Linking surface and subsurface volcanic stratigraphy in the Turkana Depression of the East African Rift system

Abstract

The Northern Kenya Rift is an important natural laboratory for understanding continental rifting processes. However, much of the current understanding of its geological evolution is based on surface outcrops within footwall highs due to a lack of subsurface geological constraints. In this paper, we present an investigation of the Cenozoic stratigraphy and volcano-tectonic relationship of the volcanic sequences within the Turkana Depression (namely the North Lokichar, North Kerio and Turkana Basins). We integrate regional seismic reflection data collected as part of ongoing petroleum exploration in the area with lithological and biostratigraphic data from new wells that were drilled in 2014 and 2015 (Epir-1 and Emesek-1). This has allowed linking and extrapolation of the detailed stratigraphy of the paleontologically important Lothagam site to the volcanic sequences within the Napedet Hills, North Lokichar, North Kerio and Turkana Basins. The site of the Plio-Pleistocene-age Turkana Fault, which separates the North Lokichar Basin from the Turkana and North Kerio Basins, appears previously to have acted as a focus of Middle Miocene volcanism c. 5 Ma prior to the main period of movement on the fault. Our study highlights how subsurface and outcrop information can be combined to give a more in-depth knowledge of the magmatic history within rift basins.

New Discovery of Oligocene Strata in the Topernawi Formation, Turkana County, Kenya

“…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). Despite significant subsurface data indicating that extension was likely widespread across the northern Turkana Depression [e.g., (Morley et al, 1999;Wescott et al, 1999;Schofield et al, 2021)], to the best of our knowledge, the Topernawi Formation is the only directly dated sedimentary section of this age in the northern Turkana Depression. The only other dated sedimentary section that outcrops in the Turkana Depression and is known to span this time is the Lokhone Formation and underlying Loperot Shale of the Lokichar Basin, in the southern part of the Turkana Depression (Morley et al, 1999;Hautot et al, 2000;Muia, 2015;Ragon et al, 2019;Morley, 2020).…”

“…The earliest stages of EARS evolution remain poorly understood [e.g., Boone et al (2019)]. Geologic evidence for the spatiotemporal regional tectonic evolution that characterizes EARS initiation consists of seismic and well data (Morley et al, 1992;Morley et al, 1999;Morley, 1999b;Wescott et al, 1999;Schofield et al, 2021), emplacement ages and spatial patterns of mafic volcanic rocks , and basement and detrital thermochronology (Boone et al, 2018;Boone et al, 2019). While it is unfortunate that in the northern Turkana Depression the seismic data lacks direct geologic constraints, these studies do provide significant seismic evidence suggesting that similar aged and possibly older syn-rift strata may be preserved at depth in numerous basins in the northern Turkana Depression.…”

“…The Turkana Depression hosts a diversity of sedimentary rocks, including Cretaceous-Paleogene sediments that mostly pre-date EARS inception (e.g., Lapur Formation) and a wide variety of richly fossiliferous Miocene and younger syn-rift sedimentary rocks [e.g., Lothidok Formation, Koobi Fora Formation, Galanaboi Formation; (Ragon et al, 2019;Morley, 2020) and citations therein]. The timing of local onset of EARS extension in the Turkana Depression is recorded in well and outcrop data by the Eocene-Oligocene sedimentary record of the Lokichar Basin (Morley et al, 1992;Morley et al, 1999;Boone et al, 2019), Eocene-Oligocene faulting east of Lake Turkana (Vetel and Le Gall, 2006), Oligocene growth of North Lokichar Basin, North Kerio Basin, and southern Turkana Basin (Morley et al, 1999;Morley, 1999b;Boone et al, 2018;Schofield et al, 2021) and two approximately north-south trending subsurface basins of possible Paleogene age (Wescott et al, 1999). Elsewhere in Turkana, EARS onset is primarily marked by widespread mafic volcanism [Asile Group: circa 34.3-15 Ma (McDougall andWatkins, 2006), Turkana Volcanics Formation: circa 38-29.7 Ma (McDougall andBrown, 2009;Ragon et al, 2019), Kalokol Formation: circa 28-18.5 Ma (Boschetto et al, 1992)].…”

PiAutoStage: An Open‐Source 3D Printed Tool for the Automatic Collection of High‐Resolution Microscope Imagery

“…In order to demonstrate that petrographic observations can be made from images collected with the PiAuto-Stage system, we present image mosaics of a mantle xenolith from Kilbourne Hole (Rio Grande Rift, United States) (Byerly & Lassiter, 2012;Perkins & Anthony, 2011;Reid, 1976) and a Cenozoic porphyritic lava from the East African Rift System, southeast of the Kenyan town of Lodwar, within the Lothagam tectonic block (East African Rift) (McDougall & Feibel, 1999;Schofield et al, 2021). Key petrographic features are evident at a range of scales in all of the images (Figures S1 and S2), comparable with observations that could be made using a standard petrographic microscope.…”

Plio-Pleistocene sedimentation in West Turkana (Turkana Depression, Kenya, East African Rift System): Paleolake fluctuations, paleolandscapes and controlling factors

“…Overlying the Turkana Volcanics Fm, Oligocene sedimentary rocks were deposited in extensional microbasins (Ragon et al, 2019); they fill the oldest syn-rift structures in this area. Upwards in the succession, Miocene rocks are exposed sparsely, but they have been identified in some wells (Schofield et al, 2020) suggesting deposition at least in the North Lake Basin after its J o u r n a l P r e -p r o o f opening 14 Ma ago. Overlying, Plio-Pleistocene sediments of the Omo Group are exposed all around modern Lake Turkana.…”