Potrok Aike, a peculiar maar among one hundred phreatomagmatic edifices. Pali Aike Volcanic Field, Southern Patagonia, Argentina.

CORBELLA, H., M. FEY, T. HABERZETTI, C -MAYR, C. OHLENDORF, M .WILLE, B. ZOLITSCHKA

Malargue, Mendoza. Argentina.

Workshop; IAVCEI 3rd International Maar Conference; 2009

IAVCEI 3rd International Maar Conference

Potrok Aike, a peculiar maar among one hundred phreatomagmatic edifices. Pali Aike Volcanic Field, Southern Patagonia, Argentina. . Corbella, H.1, Fey, M.2, Haberzettl, T.3, Mayr, C.4, Ohlendorf, C.2, Wille, M.5, Zolitschka, B.2 1 Universidad Nacional de la Patagonia Austral – Museo Argentino de Ciencias Naturales, Buenos Aires, Argentina. – hcorbel@yahoo.com.arUniversidad Nacional de la Patagonia Austral – Museo Argentino de Ciencias Naturales, Buenos Aires, Argentina. – hcorbel@yahoo.com.ar 2 Geomophology and Polar Research (GEOPOLAR), Institute of Geography, University of Bremen, Bremen, Germany. Geomophology and Polar Research (GEOPOLAR), Institute of Geography, University of Bremen, Bremen, Germany. 3 Institut des sciences de la mer de Rimouski (ISMER), University of Québec at Rimouski (UQAR),Canada. 4 GeoBio-Center and Department of Earth and Environmental Sciences, University of Munich, Munich, Germany. Institut des sciences de la mer de Rimouski (ISMER), University of Québec at Rimouski (UQAR),Canada. 4 GeoBio-Center and Department of Earth and Environmental Sciences, University of Munich, Munich, Germany. 5 Seminar for Geography and its Didactics, University of Cologne, Cologne, Germany. Seminar for Geography and its Didactics, University of Cologne, Cologne, Germany. Keywords: Phreatomagmatism, Maar, Patagonia. Phreatomagmatism, Maar, Patagonia. 1, Fey, M.2, Haberzettl, T.3, Mayr, C.4, Ohlendorf, C.2, Wille, M.5, Zolitschka, B.2 1 Universidad Nacional de la Patagonia Austral – Museo Argentino de Ciencias Naturales, Buenos Aires, Argentina. – hcorbel@yahoo.com.arUniversidad Nacional de la Patagonia Austral – Museo Argentino de Ciencias Naturales, Buenos Aires, Argentina. – hcorbel@yahoo.com.ar 2 Geomophology and Polar Research (GEOPOLAR), Institute of Geography, University of Bremen, Bremen, Germany. Geomophology and Polar Research (GEOPOLAR), Institute of Geography, University of Bremen, Bremen, Germany. 3 Institut des sciences de la mer de Rimouski (ISMER), University of Québec at Rimouski (UQAR),Canada. 4 GeoBio-Center and Department of Earth and Environmental Sciences, University of Munich, Munich, Germany. Institut des sciences de la mer de Rimouski (ISMER), University of Québec at Rimouski (UQAR),Canada. 4 GeoBio-Center and Department of Earth and Environmental Sciences, University of Munich, Munich, Germany. 5 Seminar for Geography and its Didactics, University of Cologne, Cologne, Germany. Seminar for Geography and its Didactics, University of Cologne, Cologne, Germany. Keywords: Phreatomagmatism, Maar, Patagonia. Phreatomagmatism, Maar, Patagonia. In southernmost South America, from 51° to 52°S, the Pali-Aike tectono-volcanic field is located in the Magellan Basin area 300 kilometers East of the Andean volcanic front. The volcanic outcrops, mostly of alkali-basaltic and basanitic composition, are part of a Pliocene-Holocene back-arc volcanic field. Older Miocene basaltic mesetas outcrop towards the western end of the field (Ton-That et al. 1999, Corbella 2002, Mejía et al. 2004, Corbella and Lara 2008). Most of Pali Aike volcanics rest on a soft substratum composed by fluvioglacial Patagonian Gravels lying on a Miocene terrace, or on a thick package of glacial sediments (till and glacifluvial conglomerates) belonging to one of the several glaciations that affected the area (Caldenius 1932, Coronato et al. 2004, Rabassa 2008). Below these units there are ~500 m of molassic deposits (sands, microconglomerates and tuffs poorly lithified) of the Miocene Santa Cruz Formation that in turn lie on marine sandy-siltstones and silty-claystones of the Miocene Monte León Formation. The predominant fault systems have a NW direction, later followed by faults of E and ENE strike. The NW system coincides with an