EVOLUTION OF THE EASTERN FLANK OF THE NORTH PATAGONIAN ICE FIELD: THE DEACTIVATION OF THE DESEADO RIVER AND THE ACTIVATION OF THE BAKER WATERSHED

ISLA, F.; IANTANOS, NERINA; ESPINOSA, M. A

Natal

Congreso; Quaternario: procesos naturais e antropicos; 2013

ABEQUA

Drainage reversals have affected several watersheds of Patagonia at the transition between Pleistocene Glaciation and the Holocene deglaciation when terminal moraines blocked glacial valleys. Significant volumes of water were diverted as the deglaciation progressed and the Andes tectonics styles became dominant. During the Pleistocene the Eastern flank of the North Patagonian ice field flowed towards the Atlantic Ocean via the Deseado River. The western flank was drained directly via short glaciers to the Pacific Ocean. The Eastern drainage was performed by two valleys today occupied by chain lakes. The northern valley comprised the Bertrand-General Carrera -Buenos Aires system discharging to the Fénix and Deseado rivers. The southern valley, today occupied by the Cochrane-Pueyrredon-Posadas lakes systems, was drained by the Pinturas River towards the Deseado watershed. Such a quantity of water caused the incision of Jurassic volcanic rocks into deep valleys extending from the foot of the mountains to the Atlantic coast. The end of the Pleistocene Glaciation caused the deposition of a new set of terminal moraines that, interfingered with volcanic lavas, blocked the Atlantic watershed at the foot of the Andes. Ice retreat increased during the transition Pleistocene to Holocene until the watershed became diverted to the Pacific Ocean via the Cochrane and Baker rivers. Much of the Deseado River watershed became diminished, and gorges close to the inlet became infilled. Glaciolacustrine deposits accumulated surrounding the Buenos Aires-General Carrera and Posadas lakes. The North Patagonian Ice Field became completely diverted to the Pacific Ocean. The Baker River watershed is today draining the Bertand Lake, and discharging up to 1000 m3/s, with peaks of 3000-4000 m3/s. Several glaciers remain at this Holocene watershed and subject to special monitoring in relation to hydroelectric dams projected and the risk of sudden outbursts of glacial lakes (GLOFs).