The official end of an era
In a section of rolling hills near El Kef, Tunisia, a small metal disc is fixed to a rock face. This is a Global Boundary Stratotype Section and Point (GSSP), an internationally agreed-upon marker that defines the boundary between two geological ages. This specific GSSP, ratified in 1991 by the International Union of Geological Sciences, marks the official end of the Cretaceous Period and the beginning of the Paleogene Period. It is the dividing line between the Mesozoic and Cenozoic eras—the moment the age of dinosaurs ended and the age of mammals began.
The El Kef outcrop was chosen as the official reference point because of its remarkably continuous and well-preserved sequence of marine sediments. The rock layers here were deposited on the seafloor without interruption, capturing a high-resolution record of the environmental changes caused by a cataclysmic asteroid impact 66 million years ago. The boundary itself is a thin, dark clay layer, just 1 to 3 millimeters thick. At the very base of this clay is a reddish, rust-colored lamina with iridium, an element rare in Earth's crust but abundant in asteroids. The "golden spike" is hammered into the rock precisely at the base of this reddish layer, providing a physical reference for geologists worldwide.
Evidence of impact
The thin boundary layer at El Kef contains extraterrestrial evidence. The most famous component is the iridium anomaly. Levels of this platinum-group metal spike to as high as 16.25 parts per billion in the reddish layer, a concentration hundreds of times greater than in the surrounding rock. This enrichment is the global chemical signature of the vaporized Chicxulub asteroid that struck the Yucatán Peninsula.
Alongside the iridium, the clay contains other impact markers. These include tiny spherules of goethite and sanidine, which are altered microtektites—droplets of molten rock ejected from the impact crater that cooled into glass beads as they rained down through the atmosphere. The layer also contains shocked quartz, mineral grains with microscopic fractures created by the immense pressure of the impact.
The evidence is seen in the fossil record. The marl layers directly below the boundary are filled with an array of large, complex Cretaceous planktonic foraminifera. In the layers immediately above the iridium-rich line, these species are gone. The extinction is abrupt and total. They are replaced by a few small, simple "disaster species" that survived the event. This sharp paleontological shift, visible in the microscopic fossils, mirrors the wider extinction of 75% of Earth's species, including all non-avian dinosaurs.