Glacial landscape in the lake Fagnano basin, Tierra del Fuego, Argentina.

CORONATO, A.; PONCE, J. F.; WALLDMAN, N.; SEPPÄLÄ, M.; RABASSA, J.

Santiago de Chile

Congreso; GEOSUR 2007; 2007

Universidad de Chile

Glacial landscape in the lake Fagnano basin, Tierra del Fuego, Argentina.    Coronato, A.1,2, Ponce, J. F. *1, Walldman, N.3, Seppälä, M.4 , Rabassa, J.1,2       (1) Centro Austral de Investigaciones Científicas-CONICET. Houssay 200, 9410 Ushuaia, Argentina.  (2) Universidad Nacional de la Patagonia San Juan Bosco, Sede Ushuaia. Darwin y Canga, 9410, Ushuaia, Argentina. (3) Earth Science Section, University of Geneva, Rue des Maraichers 13, 1205 Geneva, Switzerland. (4) Department of Geography, Helsinki Universit, PO Box 64, FI-00014, Finland * Presenting Author’s email: jfponce@cadic.gov.ar   Lake Fagnano, located in the centre of the Isla Grande de Tierra del Fuego (54°26’ - 54°37’ S; 68°35’ W - 66°42’; 26 m a.s.l), is one of the most important glaciated areas of southernmost South America, both because of its extension and the volume of ice contained by the Pleistocene glacier network. The lake occupies a tectonic basin in the Magallanes-Fagnano transform system along the Sudamerica and Scotia plates. This basin has been glacially eroded and modified during several Pleistocene glaciations.  A regional geomorphological survey is been performed along the southern and eastern coast of the lake. Based on the location and type of erosional and depositional glacial landforms, a first palaeoglacial reconstruction is proposed. The outlet paleoglacier flowing eastwards from the Darwin Cordillera (Chile) had more than 50 tributary glaciers. An alpine-type landscape, including arêtes, cirques, horns, truncated spurs and hanging valleys developed in the western region of the present lake, whereas a piedmont-type landscape including lateral moraines, glaciofluvial and glaciolacustrine terraces, and an ice-disintegration landscape developed in the eastern region. The glacier spread over the low ranges and lowlands through three different lobes and was drained by four main outwash basins, directly into the Atlantic Ocean. During the maximum ice-expansion, the ice-covered area is estimated at 4,000 km2; the maximum length of the main lobe at 132 km, and the general slope at 8°. The ice volume, calculated over 1 m wide cross-sections, would have varied between 5,397,092 and 165,722.9 m3 per section. Four terminal positions were identified but their chronostratigraphic relationship is not clear yet, due to the lack of datable material.  Basal dates of peatbogs located south and east from the lake shore indicate that the ice would have disappeared from the present landscape during the Late Glacial (sometime before ca. 12.3 ka B.P.). Moreover, a set of submerged frontal moraines covered by lacustrine infilling were identified by seismic stratigraphic analysis performed by high-resolution 3.5 kHz pinger system. They are interpreted as recessional stages of the palaeoglacier during the general deglaciation times, when the ice flowed headward confined into the valley, probably during a cooling period equivalent to the Antarctic Cold Reversal.