LATE-QUATERNARY STRATIGRAPHY AND PALEOLIMNOLOGY OF THE SOUTH BASIN OF LAKE TURKANA, KENYA

Fifty new sediment cores (piston cores, vibrocores, and sediment-water interface gravity cores) and over 1300 km of seismic reflection data collected in 2010 and 2011 from Lake Turkana, Kenya, East African Rift reveal a dynamic record of climate-driven lake level changes and accompanying shifts in basin lithofacies. Turkana is the largest rift valley lake in the eastern branch of the East African Rift system, and measures 250 km in length with maximum and mean depths of 120 and 30 m, respectively. The mildly saline (2500 ppm) and alkaline (pH = 9.2) water column is a consequence of its closed-basin hydrology, which has promoted the accumulation of assorted carbonate facies since the late-Neogene. Whereas deep-water sediments consist of hemipelagic muds, westward prograding delta deposits are observed in both high-resolution CHIRP and airgun seismic data, primarily near the eastern shore (flexural margin) of the southernmost half-graben.

At least 3 time-correlative sets of deltas or reworked shoreline deposits are observed in the geophysical data along the eastern shore at depths of approximately 26, 32, and 39 m below the modern lake surface. Other evidence for pronounced lake level shifts include: (1) gravel strandlines observed at elevations up to 63 m above the modern lake surface, (2) unconformities within the ~5-40 m thick acoustically transparent seismic sequence observed in the CHIRP seismic reflection data at depths of ~30-60 m below the modern lake surface, and (3) a high-amplitude reflection observed in CHIRP and airgun seismic data that is traceable throughout the South Basin and may indicate near-complete basin desiccation in the late-Pleistocene.

Lithologies observed in sediment cores include mixed siliciclastic and carbonate muds, ostracode silts, organic and diatom-rich muds, and sands and gravels comprised of Precambrian basement detritus and late-Quaternary volcanoclastics. In water depths of 5-25 m, ~0.2-0.8 cm-thick carbonate rinds encase volcanic cobbles and boulders on the lake floor, as observed in rock-dredge samples and in video collected from a remotely operated vehicle. New radiocarbon dates from the sediment cores provide constraints on sediment accumulation rates and the Lake Turkana hydrological record over the late Quaternary.