Stratigraphic and structural geology of Area 117, Koobi Fora Region, northern Kenya

The lowest four members of the Koobi Fora Formation: the Lonyumun Member, the Moiti Member, the Lokochot Member, and the Tulu Bor Member, are exposed in Areas 117, 137 and 116. These deposits include strata that were incorrectly identified as the KBS Tuff and the Surgaei tuff in Findlater's 1976 study. The composite thickness of strata within the study area is approximately 170 m, approximately 100 m of which are deposits of the Lonyumun Member. In addition to these strata, intrusions of Gombe Group basalts are exposed as well as a littoral zone of the lacustrine Galana Boi Formation of Holocene age.;A large lake from 4.2-3.99 Ma, a fluvial deltaic system from 3.99-3.40 and a large fluvial system from 3.40-3.06 Ma deposited strata of the Koobi Fora Formation in this region. This observation is consistent with Brown and Feibel's 1991 study.;Nine tuffs are exposed in the study area. Two of these tuffs, the Naibar and Aberegaiye Tuff, are newly named in this study. The Naibar Tuff was deposited by fluvial action to the shores of the receding Lonyumun Lake. Stratigraphic relationships reveal a date of ∼4 Ma. The Gombe Group basalts did not intrude the Naibar tuff. Several tuffs known from elsewhere in the Koobi Fora Region are missing from the section in Area 117, including the Moiti Tuff and the Wargolo Tuff, and the Lokochot Tuff, prominent in other locales in the region is poorly represented in small outcrops. It is believed that these units were removed by erosion attendant on incision by fluvial channels associated with the Tulu Bor Tuff. Because the Lokochot Tuff is the boundary between the Moiti Member and the Lokochot Member these two units are combined on the geological map. South of the study area proper, and also east of the study area, the upper Burgi Member unconformably overlies strata exposed in Area 117.;The area is affected by a large normal fault west of the Kokoi Highland, which is now named the Kokoi normal fault. Movement along this fault caused footwall deformation in Area 117 which manifests itself as west dipping normal faults. These normal faults bring strata of the Lonyumun Member, the Moiti/Lokochot Member, and the Tulu Bor Member into contact with each other. Movement along this fault must have been initiated after the upper Tulu Bor Member was deposited, but before the upper Burgi Member was deposited.