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volcanic rock called andesite overlain by a thick sequence of Creta-
ceous sedimentary rocks, some of them brecciated. If this interpre-
tation 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 Creta-
ceous rocks—the period ended with the impact. Thus for those who
claimed that Chicxulub was the long-sought impact crater, two crit-
ical questions arose: (1) Is the igneous rock volcanic, or an impact
melt? (2) What are the true ages and origins of the overlying sedi-
mentary 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. Unfortu-
nately, 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. 2 2 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.
AGE
Although its general stratigraphy and paleontology appeared to
define the Chicxulub structure as Cretaceous, geologists argued
about its exact age. Officer, Meyerhoff, and their colleagues said
that since Cretaceous strata lay above it, the structure must be older
than latest Cretaceous and therefore could not be the K-T impact
crater. They based their conclusions on the earlier identification by
Meyerhoff that the fossils in the breccia blocks above the melt rock
were late Cretaceous. Subsequently, however, these fossils were
redated and found to be from the Tertiary period. In any case,
impact ejecta often contains blocks of older rocks that were exca-
vated during a cratering event and blasted into the air, from whence
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