Río Cuarto craters

The Río Cuarto craters were brought to the attention of Peter Schultz Later, Lianza contacted the editors of Sky and Telescope about his observations and was put in contact with Peter Schultz. In 1991, Schultz organized an official field investigation of these depressions. The group studying these depressions consisted of Peter Schultz, (coauthor), John Grant (a field geologist), William Collins (a technician), Lianza, Alejandro Toselli (meteorite specialist from Tucuman University), an unnamed graduate student (Tucuman University), and a small group of local volunteers. they examined Argentine Air Force aerial photography in the office and examined, mapped, described, and collected samples from these depressions. observed that hundreds of similar depressions occur widely across the Pampas in the provinces of Córdoba, San Luis, La Pampa, and Buenos Aires of Argentina. These geologists explained these landforms as having been created by either water- or wind-driven processes. ==Description==

Only ten of the oblong depressions out of the hundreds found on the Pampas of the Rio Cuarto Department were proposed to be impact craters by Schulz and others. The ten depressions that comprise the Rio Cuarto craters have well-developed rims and 4:1 length-to-width ratios. They are clustered together within a long corridor that lies just north of the Rio Cuarto River and the city of Rio Cuarto, Argentina. Eight of these landforms lie within a long corridor that is wide. The largest and northernmost of the ten depressions is wide by long. The rims at its ends are poorly defined and low. Its side rims are well-defined and rise above the surrounding Pampas. Its interior lies below the surrounding Pampas. Lying about to the southwest are a smaller pair of depressions which are both wide by long. Their rims are also poorly defined and low at their ends. Their side rims also are well-defined and rise above the surrounding Pampas and their interior lie below the surrounding Pampas. Lying further to the southwest along trend of the pair of depressions are three more smaller depressions that are wide. Two more possibly related depressions occur southwest of the northern rim of the largest depression. ==Geology==

The depressions of the Rio Cuarto craters are incised into of loess that accumulated during the Pleistocene. The loess blankets crystalline, metamorphic bedrock of the Sierra de Cordoba. The closest bedrock exposures occur to the northwest. ==Impact origin==

Based solely on the field study of the morphology of these depressions, Schultz and others conclude that these landforms have certainly been modified by both wind and water. They also note the lack of specific morphological and structural evidence, such as overturned strata, that unambiguously demonstrate their extraterrestrial origin. In addition, it is admitted that regionally within the Pampas considerable evidence exists for aeolian reworking and the occurrence of innumerable deflation basins similar to Rio Cuarto craters. As a result, they concede that these depressions could easily be interpreted as aeolian in origin. However, they conclude that the unusual distribution and striking resemblance of these depressions to oblique clustered impacts modeled in laboratory experiments are compelling evidence of their extraterrestrial impact origin. Schultz insists that these landforms do have a distinct morphology that they follow a linear orientation pattern instead of the curved or arching alignments displayed by aeolian depressions. Finally, Schultz and others argue that fragments of chondrite meteorites and numerous pieces of glassy impactites found in and near the Rio Cuarto craters provide conclusive proof of their extraterrestrial origin. == Aeolian origin theory ==

Although the ten depressions that comprise the Rio Cuarto craters are included in many lists of impact crates, the theory of their creation by extraterrestrial impact processes remains controversial. Other Earth scientists. The glassy impactites that Schulz and others Meteorites In 1992, Schultz and others regard an iron-rich rock collected from the Rio Cuarto craters to be the important piece of evidence for their impact origin. When cut open, this rock and another revealed a distinctive texture that demonstrated that both are fusion crusted chondritic meteorites. It was stated that they were more than a smoking gun but instead part of the bullet that created these ten depressions. report terrestrial residency ages of 36,000 years for one of the chondrite meteorites and 52,000 years for the only achondrite meteorite. estimated a terrestrial residency age of 410,000 years for the single achondrite meteorite and deduced that this meteorite came from a pre-impact body no larger than about in diameter. Radionuclide analysis of three of the Rio Cuarto chondrite meteorites yielded cosmic ray exposure (CRE) ages of 3.5, 8.0 and 12.2 million years for the meteorites. The same radionuclide analysis further revealed that they are derived from three different parent bodies that would have been no larger than a few meters across and each parent body having fell to Earth in as a separate meteorite fall. Thus, the five meteorites collected from the Rio Cuarto craters are neither part of a single impactor, involved in a single impact or crater forming event, nor evidence of such an event. In addition, the meteorite specimens and associated data neither supports nor disproves an impact origin for the Rio Cuarto craters. However, it is consistent with the accumulation of meteorites with a range of compositions and terrestrial ages from separate meteorite falls over a period of time and their concentration into a surface lag by the removal of soil and sediment by aeolian deflation. ==Summary==

Beech concludes that although the counterarguments towards the impact origin of the Rio Cuarto craters is potentially powerful, that more data needs to be collected by field work about these depressions before their origin can be determined with ant confidence. In addition, he contends in order to resolve the origin of the Rio Cuarto craters that the geochronology, geomorphology, and sedimentology of the near-by aeolian depressions needs to be also thoroughly studied and understood. == References ==