Distribution of the marine and transitional terraces (Holocene) and geomorphological map of the coastal area between Punta Paulo and Porvenir, Tierra del Fuego, Straits of Magellan, Chile (1:50,000 scale map – Map n° 3/12 - Second Edition)

Previous studies, carried out by the Department of Geological, Environmental and Marine Sciences of the University of Trieste within Programma Nazionale di Ricerche in Antartide (P.N.R.A) and by the "Dipartimento di Scienze della Terra" of the "Università di Cagliari" within the cooperation project PVS - Regione Autonoma della Sardegna - Chile on the Straits of Magellan were mainly directed at sedimentological research on the sea bottoms and coastal belt of the eastern section (Atlantic opening). Similar wide ranging research was carried out also on the coastal belt of the western section of the Straits (Pacific opening) from seavessels, since it was inaccessible by land which is typically sheer (fiords). The first coastal studies were connected with sedimentological studies of the bottom, but were limited to research on the source and transport of the sediments as well as a regional definition of morphostructural units. Subsequent research along the coastal belt was carried out in greater detail with mapping of morphological units. During this second phase, greater attention was given to the study of palaeo-shorelines and different terrace orders of presumed marine and transitional origin (Brambati et alii 1993a, 1993b). On the basis of the initial geomorphological, geological and sedimentological results obtained, we made a zoning of the coastal belts along the Atlantic opening of the Straits which enabled us to print three sheets on a scale 1:200,000 (De Muro et alii 1995; Brambati et alii 1995a, 1995b; Di Grande et alii 1995). Detailed studies and maps (scale 1:50,000) of the terraced sequences linked to Holo-Pleistocene glacio-eustatic variations are in course. The aim is to publish an Atlas accompanied by 12 geomorphological maps (which this is the third) topographically based on 28 Chilean I.G.M. 1:50,000 scale maps of the area between Punta Dungeness and Bahía Inútil (De Muro et alii, 1996, 1996b; Di Grande et alii, 1996a, 1996b). The coastal area between Punta Paulo and Porvenir is situated on the Paso Ancho southward of the Península Juan Mazía. Since the lower Holocene it has been an important morphodynamic unit in the history of the Straits of Magellan. Its recent evolution can be easily inferred upon examination of this geomorphological map which includes Sheets, No 86, 87 and 103 (Chilean I.G.M.) and mainly concerns the study of the coastal strip. Its origin is prevalently linked to the last glaciation which produced the imposing glacial deposits of which it is formed (Drumlin, morain deposits etc). The coastal area between Punta Paulo and Porvenir, like the rest of the coast along the eastern part of the Straits, bear testimony to a different conformation in the lower Holocene. In fact, the western area of Paso Ancho (F.86) like the remaining eastern Magellanic area, shows a regular sequence of mainly marine terraced deposits, located in four orders at the respective elevations of 18-25 m, 6-11 m, 3-5 m and 1-2 m (above M.S.L) in witness of the different ancient sea levels. In the eastern area (F.87), the intermediate order (Second Order - elevation 6-11 m above M.S.L) is better represented whereas a presence of the First Order appears uncertain. From its stratigraphic position and our radiocarbon dating, the age of the oldest order can certainly be attributed to the lower Holocene although formation may have started in the late Pleistocene. Its formation environment seems to be marine-transitional. The Second Order, certainly linked to a marine environment, is also Holocene and is on average attributable to 6,000-7,000 years B.P. (Brambati et alii 1993a, 1993b, 1995a, 1995b; De Muro et alii, 1995; Di Grande et alii 1995). The third Order Terraces, with an average age of 4,000-5,000 years B.P. and the Fourth Order, with an average age of 1500-2000 years B.P., can be referred to the same environment. In addition to the above three orders, we noted the presence of other terraced surfaces, located above 25 meters and of fluvial and/or glacifluvial origin. The considerable altimetric precision of the three younger terrace orders mapped, suggests that the considerable isostatic component of the Holocene uplift was added to in parallel by a positive eustatic component which, however at the present stage of research is estimated to be less important.