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Large anabranching sand-bed rivers are characterised by dynamic lateral channel migration, bar aggradation and floodplain accretion. On the Amazon River, variations in bank and floodplain sediments exert a primary control on channel migration rates. In a reach near Tefé, Brazil, the Solimões River shows different migration dynamics along its north and south banks, suggesting that bank strength plays a role in the large-scale and long-term channel and floodplain evolution. Here we present measurements of lower bank strength and sediment resuspension in 29 locations along and across the Solimões River to investigate their spatial variability in sediments of different age and origin by means of a cohesion strength meter and a Pilcon Shear Vane. Results show that the north bank consists mainly of late Holocene sandy and silty deposits, whereas the south bank is characterised by frequent Pleistocene outcrops of cohesive muds and diagenetic iron cements and concretions. The south-bank Pleistocene deposits have on average three times higher bank strength than the younger floodplain deposits along the north bank. When comparing the locations of the Pleistocene deposits with lateral migration rates along both banks for two 40 km reaches, we observe that these sediments occur mainly where the south bank has been eroded, suggesting that they are revealed when the river migrates. Our results suggest: (1) that lateral migration uncovers the less erodible layers that can then deflect the flow towards the north bank; (2) that outcrops of resistant Pleistocene deposits might be abundant underneath the northern alluvial floodplain; and (3) spatial variations in bank erodibility exert a first-order local control on the planform morphology and lateral dynamics of the river. We suggest that such variations in erodibility are equally important for the morphodynamics of other large sand bed rivers that show evidence for the presence of resistant Pleistocene sediments, such as the Nile, Mekong and Mississippi Rivers.

Alluvial rivers are the most important agents of sediment transport in continental basins, whose fluvial deposits enclose information related to the time when rivers were active. In order to extract the most information from fluvial deposits in the sedimentary record, it is imperative to quantify the natural variability of channel patterns at the global scale, explore what controls may influence their development, and investigate whether channel pattern information is preserved in the alluvial plains in order to develop tools for recognizing them in the sedimentary record. By surveying 361 reaches of modern alluvial rivers with available water discharge data at a global scale, we present a quantitative channel pattern classification based on sinuosity and channel count index applicable to the recognition in the rock record. A continuum of channel patterns ranging from high-sinuosity single channel to low-sinuosity multichannels is documented, along with the proportion of depositional elements in their alluvial plains and their conditions of occurrence. Preserved barforms in the alluvial plains of these rivers are used to infer and quantify paleoflow directions at the channel-belt scale and result in ranges of paleocurrent circular variance that may lead to channel pattern identification in the rock record. Data from this work indicate that the recognition of channel patterns may be used to predict paleogeographic features such as the scale of drainage basin area and discharge, slope, and annual discharge regimes.

Dunes form critical agents of bedload transport in all of the world’s big rivers, and constitute appreciable sources of bed roughness and flow resistance. Dunes also generate stratification that is the most common depositional feature of ancient riverine sediments. However, current models of dune dynamics and stratification are conditioned by bedform geometries observed in small rivers and laboratory experiments. For these dunes, the downstream lee-side is often assumed to be simple in shape and sloping at the angle of repose. Here we show, using a unique compilation of high-resolution bathymetry from a range of large rivers, that dunes are instead characterized predominantly by low-angle lee-side slopes (

Although large anabranching rivers are a very common element in the greatest modern drainage basins, the recognition of such systems in the rock record is hampered by insufficient sedimentological information from active rivers. That is particularly the case of bar-top depositional environments in large anabranching rivers, motivating the survey of six selected areas of seasonally emergent bars in the Solimoes and Japura rivers (western Brazilian Amazonia) using Ground Penetrating Radar, Sub Bottom Profiler and Multibeam Echosounder, as well as direct observation sediment sampling. GPR surveys and field observations indicate these bars are dominated by 0.5 to 4 m tall subaqueous dunes with the local presence of metric angle-of-repose bar-side surfaces. Downstream accretion components are characterized by radar facies indicating horizontal to low-angle downstream-dipping cosets of dm-scale to cm-scale cross-strata. Lateral accretion components are characterized by radar facies indicating m-scale high-angle cross-strata sets passing laterally to low-angle and inclined cosets composed of dm-scale cross-strata sets. Upstream accretion components are characterized by radar facies indicating upstream-inclined cosets of m- and dm-scale cross-strata sets with local mud drapes. Preservation of accretionary bar forms in these periodically exposed areas is supported by calculation of the coefficient of variation of peak discharge, which indicates low interannual peak discharge variability over the bar tops.

Climate changes have been considered as an essential factor controlling the shaping of the recent alluvial landscapes in central Amazonia, with implications for explaining the biogeographic patterns in the region. This landscape is characterized by wide floodplains and various terrace levels at different elevations. A set of older terraces with ages between 50’000 and > 200’000 yrs occupy the higher portions of central Amanzonia, whereas multiple terraces next to floodplains occur at lower elevations and display ages of a few thousands of years. These lower terraces, referred to as mid-lower terraces, reveal what can be perceived as a stochastic pattern both in space and time. Despite the widespread occurrence of these geomorphic features, no process-oriented analysis has been conducted to explain their formation. Here, we develop a landscape evolution model referred to as SPASE to explicitly account for fluvial erosion and deposition in combination with lateral channel migration to explore the controls on terrace development. The model results show that the higher terraces were deposited under the condition of a higher base-level for the basins upstream of the confluence between the Solimões and Negro Rivers. The subsequent decrease in the base level initiated a phase of gradual incision, thereby resulting in the current fluvial configuration. The model also predicts that high-frequency climate changes yielded in the construction of mid-lower terraces at various elevations, which however, are all situated at lower elevation than the higher terrace levels. Our model shows that dry-to-wet shifts in climate, in relation to the modern situation, yield a landscape architecture where mid-lower terrace levels are better preserved than wet-to-dry changes in climate, again if the current situation is considered as reference. Finally, our results show that fast and widespread landscape changes possibly occurred in response to high-frequency climate changes in central Amazonia, at least since the Late Pleistocene, with great implications for the distribution and connectivity of different biotic environments in the region. Because of this short time scale of response to external perturbations, we suggest that the streams in Central Amazonia possibly also respond in rapid and sensitive ways to human perturbations.

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.

The Andean orogeny had a profound impact on the evolution of the Amazon drainage system, modifying the climate in South America and the influx of sediments to the interior and marginal sedimentary basins. Additionally, the subduction of the Nazca plate under the continent produced dynamic topography that perturbed the landscape and the generation of accommodation space in the interior sedimentary basins mainly in western Amazonia. Therefore, the correct interpretation of the geological evolution of the northern South America during the Cenozoic depends on the coupling of different geodynamic processes with the erosion of the continents, deposition in the sedimentary basins and the interaction with the evolving climate. Due to the great complexity of the different processes involved in the geological evolution of Amazonia, the use of numerical models is a natural way to treat this problem. The aim of this work is to present numerical scenarios for the formation and evolution of the Amazon drainage system taking into account surface processes along with geodynamic processes like Andean uplift, flexure of the lithosphere, and dynamic topography induced by mantle convection. We conclude that the Amazon drainage system was formed essentially by the asymmetric influx of sediments from the Andes, while the dynamic topography modulated the timing for the transcontinental connection between western and eastern Amazonia and the stratigraphic evolution of interior basins.

