RESUMO: Esta revisão incorpora novos dados isotópicos e químicos para correlação quimioestratigráfica de sucessões, predominantemente carbonáticas, na América do Sul (Argentina, Bolívia, Brasil, Paraguai e Uruguai). Estas sucessões, depositadas durante eventos de extensão, aparecem como espessos estratos carbonáticos/siliciclásticos em bacias epicontinentais ou foram acumulados sobre margens passivas em bordos de cratons (ex.: São Francisco, Amazonas, Bloco Rio Apa, Pampia e Río de La Plata) como três megasequências: glaciogênica, plataforma marinha carbonática (sobre diamictitos glacio-marinhos) e predominantemente continental siliciclástica. Em cinturões circunjacentes a cratons, sucessões carbonáticas contendo material vulcanoclástico/siliciclástico foram, na maioria dos casos, fortemente deformados. A idade precisa de sedimentação destas sucessões permanece em debate, mas recentes dados paleontológicos e geocronológicos têm consideravelmente restringidos os intervalos de deposição. Neste trabalho, reportamos trends isotópicos de alta resolução de C, O, Sr e S medidos em amostras bem preservadas e usamos principalmente isótopos de Sr e C em consonância com observações litostratigráficas/biostratigráficas, para estabelecer correlações detalhadas destas sucessões. Propor uma correlação quimioestratigráfica entre bacias Neoproterozoicas na América do Sul é o principal objetivo deste trabalho.

ABSTRACT: 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.

We report for the first time the occurrence of organic-walled microfossils in Ediacaran limestones and marls of the Frecheirinha Formation, Ubajara Group, and the first Precambrian acritarchs so far reported for northeastern Brazil. The assemblage of the Frecheirinha Formation represents a low-diversity microflora comprising Leiosphaeridia, Lophosphaeridium and subordinated Bavlinella (=Sphaerocongregus). Their thermal alteration index (TAI) between 4+ and 5, suggests metamorphic temperatures exceeding 200-250°C. Higher temperatures are probably related to intruding granitic plutons (Meruoca, Mucambo). Reported δ13C values of carbonates of -3.5 ‰ VPDB (Vienna-Peedee Belemnite) at the base, passing up section into a positive plateau of up to +3.7 ‰, and corresponding 87Sr/86Sr values between 0.7075 and 0.7080 suggest an Ediacaran age. The acritarch assemblage is comparable to the Late Ediacaran Leiosphere Palynoflora (LELP) or Kotlin-Rovno assemblage, in broad agreement with chemostratigraphic data. Macrofossils belonging to the Ediacara fauna were reported from the overlying Jaibaras Group, which would constrain even further the depositional age of the Frecheirinha Formation to within ca. 575-555 Ma. A more comprehensive palynological study of the Frecheirinha Formation is necessary to confirm this age assignment.

We compare the litho-, bio- and chemostratigraphy of Neoproterozoic sedimentary successions of the Río de la Plata Craton, namely the Arroyo del Soldado Group (ASG) of Uruguay and the Sierras Bayas Group -Cerro Negro Formation (SBG-CNFm) of Tandilia (Argentina). Both successions are characterized by alternation of carbonates and siliciclastics (Poiré, 1987; Gaucher, 2000), as well as a geotectonic setting corresponding to a passive continental margin. The Yerbal, Polanco and Cerro Espuelitas Formations (ASG) are correlated lithostratigraphically to the Cerro Largo, Loma Negra and Cerro Negro Formations of Tandilia. Whereas the ASG is up to 5 km thick, the SBG-CNFm represents a strongly condensed section showing numerous erosive surfaces and only 350 m in thickness. This difference may be explained by the shallower, epicontinental sedimentary environment of the SBG-CNFm. The predominantly siliciclastic Yerbal and Cerro Largo Formations represent the transgression of the Vendian (Ediacaran) sea onto the Río de la Plata Craton. The overlying Polanco and Loma Negra Formations are characterized by thick and pure carbonate (especially limestone) deposits currently mined in both countries for use in the cement, lime and fertilizer industries. An expressive palaeokarst is developed on top of these carbonates, indicating sea-level drop and platform exposure (Barrio et al., 1991; Gaucher et al., 2004b). The lower Cerro Espuelitas Formation and the Cerro Negro Formation are made up of marls at the base, passing into shales and heterolithic facies up section. We report for the first time the occurrence of Cloudina cf. C. riemkeae