Abstract

Palynologic studies and source-rock potential evaluation on fresh outcrops of an Upper Permian deposit provide the first biostratigraphic age and organic characterisation for Gondwana deposits from northern Malawi. Earlier the fluvio-lacustrine deposit was assigned an Upper Permian age based on lithological similarities to deposits in adjacent basins of southern Tanzania and northern Zambia. This study shows that the palynoflora is dominated by striated bisaccates and polyplicates comprising Taeniaesporites, Striatopodocarpites, Vittatina and Guttulapollenites, characteristic elements of the Late Permian Glossopteris flora in Gondwana. Based on the abundance of Guttulapollenites and Taeniaesporites and the paucity of spores, both features of Permo-Triassic transition floras, the age of the northern Malawi pollen flora is tentatively dated as Tatarian. The palynoflora is floristically less diverse than those from neighbouring Tanzania and Zimbabwe, but richer in bisaccate and striate pollen forms. It is closest in composition to the microflora from the adjacent Luangwa basin in northern Zambia, but also bears strong similarities to other microfloras in southern Africa, India and Antarctica. The pollen assemblage and corroborating sedimentologic evidence (arkosic lithology and illite-chlorite mixed layer clay mineral assemblages) suggest a temperate, seasonal climate and deciduous-dominated upland vegetation at the palaeolatitudes of northern Malawi (55°S) during Late Permian time.

The palynofacies is dominated by liptinic, humic and fusinitic kerogen. Thermoanalytic and microscopic studies indicate that the kerogen has not been buried deeply and is dominated by terrestrial plant matter. Rock-Eval characterisation shows that the kerogen has low hydrogen, low mean T_max (425-430°C), and is immature (R_o=0.5). Upper Permian Gondwana deposits in northern Malawi therefore possess no source-rock potential for oil. This evaluation would also apply to equivalent Upper Permian deposits from southern Tanzania and northern Zambia. However, rocks of the Madumabisa Formation from the mid-Zambezi basin appear to have better source-rock potentials. The emerging scenario for the Late Permian environment is one in which giant shallow lakes were interspersed with channels and swamps on humid fluvial plains. Forested hinterland in the drainage contributed a major part of the organic matter (δ¹³C_org =-23 to -27°) that was deposited into the lakes, although minor primary organic matter was produced within the lakes. The poor source-rock potential of Upper Permian deposits in central southern Africa can be attributed to three factors related to this depositional setting: i) dilution by siliciclastics which lowers the TOC content of the rocks; ii) poor organic matter preservation, a consequence of the shallow depths of most of the lakes; and iii) the predominantly terrestrial, hydrogen-poor nature of the organic source material. Present day high temperate latitudes (60-65°) in the northern hemisphere, with their belts of low diversity conifer forest stands and multitudes of freshwater clastic lakes, and seasonal inputs of plant debris from vegetated upland drainages may present an instructive analogy of the palaeogeographic setting of the palaeolatitudes of central southern Africa during Late Permian time.