Relative ages of loess and till in two Quaternary palaeosols in Gorges Valley, Mount Kenya, East Africa

Abstract

Two compound palaeosol profiles formed in loess and till were studied in the oldest early Quaternary end moraine (Gorges age) system on Mount Kenya. Although both tills appear to have similar weathering histories, the overlying loessic sediments, serving as secondary parent materials for the palaeosols, are considered to have different ages on the basis of colour, presence or absence of clay films, and field texture (especially percentage of silt). Palaeomagnetic determinations showed both tills to have reversed remanent magnetism, suggesting that they were emplaced during the Matuyama Chron. The overlying loessic sediments in profiles GOR55 and GOR58 show normal magnetism and grade upward into the lower zone of bioturbation in the A horizon complex. However, in GOR58 the surface loessic sediments have normal remanent magnetism with considerable secular variations, suggesting it was emplaced over a longer time frame during the Brunhes Chron (i.e. < 0.78 Ma). Profile GOR55 contains larger amounts of gibbsite, indicating more aggressive leaching over time, and an absence of secular magnetic variations suggests deposition over a shorter time interval. The study of volcanic feldspars and ash, using scanning electron microscopy, in the two profiles, shows that sand grain (63–250 μm) weathering is slightly different in the two tills; relative weathering effects (e.g. corrosion of grain surfaces and neoformation of clay minerals) are greater in profile GOR55 than in profile GOR58. Overall the data indicate that some sites in the lower tropical Afroalpine timberline environment may have aeolian covers that are considerably younger than the deposits they overlie. © 1997 by John Wiley & Sons, Ltd.