Giant tidal wave may have hit millions of years ago Scientists say meteorite hit in the Atlantic, sending huge sea wave.

October 25, 1991|By Frank D. Roylance | Frank D. Roylance,Evening Sun Staff

A federal geologist says he has found evidence that the southern Chesapeake region was devastated 40 million years ago by a giant sea wave, hundreds of feet high, that crashed ashore after a meteorite impact somewhere in the Atlantic.

C. Wylie Poag, a United States Geological Survey geologist from Woods Hole, Mass., said the titanic wave "gouged out a 200-foot-deep Connecticut-sized area in the coastal region and filled it in with a spectacular assortment of large boulders and shock-altered minerals."

The deposits consist of sedimentary boulders up to 3 feet in diameter, wrenched from beds of seven geological ages and mixed together "like plums in a pudding," Poag said in remarks yesterday at the Geological Society of America's annual meeting in San Diego.

This "impact wave deposit" first turned up during a ground water study five years ago at Exmore, Va., leading geologists to name it the "Exmore beds."

At first, Poag said, geologists thought the scrambled rocks and debris represented an ancient river channel.

But in the years since, detailed analysis of the beds has turned up quartz and tektite glass typical of meteorite impacts. Then, drilling at other sites revealed that the Exmore beds extend far beyond Exmore.

Two hundred feet thick at Exmore, the beds thin to 60 feet at Newport News, Va. They also reach some 40 miles west of Chesapeake Bay in Virginia, and north some 20 miles into Maryland's Eastern Shore.

In all, Poag estimates the wave crushed and scrambled sedimentary rock in the ancient sea bed across a 6,000- to 9,000-square-mile area, an expanse nearly the size of Connecticut. All of that now lies buried below younger sediments.

What scientists have not found is the exact spot where the meteorite struck. Offshore sonar studies have traced a grid with 50-kilometer intervals without spotting it.

If there is a crater out there, Poag said, its remains must be less than 50 kilometers in diameter and the sonar just missed it.

From that, Poag said, "we can infer that [the meteorite] was probably on the scale of a quarter-mile in diameter" when it struck.

When it hit, the meteorite would have vaporized itself and the upper layer of the rock it struck. Some of that vaporized rock would have cooled as it rose and condensed into glassy "raindrops" called tektites.

In 1983, Poag and other scientists drilling 90 miles east of Atlantic City discovered a 2-inch-thick layer of tektites buried 600 feet down. It was the first hint of fallout from a meteorite impact of that age in that area.

The impact would have sent a huge wave rippling out in all directions, initially perhaps 1,000 feet high. The wave would have crushed and stirred the seafloor as it passed, forming the Exmore beds.

Finally, the wave would have crashed onto the shore, which at that time was farther west, at the foot of the Piedmont.

"At a minimum, it could have taken an hour" for the wave to pass, he said. "On the other hand, it could have taken a few days for it all to settle down."

The Exmore meteorite strike was not large enough to have caused global climate changes such as those thought by some scientists to have caused the extinction of dinosaurs about 65 million years ago, Poag said.

But it "certainly would have destroyed everything on the sea floor and organisms living in that Connecticut-sized area, and may have affected life across an even wider area," he said.

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