Sediments from the Magadi Basin (south Kenya Rift) preserve a one-million-year palaeoenvironmental record that reflects interactions between climatic, volcanic and tectonic controls. Climate changes that impacted sedimentation include wet-dry cycles on variable timescales and an overall progressive trend towards greater aridity. Volcanic influences involved inputs of tephra to the basin, significant inflow of geothermal fluids, and the effects of weathering, erosion and transportation of clastics from trachyte and basalt terrains. Tectonic controls, which were often step-like, reflect the influence of faults that provided pathways for fluids and which controlled accommodation space and drainage directions. Intensified aridity and evaporative concentration resulted in salinity and pH increasing with time, which led to a change from calcite deposition in mildly saline lakes before 380 ka to the later formation of zeolites from reactions of volcaniclastic debris with highly alkaline lake and pore water. After 105 ka, hyperalkaline conditions led to trona accumulation and increasingly variable rare earth elements (REEs). The presence of mixed saline and freshwater diatom taxa between 545 and 16 ka indicates climate variability and episodic inputs of fresh water to saline lakes. Calcrete formed in lake marginal settings during semi-arid periods. Tectonic controls operated independently of climate, but they interacted together to determine environmental conditions. Aquatic deposition was maintained during periods of increasing aridity because fault-controlled ambient and geothermal springs continued to flow lakewards. This recharge, in turn, limited pedogenesis: palaeosols are common in other rift floor sequences. Trona formed when aridity and evapoconcentration increased, but its precipitation also reflects increased magmatic CO 2 that ascended along faults. Basin fragmentation and north-south fractures caused loss of cross-rift (east-west) drainage from rift-marginal basalts, resulting in reduced transition metals after 545 ka. The Magadi Basin demonstrates how a careful reconstruction of these complex tectono-climatic interactions is essential for accurate palaeoenvironmental reconstruction in continental rifts and in other tectonic settings.
Bibliographical noteFunding Information:
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 for downhole logging and the US National Lacustrine Core Facility. Tata Chemicals Magadi Limited and the Magadi Administrative District of Kajiado County provided local support. Research was undertaken with support of the local Maasai community. The late Jean-Jacques Tiercelin, with John Ego (NOCK) and George Muia, helped to log outcrops. Luis Buatois provided help with description of the ichnofossils. This is publication # 18 of the Hominin Sites and Paleolakes Drilling Project.
© 2019 Elsevier B.V.
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- Rift basins
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