STRATIGRAPHIC AND MINERALOGICAL RECORD OF LAKE LEVEL CHANGES IN DELTAIC SEDIMENTS OF LAKE BOGORIA, KENYA RIFT VALLEY

The most continuous stratigraphic records normally come from the deepest parts of lakes. The littoral margins, however, may record many more fluctuations in lake level, particularly in closed basins. Studies of deltaic sediments at Lake Bogoria, Kenya, coupled with observations of changes over 25 years, have revealed the complexity of the stratigraphic record of saline lake margins, and shown the influence of El Niño events on the delta stratigraphy.

Loburu, a small (0.5 km²) delta on the midwestern shoreline, is fed by ephemeral streams. Its two lobes are separated by a marshy interdistributary embayment. About 50 hot springs discharge along two N-S faults that transect the delta plain. Cores, pits and channel sections show complex interstratification of stream, sheetwash, beach, and delta front sediments, broken by semi-lithified, mud-cracked exposure surfaces and thin paleosols. The sediments are mainly feldspathic sands, silts, muds, and lava gravels. Lateral tracing of strata is difficult because of frequent erosional and depositional episodes associated with fluctuating lake level, tectonic movements, and the concurrent migration of distributary channels.

During the 1997-8 El Niño event, littoral marsh was drowned, shallow offshore sediments were winnowed, and coarser deposits were swept shoreward to form fine-gravel beach bars. These bars are stacked upon and against older bars that record earlier El Niño events. The gravels contain abundant reworked rhizoliths (opal-A, fluorite, calcite) that formed around roots of the former littoral marsh plants. These littoral gravel spreads record phases of high lake level. If high-levels are sustained, calcite stromatolites encrust stable substrates.

During prolonged periods of low lake level, many plants die and mudflats increase in area. These subaerial surfaces desiccate and become weakly cemented by calcite, opal-A, and zeolites where underlain by shallow groundwater. Once lithified, they protect underlying strata from erosion, and act as a firm substrate for sediments of later depositional-erosional cycles that may leave little permanent record. Minor cementation occurs in brief dry phases. The record of the delta, therefore, is biased toward high magnitude events during high levels, and strata that precede cementation during low phases.