||Five members of the Koobi Fora Formation: the Lonyumun, Moiti, Lokochot, Tulu Bor, and upper Burgi members, are exposed in Areas 40 and 41 (study area) northeast of Ileret in northern Kenya. Areas 40 and 41 were first mapped using tonal contrasts on aerial photographs by Key and Watkins in their 1988 study, and were not revisited until Gathogo and Brown did a reconnaissance, broadly mapping exposures in their 2006 study. The study area is located on the eastern margin of the Turkana Basin, where the base of the Koobi Fora Formation is in contact with volcanic rocks of Miocene age. Five tuffs are exposed in the study area. Four tuffs occur in the Lonyumun Member: the Guo Tuff, the Kanyeris Tuff, and the newly named Tukunan and Kisemei tuffs. This study, directly or indirectly provides dates for the tuffs in the Lonyumun Member and associated strata by means of magnetostratigraphy and 40Ar/39Ar dating methods. Paleomagnetic sampling and the known dates on the Moiti and Tulu Bor Tuffs of 3.4 and 4.0 Ma respectively, proved invaluable in assigning ages to strata, and was materially helped by an 40Ar/39Ar age of 4.06 ± 0.02 Ma on the Kisemei Tuff. Three tuffs known at many other localities south of the study area, the Lokochot, a-Tulu Bor and Burgi Tuffs, are missing from the study area. Stratigraphic correlations between exposures proved difficult due to numerous and pervasive disconformable surfaces, so that significant time gaps exist in the Lonyumun Member; all of the Moiti Member except the Moiti Tuff is missing, as is all of the lower Burgi Member. Newly discovered fossils in the Lonyumun Member have also been indirectly dated. Initial identification suggests correlation to fossils in the Apak Member at Lothagam southwest of Lake Turkana. In Areas 129 and 130 the Tulu Bor Member is in contact with the Lonyumun Member. Three tuffs, disconformable on the Tulu Bor Tuff allow correlation outside of these areas of which the Aberegaiye Tuff is dated to 2.70 ± 0.02 Ma on alkali feldspars contained in pumices. The other tuffs include a correlate to a tuff in submember D-5 of the Shungura Formation and a correlate to Area 116 to a tuff below the Ninikaa Tuff, the former being the first documented proof for deposition between 2.6-2.0 Ma in the Koobi Fora Formation.