Bad day for dinos, good day for coal? Researchers propose extraterrestrial connection for Montana coal fields

Note: Art is in Future Photos - "Science-coal"

A really bad day for dinosaurs 65 million years ago may have been a great day for coal producers in Montana and North Dakota, according to a group of Wisconsin geologists.

Peter Sheehan, curator of geology at the Milwaukee Public Museum, together with University of Wisconsin-Milwaukee colleagues John Isbell and Claudia Barreto and David Fastovsky from the University of Rhode Island, recently suggested that the massive asteroid impact that ended the dinosaurs' 160-million-year reign may have created the conditions that led to coal formation in the Williston Basin.

The basin underlies most of northeastern Montana and the western half of North Dakota. It contains a total of about 530 billion tons of lignite in several beds, according to U.S. Geological Survey estimates. The North Dakota Geological Survey says this is the largest known deposit of lignite in the world.

As outlined by Sheehan, the extraterrestrial hypothesis regarding how this deposit was formed begins with evidence that the impact burned off all the vegetation across the region.

Like the mother of all forest fires, this led to a huge increase in subsequent runoff and sedimentation rates, which blocked up stream channels, caused ponding and created numerous swampy areas. At the same time, the impact killed off any major herbivores, together with T. rex and most other reptilian predators. Consequently, once the surviving plants recolonized the area - just as they do after modern forest fires - they were free to bloom and grow and die unimpeded, forming substantial peat mires that eventually turned into coal.

"Before the asteroid hit, it was a very nice day. A few minutes later, it wasn't," he said during a recent telephone interview. "In Montana, things were pretty bad."

Most calculations say the asteroid was about 10 kilometers in diameter - meaning when the leading edge hit, the trailing edge was still about 30,000 feet high, or normal cruising altitude for a commercial jet.

The asteroid landed in a shallow seabed along Mexico's Yucatan Peninsula, releasing an estimated 100 million megatons of energy. There's some evidence that it hit at an angle, blasting a fiery vapor cloud across North America, Sheehan said. Other evidence suggests that huge quantities of debris were lofted into space, only to be pulled back by gravity, creating a global meteor storm.

"For several hours after the impact, the entire Earth was bathed with intense infrared radiation from this ballistically re-entering ejecta [debris]," University of Colorado researcher Doug Robertson noted in a recent paper. "Darkness would have been eliminated worldwide, and shadows curtailed. This global heat pulse would have killed any unsheltered animals and ignited fires wherever fuel was available."

Previous studies indicate that temperatures in the upper atmosphere would have ranged from 1,500 to 2,000 degrees Fahrenheit for several hours, Robertson said. Fires would not have spread from one place to another - they would have erupted everywhere simultaneously. Anything too big to find a hole to hide in would have fried.

"Entire continents would have been set ablaze," Sheehan said. "Of the animal species that lived on land, about 90 percent went extinct. For those that lived in streams right next to them, 90 percent survived."

It took an estimated five months for all the smoke and debris to settle out of the atmosphere, he said. During that time, it was too dark for much photosynthesis to take place. Ecosystems based on plankton and algae and other forms of primary production collapsed; species that lived off detritus and dead matter survived - including crocodiles, turtles and small mammals.

"This was a very unusual period because there were no major herbivores," Sheehan said. "There had been large tetrapods feeding on plants for 200 million years prior to this time, including some that were huge, but now they were gone. Mammals survived because they were small insectivores. Within about 5 million years, some of them had evolved into herbivores and some became carnivores."

Throughout the Williston Basin, he said, the boundary between the pre-impact Hell Creek and post-impact Fort Union rock formations is immediately apparent, as darker floodplain deposits are replaced by distinctive layers of lighter-colored sediments.

The size and amount of sediment change across the boundary as well, he said, with one previous study estimating that sedimentation rates increased by about 40 percent, from 66 meters per million years to 91 meters.

And just above the boundary, the first thin coal seam in the basin 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 have been a continuation of the Hell Creek formation."

He and his co-workers, though, may have an uphill battle convincing their colleagues that this hypothesis holds up, given that there were multiple changes taking place in Montana and the Great Plains at the time of the impact that could have affected coal formation in the basin.

For example, a 1999 U.S. Geological Survey report on the region indicates that the creation and growth of peat bogs in the basin was influenced at least to some degree by crustal distortions and changes in runoff patterns related to the formation of the Rocky Mountains.

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

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

The Cannonball Sea was a younger, smaller version of the shallow Western Interior Seaway. At its peak 90 million years ago, the seaway covered much of North America, stretching from central Montana and the Rocky Mountains to the Appalachians and connecting the Gulf of Mexico to the Arctic Ocean.

About 2 million years prior to the asteroid impact, the seaway retreated and split in two. The southern half was named the Cannonball Sea; it ran from the gulf up to North Dakota.

Johnson, who for more than 20 years has studied fossil plants from the time of the impact, is somewhat skeptical of Sheehan's coal theory. He says more recent research has shown that the coal started forming prior to the impact. He also wants to see more hard data.

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

Sheehan, who also has studied the dinosaur extinction and other mass extinctions for more than 20 years, said he recognizes that proving this theory "is going to be a long road."

"This is just a first attempt," he said. "We need to look at how fast some of these changes occurred, and we need to look at the distribution of the coal seams in relation to when herbivores returned. At this point, what we're really saying is that this type of [extraterrestrial] event is something we need to be considering."

Reporter Bill Spence may be reached at 758-4459 or by e-mail at bspence@dailyinterlake.com