TEPHROSTRATIGRAPHIC STUDIES IN EAST AFRICA AND THEIR CONTRIBUTION TO OUR UNDERSTANDING OF THE GEOLOGY, STRATIGRAPHY, AND AGES OF THE SEDIMENTS IN THE TURKANA BASIN

The East African Rift system, a significant tectonic and geological feature, experienced substantial volcanic activity during the Plio-Pleistocene. This period saw both silicic and basaltic volcanism, which left enduring evidence in the geological record.

The Gombe Group Basalts, which extensively erupted in the Turkana Basin around four million years ago, signify the onset of Pliocene deposition in this region. This volcanic activity was primarily confined to a brief period of roughly 0.3 million years. Although these basalt outcrops are known by different local names, such as Mursi, Usno, and Lothagam, they collectively form the Gombe Group. The group intermingles with the basal Omo Group's sedimentary deposits, highlighting a link between deposition processes and basaltic eruptions driven by regional tectonic activities. Simultaneously, siliceous volcanism began in the Ethiopian highlands, contributing volcanic ashes to sedimentary basins in the region. These tuffs, alongside interbedded sediments, are essential for understanding the stratigraphic relationships among the Shungura, Koobi Fora, and Nachukui Formations within the Turkana Basin.

Tephrostratigraphic studies, which utilize volcanic ash and tuff beds to arrange tephra layers, have provided isotopic ages and temporal correlations. Over 2,530 chemical analyses of tephra glass from the Turkana Basin have been conducted, identifying about 350 chemically distinct types, with 295 organized into a reliable sequence. Some of these tephra layers are widespread across different formations, with around 78 correlating between formations within the Omo-Turkana Basin. Additionally, about half a dozen tuffs have been discovered in deep-sea cores, the Hadar Formation, prehistoric sites in Gadeb Ethiopia, the Baringo Basin of Kenya, and the Western Rift Valley in Uganda, thus enabling regional correlations.

The abundance of tuffs necessitates grouping them into subsets based on widespread ash layers serving as markers. This involves discussing all investigated tephra layers from the oldest to the youngest, detailing their type localities, chemical analyses, and areal distributions, and defining previously undefined tephra layers.