Late Neogene Paleosols and Paleoclimate, Mweiga Area, Mount Kenya, East Africa
Abstract
A sequence of lavas, principally trachyandesites and tephrites of Pliocene age, forming the underpinning of the Mt. Kenya Volcanic Series and long considered to have a Pliocene/Miocene age, were recently dated by Ar/Ar, and from oldest to youngest, gave ages of 5.48, 5.38 and 5.19 Ma. All dated lavas are reversely magnetized. Outcrops of two flows, both with interbedded paleosols, and overlain with loess occur near the Amboni River north of Mweiga, Kenya, along the Nyeri/Thompson Falls Road at 0 degrees 18 minutes S; 37 degrees 48 minutes E. Mineralogy of lavas was determined using microprobe and whole rock chemistry, while sediments and paleosols were analyzed using X-ray diffraction and Energy-Dispersive Spectrometry. Sediments and paleosols were studied for lithologic uniformity, weathering characteristics, and clay mineralogy. Ages of lavas and sediments were determined by Ar/Ar dating and paleomagnetism. The compound paleosols of Late Neogene age depict a long and complex weathering history of rock alteration, leading in part to the formation of claystone and later saw the addition of loessic sediments. The paleosols appear to have been episodically deflated, initially in phase with outpourings of lavas, and later with periods of climatic deterioration when wind systems intensified. Such disjunct weathering histories within multi- component paleosol profiles are similar to those documented on nearby Mount Kenya, where well-weathered lower paleosol horizons developed on Matuyama-age till are overlain by much younger, less weathered paleosols developed on Brunhes-age loess. These first-reported Ar/Ar ages, and paleomagnetism of lavas within the very Late Miocene/Early Pliocene, provide maximum ages for initial weathering rates in a xeric tropical highland climate. Clay mineral analyses of both the weathered bedrock and surface loessic sediments indicate an initial prolonged dry climate during the Lower Pliocene, followed by more humid conditions during the Upper Pliocene and Quaternary.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSMPP71B..08M
- Keywords:
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- 0486 Soils/pedology (1865);
- 1625 Geomorphology and weathering (0790;
- 1824;
- 1825;
- 1826;
- 1886)