The Cenozoic landscape of northern South America evolved under the influence of different geodynamic processes, culminating in the formation of the largest drainage basin on Earth, the Amazon drainage system. The orogeny of the Andean cordillera and the increase in surface denudation induced an asymmetric influx of sediments in Amazonia, which caused an eastward propagation of the Andean sedimentary wedge, reversing the topographic gradient and driving the drainage flow towards the Atlantic Equatorial margin. The low-relief topography of lowlands in Amazonia is also a factor that propitiates this reversal. Concomitantly, variations in the subduction angle of the Nazca plate under the continental lithosphere during the Miocene also contributed to create dynamic topography that perturbed the surface and modified the drainage pattern through time. In the last decade, both surface processes during Andean orogeny and dynamic topography were invoked independently to explain the formation of the present Amazon drainage system connecting the Andes and the equatorial margin. However, hitherto these mechanisms were not considered in a single model, hampering the evaluation of the contribution of each factor on the evolution of the Amazon River. To quantify the combined effect of the different geodynamic processes, in the present work we used numerical models that integrate orogeny, surface processes, dynamic topography and flexural isostasy of the lithosphere. In our results, we present new numerical scenarios for the Cenozoic landscape evolution of Amazonia, showing that the dynamic topography may have affected the environmental evolution in western Amazonia, favoring the development of a megawetland in this region before the establishment of the Amazon River as a transcontinental river. We conclude that, although the formation of the Amazon drainage system can be explained without the influence of mantle convection, variations in dynamic topography may have anticipated the establishment of the transcontinental drainage system a few million years. Furthermore, the stratigraphic evolution and the total Cenozoic sedimentation preserved in the Solimoes Basin is a function of the temporal variations in the amplitude of dynamic topography in northern South America.

The modern Amazonian rainforest has a great fascination and global significance, but our knowledge of past landscape changes is still limited due to sparse data and the lack of radiometric age constrains. Precise dating in records older than the late Pleistocene are difficult to obtain and often regionally confined, therefore biostratigraphic correlations are used to estimate depositional ages across sedimentary basins. Aiming for a better understanding of the Neogene changes in western Brazil, we studied palynology and ostracod assemblages from the Solimoes Formation, a key site for paleoenvironmental models to discuss the biotic changes in Amazonia. Further, we compare the biostratigraphic information of both groups with radiometric maximum deposition ages constrained by U/Pb measurements on detrital zircon grains. Down to the depth of ~95 m, zircon populations show a maximum deposition age of 11.42 ± 0.66 Ma documenting Tortonian (late Miocene) or younger ages for the top of the Solimoes Formation. Palynological biostratigraphy show the Psilatricolporites caribbiensis zone (latest Miocene–Pliocene) down to a depth of 181.8 m, where the Grimsdalea magnaclavata zone suggests a late middle Miocene to early late Miocene age. In comparison, ostracod biostratigraphy indicates the Cyprideis cyrtoma zone (late Miocene) down to 176.0 m and the Cyprideis minipunctata (late middle to late Miocene) for the bottom of the section. Thus, both biostratigraphic concepts suggest ages close to the maximum ages indicated by the detrital zircons. This comparison of the biozonations with U/Pb maximum deposition ages represents the first independent calibration of these widely used methods for age determination in the Solimoes Basin. Palynological and ostracod data illustrate dynamic environments varying from fluvial settings with wetland forests and swamps to lakes and/or abandoned channels with temporary slightly saline conditions. Instead of a slow evolution of Neogene Amazonian environments, our results show migrating rivers and waterbodies surrounded by rainforest during the period recorded by the base and top of the studied core material, separated by a phase of more extensive lakes and swamps (core depth 90–120 m). Indicators for slightly elevated salinity are found until the topmost sample and thus persisted during the late Miocene. This highlights the importance of new age control to improve the regional biostratigraphic timescale and reconstructed environmental changes.

We identify key interdisciplinary questions and approaches to understanding the paleogeographic and biotic history of Amazonia. We discuss the importance of comparing evolutionary taxonomic units across groups and considering their particular environmental affinities as a framework for a mechanistic understanding of how the diversity of Amazonia was assembled across space and time. Although lineage diversity of the Amazonian biota is often quite old, current evidence indicates that much of Amazonia’s observed “species-level” diversity, as exemplified by birds, mammals, and butterflies, is young, mostly dating to the Quaternary. Much of the modern Amazonian landscape, including in particular its hydrological and environmental systems, was shaped over the Pliocene and Quaternary (last 5 Ma), yet significant landscape change affecting the evolution of biotas is less than 2 million years old.

The Neoproterozoic Araçuaí-West Congo orogen in Brazil and Congo represents a branch of the Brasiliano/Pan-African orogenic system that is considered to terminate northward into a confined cratonic environment defined by the horseshoe-shaped pre-Atlantic São Francisco-Congo craton. The prevailing interpretation is that this orogen formed as a result of 50 m.y. of subduction leading to a classical continent–continent collision. This subduction-collision model hinges on interpretation of 630–580 Ma granitoids in the core of the orogen as being arc-related, and of locally exposed mafic and ultramafic metamorphic rocks as being ophiolitic. We show that when tested beyond geochemical signature, the model has fundamental problems that cannot be accounted for. In particular, there is an insurmountable oceanic space problem in this confined setting that is overlooked in most of the current literature. There are also problems with subduction initiation, lack of unambiguous evidence for oceanic crust, no trace of any high-P metamorphism and the abrupt termination of an ocean with no realistic way to transfer the large amount of oceanic opening displacement and subsequent convergence required by the model. We conclude that the prevailing subduction-collision model cannot possibly work and argue that the existing data are more consistent with hot intracontinental orogeny. We stress the importance of building tectonic models on more than one type of data, making realistic restorations and palinspastic reconstructions, and taking into account modern geotectonic knowledge. publishedVersion

In this study, we challenge the multiple collision model for the tectonic evolution of the Neoproterozoic Ribeira Belt in Southeastern Brazil. New U–Pb SHRIMP data reveal Palaeoproterozoic (2153 ± 15 Ma) and Cryogenian (783 ± 6 and 768 ± 8 Ma) granitic rocks in the Embu Domain, and detrital zircon data of metasedimentary units from the Embu and Costeiro domains suggest a coherent tectonic evolution for the whole Ribeira Belt. Rather than by multiple collisions, these data are best explained by a simpler tectonic model involving continent (craton)-volcanic arc collisions in the Dom Feliciano and Brasilia belts that led to intracontinental crustal thickening of the adjacent thinned hinterland (Ribeira Belt) at ~640–610 Ma, followed by widespread post-collisional magmatism and rift-related sedimentation at ~600–540 Ma. We suggest that intracontinental orogeny is a relevant process during supercontinent assembly, as illustrated here by the evolution of significant parts of the Brasiliano orogen.

