Geology of the Lothidok Range, Northern Kenya

This study provides a stratigraphic and geochronologic context for fossils collected from the Lothidok Range, northern Kenya, that include three new hominoid genera. Age control on the strata is provided by 2 2 potassium-argon determinations at 11 stratigraphic levels. Three fossil bearing horizons in the Lothidok Range are: (1) pre-17.7 Ma, (2) between 17.7 and 16.6 Ma and (3) between 13.6 and 12.0 Ma. Fauna from the lower two horizons are equivalent to the collections from Rusinga and the highest horizon is equivalent to, or slightly younger than, those from Fort Teman. Exposed Tertiary strata of the Lothidok Range are 1500 m thick, but the base is not exposed. In ascending order the section comprises 785 m of basalts and intercalated sedimentary rocks (Kalakol basalts, > 17.7 Ma), 540 m of interbedded sedimentary, pyroclastic and basaltic rocks (lower and upper Lothidok Formation, 17.7 - 12.0 Ma), 120 m maximum of basalts (Loperi basalts, 12.0 - 10.9 Ma) and 50 m minimum of undifferentiated Tertiary sedimentary strata (< 10.9 Ma). Sedimentary rocks of the lower Lothidok Formation consist primarily of polymictic conglomerates and conglomeratic litharenites with southwest paleotransport directions, and are interpreted as braided stream-alluvial fan deposits. The upper Lothidok Formation contains polymictic conglomerates and conglomeratic and feldspathic litharenites near its base, with finer grained lithic to arkosic sandstones near the top. The lower strata are interpreted as braided stream-alluvial fan deposits, and the higher strata are interpreted as meandering stream deposits. All upper Lothidok Formation strata have easterly paleotransport directions. Pyroclastic rocks of the Lothidok Formation consist of mafic alkaline and trachytic tephra and lahars. A disconformity within the Lothidok Formation represents a time gap from -16.4 to ~13.7 Ma. An angular unconformity exists between the Lothidok Formation and the Loperi basalts, and a third unconformity may exist within the undifferentiated Tertiary strata. The structure is dominated by late Tertiary normal faults. Only one fault predates deposition of the Lothidok Formation. Other faults postdate the undifferentiaited Tertiary strata and the range front fault postdates 4.1 Ma. No source terrains for the sediments and pyroclastic rocks are known. The sedimentary and pyroclastic rock record, however, provides evidence for a proximal, early Miocene volcanic highland and mafic alkaline volcanic center close to and east of the Lothidok Range as well as middle Miocene trachytic center(s) to the south.