Germs (1972) in micritic limestones of the Loma Negra Formation. This supports the correlation of this unit with the lower Polanco Formation, where these shelly fossils also occur (Gaucher y Sprechmann, 1999; Gaucher et al., 2003b). Moreover, Cloudina is currently regarded as an index fossil of the late Ediacaran (Grant, 1990; Amthor et al., 2003). Low diversity acritarch assemblages occur both in the ASG and SBG-CNFm. In the former, two assemblages were distinguished: a Bavlinella faveolata- Soldadophycus assemblage occurring mainly in the siliciclastic units, and a slightly more diverse Leiosphaeridia-Lophosphaeridium assemblage characteristic of the Polanco Formation (Gaucher, 2000). In the SBG-CNFm, the acritarch asemblage is sphaeromorph-dominated (mainly Leiosphaeridia), and similar to the Leiosphaeridia-Lophosphaeridium microflora of the ASG. For the first time we report acritarchs occurring in greenish pelites of the Villa Mónica Formation, assigned here to Leiosphaeridia minutissima, L. tenuissima and Synsphaeridium sp. These represent the oldest fossils so far reported from Argentina. Pothe de Baldis (1983) described Chuaria olavarriensis, Leiosphaeridia sp. and Paleorivularia ontarica from the upper Cerro Largo Formation. The former two taxa can be reassigned to Chuaria circularis and Leiosphaeridia minutissima, respectively. Palynological macerations of shales of the Cerro Largo Formation carried out in the present study yielded Leiosphaeridia tenuissima as dominant species, and a single vesicle assigned to Leiosphaeridia minutissima. A detailed micropalaeontological study is due to Cingolani et al. (1991), which describe the following acritarch species from the Cerro Negro Formation: Leiosphaeridia sp., Trachysphaeridium sp., Synsphaeridium sp. and granular sphaeromorphs grouped by the authors under «type A» acritarchs. The application of the currently accepted taxonomic criteria to these acritarchs allows assigning them to Leiosphaeridia jacutica, L. minutissima, L. tenuissima, Lophosphaeridium sp. and Synsphaeridium sp. In the present study, acritarchs assigned to Leiosphaeridia tenuissima were isolated from shales of the Cerro Negro Formation. Throughout the SBG-CNFm, a tendency towards smaller sphaeromorphs from base to top is observed: from 450 µm for Villa Mónica Formation to 380 µm for the Cerro Largo Formation and finally 200 µm for the Cerro Negro Formation. This trend has been reported for other shallow-water, marine Ediacaran successions elsewhere (Butterfield, 1997). On the basis of low diversity and dominance of sphaeromorphs, the acritarch assemblage of the SBG-CNFm can be assigned either to the Simple Leiosphere Palynoflora of Grey et al. (2003) or the younger Kotlin-Rovno assemblage of Vidal y Moczydlowska (1997). The palynoflora occurring in the lower and middle ASG has been correlated to the Kotlin-Rovno assemblage (Gaucher, 2000), supporting the lithostratigraphic correlation depicted above. Available chemostratigraphic data for the ASG and SBG-CNFm also suggest that the Loma Negra Formation correlates with the Polanco Formation. Whereas 87Sr/86Sr values for limestones of the Loma Negra Formation range between 0.7069 and 0.7087 (Kawashita et al., 1999b), limestones of the Polanco Formation yielded ratios between 0.7072 and 0.7085 (Kawashita et al., 1999a; Gaucher et al., 2004c). The Loma Negra Formation is further characterized by d13C values around +4 ‰ PDB (Gómez Peral et al., in press), which are similar to d13C relationships between +5.5 and 0 ‰ PDB reported for the lower Polanco Formation (Unit A of Gaucher et al., 2004b). Regarding the Villa Mónica Formation, available data are ambiguous regarding its age assignment and correlatios. On the one hand, stromatolite assemblages described from the unit (Poiré, 1987, 1993) suggest an upper Riphean age (Tonian or Cryogenian). On the other hand, the similarity of acritarch assemblages preserved throughout the SBGCNFm suggests that there is not a large time hyatus between the Villa Mónica and Cerro Largo Formations, raising the possibility of the Villa Mónica Formation being of Ediacaran age. The correlations here suggested imply that an extensive marine shelf developed on a large area of the Río de la Plata Craton in the late Ediacaran, possibly under tropical climate. According to Gaucher et al. (2003b), this shelf also included the Corumbá Group (Mato Grosso, Brazil), being thus at least 2500 km long.

