Matheus Kuchenbecker

Quimioestratigrafia da porção basal do Grupo Bambuí no sudoeste do Cráton do São Francisco: implicações para os paleoambientes de Gondwana

The Bambui Group, the most extensive carbonate-siliciclastic cover on the Sao Francisco craton, has been a matter of debate because of its potential correlations to global glacial events. Unfortunately, most available chemostratigraphic data came from samples collected on surface rock exposures, ever susceptible to the aggressive chemical weathering that characterizes the southeastern Brazil. On the other hand, we present here high-resolution chemostratigraphic studies based on C, O and Sr isotopic data from 53 samples collected along a weathering-free, continuous, 175 m thick sedimentary succession. This succession was recovered by borehole drilling in the southwestern Sao Francisco craton, where occur the Carrancas and Sete Lagoas formations, the lowermost units of the Bambui Group. The drill cores reveal extremely irregular contacts between the basal diamictite and its basement, an Archaean foliated granodiorite. Geochronological and sedimentological data strongly suggest that the diamictite represents a lodgement till. This glaciogenic deposit is covered by a limestone succession which starts with impure carbonates showing aragonite pseudomorph fans and thin bands of black shale. The limestone pile grades to a marl-mudstone interval, which turns to a carbonate with biological components, succeeded by stromatolitic dolomite at the top. C and O isotopic signatures (referred to V-PDB) allow to the subdivision of the lower carbonate-pelite section into three intervals. The first isotopic interval corresponds to a cap carbonate, and displays negative values of δ13C (c . -4‰), and a large oscillation of the δ18O (-6 to -15‰). The Interval II shows a striking homogeneity in δ13C and δ18O, around 1‰ and -7‰, respectively. At the top, Interval III shows a large positive excursion of the δ13C (up to 8‰) and δ18O (-8 to -3‰) values. Unaltered 86Sr/87Sr ratios range from 0.7075 to 0.7077, mainly at the top of the section. The geochemistry of the carbonates is controlled by their terrigenous content (mostly quartz and clay minerals) which is concentrated in the lower units. Samples free of terrigenous contamination show Y/Ho ratios ranging from 25 to 50, suggesting a freshwater input during carbonate deposition. It is concluded that the diamictite has a glaciogenic origin and is covered by a cap carbonate. This pair has been identified along the basin and is related to one of the main Neoproterozoic glaciations. Discrepancy between the 86Sr/87Sr values and the global variation curves can be related to freshwater input during the carbonate deposition. Based on the regional tectonic context, the Bambui Basin may have been a restricted marine basin, totally or partially surrounded by mountain ranges within Gondwana, in the Neoproterozoic/Paleozoic boundary. In its early stages, the sedimentation was influenced by a global glacial event, whose melting phase was responsible by freshwater input in the basin. The gradual rise of the temperature was followed by an increase of the biological activity. Finally, a sudden increase in the biological activity could have been driven by paleogeographic changes caused by the active tectonic.Manuscript ID: 30285: Received in: 04/21/2015. Approved in: 02/12/2016. ABSTRACT: The Bambuí Group, the most extensive carbonate-siliciclastic cover on the São Francisco craton, has been a matter of debate because of its potential correlations to global glacial events. Unfortunately, most available chemostratigraphic data came from samples collected on surface rock exposures, ever susceptible to the aggressive chemical weathering that characterizes the southeastern Brazil. On the other hand, we present here high-resolution chemostratigraphic studies based on C, O and Sr isotopic data from 53 samples collected along a weathering-free, continuous, 175 m thick sedimentary succession. This succession was recovered by borehole drilling in the southwestern São Francisco craton, where occur the Carrancas and Sete Lagoas formations, the lowermost units of the Bambuí Group. The drill cores reveal extremely irregular contacts between the basal diamictite and its basement, an Archaean foliated granodiorite. Geochronological and sedimentological data strongly suggest that the diamictite represents a lodgement till. This glaciogenic deposit is covered by a limestone succession which starts with impure carbonates showing aragonite pseudomorph fans and thin bands of black shale. The limestone pile grades to a marl-mudstone interval, which turns to a carbonate with biological components, succeeded by stromatolitic dolomite at the top. C and O isotopic signatures (referred to V-PDB) allow to the subdivision of the lower carbonate-pelite section into three intervals. The first isotopic interval corresponds to a cap carbonate, and displays negative values of δ13C (c. -4‰), and a large oscillation of the δ18O (-6 to -15‰). The Interval II shows a striking homogeRESUMO: O Grupo Bambuí, mais importante unidade de cobertura do Cráton do São Francisco, tem sido alvo de intensos estudos e debates, entre outros motivos, pela possibilidade de correlação com eventos glaciais globais. A maior parte dos dados quimioestratigráficos disponíveis, no entanto, provém de amostras coletadas em afloramentos, sujeitos a expressivo intemperismo químico. Neste trabalho, é apresentado um levantamento quimioestratigráfico de alta resolução, baseado em análises de C, O e Sr realizadas em 53 amostras coletadas em 175 m de sequência sedimentar contínua, livre de intemperismo. Tal sequência foi obtida a partir de testemunhos de sondagem realizada na porção sul do Cráton do São Francisco, onde ocorrem rochas das formações Carrancas e Sete Lagoas, as mais basais do Grupo Bambuí. Os testemunhos revelaram contato extremamente irregular entre uma camada de diamictito e seu embasamento, um granodiorito foliado de idade arqueana. Dados sedimentológicos e geocronológicos indicam que o diamictito representa um tilito de alojamento, que apresenta contato brusco com a sequência carbonática sobrejacente. Essa sequência se inicia com calcário impuro, que exibe leques de cristais pseudomorfos de aragonita e delgadas camadas de folhelho negro. O calcário passa gradacionalmente para um intervalo argiloso, que por sua vez volta a gradar para uma espessa sequência de calcário com laminação microbiana, sucedido por dolomito estromatolítico no topo da coluna. As assinaturas isotópicas de C e O permitem a identificação de três intervalos distintos. O Intervalo I, basal, corresponde a um carbonato de capa, exibindo valores negativos de δ13C (c. -4‰), e grande oscilação nos valores de δ18O (-6 a -15‰). O Intervalo II exibe marcante homogeneidade nos valores de δ13C e δ18O, que se situam em torno de 1‰ e -7‰, respectiChemostratigraphy of the lower Bambuí Group, southwestern São Francisco Craton, Brazil: insights on Gondwana paleoenvironments

