Coal rank distribution and implications for structural and geothermal controls on coalification; case study from the Cevennes Basin, Massif Central, France

A detailed study of the coalification and paleogeothermal evolution of the Stephanian Cevennes basin in southeastern France is presented in this paper. This study includes several parameters: 1) the geometry of the basement, the base of the coal measures and the unconformity beneath the overlying Mesozoic series, 2) the lateral and vertical distribution of the content of volatile matter in the coal beds, 3) the relationship between these parameters and the geological boundaries. This study also addresses qualitative and semi-quantitative parameters of the degree of coalification of organic matter in the same basin. The parameters investigated are: 1) the relief of the basement, 2) the nature and thickness of the overlying Mesozoic series, and 3) the faulting in the basin.In the Cevennes basin two coalification episodes were recognized (early pre-folding and late post-folding), and their ages and intensities have been estimated. The results suggest that coalification was strongly influenced by the faulting and the thickness of the strata overlying the coal seams. The early coalification episode during the Pennsylvanian basin-filling stage was closely related to the syndepositional faulting, and the later coalification episode was affected by subsequent rapid subsidence during the Mesozoic. The ultimate maturity level of the oldest Stephanian coal was achieved by the polyphase and multiformity coalification.