THERMAL AND BURIAL HISTORY OF THE CRACOW-SILESIA REGION (SOUTHERN POLAND) ASSESSED BY CONODONT CAL ANALYSIS

Conodont colour alteration (CAI) data have been applied in the reconstruction of palaeothermal conditions in the Cracow-Silesia region, southern Poland. The analysis of conodont CAI values supplemented by electron spin resonance (ESR) data provided a complex maturity pattern which coincides well with the tectonostratigraphical divisions of this region during the Variscan and Alpine diastrophic cycles. The Palaeozoic rocks in the Upper Silesian Coal Basin attained the observed thermal maturity during Late Carboniferous or Permian time just after the tectonic inversion of the basin. Vertical maturity trends suggest that palaeo-geothermal gradients were significantly higher than the present values. The irregular maturity pattern recognized in the Cracow-Myszkow Zone is related to the thermal overprint of intrusive igneous rocks on the maturity field resulting from the relatively shallow burial. This pattern has not been modified by Mesozoic hydrothermal events. The low maturity level of Devonian and Early Carboniferous rocks in the substrate of the Carpathian Foredeep is a product of the subsidence during Miocene time. The implications for hydrocarbon generation and further exploration are discussed. The CAI data are consistent with other indicators of organic metamorphism previously used in this region. The study highlights also features about the conodont colour alteration, such as the influence of silica mobilization on conodont colour, which limit the application of the CAI method.