Boring plains belie bounty beneath.

Boring plains belie bounty beneath

Geologically speaking, the U.S. Midwest gets a bum rap. While oil companies pursue their quarry in the uppermost rock that forms a veneer across the central states, the basement rocks beneath have never captured much attention from geologists. But researchers probing the midcontinent with seismic waves are now finding surprising, layered structures hidden within this basement. In southern Illinois and Indiana, the layered rocks extend at least 180 kilometers in an east-west direction and average about 6 km in thickness, says Larry Brown of Cornell University in Ithaca, N.Y. Brown is part of the Cornell-based Consortium for Continental Reflection Profiling (COCORP), a program aimed at exploring the crust of the entire continental United States.

Previous seismic work, partly by oil companies (which rarely release their information), had hinted such layered structures might exist in the basement. But scientists traditionally have regarded this area as a province of hard, deformed rocks without an organized structure.

COCORP also has found evidence in Texas of stratified regions in the basement, suggesting these structures may be part of one massive complex, says Brown. Researchers say the layered areas must have formed more than 1.3 billion years ago, but seismic work alone cannot reveal whether the rocks are sedimentary or volcanic in origin.

Since sedimentary structures are the bearers of oil and natural gas, these basement layers could represent a new source for fossil fuels. Conversely, if they are volcanic, the rocks are relics of anicent volcanic eruptions that were previously unknown to geologists. The only way to be sure is to drill into the structures, says Brown. In the meantime, COCORP will continue working from the surface to determine the size of the basement strata.