Renato Paes de Almeida

The Ediacaran to Cambrian Rift System of Southeastern South America: Tectonic Implications

The tectonic evolution of southeastern South America from the Middle Ediacaran to the Early Cambrian is marked by a series of small fault‐bounded siliciclastic and volcaniclastic basins and voluminous coeval granites traditionally associated with the compressional or transpressional tectonics of the late stages of the Pan‐African‐Brasiliano orogeny. Most existing models consider these basins separately, with distinct tectonic evolutionary histories according to local geological settings. However, new and recently published age constraints, lithological similarities, and structural aspects point to the correlation of all Ediacaran to Cambrian basins in southeastern South America within a common basin system more than 1500 km long. The interpretation of a common origin for all Ediacaran to Cambrian basins of southeastern South America implies that all the different terranes of the Brasiliano orogenic collage in the region were already united in a single plate at approximately 600 Ma. An extensional origin for this basin system is interpreted from the recognition of basin‐forming normal faults (later reactivated as strike‐slip or inverse) feeding alluvial fans and from expressive basic to acidic volcanic successions in several basins. The occurrence of basic, intermediate, and acidic volcanic rocks and voluminous coeval granites indicates that mantle and crustal fusion were simultaneous with the extensional event. Raised temperatures may have caused the thermal weakening of the lithosphere, enabling both extensional deformation and recurring strike‐slip deformation that formed major shear zones in the region. This strike‐slip deformation has been mistaken for basin‐forming tectonics, but it occurred in the Early Cambrian, after the formation of the basins, and most probably was the result of the far‐field propagation of compressional stresses originating in younger collisional orogens at the plate margins.

Recurring extensional and strike-slip tectonics after the Neoproterozoic collisional events in the southern Mantiqueira province

In Eastern South America, a series of fault-bounded sedimentary basins that crop out from Southern Uruguay to Southeastern Brazil were formed after the main collisional deformation of the Brasiliano Orogeny and record the tectonic events that affected the region from the Middle Ediacaran onwards. We address the problem of discerning the basin-forming tectonics from the later deformational events through paleostress analysis of more than 600 fault-slip data, mainly from the Camaqua Basin (Southern Brazil), sorted by stratigraphic level and cross-cutting relationships of superposed striations, and integrated with available stratigraphic and geochronological data. Our results show that the Camaqua Basin was formed by at least two distinct extensional events, and that rapid paleostress changes took place in the region a few tens of million years after the major collision (c.a. 630 Ma), probably due to the interplay between local active extensional tectonics and the distal effects of the continued amalgamation of plates and terranes at the margins of the still-forming Gondwana Plate. Preliminary paleostress data from the Castro Basin and published data from the Itajai Basin suggest that these events had a regional nature.

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.

Paleostress Analysis in Brittle Structures of the Camaquã Copper Mines

Fault planes with striae and kinematic indicators from the Camaqua Copper Mines, Cacapava do Sul (Brazil), were analyzed through graphical methods and structural geology software in order to determine the stress-field orientation for the numerous tectonic events that affected the region, and in this way determine the paleostress fields. 94 faults with slickenside striae and kinematic indicators were acquired and interpreted in the field, and with the Tensor software. The relative chronology of the events was obtained through cross-cutting relationships of multiple overprinting fault striae, and stratigraphical analysis according to previous published papers. Field data was integrated through geographical information systems (GIS) for correlation with data obtained through remote sensing and imagery processing. We identified four distinct paleostress fields: ENE to NE extension (Cretaceous); NNE to N-S compression (Triassic); NW extension (Paleozoic); and WNW compression (Cambrian). A specific study was applied to the mineralized brittle structures, leading to the proposition of a model for the emplacement of the ore bodies of the Camaqua Copper Mines. This model is characterized by the opening and filling of fractures that were parallel to the maximum horizontal stress-field, resembling T-fracture structures, during the main deformational transcurrent event that affected the Camaqua Basin ~ 530 Ma.

