Five carbonatite complexes of probable Lower Miocene age occur along a nne–ssw line some 65 km in length in the southeast of Uganda, close to the Kenya border (King & Sutherland 1966). They are associated with the major central-type alkaline volcanoes of Mount Elgon and Kadam (Fig. 1) which may also be assigned to the Lower Miocene on the basis of recent K-Ar age determinations (Bishop, Miller & Fitch 1969).
Sub-volcanic sediments, termed the Bugisu Series, crop out sporadically on the periphery of Mount Elgon and partly underlie Nkokonjeru, a neighbouring volcanic outlier. This Series was first described by Davies (1952 p. 7), who inferred four separate lacustrine basins of sedimentary deposition, three near the edges of the main Mount Elgon volcanic pile and the fourth underlying Nkokonjeru. Later, the Geological Survey of Uganda carried out a pitting and drilling programme on the northern slopes of Nkokonjeru and located various sub-horizontal sediments classified as ‘calcareous tuffs’, ‘ashy limestones’, and conglomerates in the lower part of the sequence.
The writer made a petrographic examination of available drill-core and found that most of the rocks were calcarenites and calcirudites containing abundant magnetite and apatite with small amounts of detrital pyrochlore––all apparently derived from carbonatites. Accordingly a detailed survey of the Nkokonjeru area was carried out, accompanied by intensive geochemical stream sediment sampling. This note summarizes the results.