ABSTRACT

Lake Malawi occupies the southernmost part of the East African Rift Valley. Piston cores and gravity cores collected along the whole length of the lake were analyzed for clay minerals by X-ray diffraction and scanning electron micrography. The clays consist of smectite (8 to 96%), kaolinite (<1 to 76%) and illite (0 to 29%) with minor amounts of halloysite. Smectite is dominant in the finer-grained sediment facies whereas illite and kaolinite are more prevalent in the coarser-grained facies. Smectite also increases towards the southern parts of the lake. Scanning electron micrographs show broken and rounded crystal faces which, together with the shallow depth of deposition under low pressure and temperatures, suggest a detrital origin. Relative abundance of the three main clay groups show vertical fluctuations in the cores. Some authigenic nontronite is present in the southern part of the lake.

The spatial distribution and association with particular sedimentary facies suggest that the principal controls on clay mineral types is related to the source area (provenance) and sorting during deposition. Vertical fluctuations are caused by shifts in these variables, although influence by climatic change cannot be excluded. Further investigations of the clay mineral changes through time are needed in order to resolve these influences.