DISTRIBUTION OF THE MARINE AND TRANSITIONAL TERRACES (HOLOCENE) AND GEOMORPHOLOGICAL MAP OF THE COASTAL AREA BETWEEN PUNTA DUNGENESS AND BAHÍA SAN FELIPE STRAITS OF MAGELLAN - CHILE (1:200.000 Scale Map) MAP A - Second Edition

This map shows the results of the geological-morphological survey on the marine and transitional terraces of the coastal belt within the Straits of Magellan (Estrecho de Magallanes) between Punta Dungeness (Patagonia) and Bahía San Felipe (Tierra del Fuego). The information is mainly from the mapping and field sampling carried out during the 1991, 1994 and 2003 Italian expeditions which were organized by the “Dipartimento di Scienze Geologiche, Ambientali e Marine” of the “Università di Trieste”, within the P.N.R.A. (Programma Nazionale di Ricerche in Antartide) - Magellan Project (Coordinator: Antonio Brambati) and by the "Dipartimento di Scienze della Terra" of the "Università di Cagliari" within the cooperation project PVS - Regione Autonoma della Sardegna - Chile (Coordinator: Sandro DeMuro). Previous studies, carried out on the Straits of Magellan were mainly focused on sedimentological research on the sea bottoms and coastal belt of the eastern section (Atlantic opening). Similar 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 the sedimentological analysis of the bottom, but were limited to research on the source and transport of the sediments as well as a regional definition of vast morphostructural units. Subsequent research was carried out in greater detail with mapping of morphological units in the coastal belt (collection of morphostructural, stratigraphic, micropaleontological and sedimentological data, study of active cliffs, palaeo-cliffs, littoral dynamics, dunes, etc.). During this second phase, greater emphasis was given to the study of palaeo-shorelines and different terrace orders of presumed marine origin. On the basis of the initial geological, sedimentological and geomorphological results obtained, we made a zoning of the coastal belts along the Atlantic opening of the Straits. The map shows the distribution of the youngest terraces (prevalently Holocene), from which four orders have been mapped starting from the average sea-level. The three youngest orders are certainly of coastal origin while for oldest a transitional origin may be suggested. Generally, they are mostly depositional bodies, but very ocasionally they are just simple marine erosion surfaces (palaeo-abrasion plattforms) formed either on glacial and/or glacifluvial deposits, or on the Meso-Cenozoic bedrock. On the whole, they follow the current coastline, being apparently genetically connected to it, although in detail, they present an ancient configuration different from the present Straits. First Order terraces, presently located between 18 and 25m above p.s.l., are not well represented along the 250 km of explored and mapped coast. Landwards, they are, difficult to identify, because they tend to link up with continental terraces of diverse orders, and relative hydrographic networks, which are older (lower Pleistocene?). From the textural point of view, they are made up of stratified sands and laminated pelites. The Holocenic age is hypothesized on the basis of both stratigraphic position and relation to analogous deposits in the Straits of Magellan with radiocarbon dates. Second Order terraces lie mainly between 6 and 11 m above p.s.l.; they are much more extensive than the previous order, and sometimes contain evidence of marine palaeo-forms (Punta Catalina, Punta Dungeness). The deposits are mostly clayey and sandy, sometimes gravelly, and often fossiliferous (gasteropods, bivalves etc.). Their age (Holocene) is presumed from their stratigraphic position, their faunal content, and radiocarbon dating. Third Order terraces (Holocene) are less present than in adjacent areas (Maps B and C), although they represent larger single outcrops between 3 and 5 m a.s.l.. Fossiliferous gravels prevail, with some sands. Their age (Holocene) has been directly based on radiocarbon dating carried out on the shells that they contain. The Fourth Order shows itself to be distribuited at a height of betwen 1 to 2 metres. The Fourth Order is composed of raised palaeo-beaches containing a rich faunas of gasteropods, bivalves and balanids as recognized by C14 dating. The coastal belt is almost always made up of depositional continental units, mainly linked to the last glaciations and consisting of moraines (ground, terminal and lateral) and kame terraces, as well as fluvial terraces and drumlins. There are, moreover, erosion units, such as marginal and sub-marginal streams and ancientstream channels (first, second and third glaciations). The present coastal morphology is more uniform compared to the ancient one suggested by the Holocene terraces. Present abrasion platforms, salt-marsh areas, tidal flats and channels, stable or advancing moving coasts are better developed in areas where the terraces are more noticeable (Bahía Santiago-p.p., Bahía San Felipe, Bahía Lomas, the area between Primera Angostura and Punta Tandy), which indicates that the morphology was the same in the past. On the contrary, they are not developed where there are no terraces or where they are less evident (Bahía Santiago-p.p., Primera Angostura, the stretch between Punta Tandy and Punta Wreck). The genetic similarity between the present configuration of spits and cuspate forelands of Punta Dungeness and Punta Catalina and those of the past expressed by Holocene terraces is also notable. The main aeolian morphologies follow the dominant Magellanic wind direction at high-altitudes, whereas on lower zones they run parallel to the coast. Lakes in the area are basically bound to glacial morphogenesis. Nevertheless, there are other lakes formed by entrapment in the palaeo-geographic evolution of the marine terraces (Bahía San Felipe, Bahía Lomas, Punta Catalina). They can be, sometimes, considered different stages in the latter phase of the last deglaciation, indicating backshore environments, and relative palaeo-beach ridges formed by isostatic uplift and risen, above the base level. In some sector, they have been, or are currently being shaped in their morphologic evolution, by strong and regular West North - West winds, migrating leewards. Elucidation of the evolutionary framework of the mapped palaeo-surfaces, linked to palaeo-beach deposits, provides an important contribution to the reconstruction of the Holo-Pleistocene s.l. palaeo-geographic picture of the peri-Antarctic areas. Detailed studies (scale 1:50,000) on marine terraces linked to Holo-Pleistocene glacio-eustatic variations are in course. The aim is to publish an Atlas accompanied by about 12 Geomorphological Maps (topographically based on 28 sections 1:50,000 I.G.M.C.) of the area between Punta Dungeness and Bahía Inútil.