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The impact of benthic microbial communities in sediment dispersion and bedform preservation : a view from the oldest microbially induced sedimentary structures in South America.
(2022) Warren, Lucas Veríssimo; Varejão, Filipe Giovanini; Quaglio, Fernanda; Inglez, Lucas; Büchi, Fernanda Miranda de Siqueira; Simões, Marcello Guimarães
The influence of microbial communities upon sedimentary dynamics is an issue of increasing significance. Over the last decades, studies have revealed a particular class of sedimentary structures and textures produced by the interaction among distinct flows, marine substrate, and ben- thic microbial communities. We present evidence of the oldest record of microbially-induced sedimentary structures (MISS) in South Amer- ica, as recorded in low-grade metasedimentary rocks of the Early Mesoproterozoic (~1536 ± 33 Ma) Tiradentes Formation, state of Minas Gerais, SW Brazil. Types 1, 2, and 3 correspond to wrinkle, pustular, and dome structures related to flat or rippled bed surfaces, preserved in metasandstone deposited in shallow marine settings. Evidence supporting the microbial origin of these structures includes delicate morphol- ogy, degree of alignment, presence of original mat cover, and orientation of quartz grains indicating biostabilization. The presence of distinct MISS associated with well-preserved ripple marks, wrinkled surfaces, and flatbeds suggests deposition under varied energy conditions with different potentials for the preservation of surficial structures. Vertical growth of microbial communities influences sediment cohesion and stability. This reduces substrate roughness and, as a consequence, wave and current shear, thereby increasing the preservation potential of bedforms and delicate features of their bedding surfaces.
Evidences of seismic events during the sedimentation of Sete Lagoas Formation (Bambuí Group – Ediacaran, Brazil).
(2020) Okubo, Juliana; Warren, Lucas Veríssimo; Luvizotto, George Luiz; Varejão, Filipe Giovanini; Quaglio, Fernanda; Uhlein, Gabriel Jubé; Assine, Mario Luis
The Sete Lagoas Formation (Ediacaran), located in the central part of the São Francisco Craton (Brazil), consists of limestones and dolostones deposited in very shallow waters in the inner part of the carbonate platform. Four breccia types occur throughout the stratigraphic succession: evaporitic breccia with tepees, flat-pebble breccia, hydrothermal breccia and brecciated stromatolites. Here we combine a detailed sedimentological and strati- graphic analyses of the flat-pebble breccia in order to determine its origin and the processes and environmental conditions that originated these brecciated facies. The studied interval consists of a 20 m thick succession of tabular beds composed of flat-pebble breccia interbedded with laminated microbialites. In these breccia beds, the clasts are usually platy or oblate with angular edges and are mainly disposed horizontally within the sedi- mentary bed, suggesting that they were not transported or reworked. The presence of microbialite clasts with sharp edges and vertices in the Sete Lagoas flat-pebble breccia suggests that the lithification process started very early in diagenesis and, even the sediments exposed at the bottom were, at least, partially lithified. Some breccia levels show bidirectional imbrication and clast size analyses reveal a NE-SW long-axis clast orientation whereas square clasts tend to fill the space among oriented clasts. Breccia clasts are vertically oriented and show de- formation features increasing upwards, typically of deforming beds formed by ascendant expulsion of liquefied sediment. Disrupted layers or presenting folds and synsedimentary faults commonly occur confined between undeformed beds. Other evidences of liquefaction and soft-sediment deformation are the injection structures, as flame and load cast-like features, in the base of the brecciated beds. These structures commonly penetrate the upper bed and disrupt the sedimentary layer immediately above promoting local brecciation. These features are found both in modern and ancient deposits of seismic influence, which suggests a similar origin for the Sete Lagoas flat-pebble breccia. Thus, the processes that led to the formation of the studied flat-pebble breccia are interpreted as seismically triggered, since: a) the breccia beds are laterally continuous and extend for several kilometers; b) the breccia beds are restricted to a 20 m thick stratigraphic interval; c) the clasts of the breccia are the same lithology of non-deformed beds below and above the interval of breccia; d) the interbedding of breccia beds and laminated microbialite beds is recurrent; e) the breccia beds are subhorizontal and present irregular upper and lower contacts; f) the presence of liquefaction structures and dyke injection. Thus, this seismic-trig- gered breccia deposits represent the product of the synsedimentary tectonism occurred within the São Francisco Craton during the terminal Ediacaran and correspond to a very well-defined local stratigraphic marker in the Bambuí Basin. The seismic activities could be related to the NW regional faults in the regional Paleoproterozoic basement of the study area, which were reactivated during the deposition of the Sete Lagoas Formation in the Ediacaran Period.

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