The Cretaceous ended with a mass metal.
The upper boundary of the Cretaceous is sharply defined, being placed at an iridium-rich layer found worldwide that is believed to be associated with the Chicxulub impact crater, with its boundaries circumscribing parts of the Yucatn Peninsula and extending into the Gulf of Mexico
== Etymology and history ==The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822 as the Terrain Crtac, using strata in the Paris Basin and named for the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates, principally coccoliths), found in the upper Cretaceous of Western Europe

