The Turkana Depression has for some time been the subject of speculation regarding the geometric relationship of an obvious Miocene rift system crossing between Kenya and Ethiopia and an older sequence of rifts – oblique to the East African Rift System (EARS) – that is evident in the Mesozoic Anza Graben of Kenya and the Muglad and Melut systems of southern Sudan. Up to five phases of rifting potentially affected the area. The Early Cretaceous to Paleogene systems of the Anza, Muglad and Melut basins, (ca. 120Ma, 70Ma and 40Ma) have been overprinted by theMiocene East African Rift system (ca. 20 Ma and 10 Ma). Crucially a lack of geological or geophysical data has hampered an understanding of this critical area of interfering rift systems.

The East African Rift System of Tertiary age is predominantly located in zones of Precambrian orogenic belts, avoiding stable Archaean cratonic areas. The geometry of the rift system is largely controlled by the Precambrian mobile belt architecture; however, later extensional events of Permo-Triassic (Karoo), Jurassic, Cretaceous, and Palaeogene ages have also affected the location and orientation of the Tertiary rift systems to a greater or lesser degree. A major challenge in the area is that outcrops comprise either volcanic rocks or metamorphic basements with few sedimentar y sequences that might give indications of East African or earlier rift systems.

Seismic control was restricted to mid-1980’s vintage data shot over the Lotikipi and Gatome basins of northwest Kenya by Amoco, plus ship-borne Project PROBE data acquired over the Lake Turkana rift. There was also sparse gravity data over a large part of the depression, outside of Kenya.

A pioneering, tri-nation airborne gravity and magnetic survey that was flown offered a new insight into the complex rift geometry of the Turkana Depression. The survey revealed an interfering older rift system running from the Anza into the Muglad-Melut systems. Cretaceous rifting extends from the Anza Graben through the Turkana region and links with the Sudanese rifting of the Muglad and Melut basins. Maastrichtian rifting appears to extend northwestwards from the Anza Graben, and may be present within basins of SW Ethiopia and northern Kenya. Similar rifting occurred in the Eocene, and Oligocene-Miocene rifting is evident in the wester Turkana basins. Middle to Late Miocene East African rifting cross-cuts all of these earlier rift geometries.

Source rocks are known in the Miocene of Ethiopia and Kenya and in the Oligocene (?Eocene) of the Loperot-1 well, and they are predicted to occur in the earlier rift phases. In southwest Ethiopia, north of the Turkana Depression, Middle Miocene oil-shales and coals have been shown to have excellent source rock properties, and equivalents may be preserved within the EARS. The presence of marine Cretaceous sequences is supportive evidence of earlier source rocks. Suitable reservoirs are developed in all tectono-stratigraphic phases of rifting. The interfering rifts of the Turkana Depression provide several new exciting exploration plays and the probability of numerous trapping scenarios. Undoubted challenges are presented by the complex thermal history and the presence of volcanics and volcanic activity.

Figure 1: East African topography showing the Afar and East African Domes separatd by the Turkana Depression

Figure 2: Airborne survey flight lines over pre-survey outlines of potentail rifts

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