Abstract:
The Olorgesailie Basin in the southern Kenya Rift Valley contains a variety of lacustrine, wetland and terrestrial facies that were laid down during the last 1.2 million years. This study integrates diatom, ichnofossil, and rhizolith (silica and carbonate) evidence in order to identify and characterize shallow wetland deposition at the top (Member 14; ca. 493 ka) of the Olorgesailie Formation. A key feature of these deposits is the presence of facies with aquatic indicators (diatoms) that are overprinted by ichnofossils and rhizoliths that suggest drier conditions. Diatom floras include several fresh water Aulacoseira spp. and saline indicators such as Thalassiosira rudolfi, T. faurii, Cyclotella meneghiniana and Anomoeoneis sphaerophora. The main indicators of shallow fresh to mildly saline waters include Epithemia argus, Rhopalodia gibberula, Encyonema muelleri, and Synedra spp. Three main suites of ichnofossils indicate varying water table depths during their formation. These consist of 1) horizontal tunnels that suggest saturated substrates; 2) vertical burrows formed in substrates with relatively lower water tables; and 3) termite, hymenopteran, and beetle ichnofossils that indicate dry substrates. Member 14 of the Olorgesailie Formation also contains carbonate and siliceous rhizoliths, which occur both together and separately. Four possible models are suggested to explain the silica and carbonate rhizolith formation: 1) a complex of spring-fed marshes and dry areas; 2) ephemeral wetlands and floodplains; 3) wetlands with abundant organic decay, and 4) initial carbonate rhizoliths that were replaced by silica. Preservation of cellular structures and the reworking and incorporation of rhizoliths in overlying younger strata suggest that they formed early during diagenesis.
