ABSTRACT
During routine geological mapping of an area in Kenya north of the Equator and between longitudes 37° and 37°30ˈE, a sequence of bedded sediments and pyroclastics, referred to as the Nanyuki Formation, was noted on the northern slopes of the Mount Kenya stratovolcano. The sediments comprise a sequence of decreasing grain size consisting of poorly- to well-consolidated gravels, sands, silts and clays, and including diamictons. These are interbedded locally with trachytic ash presumed to be associated with the Ithanguni volcano to the southeast.
The component clasts in the gravels are of igneous rock types belonging to the Mount Kenya volcanic suite, and include nephelinesyenite which outcrops only on the central peaks of the mountain. In addition to clast types, a number of factors such as cross-bedding directions, overall grading, rounding of clasts, areal distribution and lack of fossils all point to an origin for the sediments as glaciofluvial material deposited by high energy meltwater streams within a piedmont fan on the margin of the Mount Kenya volcano. The disposition of the sediments indicates an apex of the fan close to the town of Nanyuki near the confluence of the modern Liki and Nanyuki rivers, whose catchments include almost the entire northwestern sector of the summit area (above 4200m) of Mount Kenya.
The Nanyuki Formation postdates the main period of volcanic activity of the Mount Kenya volcano, but is contemporaneous with the activity of the Ithanguni volcano. A trachytic tuff from close to Nanyuki, correlated with Ithanguni tuffs has been determined to be 320,000 years old. In addition, the formation predates boulder beds and fluvial deposits directly related to later glacial deposits on Mount Kenya. The most likely correlation for the formation is with probable glacial deposits pre-dating the Teleki Formation (Mahaney, 1982). The formation is considered to have been deposited following a major glaciation in the period from c400,000 years to c320,000 years ago (see Mahaney, this volume).
