Ring around the K-T crater?

Charles E. Duller and his colleagues didn't know what to make of their find. While surveying satellite images of Mexico's Yucatan peninsula in the mid-1980s, these remote-sensing experts noticed a strange semicricle of sinkhole lakes, called cenotes by local residents. From high above, it looked like a set of teeth marks from an animal with jaws the size of New Jersey.

The researchers now believe they have linked the cenotes to the disappearance of the dinosaurs. In the May 9 NATURE, they report that the semicircle of lakes sits atop an area recently fingered as a possible crater left over from a meteorite impact 65 million years ago.

Many scientists believe a meteorite wiped out a large fraction of life on Earth at the end of the Cretaceous period, or K-T boundary, and geologists have searched the world over for the scar from such a catastrophe. Last year, they began focusing on a circular structure buried by about 1,100 meters of rock in the northern Yucatan. Scientists suspect the structure is a crater because holes drilled into it have pierced glassy rocks that could have formed from the tremendous heat of an impact (SN: 11/17/90, p.319).

Because the edge of the buried structure lies below the semicircle of lakes, the cenotes provide a surface marker that outlines the crater's rim, suggests Duller, of the NASA Ames Research Center in Mountain View, Calif. He and his co-workers believe the cenote pattern formed as the crater rim collapsed over millions of years, causing cracks to develop in the overlying limestone rock. As groundwater flowed through the cracks, it created a string of sinkholes around the crater.

The cenotes add to a growing body of evidence pointing to the Yucatan structure as the K-T crater. At the Lunar and Planetary Science Conference last March, Alan R. Hildebrand and his colleagues at the University of Arizona in Tucson reported finding pieces of pressure-shocked quartz in drill cores from the buried structure. Scientists believe such fracturing of mineral grains could result from the pressure waves of an impact. Researchers are now trying to date the structure to find out whether it did indeed form at the K-T boundary.