Sedimentology of till deposits in the Lago Fagnano ice lobe, Tierra del Fuego, Argentina.

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

Mendoza, Argentina

Congreso; 18th International Sedimentological Congress; 2010

IAS - International Association of Sedimentologists

The grain-size characteristics of different till deposits in the Lago Fagnano valley (54°26′–54°37′S, 66°42′–68°35′W), Tierra del Fuego, Argentina, are herein presented and discussed. These glacigenic deposits were generated by the Fagnano Paleoglacier when it occupied a region of approximately 4000 km², flowing from W to E, as an outlet glacier of the Cordillera Darwin mountain ice-sheet, 130 km westwards from its maximum LGM extension. A Late Pleistocene age (ca. 26 cal. ka B.P.) is estimated. The glacial flow took place along the Fuegian Andes, a mountainous environment of varied lithology: plutonic rocks in the main accumulation area, a sedi - mentary-volcanic complex in the southern side of the valley and metamorphic and sedimentary rocks in the northern slopes. From a textural point of view, the following till types were analyzed: (a) subglacial till beds, corresponding to the ground moraine of the Arroyo Café, and (b) slope basal till at Cerro Jeujepén and Cerro Mi - chi. The sediment sampling was performed randomly both in exposed outcrops and by means of excavations. The grain-size distribution of the till matrix, between the Φ -3 to 4 fractions, was analyzed at the Centro de Investigaciones Geológicas (CIG, Universidad Nacional de La Plata, La Plata, Argentina). The composition of the clay minerals was also studied using whole rock and clay fraction analyses in oriented samples by means of Xray diffractometry. At the Arroyo Café site, a stratigraphic section was described, composed by the following units, from base to top: (1) a bluish-grey, lower till bed, with non-exposed base; (2) fine to medium sandy gravels, with interbedded sandy, silty and sabulitic beds, showing intense cross-bedding; (3) a level of clast-sup - ported fine to medium gravels; (4) a second till bed, with medium to coarse, rounded to subrounded clasts; (5) another level of fine to medium grain-sized gravels; finally, (6) a third basal till bed, with fine to medium grainsized larger clasts. This uppermost basal till is composed by coarse silt (31%) to coarse sand (13%) matrix, with fine gravels (38%), whereas the underlying till bed (4) is dominated by the clayey-silty fractions (90%). Both strata show high variability in the accumulation conditions within ice-contact environments. The grain-size analyses of the till samples obtained along the slopes of Cerro Jeujepén and Cerro Michi show high similarity with those of the upper basal till of the Arroyo Café outcrop. The matrix is mainly composed of coarse silt (30-36%) and coarse sand (16-20%). The content of fine gravels is close to 30 % of the bulk mass. The distribution of the fractions according to total weight shows slope curves very similar between them. At Arroyo Café, the upper till level presents an accumulation grain-size curve similar to the slope till, whereas in the second till bed the total weight curve starts to develop from the Φ 2.5 fractions. No difference between the lithological content of the gravel fraction of the various analyzed groups has been observed. The presence of allochtonous granite and granodiorite clasts in all till types is remarkable, which confirms the glacial origin of these sedimentary beds, indicating that the main feeding of the Fagnano Paleoglacier sedimentary load came from Cordillera Darwin, where these plutonic rock types are outcropping.