Quaternary history of the Lake Magadi Basin, southern Kenya Rift: Tectonic and climatic controls
Introduction
Much of the early palaeoclimate research in East Africa was devoted to understanding Plio-Pleistocene hominin and archaeological sites (see Cohen et al., 2016 and Campisano et al., 2017 for reviews). More recently, efforts have focused on climate variability from various Pliocene to Recent time-slices (Kingston et al., 2007; Owen et al., 2008; Tierney et al., 2010; Junginger and Trauth, 2013; Magill et al., 2013). A range of record types have proven useful in reconstructing palaeoclimates in the region, including Late Quaternary core-records (e.g., Verschuren and Chapman, 2008; De Cort et al., 2013, De Cort et al., 2018), outcrop data (e.g., Levin, 2015), and marine cores (e.g., deMenocal, 1995, deMenocal, 2004). More recently, drill cores from extant rift lakes have proven exceptionally useful in documenting palaeoenvironmental histories, including a 1.3-million-year Lake Malawi record (Ivory et al., 2016). In addition, a one-million-year pollen-diatom-mineralogy study of a core at Lake Magadi synthesised the regional climatic history for the southern Kenya Rift as a basis for exploring hominin evolution and mammalian change (Owen et al., 2018a). The latter paper complements this study, which is based on outcrops and two lake cores, and which focuses on aquatic sedimentation and palaeoenvironments using sedimentological, mineralogical and geochemical data, supplemented by diatom analyses.
Lake Magadi lies in the axial trough of the southern Kenya Rift ~605 m above sea level (masl), and is a seasonally-flooded, saline alkaline pan, currently floored by trona (Fig. 1). Discontinuous Quaternary sedimentary outcrops around the lake include fluvial sediments (channel deposits, alluvium), calcrete, lacustrine limestone (including microbialites), zeolitic mudstone and siltstone, sodium silicate minerals, and chert of diverse origins (Baker, 1958, Baker, 1963; Eugster, 1967, Eugster, 1969, Eugster, 1980; Hay, 1968; Herrick, 1972; Surdam and Eugster, 1976; Behr, 2002; Brenna, 2016; Felske, 2016; Leet et al., 2016). These sediments accumulated in a N-S axial rift sump, in which the northern depocentre remained a lake or wetland for most of the last million years because of spring recharge during drier periods. Lake Magadi probably united with Lake Natron in northern Tanzania as a single, relatively dilute lake for different periods during the Pleistocene and early Holocene (Eugster, 1986; Casanova and Hillaire-Marcel, 1987; Williamson et al., 1993).
Lake Magadi was cored in June 2014 by the Hominin Sites and Paleolakes Drilling Project (HSPDP), which aims to develop basin-to-regional scale palaeoenvironmental histories that can be compared with local hominin remains and artefacts to infer possible environmental influences on hominin evolution (Cohen et al., 2016). We present here results of detailed analyses of the sedimentology, major- and trace-element geochemistry and mineralogy from two Lake Magadi cores and nearby sediment outcrops (Fig. 1A), supported by diatom records, which collectively provide a history of changing aquatic environments during the past million years. Both drill cores reached the volcanic basement. This new evidence gives an opportunity to reconstruct the Pleistocene history of the Magadi basin in much greater detail than previously possible. Specifically, we aim to: 1) reconstruct the environmental history of the Magadi palaeolakes; and 2) relate that sedimentary record to evolving tectonic, volcanic and climatic controls.
Section snippets
Previous outcrop and borehole studies in the Magadi Basin
Early descriptions of sediments in the Magadi basin were provided by Parkinson (1914), Gregory (1921), Walter (1922) and Coates (as Anonymous, 1923). The oldest exposed deposits are the fluvial, spring and lacustrine ‘Oloronga Beds’, which rest upon the Magadi Trachytes (~1.4–0.8 Ma; Fig. 1B; Baker, 1958, Baker, 1963; Crossley, 1979, Herrick, 1972; Eugster, 1980; Behr, 2002). A radiometric age of 0.78 ± 0.04 Ma for an obsidian flow overlying basal Oloronga sediments (Fairhead et al., 1972;
Methods
Sediment outcrops logged between 2006 and 2018 were combined with sampling of modern sediments and waters across the catchment. Lake Magadi was drilled in June 2014 to depths of ~133 mbs (metres below surface; Site 1) and 194 mbs (Site 2) (Fig. 1A; Cohen et al., 2016). Geochemical analyses were carried out on outcrop samples (n = 76) and core sediments, with MAG14-2A sampled at 32 cm intervals, and where distinctive lithologies would have been omitted (n = 344; see Electronic Supplementary
Exposed Quaternary sediments
The Early to Middle Pleistocene Oloronga Beds overlie Magadi Trachyte in scattered outcrops north, south and west of Lake Magadi (Baker, 1958; Fig. 1A). Sections OB1 and OB2, southwest of Magadi, contain trough cross-bedded fluvial sandstone (Figs. 1A, Fig. 3, Fig. 4A, B) overlain by bedded, wavy lacustrine chert and grey zeolitic siltstone (Fig. 4C) covered locally by thin (<5 cm) dark grey limestone. A laterally extensive calcrete up to 40 cm thick, with massive, pisolitic and laminated
Geochemical stratigraphy
Lacustrine deposition began soon after volcanic eruptions in the Magadi Basin ceased, given the lack of weathering of the underlying trachytes. Zone G1 (~1056–930 ka) is characterised by ostracod-rich grainstone without diatoms. In contrast, ostracod- and gastropod-rich grainstone in the basal part of MAG14-1A and MAG14-1C (Fig. 2A) contains freshwater benthic and epiphytic diatoms (Epithemia, Rhopalodia, Encyonema), implying a marsh setting. Grainstone is absent above the G1-G2 boundary, but
Conclusions
The Magadi Basin preserves a one-million-year record of aquatic deposition under dry, tropical conditions. The basin experienced progressive increases in aridity superimposed on wet-dry cycles and step-like changes that resulted from tectonic processes. Major tectonic controls include:
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Axial rift faulting that tapped geothermal fluid reservoirs, introducing silica via springs from early in the basin history.
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Faulting of the rift floor that diverted cross-rift (E-W) rivers that reduced inputs of
Acknowledgements
Drilling was funded by ICDP and NSF grants (EAR-1123942, BCS-1241859, and EAR-1338553). Analyses were supported by the Hong Kong Research Grants Council (HKBU-201912 and 12304018). We thank the National Museums of Kenya, the Kenyan National Council for Science and Technology, the Kenyan Ministry of Mines, and the National Environmental Management Authority of Kenya for providing permits. We also thank DOSECC Exploration Services for drilling supervision, the Operational Support Group of ICDP
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