The Alemania rockfall dam: A record of a mid-holocene earthquake and catastrophic flood in northwestern Argentina

A prehistoric rockfall, probably caused by an earthquake with a magnitude of 5.0 or greater, blocked Rı́o de las Conchas 2 km upstream from Alemanı́a (Salta Province, Argentina) in the southern part of the Cordillera Oriental. The rockfall created a dam that formed a lake at least 16 km long. Shells of Biomphalaria, preserved in the lake sediments and, dated by radiocarbon, established the time of this event to be about 5500–6000 years ago. When the natural dam failed, probably either by piping or overtopping, the lake drained catastrophically, eroding a trench through the chaotic mass of sandstone rubble.