The Araçuaí Belt

The Aracuai belts extends along the curved southeastern margin of the Sao Francisco craton between the Brazilian coast and Lat 21°S, where it merges with the Ribeira belt. It represents the external, basement-involved fold-thrust belt of the Aracuai-West Congo confined orogen (AWCO), which formed due to the closure of the terminal branch of the Adamastor ocean during the amalgamation of West Gondwana in the Ediacaran and beginning of the Cambrian. Bounded to the east and southeast by the high grade and granitic core of the AWCO, the Aracuai belt involves a basement assemblage older than 1.8 Ga, the 1.7–0.9 Ga rift to rift-sag successions of the Espinhaco Supergroup, the Tonian-Edicaran rift-passive margin Macaubas Group, as well as the syn-orogenic Salinas Formation and crustal derived granitic intrusions. The Macaubas Group, the type unit of the belt, contains a glaciomarine sequence made up of thick diamictites, sandstones and Rapitan-type banded iron formations. The units exposed along the belt were metamorphosed under greenschist to amphibolite facies conditions and affected by thrusts, reverse faults and cratonward verging folds, developed between 575 and 530 Ma. The Aracuai orogenic front propagates into the craton interior and interacts with preexistent rift structures. This chapter describes the stratigraphic framework and overall structure of the Aracuai belt, emphasizing the paleogeographic and tectonic significance of its sedimentary and volcanic assemblages.