We investigate the subseismic structural expression of the major Vaza-Barris Transfer Zone in the Early Cretaceous Tucano rift basin, NE Brazil based on field observations. Subseismic structures in the Tucano rift fill encompass cataclastic deformation bands, deformation band clusters and deformation band faults. In general, these subseismic structures indicate a ∼120° extension direction and document oblique extension across the N-S Tucano Rift, consistent with the movement direction inferred from plate-scale reconstructions. The transfer zone itself is dominated by a large population of NE-SW trending deformation band structures that developed into deformation band faults making a high angle to the transfer zone. The deformation band faults are quite evenly distributed along the transfer zone, which we attribute to extension related to its arcuate cross-sectional shape with flanks dipping toward the rift margins. Additional subordinate structures, many of which are oriented parallel to the transfer zone, show strike-slip dominated motion, and indicate that the finite strain field in the transfer zone involves a component of NNW-SSE shortening in addition to the main extension along the transfer zone. In terms of subseismic fault prediction, however, the evenly distributed zone-perpendicular structures dominate and could impose restrictions on fluid flow along the zone. This macro-permeability anisotropy should be considered in an injection/production scenario in a transfer zone of the type described here. Although other transfer zones may have different geometries and therefore different subseismic characters, an important observation made here is the close correspondence between first-order geometry (in our case an arch-shaped transfer zone) and subseismic deformation (arch extension). In addition, the observation of deformation band faults in the Banzae Member of the Marizal Formation shows that rifting lasted through the Aptian and possibly into the Albian.

Neoproterozoic cap carbonates are characterized by “anomalous” sedimentary features, including sea-floor precipitates, represented by aragonite pseudomorph cement crusts and crystal fans. These features are found in restricted periods of geological history, and their generation in the context of the Neoproterozoic has been attributed to very specific postglacial conditions leading to alkalinity oversaturation. New sedimentary and geochemical results on well-preserved Neoproterozoic deposits of aragonite pseudomorph crystal fans at the base of the Sete Lagoas Formation (Bambui Group, central Brazil) are reported. The micrite-settling facies shows the highest abundance of crystal fans, arranged in centimeter-scale layers interfingered with the micrite matrix; this facies is interpreted as a result of postglacial calcium carbonate oversaturation in restricted areas characterized by marine dysoxic waters. A numerical model constrains the kinetics of formation of aragonite crystals or micrite by calculating the induction times and precipitation rates of both carbonate species for postglacial conditions of elevated temperature and high atmospheric pCO2. In these conditions, the Mg/Ca ratio of the Neoproterozoic seawater is calculated at ca. 1.2. Alkalinity oversaturation is monitored by a variable evaporation degree. Aragonite formation is kinetically favored, and the discrepancy between calcite and aragonite precipitation rate is greater as atmospheric pCO2, hence alkalinity, is higher. These modeling conditions do not preclude the contribution of incomplete organic-matter degradation to alkalinity as suggested by negative carbon isotope signatures. In addition, the large number of aragonite–micrite pairs suggests a seasonal or paleoclimatic forcing. Other cases of abiotic aragonite precipitation through time are also briefly examined by comparison.

One key element to the understanding of the dynamics of pre-vegetation fluvial systems is the reconstruction of processes operating on their floodplains given that, in modern systems, channel banks and floodplains are the environments most affected by plant colonization. Notwithstanding, few pre-vegetation floodplains have been described, and major questions regarding their most basic characteristics are still unresolved. In order to address these questions, detailed analysis of coeval channel-belt, fluvial floodplain and alluvial-fan deposits from the Santa Barbara Group (Late Neoproterozoic to Early Cambrian, southern Brazil) was performed. While floodplain facies resemble ephemeral stream deposits, being coarser-grained than modern floodplains and marked by the stacking of flood event cycles, channel-belt deposits show composite bars, which do not present conclusive evidence for high water discharge variation. The floodplain deposits show particular features common to other pre-vegetation fluvial systems, such as better preserved small-scale structures, lack of bioturbation, and abundance of cross-laminated sandstones, while other features differ from previous depositional models, namely abundant mudcracks and evidence of soil formation. The lateral variation of depositional systems recorded in the Santa Barbara Group shows contrasting signatures of water discharge variation in sand-dominated coeval environments, and offers an example of the relation between different alluvial environments before the evolution of land plants.

Process-based and facies models to account for the origin of pre-vegetation (i.e. pre-Silurian) preserved fluvial sedimentary architectures remain poorly defined in terms of their ability to account for the nature of the fluvial conditions required to accumulate and preserve architectural elements in the absence of the stabilizing influence of vegetation. In pre-vegetation fluvial successions, the repeated reworking of bars and minor channels that resulted in the generation and preservation of broad, tabular, stacked sandstone-sheets has been previously regarded as the dominant sedimentary mechanism. This situation is closely analogous to modern-day poorly vegetated systems developed in arid climatic settings. However, this study demonstrates the widespread presence of substantially more complex stratigraphic architectures. The Guarda Velha Formation of Southern Brazil is a >500 m-thick synrift fluvial succession of Cambrian age that records the deposits and sedimentary architecture of three distinct fluvial successions: (i) an early rift-stage system characterized by coarse-grained channel elements indicative of a distributive pattern with flow transverse to the basin axis; and two coeval systems from the early- to climax-rift stages that represent (ii) an axially directed, trunk fluvial system characterized by large-scale amalgamated sandy braid-bar elements, and (iii) a distributive fluvial system characterized by multi-storey, sandy braided-channel elements that flowed transverse to the basin axis. Integration of facies and architectural-element analysis with regional stratigraphic basin analysis, palaeocurrent and pebble-provenance analysis demonstrates the mechanisms responsible for preserving the varied range of fluvial architectures present in this pre-vegetation, rift-basin setting. Identified major controls that influenced pre-vegetation fluvial sedimentary style include: (i) spatial and temporal variation in discharge regime; (ii) the varying sedimentological characteristics of distinct catchment areas; (iii) the role of tectonic basin configuration and its direct role in influencing palaeoflow direction and fluvial style, whereby both the axial and transverse fluvial systems undertook a distinctive response to syn-depositional movement on basin-bounding faults. Detailed architectural analyses of these deposits reveal significant variations in geometry, with characteristics considerably more complex than that of simple, laterally extensive, stacked sandstone-sheets predicted by most existing depositional models for pre-vegetation fluvial systems. These results suggest that the sheet-braided style actually encompasses a varied number of different pre-vegetation fluvial styles. Moreover, this study demonstrates that contemporaneous axial and transverse fluvial systems with distinctive architectural expressions can be preserved in the same overall tectonic and climatic setting.

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.

