Geoscience Reference
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coincidence at a single moment in geologic time that is rare, perhaps
even unique. If Erwin is right, then the greatest mass extinction of
them all was not caused by impact. On the other hand, a single con-
firmed grain of shocked quartz at the Permian-Triassic boundary,
which Retallack says he has found, would corroborate the claim that
impact had occurred then.
THE TRIASSIC-JURASSIC AND THE EOCENE-OLIGOCENE
The Triassic-Jurassic boundary in Italy contains shocked quartz and
a set of weak iridium peaks; these need to be confirmed and repli-
cated at other localities. Two scientists concluded that the plant
extinction at the Triassic-Jurassic took less than 21,000 years, analo-
gous to the sharpness of the K-T fern spike and supportive of
impact.
1
2
On the other hand, the boundary appears to be just a lit-
tle younger than each of the two candidate craters listed in Table 4.
However, the Triassic-Jurassic boundary age of 202 ± 1 million years
was not measured directly on something like a boundary clay, but
instead is based on analysis of a volcanic rock that was interpreted
by the researchers to be just younger than the boundary. It is possi-
ble that as more measurements are taken, the accepted age of the
Triassic-Jurassic will shift.
At the Eocene-Oligocene boundary, approximately 34 million
years ago, some 35 percent of marine genera became extinct (mean-
ing that two or three times as many
species
did), as did many mam-
mal species. Deep-sea drill cores show an iridium spike near the
boundary, as well as the kind of spherules and shocked quartz found
at the K-T. Even coesite, the high-pressure form of quartz and a diag-
nostic indicator of impact, is present. Not one but two craters date to
the Eocene-Oligocene section of the geologic record. The Popigai
structure in Siberia, 100 km in diameter, is dated at 35.7 ± 0.2 mil-
lion years.
1
3
In 1994, Wylie Poag of the U.S. Geological Survey dis-
covered a large crater buried beneath Chesapeake Bay that dates
to 35.5 ± 0.6 million years.
1
4
At an estimated diameter of 85 km,
Chesapeake Bay is the largest impact structure yet discovered in the
United States. Its age, and the composition of the associated breccia,
are consistent with Chesapeake Bay being the source of the North
American tektites. Two craters the size of Popigai and Chesapeake
Bay should have been created at approximately 10-million-year inter-
vals, yet these two were struck within a few hundred thousand years
of each other, showing that the laws of chance can produce seem-
ingly improbable results.
Both craters are just older than the Eocene-Oligocene boundary,
and, if all the dates are correct, could not have caused it. The age of