Proveniência e análise sedimentar da porção basal do Grupo Bambuí em Arcos (MG)

The origin and evolution of Bambui Basin has been a matter of debate, in much intensified by the recent exploratory efforts carried out by public and private companies looking for natural gas. In the southeastern portion of the basin there are rare opportunities to access the contact between the sedimentary rocks and its basement, whose characteristics are crucial to understanding the processes of basin installation and the early sedimentation. The analysis of drill cores allowed us to describe the lowermost Bambui Group in Arcos (MG) region, including its basement. The sampled section displays as basement an archean granodiorite (ca. 2.8 - 2.9 Ga), fractured at the time of sedimentation. The Bambui Group basal unit is dm-thick, massive lodgment tillite. Grains of zircon separated from the tillite matrix were dated and show a main age peak at ca. 2.8 Ga, indicating provenance from the own basement. On the tillite rests an impure limestone that passes gradually to a muddy unit, in a retrogradational filling trend. Terrigenous fragments in the impure limestone suggests that the basement has continued to be a source of sediments in the bottom of the section. The pelitic rocks that occurs to the top has a litochemical signature compatible with acidic source rocks, and shows Sm-Nd model ages of 1.7 Ga. These data suggests that rocks from the Brasilia Belt have acted as a source for the pelitic rocks, supporting the interpretation of a foreland set for the basin.

Estratigrafia da porção basal do grupo Bambuí na região de Arcos (MG): uma contribuição a partir de testemunhos de sondagem

This study presents the first results of the sample analysis of two boreholes drilled in a quarry located in the Arcos region (MG). Petrographic analysis (macro- and microscopic observations) was performed and made possible to study in detail an approximately 175 meter-section, which shows the basement and 10 sedimentary units. The basement is represented by dark green to gray fine- to medium-grained granodiorite. At the base of the sedimentary sequence, Unit 1 is represented by a decimetric layer of polymictic diamictite and is overlain by the impure limestones of Unit 2. Unit 3 is carbonatic and contains calcilutite and layers of carbonaceous shale. This unit gradually changes to Unit 4, which is composed of marl and mudstone. Unit 5 is composed of a thick sequence of massive calcarenites, which begins to show lamellar layers and microbial structures in Unit 6. Unit 7 is similar to the previous one, but contains several intraclastic layers. The number of lamellar layers decreases sharply, giving rise to a thick interval with considerable granulometric variations in Unit 8, which grades to the oolitic calcarenite of Unit 9. At the top of the sequence, Unit 10 is a thick package of stromatolitic dolarenite. This stratigraphic arrangement allows the identification of progradational/retrogradational trends. The sedimentological features, especially those present in the basal portion, suggest that at least part of the carbonatic sequence could represent a cap carbonate, supporting a glacial origin for the diamictite. Other data also collected from the basal units suggest that changes occurred in the source area at the beginning of the filling of the basin.

O cinturão epidérmico de antepaís da Bacia de Irecê, Cráton do São Francisco: principais elementos estruturais e modelagem física analógica

Located in the central portion of Bahia state, Irece Basin displays the best exposures of neoproterozoic sedimentary cover at Northern Sao Francisco Craton. Despite of the large amount of geological studies performed there, some questions remain unsolved, especially concerning the tectonic evolution of the thin-skinned fold-and-thrust belt that involves the rocks of the basin. In order to contribute to the understanding of such evolution, the present study reviews the main structural elements of the basin and surroundings, and present new data acquired through sandbox physical analog modeling. The Thin-skinned Fold-and-thrust Belt of Irece Basin is a great curved feature, confined in the homonymous syncline, whose genesis is related to the development of orogenic belts north of Sao Francisco Craton. Its evolution was conditioned by a N-S tectonic vector, responsible by the nucleation of E-W folds and thrusts. At basin boundaries, the deformation is accommodated by strike-slip faults, which locally rotated early structures. Towards south, the belt gradually loses its expression, only remaining structures related to the Chapada Diamantina thrust-and-fold system. The sandbox analog model successfully simulated the development of the Thin-skinned Fold-and-thrust Belt of Irece Basin, and indicates that its map-view curve results from the interaction with the syncline borders, as well as substrate geometry of the foreland belt. The propagation was made through a low-friction detachment, probably conditioned by the rheological contrast between the Una Group carbonates and the underlying Espinhaco Supergroup siliciclastic rocks.