IMPRINTS OF POSTGLACIAL CLIMATES AND PALEOGEOGRAPHY IN THE DETRITAL CLAY MINERAL ASSEMBLAGES OF AN UPPER PERMIAN FLUVIOLACUSTRINE GONDWANA DEPOSIT FROM NORTHERN MALAWI

Detailed quantitative clay mineral studies on an Upper Permian fluviol acustrine Gondwana deposit (approximate to 55 degrees S) from northern Malawi (9 degrees S, 32 degrees E) in central-southern Africa show im prints of Late Permian climates on the clay mineral assemblages. The c lay mineral assemblage in the bottom 75 m is characterised by abundant illite, chlorite, regular smectite-chlorite (corrensite), regular ill ite-smectite, and vermiculite (Zone I). Between 75-240 m, illite and c hlorite are the principal clay minerals, whereas corrensite and irregu lar illite-smectite mixed-layers disappear and vermiculite is rarely p resent (Zone II). The assemblage between 240-305 m is similar to that in Zone I, but is distinguished by the absence of irregular illite-sme ctite and a decrease in corrensite (Zone III). Kerogen fluorescence an d chemistry, vitrinite reflectance, and palynomorph thermal alteration index indicate insignificant thermal diagenesis (50-70 degrees C). Th e absence of depth-temperature related variation in mixed-layer minera l composition or in illite crystallinity indicate that the original cl ay mineral composition has not been overprinted by diagenetic modifica tions. Sandstone provenance and palaeocurrent directions suggest that detritus throughout deposition was derived from the slowly uplifting N yika Plateau Basement and sedimentary rocks southwest and west of the basin. Furthermore, compositional and textural immaturity of the sands tones, as well as the fresh nature of the feldspars strongly suggest t hat the detritus was little-altered. The clay mineral assemblages in t he three zones can thus be interpreted as representing the signatures of the climate during the periods of deposition. Zones I and III repre sent warmer and more seasonal climates than Zone II, which had general ly more humid but less distinct seasons. Sedimentological features suc h as the presence of nonglacial rhythmites, sub-millimetric to millime tric laminated mudrocks, elevated CaCO3 and TOC contents in claystones and mudstones in Zones I and III, whereas organic-lean, coarse siltst ones and sandstones are common in Zone II. The three assemblage zones reflect changes perhaps associated with global Late Permian climate cy cles. The quantitative clay mineral composition of the Upper Permian d eposit is similar to those from modern temperate lake sediments from e quivalent geographic and latitudinal settings, and source rocks in the drainage basins, in the northern hemisphere. The Upper Permian clay m ineral assemblage contains substantial quantities of corrensite, which to date is documented from hypersaline or metamorphosed sedimentary e nvironments. We report here its first occurrence from a shallow-buried freshwater lacustrine deposit and present evidence for syngenetic agg radation of detrital mixed-layers in a magnesium-enriched lacustrine d epositional environment.