Sand dunes on Lake Fagnano Cliffs, Tierra del Fuego, Argentina.

CORONATO, A.; SEPPÄLÄ, M.; PONCE, J. F.; RABASSA, J. O.

La Plata

Congreso; IV Congreso Argentino de Cuaternario y Geomorfología; 2009

Asociación Argentina de Cuaternario y Geomorfología. Associação Brasileira de Estudos do Quaternário

Sand dunes have been recognized along the southern coast of the lake Fagnano, Isla Grande de Tierra del Fuego (54°26’ - 54°37’ S; 68°35’ W - 66°42´W). They are placed in subantarctic humid temperate-cold climate, where the Nothofagus sp. Southern Beech develops. The westerlies blow along the lake corridor during the whole year but they are more frequent during spring and summer. The lakes long axis is W-E; it is 110 km long and covers 609 km2 at 26 m a.s.l. Its width varies from 2.8 to 9.8 km and the maximum depth is 200 m. Basal moraines composed of till with sand and gravel layers form the cliffs along the southern and eastern coasts. The gravel beach is minor than 1 m and wave erosion at the foot of the cliff is a common process. Wavesorted sand is presently blown up the cliff and deposits over the Nothofagus sp. leaves forming the sand dunes.  Sand accumulation over glaciofluvial deposits within basal till forms dunes of 12-8 m high in the upward slope and 6 m high on the opposite. They are approximately 350 m long and 40 m with. Slopes are assymetrical. Granulometric analyses were performed in two samples of Valdez Dune and in six samples of. Ruta Vieja Dune. The abundance of deposited material varies from fine sand (31 - 55 %), very fine sand (29-45 %), silt - clay (14 - 24%) to medium sand (0,5 - 1 %) in Valdez Dune, meanwhile in the Ruta Vieja Dune the abundance varies from fine sand (36 - 70 %), medium sand (6 - 40 %), silt - clay (5 - 16%), coarse sand (0,5 - 16 %) to very fine sand (1,5 - 9,5 %). Organic layers are buried into the sands. Radiocarbon dates on organic and charcoal show that vegetation were placed over the sands around at 1,140 +/-50 yr B.P. (AA74218) in Valdez Dune and twice at 2,260 +/-45 yr. BP (AA74219) and 3,200 +/-39 yr BP (AA74220) at Ruta Vieja Dune. Considering the nearest of the lake and the narrow gravelly shore, the unique source of material to be blown and deposited in order to form the dunes is the underlying till forming the cliffs. Turbulent air flow during wind episodes removes the fine particles of till when it is dry and moves them up to short distances producing sand accumulation along the cliff edges.  Since the organic material is buried with sand layers, we support that vegetation was covered by sand accumulation repeatedly, and it was also affected by Holocene fires -natural or anthropogenic origin- at Ruta Vieja Dune. Aeolian accumulation of sands indicates that the present westerlies have acted since the Late Holocene in Central Tierra del Fuego moving and depositing materials in the temperate forest region. Although, several low active phases would have occurred allowing vegetation to cover the sands, or to reduce the flow of sediments moving up the cliff. At present the process is in an active phase.