Abstract
New field observations and 40Ar/39Ar geochronology reveal that the Topernawi Formation of the Ekitale Basin, northern Turkana Depression, Turkana County, Kenya was deposited entirely during the Oligocene between 29.7 ± 0.5 Ma and 29.24 ± 0.08 Ma. These bracketing ages are determined via new 40Ar/39Ar geochronology on a basaltic lava flow at the base of the section and a felsic ignimbrite near the top. A newly discovered basal unit and interbedded lava flow result in a new total sedimentary thickness of 92 m. The Topernawi Formation is the oldest dated syn-rift sedimentary section in the northern Turkana Depression.
Highlights
The East African Rift System (EARS) is an active continental rift that initiated during Paleogene time (Morley et al, 1992; Morley et al, 1999; Vétel et al, 2005; Vetel and Le Gall, 2006) and hosts numerous sedimentary sections that have played a key role in recording hominid evolution (Coffing et al, 1994; Leakey et al, 2001; Ducrocq et al, 2010; Roberts et al, 2012)
It is notable that the spatial extent of regionally lower elevations known as the Turkana Depression is roughly coincident with the overlap of the CARS and EARS (Figure 1)
The Topernawi Formation of the Ekitale Basin provides a sedimentological snapshot of an Oligocene phase of early EARS evolution during which much of the Turkana Depression is dominated by mafic volcanism—the Turkana Volcanics Formation, Kalakol ß, and Asile Group (Boschetto et al, 1992; Morley, 1999a; McDougall and Watkins, 2006)
Summary
The East African Rift System (EARS) is an active continental rift that initiated during Paleogene time (Morley et al, 1992; Morley et al, 1999; Vétel et al, 2005; Vetel and Le Gall, 2006) and hosts numerous sedimentary sections that have played a key role in recording hominid evolution (Coffing et al, 1994; Leakey et al, 2001; Ducrocq et al, 2010; Roberts et al, 2012). The EARS has played an integral role in the development of conceptual and analog models for the succession of rift stages from pre-inception through maturity (Cowie et al, 2000). Despite it being one of the type examples of continental rifting, the initiation and early history of the EARS is poorly known (Boone et al, 2019). In the case of the EARS, these initial conditions are characterized by the geometry, structure, and spatio-temporal evolution of the pre-EARS lithosphere. There is significant direct evidence for the pre-EARS condition in the Turkana Depression, it is difficult to fully characterize the preEARS lithospheric structure. It is notable that the spatial extent of regionally lower elevations known as the Turkana Depression is roughly coincident with the overlap of the CARS and EARS (Figure 1)
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