Chapter 6 Rift-associated sedimentary basins

Abstract

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. Fig. 6.1. Location of the rift-associated sedimentary basins relative to the major faults. 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 by the Natron boundary faulting. #### Palaeo-Lake Natron Sedimentation in the Natron Basin began at c . 1.7 Ma in a shallow, depression that may have been fault-generated. A succession of basalts and nephelinites from Oldoinyo Sambu and Mosonik, dated at 3.5–1.77 Ma (Isaac & Curtis 1974 …