The Discovery

The crater is named after a small town on the Yucatan Peninsula, Mexico (Fig. 1), that lies closest to its centre. From gravity and magnetic surveys, Hildebrand and colleagues mapped out a circular shape with approximate diameter of 180km. Little was discovered concerning the three-dimensional shape of the crater, although the sediments of Cretaceous/Tertiary age are 600 to 1100m lower than surrounding sediments of the same age, suggesting a basin to have been present at this time. The stratigraphy was described from samples retrieved from exploratory drill holes, originally referred to by Penfield and Camargo. Rock chemistries were also analysed, their high temperatures and pressures of formation showing further evidence of an impact event. Ejecta material was also observed. The age of the event could not be constrained to the Cretaceous/Tertiary boundary, although this was implied.

Fig. 1 Global position of the Chicxulub crater, found on the Yucatan Peninsula, Mexico. Courtesy of Imperial College.

As mentioned, the implications of this work were significant to the theory of extinctions, and particularly for explaining their global distribution at the Cretaceous/Tertiary (K/T) boundary. In this context, Hildebrand and colleagues suggested that extinction may have been enhanced due to the impact occurring in the carbonate target rocks of the Yucatan area; global warming would have been accelerated by shock-production of CO2 to the atmosphere. In addition, the discovery of the crater provided confirmation of much other evidence that had been accumulated over the years:

The crater is within continental crust, agreeing with rock chemistry of crater ejecta. It is an attractive source for K/T boundary tektites, with crater melt rocks having similar chemical composition, and it is also found midway between Haiti and Mimbral, northeast Mexico (Fig. 2), the locations of the two thickest K/T boundary ejecta deposits (4).

Fig. 2 Close-up map showing the location and shallow marine setting of the Chicxulub crater.

Discovery of the Chicxulub crater therefore provided a link in a chain, moulding many lines of evidence together to support the extraterrestrial theory. However, some problems relating the crater to the K/T boundary arose. Firstly, isotopic data suggested that a mantle component was present in the K/T boundary layers, although its origin at Chicxulub was unclear (4). Secondly, the presence of impact-wave deposits in some of the boundary layers also remained unexplained solely by an impact at Chicxulub; the shallow water depths, present at the time of impact, would have limited the carrying capacity of the waves (4). However, it is possible that the intensity of the impact produced sub-marine slumping of sediments (as seen at various Deep Sea Drilling Project sites), and that this caused the large waves necessary to form these deposits (4). A deep-water impact occurring at a similar time is an alternative explanation (4).

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