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A Geological History of the Turkana Basin

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A Geological History of the Turkana Basin Evolutionary Anthropology 20:206–216 (2011) Article A Geological History of the Turkana Basin CRAIG S. FEIBEL The Turkana Basin preserves a long and detailed record of biotic evolution, cultural nent quartzo-feldspathic sandstones development, and rift valley geology in its sedimentary strata. Before the formation of into the ‘‘Turkana Grits,’’ an assem- the modern basin, Cretaceous fluvial systems, Paleogene lakes, and Oligo-Miocene blage later determined to include volcano-sedimentary sequences left fossil-bearing strata in the region. These strata ranging in age from the Creta- deposits were in part related to an early system of rift basins that stretched from ceous through the Pleistocene.4 Sudan to the Indian Ocean. The present-day basin has its origins in Pliocene tectonic Reconnaissance mapping west of developments of the modern rift, with subsidence making room for more than one Lake Turkana initiated comprehen- kilometer of Plio-Pleistocene strata. Much of this sequence belongs to the Omo sive documentation of the basin with Group, richly fossiliferous sediments associated with the ancestral Omo River and a series of geological reports.5–7 In its tributaries. Modern Lake Turkana has a record stretching back more than 200 1972-1974, the Omo River Project thousand years, with earlier lake phases throughout the Plio-Pleistocene. The investigated the lower reaches of geologic history of the basin is one of dynamic landscapes responding to the Omo Valley, including mapping environmental influences, including tectonics, volcanic activity and climate. and description of its geological exposures.8 Field work undertaken between 1980 and 1986 around the eastern and southern periphery of The Turkana Basin is a hydro- have controlled accumulation and the basin9–13 provided the most com- graphic and sedimentary system exposure of the sedimentary strata prehensive mapping and geological encompassing about 131,000 km2 of for which the basin is world-famous. descriptions available for that region. northern Kenya and southern Ethio- This contribution attempts to synthe- Beginning in the mid-1960s, paleo- pia (Fig. 1). In its present configura- size the geologic history of the Tur- anthropological investigations in tion, the basin is hydrologically kana Basin with an emphasis on the 14 the lower Omo Valley, at Koobi closed and dominated by alkaline sedimentary strata associated with Fora,15,16 West Turkana,17 and else- Lake Turkana, with the Omo River the basin’s rich paleontological and where4,18,19 catalyzed geological as its primary source of water. As archeological records. research in the basin. Although this recently as the middle Holocene, it work has focused heavily on provid- had connections to adjacent rift GEOLOGICAL INVESTIGATIONS ing context for fossil and archeologi- basins and an outlet to the Nile cal discoveries, it has generated River. The present-day isolation of Exploration and description of the much of our current understanding the Turkana Basin is in part due to geological features of the Turkana of the basin as a whole. In particular, the current water balance, but has at Basin have been under way for over the development of detailed chrono- its roots the tectonic dynamics that a century. The earliest European stratigraphic frameworks4,20–23 has explorers to visit the region, Teleki helped to constrain the geological his- and von Ho¨ hnel,1 returned with valu- tory of the basin. (Note that the time Craig Feibel is a geologist at Rutgers Uni- 24 versity, where he teaches sedimentology, able observations on the geology and scale of Gradstein, Ogg, and Smith geoarcheology, and the environmental landscapes they traversed. Other is used throughout this paper). context of evolution. His fieldwork in the Turkana Basin is currently focused on expeditions followed (see bibliogra- Hydrocarbon and geothermal in- 2 linking modern and ancient depositional phy in Coppens and coworkers ), terest fueled exploration in the systems, and understanding the controls culminating in Fuchs’3 first synthesis southern reaches of the basin and on local environmental change E-mail: [email protected] of the basin’s geological history. expanded our understanding of sub- These early workers recognized surface structure, stratigraphy, and many significant aspects of the petrology.25–27 Research focused on Key words: stratigraphy; evolution; Africa; rift basin’s geology, from the complex deposits beneath the modern lake valley; hominins record of lake-level fluctuations to has also added considerably to our major tectonic and volcanic episodes. understanding of the structural VC 2011 Wiley Periodicals, Inc. One confounding generalization underpinnings of the system