Moraines and marls: Giant landslides of the Lago Pueyrredon valley in Patagonia, Argentina

Giant catastrophic landslides (>10(8) m(3)) dot the formerly glaciated mountain forelands of the eastern Patagonian Andes. From geomorphic mapping, sedimentological logs and radiocarbon dating, we infer the emplacement kinematics and approximate timing of giant landslides in moraines and other glacial deposits in the Lago Pueyrredon valley (LPV), Argentina. For the first time, we report in detail examples of giant low-gradient landslides with hummocky lobes derived from unconsolidated glacial deposits and weak bedrock. We find that at least 4.5 km(3) of debris and weak bedrock were mobilized by slope failures in an area of similar to 500 km(2) since the Last Glacial Maximum (LGM; similar to 25-18 ka). Nearly 90% of this landslide volume originated along the shores of, or as subaqueous failures in, a postglacial moraine-dammed meltwater lake. The larger landslides (>1 km(3)) detached from moraines fringing the lake, whereas other landslides displaced glacial and lake deposits either on the paleolake bed, or in a river gorge that was cut upon drainage of the glacial lake. Sequences of till, glaciofluvial and glaciolacustrine deposits overlying weak Early Miocene marlstone are mostly conducive to major landslides in the LPV. Crosscutting relationships indicate that largest landslides in the area originated during rapid glacial lake-level drops. Distribution and internal structure of hummocks within landslide lobes suggest that these landslides were emplaced as catastrophic debris avalanches. It suggests that giant catastrophic landslides in the glacier forelands of Patagonia can result from layered weak rocks, changes in meltwater-lake levels, and possibly as a consequence of strong earthquakes linked to rapid post-glacial rebound following the retreat of the Patagonian Ice Sheet (PIS). (C) 2020 Elsevier Ltd. All rights reserved.