Refinement of the age of the middle miocene Fort Ternan Beds, western Kenya, and its implications for old world biochronology

It has become increasingly obvious over the past two decades that the fossiliferous strata at Fort Ternan, Kenya, are probably somewhat younger than 14 Ma, an age which has long been attached to the deposits. This realisation flows from geological and biochronological observations. In order to test the hypothesis, resampling of all the lava flows in the region of Fort Ternan was undertaken in 2003, especially those underlying the Fort Ternan Beds in the Kipchorion Gorge where the sequence is the most complete. Samples obtained from lava flows underlying and overlying the fossil beds were analysed for anorthoclase K/Ar and 40Ar/39Ar and biotite 40Ar/39Ar age determinations. The results reveal that the age of the fossiliferous sediments is ca 13.7 +/- 0.3 Ma. Since Fort Ternan yielded the 'core fauna' that defines Faunal Set IV of the East African biochronoiogical sequence this refinement of its age will impact on age estimates of neighbouring Faunal Sets, as well as on other faunas correlated to Fort Ternan, including those in Europe belonging to MN Zones MN 5, MN 6 and MN 7/8. (c) 2006 Academie des sciences. Published by Elsevier SAS. All rights reserved.