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Exactly when did that happen? Explorationists spend a lot of time correlating events. Over the last few decades, our ability to determine geological time frames has become much more refined, because we have better tools, such as the ion microprobe and the refinement of radionuclide parent-daughter relationships. Precision still requires two things to match up: the isotopic evidence (geochronology) and the depositional history and relative location of the sample (chronostratigraphy).
Most folks know that the end of the Cretaceous Period (K) and the beginning of the Tertiary Period, known as the K-T boundary, occurred about 66 million years ago, accompanied by the mass extinction of dinosaurs. The prevailing theory is that a massive asteroid slammed into the Gulf of Mexico, just offshore Yucatan, and triggered the extinction event. However, recent study of melted rock ejected far from the Chicxulub impact crater suggests that the impact was too early to have caused the mass extinction, about 300,000 years too early.
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