One of the distinct features of the boundary between the Cretaceous and the Tertiary (paleogene) in the geologic record is found in this photo. Sandwiched in the middle of this section, you see what is called the “boundary clay” layer.
66 million years ago at the end of the Cretaceous, a rocky asteroid hit the Earth off the shore of the Yucatan peninsula. Near the site of the impact, rocks were tossed around by enormous waves creating tsunami-caused deposits several meters thick in places like Cuba.
Away from the impact site, a variety of other things happened. The ash from that impact blotted out the sun and took years to rain out, causing rapid and catastrophic shifts in the climate. The change in climate caused changes in rock types that appear and then, in some places, suddenly disappear, as you see here. Many things died in the time window represented by that small layer, but after the catastrophe was over, sedimentation returned to mostly what it was doing pre-impact.
This rock comes from an exposure of the boundary in Wyoming. One of the distinct characteristics of this extinction is that the boundary clay is enriched in elements that aren’t abundant in the Earth’s crust. When the Earth formed, many elements sank along with iron metal and sulfide into the Earth’s core, depleting the surface layers of those elements. One of those elements, Iridium, is enriched by a factor of 1000x in that layer relative to the rocks around it; measurement of that iridium anomaly was the key step in realizing that these global boundary clay layers had to be caused by an impact.
There are many mass extinctions in Earth’s history and the Cretaceous is not the biggest. However, it is the only one we can tie fairly directly to an impact. The end-Permian extinction was bigger; many more species died and life took ~10 million years to recover from that calamity, but so far there is little to no evidence of a similar “boundary clay” layer or similar chemical fingerprints of an asteroid impact associated with any other extinction. The K-Pg boundary extinction thus may be a unique event in Earth’s history, at least over the last 500 million years.