Gombe Group basalts and initiation of Pliocene deposition in the Turkana depression, northern Kenya and southern Ethiopia

Abstract

A little before 4 Ma ago, deposition of Pliocene and Pleistocene strata described as the Omo Group began in the Turkana and Omo basins of northern Kenya and southern Ethiopia. Soon after, basaltic magma erupted as thin lava flows, and intruded as dykes into the oldest Pliocene strata of the basin. These flows and intrusions are similar petrographically and geochemically, and mark a basaltic magmatic event spanning latitudes from 2° 45′ N to 6° 45′ N at a longitude of about 36° E. By 3.94 Ma, this basaltic magmatic activity had ceased. Previous researchers used these lavas as an important seismic marker in their study of the southern part of the Turkana Basin. Subsequent volcanic eruptions formed North, Central and South islands in Lake Turkana, and the Korath Range in southern Ethiopia. Thus there was a hiatus in basaltic magmatic activity of nearly 4 Ma in the area presently occupied by Lake Turkana and the lower Omo Valley, although volcanism continued on the eastern margin of the basin. Here we review the field occurrences of these basalts, their distinctive petrography, composition, age and significance to Pliocene deposition in the basin.