CLASTIC SEDIMENTARY ENVIRONMENTS AND ORGANIC PETROLOGY OF COALS IN THE ORANGE-FREE-STATE, SOUTH-AFRICA

The coal measures of the Orange Free State Coalfield occur in post-glacial Permo-Carboniferous sedimentary environments of the Karoo Basin. Pre-Karoo geology and palaeotopography are the overriding controls on sedimentation and coal distribution. The more important coal-bearing sediments occur within the Vryheid Formation of the Karoo Sequence in paralic depositional environments. Sedimentation is cyclothemic in character and four regressive cycles are recognized. Each basinward progradation is separated from the following one by a major coal seam. The basal Dwyka coal has a moderate to extremely high inertinite content and the Bottom, Middle and Top coals have high vitrinite contents. The petrographic distinction between the coals is also evident in their microlithotype compositions, the Dwyka coal having less than 40% vitrite plus clarite, the others, more than 50% vitrite plus clarite. This major variation in petrographic type may be due to climatically controlled diversity in sedimentary environments and plant type. The Dwyka seam was deposited in cold conditions associated with glaciation in the Early Permian; the overlying coals were deposited in a more temperate climate. However, within these two major climatic subdivisions, petrographic compositions vary with depositional environment. Comparison has been made of the Orange Free State coals with Permian coals from the Cooper Basin, Australia, that have formed in a post-glacial, cratonic environment. The Cooper Basin coals were similar, petrographically, to those of the Dwyka seam, irrespective of palaeodepositional environment.