Abstract High-resolution stratigraphic analysis was carried out on tidal microbial carbonates from the middle part of the Lagoa do Jacaré Formation from the Ediacaran-Cambrian Bambuí Group in the Ubaí area. In a 41 meter-thick section, sixteen lithofacies and three facies associations were interpreted as deposits of supratidal, intertidal, and subtidal settings. The tidal cycles are asymmetric and shallow upward, the high-resolution stratigraphic approach allowed inferences as to short-term and long-term fluctuations of sea level during deposition. The metric tidal ranges here described suggest strong tidal currents, which is incompatible with the giant enclosed sea model as an analog for Bambuí deposition, given that tidal ranges in suc seas are null or less than a few decimeters. Therefore, we suggest that the Ubaí carbonates were deposited in a semi-enclosed epicontinental sea subject to strong tidal forces when the formerly resticted Bambuí Basin reopened probably during deposition of the upper half of the Lagoa do Jacaré Formation, as suggested by previous isotopic data and now reinforced by our sedimentologic and stratigraphic field data.

Abstract A section investigated in the region of Paraopeba, Minas Gerais, provided detailed sedimentologic, stratigraphic and chemostratigraphic data from the Ediacaran Lagoa do Jacaré Formation, Bambuí Group, Southeast Brazil. This information allowed interpretation of tempestite facies in transgressive-regressive cycles, reinforcing the previously proposed storm-influenced sedimentation model and clarifying how it is associated with unusually high C-isotope values related to the Middle Bambuí Excursion (MIBE). Facies analysis of nine lithofacies identified at the GMD quarry based on field and petrographic descriptions showed distal tempestite facies grading upwards to a shallower, oncoidal/ooidal carbonate marine environment, then to basinal shales. The entire succession comprises a low-order transgressive hemicycle, recording the transition from a storm-influenced carbonate ramp to a siliciclastic-dominated platform. Chemostratigraphic data yielded high δ¹³C values ranging between +11.11‰ and +13.94‰. Our data contribute to the revision of the previously proposed interpretation and as well as to the understanding of paleoenvironmental conditions and C isotope signatures across the MIBE, near the Ediacaran-Cambrian boundary in the Bambuí Basin.

Abstract According to previous studies, the Palaeoproterozoic Minas-Itacolomi meta-sedimentary mega-sequence in the Quadrilátero Ferrífero metallogenic province records the transition of a continental rift to passive margin to foreland basin. From the perspective of sequence stratigraphy, this article identifies five sequences summarised in the Minas-Itacolomi Basin stratigraphic chart, providing information of the Palaeoproterozoic eustatic changes, such as the Great Oxidation Event. Interpretations were based on the systematic recognition of 41 facies and their associations, the identification of nine recurring depositional systems, nine system tracts, and five sequences (in that order). The Caraça and Itabira Groups comprise the first sequence, for which three system tracts were identified. The Piracicaba Group hosts two sequences and four system tracts. The Sabará and Itacolomi Groups are the two upper sequences, both related to a foreland basin, where the underfilled and overfilled system tracts were recognised. Sedimentary provenance was determined mainly from detrital zircon U-Pb age spectra of previous authors’ databases, as well as on palaeocurrent field data and thickness variations. All of the units of the Minas-Itacolomi mega-sequence share detrital zircons from Archaean sources (Rio das Velhas Supergroup and crystalline basement). Only the Sabará and Itacolomi Groups exhibited bimodal U-Pb age histograms, thus indicating both Archaean and Palaeoproterozoic (Mineiro Belt) provenances.

