Scientists link valley's clay soil to Mount Mazama ash spew.

Byline: Greg Bolt The Register-Guard

If you've ever slogged your way across a particularly swampy patch of Willamette Valley bottomland and wondered why this is such a boggy place, cast your eyes toward Crater Lake.

New research by a pair of Eugene scientists has turned up evidence that Mount Mazama, whose remains form the rim of Crater Lake, sent a cloud of volcanic ash into the valley during the catastrophic series of eruptions that blew the top off the mountain more than 7,000 years ago. That ash apparently formed the layer of clay that underlies much of the valley, keeping water from soaking deeper into the earth and making this one big, soggy wetland.

Karin Baitis, a soils scientist with the Bureau of Land Management in Eugene, tied the clay layer to the Mazama eruption in a paper she delivered Thursday at the Northwest Anthropological Conference being held at Valley River Center. She and geochemist Michael James of James Geoenvironmental Services, stumbled on the origin of the clay as they were doing separate surveys of soil characteristics in areas around Eugene.

Until now no one thought ash from the massive eruption of Mount Mazama made it into Western Oregon. Previous studies suggested it all went north and east of the mountain, where layers hundreds of feet thick have been found.

"We were really surprised with our findings," Baitis said after her presentation. "No one ever thought we had anything like this here."

Just about anyone who's tried to dig a hole in the Willamette Valley is probably familiar with the layer of clay. In the Eugene area, it's found beneath about 12 to 18 inches of topsoil in flatland areas west and north of the city and runs about 5 feet deep, tapering to about a foot deep near Woodburn.

Before Baitis and James came along, some scientists thought it was alluvial material swept into the valley by the great Missoula Floods about 13,000 to 15,000 years ago. Those massive events occurred when an ice dam that held back a colossal ancient lake in what is now Idaho collapsed, releasing floods that surged across Eastern Washington, down the Columbia River and into the Willamette Valley.

But when Baitis and James began analyzing samples they collected near Bailey Hill, in the west Eugene wetlands and all the way over toward Veneta and up to Harrisburg, that idea just didn't pan out. The material was just too fine, not the mixture of larger and smaller bits characteristic of a flood and not something that came from weathering of existing materials over time.

And when they thought to compare the mineralogy of their samples with known Missoula sediments, they were surprised to find that no one had ever examined the clay in detail.

"There's been this mystery layer of gray clay that people have always wondered where it came from and no one ever thought to look at the mineralogy of it," Baitis said.

So she and James worked with researchers at the University of Oregon and the U.S. Geological Survey to analyze their samples, which didn't support any of the existing theories and only added to the clay mystery. But it wasn't until they later saw the same clay turn up in an archaeological dig near Woodburn that they made the connection to Mount Mazama.

Clay from that dig had been carbon-dated, placing it around 7,500 to 7,600 years old, which matches up with the latter half of the 200-year eruptive phase of Mazama, probably after the mountain's collapse. That's when they realized the clay might have come not from a flood, but from the sky.

Their idea was confirmed when they compared the minerals of the clay and material known to have been ejected by Mount Mazama. It was a sure match. Even the chemical signature of water in Amazon Creek and Spencer Creek, which flow through the clay, matches that of water found in Crater Lake, Baitis said.

Baitis speculates that the pressure that built up under the Mount Mazama caldera after the initial eruptions caused later bursts that shot ash in different directions.

"We're thinking that one of these little squirts is what we're finding up here," she said.

The ash theory ties in with other evidence in the historical record and demonstrates the violence of the Mazama eruptions. Mount St. Helens shot about 1 cubic kilometer of ash into the air, but Mount Mazama's eruptions fired 100 cubic kilometers of material so high into the atmosphere it probably altered the region's weather.

"This would help explain why we've never found evidence of human occupation in the Willamette Valley between 6,000 and 8,000 years ago," Baitis said. "That was some catastrophic event."

The two researchers acknowledge that they haven't nailed down the entire story of how the Mazama eruption affected the Willamette Valley and realize there's likely to be some controversy over some of their theories. They hope to get additional funding to continue the research and try to confirm their findings.

But both are sure that they've found a connection between one of the largest eruptions in Northwest's long volcanic past and present-day life in the Willamette Valley, something that substantially changes the region's history books.

"We've got the evidence," Baitis said. "It's just so remarkable. I think we have a different history than what we've thought."

CAPTION(S):

Crater Lake, whose rim was once part of Mount Mazama, has the same chemical signature as water in Amazon Creek and Spencer Creek, which flow through clay thought to be formed from Mount Mazama ash. Paul Carter / The Register-Guard, 1994