Distribution of the marine and transitional terraces (Holocene) and geomorphological map of the coastal area between Seno Otway and Bahía Shoal, Patagonia, Straits of Magellan, Chile (1:50,000 scale map – Map n° 1/12 - Second Edition)

Previous studies, carried out by the “Dipartimento di Scienze Geologiche, Ambientali e Marine” of the “Università di Trieste”, within the "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 enables us to print three sheets on a scale of 1:200,000 (De Muro et alii 1995, Brambati et alii 1995a and 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 (of which this is the second) topographically based on 28 sections of the Chilean I.G.M. 1:50,000 scale maps of the area between Punta Dungeness and Bahía Inútil (De Muro et alii 1996, 1996b). This map mainly concerns the coastal strip of two sheets of the Chilean I.G.M. (No. 67, No. 68 of Section L). The primary objective of the research is the mapping of the more recent marine and transitional terraces. The outcropping substrate is prevalenly linked to the last glaciations which affected the southern Andean Cordillera (Darwin Cordillera) and the adjacent coastal areas of Patagonia and Tierra del Fuego grouped in depositional units (ground, terminal and lateral moraines, kame terraces, drumlins, eskers etc.) and erosion units (marginal and submarginal streams etc.). In the present coastal areas a series of mostly marine terraces, which can be attributed to the four principal orders, lie upon it. Deposits and terraced surfaces located at 18-25 m above M.S.L formed in a lacustrine, transitional and marine environment have been referred to the oldest order (First Order). The Second Order concerns erosion surfaces often with associated gravelly and sandy, sometimes fossilipherous deposits, located between 6 and 11 meters above M.S.L, whereas outcrops present between 3 and 5 meters above M.S.L. have been referred to the Third Order. 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. Inland there are numerous older terraced sequences above 25 meters, prevalently linked, however, to a continental environment. Due to their stratigraphic position and after cheking by both radiometric C14 ages and relative methodologies, all four mapped orders are attributable prevalently to the Holocene (Brambati et alii 1995a). The present coastal configurations are prevalently a product of the evolutionary dynamics of said terraced sequences through the continental, transitional and marine environment. Especially the lakes (lagunas) which characterize the coastal areas and were formed by morphogenesis occurring in a glacial environment, have often evolved into a littoral environment as backshore. In many cases they are raised (palaeobackshore) above M.S.L., due to imposing isostatic phenomena following the last glaciation, associated in parallel with a eustatic phase of limited intensity, with relative beach morphologies (beach ridges, berms, spits, etc.) which make up the most frequent paleo-landforms mapped on the terraced surfaces. The western coastal section of the area (Sheet 67) is characterized by vast outcroppings of the Second Order which border Laguna Cabeza de Mar and contain and a whole series of minor lakes. The western coastal area (Seno Otway) shows a more complete terraced sequence which gives witness to a more homogeneous uplift. The considerable altimetric corrispondence of the Second, Third and Fourth Order terraced outcrops in said coastal areas of the Península Brunswick suggests a prevalent and homogeneous isostatic uplift. The rare presence of eroded First Order outcrops in the western area allows us to hypothesize tilting phenomena during the relative uplift.