Stratigraphy of the Lothidok Range, northern Kenya, andK/Ar ages of its Miocene primates

Abstract

We propose a formal stratigraphic nomenclature for fossil-bearing strata exposed in the Lothidok Range of northern Kenya. About 1540 m of sedimentary and volcanic rocks are defined (in ascending order) as the Kalakol basalts (and Eragaleit beds), the Lothidok Formation (Moruorot, Kalodirr, Naserte, Lokipenata and Kalatum Members), the Loperi Basalts, and the Turkwel beds. These new units provide a stratigraphic context for fossils collected from the region since 1932, including the recently described primate taxa Turkanapithecus and Afropithecus, and also a framework for isotopic ages of the strata. Fossiliferous strata occur within the Eragaleit beds of the Kalakol basalts, the Kalodirr, Naserte and Kalatum Members of the Lothidok Fm., and the Turkwel beds of presently undifferentiated strata. On the basis of potassium-argon (K/Ar) dates on associated volcanic rocks, faunas of four ages are recognized: those from the Eragaleit beds (24–27·5 Ma; Late Oligocene), those from the Kalodirr and Naserte Members (17·8–16·8 Ma; latest Early Miocene), those from the Kalatum Member (13·8–12·2 Ma; Middle Miocene) and those from the Turkwel beds (<11·9 Ma). Turkanapithecus and Afropithecus derive from the lower part of the Lothidok Fm. (latest Early Miocene). Primate fossils have also been recovered from the Eragaleit beds (Late Oligocene), and the Kalatum Member (Middle Miocene). The assemblage of fossil higher primates from the Lothidok region appears to be distinct from those of western Kenya and Napak, and is perhaps more similar to that recovered from Buluk, northeast of Lake Turkana. Age determinations are also reported for a basalt that overlies fossiliferous strata at Loperot, situated about 90 km south of the Lothidok Range; faunas at Loperot predate 15 Ma.