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

Abstract

Disponível on-line no endereço www.igc.usp.br/geologiausp -49 - RESUMO A origem e evolução da Bacia Bambuí é tema de grandes debates, em muito intensificados pela recente corrida exploratória de empresas públicas e privadas em busca de hidrocarbonetos gasosos. Na porção sudoeste da bacia, são raras as oportunidades de se observar o contato basal das rochas sedimentares com seu embasamento, cujas características são cruciais para o entendimento dos processos de instalação da bacia e do início de seu preenchimento. Por meio de testemunhos de sondagem, teve-se acesso a toda a porção basal do Grupo Bambuí na região de Arcos (MG), incluindo seu embasamento. A seção amostrada exibe como embasamento um granodiorito arqueano (c. 2,8 -2,9 Ga), fortemente fraturado à época da sedimentação. A unidade basal do Grupo Bambuí é um tilito de alojamento maciço, polimítico, de espessura decimétrica. Grãos de zircão separados da matriz do tilito foram datados e exibem um pico principal de idades em c. 2,8 Ga, indicando proveniência do próprio embasamento. Sobre o tilito repousam calcários impuros, que passam gradativamente para um pacote pelítico, em tendência de preenchimento retrogradacional. Fragmentos terrígenos no calcário impuro sugerem que o embasamento tenha continuado a atuar como fonte durante a sedimentação dos metros iniciais da seção. As rochas pelíticas que ocorrem para o topo têm assinatura litoquímica compatível com fontes ácidas, e possuem idades-modelo Sm-Nd de 1,7 Ga. O acervo de dados sugere que rochas da Faixa Brasília tenham atuado como fonte para as rochas pelíticas, corroborando a interpretação de um caráter foreland para a bacia.Palavras-chave: Bacia Bambuí; Proveniência sedimentar; Zircões detríticos; Idade modelo; Arcos. ABSTRACTThe origin and evolution of Bambuí 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 Bambuí 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 Bambuí 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 mo...

C, O, and Sr isotopic variations in Neoproterozoic-Cambrian carbonate rocks from Sete Lagoas Formation (Bambuí Group), in the Southern São Francisco Basin, Brazil

“…The 87 Sr/ 86 Sr ratios between 0.7074 and 0.7080 indicate ambiguity in the age of the Bambuí Group (Misi et al, 2007;Babinski et al, 2007;Kuchenbecker, 2011;Alvarenga et al, 2014;Paula-Santos et al, 2015) in comparison with the seawater Sr isotope evolution curves proposed in literature (Jacobsen & Kaufman, 1999;Melezhik et al, 2001;Halverson et al, 2010;Kuznetsov et al, 2013). Considering the maximum depositional age of 560 Ma, such Sr isotope ratios are lower than the ones expected for the terminal Ediacaran.…”

“…values are correlated with those obtained by Kuchenbecker (2011), Alvarenga et al (2014), Warren et al (2014) and Paula Santos et al (2015) at the unit base. These carbonates are characterized by sedimentary structures as rhythmic lamination interlayered with pelites, clays and truncated irregular laminations, possibly associated with tidal shallow marine environment, typically attributed to Pedro Leopoldo Member described by Schöll (1976), Ribeiro et al (2003), Vieira et al (2007a, b) and Paula- Santos et al (2015), corresponding to the lower Sete Lagoas Formation.…”

“…Low radiogenic 87 Sr/ 86 Sr ratios between 0.7074 and 0.7080 have been reported for the Sete Lagoas Formation (Misi & Veizer 1998;Misi et al, 2007;Babinski et al, 2007;Kuchenbecker, 2011;Alvarenga et al, 2014, Paula-Santos et al, 2015. The latter authors suggest that the difference between the low radiogenic 87 Sr/ 86 Sr ratios commonly obtained for Bambuí Group and the ratios presented for the late Neoproterozoic (Melezhik et al, 2001;Halverson et al, 2010;Kuznetsov et al, 2013) may be due to the fact that carbonates of the Sete Lagoas Formation have been deposited in a restricted epi-continental sea, and its isotopic composition was not homogenized with contemporary global oceans (Paula- Santos et al, 2015).…”

Sedimentary provenance in the southern sector of the São Francisco Basin, SE Brazil

“…Provenance studies in the Carrancas Formation conglomerates reveal a wide range of clast assemblage varying from monomitic (Guacaneme et al 2017) to oligomitic (Kuchenbecker 2011) and polimitic , Uhlein et al 2016. The occurrence of the unit described by Guacaneme et al (2017) in Inhaúma city area is emblematic, as the conglomerate is clearly deposited in a basement valley and only has clasts of the leucogranite, which can be observed in the valley walls (Fig.…”

Quimioestratigrafia isotópica (C, O, Sr, Li, Mg) e proveniência sedimentar (U-Pb, Hf, Sm-Nd) do grupo Bambuí no sul da bacia do São Francisco

“…2, Kuchenbecker et al, 2013). Despite tectonic discontinuities (e.g., thrust faults along craton limits) the glacial unit has equivalents on the mobile belts that surround the craton: the Macaúbas Group on the Araçuaí orogen and the Ibiá Group on the Brasília Belt .…”