Se compara la lito- bio- y quimioestratigrafía de sucesiones sedimentarias del Neoproterozoico del Cratón del Río de la Plata, a saber: Grupo Arroyo del Soldado (GAS), Uruguay y Grupo Sierras Bayas - Formación Cerro Negro (GSB-FmCN) de Tandilia (Argentina). Ambas sucesiones se caracterizan por la intercalación de unidades carbonáticas y siliciclásticas, así como por un ambiente geotectónico correspondiente a un margen continental pasivo. Las formaciones Yerbal, Polanco y Cerro Espuelitas inferior se correlacionan litoestratigráficamente con las formaciones Cerro Largo, Loma Negra y Cerro Negro de Tandilia. Las formaciones siliciclásticas Yerbal y Cerro Largo representan la ingresión del mar Vendiano (Ediacarense) sobre el Cratón del Río de La Plata. Las sobreyacentes formaciones Polanco y Loma Negra se caracterizan por importantes depósitos carbonáticos, culminando con una expresiva superficie kárstica indicando regresión y exposición subaérea de la plataforma. La parte inferior de la Formación Cerro Espuelitas y la Formación Cerro Negro se caracterizan por depósitos margosos en la base, pasando a pelitas y facies heterolíticas hacia el tope. Se comunica el hallazgo de Cloudina cf. C. riemkeae de calizas micríticas de la Formación Loma Negra, apoyando la correlación de esta unidad con la parte inferior de la Formación Polanco y la asignación de ambas unidades al Ediacarense superior. Una microflora de acritarcas de baja diversidad ocurre en el GAS y en el GSB-FmCN. En el GAS se distinguen dos asociaciones de microfósiles de pared orgánica: una dominada por Bavlinella faveolata y Soldadophycus que ocurre en las formaciones siliciclásticas, y una de diversidad algo mayor denominada de Leiosphaeridia- Lophosphaeridium, característica de la Formación Polanco. En el GSB-FmCN la asociación está dominada por esferomorfos, principalmente Leiosphaeridia, siendo comparable a la asociación de Leiosphaeridia-Lophosphaeridium del GAS. Se reportan por primera vez para la Formación Villa Mónica las siguientes especies de acritarcas: Leiosphaeridia minutissima, Leiosphaeridia tenuissima y Synsphaeridium sp. Trátase de los fósiles más antiguos que se conocen en territorio de Argentina. En la Formación Cerro Largo superior aparece Leiosphaeridia tenuissima como elemento dominante y una sola vesícula de Leiosphaeridia minutissima, sumándose a previas descripciones de acritarcas de esta unidad. En la Formación Cerro Negro pudieron aislarse acritarcas asignables a Leiosphaeridia tenuissima. Se verifica una disminución del diámetro máximo de las acritarcas hacia el tope: de 450 µm para la Formación Villa Mónica a 380 µm para la Formación Cerro Largo y finalmente 200 µm para la Formación Cerro Negro. La asociación de acritarcas del GSB-FmCN puede correlacionarse con la Palinoflora de Leiosferas Simples o con la asociación Kotlin-Rovno del Ediacarense. La asociación de acritarcas del GAS inferior y medio ha sido asignada a la asociación Kotlin-Rovno y por tanto apoya una correlación entre las formaciones Cerro Largo, Loma Negra y Cerro Negro con la mitad inferior del GAS. Los datos quimioestratigráficos disponibles apoyan esta correlación, verificándose valores de 87Sr/86Sr entre 0.7069 y 0.7087 y variaciones seculares de d13C para los carbonatos entre -3.5 y +5.8 ‰ PDB. Respecto a la Formación Villa Mónica, los datos disponibles son ambiguos en cuanto a su edad y posibles correlaciones. Mientras la asociación de estromatolitos que ocurre en esa unidad sugiere una edad Neoproterozoico temprano, la similitud de la microflora allí preservada con la de las unidades suprayacentes sugiere que no existe un hiato temporal muy importante entre las mismas. Desde el punto de vista paleogeográfico, las correlaciones arriba planteadas implican el desarrollo de una extensa plataforma en el Ediacarense tardío y en condiciones tropicales, que abarcaba una gran parte del Cratón del Río de la Plata e incluía posiblemente al Grupo Corumbá en Mato Grosso, Brasil.