Palaeo-thermal and coalification history of Permo- Carboniferous sedimentary basins of Central and Western Bohemia, Czech Republic: first insights from apatite fission track analysis and vitrinite reflectance modelling

摘要

Apatite fission track analysis (AFTA) and vitrinite reflectance (VR) modelling were integrated to reveal palaeothermal evolution of the Upper Carboniferous coal-bearing basins of Central and Western Bohemia. Thermal modelling using AFTA has shown that the Carboniferous sediments were subjected to maximum temperatures ranging from ?75 °C in the SW to more than ?120 °C in the NE, which generally concluded during pre-Triassic times. Thermal records show a gradual cooling between ?280-180 Ma, followed by a long Mesozoic to Cenozoic Period of thermal stability, during which time, the sediments resided at constant temperatures of ?50-75 °C. From ?30 Ma onwards, accelerated cooling and erosion occurred, resulting in the present-day exposure of rocks on the surface. The EASY%Ro modelling of average VR values that range from 0.59% Rr in the SW to 0.77% Rr in the NE, predicted maximum palaeo-temperatures ranging from ?85 °C to ?135 °C, respectively; these were attained soon after sediment deposition. A coalification grade of about 0.40-0.50% Rr characteristic of sub-bituminous coals, was already completed during the late Carboniferous and/or early Permian times. Later post-Permian heating did not have any substantial impact on the maturation of Carboniferous organic matter. By combining VR thermal modelling with local stratigraphic information we provide evidence that the coalification process occurred very rapidly. A coalification grade of ?0.40% Rr was attained during 2-4 m.y. after peat deposition, or even earlier. In addition, abundant sub-bituminous coal clasts embedded within the Carboniferous strata suggest that coalification proceeded close to the surface. These data collectively imply extremely high thermal gradients (around or above 200 °C/km) that must have prevailed during the Permo-Carboniferous thermal climax. The syn-sedimentary volcanic and/or igneous activity combined with effects of heat advection due to hot fluids circulating through the strata could have accounted for this dramatic geothermal setting, which had vanished by the end of the Palaeozoic Era.