Very early rift sedimentation in the Turkana depression (EARS, Kenya): example of the Topernawi Formation

Abstract:

Sedimentation associated with very early phase of continental rifting remains poorly understood as related deposits lie at deepest part of basins and rarely outcrop at the surface. However, understanding of these sediments are essential first to better-constrain early extensional phase and second in term of potential resources.

The Turkana depression is a rift system active since the Paleogene, which makes the connection between Kenyan and Ethiopian domes. The southern area consists of four asymmetrical and juxtaposed grabens: the Lokichar, Turkwell, Kerio and South Lake Basin, which have been intensively documented through oil exploration. The northern part is structured into a single asymmetrical graben, the North Lake basin, less-known even oil exploration started. In this contribution, a sedimentary system preserved on the rift shoulder of the North Lake Basin is presented. Referred to as the Topernawi Fm, it is interpreted as recording the earliest phase of Cenozoic rifting in the area.

The Topernawi Fm delineates a relic sedimentary basin of limited extension (3 – 5 km). Boundaries of the basin are inherited from basement structures, more precisely from the reactivation during Late Oligocene to Early Miocene of a previous transfer zone producing N40-50° border faults. Basin fills is up to 80 m thick and includes first alluvial fan associated with the reactivation of these faults and then braided fluvial deposits from axial system. Above, several volcanic events recurrently emplaced pyroclastic deposits, repeatedly reworked by fluvial channels. Subsequently, N-S trend faulting cut the Topernawi system during the development of the North Lake Basin and led to its partial preservation over the present-day rift shoulder.

Geological maps, structural sections across Topernawi basin and sedimentary facies are presented. An integrated model is proposed to illustrate the basin evolution. Implications for rifting in Northern Turkana depression are discussed.