The Neogene history of environmental change in the Kenya Rift : Contribution to deciphering of evolution from ape to early hominid

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The research results of the project are summarized as follows ;(1)On the basis of K-Ar ages and magnetostratigraphy, the Lukeino Formation (Fm) which yielded the earliest hominid Orrorin tugenensis accumulated between 6.0Ma and 5.7Ma.(2)The anorthoclase K-Ar ages from tuff layer yielding Nacholapithecus are 16.36±0.37Ma to 15.27±0.47Ma. Combined K-Ar ages and magnetostratigraphy of the Aka Aiteputh and Namurungule Fms in Samburu Hills, indicates that the hominoid fossil Sambrupithecus kiptalami occurs in the normal polarity zone, which is correlated with C4Ar.2n (ca.9.6Ma).Drastic environmental change occurred around 10Ma between the Aka Aiteputh Fm phase and subsequent Namurungure Fm phase which is sharply recorded in the sediment succession.(3)The age of lahar deposit layer yielding Kenyapithecus in the Fort Ternan area was dated 13.7 Ma based on the anorthoclase K-Ar and ^<40>Ar-^<39>Ar ages from phonolite lavas which sandwiched the fossil layer.(4)Earliest Late Miocene hominoid, which is very important for understanding on the evolution from great ape to human, and mammalian fossils were discovered from the lahar deposits of the Nakali Fm. The Nakali Fm consists mainly of lake and fluvial deposits with tuff and lapilli tuff beds including volcanic mud flow and pyroclastic flow deposits. Combined with the ^<40>Ar-^<39>Ar ages and magnetostratigrapy, the age of the Nakali Fm ranges from 10.1Ma to 9.8Ma (within Chron C5n.2n to C5n.1n).(5)Research of middle Miocene trachyte-phonolite flood volcanism in the whole Kenya rift ; geology, K-Ar age determination with magnetostratigraphy, petrography and geochemical characteristics.(6)Comparative geochemical studies on modern lacustrine sediments from the five lakes in the Kenya rift, and sedimentary rocks 16 to 5Ma.(7)Publications to journals and book (20 papers). The symposium was held at the annual meeting of Geological Society of Japan.