underlying Jurassic palaeo-rift zone. The E and ENE structures are parallel or sub-parallel to the faults that channeled the large glacial valleys (e.g. the Magellan Strait and the Skyring, Otway, Inutil and St. Sebastian Bays). During the Upper Tertiary, these structures were caused by a NW stretching due to a new stress field in the southern flank of the Magellan Basin (Corbella 2004). Young N-S alignments seem to control the course of several valleys that drained glacial lobes. . One of the characteristics of Pali Aike, in relation to other patagonian volcanic fields, is the abundance of phreatomagmatic edifices. There, the phreatomagmatic activity has given place to the formation of multiple depressions 500 or more meters wide. But due to the scarce precipitations and the high evaporation rate produced by the wind, or because many of them were padded by fluvial and eolian sediments or have scoria cones or lava flows inside, they are almost always dry or have shallow ephemeral water bodies. In this volcanic field only two depressions at present preserve water bodies of certain depth: a very young and small one, the Laguna Azul (see another presentation in this same Conference) and one housed in the largest maaric depression of the area, Lake Potrok Aike. The Potrok Aike maar (51°57’S 70°22’W) is located in the immediate vicinity of the intersection of one of the longest ENE fractures in Pali Aike and the structural N-S alignment that channels the upper course of the Bandurrias creek and the lower course of the Robles creek. These structures may have controlled the rise of the magma. In Pali Aike volcanic field the necessary water to start and sustain the phreatomagmatic activity could have come from superficial sources such as running waters, frozen soils (permafrost) characteristic of periglacial environments, soaked sediments or from deep phreatic aquifers housed in the Santa Cruz Formation. In Potrok Aike maar the Bandurrias creek alignment and the ENE fracture could have been hydraulically active zones. Among the one hundred pheatomagmatic edifices outcropping in Pali Aike volcanic field, Potrok Aike maar stands out for: its wide dimension, the severe erosion undergone and the presence in its interior of a deep water body when all other maar depressions are padded by sediments. The Potrok Aike maar has a broad and flat morphology. The lake (113 masl) has an almost circular shape with diameters ranging between 2.4 and 2.7 kilometers. The entire depression containing the lake is ~ 4-5 km wide and the highest altitude difference between the present day lake-level and the surrounding morainic plain is ~50 m. Bathymetric surveys (Zolitschka et al. 2004) showed the morphology of the diatreme, roughly circular in the underwater upper part and N-S elongated at 100 m depth. Seismic surveys (Niessen and Gebhardt 2006) permitted to infer the continuity of the diatreme’s walls up to a depth of 300 or more Corbella et al. meters. The maar depression+diatreme ensemble has a champagne glass shape, which is characteristic of maars erupted in soft-rock environments (Lorenz 2003). On the SW side of the depression rises a dismantled scoria cone whose North flank, according to magnetometric data, seems to have collapsed towards the North. In the W sector of the lake, basaltic lava flows spilled prior to the formation of the diatreme yield a 1.3 Ma Ar/Ar age (Corbella 2002). The ring of phreatomagmatic deposits that usually surrounds most of the Pali Aike diatremes is lacking here. The only preserved phreatomagmatic deposit of some extension outcrops in the SE flank of the lake. It lies above the glacial sediments and is covered by eolic soils that hinder its being noticed. With a surface of ~ 3 km2 and a thickness of several dozens of meters it presents a fine layering containing chilled juvenile sideromelane clasts and minor tachilitic fragments, and abundant lithics such as glacial or glacifluvial pebbles, basaltic clasts, silty-sand fragments (presumably of the Santa Cruz Formation) and quartz grains. Only one Ar/Ar geochronological determination on a juvenile fragment