Late Quaternary lacustrine deposits of eastern Africa represent a suitable archive of regional to global climatic variations and their influence on sedimentary and geomorphic processes. When the Late Holocene interval (last 5 ky) is considered, palaeoenvironmental changes recorded in the stratigraphic and morphological record may reveal crucial aspects for understanding present and future environmental response to natural (climate, tectonism, etc.) and anthropogenic forcings. Lake Hayk in Northern Ethiopia bears stratigraphic, pedologic and sedimentologic signatures of rapid environmental transitions occurred in the last 4 ky in a region characterized by high geomorphic sensitivity and intense human pressure. Lake Hayk, about 300 km NE of Addis Ababa, lies in a 16 km2 wide tectonically-controlled basìn, up to 80 m deep, developed in the structurally complex belt linking the Ethiopian Plateau to the Afar-Awash depression. The basin is bordered by steep fault-controlled, N-S trending, escarpments, cut in Tertiary flood basalts with intervening tuffs, particularly developed on the eastern and northern sides. Short, seasonal rivers drain the southern margin, flowing in relatively wide floodplains; on the northern margin, characterized by steep escarpments, the drainage network is poorly developed. Late Holocene deposits, up to 10 m thick, and exposed along the lake's margins, accumulated in three distinct depositional systems developed throughout repeated fluctuation of lake's level and sediment supply. The subaerial depositional environments of the last 4 ky were, analogously to the present, alluvial plains on the southern margin and slope systems elsewhere, especially on the northern margin. On the footslopes, mass flow and subordinately sheetflow created colluvial fan deltas during lake level lowstands, followed by sediment starving and development of stromatolithic reefs during lake's highstands. Along the southern margin vertical aggradatìon of alluvial and coastal plains was punctuated by short-lived drops in lake level. Incised fluvial valleys, filled with gravels and sands, document low stands, whereas floodplain muds, peat deposits and sandy-gravelly, shoal-water or Gilbert-type, deltas point to lake transgressions and highstands. Different palaeosols, and soil sediment layers, are intercalated in the succession. Palaeosols are mostly present in the lower part, and have mostly vertisol-like character; they point to phases of geomorphic stabilìty. Soil sediments contribute to understanding of depositional and environmental dynamics. Limitedly exposed sands and organic muds with carbonate laminae attest to the marginal lake flooding coastal plains during phases of rising level. Stratigraphic architecture of the successions exposed on the opposite margins of the basin suggests seven major phases of incision and aggradation. Radiocarbon dating allows to constrain phase 1 and 2, respectively to 3900-3300 BP and 125O~950 BP, and phase 6 to 690-540 BP, Further radiocarbon analyses are planned for precise positioning of phases 3-5. This will in fact date a marked change in sediment supply, recorded in the southern margin by the change from floodplain mud and sandy delta front, typical respectively of phases 1 and 2, to gravel-dominated fluvio-deltaic deposits in phases 3-5. Phase 7 was characterized by development of alluvial fans in the last few centuries, imparting a control on present morphology and hydrography particularly on the southern alluvial plain. Changes of sediment supply, as well as lake level variations, will be discussed in the frame of possible interactions between climatic, tectonic and anthropogenic events.

Facies development of Late Holocene slope-fluvio-lacustrine depositional systems in the Lake Hayk (Northern Ethiopia) / C. BARTOLINI; BENVENUTI M; CARNICELLI S; CIAMPALINI R; FERRARI G. A; GHINASSI M; PAPINI M; SAGRI M. - STAMPA. - (2005), pp. 0-0. ((Intervento presentato al convegno Geoitalia 2005 tenutosi a Spoleto nel 21-23 Settembre 2005.

