Geologist Looks At Mystery Of Ancient Sea: Did The Iapetus Ocean Ever Exist?

During a presentation Wednesday, Oct. 22 at the annual meeting of the Geological Society of America in Salt Lake City, Huff will present evidence from ancient volcanic ash beds which he believes demonstrates a narrow Iapetus Ocean did exist during the Ordovician but closed off and disappeared by the Silurian 420 million years ago.

"If you look at the younger beds 420 million years ago there is no more Iapetus Ocean," said Huff. "The question is whether there ever was one."

The best way to determine whether the ocean existed is to look at the land masses which would have surrounded it. If large land masses were locked together, there would have been no room for an Iapetus Ocean to exist. If they were spread farther apart, the ocean would have filled the gap between ancient continents.

There are three basic tools geologists have to determine how the ancient continents were spread out over the globe: paleo- magnetic data, the fossil record, and the ash beds left by massive, explosive volcanism.

Paleo-magnetic data is difficult to interpret during the time period in question. The fossil record indicates strong similarities between what is now Texas and the southern Applachians and a portion of Argentina known as the Precordillera. Those similarities have led some geologists to argue that ancient North and South America were smashed together side by side. That would leave no room for the Iapetus Ocean.

Huff and his collaborators have examined dozens of ancient volcanic ash beds, including 30 in the Argentine Precordillera. They find no similarities between ash beds in North and South America.

"If two land masses were adjacent, the volcanoes should leave their mark on both continents," explained Huff. You ought to be able to find those ash beds and match them up. The problem is, when we go looking for volcanic ash beds down there of the same age as in the Appalachians, we don't find them."

So, did the Iapetus Ocean exist? Huff says yes. "Based on our data, it was roughly 1,000 kilometers wide..about 600 miles."

More important, Huff finds no evidence that North and South America were ever side by side. He explains the fossil similarities, using evidence collected by UC geology graduate student Maria Prokopenko. The evidence indicates part of North America broke away and eventually reassembled as part of South America. The fossils weren't formed in South America. They were carried there.

"Maria is showing something rather interesting which we didn't know before. The volcanoes change their compositions during the time in which a whole series of these ash beds were being formed. That indicates the continent was shifting positions at the time."

Huff believes the process is very similar to the modern day South Pacific where volcanic islands like Borneo and Sumatra are being driven toward Asia by plate tectonics.

Huff's collaborators include Dennis Kolata of the Illinois State Geological Survey, Stig Bergstrom of Ohio State University and Argentine geologists Carlos Cingolani and Ricardo Astini.