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Kari Fluegel March 15, 1993

Johnson Space Center

Pam Thompson

Lunar and Planetary Institute

Release No. 93-018

STRUCTURE OF CHICXULUB IMPACT BASIN IS EXAMINED AT LPSC

Scientists are gradually gaining insight into a buried impact crater in the Yucatan that was created by the catastrophic collision between a large meteorite and Earth 65 million years ago, an event that led to the extinction of about 70 percent of the planet's species, including all dinosaurs at the end of the Mesozoic era.

A presentation at the 24th Lunar and Planetary Science Conference in Houston this week focuses on gravity data used to establish the shape and deep structure of the Chicxulub Impact Basin on the Yucatan coast. The crater -- which is now covered by a half mile of limestone deposits that have protected it from erosion while also obscuring it from view -- is the leading candidate as the source crater of the catastrophe that occurred at the time known as the Cretaceous-Tertiary (K-T) Boundary.

Scientists believe that the downfall of the dinosaurs allowed mammals -- and thus humans -- to become the dominant form of life. Finding the site of a massive, K-T-aged impact has been the goal of detective work by many researchers over the last several years.

A combination of reprocessed Bouger (a technique for measuring gravity on land) and free-air gravity data reveals that the structure is a multi-ring basin, 200-km in diameter, with at least three concentric rings. The spacing of the rings follow the rule scientists have observed from studying multi-ring basins on other planets. The most highly magnetic zone lies within the central ring, which could result from deep rocks being melted and uplifted by the impact.

Researchers interpret the weakly circular northwest quadrant of the crater as the superposition of the impact onto an older linear gravity high, rather than a post-impact fault as assumed by other workers. They point out that such a feature may have resulted from processes that tore the Yucatan Peninsula away from the southern United States as the Gulf of Mexico opened during the Jurassic era.

Several lines of evidence cited by the scientists suggest that the basement rock under the crater is 500 million years old. Zircon minerals found in the geologic strata in the western United States that mark the 65 million-year-old impact event have been determined to be about 550 million years old. The authors believe that this is consistent with the emplacement of the zircons as debris from the K-T-aged impact basin.

The paper's authors are Virgil Sharpton of the Lunar and Planetary Institute, Houston; Kevin Burke and Stuart Hall, both of the Department of Geosciences at the University of Houston; Scott Lee, also of LPI; Luis Marin, Gerardo Suarez and Jaime Urutia, of the Instituto de Geofisica, Universidad Nacional Autonoma de Mexico in Mexico City; and Juan Manuel Quezada of the Gerencia Divisional de Programacion y Evaluacion, Petroleos Mexicanos, also in Mexico City.

The 24th Annual Lunar and Planetary Science Conference, set for March 15-19, is an annual gathering that gives scientists from around to world the opportunity to discuss the latest research in a variety of research areas. It is co-sponsored by the Lunar and Planetary Institute and JSC.

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