Cretaceous-Tertiary (K-T) boundary clay from Colorado, USA.

Light brownish gray-colored layer at middle = K-T boundary

Earth experienced a major mass extinction 65 million years ago, at the Cretaceous-Tertiary (“K-T”) boundary. The most famous victims of this event were the dinosaurs, but many other groups of organisms also disappeared. The cause of the K-T extinction has long been a major mystery in geology.

Starting in the late 1970s, abundant evidence has been accumulated indicating that the dinosaurs & company were wiped out by the effects and aftermath of a gigantic impact event. An approximately 6-mile large object (“bolide”) hit what is now Mexico’s Yucatan Peninsula to form the Chicxulub Crater. Enormous quantities of debris were kicked up into the atmosphere. This material settled out all over Earth to form the K-T boundary clay.

Exhibit info.: K-Pg asteroid fallout layer Late Creatceous, 66 million years ago Southern Colorado, Raton Formation

The fallout layer was preserved in fine-grained shale that formed in a river floodplain environment. It was collected during a Cranbrook Institute of Science field study in 2013.

A Bad Day for Dinosaurs

An asteroid or comet struck the Earth 66 million years ago in the Yucatan Peninsula of Mexico unleashing a force equivalent to 100 trillion tons of TNT or 1 billion Hiroshima-sized atomic bombs! This event blasted a 200 kilometer wide ringed impact structure in the carbonate and gypsum-rich bedrock, instantaneously shrouding the planet with dust and injecting up to 500 gigatons of sulfur dioxide in the atmosphere (Alvarez et al., 1980; Schulte et al., 2010). This cooled and darkened the planet, shutting down photosynthesis on land and in the oceans, triggering the mass extinction.

Impact induced seismic waves also generated larger more episodic eruptions of lava flows (up to 70% of the volcanic rocks of the Deccan Traps in India) and prolonged release of sulfur dioxide, likely contributed to delayed post-impact recovery of the biosphere (Renne et al., 2015).

The iridium-rich global fallout layer from the impact displayed here marks the end of the Cretaceous Period and in-situ non-avian dinosaur fossils have not been found above it.

[Stratigraphy of the sample shown above, from top-down:] End Cretaceous Asteroid Fall Out Layer Structure of K-Pg (T) boundary layer

Paleogene (Paleocene) coal - fern spore spike; 99% to 80% fern spores of a single genus, Cyathodites, and an abrupt disappearance of Tschudypollis angiosperm assemblage - extinctions and devastation of terrestrial vegetation

Fireball fallout - 65 to 81 degrees launch angle, iridium anomaly, impact shocked quartz, zircons linked to bedrock at Chicxulub impact structure, soot

Boundary claystone - altered glassy impact eject curtain, 30 to 45 degrees launch angle, carbonized plant remains.

Late Cretaceous (Maastrichtian) mudstone with a diverse Tschudypollis angiosperm pollen assemblage (70%) and diverse fern spores (30%).

Stratigraphy: Raton Formation, Maastrichtian Stage-Danian Stage boundary (= Cretaceous-Tertiary boundary), 65 Ma