ALTERNATING SILICA AND CALCITE LAMINAE IN PLIO-PLEISTOCENE OOLITES FROM LAKE TURKANA, KENYA

Ooids consisting of alternating silica-enriched and calcite-enriched laminae are present from the southeastern side of Lake Turkana, Kenya. Structurally, the oolite interval is located in the northern Kenya Rift system and crops out discontinuously along the upper part of a plateau. Stratigraphically, the measured section (21.7m thick) is part of the Plio-Pleistocene Koobi Fora Formation. The oolite bed pinches and swells with thicknesses ranging from 26 to 50cm, and is lacking sedimentary features such as cross-bedding. The oolite interval sits above oncolites and below large microbial balls/oncoids/stromatolites. Capping the microbial balls is a thick, cross-bedded biolitharenite composed predominately of gastropods and oyster shell hash.

The nuclei of the ooids (0.5-1 mm in diameter) vary between calcite peloids, amorphous silica, or lithoclasts. These nuclei are concentrically coated by alternating calcite and amorphous silica. Petrographically, the ooids vary greatly, with laminae that contain a micro-radial structure, a sparry calcite structure, a micritic structure, and/or an amorphous silica structure. SEM analyses reveal an even more complicated laminae structure, where the amorphous silica and crystalline calcite are tightly interlaminated with surficial halite crystals. Energy dispersive x-ray (EDX) was used to provide a qualitative assessment of the mineralogical variation between the two morphologically different laminae. Preliminary analyses show EDX spectra with laminae that have distinct silica spikes and laminae with distinct calcium spikes. Both the silica spectrum and the calcium spectrum are accompanied by Mg, Ca, Cl, Al and Fe peaks.

Only one outcrop is known to contain these silica-calcite ooids in the study area. Alternating laminae chemistry is hypothesized to be caused by local environmental changes related to hydrothermal spring activity (temperature, chemistry, alkalinity) within Lake Turkana. Future microprobe analyses will quantitatively determine the mineralogical variation of the ooids, and stable isotope analyses will be used to assess a possible hydrothermal contribution into the formation of these chemically unusual ooids.