We describe two circular morphological features representing depressed areas. This preliminary work is based on visual interpretations from Landsat ETM satellite images and SRTM Radar scans. The two geoforms are located at the north of the Sierras Pampeanas in Argentina. The La Ciénaga circular indentation is 15 km in diameter and Las Cejas is 34 km in diameter. Until now rings of these characteristics, magnitudes and diameters are unknown and unheard of in Argentina.
In the northern Sierras Pampeanas of Argentina we identified two circular natural geoforms for which we do not have any evidence to elucidate a process that gave rise to them (Figure 1). The 15 km wide La Ciénaga geoform is in the east of the Sierra de Fiambalá in the province of Catamarca (27˚25'S - 67˚W) and the 34 km wide Las Cejas geoform is located to the north of Dorsal Mujer Muerta on the border between the provinces of Tucuman and Santiago del Estero (26˚50'S - 64˚45'W).
The La Ciénaga geoform was identified from Landsat ETM imagery and the Las Cejas geoform on a Shuttle Radar Topographic Mission (SRTM) image. The following topographic maps were also used: 1:250,000 - 2766-II San Miguel de Tucuman and 2766-III Belen [1] [2] , 1:200,000 Hojas Geológicas, 12c Laguna Helada [3] , 12d Capillitas [4] , and 1:250,000 - 2766-IV Concepción [5] .
In nature, there are morphological features produced by processes or natural phenomena and whose geoforms, by themselves, allow to identify the process
that generated them. For example, alluvial fans, glacier circuses, moraines, river basins, curved structures, elliptic structures, volcanic cones, volcanic caldera, impact structures, karst geoforms, etc. (e.g., [6] - [12] ). The importance to study these geoforms lies in being able to understand the processes that formed them, abundance, geographical location, etc., in order to prevent and minimize natural risks.
These two structures are not associated with volcanic rocks, limestone, tectonic processes, glaciers, etc., with which we could link their origin; thus, it was thought that they could represent impact structures. Circular geoforms such as those studied in this work, are important not only for geomorphological analysis, because they could be associated with impact cratering, but also as a tool for analysis of the effects that the impact of large objects could produce and cause to our climate and biosphere in the affected area.
According to [13] , an object of 750 m width can generate a geoform of 15 km diameter, which would be sufficient to produce an atmospheric explosion over the impact site, reducing solar radiation, causing temperature distortion and injecting of dust into the stratosphere, with a residence time of 1 Ma, and if the target composition was right, inject 5 times more sulphur than the current content in the atmosphere and thereby destroying the ozone layer. To generate a geological landform of 34 km in diameter, it would require an object of 1700 m width, causing a much greater impact, considering the impact of an object only 50 m wide that produces a crater of 1 km in diameter could obliterate an area of several hundred km2 around the impact site [14] .
The impact structure Araguainha (Brazil) of 40 km in diameter [15] , was dated in 246 Ma (40 Ar/39 Ar) [16] . It is a complex crater that was eroded; its central part rises about 150 m with respect to the surrounding surface, marked by rings of about 8 km in diameter and an internal elliptical depression of about 3 to 4.5 km [16] .
Argentina has been alleged to have seventy-four meteorite impacts and seven other pseudo-meteorite impact sites [17] [18] [19] . In the strewn field of the Campo del Cielo meteorite in Chaco, 20 small craters are located within an area of 15 km length [20] [21] , and the structures at Rio Cuarto (Cordoba) consist of elongated depressions that stretch over 40 km [22] . These meteorite impacts are arranged into five bands of NNE strike with a gap in the Santa Rosa region between 35˚ and 37˚S and another in the Río Gallegos region (Figure 2). We only know the age of 27 meteorite fall. The oldest are Luján in the province of Buenos Aires (50,000 - 20,000 years) and Campo del Cielo in the province of Chaco (4000 ± 80 years, determined by radiocarbon) [19] [23] - [43] . The age of the other meteorite fall is between 1879 and 2008 (Table 1).
