DISTRIBUTION OF THE MARINE AND TRANSITIONAL TERRACES (HOLOCENE) AND GEOMORPHOLOGICAL MAP OF THE COASTAL AREA BETWEEN PORVENIR AND PUERTO YARTOÚ STRAITS OF MAGELLAN - CHILE (1:200.000 scale map) MAP C - Second Edition

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 Porvenir and Puerto Yartoú (Tierra del Fuego) are reported. 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 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 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 attention 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 the oldest a transitional origin may be feasible. Generally, they are mostly depositional bodies, but very ocasionally they are just simple marine erosion surfaces (palaeo-abrasion platforms) 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. Bahía Inútil is a marine morphological unit located in an original Quaternary glacial valley, and its morphogenesis is probably linked to several of the latter glaciations. It became marine dominated just at the end of the last glaciation. In the surveyed area, from the zone North of Porvenir to South of Punta Yartoú, for about 200 km., the coasts of the bay impinge on the local Meso-Cenozoic basement, the glacial cover s.l., or marine-transitional terraced deposits. The Meso-Cenozoic basement, which is well represented along the whole coast, is mainly formed from the sediments of the Grupo Bahía Inútil units or of Agua Fresca, Chorrillo Chico or Cerro Cuchilla formations. In the southern area of the bay, up to Puerto Arturo, it is formed from a sequence of folds, sometimes faulted with axial direction West North West - East South East (E.N.A.P., 1977; Mapa Geológico de Chile - Instituto de Investigacion Geológicas, 1980). Glacial deposits s.l. result from superimposition of the latter glaciations or just local episodes of them. They are part of continental geomorphological units, both depositional and erosive. These units are expressed by: moraines (ground, terminal or lateral), drumlins, erratics, kame terraces, fluvial terraces, kettle holes, marginal and submarginal channels, etc., which evidence advancement of the glaciers towards Bahía S. Sebastián (East). They are intimately connected to broad, frequent glacifluvial inter-morainal or alluvial plains at different heights, which represent either simple erosion surfaces or depositional bodies. Altogether, these glacial and alluvial deposits are formed from gravels, sands and well laminated pelites often affected by tectonics due to the ancient glacier actions. The terraced marine and transitional deposits belong basically to four orders, and they are mainly represented by depositional bodies, but sometimes only by erosion surfaces lying on the Meso-Cenozoic basement (abrasion palaeo-platforms) or on Quaternary glacial and glacifluvial deposits. Their sequence is evident in the vicinity of Porvenir. The First, more ancient Order, shows terraces generally located between 18 and 25 m above p.s.l., mostly around 20 m., constituted by sands and pelites characterized by an intense stratification and parallel lamination. There is no proof that it belongs to a marine environment, although its strict distribution along the present coast and not beyond it, suggests this. Landwards, especially in the eastern area of the bay and to the North East of Cabo Boquerón, where it shows an inner step, its upper surface connects with palaeo-alluvial fans. It suggests, on the whole, a transitional environment. There are neither absolute nor relative chronological data. Its superimposition on last glaciation deposits suggests that it belongs to the lowest stage of the Holocene or, perhaps, to the upper Pleistocene. Second-Order terraces are definitely coastal marine, as proved by lithology and biofacies. At Bahía Inútil and in its neighbourhood, they are mainly made up of gravels and sands. They are mainly located around 10 m, ranging from 6 to 11 m above p.s.l.; to the north of Porvenir they are higher, containing there or at Cabo Boquerón, some inner steps. According to the bibliography, their Holocene age has been confirmed by dating with radiocarbon and by their superimposition on last glaciation deposits and the current fauna. Third-Order terraces run mostly parallel with the present coast, with the exception of the ancient deltaic areas, where they follow a landward tendency, becoming transitional terraces in the contact zones. They are much more evident than they seem on this map, because of the scale, especially in small inlets to the mouths of creeks. They are mainly made up of fossiliferous gravels (gasteropods, bivalves etc.), and locally, sands and laminated pelites. They are distributed between 3 and 5 m above p.s.l. 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. From the evolutional palaeo-geographic point of view, all four terrace orders are post last-glacial. The First Order is probably bound to the morphological situation at the beginning of the last glaciation, whereas the Second, the Third and the Fourth Orders belong to pauses in the isostatic uplift, together with a positive eustatic component subsequent to the glacial phase and currently considered of minor importance to the isostatic component. The presence of contemporary tectonic movements cannot be exluded. Detailed studies aimed at distinguishing the eustatic effect from the isostatic and tectonic ones are in course. The present morphologic conditions support the described palaeo-geographic evolution since they repeat the conditions of the past shown by the terraces. In this sector also, many of the coastal lakes were connected, in their first phase of formation only, to glacial morphogenesis; subsequently, their evolution in an advanced phase of deglaciation (the last) was strictly linked to marine action and thus closely linked to the formations of the terraces. The lakes were, in fact, trapped between those terraces following their isostatic uplift (Cabo Monmouth, Laguna Barrosa, Puerto Nuevo, Caleta Josefina, Laguna Ema, Puerto Yartoú).