Abstract The Serra da Saudade Formation, Ediacaran/Cambrian Bambuí Group, cropping out along the ridge of the same name in Central-Western Minas Gerais, Brazil, contains important deposits of glauconitic siltstones associated with phosphatic rhythmite as well as other siliciclastic, volcaniclastic and carbonate lithofacies. Lithofacies indicate deposition by proximal and distal storm waves on a continental marine shelf. The basal gray siltstone and part of the green siltstone represent a transgressive systems tract. The glauconitic siltstone indicates sea level rise, with the maximum flooding surface located approximately at the interval of highest K2O content. Biogenic carbonate and an upward increase in sandstone beds are interpreted as a highstand systems tract. Gradual lateral facies changes reflect the transition from foredeep (to the west) to forebulge (to the east) deposition in a foreland basin as the final stage of a shallowing-upward second-order sequence. Black organic-rich limestone has δ13C values between -8.78 and 9.79‰, negative δ18O values from -11.50 to -9.35‰, and 87Sr/86Sr around 0.7075. This isotopic signature, similar to that of other Bambuí carbonates, indicates a probably-restricted depositional environment with organic sedimentation, methanogenesis and high evaporation rates from the Ediacaran through the early Cambrian period.

Abstract Orthogneisses associated with metavolcanosedimentary successions related to the 1000 - 920 Ma Cariris Velhos event occur mainly in a ca. 700 km-long sigmoidal-shaped belt that crosscuts the Transversal Zone of the Borborema Province and extends to the fold belts located in its southern or external zone (part of the Riacho do Pontal and Sergipano orogens). Despite its importance, the tectonic setting and the role of those rocks in the geological evolution of the Borborema Province are contentious and not yet well understood. New zircon U-Pb SHRIMP data on an augen-gneiss sill of the Afeição Suite intruding mica-schists mapped as part of the Santa Filomena Complex yielded a Concordia Age of 974 ± 11 Ma, indicating that at least part of the metasedimentary rocks in the internal zone of the Riacho do Pontal Orogen are Tonian or older and possibly related to the Cariris Velhos event. Hf-in-zircon isotope data are presented for the first time for Cariris Velhos-related ortho-derived rocks of the Afeição Suite. Analyzed samples yielded εHf(t) in a narrow range between -1.51 and +2.41, with associated TDMHf of 1.6-1.4 Ga, similar to previously obtained Nd isotope data with εNd(t) = -1.0 to +3.1 and TDMNd of 1.5-1.2 Ga. A possible scenario to explain both the geochemical features and the moderately juvenile to slightly evolved, near-chondritic Hf and Nd isotope signatures is a continental arc setting, where fractionated melts produced in the supra-subduction zone mantle wedge carrying a Tonian juvenile signature became contaminated with discrete amounts of Archean-Paleoproterozoic continental crust during ascent, producing Mesoproterozoic model ages which represent the mixture of those two end-members.

Abstract We report a case of abdominal actinomycosis, a chronic suppurative infection caused by bacteria of the genus Actinomyces, simulating colon cancer, and presenting with abdominal pain and leukocytosis. Computed tomography revealed a mass lesion with irregular contours, infiltrative aspect, with extension to omental fat and abdominal wall, in the transverse colon. A surgical intervention was performed due to the suspicious of a colonic tumor. In the post-operative period, anatomopathological examination showed suppurative nodules associated with actinomycetes colonies, confirming the diagnosis of abdominal actinomycosis. After surgery, the patient was submitted to antibiotic treatment and no relapse was observed.

Resumen Reportamos un caso de actinomicosis abdominal, una infección supurativa crónica causada por bacterias del genero Actinomyces, que simula el cáncer de colon y se manifiesta con dolor abdominal y leucocitosis. La tomografía computada reveló una lesión sólida con contornos irregulares y aspecto infiltrativo en el colon transverso, con extensión al epiplón y la pared abdominal. La intervención quirúrgica fue realizada debido a la sospecha de un tumor de colon. En el posoperatorio, el examen anatomopatológico mostró la presencia de nódulos supurativos asociados con colonias de actinomicetos, lo que confirma el diagnóstico de actinomicosis abdominal. Posteriormente a la cirugía el paciente recibió antibioticoterapia y no presentó Recidivas.