Age of meteorites fall in Argentina. WL: west latitude. SL: south latitude
ReferencesInstituto Geográfico Militar (1991a) Carta Topográfica 1:250.000 2766-II San Miguel de Tucumán (provincias de Tucumán, Salta, Santiago del Estero y Catamarca). Buenos Aires, Argentina.Instituto Geográfico Militar (1991b) Carta Topográfica 1:250.000 2766-III Belén (provincias de Catamarca y Tucumán). Buenos Aires, Argentina.Ruíz Huidobro, O.J. (1975) Carta Geológico-Económica Laguna Helada-12c, provincia de Catamarca, escala 1:200.000. Ministerio de Economía, Servicio Nacional Minero Geológico, 146, 68 p.González Bonorino, F. (1947) Carta Geológico-Económica Capillitas-12d, provincia de Catamarca, escala 1:200.000. Ministerio de Economía, Dirección General de Minas y Geología, 65, 72 p.DalMolin, C.N., Fernández, D. and Escosteguy, L. (2003) Hoja Geológica 2766-IV Concepción, escala 1:250.000 (provincias de Tucumán, Catamarca y Santiago del Estero). Servicio Geológico Minero Argentino, 342, 41 p.Summerfield, M.A. (1991) Global Geomorphology: An Introduction to the Study of Landforms. Pearson, Prentice Hall, England, 537 pp.French, B.M. (1998) Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures. Lunar and Planetary Institute, Houston, 120 pp.Gutiérrez A.A. ,et al. (1999)Tectonic Geomorphology of the Ambato Block (Northwestern Pampeanas Mountain Ranges, Argentina) 1, 307-310.Tsikalas, F. (2005) Mjolnir Crater as a Result of Oblique Impact: Asymmetry Evidence Constrains Impact Direction and Angle. In: Koeberl, C. and Henkel, H., Eds., Impact Tectonics, Springer, Berlin, 285-306. https://doi.org/10.1007/3-540-27548-7_10Cunningham, W.D. (2007) Structural and Topographic Characteristics of Restraining Bend Mountain Ranges of the Altai, Gobi Altai and Easternmost Tien Shan. In: Cunningham, W.D. and Mann, P., Eds., Tectonics of Strike-Slip Restraining and Releasing Bends, Geological Society Special Publication, London, 219-238.https://doi.org/10.1144/SP290.7Gutiérrez, A.A. and Mon, R. (2008) Macroindicadores cinemáticos en el Bloque Ambato, provincias de Tucumán y Catamarca. Revista de la Asociación Geológica Argentina, 63, 24-28.Zampieri, D., Gutiérrez, A.A., Massironi, M. and Mon, R. (2012) Reconciling Opposite Strike-Slip Kinematics in the Transpressional Belt of the Sierra Pampeanas (Argentina). European Geosciences Union General Assembly, Viena, 2 p.Chapman, C.R. and Morrison, D. (1989) Cosmic Catastrophes. Plenum Press, New York, 302 p. https://doi.org/10.1007/978-1-4899-6553-0Kring, D.A. (1997) Air Blast Produced by the Meteor Crater Impact Event and a Reconstruction of the Affected Environment. Meteoritics & Planetary Science, 32, 517-530. https://doi.org/10.1111/j.1945-5100.1997.tb01297.xDietz, R.S. and French, B.M. (1973) Probable Astroblemes in Brazil. Nature, 244, 561f. https://doi.org/10.1038/244561a0Engelhardt, W.V., Matthai, S.K. and Walzebuck, J. (1992) Araguainha Impact Crater, Brazil. I. The Interior Part of the Uplift. Meteoritics, 27, 442-457.Rocca, M.C.L. (2003) El Cráter en la Meseta de la Barda Negra, Neuquén: Un potencial nuevo impacto de meteorito en Patagonia, Argentina. Cambridge Conference Network, Vol. 116, 7 p.Acevedo, R.D., Valín-Alberdi, M.L. and Villar, L.M. (2002) Hallazgo del mineral fosfuro de Níquel en una octahedrita IAB de Rubín de Celis (Campo del Cielo, Argentina). 1 Congreso Ibérico de Meteoritos y Geología Planetaria, Resúmenes, Museo de las Ciencias de Castilla-La Mancha, Cuenca.Acevedo, R.D. and Roca, M.C.L. (2008) Catálogo de los meteoritos hallados en territorio argentino. Actas del 17 Congreso Geológico Argentino, Vol. 3, 1317-1318, San Salvador de Jujuy.Cassidy, W.A. and Wright, S.P. (2003) Small Impact Craters in Argentine Loess: A Step up from Modeling Experiments.Workshop on Impact Cratering, 8004 p.Bocanera, R. (2006) Algunas observaciones sobre meteoritos y meteoros. Petrotecnia, 80-88.Bland, P.A., De Souza, F.C.R., Hough, R.M., Pierazzo, E., Coniglio, J., Pinotti, L., Jull, A.J.T. and Evers, V. (2001) The Río Cuarto Crater Field Revisited: Remote Sensing Imagery Analysis and New Field Observations. 