Koobi Fora Stratigraphy

STRATIGRAPHY
OF KOOBI FORA

All of the Plio-Pleistocene deposits and interbedded tuffs [volcanic materials deposited by nearby volcanoes] found at Koobi Fora are placed within what is termed the Koobi Fora Formation. This is divided into eight successive members, each named for a tuff located at its base. These are: Lonyumun (4.3 to 4.1 m.y.a.); Moiti (4.1 to 3.5 m.y.a.); Lokochot (3.5 to 3.35 m.y.a.); Tulu Bor (3.35 to 2.6 m.y.a.); the Lower Burgi (2.6 to 2.0 m.y.a.); Upper Burgi (2.0 to 1.89 m.y.a.); KBS (1.89 m.y.a. to 1.64 m.y.a.); Okote (1.64 to 1.39 m.y.a.); and Chari (1.39 to 0.6 m.y.a.).

The various soil deposits making up the different members vary from region to region within the study area depending upon the geologic and topographic setting. This means that different soil deposits and different depths of deposits can be found throughout the study area. This is similar to the types of soil deposits found in the Phoenix Basin around us here in the Mesa area. Soils deposited along the Salt River will be different from those along the flanks of South Mountain or the McDowell Mountains for example. They will be deposited at the same time but by different geological processes.

It is only the tuff deposits created by volcanic eruptions that can dated using a method known as potassium-argon dating.

The majority of the Hominid fossils come from deposits overlying the KBS tuff dated to about 1.89 m.y.a. Fossils clearly representing a robust australopithecine have been found mainly in this member. However, they are found below it as well. Fossils commonly attributed to an early Homo have been recovered from the member just below the KBS tuff. The most famous of these is KNM-ER 1470. Fossils clearly representing Homo Erectus have also been found at Koobi Fora. These commonly are found above the Okote tuff. There is yet another Hominid that has been found in and below the KBS member. It is unclear as to what this Hominid is exactly. It may represent a small brained early Homo or something similar to Australopithecus Africanus.