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 elastics 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 CO2 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.
1.Hong Kong Baptist Univ, Dept Geog, Hong Kong, Peoples R China 2.Univ Saskatchewan, Dept Geol Sci, Saskatoon, SK S7N 5E2, Canada 3.Natl Museums Kenya, Palynol & Palaeobot Sect, Dept Earth Sci, POB 40658, Nairobi 00100, Kenya 4.Rutgers State Univ, Dept Chem, Newark, NJ 07102 USA 5.SUNY Binghamton, Dept Geol Sci, Binghamton, NY 13902 USA 6.Georgia State Univ, Dept Geosci, Atlanta, GA 30302 USA 7.Natl Cheng Kung Univ, Dept Earth Sci, Tainan 701, Taiwan 8.Berkeley Geochronol Ctr, Berkeley, CA 94709 USA 9.Univ Arizona, Dept Geosci, Tucson, AZ 85721 USA 10.Univ Oxford, Dept Earth Sci, Oxford OX1 3AN, England 11.Ctr Nacl Invest Evoluc Humana, Burgos 09002, Spain 12.Arizona State Univ, Inst Human Origins, Sch Human Evolut & Social Change, Tempe, AZ 85287 USA 13.Natl Taiwan Univ, High Precis Mass Spectrometry & Environm Change L, Dept Geosci, Taipei 10617, Taiwan 14.Natl Taiwan Univ, Res Ctr Future Earth, Taipei 10617, Taiwan 15.Tata Chem Magadi, Box 1, Magadi, Kenya 16.Univ Minnesota Duluth, LLO, Duluth, MN 55812 USA
Recommended Citation:
Owen, R. Bernhart,Renaut, Robin W.,Muiruri, Veronica M.,et al. Quaternary history of the Lake Magadi Basin, southern Kenya Rift: Tectonic and climatic controls[J]. PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY,2019-01-01,518:97-118