Evolução Estratigráfica e paleogeográfica do grupo Santa Bárbara (Ediacarano) na sub-bacia Camaquã Central, RS

As exposicoes do Grupo Santa Barbara (Ediacarano) na Sub-bacia Camaqua Central sao representadas por uma sucessao de depositos arenosos e ritmicos marinhos/transicionais e depositos conglomeraticos de leques aluviais, que compoem ciclos progradacionais-retrogradacionais. Nesta sub-bacia, o Grupo Santa Barbara apresenta mais de 2.000 m de espessura, com excelentes exposicoes nas regioes das Minas do Camaqua e de Bom Jardim. Estudos estratigraficos de paleoambientes, paleocorrentes e de proveniencia realizados nestas regioes possibilitaram a seguinte subdivisao do Grupo Santa Barbara na Sub-bacia Camaqua Central, a partir da base: 1. Formacao Passo da Capela: unidade areno-siltitica subdividida em turbiditos distais de leque subaquoso (Outer-fan), e turbiditos proximais a distais de leque subaquoso (Inner-Fan a Outer-fan) e tempestitos de costa-afora (offshore); 2. Formacao Seival: arenitos e siltitos de depositos litorâneos e de planicie de mares; 3. Formacao Rincao do Mouras: unidade areno-conglomeratica formada por depositos de leques aluviais e de sistemas fluviais entrelacados e 4. Formacao Joao Dias: composta principalmente por arenitos interpretados como depositos litorâneos de antepraia e de face litorânea e tempestitos de face litorânea. Estes estudos permitiram ainda o reconhecimento de uma notavel correlacao entre as unidades encontradas nestas regioes legitimando, desta forma, a proposta de englobar os depositos da regiao das Minas do Camaqua, classicamente relacionadas a Formacao Arroio dos Nobres, no redefinido Grupo Santa Barbara.

Hot Versus Cold Orogenic Behavior: Comparing the Araçuaí‐West Congo and the Caledonian Orogens

Observations and modeling show that temperature controls crustal rheology and therefore also the orogenic evolution of continent-continent collision zones and the associated tectonic style. In order to explore the effect of temperature in a natural environment, we compare eroded sections through the unusually cold lower-Paleozoic North Atlantic Caledonian (Scandian) collision zone and the very hot Brasiliano/Pan-African Aracuai-West Congo orogen. A cold and stiff subducting Caledonian continental margin was able to subduct as a rather coherent unit to ultrahigh pressure conditions, twice as deep as the Pan-African/Brasiliano crust that was quickly heated and softened and got involved in partial melting. Furthermore, the Caledonian collision developed large coherent thrust sheets that were transported hundreds of kilometers toward the foreland. This was never achieved in the hot Aracuai-West Congo orogen, where much of the tectonic stress was absorbed by the partially molten central part of the orogen through magmatic state deformation. Major mylonite zones (thrusts) such as those seen in the Caledonides are therefore less common in the Aracuai-West Congo orogen. Further, the deep continental subduction in the Caledonides developed a strongly asymmetric collision zone, with rapid variations in pressure and temperature. In contrast, the Aracuai-West Congo orogen soon developed into a more symmetric geometry due to its easily flowing hot crust, with a relatively flat base and a corresponding plateau in its upper part. Deformation of the cold Caledonian crust was controlled by plate-tectonic stress, while gravitational forces more strongly influenced the hot Brasiliano/Pan-African example.

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.

A Formação Sete Lagoas em sua área-tipo: fácies, estratigrafia e

Detailed facies analysis and stratigrafic measurement, description and interpretation of 12 quarry and road-cut sections across a 80.5 km long transect lead to recognition of 11 sedimentary facies of the Sete Lagoas Formation, 2 facies of Carrancas Conglomerado and 3 facies at the base of the Santa Helena Formation. Such facies are comprised into 9 facies associations regarding 3 depositional sequences. The described stratigraphic sequences are characterized by transgressive and higstand systems tracts, with minor preservation of one lowstand system tract, being the first two correlated with the carbonatic Sete Lagoas Formation and the last to the overlying siliciclastic Serra de Santa Helena Formation. The stratigraphic distribution of facies associations reveals an evolution from a CaCO 3 oversaturated carbonate ramp to a storm dominated ramp that progressively becomes distally steepened, and finely a clastic-dominated margin in the last sequence. The geographic distribution of facies associations reveals a transition from shallow water environments in the West to deeper water environments towards the East of the studied area.