Evidence of wildfires during deposition of the Upper Silesian Keuper succession, southern Poland

Abstract

Charcoals from the Upper Triassic vertebrate-bearing clays of the Zawiercie area (Upper Silesia, S-Poland) were analyzed using petrographic methods, to reconstruct burning temperatures as well as taphonomic processes. SEM and reflected light microscopy show excellent preservation of charcoals most probable connected with early diagenetic permineralization by calcite. The charcoal was assigned to three morphotypes, probably corresponding to three different fossil taxa. Fusinite reflectance data suggest, that the highest temperature reached above 600 °C (fusinite reflectance of 3.59%), what counterparts to the lower limit crown fire temperature. The values for most of the samples are lower (ca. 1% to 2.5%) what is typical for surface fires. In many cases fusinite reflectance values depends on the measured zone within the sample. Such zonation formed due to charring tem- perature differences. In zones remote from the potential fire source, reflectance values gradually decreases. It implies that calculation of fire temperatures based on average fusinite reflectance values might be too far-reaching simplification. Occurrence of fungal hyphae within the charcoal supports the interpretation of a predomination of surface fire, consuming dead twigs and stems.

The low content of micro-charcoals in charcoal-bearing rocks as well as rounded to sub-rounded shapes of large specimens indicates that they were transported after burning, deposited away from the burning area, and fi- nally early diagenetic mineralization.