Depositional cyclicity and paleoecological variability in an outcrop of Rio Bonito formation, Early Permian, Parana Basin, Rio Grande do Sul, Brazil

摘要

This article integrates faciological, paleobotanical, and palynological analyses to establish the relationship between depositional cyclicity and paleoecological patterns for the (Early Permian) Quiteria outcrop, Rio Bonito Formation, southern Parana Basin, Rio Grande do Sul state. The basal section of this outcrop represents a coastal lagoon depositional system protected by barriers in microtide conditions, where peat-forming conditions developed in lowlands with ingression of distal alluvial fan deposits. The upper clastic section represents different environmental conditions, originated by the barrier sectioning brought by washover fans. The palynoflora identified in the basal section present a dominance of spores produced by arborescent and herbaceous lycophytes, as well as by sphenophytes and filicophytes, complementary forms of gymnosperm pollen grains. Algae or algae-elements, indicative of fresh, brackish, or marine water, are recorded together with terrestrial spores and pollen grains. The palynological content of matrix-supported conglomerates suggests a close, qualitative similarity with the coaly facies; however, the increase in gymnosperm pollen grains accompanied by a decrease in spores produced by pteridophyte vegetation is remarkable. The autochthonous roof-shale flora related to the clastic upper section is composed of basal stumps of arborescent cormose lycophytes (Brasilodendron pedroanum), understory vegetation (Botrychiopsis valida, Lycopodites sp.), small trees (Coricladus quiteriensis), undetermined filicoid rodheopterid fronds, and parautochthonous elements (Rubidgea sp., Cordaites sp.). Palynofloras point to floristic similarity between the roof-shale floras and the subjacent coal-forming parautochthonous floras. Correlations between the floristic data and those from high-resolution sequence stratigraphic methods indicate that this interval is associated with the final parasequences of the transgressive tract of Rio Bonito Formation, where coal levels are thin. The record in some coal palynofloras of Striadopodocarpites fusus, a component of the Hamiapollenites karrooensis subzone, as defined in the palynostratigraphic framework for the Parana Basin, indicates a Kungurian age for the palynoflora.