Geology of the Neogene System in and around the Samburu Hills, Northern Kenya

Abstract

The Neogene System in and around the Samburu Hills, northern Kenya, is divided into seven main formations on the basis of stratigraphy and geochronology, which in ascending order are the Nachola, Aka Aiteputh, Namurugungule, Nanyangaten, Kongia, Nagubarat and Tirr Tirr Formations. The Nachola and Aka Aiteputh Formations of Makinouchi et al. (1984) have been redefined. The Nachola Formation, which is widely distributed along the Baragoi River, is the lowest part of the Neogene System and either unconformably overlies the Precambrian Basement Complex or is in fault contact with it. It consists mainly of basalts, phonolitic trachytes and their pyroclastics, and sediments. The Aka Aiteputh Formation conformably overlies the Nachola Formation and occurs in the Nachola area and the Samburu Hills. It is mainly composed of basalts intercalated with trachytes, trachytic pyroclastics and sediments. The Namurungule Formation consists chiefly of clastic sediments with intercalations of mud flow deposits and tuffs, and covers the Aka Aiteputh Formation. No structural gap between the Aka Aiteputh and Namurungule Formations is recognizable. The Nanyangaten Formation, consisting of basalt lavas, which is one of the "undifferentiated basalts" of Mainouchi et al. (1984), unconformably overlies the Nachola and Aka Aiteluth Formations. Kenyapithecus occurs in the Aka Aiteputh Formation at Site BG-X of the Nachola area (Ishida et al., 1984). The bed containing Kenyapithecus accumulated between 15.4 Ma and 12.8 Ma according to K-Ar dating of volcanic rocks (Itaya & Sawada, 1987). On the other hand, the Samburu hominoid fossil occurs in the Namurungule Formation (Makinouchi et al., 1984; Ishida et al., 1984) whose K-Ar age is between 10.8 Ma and 7.3 Ma (Itaya & Sawada, 1987). A major structural gap is recognized between the Namurungule and Nanyangaten Formations indicating that a significant tectonic disturbance in the (northern Kenya Rift) took place between about 10 Ma and 7 Ma based on geological data obtained by this work and Makinouchi et al. (1984), and geochronological data of Itaya & Sawada (1987).