LACUSTRINE CARBONATE STRATIGRAPHY OF SOUTHEASTERN LAKE TURKANA, KENYA

The Plio-Pleistocene Koobi Fora Formation, which is well-characterized along the northeastern shores of Lake Turkana as a result of intensive exploration for hominid fossils, is also present along the southeastern shores. The Tulu Bor and Burgi members of the formation crop out to the southeast, as determined by correlation of tuff layers within the sedimentary sections. Most of the area is covered with volcanics, and accordingly, the Koobi Fora Formation is discontinuous and many of the exposures are observed in small quarries. Although expansive outcrops are rare, numerous in-situ carbonate-bearing outcrops are observed within the study area. Key lithologies include grainstones, packstones, wackestones, calcite-cemented biolitharenites, as well as microbialites, including stromatolites and oncolites. The grainstones are the most prevalent of the carbonate rocks and commonly contain ostracods. Stromatolites and oncolites vary in diameter from ~3cm to ~75cm and the stromatolites are found primarily in association with basalt or large calcite veins. Some of the largest stromatolites are flat-topped, coalescing specimens which average 0.5m in diameter. At several locations, upwardly coarsening oncolite sequences are observed. Additionally, three large (up to 7.5m thick), rounded to horse-shoe shaped, mounded stromatolitic outcrops were identified. Preliminary analyses of the large mounds suggest hydrothermal influence to the depositional environments, the first of which to be described from this area. Evidence includes the superficial shape of the outcrops and the meso-structure of the microbialites. The general stratigraphic sequence of the carbonate facies observed is basalt cobbles encrusted by microbialites overlain by either biolitharenites or ostracod grainstones or packstones, indicating fluctuating lake levels.