LUIZ SUGIMOTO

A The insight of a forestry engineer contributed to the discovery in Brazilian territory of a new crater formed by the impact of a celestial body. When reading an article about the Vargeão crater (SC) in a magazine, reproduced from Journal of Unicamp nº 225, reporting the research of professor Álvaro Penteado Crósta, from the Institute of Geosciences, Osmar Eugenio Kretschek contacted the researcher informing him about a feature he had observed in southwestern Paraná. According to the engineer's report, the area had characteristics very similar to those described in the report. It is a circular depression with more than 9 km in diameter, in the rural neighborhood of Vista Alegre, municipality of Coronel Vivida.

Almost all the indications of possible craters that Alvaro Crósta receives from people who are aware of his work end up being discarded because the locals do not present any characteristic of impact phenomenon. But, as the professor checks all the indications, he initially chose to analyze the satellite images from the IG database, looking for geomorphological evidence that would suggest the occurrence of the phenomenon in the location pointed out by engineer Kretscheck. “Both in the Landsat-7 satellite image and in the topographic model image obtained by the space shuttle, both from NASA, that area corresponded to a clear circular depression, with sharply steep edges, with a topographic protrusion in the center [see image on this page ]. These are typical characteristics of eroded craters, although they are not conclusive evidence of impact origin alone”, says the geologist.

A few months later, Professor Álvaro traveled to Vista Alegre in the company of two Unicamp students, César Kazzuo Vieira and Guilherme Duch Crósta, in search of direct evidence of the impact on the rocks there. “At the second stop we found impact breccias – fragments of rocks and minerals that only form in craters of this type. In the midst of these gaps we also observed a type of structure called shatter cones. These features, together with the geomorphological characteristics of the structure, constitute irrefutable evidence of its origin by impact”, says Alvaro Crósta. Analysis of the breccias under a microscope revealed that they were made up of fragments of various types of rock existing at the (still undetermined) time of the impact. “These rocks were thrown into the air and fell back into the crater, mixing with dust and material molten by the heat, sedimenting to form the breccias.”

Another piece of evidence is the occurrence of highly deformed sandstone blocks, found near the center of the crater. “The blocks were covered by soil with soybeans and we were only able to locate it thanks to Adroaldo Weber, a local farmer. These sandstones are correlatable to the Botucatu or Pirambóia formations, from the Jurassic and Triassic periods, which would normally be hundreds of meters deep, having been brought to the surface during the final process of crater formation,” he explains.

Studies of the new crater are still at an early stage, under the responsibility of Álvaro Crósta, professor Alfonso Schrank and master's student César Kazzuo Vieira, all from the IG. Crósta will officially announce the discovery during the Congress of the International Meteoritics Society, in August, in Rio de Janeiro. The last crater proven to have been formed by impact in Brazil had been discovered in the early 1980s. The Vista Alegre crater joins the four already proven in Brazil, in addition to seven others for which this origin is suspected (see table).

Twin craters – What Osmar Kretschek may not have suspected is that the Vargeão and Bela Vista craters could be twins, the result of a double impact. About 100 km apart, they have several geological similarities and a very similar degree of erosion. The samples collected from the two craters will undergo isotopic dating work, aiming to establish their formation ages. “When it collides with the atmosphere, the meteorite, asteroid or comet can break into two or more fragments, which will reach the surface in different, but generally close, locations. Canada has a beautiful example of twin craters, called Clearwater East and Clearwater West, in the province of Quebec. In Brazil, the craters of Serra da Cangalha (TO) and Riachão (MA), only 45km apart, represent another possible example of this phenomenon”, says the researcher.

With a PhD in geology from the University of London, Alvaro Crósta is one of the few Brazilian specialists in this area. He began his research in the 1970s, when the first Earth imaging satellites were launched, having done his master's studies on the Araguainha crater (MT), the largest in South America, measuring 40 km in diameter. Previously, Araguainha appeared on geological maps as being of volcanic origin, according to the professor due to lack of knowledge of impact phenomena, whose studies took shape in the 1960s and 1970s. In 1982, Crósta had already placed the Vargeão crater among the known structures in the Brazil as probable “astroproblems”. Astroblema is the technical name for an ancient crater that has already been partially eroded by erosion, produced by the impact of a large celestial body (asteroid or comet). The term comes from Latin, meaning a scar (blema) caused by the fall of a celestial body (star).

Craters can store oil Professor Alvaro Crósta points out a good economic reason to encourage the study of impact craters buried in sedimentary basins. He recalls that rocks fragmented by the impact form excellent reservoirs for oil and gas. “Hydrocarbons require porous material to accumulate. The volcanic rock called basalt, for example, common in the Paraná sedimentary basin, is compact, without spaces or fissures, and would normally not store oil or gas. If impact breccias have formed in these basalts, as occurs in the craters of Vargeão and Vista Alegre, the rock begins to have favorable characteristics for this. Oil tends to migrate to where there are spaces and these craters become potentially interesting sites”, he ponders. According to Crósta, this model of oil exploration has been used successfully in countries such as the USA, Canada and Mexico. He suggests that the exposed craters can serve as a model for studies using seismic data (a geophysical technique widely used in oil exploration), aiming to locate structures with similar characteristics, located hundreds or thousands of meters below the surface. He recently forwarded a suggestion to the National Petroleum Agency (ANP) to offer incentives for projects in this area, through future notices from the Oil and Natural Gas Sector Fund – CT-Petro. Alvaro Crósta also reports on the use of existing seismic data on the Vargeão crater, obtained in the 1980s by the Paulipetro consortium. “The technicians considered the site favorable, although they probably did not know that they were on top of an impact crater. This data was never processed and interpreted. We obtained the original data from the ANP and, in collaboration with Professor Martin Tygel (from the Institute of Mathematics, Statistics and Scientific Computing at Unicamp), a specialist in seismic processing, we will try to establish the structural conformation of the crater in the sub-surface, thus contributing to generate models for oil and gas prospecting in buried craters.” Bolívia – Although he does not yet have details, Alvaro Crósta reveals his most recent discovery: a crater without any record in the literature, in the province of Potosi, northwest of Bolivia, a region of volcanic cones and salty and dry lakes in the Andes Mountains. Showing the images obtained by satellites, the researcher explains why he suspects that this is not one of the frequent volcanic craters in the region: “It is approximately 3km in diameter and, unlike craters formed by volcanic explosions, it is located at the base and not at the bottom. top of a large volcano. Furthermore, its dimensions are too large to have been formed by the explosion of gases at the base of the volcano.” In fact, in the 3-D perspective image, you can see a huge circular hole between the mountains, with almost vertical walls up to 400 meters high, which could have been opened by a large body coming from the sky.

Top