Laguna Azul: an unique volcanic lagoon in Pali Aike Holocene volcanic terrains, Patagonia Austral, Argentina

CORBELLA, H., B. ERCOLANO, P. TIBERI

Malargue, Mendoza, Argentina

Workshop; IAVCEI 3rd International Maar Conference; 2009

IAVCEI 3rd International Maar Conference

  IAVCEI – CVS – IAS 3IMC Conference Malargüe, Argentina, 2008 Laguna Azul: a unique volcanic lagoon in Pali Aike Holocene eruptive terrains, Patagonia Austral, Argentina. Hugo Corbella1, Bettina Ercolano2, Pedro Tiberi2 1 UNPA-MACN Av.Angel Gallardo 470. 1405 Buenos Aires, Argentina. hcorbel@yahoo.com.ar UNPA-MACN Av.Angel Gallardo 470. 1405 Buenos Aires, Argentina. hcorbel@yahoo.com.ar 2 UNPA. .Lisandro de la Torre 1070. 9400 Río Gallegos, Argentina. UNPA. .Lisandro de la Torre 1070. 9400 Río Gallegos, Argentina. Keywords: Volcanic lagoon, Holocene, phreatomagmatism. Volcanic lagoon, Holocene, phreatomagmatism. 1, Bettina Ercolano2, Pedro Tiberi2 1 UNPA-MACN Av.Angel Gallardo 470. 1405 Buenos Aires, Argentina. hcorbel@yahoo.com.ar UNPA-MACN Av.Angel Gallardo 470. 1405 Buenos Aires, Argentina. hcorbel@yahoo.com.ar 2 UNPA. .Lisandro de la Torre 1070. 9400 Río Gallegos, Argentina. UNPA. .Lisandro de la Torre 1070. 9400 Río Gallegos, Argentina. Keywords: Volcanic lagoon, Holocene, phreatomagmatism. Volcanic lagoon, Holocene, phreatomagmatism. One of the remarkable characteristics of Pali Aike volcanic field is the abundance of phreatomagmatic manifestations. One hundred maars and cinder or lapilli rings, frequently appearing along preferential structural alignments, stand out among the Plio-pleistocene volcanic outcrops (Corbella 2002). In contrast, among the younger volcanics -presumably Holocene- located in the SE section of Pali Aike phreatomagmatic outcrops are rare. There, the most frequent elevations are scoria cones aligned along faults that are the emission points of extensive basaltic flows covering 150 km2 of land chiefly of glacial origin. At 52°04.6' S 69°34.9' W, next to Monte Aymond international border pass, the Laguna Azul (Blue Lagoon) volcanic complex is locally well known as a tourist site and provincial geologic reserve. For some earth scientist the most relevant interest resides in that it is one of the two existing deep and permanent water bodies of Pali Aike (the other one is Laguna Potrok Aike) able to store an more or less continuous succession of sedimentary deposits (Schäbitz et al. 2003; Zolitschka et al. 2004, 2006). Lying and incised on glacigenic terrains, it is part of the group of volcanic edifices of Holocene age most of them aligned along fractures. In the most eastern fracture this volcanic complex occupies the North end of a 5 km long alignment composed by five scoria cones and other smaller elevations. Laguna Azul is located at the intersection of two fracture zones. The first one, fractures N40°W, controls the alignment of scoria cones that rise towards the South. The second, visible to the WSW of the complex, consists of two parallel faults of N75°W direction causing a graben 6 km long by 600 meters wide. The NW fracture system is the best represented in Pali Aike and the system from which most volcanic eruptions occurred. Presumably it belongs to an old tectonic event linked to the existent strength field 130 Ma ago during the break apart of South America from Africa (Corbella 2004). With regard to the ENE fractures, they are parallel to another, 6 km to the SW, that controlled the emission of a big lapilli ring and several scoria cones 0.36 Ma ago. The ENE fractures that limit the graben are younger or at least have been reactivated in recent times since they cut and modify the direction of the basaltic lava flows coming from the Holocene scoria cone Morada del Diablo. In contrast with the scoria cones of the same alignment that rise 80 meters above the ground, the relief of Laguna Azul alkali basaltic complex is formed by a 850 m diameter ring of lapilli scarcely 15 m high with a 10° external slope. The central area consists of a 40 meters depression measured from the mean level of the surrounding terrain to the