Abstract Soft-sediment deformation (SSD) structures are widespread in the sedimentary record, and numerous triggering mechanisms can induce its development, including glaciation, earthquakes, overloading, ground-water fluctuations, and wave movement. The Late Cretaceous Uberaba Formation preserves SSD structures as small- and large-scale load casts and associated flame structures, pseudonodules, and convolute laminations observed in the contact of three well-defined intervals among fine- to coarse-grained lithic and conglomeratic sandstone with fine-grained arkose and mudstone beds. Based on the morphology of the SSD structures, sedimentary facies of the Uberaba Formation, and similarities with previous observations in the geological record and laboratory models, these features are assigned to liquefaction-fluidization processes as the major deformational mechanism triggered by seismic and aseismic agents. We propose that a deformation occurred just after the sedimentation triggered by seismic shock waves and overloading, induced by the sudden deposition of coarse-grained sandy debris on fine-grained sediments. Some of these structures can be classified as seismites, providing evidence of intraplate seismicity within the inner part of the South American Platform during the Late Cretaceous. This seismic activity is likely related to the uplift of the Alto Paranaíba High along reactivations of regional structures inherited from Proterozoic crustal discontinuities and coeval explosive magmatism of the Minas-Goiás Alkaline Province.

RESUMO: Após o aparecimento dos metazoários, o início da bioclasticidade e a competição pelo substrato marinho, a saga das esteiras microbiais é marcada por um progressivo declínio. Pontuais aumentos em sua diversidade e na produção de carbonatos durante o Paleozoico em geral estão associados a eventos globais de extinção em massa. Ao longo do Fanerozoico, as colônias microbiais ocuparam nichos ecológicos específicos e ambientes livres de organismos predadores caracterizados por condições ecológicas extremas, como aqueles encontrados em baías hipersalinas, lagos alcalinos e locais sujeitos a hidrotermalismo. Neste trabalho é reportada uma das mais antigas ocorrências de um campo de exudação de metano no registro geológico associado com microbialitos e estruturas sedimentares induzidas por atividade microbial. Nesse sistema, durante a atividade de exalação de metano, instalaram-se condições de extrema anoxia e alta salinidade, proibitivas para a manutenção da vida animal, permitindo, desse modo, o florescimento de colônias microbiais. Por causa das severas imposições do ambiente, os organismos microbiais tornaram-se altamente adaptáveis às condições adversas então vigentes. Nós sustentamos a hipótese de que aumentos na diversidade e na quantidade de colônias microbiais na história da Terra não estão apenas restritos a períodos que sucedem à extinção em massa de organismos predadores de esteiras, mas ocorrem em resposta a condições ambientais localizadas que proporcionam o crescimento de colônias microbiais. De fato, o registro geológico Fanerozoico é pontuado de exemplos semelhantes ao aqui reportado, simulando uma espécie de jogo de esconde-esconde das condições ambientais vigentes no Éon Pré-cambriano.

ABSTRACT: Following the rise of metazoans, the beginning of bioclasticity and substrate competition, the saga of microbial mats was in a fluctuating decline in the end of the Neoproterozoic era. Increases in diversity during the Phanerozoic and punctual upturns in the microbial carbonate production occured after the events of global mass extinctions. Gradually along the Phanerozoic, the microbial colonies occupied isolated niches and grazers-free environments, characterized by physically and/or geochemically stressful conditions, such as those found in saline bays, alkaline lakes and hydrothermal or cold seep vents. Here we report one of the oldest occurrences of a vent camp coupled with cold seepage of methane in the geologic record, associated with well-preserved microbialites and elephant skin structures. During the seep activity, oxygen depletion and high salinity conditions are prohibitive for complex animal life, clearing the way to microbial colonies to flourish. Due to the co-occurrence of high adaptability and low competitiveness of microbial forms, they became highly specialized in stressful conditions. We argue that the sporadic microbial mat upturns in Earth’s history are not restricted to geological periods, following massive death of metazoan species; they also may occur in response to punctual paleoenvironmental conditions that enable microbial colonies to growth. Indeed, the Phanerozoic geological record is punctuated of these examples, in a kind of hide-and-seek game of Precambrian times.