Late Holocene environmental changes in the eastern Africa: the Lake Hayk sedimentary succession

Fluvio-lacustrine deposits of East Africa contain an excellent archive to investigate Late Quatemary environmental changes. Lake Hayk, 80 m deep and 16 km: wide. lies in a tectonically-generated basin located 300 km NE of Addis Ababa. Late l-lolocene deposits. up to 10 m thick, accurnulated in fluvial, slope and lacustrine environment because of repeated fluctuation of'lake'S levcl and sediment supply. The Late Holoóene, as well as the present, subaerial deposilional environments arc mostly alluvial plains ori the southern margin and slope systems elsewhere. In the lattcr, colluvial fan deltas developed during lake level lowstands, while terrigenous sediment starvation ,and stromatolithic reef build up occurrcd during highstands. Similarly, valley incision and vertical aggradation of alluvial plaíns occurred respectively during lake low- and highstands. Palaeosols, mostly present in the lower part of the alluvial successíon, have vertisol-like character and point to phases of geomorphic stability. Organic mud with carbonato laminae point to coastal plain flooding during lake highstands. Stratigraphic architecture of the Late Holocenc successíon suggests six major phases of incision and aggradation. Radiocarbon dating allows to constrain phase l and 2, respectively to 3900-3300 BP and 1250-950 BP, and phases 3 to 5 to 320-270 BP. Phase 6 is dated as modern. Changes in sediment supply, as well as lake level variations, resultcd from imeractions between climatìc, tectonic and anthropogenic factors.