O Grupo Guaritas (Cambriano, RS) abriga depósitos de ambientes aluviais dominados por carga de fundo, aluviais de carga mista, campo de dunas eólicas e leques aluviais, constituindo um raro registro sedimentar de idade Cambriana no leste da América do Sul. Nesse contexto, sua unidade de topo, designada Formação Serra do Apertado, constitui o último registro das fases de estabilização do Gondwana no Sul e Sudeste do Brasil. Apesar disso, não há estudos detalhados de suas fácies e arquitetura deposicional publicados, e essa unidade foi considerada como semelhante à Formação Guarda Velha desde sua definição original. Com base em estudos de fácies sedimentares e arquitetura deposicional da Formação Serra do Apertado e da porção superior da unidade imediatamente sotoposta, Formação Pedra Pintada, foram caracterizados seus ambientes de sedimentação e estabelecida uma base descritiva para discriminação das demais unidades do Grupo Guaritas. A Formação Serra do Apertado é caracterizada pelo elemento arquitetural EL (Enchentes em lençol), no qual predominam associações de fácies que remetem a um ambiente fluvial dominado por grandes variações de vazão e esporádico retrabalhamento eólico de barras emersas. A porção superior da Formação Pedra Pintada apresenta intercalações entre os elementos arquitetônicos DB (Dunas barcanoides) e IF (Interdunas fluviais). A exclusividade do elemento EL na Formação Serra do Apertado contrasta com descrições anteriores da Formação Guarda Velha, que relatam elementos de preenchimento de canais e abundância de lentes conglomeráticas. Tais diferenças revelam maior retrabalhamento eólico na Formação Serra do Apertado e, possivelmente, fluxo mais episódico, sugerindo, de forma preliminar, clima mais árido que aquele da Formação Guarda Velha.

Made available in DSpace on 2018-11-26T15:45:58Z (GMT). No. of bitstreams: 0 Previous issue date: 2017-12-01 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Petrobras This study, based on detailed sedimentologic and stratigraphic analysis of the Aptian succession preserved in the Reconcavo-Tucano-Jatoba Rift System (RTJ), present new elements for biostratigraphic correlation and paleogeographic reconstruction in the mid-Cretaceous South Atlantic realm, supporting novel interpretations on the tectonic and sedimentary evolution related to the W-Gondwana breakup. The Aptian sedimentary succession in the RTJ has been referred to as Marizal Formation, and interpreted as post-rift deposits. Detailed sedimentologic and stratigraphic studies of these deposits enabled the recognition and individualization of two distinctive sedimentary units that can be traced in the entire RTJ. These units are here described and named Banzae and Cicero Dantas members of the Marizal Formation. Their contact is locally marked by the fossiliferous successions of the here proposed Amargosa Bed, lying at the top of the Banzae Member. Both members of the Marizal Formation record large river systems captured by the Tucano Basin with the local development of eolian dune fields and fault bounded alluvial fans. The Amargosa Bed represents a regional-scale base level change preserved between the Aptian fluvial successions along the RTJ. Hence, the studied sedimentary record presents important implications for the timing and direction of marine ingressions affecting NE-Brazil interior basins during the Aptian. A remarkable contrast in preserved fluvial architecture between the Banzae Member, characterized by connected channel bodies, and the Cicero Dantas Member, characterized by isolated channel bodies within overbank fines, is here reported. The main interpreted control for the observed contrast in fluvial stratigraphy is sedimentary yield variation. The interval is also subject to the interpretation of a regional shift in the mechanism responsible for the subsidence of the basins formed during the Cretaceous break-up of the Central South Atlantic. This view is challenged by our results which reveal that basin forming extension continued throughout the Aptian. As a conclusion, the detailed stratigraphy of the Marizal Formation forward alternative geodynamic interpretations for the Aptian successions in northeastern Brazil, bringing new elements to the mid-Cretaceous biogeographical, paleogeographical and tectonic reconstructions of western Gondwana. (C) 2017 Elsevier Ltd. All rights reserved. Univ Estadual Campinas, Fac Tecnol, Rua Paschoal Marmo 1888, BR-13484332 Limeira, SP, Brazil Univ Sao Paulo, Inst Geociencias, Rua Lago 562,Cidade Univ, BR-05508900 Sao Paulo, SP, Brazil Univ Fed Sergipe, Dept Geol, Av Marechal Rondom S-N, BR-49100000 Sao Cristovao, SE, Brazil CPRM Serv Geol Brasil, Rua Costa 55, BR-01304010 Sao Paulo, SP, Brazil Univ Estadual Paulista, Inst Geociencias & Ciencias Exatas, Ave 24 A,1515, BR-13506900 Rio Claro, SP, Brazil Univ Estadual Paulista, Inst Geociencias & Ciencias Exatas, Ave 24 A,1515, BR-13506900 Rio Claro, SP, Brazil FAPESP: 2009/53363-8 FAPESP: 2013/01825-3 FAPESP: 2014/16739-8 FAPESP: 2016/03091-5 FAPESP: 2016/19736-5 FAPESP: 2010/51559-0 CAPES: PROEX-558/2011 CNPq: 301775/2012-5 Petrobras: 2014/00519-9

Soft-sediment deformation (SSD) is widely described in the literature, but there is no clear consensus regarding its origin and significance. Existing models for SSD in fluvial sediments do not clearly demonstrate a relationship between the structures, preserved facies expression, and larger-scale depositional architecture. In this study several types of SSD structures are recorded in Cambrian fluvial deposits and these occur interbedded with undeformed strata throughout the entire stratigraphic interval. The random distribution of these features in relation to primary facies types and fluvial forms indicates that they have neither a direct nor indirect relationship with any depositional processes or bedform type. We propose that the relationship of SSD at the bed-set-scale to larger-scale depositional architecture, combined with tectono-stratigraphic analysis allows the determination of both short-term fluvial hydraulic conditions in ancient stream systems, such as the nature of the flow regime responsible for depositing ancient fluvial stream successions, and the long-term subsidence rates, in the form of mean recurrence interval of the seismic events responsible for triggering the generation of SSD in tectonically active basins.

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.

During the Ediacaran, southern Brazil was the site of multiple episodes of volcanism and sedimentation, which are best preserved in the 3000 km2 Camaqua Basin. The interlayered sedimentary and volcanic rocks record tectonic events and paleoenvironmental changes in a more than 10 km-thick succession. In this contribution, we report new U–Pb and Sm–Nd geochronological constraints for the 605 to 580 Ma Bom Jardim Group, the 570 Ma Acampamento Velho Formation, and a newly-recognized 544 Ma volcanism. Depositional patterns of these units reveal the transition from a restricted, fault-bounded basin into a wide, shallow basin. The expansion of the basin and diminished subsidence rates are demonstrated by increasing areal distribution and compressed isopachs and increasing onlap of sediments onto the basement to the west. The Sm–Nd isotopic composition of the volcanic rocks indicates mixed sources, including crustal rocks from the adjacent basement. Both Neoproterozoic and Paleoproterozoic sources are indicated for the western part of the basin, whereas only the older Paleoproterozoic signature can be discerned in the eastern part of the basin.

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...

