SEISMIC STRATIGRAPHY OF LATEGLACIAL AND NEOGLACIAL ICE FLUCTUATIONS IN LAGO ARGENTINO (PATAGONIA, ARGENTINA)

We present the results of a marine seismic survey that imaged lacustrine sedimentary and geomorphic features in Lago Argentino and its adjacent glacial valleys, which drain 8 major outlet glaciers of the Southern Patagonian Icefield (SPI). Since the Last Glacial Maximum, the SPI in this region has experienced a number of glacial fluctuations, which have been mapped on land across the present-day basin. Here we present the marine record of the glacial variations, as preserved in ice-proximal and -distal sediments of the proglacial lake and as imaged by high-resolution multi-channel seismic (MCS) and CHIRP data. The data were used to map the basement, measure the total sediment thickness in the glacial sub-basins, and to characterize the basin fill, which in places reaches the thickness of ~600 m and consists of glacial and glaciolacustrine sediments. Data interpretation includes seismic facies analysis that defines 5 predominant depositional units across the area's sub-basins: ice-contact moraines (M), unstratified ice-proximal deposits (P1), stratified ice-proximal drift (P2), ice-distal stratified drift (D), and periglacial flows, slides, and slump deposits (S). Our interpretation indicates that the sequence filling the bedrock valley of Lago Argentino records glacial frontal fluctuations in both the ice-proximal and -distal environments. Within the ice-distal glaciolacustrine units, we observe variations in sub-units characteristics, depending on the position in the basin and proximity to the ice front. In Brazo Cristina, a narrow glacial valley proximal to the Neoglacial ice positions, the data record acoustically chaotic units (P1) in contact with the basement, marking the remnant of the glacial deposits associated with the 1,100-5,500 yr BP Neoglacial advance. Above we interpret poorly stratified units (P2), representing gravel and sands deposited by meltwater circulation at the bottom of the glacial valley. The units above (D1-D5) are thick, stratified units marking the beginning of deglaciation and the establishment of the glaciolacustrine environment. Some of the D units are bounded by erosion surfaces, which we interpret as corresponding to smaller readvances of the glacier front, including the Little Ice Age advance, as recorded by moraine deposits mapped and dated on land.