Rock Types

In '992, the indefatigable Officer and Drake weighed in.2' Along with the late Arthur Meyerhoff, an American geologist who had been a consultant to PEMEX in the '960s when the Chicxulub structure was drilled, they published an article in GSA Today, whose title, "Cretaceous-Tertiary Events and the Caribbean Caper," suggested that, far from having capitulated and accepted Chicxulub as the K-T impact crater, the authors intended instead to treat the notion as risible. Meyerhoff had been one of the most bitter opponents of plate tectonics and had made fossil identifications that, if correct, falsified the claim that Chicxulub was the K-T impact crater. He had a great deal at stake.

PEMEX's drilling of the Chicxulub structure in the '960s and '970s uncovered what their specialists at the time interpreted as a volcanic rock called andesite overlain by a thick sequence of Cretaceous sedimentary rocks, some of them brecciated. If this interpretation is correct, the claim that Chicxulub is the K-T crater would appear to be falsified on two counts: (1) it contains volcanic rather than melt rocks, and (2) a 10-km meteorite striking at the exact end of the Cretaceous would not leave a structure capped with Cretaceous rocks—the period ended with the impact. Thus for those who claimed that Chicxulub was the long-sought impact crater, two critical questions arose: (1) Is the igneous rock volcanic, or an impact melt? (2) What are the true ages and origins of the overlying sedimentary rocks? Only a detailed examination of the rocks could give the answers.

The required data could be obtained in two ways: by examining the older PEMEX drill cores and by drilling new holes. Unfortunately, the Coatzacoalcos, Mexico, warehouse in which the original PEMEX cores had been stored was destroyed in a fire, apparently leaving the notes that Meyerhoff had made on the specimens two decades earlier as the only extant reference source.22 It eventually came to light, however, that a PEMEX employee had shipped a number of samples of the andesite to a colleague in New Orleans. Penfield then arranged to have some of those specimens sent to Hildebrand, whose associate David Kring immediately found that they contained shocked quartz. Meanwhile, new drilling efforts had begun to make new cores available for study; they showed that the Chicxulub rocks have relatively high concentrations of iridium and osmium, and reversed magnetism. Cores extracted from several holes drilled in the mid-1990s revealed the typical impact breccias, melt rocks, and structures of known impact craters. In particular, the Chicxulub sequence closely resembles that at Ries Crater.

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