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).