KALISPELL, Mont. - The asteroid thought to have killed the dinosaurs when it slammed into Earth 65 million years ago also may have created the coal-rich Williston Basin, a group of geologists suggest.

The basin underlies most of northeastern Montana and western North Dakota. It contains one of the largest lignite coal deposits in the world at about 540 billion tons, according to U.S. Geological Survey estimates.

The asteroid killed off most major predators, meaning plants in the basin would have grown and died unimpeded for years, a team of geologists from the University of Wisconsin-Milwaukee, the Milwaukee Public Museum and the University of Rhode Island say in their research.

The uninhibited plant growth created substantial peat mires that eventually turned into the coal fields that pepper the basin today, said Peter Sheehan, curator of geology at the Milwaukee Public Museum.

The boundary between pre-impact and post-impact rock formations in the basin is obvious, researchers said, with darker floodplain deposits replaced by distinctive layers of lighter-colored sediments.

Just above the boundary, the first thin coal seam appears.

"We're suggesting that the impact caused these changes," Sheehan said. "If not for that and the associated change in vegetation and animal life, there would not have been a continuation of the (pre-impact) formation."

Other theories on coal formation abound, however, and some colleagues are already disputing the group's suggestion.

Kirk Johnson, chief curator at the Denver Museum of Nature and Science, said the resurgence of a major inland sea may also have played a role in coal.

"We think the coal formation could be related to the Cannonball Sea," he said. "As sea levels rose, it may have raised groundwater levels and created swamps."

The Cannonball Sea split off from a much larger waterway about 2 million years before the asteroid impact, running from the Gulf of Mexico to present-day North Dakota.

Johnson was somewhat skeptical of Sheehan's theory, saying he wanted to see more data. More recent research shows that coal started forming in the basin before the asteroid impact, he said.

"That's the real question for Peter," Johnson said. "Where's the data?"