Rift-associated sedimentary basins
The warping of the mid-Tertiary land surface and the subsequent faulting gave rise to a series of half grabens. Due to the disruption of earlier river systems, inland drainage basins formed within the half grabens and, in these basins, lake sediments were deposited, often intercalated with lava flows and ash-fall deposits, the thickest sedimentary successions forming in depocentres immediately adjacent to the boundary faults.
The most prominent of the basins are along the foot of the east-facing Pleistocene escarpment, being, from north to south, Natron, Engaruka, Manyara, Balangida and Balangida Lelu (Fig. 6.1). Unlike the half graben rift basins in Lake Tangayika (Rosendahl 1987) and in parts of Kenya, the northern Tanzania basins are not enclosed within opposed faults, nor are they linked by accommodation zones. These basins, together with the Eyasi half graben basin, now contain saline lakes. Other sedimentation basins are those of Olduvai/Laetoli and Amboseli, and some small ones in the Monduli area.
The Natron Basin in its early stages was probably joined to the Magadi Basin to the north. The maximum depth of the basin, estimated from Euler deconvolution (Ebinger et al. 1997), is around 3.3 km, which is similar to the 3.5 km depth to the basement derived from seismic refraction data for the Magadi Basin (Birt et al. 1997). Much of the basin is underlain by the Sambu volcanics and the Peninj Beds which were deposited in the palaeo-Lake Natron until c.1.2 Ma when the sedimentation was ended
Figures & Tables
The structure and volcanic activity of the northern Tanzania sector of the Gregory Rift Valley have hitherto been described less than those in Ethiopia and Kenya. This book focuses on northern Tanzania where, although the volcanic area is smaller than those to the north, there are major features such as Kilimanjaro, the highest mountain on the African continent, Ngorongoro, one of the largest calderas on Earth, and Oldoinyo Lengai, the world’s only active carbonatite volcano. Following an account of the discovery and early exploration of the Rift Valley, there are descriptions of the individual volcanoes. These are set within the context of the regional geology and geophysics of the rift valley and in relation to the structural evolution of the rift and its associated sedimentary basins which include Olduvai, an important site in the history of human evolution. The volume concludes with a discussion of the volcanism in relation to the plume-related African Superswell.