di Scienze della Terra" of the "Università di Cagliari" within the cooperation poject 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 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 on the area between Punta Dungeness and Bahía Inútil (Di Grande et alii 1996a, 1996b, 1996c; De Muro et alii 1996). The Península Juan Mazía is situated on the Segunda Angostura and 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 the geomorphological map which includes its Sheets No. 69, No. 70 and No. 51 of Section L - Chilean I.G.M. and which mainly concerns the study of the marine and transitional terraced sequences surveyed along the coastal strip. Its origin is prevalently linked to the last glaciation which produced the glacial deposits of which it is formed (Stage-Juan Mazía, 29,000 - 24,000 years B.P., according to Clapperton, 1992). With the exception of the Segunda Angostura zone, the coasts of the Península Juan Mazía, like the rest of the coast along the eastern part of the Straits, bear clear testimony to a different conformation in the lower Holocene. In fact the western area of this Península (F.69) 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.70), the intermediate order (Second Order - Elevation 6-11 m) is better reppresented 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 attribued to the lower Holocene (9,000 - 8,000 B.P.) although formation may have started in the late Pleistocene. Its formation enviroment seems to be lacustrine, evolving to transitional and marine. The Second Order, certainly linked to a marine enviroment, is also Holocene and is on average attributable to 6,000-7,000 years B.P.. 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 1,500-2,000 years B.P. can be referred to the same enviroment. In addition to the above four orders, we noted the presence of other terraced sequences, located above 25 meters and very probably of fluvial and/or glacifluvial origin. Another order of marine enviroment, not mapped due to problems of scale, is locally present on the coastal strip less than 3 meters above M.S.L.. The considerable altimetric precision of the four younger terrace orders mapped, suggests that the considerable isostatic component of the Holocene uplift was added to in parallel by the positive eustatic component which however, at the present stage of research, is estimated to be less important. However the anomaly in the distribution of the First Order Terraces, consisting principally in their absence or scarcity in the eastern area (F70), leads us to hypothesize also a certain tectonic activity (of which however there is no sure evidence) with temporary tilting accompanied by erosion and demolition of the terraces themselves.
DISTRIBUTION OF THE MARINE AND TRANSITIONAL TERRACES (HOLOCENE) ALONG THE COAST OF PENÍNSULA JUAN MAZÍA, STRAITS OF MAGELLAN, CHILE (1: 50,000 SCALE MAP) Map 2/12 First Edition
DEMURO, SANDRO;
1996-01-01
Abstract
di Scienze della Terra" of the "Università di Cagliari" within the cooperation poject 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 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 on the area between Punta Dungeness and Bahía Inútil (Di Grande et alii 1996a, 1996b, 1996c; De Muro et alii 1996). The Península Juan Mazía is situated on the Segunda Angostura and 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 the geomorphological map which includes its Sheets No. 69, No. 70 and No. 51 of Section L - Chilean I.G.M. and which mainly concerns the study of the marine and transitional terraced sequences surveyed along the coastal strip. Its origin is prevalently linked to the last glaciation which produced the glacial deposits of which it is formed (Stage-Juan Mazía, 29,000 - 24,000 years B.P., according to Clapperton, 1992). With the exception of the Segunda Angostura zone, the coasts of the Península Juan Mazía, like the rest of the coast along the eastern part of the Straits, bear clear testimony to a different conformation in the lower Holocene. In fact the western area of this Península (F.69) 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.70), the intermediate order (Second Order - Elevation 6-11 m) is better reppresented 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 attribued to the lower Holocene (9,000 - 8,000 B.P.) although formation may have started in the late Pleistocene. Its formation enviroment seems to be lacustrine, evolving to transitional and marine. The Second Order, certainly linked to a marine enviroment, is also Holocene and is on average attributable to 6,000-7,000 years B.P.. 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 1,500-2,000 years B.P. can be referred to the same enviroment. In addition to the above four orders, we noted the presence of other terraced sequences, located above 25 meters and very probably of fluvial and/or glacifluvial origin. Another order of marine enviroment, not mapped due to problems of scale, is locally present on the coastal strip less than 3 meters above M.S.L.. The considerable altimetric precision of the four younger terrace orders mapped, suggests that the considerable isostatic component of the Holocene uplift was added to in parallel by the positive eustatic component which however, at the present stage of research, is estimated to be less important. However the anomaly in the distribution of the First Order Terraces, consisting principally in their absence or scarcity in the eastern area (F70), leads us to hypothesize also a certain tectonic activity (of which however there is no sure evidence) with temporary tilting accompanied by erosion and demolition of the terraces themselves.
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