T Esting C Hicxulub

The geophysical maps of the Chicxulub feature (Figure '4) showed it to have the form of a buried crater. The announcement of its discovery by a card-carrying pro-impactor, Alan Hildebrand, galvanized his colleagues into furious activity. Today, only a few years later, a vast amount of information has been assembled about the Chicxu-lub structure, more than enough to test the proposal that it is the K-T impact crater. Following the list of predictions given earlier, I will summarize the results of the many person-years of work on Chicxulub that have been crammed into the brief period from '99' to '997.

figure 14 The cenote ring at Chicxulub (dark circles) superimposed on the gravity anomaly map. Note how the cenotes to the middle and left trace out part of a nearly perfect circle. [Photo courtesy of Alan Hildebrand and Geological Survey of Canada. For this and other images, see web page at http://dsaing.uqac. uquebec.ca/-mhiggins/ MI AC/chicxulub. htm. ]

figure 14 The cenote ring at Chicxulub (dark circles) superimposed on the gravity anomaly map. Note how the cenotes to the middle and left trace out part of a nearly perfect circle. [Photo courtesy of Alan Hildebrand and Geological Survey of Canada. For this and other images, see web page at http://dsaing.uqac. uquebec.ca/-mhiggins/ MI AC/chicxulub. htm. ]

s IZE AND s HAPE

Although the Chicxulub structure lies buried beneath a kilometer of younger sedimentary rocks, gravity anomaly maps clearly reveal it (see Figure 14). (Structural domes, which often contain petroleum deposits, also produce concentric geophysical patterns, thus explaining the decades-long interest that Chicxulub held for PEMEX.) The edge of the gravity anomaly indicates a structure at least 170 km in diameter, consistent with an impactor of 10 km in diameter. If Chic-xulub is of impact origin, and if the outer perimeter of the gravity pattern represents the true outer rim of the crater, then that is its diameter. On the other hand, if the edge of the gravity pattern instead represents one of the inner concentric structures of a complex crater, for example, a collapsed terrace rim, then the structure might be larger, even much larger.

Still another possibility is that Chicxulub is not a complex crater of the kind shown in Figure 7 but something more: a giant, multi-ringed basin like the Orientale Basin on the moon. In any case, the original Alvarez calculation of 10 km was sufficiently imprecise that a greater radius for the crater is certainly possible. Virgil "Buck" Sharpton and his colleagues at the Lunar and Planetary Institute argue that Chicxulub may be as large as 300 km.1' At 170 km, it would be one of the largest craters yet discovered on the earth; at 300 km, Chicxulub would be one of the largest impact basins formed in the inner solar system in the last 4 billion years. Since a 300-km crater would have released far more energy and been far more destructive to life than one just over half as wide, measuring the true size of Chicxulub is crucial to understanding the extinction. Drilling, geophysical studies, and topographic work of the kind being done by Kevin Pope and Adriana Ocampo of the Jet Propulsion Laboratory in Pasadena should soon resolve this dispute."

The distribution of impact ejecta of various sorts can be used to learn more about how Chicxulub formed. Why, for example, is the shocked quartz in the North American sites only in the upper of the two impact layers, and why does more of it exist to the west than to the east of Chicxulub? Walter Alvarez, planetary-impact specialist Peter Schultz of Brown, and their colleagues are addressing both questions by studying the trajectory of the impactor and the mechanism of fireball creation and ejecta formation.

Note in the gravity anomaly map that the structure appears breached on the northwest side, an effect sometimes observed in craters on other bodies in the solar system. Peter Schultz believes that the K-T impactor was 10 km to 15 km in diameter and was descending toward the northwest at an angle of about 30 degrees from the horizon. According to Schultz, an impact from that angle and direction would explain the observed distribution pattern of shocked quartz, tektites, and iridium.

How important to the K-T event was the particular geology of ground zero in the Yucatan? Had the target area not been composed of limestones and sulfate deposits, would the extinction have been so great? Understanding the importance of the target rock types is critical to comparing Chicxulub with other potential impact-induced mass extinctions.

Was this article helpful?

0 0

Post a comment