POST LAST GLACIAL MAXIMUM ICE RETREAT FROM GLACIOLACUSTRINE STRATIGRAPHY IN LAGO ARGENTINO (PATAGONIA, ARGENTINA)

Tectonically active and glaciated mountain ranges experience a complex geomorphic response to the interaction of glaciers with tectonic and isostatic stresses. Sediments in quiet proglacial basins provide an understanding of the processes influencing the syn- and post-glacial evolution of the landscape (e.g. glacial advance/retreat, faults, erosion). In this study, we examine the stratigraphy and structure of the lacustrine deposits in Lago Argentino, a proglacial lake east of the Southern Patagonian Icefield that has been accumulating sediments from eight alpine lake glaciers since ice retreated from its Last Glacial Maximum (LGM) extent ~21 kyrs BP. For this study, we use 350 km of newly acquired seismic CHIRP data imaging soft sediment with decimeter-scale vertical resolution to a depth of 50 m. Data were frequency filtered (5-100-5000-6000 Hz), depth converted (1500 m/s constant velocity), and gain corrected. Across the moraine-bound sub-basins of Lago Argentino, we identify and interpret four seismic facies: 1) strongly layered, sub-horizontal glaciolacustrine sediments, 2) high amplitude, internally chaotic moraines, 3) hummocky, discontinuous ice-proximal deposits, and 4) chaotic lenses ~5 m beneath the lake floor that appear to deform glaciolacustrine sediments above and cause acoustic blanking below, suggesting either the presence of gas or material related to mass wasting deposits. Stratigraphic units within the glaciolacustrine sediments are separated by distinctive reflectors correlatable ubiquitously in the main lake basin across systems of moraines. The shallowest of these reflectors is the top of a high amplitude reflector “doublet” that shows consistent 1 m thickness and 4-7 m depth below lake floor. We interpret this marker as the older of a sequence of tephra deposits, preliminarily dated ~1300 yrs BP based on varve stratigraphy in gravity cores retrieved as part of the project. A gentle, angular unconformity bounding the deepest imaged unit 15 m below the lake floor exhibits numerous >5 m deep erosional/scouring structures. Identified spatial variations in unit thicknesses are interpreted to be related to differences in sedimentation rates with distance from glacial fronts, bedrock morphology, and differential uplift linked to glacial isostatic adjustment stages since the LGM.