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Evidence of a Holocene outburst flood from a landslide-dammed lake, Laguna Blanca, San Juan, Argentina
Encuentro; IAL-IPA 2022 joint meeting; 2022
Institución organizadora:
International Association of Limnology and International Paleolimnology Association 2022 joint meeting
specifically in the Central Andes of Argentina, these events are grossly understated. But, even if there are few historical cases of such events, the hazard associated with an outburst flood from these types of natural impoundments is tremendous. The reason for this underestimation could be the remoteness of the area, the lack of inventories for large landslide-dammed lakes as they can only be detected through remote sensing, or because their outburst floods sometimes are generically named ‘aluvión’ without further questioning its origin. Laguna Blanca is a landslide-dammed lake formed 12.8 ka ago after a rock avalanche blocked the Laguna Blanca River in a narrow and deep area of the valley. It is still standing and it drains both superficially and through piping, generating a permanent flow. The avalanche came from the southern flank mobilizing Rhyolites, Granites, and moraine deposits both from the outcrop and the valley bottom. Two palaeo-lake levels were mapped in the surroundings, one 70 m above the present-day lake and the other one only 2 m above it. The lower and younger palaeo-lake was dated through luminescence in 2.5 ka on the bottom and 2.12 ka on the middle sections of the 2 m high sequence. This palaeo-lake was estimated to drain catastrophically sometime after 2.12 ka, as stated by an outburst level found on top of a fluvial/alluvial terrace located less than a kilometre downstream. This terrace indicates a permanent flow prior to this outburst event which means that the lake should have been in a relatively stable state. It is considered, then, that an external event caused this partial drainage of the lake, either by excess precipitation or by a rock avalanche that fell into the lake based on fresh scars in the surrounding outcrops.