The Jurassiccretaceous

Impact craters have several ways of escaping notice. They may hide beneath seafloor sediments, arctic ice, flood basalt flows, and younger sediments. Tectonic plates may carry them down to oblivion. Or, more mundanely, erosion may obliterate them. But recently yet another hiding place has been discovered.

In the southwestern part of Africa lies the great Kalahari Desert. Until the advent of four-wheeled drive vehicles, travel in the Kalahari was next to impossible as the region is covered by a layer of sand 100 m thick. In the early 1990s, aerial gravity and magnetic surveys, of the kind done in the search for oil-bearing structures in the Yucatan, revealed a nearly circular structural dome buried beneath the Kalahari sands near the South African town of Morokweng. Drills sent down into the underlying bedrock brought back melt rock containing an iridium anomaly and shocked quartz. Although at first the structure appeared to be 70 km wide, further data analysis suggests a diameter of 340 km; if so, it would be even larger than Chicxulub. Zircons from the Morokweng melt rock give an age of 145 million years.4 The accepted age of the boundary between the Jurassic period and the overlying Cretaceous (see Figure 2, page 8), at which, according to Sepkoski, 38 percent of species became extinct, is also 145 million years.

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