The Ediacaran is one of the most important periods on Earth evolution, including the first appearance of soft-bodied macrofossils, major climatic changes and a supposed rise in free oxygen. In southernmost Brazil, this period is represented by Camaqua Supergroup, including the Bom Jardim Group and the Acampamento Velho Formation, both of which record continental palaeoenvironmental changes in a more than 5000 m thick stratigraphic succession. Age constraints are given by seven Ar-Ar and U-Pb determinations on volcanic rocks, which bracket these units between c. 605 and 574 Ma, revealing the best dated and most continuous documented Ediacaran continental succession to date. Depositional systems evolution supports a Phanerozoic-type glacial context during the last Neoproterozoic glacial event and presents the Picada das Gracas Formation (580 ± 3.6 Ma) as the first dated non-glacial unit coeval to the Gaskiers Formation.

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.

This study characterizes and provides ages for an extensive sedimentary record occurring in Lapa Doce and Torrinha caves, NE Brazil. With > 40 km of surveyed passages, these caves integrate a distributary cave system fed by allogenic recharge from the surrounding sandstone plateaus. Sediment petrography together with descriptions of depositional facies and architectural elements shows four depositional units related to fluvial and standing water environments. These include, from bottom to top: (1) a channel unit including lateral bars deposited during an ordinary flood regime; (2) a sandy flood unit including minor channel and scour fills derived from bank-full equivalent flood events; (3) mud caps deposited in standing water that often reach the ceiling; and (4) intraclast breccias associated with collapse of the mud caps under saturated conditions. The deposits were dated using a combination of cosmogenic nuclide burial dating and U-series dating of flowstone. Cosmogenic nuclide data point to fluvial aggradation being active since 1.91 + 0.12 My and extending until 0.36 ± 0.08 My, with intensive cave and valley aggradation events between 0.78 ± 0.10 My and 0.44 ± 0.12 My. Long term alluviation of the cave system seems to be important in forming passages, determining their configuration, and setting up a general distributary pattern evident in passage morphology and sedimentary sequences. Mud caps overlapping the fluvial deposits are interpreted as the products of successive rising and lowering of the water table (static level). Radiometric ages of interstratified flowstones and speleothems show that these oscillations were active at least since 115 ky ago and finally ceased around 12 ky ago, indicated by the recurrent age obtained from uneroded capping flowstones. These long-term water table oscillations may drive paragenetic expansion of the whole cave system. Valleys, caves and other landforms in our study area are part of an ancient landscape with multiple episodes of burial and exhumation. Despite the possibility of much older sediment being preserved somewhere in the cave system, our data indicate a younger Quaternary age for the bulk of the sediment filling the study caves.

The interpretation of fluvial styles from the rock record is based for a significant part on the identification of different types of fluvial bars, characterized by the geometric relationship between structures indicative of palaeocurrent and surfaces interpreted as indicative of bar form and bar accretion direction. These surfaces of bar accretion are the boundaries of flood-related bar increment elements, which are typically less abundant in outcrops than what would be desirable, particularly in large river deposits in which each flood mobilizes large volumes of sediment, causing flood-increment boundary surfaces to be widely spaced. Cross-strata set boundaries, on the other hand, are abundant and indirectly reflect the process of unit bar accretion, inclined due to the combined effect of the unit bar surface inclination and the individual bedform climbing angle, in turn controlled by changes in flow structure caused by local bar-scale morphology. This work presents a new method to deduce the geometry of unit bar surfaces from measured pairs of cross-strata and cross-strata set boundaries. The method can be used in the absence of abundant flood-increment bounding surfaces; the study of real cases shows that, for both downstream and laterally accreting bars, the reconstructed planes are very similar to measured bar increment surfaces.

Made available in DSpace on 2018-11-27T20:01:12Z (GMT). No. of bitstreams: 0 Previous issue date: 2016-12-01 Petrobras Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Center for Geosciences Applied to Petroleum Geology - UNESPetro, IGCE/Unesp/Rio Claro Training of Human Resources in Petroleum Geology Program - UNESP The evolution of the Cretaceous basins of the Brazilian northeastern hinterland was associated with the Gondwana rifting and opening of the South Atlantic Ocean. The first marine ingression in northeastern Brazil occurred in the late Aptian and was recorded as the Santana Group of the Araripe Basin, which is currently an isolated basin, located hundreds of kilometers away from the Brazilian marginal basins. Bellow the first upper Aptian marine deposits, an important section of fossiliferous limestone (Lagerstatte) was deposited and preserved in the Crato Formation transitioning upward into evaporites of the Ipubi Formation. The direction of the marine ingression is controversial, with several possibilities being suggested, mainly due to the absence of other areas of upper Aptian marine sections within the hinterland. Serra do Tona is a sedimentary mesa with scarped edges where the upper part of the Marizal Formation crops out, displaying laminated limestones, litho-and chrono-correlated with those of the Crato Formation, is preserved. Therefore, this mixed upper Aptian section, at the North Tucano Basin (Serra do Tona), is a unique occurrence of utmost importance to the definition of sedimentary events and paleogeographical reconstruction of northeastern Brazil during the late Aptian. A detailed stratigraphic analysis allowed the definition and characterization of two upper Aptian depositional sequences bounded by regional disconformities. Both sequences are dominantly transgressive and carbonatesiliciclastic in composition. The lower sequence comprises the basal portion of the Marizal Formation and consists of a succession of fluvial sandstones, ending on a laterally continuous thin interval (

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 CaCO3 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.

As exposições do Grupo Santa Bárbara (Ediacarano) na Sub-bacia Camaquã Central são representadas por uma sucessão de depósitos arenosos e rítmicos marinhos/transicionais e depósitos conglomeráticos de leques aluviais, que compõem ciclos progradacionais-retrogradacionais. Nesta sub-bacia, o Grupo Santa Bárbara apresenta mais de 2.000 m de espessura, com excelentes exposições nas regiões das Minas do Camaquã e de Bom Jardim. Estudos estratigráficos de paleoambientes, paleocorrentes e de proveniência realizados nestas regiões possibilitaram a seguinte subdivisão do Grupo Santa Bárbara na Sub-bacia Camaquã Central, a partir da base: 1. Formação Passo da Capela: unidade areno-siltítica 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. Formação Seival: arenitos e siltitos de depósitos litorâneos e de planície de marés; 3. Formação Rincão do Mouras: unidade areno-conglomerática formada por depósitos de leques aluviais e de sistemas fluviais entrelaçados e 4. Formação João Dias: composta principalmente por arenitos interpretados como depósitos litorâneos de antepraia e de face litorânea e tempestitos de face litorânea. Estes estudos permitiram ainda o reconhecimento de uma notável correlação entre as unidades encontradas nestas regiões legitimando, desta forma, a proposta de englobar os depósitos da região das Minas do Camaquã, classicamente relacionadas à Formação Arroio dos Nobres, no redefinido Grupo Santa Bárbara.

