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Distribution of the marine and transitional terraces (Holocene) and geomorphological map between Punta Arenas And Rio Quema Angusta, Peninsula Brunswick, Patagonia, Straits of Magellan, Chile (1:50,000 scale map - Map n° 6/12 - First Edition)
No full textThe 6th map of the Coastal Atlas of the Strait of Magellan (1:50,000 scale) refers to sheets 84 and 101 of the Chilean I.G.M., and was made using the preliminary cartography by E.NA.P. (Empresa Nacional de Petroleo) and the Chilean I.G.M., aerial photos by S.A.F. (Servicio Aerofotogrammetrico Fuerza Aerea de Chile) and T.M. and M.S.S. Landsat satellite images.
This research is aimed mainly at locating and mapping the marine and transitional terraces in Patagonia, along the eastern coasts of the Strait of Magellan, between Punta Arenas and Cabo Froward.
Methodologicaly, we followed both the criteria of several more recent cartographic works produced by Programma Nazionale Ricerche in Antartide (Magellan Project), as well as the general criteria internationally adopted in coastal geological cartography.
The coastal cartography of Quaternary deposits in the area south of Punta arenas (Tierra del Fuego - Strait of Magellan - Chile) is part of a wider ongoing research program that started in 1991 on the recent evolution of the Strait of Magellan. To this purpose, within the Programma Nazionale di Ricerche in Antartide (P.N.R.A.) (National Research Programme in Antarctica), 3 campaigns were carried out on land. The first was in 1991 and dealt with the sedimentology and geology of coastal Pleisto-Holocene deposits, while the second and the third, in 1994 and 2003 that is after the cartographic and geomorphological data had been processed, were intended as a check of the cartography of the old coastlines and postglacial, Quaternary, prevalently marine deposits.
Moreover, the study also involves the the project RAS-PVS (Regione Autonoma Sardegna - Paesi in Via di Sviluppo - Cartografia Geologico - Morfologica delle Coste dello Stretto di Magellano).
Among the objectives that developed during the research is the publication of an Atlas of the coasts of the Strait of Magellan on geological and geomorphological elements, prevalently Holocenic, that can be useful to formulate an evolutionary hypothesis of the Strait in very recent times. The Atlas is at present being refined while its publication depends exclusively on the obtaining of funds to print the maps in colour.
The Atlas is configured not only as a theme cartography of the evolutionary characteristics of the Late Pleistocene-Holocene sea, but also as a cartographic document of geomorphologic and evolutionary elements of the continental and transitional deposits underlying the marine deposits. Three thematic general maps at the 1:200,000 scale on the eastern region of the Strait of Magellan have already been published (Brambati et Al., 1995a; De Muro et Al, 1995; Di Grande et Al., 1995).
Marine and transitional deposits.
Marine and transitional deposits and forms regularly occupy the coastal belt from Punta Arenas to Cabo Foward. They are mainly represented by gravel, sand and silt deposits, or more rarely, by simple erosion surfaces.
Four main orders of terraced sequences have been mapped. The first, and oldest, was found regularly between altitudes of 18 and 25 meters. Sand and silt deposits prevail here. Because of its stratigraphic position and by radiocarbon dating this first order was referred to the Lower Holocene without the exclusion that its base may belong to the Upper Pleistocene, while the depositional environment is transitional.
The Second Order, is distributed between 6 and 11 metres, and is mainly represented by fossiliferous gravel deposits; sands and silts are also present. It regularly covers most of the coastal stretch and extends in the backland at the level of the main fluvial and lacustrine depressions. At times, it presents well-preserved complex sedimentary structures of paleobeach environments (berms, spits and/or bars). The age of formation of the marine deposit, by C14 dating, is on average between 6,OOO and 7,OOO years B.P.
The Third Order strictly follows the present beach, between altitudes of 3 and 5 metres, at the foot of the Second Order terrace. It is made up of gravels and sands and refers preferably to a marine environment. At a slightly lower elevation, presence of small, terraced (Fourth order), marine remnants, younger than in the Third Order, was rarely noticed.
