SAN FRANCISCO -- A NASA-sponsored study found strong new evidence that Upheaval Dome in Canyonlands National Park was not formed by a rising blob of underground salt, but was created when a comet or asteroid walloped Utah eons ago.

"We're convinced to the extent we can be in any scientific endeavor that it's a meteorite impact crater," said Jeff Plescia, manager of planetary geology at the National Aeronautics and Space Administration in Washington.

Canyonlands is noted for its flat layers of rocks. Upheaval Dome is mysterious because its rock layers were warped upward into a dome several hundred feet high and a few miles wide, with a crater in the center.

Geologists believed for many years Upheaval Dome was formed when a big bubble of salt slowly floated upward from the Paradox Formation, a thick underground salt layer left by seas that once covered eastern Utah.

Plescia and other scientists descended on Canyonlands in January and began several experiments for a $150,000 study to settle debate on whether Upheaval Dome is the eroded remnant of either a salt dome or a meteorite crater.

John Louie, a seismologist at the University of Nevada, Reno, will present partial findings here today during the American Geophysical Union's national meeting.

Louie sent thousands of seismic waves through Upheaval Dome by thumping the ground 4,000 times with a 700-pound weight on a ``thumper trailer'' owned by the University of Utah. Special listening devices measured the speed of the waves.

Seismic waves generally travel faster through salt than through rocks. Yet the waves traveled slowly right under Upheaval Dome, showing there is no buried blob of salt that could have pushed up overlying rock to create a salt dome.

"Upheaval Dome is the remnant of a crater from a meteorite impact because everything underneath it looks like it was punched out by a force from above", Louie said.

The findings "provide one more nail in the coffin of the salt-dome hypothesis for Upheaval Dome", said Gene Shoemaker, a U.S. Geological Survey geologist who first argued Upheaval Dome was the Colorado Plateau's largest impact crater.

However, Mike Hudec, a geologist at Exxon Production Research Co. in Houston, replied: "I don't believe this evidence settles the issue".

Hudec said the lack of salt right under Upheaval Dome doesn't rule out the salt-dome theory. He said no salt would be found if Upheaval Dome was the pinched-off stem of a salt dome that eroded away along with hundreds of feet of overlying rock somewhat like a mushroom with its cap chopped off.

Louie and Shoemaker replied a rising salt blob would have left in its wake chunks of salt and rock from deeper underground, yet no such fragments have been found.

Plescia said Louie's findings are particularly convincing when added to the 1993 discovery that rocks at Upheaval Dome contain shatter cones, which are cone-shaped rock fractures created when a big meteorite slams into the ground.

At a viewpoint on Upheaval Dome, two interpretive signs explain the competing meteorite-impact and salt-dome theories. Despite the new findings, Canyonlands Superintendent Walt Dabney said he isn't quite ready to dismantle the salt-dome sign.

"It's premature", Dabney said. "I'd like to see the results of the other tests come in .... I've heard very persuasively from both sides that theirs is the obviously right theory."

Nevertheless, Dabney admitted, "the thought of a meteor strike is more exciting than a salt dome".

Shoemaker helped discover the comet that smashed into Jupiter in 1994. He was the first to propose that a 1,700-foot-wide meteorite slammed into Canyonlands sometime between 10 million and 100 million years ago, creating a 5-mile-wide crater called Upheaval Dome. Shoemaker originally suspected more than a half-mile of overlying ground subsequently eroded away.

But Shoemaker said Monday he believes erosion has been less extensive. So he now figures the original crater was 3 miles wide and was formed by the impact of a 1,000-foot-wide meteorite -- either a comet or asteroid -- within the last 30 million years.

For another part of the NASA-funded study, Upheaval Dome's rock layers are being mapped by Shoemaker and Ken Herkenhoff, a planetary scientist at the space agency's Jet Propulsion Laboratory in California.

They found certain layers are thicker beneath the dome than in the areas around it. That's exactly what happens in a meteorite crater, which often has a thicker pile of debris in its center because the meteorite rebounds an instant after impact.

Yet Hudec said thicker layers could form when rocks flow into the void left beneath a rising salt blob. Shoemaker and Louie said there's no sign that happened.

Louie has yet to analyze data from the study's third experiment -- called seismic reflection -- in which sound waves produced by thumping the ground can be used to make a picture of underground rock layers. He expects the results late next year.

If the picture shows the buried Paradox Formation is flat, "I would concede the salt-dome model becomes untenable" because it would rule out any possibility a salt blob rose upward from that formation, Hudec said.

For more information, please see http://www.seismo.unr.edu/ftp/pub/louie/dome/
and
http://www.seismo.unr.edu/ftp/pub/louie/dome/98seismo/index.html