Le oscillazioni tardo-quaternarie del Lago Shala (rift etiopico): analisi dell'evoluzione ambientale dall'integrazione di evidenze morfologiche, sedimentarie e cronologiche

Abstract

New data on the late Quaternary environmental evolution of the Ziway-Shala lakes region (Main Ethiopian Rift) are presented in this paper. Geomorphological, stratigraphicaS and sedimentological studies on fSuvio-lacustrine deposits are used to improve the geological knowledge of the Holocene evolution on the eastern coast of the Shaia Lake (Ajewa embayement). This part of the Shala basin was considered a sample area to test morphologic and stratigraphic evidences of the late Quaternary lacustrine fluctuations recognized in the entire lake region and to detect possible causes for environmental changes. Several new radiocarbon dates of shells, wood and organic mud have been integrated with those from previous studies in the same area to reconstruct the chronology of these changes. The integration of geomorphological and chronological data suggest that lake level fluctuations were governed both by climate through arid/humid oscillations (as already hypothesised), and also by volcano-tectonic activity. This combined action markedly influenced Holocene morphological and depositional dynamics. It can thus be concluded that: 1) the controlling action of the Main Ethiopian Rift volcano-tectonic dynamics cannot be underevaluated for a correct interpretation of recent environmental changes in the Ziway-Shala lakes region; 2) the lacustrine fluctuations have to be seen as resulting from the interplay of climatic and geodynamic controls; 3) care is needed when correlating lacustrine deposits or lacustrine surface and their radiocarbon ages with effective lacustrine stillstands. It is not obvious to consider water level rising or falling as only due to climatic variations.