Chicxulub Crater

The Chicxulub crater is an approximately 180-kilometer-diameter structure, which is now buried by a nearly 1-kilometer-thick sequence of carbonate sediments, on the northern edge of the Yucatan Peninsula (southeastern Mexico). The name Chicxulub was selected because a small town, Puerto Chicxulub, is located above the center of the structure. The best-known surface expression of the crater is a concentric pattern of karst sinkholes called the Ring of Cenotes (cenote is the local Maya word for sinkhole). The Ring of Cenotes is generated by progressive subsidence of the crater's wall and is visible in satellite images of the region. The formation of the Chicxulub crater represents a significant event in the Phanerozoic's history: it was made about 65 million years ago when an approximately 10-kilometer-diameter extraterrestrial body hit the earth, causing the great mass extinction at the end of the Cretaceous period.

[Page 172]The Chicxulub structure was described initially in the late 1940s as a set of nearly circular geophysical anomalies discovered during oil surveys. In the next decades, three exploratory wells drilled into the structure by Petróleos Mexicanos (PEMEX, the Mexican state-owned oil company) penetrated late Cretaceous rocks (andesite). This andesite was originally interpreted as having a volcanic origin, and the Chicxulub structure was considered a giant end-Cretaceous volcano.

This interpretation changed at the beginning of the 1990s, when certain evidence confirmed an impact origin for the Chicxulub structure: impact melt rock in the crater, an extensive field of tektites around the Gulf of Mexico, and a millimeter-thick layer worldwide that contains material of the mete-oritic impact (iridium anomaly, microdiamonds, nickel-rich spinels, microtektites, and shocked quartz grains). This millimetric key-bed represents the dust and fine éjecta that covered the atmosphere after the Chicxulub impact and deposited slowly, probably over months or a few years after the impact.

The environmental perturbations of the Chicxulub impact event included a temporary absence of sunlight caused by dust and aerosols ejected into the atmosphere, impact winter (similar to nuclear winter), acid rain production, worldwide wildfires, and the largest tsunamis and earthquakes of Earth's most recent history. These disruptions caused the second, more severe biological crisis of the Phanerozoic: the Cretaceous-Tertiary mass extinction event. About 75% of all uppermost Cretaceous species were rendered extinct in this event, including the popular nonavian dinosaurs.

Chicxulub crater is the youngest and best-preserved large impact crater on Earth and is relatively untouched by the forces of erosion or tectonic deformation. For these reasons, it is of great interest for the study of impact cratering, including environmental and climatic effects and their biological consequences. The images obtained by the seismic reflection survey will be used to construct models of crater formation that can be applied throughout the solar system.