PLEISTOCENE GLACIOLACUSTRINE SEDIMENTATION AT LAGO-FAGNANO, ANDES OF TIERRA-DEL-FUEGO, SOUTHERNMOST SOUTH-AMERICA

A Pleistocene glaciolacustrine sequence is exposed at the southeastern end of Lago Fagnano, Fuegian Andes, Argentina. These sediments are in terpreted as proglacial delta facies, which rest upon a till. The prog lacial delta consists of (i) bottomset beds, composed mainly of greyis h blue silts, with dropstones; (ii) foreset beds, mostly cross-stratif ied gravel and coarse sands, and (iii) uppermost, fine-grained lacustr ine deposits, that include diatomites and peat layers. Overlying, hori zontally-stratified gravel beds may be interpreted as glaciofluvial pl ain deposits. Pollen and spore analysis of fossil peat beds indicates regional steppe conditions immediately after the deposition of the for eset beds, represented by predominant Gramineae and Empetrum. A very l ow content of tree pollen grains (Nothofagus spp.), compared to modern surface samples, supports the interpretation of steppe/tundra environ ments. The palaeoenvironmental setting at the time when the delta was built was much colder and drier than today at Lago Fagnano. This is co nfirmed by the diatom analysis which suggests a fresh-water, cool, tem perate, low energy environment. The palaeoenvironmental analysis confi rms the sedimentological interpretation of the studied units, as forme d in an ice-contact, proglacial lake, during stadial termination or in terstadial conditions. C-14 dating of the peaty lacustrine deposits in terbedded between the upper gravels and the foreset beds suggests an E arly Wisconsinan age (Oxygen Isotope Stage 4) for this sequence, altho ugh a pre Wisconsinan age (Stage 6?) cannot be ruled out. (C) 1997 Els evier Science Ltd.