Late quaternary evolution of a subantarctic paleofjord, Tierra del Fuego

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Quaternary Science Reviews, Vol. 12, pp. 889-897, 1993.

~ t 0 1994 Elsevier Science Ltd. Primed in Great Britain. All rights msava~ 0277-3793/93 $26.00

Pergamon

LATE QUATERNARY EVOLUTION OF A SUBANTARCTIC PALEOFJORD, TIERRA DEL FUEGO Sandra Gordillo, Andrea M.J. Coronato and Jorge O. Rabassa Centro Austral de lnvestigaciones Cientfficas ( CADIC, CONICET) C.C. 92 (941 O) Ushuaia, Tierra del Fuego, Argentina I.ago Rcr.a-Lapataia valley (54"50"S, 68"34'W) is a paleoi]ord that was occupied by a valley-glacier system during the glacial maximum of the late Pleistocene (estimated ca. 18-20 ka BP). Deglaciation began before 10,080:1:270 BP. "l'ne marine fauna in several marine terraces found in the area shows that early-middle Holocene climatic conditions were basically the same as at present. Species found are characteristic of cold and shallow waters, although minor temperature fluctuations cannot be ruled out for this period. A recent radiocarbon date of 7518 + 58 BP on Chlamys patasonica (NZ # 7730) confirms that La8o Roca was transformed into a fjord ca. 7500-8000 BP. The sea reached its maximum level of 8-10 m a.s.l, amend 6000 BP and at 4000-4500 BP was at least above 6 + I m a.s.l. Later, when sea level fell, Lago Roca was occupied by fresh water and was no longer tidal. The relative land-sea positions during this period are a consequence of combined eustatic and neotectouic processes.

INTRODUCTION The Lago Roca-Lapataia valley (Fig. I), a typical drowned glacial landscape, is located in Argentine Isla Grande de Tierradel Fuego (54° 50'S,68° 34'W). The area lies at the foot of the Fuegian Andes, with summits reaching not more than 1200-1400 m a.s.l.It is located 50 k m east of the higher peaks of the Cordillera Darwin with summits above 2000 in a.s.l.A large mountain ice-sheetisstillpresentin these mountains, with many outlet glaciers calving into the western area of the Beagle Channel. Bedrock is composed of high-grade, Upper Paleozoic-Lower Mesozoic metamorphic schists of the Lapataia Formation (Borrello, 1969; Caminos et al., 1981). This formation consists mainly of thinly layered, highly folded phyllites, injected with quartz veins. A cold, humid climate, with regular precipitations throughout the year and a long winter season allows the maintenance of a dense rainforest, from sea level to its upper limit at 700-800 m a.s.l. It is composed of evergreen southern beech (Nothofagus betuloides) and species of deciduous beech (Nothofagus pumilio and Nothofagus antarctica). Different species of shrubs (Berberis sp., Chiliotrichum diffusum) constitute the lower layer of vegetation. In low, wet areas, swamps and peat bogs are developed with Sphagnum magellanicum as the dominant species.

The paleoenvironmental reconstruction presented in this work is based on the geomorphological interpretation of Lateglacial and Holocene landforms and on the faunal composition of the marine deposits.

definition of the bedrock. Foremost are contributions by Borrello (1969) and Caminos et al. (1981). A first limnological study of the lakes of Tierra del Fuego, including l.,ago Roca, was made by Mariazzi et al. (1987) who found that chemical composition of the Lago Roca is different from that of the other lakes. Also, its waters are less transparent when compared with those of other lakes, because of the presence of suspended sediments. A description of the Quaternary geology has been presented by Rabassa et al. (1987), showing general morphological conditions and the different localities where shells have been radiocarbon dated. In a pollen analysis of a peat bog called Turbera Lapataia, a sample of basal organic matter was radiocarbon dated, suggesting that the ice had retreated further west, at a minimum age of 10,080 BP for this area (Heusser, 1987). Rabassa et al. (1986) have presented a sea-level curve for Tierra del Fuego, adding the data of other authors to their own, which have been obtained from radiocarbon dating of marine shells from the deposits at Bahia Lapataia. The archaeological work of Figuerero and Mengoni (1986) in a shell-midden of Isla E1 Salm6n, in Rio Lapataia, offers a minimum age of 1765 + 25 BP for the underlying marine deposits. It is located 1.7 km inland from the present coast line. At present, paleoecological studies of the faunal composition of the marine deposits are being conducted along the northern coast of the Beagle Channel. The results have been partially presented by Gordillo and Pifiero (1989) and Gordillo (1989, 1990a,b, 1991).

PREVIOUS WORK Only a few geological studies are available from this area. Geological papers have included it in general descriptions of the regional conditions; they have shown (without significant detail) the lithological and stratigraphical

THE~ LATE QUATERNARY ENVIRONMENTAL Com)mONS Most of the glaciated valleys of the region were occupied by outlet glaciers from the Cordillera de los Andes. In other areas of Patagonia and Tierra del Fuego, different stages of

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