A evolução geológica do sudeste da América do Sul do Ediacarano ao Ordoviciano é marcada pelo desenvolvimento de bacias sedimentares que registram os eventos geológicos do chamado Estádio de Transição entre o metamorfismo e intensa deformação da Orogenia Brasiliana e a relativa estabilidade tectônica da sedimentação meso e neopaleozóica das sinéclises cratônicas. A presente tese aborda o registro de eventos tectônicos preservado nas sucessões sedimentares do Estádio de Transição, com o objetivo de testar e complementar os modelos geológicos, abundantes na bibliografia, derivados do estudo de rochas plutônicas e metamórficas neoproterozóicas do sudeste da América do Sul. Os estudos apresentados têm como objetos o Supergrupo Camaquã (Ediacarano a Eocambriano, porção centro sul do estado do Rio Grande do Sul) e o Grupo Caacupé (Neo-ordoviciano, Paraguai Oriental), tomados como exemplo do registro sedimentar do intervalo entre o Ediacarano e o Neo-Ordoviciano. O reconhecimento dos eventos tectônicos relacionados à formação e deformação dessas bacias e a caracterização dos padrões de resposta sedimentar a esses eventos basearam-se em diversas abordagens e técnicas, principalmente levantamentos estratigráficos de detalhe, análise de fácies sedimentares e elementos arquiteturais, mapeamento geológico, análise de paleocorrentes, análise de proveniência macro e microscópica, análises geocronológicas, análise de estruturas tectônicas rúpteis e reconstituição de paleocampos de tensão. O Grupo Santa Bárbara (Neo-Ediacarano, Supergrupo Camaquã) apresenta, em sua área-tipo, ciclos de variação granulométrica de dezenas a centenas de metros de espessura, decorrentes de variações na proporção de depósitos de planícies de inundação distais e depósitos de correntes trativas em sistemas fluviais efêmeros. O reconhecimento da influência do soerguimento de um alto do embasamento durante a deposição da unidade, separando duas sub-bacias, baseou-se em trabalhos de mapeamento geológico e foi documentado por análises de proveniência macro e microscópica e análise de paleocorrentes. A identificação do nível estratigráfico relacionado ao início da contribuição detrítica do Alto de Caçapava do Sul na sedimentação do Grupo Santa Bárbara permitiu a comparação com as variações verticais de fácies e sistemas deposicionais documentadas em seções estratigráficas de detalhe. Constatou-se, desta forma, que o evento de reativação da falha, de caráter predominantemente normal, relacionada ao soerguimento do alto não coincide com um aumento imediato na taxa de geração de espaço de acomodação, como previsto pelos modelos vigentes, e sim com um aumento de granulação dentro de uma sucessão de depósitos de rios efêmeros arenosos. Essa constatação revela que características particulares de um evento tectônico, no caso o soerguimento de um alto interno à bacia, podem modificar o padrão de resposta sedimentar, com a possibilidade de variações das taxas de subsidência e aporte sedimentar em diferentes áreas da bacia em diferentes tempos. Levantamentos estratigráficos acompanhados por interpretações dos sistemas deposicionais e considerações sobre os controles tectônicos e climáticos nos padrões de variação vertical foram realizados na unidade superior do Supergrupo Camaquã, designada Grupo Guaritas, aqui datado no Eocambriano (535, 2\'MAIS OU MENOS\'1,1 Ma, Ar-Ar step heating em subvulcânica rasa que afeta a unidade, rocha total), levando a uma revisão da coluna estratigráfica da unidade e a modelos de resposta sedimentar a eventos de reativação de altos adjacentes à bacia, sobrepostos por oscilações climáticas. A unidade é dominada por sistemas fluviais efêmeros, com ciclos de variação na proporção entre depósitos de planícies distais e depósitos areno-conglomeráticos de correntes trativas que ocorrem em várias escalas, contando ainda com a presença de expressivos depósitos eólicos. A distinção entre ciclos de origem autogênica e alogênica baseou-se em estudos de elementos arquiteturais, perfil de eletrorresistividade de alta resolução, correlações entre seções colunares de detalhe e reconhecimento de superfícies estratigráficas com significado cronológico, identificando-se como autogênicos os ciclos relacionados à migração lateral de sistemas deposicionais e como alogênicos aqueles correlacionáveis em escala bacinal e vinculados a superfícies estratigráficas maiores. É proposto um modelo tectônico para a ciclicidade alogênica dos depósitos fluviais, relacionado a variações nas taxas de subsidência em função de eventos de reativação das falhas de borda. Já a alociclicidade relacionada a intercalações entre depósitos fluviais e eólicos é interpretada como decorrente de oscilações climáticas. Uma origem decorrente de esforços tectônicos distensionais para a Bacia do Camaquã é documentada por análises de proveniência de depósitos proximais do Grupo Santa Bárbara (Neo-Ediacarano) em sua área-tipo, que revelam a ausência de deslocamento lateral expressivo na falha de alto ângulo que delimitou a borda ativa da bacia, considerada como transcorrente em trabalhos anteriores. Essa caracterização é confirmada por análises de paleocampos de tensão realizadas separadamente em cada intervalo estratigráfico do Supergrupo Camaquã e unidades posteriores, que revelam eventos distensionais de direção NE-SW e SW-NE durante a formação da bacia e eventos causadores de transcorrência apenas durante a deformação das sucessões. Como tais eventos distensionais precedem as deformações transcorrentes, reconhecidas também no embasamento e granitos de áreas adjacentes à bacia, o suposto vínculo entre a movimentação de zonas de cisalhamento e a origem da bacia, em contextos pós-orogênicos, é descartada. Propõe-se que a origem da Bacia do Camaquã está relacionada a um grande sistema de bacias distensionais posteriores ao metamorfismo da Orogenia Brasiliana e sem relação direta com os processos orogênicos. A hipótese vigente de que o Grupo Caacupé (Neo-Ordoviciano, Paraguai Oriental) teria sido formado em uma bacia distensional ou transtrativa motivou uma análise comparativa com os depósitos do Supergrupo Camaquã. Análises de fácies, sistemas deposicionais, paleocorrentes, proveniência macroscópica e icnofósseis realizadas no Grupo Caacupé (Neo-Ordoviciano, Paraguai Oriental) caracterizam sistemas deposicionais fluviais conglomeráticos distais sobrepostos por sistemas costeiros dominados por marés. Não foram reconhecidas, nos depósitos aluviais, características que possam levar a uma interpretação de sistema de leques aluviais, não havendo evidências de campo que suportem a suposição de escarpas tectônicas proximais limitando a bacia. Sugere-se que a unidade depositou-se em uma bacia ampla, com conexão marinha a oeste, representativa da primeira fase sinéclise da Bacia do Paraná. Desta forma, propõe-se que o intervalo entre o Eocambriano e Neo-Ordoviciano marque o fim dos processos distensionais registrados nas bacias da Província Mantiqueira e o início dos ciclos de subsidência do tipo sinéclise, sendo o primeiro aflorante na borda oeste da bacia. Assim, os processos do Estádio de Transição, considerados como cambro-ordovicianos na proposta original (Almeida 1969), não ultrapassam o Mesocambriano, e