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Impact Craters in South America, by Acevedo R. D., Rocca M. C. L., Ponce J. F., and Stinco S. G. Heidelberg: Springer, 2015. 104 p. SpringerBriefs in Earth Sciences: South America and the Southern Hemisphere. ISBN 978-3-319-13092-7.

2016; Wiley; Volume: 51; Issue: 5 Linguagem: Inglês

10.1111/maps.12632

1945-5100

Álvaro Penteado Crósta, W. U. Reimold,

Space Exploration and Technology

SpringerBriefs in Earth System Sciences present concise summaries of cutting-edge research and practical applications. The series focuses on interdisciplinary research linking the lithosphere, atmosphere, biosphere, cryosphere, and hydrosphere building the system earth. It publishes peer-reviewed monographs under the editorial supervision of an international advisory board … (From: http://www.springer.com/series/10032) According to the authors of this publication, the book is aimed at presenting an extensive and exhaustive catalog of all geologic structures that have been mentioned or suggested as impact craters in South America. This is probably the main flaw of the book, namely mixing scientifically proven impact structures with a large number of generic and dubious circular structures, for which no evidence linking their origin to the impact process has been produced. To quote the authors (p. 2): “In our opinion, in the absence of meteorite remains or proof-of-shock metamorphism, geological evidence can prove the existence of an impact site and support these interpretations” (Acevedo et al. 2009). And there we must disagree—in our view it is mandatory to establish recognition criteria first in order to avoid misinterpretations before they enter the literature and can confuse newcomers to the research discipline and falsify databases. The book is organized in 14 short chapters, listing a large number of structures in each of 11 countries, with the largest number in Argentina. Also included in the book is the Eltanin impact event, which took place in the South Pacific 2.5 million years ago, some 1500 km SW off the coast of Chile, and to which no impact structure has been assigned so far (likely because the impact was such that no crater was generated on the sea-floor). Thus, the title of the book is inappropriate, as not only impact structures are covered, but also some supposed indications of impact that did not produce or leave craters in evidence. After a short introductory chapter, the authors begin their account with Argentina (Chapter 2), a country in which just one very well-known meteorite strewn field, Campo del Cielo, is known but no sizable and proven impact crater has been recorded (www.unb.ca/fredericton/science/research/passc/) to date. Campo del Cielo comprises several small-sized craters with a large number of associated meteorite fragments. Acevedo and co-authors proceed to list another eight sites as being of impact origin, but present no new evidence in the form of meteorite fragments, occurrence of shock metamorphic features, or unambiguous chemical traces of meteoritic projectiles. The lack of such indicators, as discussed by French (1998), French and Koeberl (2010), Reimold et al. (2014), Reimold and Koeberl (2014a,2014b), among others, does not allow a circular structure to be considered of impact origin. The claims made by the authors of this book are typified by the case of the controversial Bajada del Diablo, an alleged strewn field of >200 small circular structures, occurring in a volcanic province, which have been claimed by the authors and their co-workers to all be of impact origin. This controversy has been debated by Reimold and Koeberl (2014a, 2014b) and Acevedo et al. (2014a, 2014b); however, we find that no acceptable or even circumstantial evidence has been produced that would demonstrate the impact origin of the Bajada del Diablo features. The same applies to all of the other eight structures listed in this chapter. Among these, the Rio Cuarto oval structures have also been the source of a long controversy regarding their origin. Schulz and Lianza (1992) considered a meteorite strewn field formed by grazing impacts from a 200 m asteroid that entered the atmosphere on a very low angle and split into several smaller fragments, thus producing a cluster of strongly elongated, shallow depressions. These authors pointed out, as evidence for an impact origin, the occurrence of glass and fragments of a chondrite meteorite associated with the depressions. Bland et al. (2002) then showed that there were >400 of these structures in the region and that there were fragments of different types of meteorites, thus favoring an eolian origin for these depressions. Even after going through this evidence, Acevedo and co-authors still list Rio Cuarto among the impact sites of Argentina. They also list another six circular or near-circular structures, most with very short descriptions, and some accompanied by poor-quality illustrations from satellite images. Again, no evidence supporting an impact origin is presented for these structures that would allow to even classify them as “possible impact craters,” although in some cases even an impact age is suggested! A further apparent contradiction must be pointed out: after claiming a possible impact nature for these structures, some of the descriptions presented by the authors state that “… probably they may be collapse structures …” (p. 15), or “… the site may be related to plutonic processes” (p. 18). For some of these structures, the authors state that they need in situ research to be “confirmed” as impact craters, contradicting the idea that they are possible impact structures. We are concerned that nonimpact researchers and newcomers to the discipline would be left confused about what could be, and what should not be considered a confirmed or even purely a possible impact structure. In the last part of the chapter on Argentina, there are two sections with rather puzzling titles: “Structures Which May Not Be Impact Craters”, which includes the Malvinas/Falkland Islands geophysical anomaly (Rampino 1992), and “Problematic Structures Found in the Patagonian Volcanic Landscapes.” These are odd categories of classification to be found in a book about impact craters, as they do not follow the system in use for decades by the crater research community, which has generally adopted the categories of “proven,” “probable, and “possible” impact structures (e.g., Grieve and Robertson 1979; Reimold et al. 2014). Chapter 3 concerns two circular structures in Bolivia, both with short descriptions based on conference abstracts, accompanied by satellite views. The hypothetical impact nature of one of the structures, Iturralde, was proposed based only on remote sensing and gravimetric data. The second structure, Llica, has been field checked in 2005 by one of the authors of this