64th Annual Meteoritical Society Meeting, 5319 p.Ameghino F. ,et al. (1914)Aerolito fósil 2, 276-279.Herrero Ducloux, E. (1914) Nota sobre el meteorito carbonoso de Nogoyá. AnalesMuseo Nacional de Historia Natural, Buenos Aires, 26, Mineralogía, Petrografía, 3, 99-116.Herrero Ducloux, E. (1925) Nota sobre el meteorito de La Colina. Anales MuseoNacional de Historia Natural, Buenos Aires, 33, 287-295.Herrero Ducloux E. ,et al. (1926)Nota sobre el meteorito de Santa Isabel 4, 23-29.Herrero Ducloux E. ,et al. (1940)Nota sobre el meteorito de Gualeguaychú. Anales Museo Argentino de Ciencias Naturales. Buenos Aires. 40, Mineralogía 14, 123-127.Pastore, F. (1925) Aerolito de La Colina. Anales Museo Nacional de Historia Natural. Buenos Aires 33, Mineralog., Petrogr. Vol. 6, 297-306.Herrero Ducloux, E. and Pastore, F. (1929) El meteorito de Renca. Revista Facultad Química Farmacia La Plata, 5, 111-120.Herrero Ducloux, E. and Pastore, F. (1930) El Meteorito de Ishtilart. Anales Museo Argentino de Ciencias Naturales. Buenos Aires 36, Mineralogía. Petrografía, 9, 313-330.Olsacher J. ,et al. (1951)Condrita de Achiras 39, 261-267.Olsacher J. ,et al. (1951)Condrita de Quebrada de la Aguada 39, 268-273.The Permanent Commission on Meteorites of the International Geological Congress (1961) Meteorites Not Included in the Prior-Hey Catalogue of Meteorites, 1953. The Meteoritical Bulletin, 21, 1-3.The Permanent Commission on Meteorites of the International Geological Congress (1962) Meteorites Not Included in the Prior-Hey Catalogue of Meteorites, 1953. The Meteoritical Bulletin, 24, 1-6.The Permanent Commission on Meteorites of the International Geological Congress (1962) Meteorites Not Included in the Prior-Hey Catalogue of Meteorites, 1953. The Meteoritical Bulletin, 25, 1-3.The Permanent Commission on Meteorites of the International Geological Congress (1964) Discovery of Arbol Solo Stony Meteorite, Argentina. The Meteoritical Bulletin, 32, 1-6.Teruggi, M.E. (1968) El Meteorito condriticoChajari. Revista del Museo de La Plata, 6, 21 p.Giacomelli, L.O. (1969) Guía de Meteoritos de la Argentina. Revista Museo Argentino de Ciencias Naturales, Geología, 7, 1.Graham, A.L. (1986) Fall of the La Criolla, Stony, Argentina. The Meteoritical Bulletin, 64, 310.Wlotzka, F. (1994) The Meteoritical Bulletin, No. 77. Meteoritics, 29, 891-897. https://doi.org/10.1111/j.1945-5100.1994.tb01104.xGrossman, J.N. (1998) The Meteoritical Bulletin, No. 82. Meteoritics and Planetary Science, 33, A221-A239. https://doi.org/10.1111/j.1945-5100.1998.tb01336.xGrossman, J.N. (1999) The Meteoritical Bulletin, No. 83. Meteoritics and Planetary Science, 34, A169-A186. https://doi.org/10.1111/j.1945-5100.1999.tb01762.xWeisberg, M.K., Smith, C., Benedix, G., Herd, C.D.K., Righter, K., Haack, H., Yamaguchi, A., ChennaouiAoudjehane, H. and Grossman, J.F. (2010) The Meteoritical Bulletin, No. 97. Meteoritics and Planetary Science, 45, 449-493. https://doi.org/10.1111/j.1945-5100.2010.01036.xSasso, A. and Clark, A. (1999) El Grupo Farallón Negro: Evolución magmática, hidrotermal y tectónica e implicancias para la metalogenia de cobre-oro en el retroarco andino, Catamarca. In: Zappettini, E.O., Ed., Recursos Minerales de la República Argentina, Instituto de Geología y Recursos Minerales SEGEMAR, Anales, Vol. 35, 1437-1450, Buenos Aires.Bossi, G.E. and Wampler, M. (1969) Edad del Complejo Alto de las Salinas y Formación El Cadillal según el método K-Ar. Acta Geológica Lilloana, 10, 141-160.Melosh, H.J. (1989) Impact Cratering. Oxford University Press, 245 p.Elston, W.E. (1992) Does the Bushveld-Vredefort System (South Africa) Record the Largest Known Terrestrial Impact Catastrophe? International Conference on Large Meteorite Impacts and Planetary Evolution, Canadá, 23-24.Crawford, D.A. and Schultz, P.H. (1992) Enhanced Magnetic Field Production during Oblique Hypervelocity Impacts. International Conference on Large Meteorite Impacts and Planetary Evolution, Canadá, 18-20.