Sedimentation by termites in the Malawi Rift Valley

Abstract

Summary Homogeneous red clayey sand sheets, averaging 5 m thick, cover a discontinuous area totalling 8800 km 2 in the Karonga section of the Malawi Rift Valley floor. The sheets accumulated in a terrestrial environment, sedimentation was slow and gradual, and fossil termite mounds occur at the base of the sheet in one area. Topographic considerations rule out deposition by overland flow, and texture is incompatible with aeolian sedimentation. Modern termites ( Macrotermes falciger ) are transporting sediment from beneath the sand sheets onto the ground surface. The textures of fossil and modern Macrotermes mound deposits are very similar to the textures of the sand sheets. It is concluded that the sand sheets have accumulated through vertical transfer of sediment by Macrotermes . Scrutiny of the literature suggests that similar deposits may be widespread in African savanna and rainforest areas, both within the African Rift System and in epeirogenic basins. Equivalent sediments must occur in ancient tropical sequences since fossil termites are first recorded in the mid-Cretaceous.