SEDIMENT RECYCLING IN EAST AFRICAN RIFT BASINS: IMPLICATIONS FOR THE EVOLUTION OF THE TURKANA DEPRESSION

The Turkana Depression is a wide, topographically low, segment of the East African Rift System (EARS) between the Ethiopian and East African Domes. The anomalous architecture of the Turkana Depression may be attributed to pre-existing lithospheric thinning related to Cretaceous-early Paleogene Anza-South Sudan rifting, inherited basement structure, or early onset of EARS extension. U-Pb and U-Th/He ages of detrital zircons, and fission track data from detrital apatites from Paleogene to Recent sandstones and river sands define the provenance of sediment in the Turkana Depression. These data have implications for the timing of rift initiation in Turkana, and Cretaceous-early Paleogene extensional tectonics in Central Africa and along the Indian Ocean margin. Detrital zircons from Cretaceous-Paleogene and late Paleogene-Miocene strata consist of various Precambrian age ranges, with the majority falling into the 570-610 and 760-790 Ma ranges. Modern river detrital zircons share similar age distributions with Cenozoic sediments, with the following variations: 1) sediment in drainages proximal to the SE rift flank contain a higher proportion of Archean grains compared to the larger rivers, which lack these older grains, and 2) Cenozoic detrital zircons comprise <0.1% of the total population. Fission-track ages from detrital apatites fall generally into three groups: ~200, ~80, and 35-16 Ma. This is due to extensive Late Cretaceous-Paleogene erosional exhumation followed by late Paleogene burial and heating, and then renewed denudation and cooling starting in Miocene time. The similarity between the detrital zircon age populations of Cretaceous-Cenozoic strata and modern sands suggests, recycling of sediment eroded from the basement in Late Cretaceous-Paleogene time. This is consistent with the general lack of significant erosion of the EARS flanks in Miocene-Recent time. These results suggest that much of Turkana may have been a topographic high in the Late Cretaceous-early Paleogene that separated the Anza and South Sudan rifts, that thinning and significant subsidence in Turkana started in Eocene-Oligocene time, and that localized uplift of the flanks of Miocene-Recent normal faults resulted in renewed erosion and recycling of sediment in the basin.