INSTITUTO ARGENTINO DE NIVOLOGIA, GLACIOLOGIA Y CIENCIAS AMBIENTALES
Unidad Ejecutora - UE
capítulos de libros
Geocryology of Southern South America
DARIO TROMBOTTO LIAUDAT
Late Cenozoic of Patagonia and Tierra del Fuego
Lugar: Amsterdam, The Netherlands; Año: 2008; p. 255 - 268
Geological evidence indicates that during the Ice Ages the ground remained frozen for long periods of time. Cryogenic phenomena may be observed both in the mountainous or Andean region, and also the Patagonian steppe. According to Mercer (1976, 1985) glaciation during the Plio-Pleistocene left traces at Cerro Fraile, in the Patagonian Andes. Other, even older, traces occur at the Meseta of Lago Buenos Aires, which Mercer linked to the Messinian (Tertiary) Age. In his publication (1976), he refers to the most important glacial advance as the Greatest Patagonian Glaciation (GPG) (compare Clapperton, 1983, 1993; Sylwan, 1989) with an approximate age of 1-1,2 Ma. This glaciation reached the Atlantic Ocean in southern Patagonia. Other authors (see Schellmann, 1998), however, found glacial deposits in the Patagonian lowlands or steppe, where glaciers extended beyond the Southern ~ or Humid Andes (divided here in Lakes Region, between 35° and 45° 30S; Patagonian Andes between 45° 30 S and 53° 30 S and Fuegian Andes between 53° 30 and 55° S approximately), also during the Neogene. With such findings the possible occurrence of even older cold episodes cannot be ruled out. The cold episodes must have been of different glaciations of activity, regarding their expansion and duration. All those episodes must have been decisive for the creation of permafrost, in order to shape the Patagonian landscape. Even the Neoglacials of the Holocene demonstrate reactivation of periglacial Andean processes at numerous sites in the Cordillera despite the fact that they appear very discontinuous, or restricted to certain localities. What is important to note here, regardless of the number of glaciations, is that there is a larger number of cold episodes. These date from the Neogene to the Younger Dryas, or even from the Holocene/Pleistocene transition, and possibly from the Holocene. Many are documented as minor events, the Holocene ones defined as Neoglacials (Mercer 1976, 1985, Garleff & Stingl, 1984, Stingl & Garleff, 1985; Villalba, 1990; Rabassa et al., 1992). During these cold episodes the palaeoenvironmental conditions were favourable for the formation of permafrost in Patagonia and the Southern Andes. The hypothesis glacial = glaciation = permafrost, allows us to calculate an approximate age, but humidity as a determinating factor for glacial expansion, may not correspond to a glacial stage, that is to say, that the increase in ice volume occurs without a considerable drop in the mean annual air temperature (MAAT), as may be observed in some parts of the Earth at present. The second parameter to be considered is the probability that the Andean Patagonian environments have climatically been very different from the presentday climate and also highly variable. The present paper covers the most important periglacial or cryogenic traces and gives a chronological account of those traces during at least two important cold episodes, with considerable falls in MAAT, over a long period of time, which would correspond to glaciations. One of these episodes in Patagonia pertains to the Greatest Patagonian Glaciation (GPG) mentioned above. Another cold episode is related to the latest glaciation. It affects the most recent sediments of the Patagonian stratigraphy. The ice limit of Caldenius (1932), indicated in Fig. 1 is a historic maximum. The relict system is reconstructed mainly in accordance with characteristics and morphodynamics of presentday cold environments. Equating glaciation with cryogenics is only preliminary and requires further thorough research. Subdivision of the cold episodes sensu stricto into different categories and ages may also be carried out with the help of cryostratigraphy. As a result, major changes may be expected both vertically in the profiles and horizontally in the exposition of the ground. Many aspects of geocryology in Patagonia have been summarised separately by Arturo Corte (1997). His book was written in the early Eighties but, after numerous setbacks, it took until 1997 for it finally to be published in Mendoza. This is why Corte first attempted a cryostratigraphical model for Argentina as early as 1991. Two years later, in 1993, Clapperton briefly summarised the Patagonian periglacial features. Trombotto (1994, 1998, 2002) presented the first inventories of cryogenic landforms with fossil cryogenic evidence and established a new chronological order of cold episodes. For the present work bibliographical data have been assembled and field data have been collected using standard geocryological and geological procedures both in the field and in the laboratory (Trombotto, 1991, Trombotto & Stein 1993, a and b, Trombotto & Ahumada 1995 a and b; Schäbitz et al., 1999).