Alexandre Uhlein

Proterozoic rifting and closure, SE border of the São Francisco Craton, Brazil

Abstract This article presents an overview of the present knowledge of the tectonic evolution of the transition zone between the Sao Francisco Craton and the AracuaiFold Belt in Minas Gerais State. Two extensional events with rift formation and sedimentation and one compressional orogenic event were identified. The first extensional event, dated at 1750–1700 Ma, is represented by the N–S Espinhaco basin formation along a continental rift. The lower part of the sedimentary infill (Lower Sequence of the Espinhaco Supergroup) is made up of acid to intermediate metavolcanics and fluvial metasediments, with strong fault-block activity related to the initial rift phase (with mechanical subsidence). The Middle Sequence is composed of quartzite with large-scale cross-bedding and the Upper Sequence consists of an alternation of quartzite and metapelitic rocks deposited in a shallow marine setting related to a subsequent flexural phase (with thermal subsidence). The second extensional phase corresponds to the Aracuairift formation (1000–900 Ma), accompanied by a mafic dike swarm intruded into the Espinhaco Supergroup. On the highest uplifted blocks, continental glaciogenic facies and a glacio-marine sequence (JequitaiFormation) were deposited. To the east, in the Aracuaibasin, they grade into marine deposits with gravitational sedimentation (debris-flows and turbidites) organized in a probably prism of passive continental margins. To the West on the craton, the younger BambuiGroup was deposited directly on the basement. No equivalents of this group were identified in the Aracuaibasin, so either they were not deposited or they were eroded. The compressional orogenic event (650–550 Ma) is characterized by zones with asymmetric folds showing westward vergence separated by thin belts of ductile shearing (Dp phase). An east dipping Sp cleavage with down-dip stretching lineation is consistent with a relative westward tectonic transport, towards the Sao Francisco Craton. Regional Barrovian metamorphism is associated with Dp, increasing from anchizone to amphibolite facies. During this orogenic event the Sao Francisco Craton and the Aracuaiand Paramirim Fold Belts were formed as individual geotectonic units.

Neoproterozoic oceanic crust remnants in northeast Brazil

The Borborema Province of northeast Brazil occupies a strategic position in the central portion of West Gondwana, linking three of its major cratonic constituents: the Sao Francisco–Congo, Amazon, and West Africa cratons. The southern portion of the province, the Riacho do Pontal fold belt, comprises an association of exhalative rocks with metabasalts having transitional mid-oceanic-ridge basalt geochemistry (Monte Orebe Complex) that suggests the preservation of remnants of Neoproterozoic oceanic crust. This view is supported by geophysical data: the Riacho do Pontal fold belt corresponds to the inflexion of a paired positive-negative Bouguer anomaly similar to other Precambrian suture zones. Sm-Nd isotope data for the metabasalts yield a whole-rock isochron age of 819 ± 120 Ma with an initial e Nd (t) = +4.4, indicating derivation from a depleted mantle source. The onset of subduction within the Borborema Province (ca. 630 Ma) caused the inversion of basins and obduction of slices of oceanic crust. In this scenario, the Riacho do Pontal fold belt represents a complete late Neoproterozoic plate tectonics cycle involving the collision of the Sao Francisco craton (lower plate) with the Pernambuco-Alagoas block (upper plate). This interpretation challenges current views that the Borborema Province has acted as a coherent block since Paleoproterozoic time (part of the Atlantica supercontinent), suggesting instead a dynamic setting, where multiple plates interacted during the Proterozoic. The Monte Orebe ophiolite provides a link with other Cryogenian oceanic crust occurrences in central Brazil and West Africa, indicating the preservation of a transcontinental Neoproterozoic suture zone in the heart of West Gondwana.

