André Marconato

Revised Wonoka isotopic anomaly in South Australia and Late Ediacaran mass extinction

The global Late Ediacaran Shuram–Wonoka carbon isotope anomaly has been regarded as the largest and longest known isotopic anomaly in the ocean, assuming that all Ediacaran carbonate is marine. Disregarding carbonate in South Australia shown here to be palaeosol or palaeokarst, the synchronous marine organic–carbonate excursion is only −8‰ for δ13C organic and −6‰ for δ13C carbonate, and lasted less than a million years. This revised magnitude and duration is comparable with perturbations across the Permian–Triassic boundary, and correlative with a global Late Ediacaran acritarch mass extinction. Like Permian–Triassic isotopic excursions, the revised organic–carbonate Wonoka excursion may also have been a greenhouse palaeoclimatic warm spike, which terminated valley incision and glacioeustatic drawdown during the mid-Ediacaran Fauquier Glaciation, and preceded chill of the Late Ediacaran Billy Springs Glaciation. Supplementary material: Measured sections and tables of mineral and grain-size proportions, major element and stable isotope analyses are available at www.geolsoc.org.uk/SUP18756.

Alluvial-eolian interaction in a Cambrian rift margin: the Pedra das Torrinhas and Pedra Pintada formations (Guaritas Group, RS)

This work presents a study of selected outcrops from the Pedra das Torrinhas Formation of the Guaritas Group (Cambrian, Camaqua Basin), near the basin bordering Encantadas Fault Zone. The studied succession includes alluvial fan deposits that pass laterally into eolian deposits. Sedimentary facies and architectural element analysis were performed, followed by sedimentary petrography and microscopic porosity analysis, aiming to characterize the porosity of the deposits and its spatial distribution. The main objective was to contribute to a better understanding of the porosity spatial distribution in depositional systems characterized by the interaction between alluvial and eolian processes, with special reference to deposits formed prior to the development of terrestrial plants. Porosity values are related to depositional processes, with higher porosities associated to eolian dune deposits (mean of 8.4%), and lower porosity related to interdunes (mean of 3.4%) and alluvial fans (mean of 4.3%). Architectural elements analysis revealed the spatial relationships of these deposits, a response to the interplay of the eolian and alluvial processes. The integration of porosity data reveals that the interaction of alluvial and eolian processes results in heterogeneous distribution of porosity at the facies association scale. Eolian reworking of alluvial facies increases porosity whereas sheet-flood and other alluvial processes in the interdune areas reduce porosity.

The ancestors of meandering rivers

Modern alluvial plains, in contrast to their pre-Silurian counterparts, are characterized by the presence of meandering rivers in low-downstream-gradient areas, constituting efficient transport systems that maintain high bottom shear stresses in deep channels, which are made possible by bank stabilization, most commonly provided by vegetation in Earth’s recent history. Here we show, through numerical modeling and field-based description of large-scale exposures in Mesoproterozoic successions, that prevegetation rivers in low-downstream-gradient areas were markedly different from both younger meandering rivers and the common prevegetation sheet-braided rivers, showing deeper braided channels and greater floodplain preservation than the latter. These systems were less frequent and had lower transport efficiency than modern meandering rivers, implying differences in global-scale Earth-surface dynamics, from the weathering of silicate minerals in floodplains to the grain size distribution in all clastic depositional systems.

Análise de proveniência dos arenitos conglomeráticos do Grupo Guaritas (RS): implicações para o paleoclima e a paleogeografia da sub-bacia Camaquã Central no Eocambriano

The Camaqua Supergroup, located in the central-south region of Rio Grande do Sul, Brazil, constitutes a rift-type post-orogenic sedimentary basin, whose deposition occurred in a continental environment between the Ediacaran and the Eocambrian. The upper succession of the Camaqua Supergroup is represented by the Guaritas Group, a unit formed by fluvial, eolian and alluvial fan deposits that keeps important records of the sedimentation right after the end of the neoproterozoic orogenesis that gave rise to the Gondwana supercontinent. The objective of the present work was to apply sedimentary provenance analysis in conglomeratic arenites and conglomerates of the Guaritas Group, in order to explore the climatic and tectonic evolution history of this unit. Based on the pebble compositional data, two main source areas were recognized for the deposits of this unit, a more distal one located to the north, related with a trunk river system parallel to the basin main axis, and a more proximal one located to the east, related to transversal fluvial systems and alluvial fans at the border of the basin. The comparison of the provenance data with previous studies on facies and paleocurrents suggests that, during the entire evolution of the east border of the basin, there was a same transversal fluvial system, whose catchment area suffered significative reductions due to the reactivation of the east border fault during the deposition of the Varzinha and Pedra Pintada Formations. The Serra do Apertado Formation, the upper unit of Guaritas Group, shows a high correlation between the variation of quartzose and non quartzose pebbles composition, and it was attributed to a variation between more humid and more arid climatic conditions.

Fácies sedimentares e elementos arquitetônicos das Formações Serra do Apertado e Pedra Pintada na Região das Minas do Camaquã - RS

O Grupo Guaritas (Cambriano, RS) abriga depositos de ambientes aluviais dominados por carga de fundo, aluviais de carga mista, campo de dunas eolicas e leques aluviais, constituindo um raro registro sedimentar de idade Cambriana no leste da America do Sul. Nesse contexto, sua unidade de topo, designada Formacao Serra do Apertado, constitui o ultimo registro das fases de estabilizacao do Gondwana no Sul e Sudeste do Brasil. Apesar disso, nao ha estudos detalhados de suas facies e arquitetura deposicional publicados, e essa unidade foi considerada como semelhante a Formacao Guarda Velha desde sua definicao original. Com base em estudos de facies sedimentares e arquitetura deposicional da Formacao Serra do Apertado e da porcao superior da unidade imediatamente sotoposta, Formacao Pedra Pintada, foram caracterizados seus ambientes de sedimentacao e estabelecida uma base descritiva para discriminacao das demais unidades do Grupo Guaritas. A Formacao Serra do Apertado e caracterizada pelo elemento arquitetural EL (Enchentes em lencol), no qual predominam associacoes de facies que remetem a um ambiente fluvial dominado por grandes variacoes de vazao e esporadico retrabalhamento eolico de barras emersas. A porcao superior da Formacao Pedra Pintada apresenta intercalacoes entre os elementos arquitetonicos DB (Dunas barcanoides) e IF (Interdunas fluviais). A exclusividade do elemento EL na Formacao Serra do Apertado contrasta com descricoes anteriores da Formacao Guarda Velha, que relatam elementos de preenchimento de canais e abundância de lentes conglomeraticas. Tais diferencas revelam maior retrabalhamento eolico na Formacao Serra do Apertado e, possivelmente, fluxo mais episodico, sugerindo, de forma preliminar, clima mais arido que aquele da Formacao Guarda Velha.