Stressing environmental conditions in the “petrified forest” from the Mata Sequence in the Triassic context of the Paraná Basin

Abstract

Listen

Translate article

This study deals with the Piscina Outcrop, a Rhaetic reddish conglomeratic sandstone containing silicified logs which corresponds to the Mata Sequence in the southernmost Brazilian Paraná Basin. We performed ground-penetrating radar (GPR) analysis and the results ratified previous sequence stratigraphy analysis indicating that these deposits correspond to architectural elements exclusively related with the filling processes of fluvial channels. The erosive unconformity identified at the boundary between the Mata Sequence and the underlying rock packages could represent a considerable depositional hiatus. The integration of the GPR results with sedimentological and paleovertebrate data from a nearby outcrop at slightly lower topographic level indicated that the layer with the fossil woods is superimposed through an erosive unconformity to the stratigraphic levels corresponding to the Hyperodapedon Assemblage Zone of Carnian age from the basal portion of the Candelária Sequence. The wood assemblage was composed exclusively of araucarian coniferous stems identified as Agathoxylon (cf. Agathoxylon). The wood growth patterns were mainly characterized by woods showing indistinct growth rings, with latewood mostly restricted to one cell in each ring, suggesting weak seasonality and short-term, periodic droughts. Rare specimens with absent growth rings were also present. Oscillations in growth ring width and the common occurrence of false rings were likely linked to plant exposure to climatic instability. Anatomical evidence pointed to active biochemical responses to intense and extensive biological attack of fungi and arthropods. The dominant presence of carbon in the elemental composition of a dark substance filling many tracheid lumens suggested a very likely affinity with polyphenol remnants in biochemical barriers. The set of wood signatures seems to be linked to regional stressing conditions rather than to the paleolatitudinal location when compared to other key South American basins during the Rhaetic. An allochthonous taphocoenosis is the most likely taphonomic interpretation for this wood assemblage. The logs were probably incorporated into the stream through the collapse of riparian areas and buried in the fluvial channel by high-energy flows after biotic and abiotic decay processes during lifetime and secondarily through exposition before entombment in a river channel.