Microbiota fóssil em sílex da Formação Assistência (Subgrupo Irati, Permiano, Bacia do Paraná) no Estado de São Paulo

Abstract

The study of petrographic thin sections of early diageneteic chert from diverse levels and localities of the Assistência Formation in the state of São Paulo, Brazil, revealed for the first time an exceptionally well-preserved assemblage of delicate fossil microorganisms in this important stratigraphic and paleontological Early Permian marker unit of the Paraná Basin. This assemblage consists primarily of delicate colonial and solitary unicellular microfossils, dominated by cyanobacteria, without any evidence whatsoever of filamentous microorganisms. It also includes a probable cocoidal chlorophyte, pollen grains and phytoclasts, as well as several less common microfossils of uncertain biological affinity. None of the delicate microfossils of this assemblage have ever been detected in palynological analyses of organic residues from this formation. The study of thin sections made it possible to observe not only all of the fossil microorganisms in three dimensions within the rock but also their original spatial distribution and relationships with other components of the rock. This facilitated evaluation of the morphological diversity of the fossil microorganisms and permitted inferences as to possible ontogenetic patterns. Fourteen morphotypes were recognized among the more delicate microfossils and attributed to five species (all new) of known biological affinities (four species of cyanobacterias and one chlorophyte) and five taxa of Incertae Sedis. The fossil microbiota occurs principally at the base of the formation as dense, voluminous masses interpreted as remains of an in situ benthonic microbiota of photosynthetic microorganisms capable of forming microbial mats and small stromatolites. The sedimentology, together with the widespread distribution, habit, abundance and exclusively unicellular nature of the fossil microorganisms and the paleoenvironmental implications suggested by analogous modern examples, are consistent with a shallow aquatic habitat of high and perhaps variable salinity for the microbiota.

Petrographic approach to the study of organic microfossils from the Irati Subgroup (Permian, Paraná Basin, Brazil)

“…Early diagenetic chert is the medium that best conserved the original form, orientation, and position of benthic microorganisms, palynomorphs, and amorphous organic matter within the Assistência Formation, especially within cyanobacterial mats in the Brecciated Evaporite Bed of the Morro do Alto Member (Figs. 2b and 3d) (Calça and Fairchild, 2005;Calça, 2008). This chert clearly formed very quickly, very early, and at very shallow depths in the sediment, as shown by the compaction of laminae around nodules of chert in both carbonate and shale, the presence of chert intraclasts within synsedimentary breccias (Fig.…”

A stratigraphic chart of the Late Carboniferous/Permian succession of the eastern border of the Paraná Basin, Brazil, South America

“…Cyanophyta and Chlorophyta microfossils were discovered in early diagenetic silex formed in the basal contorted carbonatic interval of the Assistência Member in São Paulo (Calça and Fairchild, 2005;Calça, 2008). In the same interval, microbialites were recognized by Araújo (2001).…”

Tafonomia dos Pygocephalomorpha (Crustacea, Peracarida, Malacostraca), Permiano, bacia do Paraná, Brasil, e seu significado paleoambiental

“…Assim sendo, esse esquema tem sido especialmente útil em campo, levando vários autores a adotarem a mesma decisão metodológica (e.g. Calça, 2008;Ricardi-Branco et al, 2008). Nessa porção da bacia do Paraná, a parte basal da Formação Assistência, Membro Morro do Alto (Figura 1), apresenta a Camada de Dobras Enterolíticas, composta por folhelhos e carbonatos altamente perturbados (Hachiro, 1991).…”