Ancient Meteorite Collapsed Margin, Spawned Giant Submarine Avalanches

March 31, 1998

By DAVID WILLIAMSON
UNC-CH News Services

CHAPEL HILL --After untold years of streaking across the galaxy, a giant meteorite smacked the Earth 65 million years ago with the force of a million atomic bombs.

The collision, which scientists believe led dinosaurs and many other species to die off within a few years, also caused massive landslides along the edge of the continent north of Mexico's Yucatán Peninsula, new evidence suggests. Six miles wide, the rock and metal chunk cut a hole 120 miles across and devastated an area from northeast Mexico to what is now the U.S. Gulf Coast.

"Until now, little has been known about the effect of the impact on the continental margin closest to the crater, which is directly north of the Yucatán," said Dr. Timothy Bralower, professor of geology at the University of North Carolina at Chapel Hill. "Because it happened so long ago, you can't even see the crater from the air or from space."

In a paper published in the April issue of Geology, Bralower and colleagues report evidence that the impact caused parts of the Yucatán margin to slide into the deep sea. Co-authors are Drs. Charles K. Paull, professor of geology at UNC-CH and R. Mark Leckie of the University of Massachusetts at Amherst.

"In cores of sediment taken from the base of the margin by the Ocean Drilling Program, we found fragments of several types of rock and fossils of various ages matching the strata from different levels on the margin," Bralower said.

Fragments were mixed with minute, round melted rock beads known as spherules flung from the impact site, he said. Such beads are direct evidence of an impact blast.

"The blast shattered large chunks of the edge of the margin, and they literally slid down to the depths of the ocean," Bralower said.

Collapsing margins triggered giant submarine avalanches that moved at up to hundreds of miles an hour and spread across the Gulf of Mexico and into the Caribbean.

"We find the same mixture of fossils, rocks and spherules in Haiti and in the central part of the Caribbean suggesting that the submarine avalanches traveled great distances," Bralower said.

The geologists cannot rule out the possibility that other margins collapsed because of the blast and that the avalanches came from a number of sources, he said. But it is clear that the impact had a huge effect on Gulf of Mexico topography.

Among the more spectacular results of the margin collapse, which suddenly displaced billions of gallons of water, were giant tidal waves that inundated coastlines around the gulf.

Scientists named the event, which was like a cannon ball hitting a duckpond -- except on a vastly larger scale --the Chicxulub impact, after the Yucatán crater site. Bralower and colleagues dubbed the mixture of microfossils, rock fragments and impact-created materials they studied the "Cretaceous-Tertiary boundary cocktail."

Core sampling to about 3,000 feet took place aboard the 470-foot JOIDES Resolution, the world's largest scientific drill ship.

The National Science Foundation and the Texas A&M University-based Ocean Drilling Program supported the research.

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Note: Bralower can be reached at (919) 962-0704.
Contact: David Williamson or Bret Johnson, 962-8596.
-end-


University of North Carolina at Chapel Hill

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