M Anson

In proposing that the target was within 3,500 km of Montana, Bevan French went on to identify two possible craters, one of which was the Manson structure in Iowa. Because it is covered by 30 m to 90 m of glacial debris, Manson is not visible at the surface. Such hidden geological structures are detected using geophysical techniques that rely on magnetism, gravity, and seismic waves. For example, rocks with more magnetic minerals than average produce a positive magnetic anomaly; those that are more dense give rise to positive gravity anomalies. Using such methods, geophysicists can tell a great deal about rock structures that they cannot see; indeed, this is the way they discover buried structures that may contain oil. The geo-physicists confirmed that the Manson structure is about 35 km in diameter, smaller than the predicted size of a crater resulting from the impact of a 10 km projectile. On the other hand, Manson is of late Cretaceous age and the basement rocks in the area are granitic. Although Officer and Drake concluded from their analysis that impact did not create the Manson crater (as they also but incorrectly concluded for Sudbury and Vredefort), the discovery of shocked quartz with planar deformation features at Manson, as well as the overall form of the structure, showed that it should be added to the lengthening list of terrestrial craters. Recent seismic studies show that it has a structural central peak (not visible at the surface) nearly 3 km high.4

As the attention of the crater hunters turned increasingly to North America, Manson loomed as the natural candidate. Their interest seemed to be justified when the first argon-argon age determination from Manson returned an age of 65.7 ± '.0 million years, a range that included the age of the K-T boundary.5 As Manson drew more attention, however, its small size continued to cast doubt. The '0-km impactor predicted by the Alvarezes on the basis of the worldwide iridium levels would create a crater five times Manson's size; one no larger was unlikely to have been able to produce the observed impact effects. Doubt increased when the Izett group obtained additional drill core samples from Manson and dated (by the argon-argon method) unaltered feldspar grains that appeared to have crystallized from the impact melt and which therefore should give the structure's true age. The age came back not at 65 million years, but at 73.8 ± 0.3.6 The discovery that Manson is normally magnetized and thus cannot belong to K-T Chron 29R confirmed that it is not of K-T age. To pin the matter down, Izett and colleagues, in a neat piece of work, journeyed to nearby South Dakota, where, at the stratigraphic level equivalent to an age of 73.8 million years, they found a zone of shocked minerals.7 Thus as more evidence has accumulated, Manson has been confirmed as an impact crater, but one that formed at a time different from the K-T.

How TO RECOGNIZE AN IMPACT CRATER

If the K-T crater was located on a continent and easy to spot, it would have been discovered long ago; if it exists, it must either be so eroded or covered by younger sediments, as at Manson, that it is detectable only through the use of geophysical methods. Unfortunately, geologists could not turn to earthly examples to learn how to recognize such huge and obscure structures, for it is rare to find terrestrial craters larger than '00 km. Those that do occur, as at Sud-bury and Vredefort, are ancient, distorted, and eroded. Other bodies in the solar system, however, provide ready examples. As exempli fied by Tycho (see Figure 7), the moon has many complex craters, with central peaks, collapsed rim terraces, and internal zones of melt rock and impact breccia. Their ejecta can be seen to be scattered over hundreds or thousands of kilometers. From studying smaller craters on the earth, and larger ones on other bodies in the solar system, geologists have built up an accurate and detailed picture of what the K-T crater and its associated features would be like.

• If the impactor were 10 km in diameter, as the Alvarezes calculated, then based on studies of nuclear explosions and cratering on other bodies, the crater would have a diameter of about 150 km. However, if some of the impact material had been blasted completely out of the earth's gravity field, then the boundary clay would not contain all the ejecta, and the resulting calculation would give too small a size for the meteorite and the resulting crater.

• Magnetic, gravity, and seismic anomalies will reveal a circular, bull's-eye pattern and a buried central peak.

• If the crater is not too deeply buried, it might have an obscure but recognizable surface topography, perhaps represented by concentric, arc-shaped ridges and valleys.

• The structure will contain impact breccia and once-molten rock. (However, volcanic rocks were also once molten and can be mistaken for impact melts.)

• The melt rock will be enriched in iridium, display reversed magnetism, and have a radiometric age indistinguishable from 65 million years.

• Glass spherules resembling tektites and dating exactly to the K-T boundary will be found in the vicinity.

• Ejecta layers located farther away will contain shocked minerals and iridium. If they can be dated, they too will be 65 million years old.

• If the impact occurred in the oceans near a continent, say on a continental shelf, it might have given rise to giant waves that would leave unusual sedimentary rocks behind.

This is a long list. Were any putative K-T impact structure to meet even most of these predictions, the search would be over. A site that met them all would close the case for impact.

Having concluded that the crater is likely to be in North America narrows the field, but it still leaves an impracticably large area to explore. When confronted with such a task, the intelligent geologist heads not for the field but for the library, there to scour the literature for reports of unusual K-T deposits and descriptions of circular geophysical patterns. The most diligent crater sleuth and literature searcher was Alan Hildebrand, a doctoral candidate at the University of Arizona who is now with the Geological Survey of Canada.8 One report in particular caught his eye: the description by Florida International University's Florentin Maurrasse of a set of peculiar, late Cretaceous rocks exposed at the top of the Massif de la Selle on the southern peninsula of Haiti.' Maurrasse depicted a thick limestone sequence, the Beloc Formation, that contained a 50-cm layer that he thought was volcanic. In June 1989 Hildebrand visited Maurrasse and, as soon as he saw the Beloc samples, recognized them as altered tektites of impact rather than volcanic origin. Hildebrand then went to Haiti himself to collect from the Beloc Formation, and found the dual K-T layering that by that time had been described at many North American sites. In this case, however, the lower ejecta layer was about 25 times as thick as elsewhere and contained the largest tektites and shocked quartz grains ever found, suggesting that the Haitian site was close to the K-T target. Hildebrand and his colleagues estimated that ground zero was within 1,000 km of Haiti.

