Palynology of a coal seam in Karoo deposits of Botswana and correlation with southern African coal-bearing strata

摘要

A significant amount of palynological work has been done on southern African coaludseams in the Ecca Group, but as yet there is little consensus on how these areas relate toudeach other. This study investigated the palynology of a coal seam from Mmamantsweud(Mmamabula area), Botswana, approximately 70 km north-east of Gaborone. A total ofud124 samples were taken from two borehole cores and subjected to acid preparation,udoxidation and acetolysis. Coal samples were found to be barren of palynomorphs. Fiftyudcarbonaceous mudstones and siltstone samples yielded twenty-two productive samples.udA thermal alteration index of 3.0-3.5 was assigned for the sediments. Palynomorphuddiversity was high, with 64 genera and 90 species present, dominated by trilete and aleteudspores. This indicates a parent flora of mostly lower order lycopods, sphenophytes andudferns. Non-taeniate bisaccate and monosaccate pollens were scarce, and striatesudextremely rare (only two species), suggesting an autochthonous origin for the coaludswamp. The Mmamantswe core was sub-divided into five microfloral assemblageudzones. A transition from monosaccate dominance in the lower part of the core, to equaludnumbers of monosaccates and non-taeniate bisaccates in the upper part of the core, wasudseen. As the Mmamantswe palynoflora possesses elements of both the LateudCarboniferous glacial floras and the mid-Permian coal floras, it is thought to represent audcross-over assemblage dating to soon after the Permo-Carboniferous boundaryud(Sakmarian and Early Artinskian). The Mmamantswe assemblage can be correlatedudwith Assemblage Zones II and III of Falcon (1975a); Biozones B and C of MacRaeud(1988); and Zones 1, 2 and 3 of Anderson (1977) but does not fit well into any existingudbiozonation. The Mmamantswe palynoflora was most similar to that of Milorgfjella,udDronning Maud Land, Antarctica (Larrson et al. 1990) and the No. 2 Seam, Witbank,udSouth Africa (Falcon 1989). Taphonomic controls on palynomorph preservation suggestudthat future studies should also attempt to focus on Permian sediments not containingudcoal, as microfloral assemblages from coal seams tend to be autochthonous, and subjectudto local climatic influences. Accordingly they are not as useful for inter-basinaludcorrelation across Gondwana.