Facies development of Late Holocene slope-fluvio-lacustrine depositional systems in the Lake Hayk (Northern Ethiopia)

BARTOLINI, CARLO;BENVENUTI, MARCO;CARNICELLI, STEFANO;PAPINI, MAURO;SAGRI, MARIO
2005

Abstract

Late Quaternary lacustrine deposits of eastern Africa represent a suitable archive of regional to global climatic variations and their influence on sedimentary and geomorphic processes. When the Late Holocene interval (last 5 ky) is considered, palaeoenvironmental changes recorded in the stratigraphic and morphological record may reveal crucial aspects for understanding present and future environmental response to natural (climate, tectonism, etc.) and anthropogenic forcings. Lake Hayk in Northern Ethiopia bears stratigraphic, pedologic and sedimentologic signatures of rapid environmental transitions occurred in the last 4 ky in a region characterized by high geomorphic sensitivity and intense human pressure. Lake Hayk, about 300 km NE of Addis Ababa, lies in a 16 km2 wide tectonically-controlled basìn, up to 80 m deep, developed in the structurally complex belt linking the Ethiopian Plateau to the Afar-Awash depression. The basin is bordered by steep fault-controlled, N-S trending, escarpments, cut in Tertiary flood basalts with intervening tuffs, particularly developed on the eastern and northern sides. Short, seasonal rivers drain the southern margin, flowing in relatively wide floodplains; on the northern margin, characterized by steep escarpments, the drainage network is poorly developed. Late Holocene deposits, up to 10 m thick, and exposed along the lake's margins, accumulated in three distinct depositional systems developed throughout repeated fluctuation of lake's level and sediment supply. The subaerial depositional environments of the last 4 ky were, analogously to the present, alluvial plains on the southern margin and slope systems elsewhere, especially on the northern margin. On the footslopes, mass flow and subordinately sheetflow created colluvial fan deltas during lake level lowstands, followed by sediment starving and development of stromatolithic reefs during lake's highstands. Along the southern margin vertical aggradatìon of alluvial and coastal plains was punctuated by short-lived drops in lake level. Incised fluvial valleys, filled with gravels and sands, document low stands, whereas floodplain muds, peat deposits and sandy-gravelly, shoal-water or Gilbert-type, deltas point to lake transgressions and highstands. Different palaeosols, and soil sediment layers, are intercalated in the succession. Palaeosols are mostly present in the lower part, and have mostly vertisol-like character; they point to phases of geomorphic stabilìty. Soil sediments contribute to understanding of depositional and environmental dynamics. Limitedly exposed sands and organic muds with carbonate laminae attest to the marginal lake flooding coastal plains during phases of rising level. Stratigraphic architecture of the successions exposed on the opposite margins of the basin suggests seven major phases of incision and aggradation. Radiocarbon dating allows to constrain phase 1 and 2, respectively to 3900-3300 BP and 125O~950 BP, and phase 6 to 690-540 BP, Further radiocarbon analyses are planned for precise positioning of phases 3-5. This will in fact date a marked change in sediment supply, recorded in the southern margin by the change from floodplain mud and sandy delta front, typical respectively of phases 1 and 2, to gravel-dominated fluvio-deltaic deposits in phases 3-5. Phase 7 was characterized by development of alluvial fans in the last few centuries, imparting a control on present morphology and hydrography particularly on the southern alluvial plain. Changes of sediment supply, as well as lake level variations, will be discussed in the frame of possible interactions between climatic, tectonic and anthropogenic events.
Epitome 2005
Geoitalia 2005
Spoleto
21-23 Settembre 2005
C. BARTOLINI; BENVENUTI M; CARNICELLI S; CIAMPALINI R; FERRARI G. A; GHINASSI M; PAPINI M; SAGRI M
File in questo prodotto:
File Dimensione Formato  
geoitalia001.pdf

Accesso chiuso

Tipologia: Pdf editoriale (Version of record)
Licenza: DRM non definito
Dimensione 104.67 kB
Formato Adobe PDF
104.67 kB Adobe PDF   Visualizza/Apri   Richiedi una copia

I documenti in FLORE sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2158/236662
Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus ND
  • ???jsp.display-item.citation.isi??? ND
social impact