The KBS Tuff (Kay Behrensmeyer Site Tuff) is an ash layer in East African Rift Valley sediments, derived from a volcanic eruption that occurred approximately 1.87 million years ago (Ma). The tuff is widely distributed geographically, and marks a significant transition between water flow and associated environmental conditions around Lake Turkana shortly after 2 Ma.
Between 1970-1985 the age of the tuff was the subject of intense academic dispute, with a variety of dates proposed by different geochemical and paleontological laboratories. This dispute came to be known as the KBS Tuff Controversy.
The KBS Tuff has been described as "the Turkana Basin’s most celebrated tephrostratigraphic marker."[1]