Relationships between major structures, salic volcanism and sedimentation in the Kenya Rift from the equator northwards to Lake Turkana

Abstract

Summary At the equator, the Kenya (Gregory) Rift Valley is a half-graben, faulted on the eastern side. It passes northwards into a deep graben with a double western margin, and then into what is essentially a downwarped depression. The deep graben sector (0°15′–1°00′N) is virtually devoid of late Miocene-Quaternary trachytic central volcanoes: their counterparts in areas of strong faulting are salic flood lavas. Moreover, the scarcity of pyroclastics in early-late Miocene ‘plateau’ phonolite sequences of the deep graben suggests that the thick tabular lava units are some distance from source. The phonolites exposed on the western side of the rift (Kamasia-Uasin Gishu) probably came from eruptive centres beyond the deep graben, whereas on the eastern side (Laikipia) a substantial part of the sequence could well belong to a lava shield on the plateau. In contrast to intra-volcanic pyroclastic sediments that occur predominantly N of latitude 1°N, the formations in the Kamasia region consist of waterlaid deposits intercalated with tabular lava flows. This spectacular development of waterlaid sediments is attributable to the absence of major eruptive centres and the existence of tectonic-geomorphic sediment traps in the deep graben sector of the rift.