In May 1990, Hildebrand and William Boynton reported that the only crater candidate their literature search had turned up was a vaguely circular structure lying beneath 2 km to 3 km of younger sediment in the Caribbean Sea north of Colombia.10 They acknowledged, however, that an impact at this seafloor site probably could not have provided the continental grains and rock fragments found in the boundary clay. Hildebrand and Boynton did note, almost as an afterthought, that in 1981, at the annual meeting of the Society of Exploration Geophysicists, geologists Glen Penfield and Antonio Camargo had reported circular magnetic and gravity anomalies from the northern Yucatan Peninsula and had speculated that buried there, beneath younger sedimentary rocks, might lie an impact crater.

By 1990, a peculiar kind of sedimentary rock deposit of K-T age had been found at several sites rimming the Gulf of Mexico. To the pro-impactors it appeared that these rocks were formed by giant waves, or tsunami, of the kind that a meteorite splashdown in the ocean would have produced. Since the Colombian basin site turned out to be the wrong age, the giant wave deposits helped to persuade

Hildebrand and Boynton that the Yucatan peninsula location identified by Penfield and Camargo was the more likely candidate. Soon after Hildebrand and Boynton announced in an article in Natural HistoryA' that it was indeed the K-T crater and claimed partial credit for the discovery: "In 1990, we, together with geophysicist Glen Penfield and other coworkers, identified a second candidate for the crater. It lies on the northern coast of Mexico's Yucatan Peninsula, north of the town of Merida. The structure, which we named Chicxulub [pronounced Cheech-zhoo-loob] for the small village at its center, is buried by a half mile of sediments."*

In a letter to Natural History a few months later, reflecting the scientist's vital interest in priority, Penfield took exception to Hildebrand's claim, noting that he had "identified" the structure in 1978, and reminding readers of the words with which he and Camargo had closed their 1981 presentation: "We would like to note the proximity of this feature in time to the hypothetical Cretaceous-Tertiary boundary event responsible for the emplacement of iridium-enriched clays on a global scale and invite investigation of this feature in the light of the meteorite impact-climatic alteration hypothesis for the late Cretaceous extinctions."12

Whew! In 1981, only months after the Alvarez theory appeared, and in public at a scientific meeting, a prime candidate for the K-T crater had been identified, but no one had noticed. The Yucatan structure thus had to be rediscovered a decade later, after hundreds of person-years had been spent in the search. Poor timing may be a partial explanation: The meeting at which Penfield and Camargo presented their paper took place in the same week as the Snowbird I conference, to which the pro-impactors naturally were drawn. It turned out that the circular nature of the Chicxulub structure had been discovered in the 1950s by Petroleos Mexicanos (PEMEX) through the use of geophysical techniques. In the 1960s and 1970s, probing for possible oil-bearing structures, PEMEX drilled the structure and extracted rock cores. According to the account of Gerrit Verschuur, Penfield wrote to Walter Alvarez in 1980, right after he read the original Alvarez paper, to tell him that the crater might be located in the Yucatan, but he never heard back.13 (At that time,

"Ever since the crater was named, uncertainty has prevailed over the proper translation of the Mayan word, Chicxulub. According to Mayan specialist George Bey of Millsaps College in Jackson, Mississippi, there are two acceptable meanings: "the place of the cuckold" and "the red devil." Since "red devil" is so evocative of the actual event, it seems the better choice.

however, Walter was focusing on the belief that the crater was located in an ocean basin and might not have taken a proposed continental site seriously.'4)

The failure of the geological community to jump at the Penfield-Camargo suggestion and save everyone a decade becomes even more poignant when we learn that a reporter for the Houston Chronicle, Carlos Byars, interviewed Penfield and Camargo in '98' and wrote an article about their work.'5 In March '982, an account of the Penfield-Camargo finding was published in Sky and Telescope: "Penfield . . . believes the feature, which lies within rocks dating to Late Cretaceous times, may be the scar from a collision with an asteroid roughly '0 km across."'6 How could it have been more clear that here was a lead that demanded to be followed up? And yet it was not. At least a few pro-impactors must have read that issue of Sky and Telescope; if so, they did not take the report seriously. Byars continued to attend meetings of pro-impactors to push the idea that the crater might lie underneath the Yucatan, but no one paid any attention. Perhaps it was necessary for the crater to be discovered, or rediscovered, not by a journalist or oil geologist, but by a bonafide member of the pro-impact research community.

Walter Alvarez argues that not finding the crater for '0 years "was actually a blessing" because it forced the pro-impactors to confront the repeated challenges from the Officer-Drake school.'7 Since one cannot rerun history and thereby learn what would have happened had he and others paid attention to Penfield and Camargo, it is hard to know whether he is right. In the long span of scientific history, a decade is not much, surely, and yet one can wonder what advances might have been made had the crater been confirmed in '980 or '98' and the next '0 years been spent differently.

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