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The rise and fall of the giant stromatolites of the Lower Permian Irati Formation (Paraná Basin, Brazil) : a multi-proxy based paleoenvironmental reconstruction.
(2022) Antunes, Gonçalo Cruz; Warren, Lucas Veríssimo; Okubo, Juliana; Fairchild, Thomas Rich; Varejão, Filipe Giovanini; Uhlein, Gabriel Jubé; Inglez, Lucas; Poiré, Daniel Gustavo; Rumbelsperger, Anelize Manuela Bahniuk; Simões, Marcello Guimarães
Giant stromatolites are meter-scale laminated carbonate biosedimentary deposits formed by the action of benthic microbiota under very specific conditions. Although occurrences of giant stromatolites are relatively common in Precambrian deposits, the Phanerozoic record is still sparse. Here, we carried out an integrated analysis of the Lower Permian Santa Rosa de Viterbo giant stromatolite field, developed in a mixed carbonate-siliciclastic depositional system within a restricted intracontinental basin (Irati Formation, Paran ́ a Basin, Brazil). Using available and new descriptions of stromatolite morphology and associated facies, we applied a multi-proxy approach based upon sedimentological, paleontological, geochemical, and isotopic data to develop a detailed paleoenvironmental model for this particular occurrence. The NE-SW elongated giant stromatolites – of >3 m in height, > 7 m in length, and > 1 m wide – have variable external shape and internal morphology, indicating changing growth strategy due to variations in the hydrodynamic conditions, bathymetry, and terrigenous input. Increasing δ13C values towards the top of the succession are related to intense microbial activity, increased nutrient supply, and enhanced primary productivity, with the higher δ13C values matching the global Permian seawater signal. Y/Ho and La/La* ratios indicate a less restricted setting towards the top of the succession, which is consistent with the increasing water depth recorded by the stromatolite morphology. The combination of smooth lamination, exclusively coccoidal microbial community, and fossil content, point to deposition under high (possibly hyper) salinity conditions for most of the succession. Our data suggests that the existence of stressful conditions (i.e., strong currents and high salinity) protected the benthic microbial communities from predation and favored EPS production, generating the ideal conditions for the growth of giant stromatolites, a very uncommon situation in the Phanerozoic.
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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