fig. 2), Iñiguez and Zalba (1974) and Iñiguez et al. (1989) described and named this unit in well-exposed quarries faces. They indicated that it was mostly composed of siliciclastics (reddish mudstones and sandstones), with subordinated marls in its basal section. In the nearby subsurface area, some authors mentioned the presence of a relatively thick succession of dark fine-grained rocks overlying the Sierras Bayas Group (Amos et al., 1972; Cuomo et al., 1983), but the relationship with the Cerro Negro Formation described in outcrops was unclear. The aim of this contribution is to present the sedimentological attributes and stratigraphic relationships of the outcrop and subsurface succe ssions originally reported as Cerro Negro Formation, in order to integrate this upper section of the Neoproterozoic cover in a common evolutionary model. To do so, more than 200 m of cores were described (Fig. 3), and combined with sections measured in the quarries. Conventional sedimentological description (textures, physical structures, bed thickness, etc.), was combined with X-Ray diffraction and standard petrography, in order to define mineralogical composition and proportion of main components. In this context, 181 samples were analyzed by XRD using standard procedures, and 50 thin sections, mostly from siltstones and sandstones, were qualitatively and quantitatively described. This information was integrated in a facies analysis. The facies analysis allowed to identify and interpret 10 sedimentary facies, based on texture, structures and colors, which are grouped in four categories (Figs. 4-7): marls (massive and lamina ted marls), siliciclastic mudstones (dark fissile mudstones, red massive mudstones, and grey massive mudstones), mudstone-sandstone with heterolithic bedding (grey heterolithics with lenticular bedding, grey heterolithics with wavy bedding, and red heterolithics with wavy bedding), and finally, sandstones (cross-laminated sandstones and massive sandstones). Broadly speaking, marls and siliciclastic mudstones are interpreted as the result of settling from suspension, either in oxygen-rich (marls and red massive mudstones) or oxygen-depleted (dark fissile mudstones) sea floors. Heterolithic intervals with lenticular and wavy bedding represent recurrent alternation of settling and traction, the latter mostly as a result of ripple migration. Finally, the sandstones are interpreted as the product of wave-generated ripple migration (cross-laminated sandstones) or deposition under high-concentrated flows (massive sandstones). The ten recognized facies occur in a systematic order in the investigated succession. Therefore, three distinctive facies successions were defined (facies successions I, I, and III), and correlated when possible across subsurface or outcrop areas (Fig. 8). Facies successions I (4-25 m thick) comprises laminated and massive marls at the base, which grade vertically into red massive mudstones. Mudcracks are occasionally observed. Clay minerals are mostly illitic (illite and illite/smectite). This succession overlies the regional erosive karstic surface sculpted at the top of the Sierras Bayas Group (named Barker surface) and marks the resumption of mixed carbonate/siliciclastic sedimentation in the area. This facies succession is interpreted as deposited under supratidal conditions in a tidalflat depositional environment, probably under well oxygenated conditions. Facies succession II (0-150 m thick) conformably overlies the previous one in many of the investigated cored wells and is composed of dark fissile mudstones and massive grey siltstones, as well as grey heterolithics with lenticular and wavy bedding. In the fine fraction, clays are now dominated by chlorite and illite in similar proportions (Fig. 8b). This facies succession is interpreted to represent low-energy subtidal settings, likely with suboxic to anoxic bottom conditions. Facies succession III (up to 75 m thick) unconformably rests upon the previous, and represents an abrupt change to red heterolithics with wavy and flaser bedding, together with cross-laminated and massive, fine- to medium-grained sandstones (Fig. 8b). Mudcracks, scour marks and flutes are common in this succession. In these facies quartz and plagioclases are abundant, with subordinated clay minerals, which are composed mainly of illite and less proportion of chlorite. This succession is interpreted to represent subtidal to intertidal settings, probably with well-oxygenated substrates. The top of this succession has been removed by erosion both in subsurface and outcrops. The spatial (vertical and lateral) distribution of facies successions and their basal boundaries, together with the faults recognized by well correlation, provide the framework for the proposed tectono-sedimentary model for the investigated Neoproterozoic succession (Fig. 9). This begins with the stage I which coincides with the deposition of the facies succession I over the karstic surface of the top of Sierras Bayas Group. Then, the facies succession II is deposited conformably onto the previous one, and is related here to a sea level drop referred as stage II, which is followed by a transgressive event. Stage I was dominated by marly to clay-rich sedimentation under oxic conditions which turns to anoxic and purely siliciclastic during the stage II. At the end of the stage II the complete succession, including the Sierras Bayas Group strata, was affected by the reactivation of vertical to subvertical falls from the basement with modified the relief by uplift, followed by intense erosion and peneplanation. This period is defined as stage III. Finally, the stage IV is constituted by the deposition of the facies succession III in which the top is not preserved. The lithological and stratigraphic relationships defined in this contribution for the Neoproterozoic upper strata of the northwestern Tandilia System make necessary to postulate a new lithostratigraphic scheme for them. As mentioned before, the facies succession II is composed of up to 150 meters of mainly black shales and was almost unknown till the present study. This stratigraphic division is lithologically distinctive compared to facies successions I and III and is truncated at the top by an erosional unconformity. This interval is therefore proposed as a new formation here, complying the provisions of the article 28 of the Argentine Stratigraphic Code. This definition drives necessary the re-location of the facies succession I, not as the base of the Cerro Negro Formation, but as a new lithostratigraphic unit as well. In this sense, the facies succession I is here defined as Avellaneda Formation, while facies succession II is termed Alicia Formation (Fig. 10). The name Cerro Negro is here attributed solely to facies succession III, the reddish heterolithics that represent the uppermost part of the investigated strata. Moreover, these three lithostratigraphic units, conforming a discrete depositional record within a single basin, are grouped into a new lithostratigraphic unit defined as La Providencia Group (Fig. 10).
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.
