--> ABSTRACT: Seismic Stratigraphy and Lake Level Change in a Desert Rift Lake: Observations from New High-Resolution Seismic Reflection Data and Sediment Cores from South Basin, Lake Turkana, East African Rift, by Morrissey, Amy; Scholz, Christopher; #90142 (2012)

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Seismic Stratigraphy and Lake Level Change in a Desert Rift Lake: Observations from New High-Resolution Seismic Reflection Data and Sediment Cores from South Basin, Lake Turkana, East African Rift

Morrissey, Amy *1; Scholz, Christopher 1
(1) Department of Earth Sciences, Syracuse University, Syracuse, NY.

Lake Turkana is the largest lake in the world in a desert and is prone to frequent lake level changes because of its extreme climate. A new stratigraphic framework of the Lake Turkana Rift spanning that last ~20 ka was developed using 1300 km of high-resolution (CHIRP) and 1 in3 airgun seismic reflection data and over 40 sediment cores collected in 2010 and 2011. Two high-amplitude reflections that extend throughout the entire basin correlate with marked decreases in total organic carbon and increases in calcium carbonate, and are interpreted to have formed during major lake lowstands. These surfaces likely indicate near desiccation of this rift lake during the late Pleistocene, at ~15 and ~11 ka BP. In the overlying Holocene section, onlapping sequences and thick packages of flat-lying reflections indicate a major transgression during the early Holocene African humid period, when the lake level increased by ~175 m. During stillstands in the transgression, several progradational delta deposits accumulated along the eastern (flexural) margin of the South Basin at different subsurface depth and stratigraphic levels. Westward progradational reflection packages are observed within these structures in east-west airgun profiles. Lake Turkana was hydrologically-open during much of the Early and Mid-Holocene until ~4,000 years ago, when lake level fell below the catchment spill point (~90 m above modern lake level). An unconformity ~45 m below the current lake surface in the Late Holocene section suggests another drop in lake level at approximately 3 ka BP, after which lake level rose to its current stage. Water levels of Lake Turkana dropped approximately 15 m in the last 3 decades, indicating that dynamic hydrologic behavior has continued through the present day. Radiocarbon dated sediment cores suggest that sedimentation rates were highest in the latter part of the African Humid period.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California