New potassium-argon age determinations relevant to the Miocene fossil mammal sequence in east Africa

Abstract

The results of 57 potassium-argon and eight ' 10 Arj 30 Ar isotopic age cleter-minations macle at the Cambriclge Geochrono!ogy Laboratory on 27 East African rocks, related to fossil mammal localities and of presumecl Miocene age, are presentecl. The ages obtained are compared with 12 isotopic dates for rocks from the same area published previously and with four other hitherto unpublished ages. The majority of the apparent ages fall into the following groups: (I) nephelinitic (carbonatitic) volcanics associated with the Koru and Songbor fossilmammal assemblages-19.5 ± 0.3 to 19.9 ± 0.6 m.y.; (2) the fossil mammal assemblages aL Napak I, IV, V, and IX occur within tulfs yielcling minimum ages of 17.8 ± 0.4 ancl 19 ± 2.0 m.y.; (3) the main Rusinga fossil mammal assemblagcs are probably younger than 19 to 19.3 ± 0.3 m.y. but older than 15.9 ± 1.5 m.y., whilc the Karungu fossil mammal assemblage is dated at 22.5 ± 0.4 m.y.; (4) tuffs containing the Fort Ternan mammalian assemblage are confirmed as having an age of approximately 14' m.y.; (5) the main extrusions of plateau phonolites overlying various fossil mammal localities yield ages rauging from 13.6 ± 0.6 to 12.0 ± 0.3 m.y., and the basalt overlying the Moroto fossil assemblages is aged 12.5 ± 0.3 m.y.; (6) a new fossil mammal locality ncar ßukwa, Mt. Elgon, is older than 22.0 ± 0.2 m.y. The problems of potassium-argon dating· these later Tertiary rocks ancl some possible sources of error are cliscussecl Iogether with the wider implications of the ages established. INTRODUCTION AND AIMS