Illite crystallinity and vitrinite reflectance in Paleozoic siliciclasticsin the Se Bohemian Massif as evidence of thermal history

The thermal maturity of Paleozoic rocks in the SE part of the Bohemian Mass if is characterized by clay minerals and organic matter. The expandability of illite-smectite (S in I-S), illite crystallinity index (IC) and reflecta nce of (R-r) were measured and their regional distribution was evaluated. T he mutual correlation of IC and R-r from diagenesis to very low-grade metam orphism is compared with the published data and used to distinguish data wi th more reliable paleogeothermal information from those affected by other f actors. In the SE part the Paleozoic units have illite-smectites with an ex pandable component of 15-35 % S. The reflectance values (R-r of 0.55-1.1 %) are in good agreement with the expandability and suggest the oil window ra nge of catagenesis with paleotemperature close to 100 degrees C. In the NNW part of the area the clays contain no expandable layers in illite. The ill ite crystallinity (IC of 0.36-0.24 Delta degrees 2 Theta) and vitrinite ref lectance (R-r from 3.17 to 5.23 %) indicate very low-grade metamorphic cond itions with probable maximum paleotemperatures of 240-300 degrees C. The sy stematic change in both clay and organic parameters shows the gradual decre ase in thermal exposure towards the front of the Variscan orogenic zone in the S and SE and suggest extensive erosion in the NNW.