Stratigraphy of the Koobi Fora Formation (Pliocene and Pleistocene) in the Loiyangalani region of northern Kenya

摘要

Tuffaceous marker beds in the Loiyangalani region were previously identified as correlative with similar beds in the Shungura and Koobi Fora Formations of southwestern Ethiopia and northwestern Kenya. Associated sedimentary strata correlate with the Koobi Fora Formation, and include intercalated basalt flows. In the Loiyangalani region the Koobi Fora Formation has a composite thickness of at least 257 m, and records one time interval for which no strata are known in the Koobi Fora region. Four members of the Koobi Fora Formation (Lonyumun, Tulu Bor, Burgi, and KBS) are recognized in the region, and a composite member (Moiti-Lokochot) proposed because the Lokochot Tuff, the formal boundary between the Moiti and Lokochot Members has not been identified. This extends the known geographic area of the Koobi Fora Formation 100 km southward. These strata, together with others, were previously termed 'Loiyangalani formation (or Formation)'. Deposition in the Loiyangalani region began at about the same time as in the Koobi Fora and Nachukui Formations. Newly named basalts are Kankam (3.2-3.3 Ma), Lenderit (2.02 to 2.18 ± 0.02 Ma), and Balo (1.79 ± 0.02 Ma). We also report K/Ar ages on basalts from Mt. Kulal (2.04 ± 0.02 to 2.40 ± 0.03 Ma) east of the region and from Serima Gorge (0.77 ± 0.01 Ma) south of the study area.