Geologic formations in southern Utah may have driven a period of global warming 6 million years ago.

A University of Utah graduate student and her colleagues have discovered that so much hydrocarbon leaked from the Navajo sandstone formation in that area that it may have caused a greenhouse effect.

The familiar sandstone often is in the form of hills, flat stretches, domes and dunes of rock. It is exposed throughout the southern part of the state, usually tawny, light gray or reddish.

The top of the layer looks like dunes because that's what it once was, a field of sand dunes. Minerals in the sand cemented the grains together and turned the deep formation to stone. Sometimes the direction of winds that blew millions of years ago can be detected by studying the layering and grain sizes of the petrified dunes.

"It extends from up in Idaho and Wyoming, down to Arizona," Said Brenda Beitler, lead author of the study. "It covers a huge area. . . . It's comparable to the Sahara Desert, but even bigger."

According to a paper by Beitler, Marjorie A. Chan and William T. Parry, Navajo sandstone represents the largest dune field ever, either in modern times or in the geologic record. The sand was heaped up during lower Jurassic times, about 190 million to 200 million years ago.

Their paper is titled "Bleaching of Navajo Sandstone on Colorado Plateau Laramide Highs: Evidence of Exhumed Hydrocarbon Supergiants?" It was published in the December edition of Geology, the journal of the Geological Society of America.

Beitler is working on her Ph.D. in geology. Chan, professor and chairman of the U.'s department of geology and geophysics, is her adviser, while Parry is a geochemist and professor emeritus in the department.

Their inspiration was simply curiosity about the Navajo sandstone. "We got to thinking, why it is so many different colors?" Beitler said during a telephone interview.

Originally the sandstone was red. Each sand grain is covered by a thin coating of hematite, or iron oxide, she said. Studying the rock with a microscope, "you can see the coating in the parts that are red. . . .

"In the white rocks, that coating is removed."

After it was formed, the sandstone was buried by later sediments. It lay underground for more than 100 million years. Hydrocarbons migrated through the porous sandstone.

Where did the hydrocarbons originate? "That's something else we're working on," she said.

The hydrocarbons resulted from decomposition of organic material, like ancient swamps and forests. Other known petroleum systems in the region have hydrocarbons that may have originated in the Paradox Basin.