The collision between a large extraterrestrial object (probably a meteor) on the surface of the Earth 60 million years ago in the Yucatan led to the extinction of about 70 percent of the planet's species, including all dinosaurs, at the end of the Cretaceous Period. The Chicxulub impact basin on the northern coast of Yucatan appears to be the site of this catastrophic impact event. The crater is completely buried by Cenozoic platform rocks, so conventional geophysical techniques and drilling must be employed to study the style and extent of deformation associated with its formation.

Bouguer gravity anomalies indicate a configuration of multiple concentric rings, the largest of which has a diameter of nearly 300 km. Recently acquired seismic data substantiates this morphology and delineates in remarkable detail the smaller central basin formed by inward slumping of the excavation crater walls. Central crater diameter is of nearly 200 km and modest extensional deformation extends the area to approximately where the outer gravity ring is located.

The weakly circular northwest quadrant of the crater is interpreted as the superposition of the impact onto an older linear gravity high, rather than a post-impact fault as assumed by other workers. Such a linear 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.

Several lines of evidence suggest that the basement rock under the crater is 500 million years old. Zircon minerals found in the 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, consistent with the emplacement of the zircons as debris from the K-T-age impact basin.

Stratigraphic and paleo-environmental constraints derived from deep exploration drilling by Pemex and shallow drill coring by UNAM indicate that the higher-standing topography associated with the inner basin flanks has undergone substantial deformation. Large blocks, some exceeding 50 m in thickness, were ejected more than 70 km from the edge of the excavation cavity. The breccias and melt rocks are mixtures of Pan-African crystalline rocks and Cretaceous sediments. The abundance of anhydrite and carbonate in the platform section, which was melted and vaporized by this impact event, contributed to the destructiveness of the impact deformation.

End_of_Record - Last_Page 7---------------