The K-T boundary, now referred to as the Cretaceous-Paleogene (K-Pg) boundary, marks the transition between the Cretaceous and Paleogene periods around 66 million years ago. It is associated with a mass extinction event that wiped out approximately 75% of Earth’s species, including the dinosaurs. The boundary is often identified by a layer of iridium-rich clay.
Source Schulte, P., et al. (2010). “The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary.” Science, 327(5970), 1214-1218.