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)

The 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.