The Triassic-Jurassic boundary in Italy contains shocked quartz and a set of weak iridium peaks; these need to be confirmed and replicated at other localities. Two scientists concluded that the plant extinction at the Triassic-Jurassic took less than 21,000 years, analogous to the sharpness of the K-T fern spike and supportive of impact.12 On the other hand, the boundary appears to be just a little 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 possible 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 (meaning that two or three times as many species did), as did many mammal 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 diagnostic 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 million years.13 In 1994, Wylie Poag of the U.S. Geological Survey discovered a large crater buried beneath Chesapeake Bay that dates to 35.5 ± 0.6 million years.14 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 intervals, yet these two were struck within a few hundred thousand years of each other, showing that the laws of chance can produce seemingly 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 the boundary, however, has been argued at great length in the literature, and may not be completely firm.15 But the impact markers also appear to be just slightly older than the boundary, and no mass extinction lines up with the impact evidence.
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