Conclusion
The 6th map of the Coastal Atlas of the Strait of Magellan provides a zoning of the area distribution of the most significant deposits, forms and processes relating to the coastal belt between Punta Arenas and Rio Quema Angusta - Peninsula Brunswick.
From the Map it can be seen that the morphogenesis of the area was strictly controlled by the processes associated with the advance and retreat of the Magellan Glacier, and subordinately, by the main morphodynamic event represented by post deglaciation marine ingression.
The Holocene paleogeographic evolutionary picture of the Patagonia-Tierra del Fuego area, is mainly controlled by the imposing phenomenon of glacio-isostasy, though it cannot be ruled out that the morphogenesis subsequent to deglaciation may have been affected by mild neo-tectonic movements.
As previously observed (DeMuro et Al., 1995b), marine ingressions that produced terracing of the coasts and backland presumably depend on prevalently positive movements of the whole coastal system, related to the final deglaciation of the region.
It is deduced that parallelly following deglaciation, a strong parallel uplift of the mean sea level was produced with initially transgressive stratigraphic effects. Isostatic rebound, that probably occurred rather late compared to post-glacial transgression, is superimposed with very complex and articulate phenomena, that are under study at present and will be the subject of future works.
From an examination of map n. 6/12 of the Atlas of the coasts of the Strait of Magellan, it emerges that all marine and transitional terraces that have been found are later than the last glaciation, and are superimposed on its products with clear angular discordance and sharp erosive truncation. This is deduced both from the stratigraphy of the outcrops and from strata geometry and C14 dating
HOLOCENE EVOLUTION OF THE PRIMERA ANGOSTURA BASED ON MAP 5/12 OF THE GEOMORPHOLOGICAL ATLAS OF THE STRAIT OF MAGELLAN (SOUTHERN CHILE). (+1:50.000 SCALE MAP)
No full textMapping of coastal Quaternary deposits in the Primera Angostura area forms part of an ongoing comprehensive research programme, started in 1991, concerning the recent evolution of the Strait of Magellan. Two field campaigns were carried out on land within the framework of the Italian Programma Nazionale di Ricerche in Antartid e (PNRA). The first , in 1991 concerned the sedimentology and geology of coastal Pleisto-Holocene deposits, while the second, carried out in 1994 once the cartographic and geomorphological data had been processed, was to check the maps of the old coastlines and postglacial, Quaternary and chiefly marine, deposits.
The research also embraces two other projects , "International Geological Correlation Programme" TGCP 367 "Late Quaternary Coastal Records of Rapid Change" and "Coastal Geomorphology Laboratory at the Centro Austral Antartico" (RAS-PYS project) funded by the Autonomous Region of Sardinia within its "Developing Countries" programme.
One of the objectives of the research is the publication of an Atlas of the mainly Holocene geological and geomorphological features of the Strait of Magellan coasts, which will be useful for formulating evolutionary hypothesis of the Strait in very recent times. The Atlas is currently being completed but its publication will depend on the retrieval of funds to print the maps in colour.
The Atlas, that includes the colour map presented here (n. 5112) and three other published maps (De Muro el AI., 1996a; Di Grande et al., 1996; DeMuro et al., 1996b), will consist of 12 sections at the 1:50.000 scale of the east coast of the Strait of Magellan.
It will not only contain thematic maps of the evolutionary characteristics of the Late Pleistocene-Holocene sea, but is also intended as a cartographic document of the geomorphologic and evolutionary features of the continental and transitional deposits underlying the marine deposits. Three general thematic maps 1:200.000 in scale of the eastern region of the Strait of Magellan have already been published (Brambati et al., 1995a; DeMuro et al., 1995; Di Grande et al., 1995).
In the main the preparation of these maps, at least as regards the cartographic basis, has been far from easy.
