Timing of coalification of the upper carboniferous sediments in the upper silesia coal basin on the basis of by apatite fission track and helium dating

The apatite fission track and helium dating were used to provide a temporal framework for the coal rank data in the Upper Silesia Coal Basin. Measured apatite fission-track (AFT) central ages from sandstones and tonsteins in the USCB range from 259±11 (Permian) to 103±6 Ma (Early Cretaceous), with mean track lengths ranging from 11.7±0.2 to 13.7±0.1 μm. All AFT ages are younger than sample stratigraphic ages, indicating substantial post-depositional annealing. Samples from the western and central part of the USCB yield AFT ages of Permian to Late Triassic (259±11 to 214±10 Ma). Mean track lengths and unimodal track length distributions of these samples are indicative of a single relatively rapid Variscan cooling event to below 60°C consistent with erosion during the Asturian inversion of the basin. This was followed by slower cooling during the Mesozoic. The samples from the eastern and NE part of the USCB have AFT ages from Late Triassic to Early Cretaceous (210±10 Ma to 103±6 Ma). The relatively shorter mean track length and higher standard deviation, combined with a bimodal and/or mixed fission track length distribution in some samples, is indicative of amore complex thermal history with possibly a thermal event separated by a prolonged period in the PAZ. Apatite helium ages of samples from across the basin range from 144.1±11 to 108.1±8Ma (Early Cretaceous) indicating rather slow, post-Variscan inversion cooling or the possible mid-Mesozoic re-heating where temperatures reached only to 60-70°C. It was high enough for partial He loss from the apatite but not enough to anneal fission tracks in the most areas of the USCB. Only in the NE part of the USCB Mid-Mezozoic re-heating could be able to increase temperature to ~70-85°C causing partially resetting AFT (particularly during long stay in PAZ). Mid-Mesozoic re-heating would be caused by a hot fluid circulation related to extensional tectonic development. The timing and temperature range from thermochronological analysis imply that major coalification processes occurred in the latest Carboniferous period.