RESUMO: Duas importantes sequências metassedimentares proterozoicas ocorrem ao longo da margem noroeste do Cráton do São Francisco e a porção setentrional do Corredor do Paramirim, os grupos Rio Preto e Santo Onofre, respectivamente. O Grupo Rio Preto, que constitui a faixa homônima situada no limite noroeste cratônico, é composto pelas formações Formosa (xisto granatífero, quartzo-mica xisto, quartzito, clorita-sericita xisto e quartzo xisto ferrífero) e Canabravinha (quartzito, quartzito micáceo, metaritmito, filito, xisto e metaturbidito). O Grupo Santo Onofre ocorre exclusivamente na região do Corredor do Paramirim, e é composto por quartzitos e subordinadamente filitos sericíticos, carbonosos ou manganesíferos. As duas unidades estratigráficas registram a sedimentação associada a um ambiente marinho raso a profundo, e são interpretadas como bacias rift invertidas e metamorfisadas na fácies xisto verde durante a orogênese brasiliana. Neste artigo apresentamos 427 idades U-Pb em zircões detríticos para essas unidades. Os novos dados apresentaram idades máximas de deposição de 912 Ma para Formação Canabravinha, 965 Ma para a Formação Formosa e 971 Ma para o Grupo Santo Onofre. Portanto, é sugerida a existência de duas bacias rift neoproterozoicas formadas pelos Grupos Rio Preto e Santo Onofre. O Grupo Santo Onofre, predominantemente psamítico, representaria um ambiente marinho raso plataformal relativamente estável. O Grupo Rio Preto, com diamictito, quartzito, pelito e ritmito, representaria um ambiente marinho raso a profundo associado a fluxos gravitacionais. Ambos os grupos teriam sido depositados no Toniano tardio, e são possivelmente correlacionados com as unidades inferiores (pré-glaciais) do Grupo Macaúbas, localizado na Faixa Araçuaí.

ABSTRACT: Two important Proterozoic metasedimentary sequences, the Rio Preto and Santo Onofre Groups, crop out along the northwestern margin of the São Francisco craton and in northern Paramirim corridor, respectively. The Rio Preto Group, involved in the eponymous fold-thrust belt along the northwestern cratonic boundary, comprises the Formosa (garnet schist, quartz-mica schist, quartzite, chlorite-sericite schist and ferriferous quartz schist) and Canabravinha (quartzite, micaceous quartzite, metarhytmite, phylite, schist and metaturbidite) formations. The Santo Onofre Group occurs exclusively in the Paramirim corridor, and is composed of quartzite and minor carbonaceous or Mn-rich phylite. These units record sedimentation in shallow to deep-water marine settings related to rift basins, and were deformed and metamorphosed under greenschist facies conditions during the Brasiliano orogeny. Here we present 427 new detrital zircon U-Pb ages, which constrain the maximum depositional ages of ca. 971 Ma for the Santo Onofre Group, ca. 912 Ma for the Canabravinha Formation, and ca. 965 Ma for the Formosa Formation of the Rio Preto Group. Our data suggests that the Santo Onofre and the Rio Preto Groups accumulated in two distinct basin settings. The latter, composed mostly of sandy rocks, would represent a relatively stable, shallow-marine shelf environment. The Rio Preto Group, with metadiamictite, quartzite, pelitic and rhythmitic rocks, represents a shallow to deep marine environment influenced by gravity flows. Both groups were probably deposited in the Late Tonian, and are potential correlatives of the lower (pre-glacial) units of the Macaúbas Group of the Araçuaí belt.

