Distribution of the marine and transitional terraces (holocene) and geomorphological map of the coastal area between Bahía San Felipe and Paso Ancho Straits of Magellan - Chile (1:200.000 Scale Map) map B - Second Edition

The results of the geological survey on the marine and transitional terraces of the coastal belt within the Straits of Magellan (Estrecho de Magallanes) between Bahía San Felipe and Paso Ancho 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 suggest. 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. The approximately 300 km of coast currently explored show mainly unaltered stretches (stable) in the sectors of Rio Leña Dura, Bahía Pecket, Bahía Gregorio-p.p., Ensenada Susana, Bahía San Felipe-p.p., Bahía Lee, and Bahía Gente Grande. Other sectors are mainly receding, such as Isla Isabel, Bahía Santiago, Bahía Gregorio-p.p., Segunda Angostura-p.p., Bahía Gente Grande North, Paso Ancho East, and others are moving forward, like Bahía Pecket (p.p.) and Bahía San Felipe. The most evident tidal flats, often including salt-marsh areas, are Bahía Shoal - Punta Tumba, Bahía Pecket, Ensenada Oazy, Ensenada Susana, Bahía Gregorio, Bahía San Felipe and Caleta Hobbs. Aeolian deposits are present at Cabo Gregorio, on the northern coast of Península Juan Mazía, at Bahía San Felipe and at Bahía Lee. These morphologies can be located in a dynamic-evolutive context similar to the palaeo-geography hypothesized for the Holocene marine terraces. In fact, the ancient coastal morphology is inferred from three orders of basically Holocene marine terraces (Second, Third and Fourth order), and from one mainly transitional (First order), slightly older, but still Holocene terrace. A complete sequence of the three orders can be seen at Bahía Lee, Bahía Gente Grande and Seno Otway. First Order terraces are on average present at about 20 m elevation, with maximum altitude between 18 and 25 m. Their identification is difficult in some sectors since they are sometimes linked to ancient depositional surfaces of alluvial character. They are clearly present in the vicinity of Bahía Shoal, West of Bahía Pecket and Ensenada Susana. They are mostly sedimentary bodies, superficially eroded and flattened, and made up mainly of sands and, to a lesser degree of gravels and laminated pelites. Sometimes they are linked hillward by alluvial plains or lie unconformably on them, and sometimes they characterize the evolution of lacustrine and transitional depositional surfaces, dating back to phases immediately after the last glacial period. Lithology, distribution according to the coasts of the Straits, and links with ancient alluvial plains and palaeo-alluvial fans suggest a mainly transitional environment for this order of terraces. Due to the stratigraphy and its relationship with the last glaciation morainal deposits, a Holo-Pleistocene age is suggested. The Second Order is the most evident of the three, both for size and for the wealth of palaeo-forms and marine deposits. Its terraces are located between 6 and 11 m above p.s.l.. They often show ancient beach ridges or even very well preserved palaeo-beaches, with their sedimentary accretionary macrostructures (Punta Arenas, Bahía Laredo, Bahía San Felipe, Punta Valle), mainly transversal to the present coast. In the western area of Península Juan Mazía, some inner steps are present, whereas in the north-eastern part of the Península, huge beach ridges are perfectly preserved. Lithology is changeable: sometimes, partly fossiliferous gravels prevail (Bahía San Felipe), otherwise sands stratified and laminated pelites are to be found. The appartenance of this order to the littoral marine environment is proved by the fossil (gasteropods, bivalves etc.) and sedimentological content. Its age (dating back to the Holocene) is inferred from its stratigraphic position, from the modernity of the fauna, and from radiocarbon datings. Of the Third Order, only the most characteristic terraces, distributed between 3 and 5 m above p.s.l. have been mapped. Its parallelism with the present coast and, sometimes its continuity, are noticeable, despite its reduced thickness (South of Punta Arenas, at Bahía Gente Grande, etc.). Presently, in some sectors, erosion is acting strongly on it, and tends to obliterate it completely. Samples of the present abundant fauna date it to the Holocene. 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 mainly made up of continental units, both depositional and erosive (ground, lateral, terminal moraines, drumlins, kettle holes, marginal and submarginal streams, etc.) linked to the last glaciation and previous ones, and associated with ancient glacifluvial (kame) or alluvial bodies. Sometimes drumlins fields mark the morphology in a strict Northeast - Southwest direction, like to the Southwest of Bahía Pecket and to the West of Caleta Hobbs, where lakes of various dimensions pertaining mainly to the glacial morphogenesis are trapped. Especially relevant is the distribution of the numerous lakes, whose morphogenetic origin seems to be linked to a first glacial phase and, subsequently, to an evolution of littoral and backshore environments, referred to the relative base levels of both the Second-Order Terraces (Laguna Deseada, Lago Serrano, Lago de los Cisnes, Laguna Verde, etc.) and the Third Order. Analogously, it is obvious that the conformation of the numerous bays and their morpho-sedimentological role are linked to the ancient equilibrium profiles. Within the framework of local palaeo-geographic evolution, terraces of Second and Third Order could correspond to pauses in the isostatic readjustment subsequent to the end of the last glaciation, although, it cannot be excluded that a positive eustatic component, currently considered of minor importance to the isostatic component, was added to it in parallel. The most ancient terrace is also presumably linked to the final phase of the last glaciation