At best we could refer to the 1:50.000 scale maps compiled by the Chilean l.G.M., which, besides the drawback of not covering the whole area, had a contour spacing of 50 metres. This created even greater difficulties in evaluating minor height differences, which was not infrequent. For areas not covered by the 1:50.000 scale sections, we resorted to the 1:100.000 preliminary maps drawn up by the Chilean E.Na.P. (Empresa Nacional de Petroleo), the 1:250.000 maps of the Chilean l.G.M. and at times also to other l.G.M maps in smaller scales. In some instances, in solving objective difficulties it was necessary to use different scales at the same time, in addition to the various scale nautical charts for near coastal regions.
We also used approximately 1:60.000 scale aerial photos provided by the Servicio Aerofotogrammetrico Fuerza Aerea de Cile (S.A.F.) as well as Landsat TM and MSS satellite images acquired with funds of the Italian Programma Nazionale di Ricerche in Antarticle (PNRA) and processed at Cagliari University's Photogeology and Remote Sensing Laboratory, using funds from the RAS-PVS Project for setting up a Remote Sensing Laboratory at the Centro Austral Antartico" (see satellite images on attached map).
During surveying the attention was focused on those forms, processes and deposits associated with the action of the Holo-Pleistocene sea, and, for a better regional perspective of the problems, with zonation of old glaciation. Moreover, the extremely intense coastal and eolian dynamics in progress today have also been taken into account. For completenss the general features of the fluvial and lacustrine landforms and deposits have also been mapped.
The mapping criterion follows the example of recent works by the National Group (Italy) of Physical Geography and Geomorphology for various themes (coastal, submarine, continental).
Moreover, for certain graphic and explanatory aspects, the cartography of the Atlas of the Coasts of the Strait of Magellan is innovative and represents a new and modern contribution to thematic mapping
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
No full textThe 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)
Geomorphological and textural map of the coastal area of Bahía Inútil - Straits of Magellan – Chile. (1:100.000 - scale map) Map n.2
No full textMARINE AND TRANSITIONAL TERRACES (HOLOCENE) ALONG THE COAST OF SENO OTWAY AND BAHIA SHOAL. PATAGONIA, STRAITS OF MAGELLAN – CHILE
No full textDISTRIBUZIONE DEI TERRAZZI MARINI E TRANSIZIONALI (PLEISTOCENE? - OLOCENE) E CARTA GEOMORFOLOGICA DELLA FASCIA COSTIERA TRA PUNTA DUNGENESS E BAHIA SAN FELIPE, STRETTO DI MAGELLANO - CILE - FIRST EDITION. - SCALA 1:200000” [MAP B]
No full textThe 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 poject 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
DISTRIBUZIONE DEI TERRAZZI MARINI E TRANSIZIONALI (PLEISTOCENE? - OLOCENE) E CARTA GEOMORFOLOGICA DELLA FASCIA COSTIERA TRA PORVENIR E PUERTO YARTOU, STRETTO DI MAGELLANO – CILE - FIRST EDITION. - SCALA 1:200.000 [MAP C]
No full textThe 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 poject 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)
DISTRIBUTION OF THE MARINE AND TRANSITIONAL TERRACES (HOLOCENE) AND GEOMORPHOLOGICAL MAP OF THE PRIMERA ANGOSTURA, PATAGONIA, TIERRA DEL FUEGO, STRAITS OF MAGELLAN, CHILE (1:50,000 SCALE MAP) First Edition
No full textPrevious 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 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 Strait (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 (Bramati at alii, 1993a; 1993b).
On the basis of the initial geomorphological, geological and sedimentological result obtained. We made a zoning of the coastal belts along the Atlantic opening of the Strait which enabled us to print three sheets on a scale of 1:200,000 (De Muro et alii 1995, Bramati 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 twelve geomorphological maps (of which this is the fifth) topographically based on 28 of the Chilean I.G.M. 1:50,000 scale maps of the area between Punta Dungeness and Bahía Inútil (Di Grande et alii 1996a, 1996b and 1996c, De Muro et alii 1996 and 1997)
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)
No full textPrevious 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
DISTRIBUTION OF MARINE AND TRANSITIONAL TERRACES IN THE COASTAL AREA BETWEEN PORVENIR AND PUERTO YARTOU - STRAITS OF MAGELLAN CHILE. (1:200.000 - MAP C)
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