present water level, plus 56 meters below it to the bottom (Zolitschka et al. 2006) where lacustrine sediments peacefully deposited lie on basaltic scoria and lapilli. The morphology of these outcrops look like a scoria cone at the initial stages of its eruption (McGetchin et al. 1974). The central depression is presumably due to gravitational collapses caused by phreatomagmatic explosions occurred during the very early phase of its development. Long time before the Laguna Azul of crystalline waters that we know today was formed, this depression contained a lava lake. Further magma contributions gradually raised the level of the lava lake to overflow the cavity through a drain, 30 meters wide at the crater´s North lip, carved and polished by the lava stream and still well conserved. These lavas spilled out on the northern slope of the complex as flows of pahoehoe texture in the central area and aa texture at the periphery, spreading over 2.3 km2. The lava flow has a 1:200 slope and 5 m mean thickness. When the lava supply diminished and the pressure in the feeding vent decreased, the lava flows stopped spilling out and the level of the lavas inside the cavity descended to the base. In the meanwhile, the surface of the lava lake exposed to the atmosphere had partially solidified and therefore was not able to be spilled outwards nor suctioned into the vent interior. That rind, scarcely few At 52°04.6' S 69°34.9' W, next to Monte Aymond international border pass, the Laguna Azul (Blue Lagoon) volcanic complex is locally well known as a tourist site and provincial geologic reserve. For some earth scientist the most relevant interest resides in that it is one of the two existing deep and permanent water bodies of Pali Aike (the other one is Laguna Potrok Aike) able to store an more or less continuous succession of sedimentary deposits (Schäbitz et al. 2003; Zolitschka et al. 2004, 2006). Lying and incised on glacigenic terrains, it is part of the group of volcanic edifices of Holocene age most of them aligned along fractures. In the most eastern fracture this volcanic complex occupies the North end of a 5 km long alignment composed by five scoria cones and other smaller elevations. Laguna Azul is located at the intersection of two fracture zones. The first one, fractures N40°W, controls the alignment of scoria cones that rise towards the South. The second, visible to the WSW of the complex, consists of two parallel faults of N75°W direction causing a graben 6 km long by 600 meters wide. The NW fracture system is the best represented in Pali Aike and the system from which most volcanic eruptions occurred. Presumably it belongs to an old tectonic event linked to the existent strength field 130 Ma ago during the break apart of South America from Africa (Corbella 2004). With regard to the ENE fractures, they are parallel to another, 6 km to the SW, that controlled the emission of a big lapilli ring and several scoria cones 0.36 Ma ago. The ENE fractures that limit the graben are younger or at least have been reactivated in recent times since they cut and modify the direction of the basaltic lava flows coming from the Holocene scoria cone Morada del Diablo. In contrast with the scoria cones of the same alignment that rise 80 meters above the ground, the relief of Laguna Azul alkali basaltic complex is formed by a 850 m diameter ring of lapilli scarcely 15 m high with a 10° external slope. The central area consists of a 40 meters depression measured from the mean level of the surrounding terrain to the present water level, plus 56 meters below it to the bottom (Zolitschka et al. 2006) where lacustrine sediments peacefully deposited lie on basaltic scoria and lapilli. The morphology of these outcrops look like a scoria cone at the initial stages of its eruption (McGetchin et al. 1974). The central depression is presumably due to gravitational collapses caused by phreatomagmatic explosions occurred during the very early phase of its development. Long time before the Laguna Azul of crystalline waters that we know today was formed, this depression