Depositional systems and stratigraphic review proposal of the Rio Preto Fold Belt, northwestern Bahia/southern Piauí

A Faixa Rio Preto bordeja a margem noroeste do Craton do Sao Francisco, na regiao noroeste da Bahia. A bacia precursora da faixa dobrada e representada pela Formacao Canabravinha, de espessura quilometrica, composta por quartzito litico, filito, metaritmito areno-pelitico, metadiamictito, e subordinadamente, metamarga. Sua deposicao ocorreu em um ambiente submarino do tipo slope-apron , dominado por fluxos gravitacionais tais como fluxos de lama e correntes de turbidez em um rift assimetrico ou hemi-graben de direcao leste-oeste, com borda falhada a sul e possivel borda flexural a norte. O aporte de sedimentos foi predominantemente de sul para norte, ou seja, do Craton do Sao Francisco talude abaixo para o rift . Na porcao norte da faixa dobrada, a Formacao Formosa representa, possivelmente, parte do embasamento paleoproterozoico, composta por xisto granatifero, quartzito, metachert ferro-manganesifero, xisto verde, e localmente, intercalacoes de orto-anfibolito. A Formacao Formosa provavelmente foi depositada em um bacia relacionada a arco magmatico ha aproximadamente 1.9 Ga atras. Uma proposta de revisao estratigrafica e baseada em dois pontos: 1) Retirada da Formacao Canabravinha da base do Grupo Bambui e seu posicionamento como equivalente cronoestratigrafico as unidades portadoras de diamictitos das outras faixas brasilianas marginais ao Craton do Sao Francisco; 2) Substituicao do termo Grupo Rio Preto pelo termo Formacao Formosa, uma vez que suas rochas nao podem ser cartograficamente individualizadas em formacoes. A sedimentacao pre-orogenica na Faixa Rio Preto ocorreu entre 850 e 600 Ma atras. Durante a Orogenese Brasiliana (600-540 Ma), a bacia sedimentar foi metamorfisada na facies xisto-verde e invertida, assumindo uma estruturacao quilometrica em leque assimetrico de dupla vergencia.

C-, Sr-isotope stratigraphy of carbonate rocks from the Southern Espinhaço Ridge, Minas Gerais, southeastern Brazil

Neoproterozoic carbonate rocks comprise different stratigraphic units in the southern part of the Espinhaço Ridge, Minas Gerais, Brazil. C, O- and Sr-isotope analyses were carried out along four selected stratigraphic sections across these formations. These are: (i) the Rio Pardo Grande Formation in the upper portion of the Espinhaço Supergroup, sampled in section 3; (ii) Macaúbas Group laminated limestones (Tijucuçu Farm) and dolostone layers (Domingas Formation) have been respectively sampled along the so-called sections 1 and 2, and (iii) the lower stratigraphic units of the Bambuí Group, sampled in section 4. Laminated limestone samples from the Macaúbas Group have δ13C values as high as 10.9‰ decreasing up section to -1.1‰ and 87Sr/86Sr values vary from 0.7072 to 0.7076, a range commonly observed in Cryogenian rocks. In section 2, dolomitic samples exhibit 87Sr/86Sr from 0.7076 to 0.7077 while in section 3, 87Sr/86Sr from 0.7074 to 0.7079. In section 4, 87Sr/86Sr values are around 0.7080. The values of 87Sr/86Sr observed in carbonate samples from the Macaúbas Group are similar to those observed in the Sr-isotope secular curve for the Neoproterozoic. Carbonate samples from the base of the Bambuí Group correlate with Ediacaran fingerprints, after the Marinoan (ca. 635 Ma) glaciation.

Chapter 3 The São Francisco Palaeocontinent

Abstract The Sao Francisco and the Congo cratons represent sectors of a Neoproterozoic palaeocontinent preserved from the Brasiliano-Pan African orogeny, recorded in their marginal belts. The boundaries between the Sao Francisco Craton and the surrounding belts are marked by intensive folding and overthrusting. In the interior of the craton, Neoproterozoic sedimentary covers are horizontal or gently deformed, forming isolated basins. The term Sao Francisco Basin includes Palaeo- and Mesoproterozoic successions but in this chapter it is restricted to the Neoproterozoic sedimentary successions of the Sao Francisco Supergroup, characterised by a glaciogenic unit at the base and an argillaceous–carbonatic–arkosic unit at the top. The Neoproterozoic evolution of the Sao Francisco Craton, including its marginal belts and sedimentary cover, is reviewed. The lithostratigraphy, Neoproterozoic mineralisations (lead, zinc, fluorine, barium and phosphates) and geochronological and stable isotope studies are also presented. The geological record of glacial events and stable isotope data of related cap carbonates are also discussed.