RESUMO: Rochas sedimentares da Formação Sete Lagoas afloram na margem esquerda do Rio São Francisco, na região de Januária-Itacarambi, Minas Gerais, em seções relativamente contínuas, tanto lateralmente como no empilhamento vertical. Boas exposições associadas à preservação de estruturas/texturas primárias favorecem o estudo de fácies, e de estratigrafia sequencial. Este estudo estratigráfico foi realizado com a integração de dados de mapeamento geológico, petrografia macro e microscópica e de três perfis estratigráficos chaves, onde foram realizados levantamentos quimioestratigráficos (C e O) de alta resolução. Foram reconhecidas oito fácies e quatro associações de fácies distribuídas em uma sequência composta por um trato de sistema transgressivo na base, e um trato de sistema de mar alto na porção superior, separados por uma superfície de inundação máxima. O padrão de empilhamento do trato de mar alto apresenta uma progradação das fácies, do alto do embasamento, em direção a áreas onde o topo do embasamento é progressivamente mais profundo. Este arcabouço estratigráfico, associado com outras feições estratigráficas e isotópicas, indica que o alto de Januária atuou como um alto do embasamento durante o evento sedimentar da Formação Sete Lagoas. Estudos quimioestratigráficos e petrográficos permitiram caracterizar o carbonato basal da Formação 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 até -5‰. Registros do fóssil Cloudina, recuperados no trato de mar alto, indicam uma idade associada ao Ediacarano Superior para a porção superior da sequência estratigráfica estudada.

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 stacking 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 microscopic petrography and high-resolution C and O isotope analysis. 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 basement areas. This stratigraphic framework, associated with others 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.

A faixa dobrada Rio Preto bordeja a margem noroeste do cráton do São Francisco e mostra uma estrutura singular quilométrica de leque assimétrico de dupla vergência, com evolução estrutural polifásica atribuída exclusivamente à Orogênese Brasiliana (∼600-540 Ma). A faixa dobrada pode ser subdividida em três compartimentos estruturais: Os Compartimentos Norte e Sul com uma tendência geral na direção NE-SW, separados pelo Compartimento Central de tendência aproximadamente E-W. A mudança de mergulho de S2, uma clivagem de crenulação apertada que é a principal estrutura da faixa dobrada, entre os três compartimentos, caracteriza a estrutura em leque. O Compartimento Central é caracterizado por quartzitos miloníticos sub-verticais, que materializam um sistema de zonas de cisalhamento direcionais de baixa-T (Zona de Cisalhamento de Malhadinha – Rio Preto) que corta a porção central da faixa dobrada. Em comparação a modelos análogos publicados, considera-se que a estrutura única da faixa Rio Preto foi gerada pela interação oblíqua destrógira entre o bloco de Cristalândia do Piauí a norte e o cráton do São Francisco a sul.

The Rio Preto fold belt borders the northwestern São 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 Piauí block to the north and the São Francisco craton to the south.

Rochas carbonáticas do Neoproterozóico ocorrem em diferentes unidades estratigráficas na parte meridional da Serra do Espinhaço, Minas Gerais, Brasil. Análises isotópicas de C, O e Sr foram realizadas ao longo de quatro seções estratigráficas selecionadas. São elas: (i) Formação Rio Pardo Grande, na porção superior do Supergrupo Espinhaço, amostrada na seção 3, (ii) calcários laminados (Fazenda Tijucuçu) e camadas de dolomitos estromatolíticos (Formação Domingas) do Grupo Macaúbas, amostrados respectivamente ao longo das seções 1 e 2, e (iii) calcários laminados das unidades estratigráficas inferiores do Grupo Bambuí, amostrados na seção 4. Amostras de calcário laminado do Grupo Macaúbas têm valores δ13C tão elevados quanto 10,9‰, diminuindo no final da seção para -1,1‰ e os valores de 87Sr/86Sr variam entre 0,7072-0,7076, faixa comumente observada em rochas do Criogeniano. Na seção 2, as amostras de dolomito apresentam 87Sr/86Sr entre 0,7076-0,7077 enquanto na seção 3 87Sr/86Sr ocorrem valores entre 0,7074-0,7079. Na seção 4, os valores de 87Sr/86Sr estão em torno de 0,7080. Os valores de 87Sr/86Sr observados em amostras de carbonato do Grupo Macaúbas são similares aos observados na curva secular dos isótopos de Sr para o Neoproterozoico. As amostras de carbonato da base do Grupo Bambuí correlacionam-se com a assinatura isotópica do Ediacarano, após a glaciação Marinoana (ca. 635 Ma).

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.