of this outcrop was done and it yielded an age of 770 Ka (Zolitschka et al. 2006). Due to the diverse nature of the clasts observed, other determinations will be done in order to better define the age of the phreato-magmatic events. Two transitory creeks and several minor gullies end at the Potrok Aike lake supplying, principally during the thaw season, its sedimentary load. This modern sedimentary contribution is being monitored by sediment traps at various depths. One of the principal affluents of the lake and sediment contributor is the Bandurrias creek. Before the Potrok Aike phreatomagmatic events occurred the former Bandurrias creek joined the Robles creek, tributary of the Gallegos river. They both drained to the North waters coming from glaciers lobes detached from the Magellan Strait glacier. The present maar lake occupies the East side of the flat Bandurrias creek valley of S-N trend mainly carved on morainic terrains. After the maar eruption, the creek waters were captured by the Potrok Aike basin, abandoning the primitive course. The Potrok Aike singularities could imply a complex subsidence structure; a peculiar sedimentation regime and a volcanic history with probable young explosive events. To better understand these topics it will be necessary to consider the eruptive history mostly registered in the phreatomagmatic deposit preserved in the SE flank; to investigate the structural framework of the complex; to obtain geophysical information about possible late volcanic and sub-volcanic events; to define which was the sedimentation regime throughout the times and how it was influenced by the evolution and capture of the Bandurrias creek and the establishment of periglacial climatic conditions in the catchment area. References Caldenius, C., 1932. Las glaciaciones Cuaternarias en la Patagonia y Tierra del Fuego. Dirección General de Minería y Geología, Publicación 95, 152 p. Argentina. Corbella, H., 2002. El campo volcano-tectónico de Pali Aike. In: M. Haller (Ed.) Geología y Recursos Naturales de Santa Cruz. Asociación Geológica Argentina, Buenos Aires. I(18)285-302. Corbella, H., 2004. Structural Control of the Pali-Aike Lavas and Maars. Second International Maar Conference, IAVCEI. Budapest, Hungary. Abstract Volume p. 49. Corbella, H., Lara, L.E., 2008. Late Cenozoic-Quaternary Volcanism in Patagonia and Tierra del Fuego. In: J. Rabassa (Ed.) The Late Cenozoic in Patagonia and Tierra del Fuego. Developments in Quaternary Sciences, 11(6)96-119. Elsevier. Coronato, A., Martínez, O., Rabassa, J., 2004. Pleistocene Glaciations in Argentine Patagonia, South America. In: Ehlers and P. Gibbard (Eds.) Quaternary Glaciations, Extent and Chronology. Part III. Quaternary Book Series 49-67. Elsevier. Lorenz, V., 2003. Maar-Diatreme Volcanoes, their Formation, and their Setting in Hard-rock or Soft-rock Environments. Geolines, Journal of the Geological Institute of AS Czech Republic, 15:72-83. Mejía, V., Opdyke, N.D., Vilas, J.F., Singer, B.S., Stoner, J.S., 2004. Plio-Pleistocene time-averaged field in southern Patagonia recorded in lava flows. Geochemistry, Geophysics and Geosystems 5(3)1-15. Rabassa, J., 2008. Late Cenozoic glaciations in Patagonia and Tierra del Fuego. In: J. Rabassa (Ed.) The Late Cenozoic of Patagonia and Tierra del Fuego. Developments in Quaternary Science 11:151-203. Elsevier. Ton-That, T., Singer, B., Morner, N.A., Rabassa, J., 1999. Datación de lavas basálticas por 40Ar/39Ar y geología de la región del Lago Buenos Aires, Provincia de Santa Cruz, Argentina. Revista de la Asociación Geológica Argentina 54:333-352. Zolitschka, B., Schäbitz, F., Lucke, A., Wille, M., Mayr, C., Ohlendorf, C., Anselmetti, A., Ariztegui, D., Corbella, H., Ercolano, B., Fey, M., Haberzettl, T., Maidana, N., Oliva, G.E., Paez, M., Schleser, G.H., 2004. Climate Changes in Southern Patagonia (Santa Cruz, Argentina) Inferred From Lake Sediments: The Multi-Proxy Approach of SALSA. Pages News 12:9-11. Zolistchka, 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., 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. Only one Ar/Ar geochronological