Mysterious granite slab slows probe of bay crater

Discovery puzzles team drilling into ancient site

December 09, 2005|By DENNIS O'BRIEN | DENNIS O'BRIEN,SUN REPORTER

Scientists are rethinking their theories about the Chesapeake Bay impact crater after they drilled deeper into it than ever before and found something unexpected: a huge slab of granite.

Over several months, crews penetrated 5,795 feet at a site about five miles north of Cape Charles, Va. They're trying to piece together what happened 35 million years ago, when a meteorite smashed into what is now the mouth of the bay.

The mile-wide meteorite incinerated everything in its path and created a tsunami when it splashed into the sea, leaving a hole the size of Rhode Island.

Drilling at the site, which began in September at a cost of $1,100 a day, ended this week. An international team of scientists will analyze the core samples that the drill unearthed.

The drills initially penetrated clay, sand and sediments before they reached pulverized stones known as suevite that were melted by the impact. When one drill bit got stuck halfway down, two of them had to be replaced, which meant crews weren't able to reach their initial target depth of 7,200 feet.

But the researchers were surprised by what stalled them between the sediment and stones: a huge slab of granite that starts at 3,600 feet and extends down to about 4,500 feet. How it got there remains a mystery.

"The granite was a complete surprise to everybody," said J. Wright Horton, a bay impact crater expert for the U.S. Geological Survey. "We had never anticipated this 900-foot block of granite, and we're going to have to rethink and reinterpret some pretty big things about the crater structure."

The granite is lodged between sedimentary material that washed into the area after impact and a layer of crushed stone that was partially melted by the meteorite. One issue ripe for review is exactly what type of debris filled the hole in the sea floor moments after the meteorite hit, Horton said.

"It could mean new models of crater formation," he said.

Experts are unsure if the granite slid into the crater bed from the rim or was pushed there by massive shifting of the earth when the meteorite hit. Scientists also are unsure whether they've actually reached the bottom of the crater. Further study may resolve such questions.

"We don't exactly know what we have right now, even though the drilling phase is over," Horton said. "We still have to go through the scientific phase and analyze all the material."

The $1.5 million drilling project was funded by the Geological Survey, NASA and the International Continental Scientific Drilling Program, a consortium of scientists that funds drilling projects at fault lines, volcanoes and craters around the world.

Some 44 scientists from around the world will come to the U.S.G.S. headquarters in Reston, Va., this spring to collect core samples for further study. The studies will focus on issues that include how prehistoric climates changed, whether the meteorite was an asteroid or comet and what types of microbes thrived in the boiling habitats created by the impact.

The bay crater is the largest in the United States and the sixth-largest of 170 known impact craters in the world.

dennis.obrien@baltsun.com

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