o controle da margem proto-andina nos ciclos de subsidência de grandes áreas no paleozóico inicia-se já no Neo-Ordoviciano. A possibilidade de relação entre a distensão formadora do Grupo Guaritas e a origem da Bacia do Paraná é improvável devido à idade eocambriana desse grupo e à ausência de depósitos da Bacia do Paraná anteriores ao Permiano sobre a unidade, no Rio Grande do Sul, implicando em um adiamento de cerca de 240 milhões de anos da fase de subsidência termal em relação à de subsidência tectônica. The geological evolution of southeastern South America from Ediacaran to Ordovician is characterized by the development of sedimentary basins that register the geological events of the so called Transitional Stage between the metamorphism and intense deformation of the Brasiliano Orogeny and the tectonic stability of the Middle to Late Paleozoic cratonic basins. The present thesis focus on the sedimentary record of the tectonic events of the Transitional Stage, aiming to test and develop the geological models based on the study of the neoproterozoic metamorphic and plutonic rocks of the same region. The objects of the present study are the Camaquã Supergroup (Ediacaran to Early Cambrian, south-central region of Rio Grande do Sul State, Southern Brazil) and the Caacupé Group (Late Ordovician, Eastern Paraguay), considered as examples of the sedimentary record of eastern South America from Ediacaran to Late Ordovician. Several methods and approaches were used in the recognition of the tectonic events responsible for the origin and posterior deformation of these basins, and in the characterization of the patterns of sedimentary response to tectonic activity. These included measurement of detailed stratigraphic sections, facies and architectural elements analyses, geological mapping, paleocurrent analysis, macroscopic and microscopic provenance analyses, geochronological analysis, analysis of brittle tectonic structures and reconstitution of paleostress fields. The Santa Bárbara Group (Late Ediacaran, Camaquã Supergroup) shows tens to hundreds of meters thick cycles of grain-size variation due to varied preservation of distal flood-plain and stream-dominated deposits of ephemeral river systems. The uplift of a basement highland during the deposition of the unit came to separate two isolated sub-basins, as first suggested by geological mapping of depositional systems and later characterized through provenance and paleocurrent analysis. The recognition of the stratigraphic level that records the first detrital contribution of the uplifted highland enabled the interpretation of the vertical facies changes recorded in the stratigraphic sections in terms of tectonic controls. It was observed that there is no correlation of the tectonic reactivation responsible for the uplift of the internal highland with an event of increased depositional space, as predicted by many theoretical models, but instead with grain-size coarsening of a sandy ephemeral stream succession. This result reveals that particularities in the history of reactivation, in this case the uplift of a highland internal to the basin, may imply in a diverse stratigraphic response than the predicted flooding surface, maybe due to different subsidence and sedimentation rates in different basin areas. Stratigraphic analysis of the Guaritas Group (here dated in 535,2\'MAIS OU MENOS\'l,l Ma, whole-rock Ar-Ar step heating method in a sample of shallow sub-volcanic rock that cuts through the unit) were undertaken aiming the reconstitution of depositional environments and the recognition of the tectonic and climatic controls on the sedimentation. The results include the revision of the stratigraphic column of the group and the development of models of sedimentary response to tectonic reactivation events and uplift of basin-margin highlands, superposed to climatic change. The Guaritas Group is composed of fluvial stream ephemeral deposits showing cycles of grain-size variation caused by lateral migration of distal flood-plain dominated deposits and sandy to pebbly proximal stream deposits, also including thick successions of aeolian deposits. A distinction between allogenic and autogenic cycles was based on architectural element analysis, high-resolution electro-resistivity pseudo-section, correlation among detailed stratigraphic sections and recognition of major bounding stratigraphic surfaces. Thus, cycles related to lateral migration of depositional systems were identified as autogenic and those related to basin-scale bounding surfaces were considered as allogenic. A model of tectonic control (through subsidence rates) on the generation of alluvial allogenic cycles is proposed. The aeolian-alluvial allogenic cycles are interpreted as climatically controlled. An extensional origin for the Camaquã Basin is interpreted from provenance analysis of the Santa Bárbara Group, which reveals the absence of lateral migration of alluvial fans with respect to their sources. Paleostress reconstitution of brittle tectonic structures of each stratigraphic unit of the Camaquã Basin and younger deposits of the region confirm this hypothesis, with extensional NE-SW and SW-NE paleostress fields being recognized as sin-depositional for the Camaquã Basin. As the strike-slip deformational events are related only to basin deformation, the supposed link between activation of basement shear zones and the origin of the Camaquã Basin, in a post-orogenic context, is refuted. The proposed model considers a great system of extensional basins formed after the Brasiliano Orogeny, with no direct relation to the previous orogenic processes. The hypothesis of a extensional or transtensional origin for the Caacupé Group (Late Ordovician, Eastern Paraguay) lead to a comparison of this unit with the deposits of the Camaquã Supergroup. Facies, depositional systems, paleocurrent and ichnofossil analyses of the Caacupé Group characterize a pebbly distal braided fluvial depositional system overlain by tide-dominated coastal systems. No diagnostic feature of alluvial fans were recognized and there is no support for the hypothesis of proximal border faults. It is suggested that the Caacupé Group was deposited in a wide basin with a marine connection to the west, recording the first intra-cratonic sag phase of the Paraná Basin. Thus, it is proposed that the period between Early Cambrian and Late Ordovician marks the end of the extensional processes recorded in the basins of southeastern South America and the onset of the intra-cratonic sag depositional cycles. So, the tectonic and depositional processes of the Transitional Stage, considered as of Cambrian to Ordovician age in the original definition (Almeida 1969), do not reach Late Cambrian, and the control of the proto-andean continental margin on the paleozoic intra-cratonic subsidence cycles of South America begins in Late Ordovician. The hypothesis of relationship between the basin-forming extension recorded in the Guaritas Group and the origin of the Paraná Basin is refuted due to the absence of post-rift deposits above the rift prior to the Permian, implying in a 240 million year gap between the tectonic subsidence and the supposed thermal subsidence phase.

