Charcoalified logs as evidence of hypautochthonous/autochthonous wildfire events in a peat-forming environment from the Permian of southern Paraná Basin (Brazil)

Abstract

Evidences of paleowildfires from the upper coal seam S from the Faxinal Coalfield have been analyzed through petrographic, fluorescence, and scanning electron microscopy (SEM) specifically at the base (1) and top (2) boundaries of an interlayered tonstein bed and revealed recurrent charcoal deposition in the peat-forming environment. An unusual accumulation of adjacent, large-sized (21.8×13.4cm and larger) charred logs occurs throughout the coal seam area (1.6km2) beneath the tonstein bed (boundary 1) reflecting an extensive wildfire event in hypautochthonous/autochthonous conditions. The logs display a semifusinite–vitrinite–semifusinite sandwich-like structure (sensu Jones et al., 1993) indicating burning in growth position, and reflectance measurements below 1%Ro suggest that this was a low-temperature fire. Dryness in the peatland during the fire event was clearly detected by altered fluorescence of spores and algae. Corroborating previous studies, the paleoecological setting can be characterized as a swamp forest based on the presence of pollen clusters and organs observed under fluorescent light and on the identification under SEM of the morphogenus Agathoxylon Hartig as a single wood type, reflecting the dominance of arborescent gymnosperms (glossopterids) in the peatland. Homogenized cell walls observed under SEM ratified the recognition of the material as charcoal. The reestablishment of regular conditions of peat deposition following the major fire episode is represented by alternate bands of vitrinite and semifusinite in both boundaries 1 and 2, suggesting an oscillation from dry to wet conditions in the climate pattern and the occurrence of wildfires as common and systemic events. Fire ignition, activity, and spread in the peat-forming environment resulted from a combination of lightning strikes, dryness, fuel abundance, and high atmospheric oxygen levels.