Palynological analysis (biochronostratigraphy and paleoenvironments) from a fossil-Lagerstätte section within the Late Paleozoic Ice Age unit of the Paraná Basin (Itararé Group), Western Gondwana

Abstract

A diverse and exceptionally preserved fossil association occurs in Campáleo Outcrop black shales of the upper Itararé Group, correlated to the Lontras Member, considered representative of the Pennsylvanian–Permian transition in the Paraná Basin, Brazil. In this paper, we present the palynological records from 20 stratigraphic horizons of these shales and related strata, which revealed 21 taxa of spores, 33 of pollen grains, two species of fungi, one species of Chlorophyceae and two species of Prasinophyceae. The section is correlated to the Vittatina costabilis Zone, due to the records of certain guide species, such as Converrucosisporites confluens, Illinites unicus and several species of Vittatina. An additional subsurface section (TC-4 core) partially correlated with the Campáleo Outcrop and comprising the passage of the Crucisaccites monoletus and Vittatina costabilis palynozones was also studied in order to support the stratigraphic positioning. Once combined with the stratigraphic positioning in the basal portion to the Lontras Shale, it allows us to consider it around the Pennsylvanian–Permian boundary, probably in the Gzhelian to age of the Campáleo Outcrop. We observed distinct palynofloristic compositions from (i) the rhythmic shale with dropstones (basal portion) and from (ii) the fossiliferous black shales (upper portion). The former presents a large dominance (about 90 %) of pollen grains (xerophylous elements) and low frequency (about 7 %) of spores (meso-hygrophylous), while the results of the latter are conversely. Marine palynormorphs were recorded herein only from the rhythmic shale. The palynological assemblages on the base of the Campáleo Outcrop demonstrated a more intense sediment supply, bringing sporomorphs from more distant regions being represented mainly by xerophylic flora, indicating the existence of regions devoid of ice cover, where forests could develop, as occur in seasonal climate regions. The upper level of the Campáleo Outcrop characterizes one of the predominance of spores (hygrophylic communities) in these palynological associations supported by the large number of insects found that indicates proximity to the coastline, little transport (spore tetrads), poor contribution of more distant continental sediments (low expressiveness of pollen grains) and low salinity water, due to the absence of marine palynomorphs, representing a reducing restricted marine environment. This data integrated with the stratigraphic context and other geological data evidenced changes in sea level due to the climatic oscillations in the course of the LPIA that influenced the sediment supply and the distance of the coastline in this region of the Paraná Basin.