Giant crater may hold secret to bay's birth

September 05, 1994|By Douglas Birch | Douglas Birch,Sun Staff Writer

The nation's richest estuary may owe its existence to a mountain-sized meteorite that slammed into what is now the coast of Virginia 35 million years ago, geologists say.

C. Wylie Poag and three colleagues with the U.S. Geological Survey reported in the August edition of the journal Geology that they found the remains of a 50-mile-wide, mile-deep crater at the mouth of the Chesapeake Bay.

Dr. Poag said his team thinks that over the past 35 million years, the crater acted like the drain in a bathtub when sea levels were low -- giving the region's scattered rivers a single outlet to the ocean.

"We think that the sea-floor depression that the impact made was never [completely] filled up" with sediments, he said. "The bay's river systems were attracted to this low area. That's why they are where they are now."

When the meteorite landed, he said, the impact occurred in shallow ocean waters off what is now North America. The Atlantic shoreline was near the current site of Richmond.

When sea levels fell during past ice ages -- as a greater portion of the world's water was locked up in glaciers and the polar ice caps -- the depression caused by the crater continued to drain the many rivers that feed the bay region.

When sea levels rose again in warmer periods, Dr. Poag said, those low-lying river channels were inundated, creating the bay's broad, shallow waters. (The Delmarva Peninsula was built out of sediments washed down by the Susquehanna and other rivers, partially filling the crater).

Object was a mile in diameter

Dr. Poag estimated that the hurtling object -- either a rocky asteroid or an icy comet -- might have been more than a mile in diameter, or about the size of Gibson Island. It would have vaporized as it landed, instantly evaporating many cubic miles of ocean, penetrating more than a mile beneath the sea floor and blasting the equivalent of a mountain range of rock skyward.

After the initial shock, the waters would have roared back into this hole in the ocean, Dr. Poag said, rebounding in a huge wave called a tsunami. Another geologist estimated that the tsunami might have been more than a mile high.

The Chesapeake Bay object, Dr. Poag noted, would have been about the same size as fragment "G" of Comet Shoemaker-Levy 9, the largest of 21 pieces of the comet that struck Jupiter in July.

The bay crater, he suggested, could have been caused by a Shoemaker-Levy-style bombardment. There is evidence that another, smaller object hit the ocean 90 miles off Atlantic City, N.J., 35 million years ago. That event excavated a 10-mile-wide crater. Both the Chesapeake and Atlantic City craters, he speculated, may have been caused by fragments of the same object or a train of objects. "It's difficult to say," he said. "We're just guessing about that."

Dr. Poag, who is based in Woods Hole, Mass., began studying the lower Chesapeake in 1990, when he looked at an unusual layer of fragmented and jumbled rocks -- called breccia -- found in cores drilled on the bay's Virginia shores. This material, he said, indicated some sort of impact. The core samples also contained small amounts of quartz and other mineral crystals that had been placed under tremendous heat and pressure.

Scientists think these "shocked" quartzes can be produced only by meteorite collisions.

In 1992, Dr. Poag published an article concluding that this layer of shattered rocks was formed when a tsunami kicked up by the impact of the Atlantic City meteorite smacked the East Coast.

But he changed his mind, he said, after he studied portions of the seismic profiles of the bay bottom made by Texaco and Exxon, which searched for oil deposits in the lower Chesapeake region from about 1989 to 1992.

In seismic studies, geologists use reflected sound waves to make pictures of underlying rock strata -- or layers -- the way a sonogram produces a picture of a fetus.

Richard A. F. Grieve, who studies craters for the Geological Survey of Canada in Ottawa, recently told the publication Science News that Dr. Poag and his co-authors -- David S. Powars, Lawrence J. Poppe and Robert B. Mixon -- had not proved they had discovered a new crater.

He said he was still looking through rock samples Dr. Poag sent him for signs of shocked quartz. He was also quoted as saying he was not impressed with the seismic evidence.

Dr. Grieve could not be reached last week.

Scientists disagree

But Glen A. Izett, a retired geologist with the U.S. Geological Survey, said Friday that he has found "one good grain" of shocked quartz from a bay core sample. And he thinks the seismic data strongly suggest a crater-shaped feature lies beneath the sediments.

Dr. Izett, a former colleague of Dr. Poag, said he would expect to find a peak or "uplift" in the deep rocks near the center of the bay impact site. Typically, he said, the earth's surface rebounds slightly from the tremendous shock caused by such collisions.

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