Na região centro-sul do Rio Grande do Sul, sobre rochas metamórficas e ígneas do Escudo Gaúcho, ocorrem coberturas não metamorfizadas de idade neoproterozóica a cambriana, agrupadas sob a denominação de Grupo Camaquã. De acordo com a proposta de Fragoso Cesar et al. (\'2000 POT. b\'), esse grupo é composto por três formações: Maricá (siliciclástica basal), Crespos (vulcânicas andesíticas e riolíticas, piroclásticas e rochas sedimentares associadas), e Santa Bárbara (siliciclástica superior). A Formação Santa Bárbara é caracterizada por uma espessa sucessão (mais de 4000 metros) de conglomerados e arenitos, com siltitos subordinados, de ambientes aluviais e marinhos costeiros. Essa unidade aflora em três sub-bacias contíguas limitadas por falhas de direção NNE, denominadas Camaquã Ocidental, Central e Oriental, compreendendo diversas ocorrências isoladas por coberturas mais recentes (formações Pedra Pintada e Guaritas da Bacia do Paraná) em uma faixa de aproximadamente 70 km de largura por 150 km de extensão. Na sub-bacia Camaquã Ocidental, a Formação Santa Bárbara aflora continuamente em uma área de mais de 500 km² na região dos Arroios do Seival, Lanceiros e Santa Bárbara que constitui a área-tipo da formação. Essa exposição foi estudada através da aplicação de diversas técnicas da geologia sedimentar visando a caracterização dos ambientes deposicionais, da evolução paleogeográfica e da estratigrafia de seqüências. A Formação Santa Bárbara na sub-bacia Camaquã Ocidental é composta por cinco unidades litoestratigráficas, incluindo conglomerados, arenitos e ritmitos arenosos e sílticos. A petrografia sedimentar revela o caráter imaturo desses sedimentos, com o predomínio de litoarenitos feldspáticos nas unidades de arenitos e na matriz dos conglomerados. Três seqüências deposicionais sensu Vail et al. (1977) foram identificadas com base no reconhecimento das principais superfícies erosivas que limitam pacotes com afinidade genética e no reconhecimento dos tratos de sistemas deposicionais. Os padrões de empilhamento e as variações de espessura dos tratos de sistemas sugerem um aumento gradual das taxas de subsidência da primeira para a segunda seqüência. A terceira seqüência apresenta evidências de um controle tectônico na arquitetura deposicional, condicionada pelo soerguimento de um alto interno à bacia. Os ambientes deposicionais da formação podem ser agrupados em dois grandes conjuntos: ambientes aluviais com abundantes fácies de rios entrelaçados rasos e amplos e de leques aluviais com predomínio de processos de enchentes em lençol; e ambientes marinhos rasos e transicionais, com fácies estuarinas com ação de marés, além de fácies lagunares e costeiras com ação de ondas. As análises de paleocorrentes indicam dois padrões de transporte sedimentar aluvial: um transversal à estruturação da bacia, associado aos leques aluviais; e outro paralelo ao eixo da bacia, com transporte axial para norte em planícies de rios entrelaçados. As paleocorrentes das fácies marinhas costeiras indicam bimodalidade de correntes de maré, com predomínio das correntes de vazante para norte e com correntes de enchente subordinadas para sul, além da presença, no trato transgressivo da terceira seqüência, de correntes de deriva litorânea para sudoeste, associadas à ação de ondas. A integração das análises de proveniência de clastos com as interpretações dos ambientes deposicionais e as análises de paleocorrentes levaram a duas conclusões. Primeiramente observou-se que há grande correlação entre os litoclastos presentes na bacia e os litotipos do embasamento hoje adjacente, indicando ausência de grandes movimentações transcorrentes nas falhas de borda. Além disso, identificou-se que a contribuição detrítica do alto de Caçapava do Sul, constituída de filitos, metabasitos e diversas fácies do Granito de Caçapava do Sul, passa a ocorrer apenas na terceira seqüência deposicional, fato interpretado como indício de que o isolamento entre as bacias Camaquã Ocidental e Camaquã Central ocorreu apenas em um período tardio de evolução da Formação Santa Bárbara. A integração de todas as informações obtidas sugere que a Formação Santa Bárbara depositou-se em uma bacia extensional tipo rift, com falhas limitantes de rejeito normal ou oblíquo, porém sem grandes rejeitos direcionais, com preenchimento sedimentar controlado principalmente pela subsidência tectônica, aporte clástico e padrões de transporte sedimentar, que modificam as respostas à variações eustáticas. The Camaquã Group crops out in the central region of the state of Rio Grande do Sul, laying above the igneous and metamorphic rocks of the Gaúcho Shield. This group is composed of thick sedimentary successions of neoproterozoic to cambrian age, divided into three formations: Maricá (lower siliciclastic), Crespos (volcanic and volcano-sedimentary) and Santa Bárbara (upper siliciclastic). The Santa Bárbara formation is characterised by a thick (over 4000 meters) succession of conglomerates and sandstones, with minor siltstones, of alluvial and coastal marine environments. It crops out in three contiguous sub-basins limited by NNE trending faults, named Camaquã Ocidental (Western Camaquã), Camaquã Cental (Central Camaquã) e Camaquã Oriental (Eastern Camaquã), including various occurrences isolated by younger sedimentary deposits (Pedra Pintada and Guaritas formations of the Paraná Basin) in a belt 70 km wide and 150 km long. The Camaquã Ocidental sub-basin has a continuous exposition of the Santa Bárbara formation of more than 500 km². The eastern part of this sub-basin, namely the region of Seival, Lanceiros and Santa Bárbara rivers, is the type-area of the formation. The present study has focused on this area and was based on the application of a series of techniques of sedimentary geology to characterise the depositional environments, paleogeographic evolution and sequence stratigraphy of the unit. The Santa Bárbara formation in the Camaquã Ocidental sub-basin is composed of five lithostratigraphic units, including conglomerates, sandstones and sandstone/siltstone rhythmites. Sedimentary petrography analysis have revealed the immature nature of the sandstones and the sandy matrix of the conglomerates, characterised as feldspathic lithoarenites. A sequence stratigraphy approach has been applied to understand the evolution of the sedimentary infill through the identification of erosional unconformities bounding units with chronostratigraphic significance. Three stratigrahic sequences were recognised. The first two represent eustatic cycles preserved in a context of rising subsidence rates, as deduced from the stacking pattern and the variation of thickness of the systems tracts. The third stratigraphic sequence shows evidences of tectonic control upon the depositional architecture, with both the subsidence end the volume of detritus being strongly influenced by the uplift of the Caçapava do Sul Highland. The depositional environments of the Santa Bárbara formation can be grouped into two categories: alluvial environments, mainly shallow and wide braided rivers and sheetflood-dominated alluvial fans; and shallow marine / coastal environments, with tide-dominated estuarine facies, lagoons and wave-dominated shorelines. Paleocurrent analysis indicate two main patterns of alluvial sedimentary transport: one transversal to the basin axis and related to the alluvial fans, and another parallel to the axis, with axial transport towards north related to braided river distal plains. The paleocurrents of the coastal marine facies indicate bimodality of tidal currents, with the ebb current being the dominant one (towards north) and the flood current the subordinate (towards south). Longshore currents related to a wave-dominated shoreline were identified in the transgressive tract of the third sequence, with currents towards southeast. The comparison between the provenance analysis and the basement lithologies, considering also the depositional environments and paleocurrent data, has lead to two main conclusions: l) There is great correlation between the composition of the lithoclasts of the basin and the basement lithologies that crop out today at their vicinity, suggesting that there were no great transcurrent movements at the border faults. 2) The clastic contribution from the Caçapava do Sul Highland, consisting mainly phylites, metabasics and various facies of the Caçapava do Sul Granite, is present only in the third sequence, indicating that the isolation of the Camaquã Ocidental sub-basin from the Camaquã Central sub-basin occurred only at the last stages of the evolution of the Santa Bárbara formation. All of the data and interpretations obtained suggest that the Santa Bárbara formation has been deposited in an extensional rift basin, with normal (dip-slip) border faults, maybe with some oblique movement but without great strike-slip displacement. The sedimentary architecture seems to be controlled mainly by tectonic subsidence, sedimentation rates and transport patterns, which have modified the response to eustatic fluctuations.