and of DOI 10.1002/evan.20331 Published online in Wiley Online Library from this early work, however, was both Neogene strata and Holocene (wileyonlinelibrary.com). the amalgamation of many promi- sedimentation.28,29 Article A Geological History of the Turkana Basin 207 Jurassic-Paleogene Central African Rift System (CARS),30 which trans- ected the area as a northwest-south- east basin complex. The CARS basins were responsible for sedimen- tary accumulations preserved from the Late Cretaceous,31 and may have remained an important structural influence into Miocene times. The CARS continued to affect sediment accumulation patterns through the Neogene as preexisting rift struc- tures and topographic lows con- strained both basin morphology and fluvial corridors, particularly the Anza Graben leading to the Indian Ocean.32 Tectonic activity delineating the modern Turkana Basin began in Early Pliocene times, with subsi- dence related to the EARS that quickly took on a pattern of alternat- ing half-graben trending north- south.33 Several significant pulses of tectonic activity are recorded in ear- liest Pleistocene times, when the Hamar Uplift developed, and in a Middle Pleistocene phase of activity that culminated in the modern basinal configuration. During this most recent phase, subsidence along the basin axis has left structural blocks along the margin in topographically higher positions and subject to the erosion that has exposed Plio-Pleistocene sediments, fossils, and sites. Biogeo- graphic34 and sedimentary patterns suggestthatuptotheMiddlePleisto- cene reorganization, the Turkana Basin preserved elements of the CARS hydrography, including an outlet to the Indian Ocean. Only in the last few hundred thousand years has the basin become a contributory system to the Nile drainage. Following extensive research, including seismic analyses tied to hydrocarbon exploration,25 the detailed structural configuration of the basin is now well understood in many areas. However, recognition of structural bounds and active sub-basins from spe- Figure 1. Locality map of the Turkana Basin. A. Extent of drainage network and major cific episodes of the past is often com- geographic features. B. Local geographic terms, fossil localities, and archeological sites. plicated. Though sometimes referred to as the Omo-Turkana Basin, there are at least nine structural sub-basins TECTONIC HERITAGE domes, associated with the East within the tectonic complex associated African Rift System (EARS). The Much of the Turkana Basin lies with EARS development, and a further within the topographic saddle region has a long history of basin seven or more older basins related to between the Ethiopian and Kenyan development, beginning with the either the CARS or early EARS. 208 FEIBEL Article trachytes, rhyolites, and other modern basin, an integrated deposi- types.39,40 At Lothidok, Boschetto41 tional system was established over a described approximately 785 m of much broader area, though the con- the Kalakol basalts deposited in lat- tinuity of these deposits has subse- est Oligocene and Early Miocene quently been disrupted by younger times. A pulse of nearly basin-wide tectonic events and the formation of basaltic effusion produced the the modern lake. Gombe Group,40,42 an important Early Pliocene stratigraphic marker. Cretaceous Windows In general, however, after the early Late Miocene, volcanism was con- The oldest sedimentary strata cur- siderably less common and more rently recognized within the Turkana localized within the basin. The vol- Basin are the Cretaceous sandstones canic centers associated with Mount and conglomerates of the Lapur Kulal developed after about 2.5 Ma. Formation and its correlates. The The Barrier complex separating Tur- best exposures are on the slopes of kana from the Suguta Basin was Lapur itself, and presumed correlates are known from Muruanachok, emplaced beginning about 1.4 Ma, 26 and the islands within the modern Lariu, and Sera Iltomia. Originally lake are of Late Pleistocene to Holo- included within the poorly con- cene age. With the exception of the strained ‘‘Turkana Grits,’’ these sedi- detailed studies by Watkins40,43–45 ments attain thicknesses of about around the Suregei region and an 500 m near Lokitaung Gorge, and are preserved as thinner packages on investigation of the islands within the metamorphic basement in other Lake Turkana,46 most of the basin’s parts of the basin.31 Lapur Forma- volcanic rocks are known primarily tion sediments are exclusively from individual sampling localities quartzo-feldspathic material derived and isotopic ages, and through from the metamorphic basement, broad reconnaissance mapping. representing alluvial fans, braided However, these volcanics shaped rivers, and associated paleosols.48 much of the basin’s landscape, are They may span up to 50 million
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