CHARACTERIZATION OF CONGLOMERATES IN THE EARLY MIOCENE LOTHIDOK FORMATION OF WEST TURKANA, KENYA

The Lothidok Formation of West Turkana, Kenya contains sedimentary and volcaniclastic deposits that preserve significant Early Miocene hominoid fossils and their associated paleoenvironments. In this study, our goal was to characterize sedimentary clast composition and depositional modes for conglomerates of the Moruorot, Kalodirr, and Naserte Members of the Lothidok Formation. We analyzed a total of 20 conglomerate units, which include both fluvial (clast-supported) and lahar (matrix-supported) deposits. In each unit, 50 clasts were selected at random and clast diameter, lithology, presence of weathering, and roundness recorded. In general, clasts were most commonly pre-weathered, subrounded basalt or phonolite; other clasts include syenite, tuff, pumice, amphibole, and reworked conglomerate. The dominance of pre-weathered basalt and phonolite throughout the succession indicates a persistent, nearby bedrock source. Clasts of pumice and euhedral amphibole crystals are often found unweathered within lahars. The introduction of syenite (likely a xenolith), tuff, and amphiboles in the upper Moruorot Member suggests an initiation of local volcanic activity and is associated with the first lahars in the stratigraphic sequence. Whereas conglomerates of the Moruorot Member are dominated by alluvial deposition, the Kalodirr Member contains a larger fraction of lahars, likely due to higher volcanic activity. The Naserte Member marks a further change in the depositional system, with the introduction of reworked, intra-basinal sedimentary clasts possibly indicating lowering of relief. Our ongoing analyses will include sandstone petrography, geochemical analysis of representative gravel clasts and fine fractions (mudstones) from each member.