Pleistocene glaciolacustrine sedimentation at Lago Fagnano, Andes of Tierra Del Fuego, Southernmost South America

Abstract

A Pleistocene glaciolacustrine sequence is exposed at the southeastern end of Lago Fagnano, Fuegian Andes, Argentina. These sediments are interpreted as proglacial delta facies, which rest upon a till. The proglacial delta consists of (i) bottomset beds, composed mainly of greyish blue silts, with dropstones; (ii) foreset beds, mostly cross-stratified gravel and coarse sands, and (iii) uppermost, fine-grained lacustrine deposits, that include diatomites and peat layers. Overlying, horizontally-stratified gravel beds may be interpreted as glaciofluvial plain deposits. Pollen and spore analysis of fossil peat beds indicates regional steppe conditions immediately after the deposition of the foreset beds, represented by predominant Gramineae and Empetrum. A very low content of tree pollen grains ( Nothofagus spp.), compared to modern surface samples, supports the interpretation of steppe/tundra environments. The palaeoenvironmental setting at the time when the delta was built was much colder and drier than today at Lago Fagnano. This is confirmed by the diatom analysis which suggests a fresh-water, cool, temperate, low energy environment. The palaeoenvironmental analysis confirms the sedimentological interpretation of the studied units, as formed in an ice-contact, proglacial lake, during stadial termination or interstadial conditions. 14C dating of the peaty lacustrine deposits interbedded between the upper gravels and the foreset beds suggests an Early Wisconsinan age (Oxygen Isotope Stage 4) for this sequence, although a pre-Wisconsinan age (Stage 6?) cannot be ruled out.