determination on a juvenile fragment of this outcrop was done and it yielded an age of 770 Ka (Zolitschka et al. 2006). Due to the diverse nature of the clasts observed, other determinations will be done in order to better define the age of the phreato-magmatic events. Two transitory creeks and several minor gullies end at the Potrok Aike lake supplying, principally during the thaw season, its sedimentary load. This modern sedimentary contribution is being monitored by sediment traps at various depths. One of the principal affluents of the lake and sediment contributor is the Bandurrias creek. Before the Potrok Aike phreatomagmatic events occurred the former Bandurrias creek joined the Robles creek, tributary of the Gallegos river. They both drained to the North waters coming from glaciers lobes detached from the Magellan Strait glacier. The present maar lake occupies the East side of the flat Bandurrias creek valley of S-N trend mainly carved on morainic terrains. After the maar eruption, the creek waters were captured by the Potrok Aike basin, abandoning the primitive course. The Potrok Aike singularities could imply a complex subsidence structure; a peculiar sedimentation regime and a volcanic history with probable young explosive events. To better understand these topics it will be necessary to consider the eruptive history mostly registered in the phreatomagmatic deposit preserved in the SE flank; to investigate the structural framework of the complex; to obtain geophysical information about possible late volcanic and sub-volcanic events; to define which was the sedimentation regime throughout the times and how it was influenced by the evolution and capture of the Bandurrias creek and the establishment of periglacial climatic conditions in the catchment area. References Caldenius, C., 1932. Las glaciaciones Cuaternarias en la Patagonia y Tierra del Fuego. Dirección General de Minería y Geología, Publicación 95, 152 p. Argentina. Corbella, H., 2002. El campo volcano-tectónico de Pali Aike. In: M. Haller (Ed.) Geología y Recursos Naturales de Santa Cruz. Asociación Geológica Argentina, Buenos Aires. I(18)285-302. Corbella, H., 2004. Structural Control of the Pali-Aike Lavas and Maars. Second International Maar Conference, IAVCEI. Budapest, Hungary. Abstract Volume p. 49. Corbella, H., Lara, L.E., 2008. Late Cenozoic-Quaternary Volcanism in Patagonia and Tierra del Fuego. In: J. Rabassa (Ed.) The Late Cenozoic in Patagonia and Tierra del Fuego. Developments in Quaternary Sciences, 11(6)96-119. Elsevier. Coronato, A., Martínez, O., Rabassa, J., 2004. Pleistocene Glaciations in Argentine Patagonia, South America. In: Ehlers and P. Gibbard (Eds.) Quaternary Glaciations, Extent and Chronology. Part III. Quaternary Book Series 49-67. Elsevier. Lorenz, V., 2003. Maar-Diatreme Volcanoes, their Formation, and their Setting in Hard-rock or Soft-rock Environments. Geolines, Journal of the Geological Institute of AS Czech Republic, 15:72-83. Mejía, V., Opdyke, N.D., Vilas, J.F., Singer, B.S., Stoner, J.S., 2004. Plio-Pleistocene time-averaged field in southern Patagonia recorded in lava flows. Geochemistry, Geophysics and Geosystems 5(3)1-15. Rabassa, J., 2008. Late Cenozoic glaciations in Patagonia and Tierra del Fuego. In: J. Rabassa (Ed.) The Late Cenozoic of Patagonia and Tierra del Fuego. Developments in Quaternary Science 11:151-203. Elsevier. Ton-That, T., Singer, B., Morner, N.A., Rabassa, J., 1999. Datación de lavas basálticas por 40Ar/39Ar y geología de la región del Lago Buenos Aires, Provincia de Santa Cruz, Argentina. Revista de la Asociación Geológica Argentina 54:333-352. Zolitschka, B., Schäbitz, F., Lucke, A., Wille, M., Mayr, C., Ohlendorf, C., Anselmetti, A., Ariztegui, D., Corbella, H., Ercolano, B., Fey, M., Haberzettl, T., Maidana, N., Oliva, G.E., Paez, M., Schleser, G.H., 2004. Climate Changes in Southern Patagonia (Santa Cruz, Argentina) Inferred From Lake Sediments: The Multi-Proxy Approach of SALSA. Pages News 12:9-11. Zolistchka, 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., 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.