review, A.P. Crósta, subsequently to the publication of the abstract cited in the review (Crósta 2004). It yielded no meteoritic or shock metamorphic evidence. Therefore, Llica should not be considered an impact crater, as it is more likely a volcanic feature. Chapter 4 is about the Brazilian impact structures and here the authors adopt the following categories: “proven,” “possible,” “features suspected to be of impact origin,” and “more doubtful structures,” thus introducing again some amendments to the traditional classification system in use. In the case of the proven Brazilian impact structures, the authors do present an adequate summary of what is currently known, based on an up-to-date, comprehensive literature review. The only out-of-date information is that the Santa Marta structure, listed by them as a possible impact, has convincingly been shown to be of impact origin (Crósta and Vasconcelos 2013; Uchôa et al. 2013; Oliveira et al. 2014). The other Brazilian structures mentioned in the book as possible impact structures are a mixture of possible ones (such as Cerro Jarau and Colônia), with others that have not provided any evidence of impact (i.e., São Miguel do Tapuio—see MacDonald et al. 2006 and Vasconcelos et al. 2010), or that have not been investigated in situ at all (i.e., Inajah). Finally, there are two sections with unusual categories, listing 11 structures, the majority of which, based on the nearly four decades’ experience of the first author of this review of working with the Brazilian impact record, are unknown to him. For some of these features, the authors provide satellite images of very poor quality, making it almost impossible to actually distinguish any information. Again, they present very ambiguous information, as in the case of item 4.3.2, “Late Pleistocene Shallow Paleolakes,” in a section dedicated to “Features Suspected to Be of Impact Origin.” Looking at the images with some visible features, several of them really look like lakes or dry lakes, without any indication of any morphological feature that might be even remotely related to impact craters. It has been demonstrated that a host of other geological processes (such as emplacement of kimberlite, sinkhole formation, glacial erosion, inter alia) can produce circular surface features. The question that remains, after reading these topics, is—what criteria were used to define the impact nature of these structures? The answer to this question cannot be found in the book, as no references are cited for many of these structures, and even the credits for the satellite images are quite generic and incomplete. It is understood that many of these structures were never checked in the field by a geologist or anyone trained in recognizing impact-related features. Chilean structures are introduced in Chapter 5. Among them is the well-known Monturaqui impact crater, the only structure of its kind confirmed in Chile, together with two meteorite strewn fields, Vaca Muerta and Quillagua, comprising clusters of small depressions. A fourth structure in Chile is Imilac, a small bowl-shaped depression. The information provided by the authors does not mention any finding of meteoritic material associated with Quillagua, leaving doubts about the meteorite impact origin of this supposed strewn field. Chapter 6 concerns Colombia and only one structure is listed, Rio Vichada. This listing deserves some comment, as the impact origin of this structure has been discussed since it was first mentioned by Rocca (2004) in a conference abstract, based on satellite observations. There has been some conjecture about the impact nature of this structure, but no conclusive evidence has ever been produced. In 2007 and 2008, a couple of conference abstracts were presented by collaborating groups from Universidad Nacional de Colombia and Ohio State University, in which very low-resolution satellite gravimetric data were used to model the Rio Vichada feature, claiming to have found “geophysical evidence of its impact origin” (Hernández et al. 2007; Hernández and von Frese 2008). Subsequently, Hernández et al. (2009) allegedly attested the impact origin of Rio Vichada. A cursory analysis of the results shows that the authors did not take into account the potential uncertainties of the models, considering the low resolution of the data they used. However, the authors clearly recommended that the gravimetric anomalies they found in association with this structure should be confirmed by ground gravity and magnetic follow-up surveys which, to our knowledge, have not been conducted yet. And in any case, geophysical observations or modelling alone does not constitute bona fide evidence of impact. This very uncertain status of the origin of Vichada did not prevent the authors of this book to consider it the largest impact structure in South America, stating that “Only one proven structure from South America (Vichada, in Colombia) is present in the list of 16 impact craters [in the world—the authors of this review] larger than 50 km” (p. 102). The book continues in the subsequent chapters by introducing 12 highly dubious circular features in Paraguay, Uruguay, Venezuela, Guyana, and Suriname, as well as the well-known and well-documented Carancas impact crater in Peru (Chapter 8). This is followed by a one-page chapter on the Eltanin impact event in the Pacific Ocean, and a chapter on “Astrobleme-Wrong,” with six structures in Argentina, Bolivia, Chile, and Colombia. The book closes with a chapter on final remarks, with some generalities on impact research, and some more misleading pieces of information, such as placing the confirmed Araguainha impact structure of Brazil onto a Precambrian shield (it is actually formed on Paleozoic sedimentary sequences, having reached down to the Precambrian crystalline basement). As pointed out in this review, the book has several flaws in relation to its contents and concepts of what is a proven or possible impact structure, leaving considerable room for speculation. Furthermore, it conveys a very wrong message to those interested in impact craters, in the sense that it would be possible to discover new craters by just searching web-based satellite image services to find circular structures that indicate the occurrence of impact craters. Proving the true nature of an impact crater demands a lot more work, starting with appropriate geological ground truthing, and proper field analysis, in order to search and collect the necessary evidence, in the form of meteoritical components (fragments or geochemical traces) or shock-metamorphic features. Finally, it is the opinion of the reviewers that, with this book, Springer has failed in its intent to present to the readers a concise summary of cutting-edge research in impact cratering, as stated in its mission for the “Briefs in Earth System Sciences” series.