THE CHICXULUB IMPACT CRATER AND THE CRETA CEOUS-TERTIARY BOUNDARY IN THE GULF-OF-MEXICO REGION

Geophysical anomalies clearly indicate a vast circular structure burie d under similar to 1 000 m of Cenozoic sediments of the Yucatan platfo rm (SE Mexico). Cores drilled in the structure indicate the presence o f suevite-like impact breccia, with abundant shocked minerals, and of a melt-breccia dated by Ar-40/Ar-39 at similar to 65 Ma. The lithology and age show that the Yucatan structure is thus the long sought Creta ceous-Tertiary (KT) impact crater. The suevite and melt-breccia are de rived from the fracture and fusion of the lithologies present under th e Yucatan platform at the time of impact, a succession of approximatel y 3 km of carbonate and evaporite sediments overlying a Pan African ag e (550 Ma) silicate rich basement. The Chicxulub melts are chemically similar to the fragments of impact glasses found at the KT boundary al l around the Gulf of Mexico. Impact glasses and shocked quartz form th e base of a 2 and 4 metres thick coarse clastic sequence which marks t he KT boundary from Alabama to Guatemala. These sands and silts were p robably deposited, over a short period of time (a few days) by the gig antic tsunami waves triggered by the Chicxulub impact. Because of the target lithology, the Chicxulub event must have almost instantaneously released into the atmosphere huge quantities of water vapor, CO2 and SO2. These components must have played a key role in the perturbation of the global Earth system and mass extinction taking place at the KT boundary.