Structural analysis of the Rio Preto fold belt (northwestern Bahia / southern Piauí), a doubly-vergent asymmetric fan developed during the Brasiliano Orogeny

The Rio Preto fold belt borders the northwestern Sao Francisco craton and shows an exquisite kilometric doubly-vergent asymmetric fan structure, of polyphasic structural evolution attributed exclusively to the Brasiliano Orogeny (∼600-540 Ma). The fold belt can be subdivided into three structural compartments: The Northern and Southern compartments showing a general NE-SW trend, separated by the Central Compartment which shows a roughly E-W trend. The change of dip of S2, a tight crenulation foliation which is the main structure of the fold belt, between the three compartments, characterizes the fan structure. The Central Compartment is characterized by sub-vertical mylonitic quartzites, which materialize a system of low-T strike slip shear zones (Malhadinha – Rio Preto Shear Zone) crosscutting the central portion of the fold belt. In comparison to published analog models, we consider that the unique structure of the Rio Preto fold belt was generated by the oblique, dextral-sense interaction between the Cristalândia do Piaui block to the north and the Sao Francisco craton to the south.

Environment, tourism and land use planning – Riachinho Basin, Brazil

Santana do Riacho County, in Minas Gerais State, Brazil, is located in an area known for its natural beauty. The growth of tourism has been the main economic activity of this area. In order to achieve organized land occupation and self‐sustained development, thematic maps of the physical environment were elaborated. Analysis of the water sources supply and recreation have been completed. The risk of water contamination has increased, caused by the increase of tourism activities in this area, added to by local authorities policy. The analysis of all the thematic maps and of the quality of the water sources supply of the area, lead us to develop proposals for land use planning and zoning, adequate for the local conditions and the vulnerability of the underground water contamination. These issues are important concerning the local development and also land use, caused by the tourism activities.

Análise de fácies, estratigrafia de sequências e quimioestratigrafia da Formação Sete Lagoas (Grupo Bambuí), norte do Estado de Minas Gerais, Brasil: Evidência de um carbonato de capa depositado sobre o Alto de Januária.

Manuscript ID: 20160112. Received in: 09/14/2016. Approved in: 01/19/2017. ABSTRACT: Sedimentary rocks of the Sete Lagoas Formation, exposed in the left margin of the São Francisco river (Minas Gerais State, Brazil), were deposited on the Januária‐Itacarambi basement high. They show both lateral and vertical rock stack‐ ing along continuous outcrops, allowing us to carry out detailed facies analysis and sequence stratigraphy studies. Our studies also integrate data from geological mapping, macro and micro‐ scopic petrography and high‐resolution C and O isotope analy‐ sis. Eight facies and four facies associations make up a sequence composed by a transgressive tract in the base, and a high stand tract in the upper portion, separated by a maximum flooding surface. The high stand tract shows a progradation stacking from the basement high apex towards progressively deeper base‐ ment areas. This stratigraphic framework, associated with oth‐ ers stratigraphic and isotopic features, indicates that the now exposed Januária basement high also represents a paleo‐high during the sedimentation event. Aragonite pseudomorphs and dolomites coupled with δ13C values of ‐5‰ characterize the basal carbonate of the transgressive tract as a cap carbonate. Records of the Cloudina fossil, recognized in the high stand tract, indicate a Late Ediacaran age for the upper portion of the studied stratigraphic sequence.Rochas sedimentares da Formacao Sete Lagoas afloram na margem esquerda do Rio Sao Francisco, na regiao de Januaria-Itacarambi, Minas Gerais, em secoes relativamente continuas, tanto lateralmente como no empilhamento vertical. Boas exposicoes associadas a preservacao de estruturas/texturas primarias favorecem o estudo de facies, e de estratigrafia sequencial. Este estudo estratigrafico foi realizado com a integracao de dados de mapeamento geologico, petrografia macro e microscopica e de tres perfis estratigraficos chaves, onde foram realizados levantamentos quimioestratigraficos (C e O) de alta resolucao. Foram reconhecidas oito facies e quatro associacoes de facies distribuidas em uma sequencia composta por um trato de sistema transgressivo na base, e um trato de sistema de mar alto na porcao superior, separados por uma superficie de inundacao maxima. O padrao de empilhamento do trato de mar alto apresenta uma progradacao das facies, do alto do embasamento, em direcao a areas onde o topo do embasamento e progressivamente mais profundo. Este arcabouco estratigrafico, associado com outras feicoes estratigraficas e isotopicas, indica que o alto de Januaria atuou como um alto do embasamento durante o evento sedimentar da Formacao Sete Lagoas. Estudos quimioestratigraficos e petrograficos permitiram caracterizar o carbonato basal da Formacao Sete Lagoas como um carbonato de capa na base do trato transgressivo, onde ocorrem dolomitos e pseudomorfos de aragonita associados a valores de δ13C de ate -5‰. Registros do fossil Cloudina, recuperados no trato de mar alto, indicam uma idade associada ao Ediacarano Superior para a porcao superior da sequencia estratigrafica estudada.

