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 coalseams in the Ecca Group, but as yet there is little consensus on how these areas relate toeach other. This study investigated the palynology of a coal seam from Mmamantswe(Mmamabula area), Botswana, approximately 70 km north-east of Gaborone. A total of124 samples were taken from two borehole cores and subjected to acid preparation,oxidation and acetolysis. Coal samples were found to be barren of palynomorphs. Fiftycarbonaceous mudstones and siltstone samples yielded twenty-two productive samples.A thermal alteration index of 3.0-3.5 was assigned for the sediments. Palynomorphdiversity was high, with 64 genera and 90 species present, dominated by trilete and aletespores. This indicates a parent flora of mostly lower order lycopods, sphenophytes andferns. Non-taeniate bisaccate and monosaccate pollens were scarce, and striatesextremely rare (only two species), suggesting an autochthonous origin for the coalswamp. The Mmamantswe core was sub-divided into five microfloral assemblagezones. A transition from monosaccate dominance in the lower part of the core, to equalnumbers of monosaccates and non-taeniate bisaccates in the upper part of the core, wasseen. As the Mmamantswe palynoflora possesses elements of both the LateCarboniferous glacial floras and the mid-Permian coal floras, it is thought to represent across-over assemblage dating to soon after the Permo-Carboniferous boundary(Sakmarian and Early Artinskian). The Mmamantswe assemblage can be correlatedwith Assemblage Zones II and III of Falcon (1975a); Biozones B and C of MacRae(1988); and Zones 1, 2 and 3 of Anderson (1977) but does not fit well into any existingbiozonation. The Mmamantswe palynoflora was most similar to that of Milorgfjella,Dronning Maud Land, Antarctica (Larrson et al. 1990) and the No. 2 Seam, Witbank,South Africa (Falcon 1989). Taphonomic controls on palynomorph preservation suggestthat future studies should also attempt to focus on Permian sediments not containingcoal, as microfloral assemblages from coal seams tend to be autochthonous, and subjectto local climatic influences. Accordingly they are not as useful for inter-basinalcorrelation across Gondwana.

[发布机构] University of the Witwatersrand