Marcelo A. Martins-Neto

Tectonics and sedimentation in a paleo:mesoproterozoic rift-sag basin (Espinhaco basin, southeastern Brazil)

Abstract Sedimentologic, paleogeographic, stratigraphic, structural and tectonic studies in a paleo/mesoproterozoic metasedimentary succession (Espinhaco Megasequence, southeastern Brazil) indicates deposition in a rift-sag basin. Four basin evolution stages are recognized (prerift, rift, transitional and flexural). The four stages can be represented by six unconformity-bounded tectonosequences. The unconformities are recognized in the field and mappable even on a regional scale. The prerift and rift stages of the Espinhaco basin were filled by products of continental depositional systems. The prerift stage probably represents the first product of the rifting process, before the development of the half-grabens that characterize the rift stage. During the rift stage, mechanical subsidence due to lithospheric stretching was predominant and led to episodic rising of the depositional base level. As a result, the basin fill is characterized by coarsening-upward intervals. Paleocurrent patterns indicate that block tilting and half-graben subsidence/uplift controlled sediment dispersion. The first marine incursion within the Espinhaco basin marks the change in the subsidence regime of the basin. The evolution of the transitional and flexural stages was probably controlled by thermal subsidence due to thermal contraction of the lithosphere during cooling. The transitional stage was characterized by relatively low subsidence rates. Higher subsidence rates and a consequent sea-level rise characterize the flexural stage of the Espinhaco basin, in which three second-order transgressive-progradational sequences can be recognized.

Positive inversion of extensional footwalls in the southern Serra do Espinhaço, Brazil - insights from sandbox laboratory experiments

Analogue experiments were carried out to get insights into the processes governing positive inversion during the foreland propagating thrust tectonics in the southern Serra do Espinhaço, a Brasiliano/Panafrican foldthrust belt in southeast Brazil. In particular, model listric half-grabens were inverted by applying contractional displacement to the footwall blocks. We investigated two different inversion conditions in listric half-grabens: (i) extensional and contractional detachments at the same level and (ii) at different positions. The models revealed that the development of a forward-breaking thrust system occurs in the basin synrift deposits, by contractional translation of the extensional footwall block when the extensional and contractional master faults do not coincide. Our experiments show the tectonic imbrication between basement and synrift sequences which characterizes the southern Serra do Espinhaço, and support the location in the eastern mountain range domain of the Espinhaço rift master fault system, which is not exposed at the surface.

CLASSIFICAÇÃO DE BACIAS SEDIMENTARES: UMA REVISÃO COMENTADA

During the evolution of the knowledge regarding sedimentary basins, many authors have discussed criteria and proposed schemes for their classification, most of them driven by geotectonic theories like Geossinclinal and Plate Tectonics. Firstly, the present paper discusses the definition of a sedimentary basin, since there are two different approaches in the literature. The first is based on geodynamic criteria, where basins are defined by their linked subsidence mechanisms, related to a single tectonic regime or tectono-thermal event, responsible for the development of a first-order basin cycle. According to the second approach, a basin is defined as a region characterized by the accumulation of a thick sedimentary package during a long time span, regardless the existence of significative hiatuses and/or unconformities inside the package and the stacking of more than one first-order cycle. After a historical synthesis about the classification of sedimentary basins, the merits and efficiency of classification proposals are discussed, including the particular case of the intracratonic basins, as well as perspectives for the future. It is concluded that the basic classification criteria proposed by Dickinson (1974), although incomplete and/or deficient in some aspects, are very efficient for practical purposes, mostly by emphasizing the type of plate interaction (tectonic regime) during sedimentation (if divergent, convergent or strike-slip). The approach explaining the evolution of basins based on plate interactions solved the major problem of the previous classification schemes that was the proliferation of names for particular cases, as well as has direct implications in terms of stress field, structural and stratigraphic styles, being very efficient in the definition of the type of basin and having a predictive character in terms of processes and products.

REPLICA DA DISCUSSÃO DE ALMEIDA-ABREU, P.A. e RENGER, F.E. - Sobre o Supergrupo Espinhaço em Minas gerais: registro de uma bacia rifte-sag do Pelo-/Mesoproterozóico

Em primeiro lugar, gostaria de discordar dos senhores Almeida Abreu & Renger em relacao ao constrangimento em contestar o trabalho de outros colegas, na medida em que acho qualquer discussao cientifica valida e, sobretudo, porque ninguem e dono da verdade, principalmente em se tratando de geologia do Pre-Cambriano, onde sempre se carrega uma grande dose de interpretacao. Acho que evoluimos quando conseguimos, em uma primeira instância, separarmos hipoteses de trabalho de modelos e estes de fatos cientificos e, em segunda instância, quando conseguimos assimilar nossas deficiencias e aprendemos com nossos colegas a trilhar com mais consciencia o desvendar dos misterios geologicos, sobretudo tendo a capacidade de avancar, recuar, mudar de opiniao, etc., de crescer junto com nossas ideias.