Chapter 51 The glaciogenic Jequitaí Formation, southeastern Brazil

Abstract Glaciogenic deposits of the Jequitaí Formation (Fm.) are well exposed along the margins of the Serra do Cabral on the São Francisco Craton, southeastern Brazil. The Jequitaí Formation is thin (0–150 m thick), lenticular and overlies the Espinhaço Supergroup on a discrete unconformity. Sandstones show subglacial erosional structures such as grooved and striated pavements oriented ENE–WSW. The Jequitaí Fm. consists of massive and stratified diamictites with granules, pebbles and boulders of gneiss, granite, quartzite and carbonate. At the base, the diamictites are massive, whereas the upper part contains many alternating beds of clast-rich and -poor diamictites. They also contain discontinuous, fine-grained sandstones and a few laminated siltstone–mudstone intercalations. This diamictite association indicates glaciomarine sedimentation. The Jequitaí Fm. covers the São Francisco cratonic domain and its equivalent extends eastward over the Araçuaí fold belt where it is part of the metasedimentary Macaúbas Group, a thick Neoproterozoic unit with metadiamictites, quartzites and schists. The diamictite–turbidite association of the Macaúbas Group was deposited on the border of the Pan-African–Brasiliano rift as gravity flows.

The Rio Preto and Riacho do Pontal Belts

Together, the Rio Preto and Riacho do Pontal belts form a 600 km-long orogenic system developed along the northwestern and northern margins of the Sao Francisco craton during the Neoproterozoic Brasiliano orogeny. Involving the Paleoproterozoic (~1.9 Ga) Formosa Formation (schist, quartzite, greenschist and amphibolite) and the Neoproterozoic (900–600 Ma) Canabravinha Formation (metadiamictite, metawacke, metaturbidite), the Rio Preto fold belt, exposed in Bahia and Piaui states, borders the craton to the northwest. Neoproterozoic deformation between 600 and 540 Ma originated a complex, asymmetrical and double-verging thrust wedge, whose southern branch propagated for over 100 km into the craton interior in form of a thin-skinned deformation front. The Rio Preto belt probably represents an inverted Neoproterozoic hemi-graben developed along the northern margin of the craton. The Riacho do Pontal fold belt occupies the northern margin of the craton. Its external zone is made up of a south-verging thin-skinned nappe system detached along the basement-cover contact. Ages of syn- to late-collisional granitic intrusions suggest that the main deformation phase in the Riacho do Pontal belt occurred between 667 and 555 Ma. The Barra Bonita Formation (quartzite, schist and marble), a correlative of the Una Group in craton interior (Paramirim aulacogen), represents a platformal unit, deposited on the northern Sao Francisco passive margin. The Monte Orebe metabasalts, exposed further north in the central sector of the belt, might represent remnants of a Neoproterozoic oceanic crust.