Publication Date:
2011-10-13
Description:
Two lava flows with interbedded palaeosols outcrop c. 40 km SW of Mount Kenya, near the Amboni River north of Mweiga, Kenya along the Nyeri/Thompson Falls Road, at 0°18'S; 37°48'E. These flows, overlain by loess, are principally trachyandesite and form the base of the Mount Kenya Volcanic Series which, in the early literature, is described as being of probable Miocene/Pliocene age. Here we report 39Ar/40Ar dates (c. 5.2–5.5 Ma) and reversed magnetizations which establish a Latest Miocene to Earliest Pliocene age for these flows. Weathering characteristics of palaeosols interbedded with the lavas indicate generally dry climatic conditions during the Late Miocene, punctuated with humid events during the Pliocene and Quaternary. These Late Miocene–Quaternary palaeosols depict a relatively long and complex weathering history, followed by loess deposition. The palaeosols appear to have been episodically deflated, initially in phase with the deposition of lavas when surfaces were devoid of vegetation and later during periods of climatic deterioration when wind systems intensified. Such weathering histories within palaeosol profiles are also documented on nearby Mount Kenya, where well-weathered lower palaeosol horizons developed on Matuyama-age tills are overlain by much younger less-weathered horizons developed on Brunhes-age loess. The geochronology of Late Miocene lavas reported here provides maximum ages for weathering histories of palaeosols formed in a xeric tropical highland climate.
