Sedimentology of the Psammo-pelitic-carbonate Unit, Paranoá Group, and Sete Lagoas Formation, Bambuí Group: examples of mixed carbonatesiliciclastic sedimentation in the Proterozoic of the Brasília Fold Belt

Abstract

The mixed carbonate-siliciclastic platforms are still poorly studied, although they represent important regional sedimentary successions in the Brasília Fold Belt. The aim of the present paper was to determine the deposition conditions of the Psammo-pelitic-carbonate Unit (top of the Paranoá Group) and the Sete Lagoas Formation (base of the Bambuí Group) in the Brasília Fold Belt, central Brazil. The sedimentation of the two sequences occurred in a mixed platform controlled by the bottom paleogeography where carbonate facies were deposited in shallow environment and detrital facies in deeper water. In the Paranoá Group, small limestone and dolomite lenses are found in the highest areas, whereas pelites were deposited in depressions and psammites inplatform channels. In the Sete Lagoas Formation, the major carbonate lenses were deposited over the highs derived from the post-glacial erosion surface that marks the unconformity between the Paranoá and Bambuí groups. In both units, micritic mud predominates, and reworking by waves occurred sporadically and associated with progradationretrogradation. The aggradation of terrigenous material caused deposition of pelitic facies. The dimensions, thickness, and distribution of carbonate lenses show that the paleorelief was more pronounced during the deposition of the Sete Lagoas Formation, when compared with the Psammo-Pelitic-Carbonate Unit. On the other hand, the sedimentation rate of Psammo-Pelitic-Carbonate Unit was greater than the one of the Sete Lagoas Formation.Keywords: Carbonate-siliciclastic platform; Bambuí Group; Paranoá Group. ResumoSedimentologia da Unidade Psamo-pelito-carbonatada, Grupo Paranoá e da Formação Sete Lagoas, Grupo Bambuí: exemplos de sedimentação mista carbonática-siliciclástica no Proterozoico da Faixa Brasília. Apesar de muito importantes e de ocorrerem em várias sucessões litoestratigráficas na Faixa Brasília, as plataformas mistas de sedimentação silicilástica-carbonática ainda são pouco estudadas. O presente trabalho objetivou a determinação das condições deposicionais da unidade de topo do Grupo Paranoá (Unidade Psamo-Pelito-Carbonatada) e da Formação Sete Lagoas (base do Grupo Bambuí) na Faixa Brasília. A sedimentação das duas sucessões se deu em uma plataforma mista controlada pela paleogeografia de fundo em que as fácies carbonáticas foram depositadas em águas mais rasas e apenas ocasionalmente mais agitadas e as fácies terrígenas ocupavam as águas mais profundas. No Grupo Paranoá, as pequenas lentes de calcário e dolomito ocupavam as áreas mais elevadas, enquanto os pelitos se depositavam nas depressões e os psamitos correspondem aos canais na plataforma. Na Formação Sete Lagoas, a deposição das maiores lentes de rochas carbonáticas se dava sobre as elevações herdadas da superfície erosiva pós-glacial que marca a discordância entre os grupos Paranoá e Bambuí. Nas duas unidades, o processo de precipitação de lamas micríticas foi predominante nas fácies carbonáticas, enquanto o retrabalhamento por base de ondas ocorria de forma...

Contrasting nutrient availability between marine and brackish waters in the late Mesoproterozoic: Evidence from the Paranoá Group, Brazil

“…The sedimentary strata of the Paranoá Group are approximately 1000 m thick and divided into nine lithostratigraphic units that were deposited during two transgressive and one regressive cycle (Alvarenga et al, 2014;Campos et al, 2012). Most of the Paranoá Group is made up of siliciclastic rocks including sandstones, siltstonesandstone rythmites and shales.…”

“…These lenses are several tens of metres thick and extend over hundreds of metres along strike (Alvarenga et al, 2014). They are interfingered with pelitic and psammitic rocks, which also occur as intraclasts (Campos et al, 2012). In outcrop, the carbonate units are classed as mudstones and intraclastic grainstones, packstones and floatstones with local conical or columnar and rarely dome-shaped stromatolites.…”

“…In outcrop, the carbonate units are classed as mudstones and intraclastic grainstones, packstones and floatstones with local conical or columnar and rarely dome-shaped stromatolites. Campos et al (2012) proposed that the carbonate lenses represent topographic highs during the time of deposition. Tidal indicators within the siliciclastic sedimentary rocks, as well as micritic rip-up clasts from some of the carbonates (Campos et al, 2012) suggest that-with important exceptions highlighted below-sediment deposition largely occurred on an open platform environment with frequent storm events and sporadic subaerial exposure (de Morrison Valeriano, 2016).…”

“…Campos et al (2012) proposed that the carbonate lenses represent topographic highs during the time of deposition. Tidal indicators within the siliciclastic sedimentary rocks, as well as micritic rip-up clasts from some of the carbonates (Campos et al, 2012) suggest that-with important exceptions highlighted below-sediment deposition largely occurred on an open platform environment with frequent storm events and sporadic subaerial exposure (de Morrison Valeriano, 2016). The stromatolitic strata investigated in this study come from the upper part of the Paranoá Group and were deposited during the second transgression cycle (Campos et al, 2012).…”

Facies Association, Lithochemistry and Isotope Geology of the Lagamar Formation (Vazante Group, Minas Gerais): A Carbonate Platform in Mesoproterozoic

“…Um exemplo de uma estrutura recifal bem preservada em rochas mesoproterozoicas é a Formação Victor Bay (Canadá), onde um recife estromatolítico apresenta quatro tipos de recifes distintos, refletindo diferentes fatores dinâmicos paleoambientais e do nível do mar (Narbonne & James, 1996). Outro exemplo regional é o Grupo Paranoá, onde Campos et al (2012) descrevem plataforma mista carbonato-siliciclástica durante um ciclo regressivo/transgressivo. Santos et al (2000) Em ambas as seções, os elementos terras raras e ítrio (∑ETR+Y), são entre 15,52 e 47,00 ppm e a razão Y/Ho varia de 28,05 a 39,00.…”