Evidence of successive discharges on a landslide-dammed lake in the Central Andes of Argentina (31ºS).

JEANNERET, P.; MOREIRAS, S.M.; ORGEIRA M.J.

Encuentro; Annual Meeting. British Society of Geomorphology (virtual); 2021

Landslide dams are typical features from the Central Andes of Argentina hosting the higher peaks of the Andes Cordillera between latitudes 30° and 35°S. These natural dams are responsible of modifying the valley?s hydrological dynamic and are also extremely prone to partial or complete catastrophic releases that end up in large outbursts and floods downstream with high debris content. One of those lakes is Laguna Blanca, located in the San Juan province (31°53? S - 69°58? W) around 3,000 m asl. It was dammed by a slump from the north facing slope that evolved into a rock avalanche, blocking the Laguna Blanca River and filling up the valley for several kilometres downstream to the east, which included moraine deposits from the outcrop and mobilized moraines from the bottom of the valley. The lake shows evidences of being larger in the past based on two relict levels of paleolakes that were found on valley margins. The highest and oldest one is found around 120 m above the present day water level, while the youngest one is located in the vicinity of the lake. The oldest level is very conspicuous and is preserved in both sides of the valley equally, which enables to assume that a very large but unstable paleolake was formed immediately after the blockage and it should have drained shortly after, causing a large outburst downstream that mobilized a large portion of the initial avalanche deposit. The youngest paleolake is an evidence of a second blockage that was at least 2 m higher than the current one, which catastrophically drained. An outwash terrace was found immediately downstream of the dam. Such terrace also has fluvial deposits on the bottom of the sequence indicating that the younger paleolake had found a period of stability with natural drainage through the lower points of the dam, which lasted for a considerable time based on the thickness of such deposits. On the other hand, the presence of scarps on the south-facing valley slopes that are immediately above the lake is probable an evidence to understand why this paleolake collapsed even when it found a stage of equilibrium. The large scarps indicate landslides that fell into the lake likely causing large displacement waves that affected the paleolake?s stability and drained catastrophically, lowering the water level to the present-day level.