LATE QUATERNARY EVOLUTION OF A SUB-ANTARCTIC PALEOFJORD, TIERRA-DEL-FUEGO

Lago Roca-Lapataia valley (54-degrees-50'S, 68-degrees-34'W) is a pale ofjord that was occupied by a valley-glacier system during the glacial maximum of the late Pleistocene (estimated ca. 18-20 ka BP). Deglacia tion began before 10,080 +/- 270 BP. The marine fauna in several marin e terraces found in the area shows that early-middle Holocene climatic conditions were basically the same as at present. Species found are c haracteristic of cold and shallow waters, although minor temperature f luctuations cannot be ruled out for this period. A recent radiocarbon date of 7518 +/- 58 BP on Chlamys patagonica (NZ # 7730) confirms that Lago Roca was transformed into a fjord ca. 7500-8000 BP. The sea reac hed its maximum level of 8-10 m a.s.l. around 6000 BP and at 4000-4500 BP was at least above 6 +/- 1 m a.s.l. Later, when sea level fell, La go Roca was occupied by fresh water and was no longer tidal. The relat ive land-sea positions during this period are a consequence of combine d eustatic and neotectonic processes.