A drastic Late-Holocene lake level rise of Lago Llanquihue, Chile´s second largest lake

South-Central Chile is a geodynamically very active area that experiences great subduction megathrust earthquakes, mass-wasting processes and frequent volcanic eruptions. These processes can induce sudden landscape changes that leave long-lasting geomorphological and sedimentary traces. They can also lead to significant lake level rise, but evidence for this is exclusively found under water and hardly investigated. Lago Llanquihue (41.156°S; 72.816°W) is a large and deep (40 x 40 km; 317 m deep) piedmont lake at the western foot of the Andes, and forms an important touristic hotspot. Its current outflow is at the western edge and its small catchment is dominated by two active volcanoes: Osorno and Calbuco. Based on previous geomorphological studies in the 1960-70s, it was hypothesized that the lake level of Lago Llanquihue was much lower during most of the Holocene and rose to its present level only in the Late Holocene. According to the hypothesis, lahars and/or lava flows in the late Holocene dammed the outflow in the eastern part of the lake system leading to lake level rise and subsequent drainage towards the west through the frontal moraine belts.

In this study, we aim to test and constrain this hypothesis by investigating the submerged geomorphological features and sedimentation patterns in Lago Llanquihue. We complement this data by a sedimentary study of Laguna La Poza, a small lake (1.7 x 0.2 km; 6 m deep) in a fluvial valley that is believed to have been inundated by the Late-Holocene lake level rise. Seismic-stratigraphic analysis of Lago Llanquihue reveals submerged lake level terraces (ca. 35-55 m water depth) of which individual levels can be traced in different parts of the lake. These terraces are constituted of erosional unconformities and delta topset geometries, and are covered only by a thin (0.5-3 m) drape of lacustrine sediments. Short sediment cores show different lithologies below these unconformities: fine-grained grey laminated sediments with dropstones, or coarse sand with shell fragments. These are interpreted as glaciolacustrine sediments from Late Glacial times and as sandy beach environments, respectively. Additionally, Laguna La Poza shows a significant change in sedimentation at about 1 m depth, where a laterally-variable fining-upwards medium sand to gravel unit with pronounced cross-stratification is covered by horizontally-stratified fine-grained organic-rich lake sediments. We interpret this sequence as evidence for a sudden inundation of a fluvial valley by a rising Lago Llanquihue. Given the rather thin drape of lake sediments over the unconformity (Llanquihue) and fluvial sediments (La Poza), a Late Holocene timing of lake level rise is plausible. Current investigations aim at correlating both lacustrine sedimentary records by geochemical analysis of tephra marker layers and use ¹⁴C dating to constrain the timing of the sudden lake level rise.