contained a lava lake. Further magma contributions gradually raised the level of the lava lake to overflow the cavity through a drain, 30 meters wide at the crater´s North lip, carved and polished by the lava stream and still well conserved. These lavas spilled out on the northern slope of the complex as flows of pahoehoe texture in the central area and aa texture at the periphery, spreading over 2.3 km2. The lava flow has a 1:200 slope and 5 m mean thickness. When the lava supply diminished and the pressure in the feeding vent decreased, the lava flows stopped spilling out and the level of the lavas inside the cavity descended to the base. In the meanwhile, the surface of the lava lake exposed to the atmosphere had partially solidified and therefore was not able to be spilled outwards nor suctioned into the vent interior. That rind, scarcely few Corbella et al. decimeters thick, can be found as torn pieces upholstering the pyroclastic East, North and West interior flanks of the depression. Apparently controlled by both structural alignments, in the interior and the immediate environment of the depression, several eruptive centres of strombolian emissions can be noticed. The batimetry of the elliptical lagoon shows three sub-basins overlapping laterally. These underwater vents are aligned along the NW fault zone with a 4th vent outcropping to the SE. A 5th vent, probably controlled by the ENE faults, is located 500 meters to the NE. The central vent, 400 m wide, is surrounded by spatter or agglutinated scoria walls rising in high angles (70-75°) 50 meters above the present water level. The other vents have given place to the formation of small lapilli accumulations and scoria cones that rise up to 30 m above the landscape. Among the spatter, scoria and lapilli deposits, pebbles, or fragments of them, belonging to the morainic and glacifluvial countryrock are also present in moderate proportions; inside of the depression cored bombs and bombs showing abundant pebbles are not infrequent. At present, most volcanics of Laguna Azul start to be covered by aeolian sediments and colonized by vegetation. The contrasting abundance of phreatomagmatic events during the Plio-Pleistocene and the scarcity during the Holocene, could be linked to the different climate, hydrological conditions and water availability during these periods. References Corbella,H. 2002. El campo volcano-tectónico de Pali Aike. In Geología y Recursos Naturales de Santa Cruz. (Edit. M. Haller). Asociación Geológica Argentina. Buenos Aires. Capítulo I- 18:285-302. (ISBN 987-20190-0-2) Corbella, H. 2004. Structural Control of the Pali -Aike Lavas and Maars. Second International Maar Conference. IAVCEI. Budapest, Hungría. Abstract Volume p. 49. ISBN 963-671-240-9 McGetchin,T,R., Settle,M. and Chouet,B.A. 1974. Cinder cone growth modeled after northeast crater, Mount Etna, Sicily. Journal of Geophysic Research 79:3257-3272 Schäbitz, F., Paez.M.M., Mancini,M.V., Quintana. F.A., Wille,M., Corbella, H., Haberzettl, T., Lucke, A., Prieto, A.R., Maidana, N., Mayr, C., Ohlendorf, C., Scheleser, G.H., & Zolischka, B. 2003. Estudios paleoambientales en lagos volcánicos en la Región Volcánica de PaliAike, sur de Patagonia (Argentina): palinología. Revista del Museo Argentino de Ciencias Naturales n.s. 5(2)301-316 Zolischka, B., Schäbitz, F., Lucke, A., Wille, M., Mayr, C., Ohlendorf, C., Anselmetti, A., Ariztegui, D., Corbella, H., Ercolano, B., F.,Fey, M., Haberzettl, T., Maidana, N.I., Oliva, G.E., Paez, M. & Scheleser, G.H. 2004. Climate Changes in Southern Patagonia (Santa Cruz, Argentina) Inferred From Lake Sediments: The Multi-Proxy Approach of SALSA. Pages News 12(2)9-11 Zolischka,B., Schäbitz, B., Lücke,A., Corbella,H., Ercolano,B., Fey,M., Haberzettl, T., Janssen,S., Maidana,N., Mayr,C., Ohlendorf,C., Oliva,G., Paez,M., Schleser,G. H., Soto,J., Tiberi,P And Wille,M. 2006. Crater lakes of the Pali Aike Volcanic Field as key sites for paleoclimatic and paleoecological reconstructions in southern Patagonia, Argentina. Journal of South American Earth Sciences. 21:294-309.