SIU research shows Mississippi River more than 3 times older than thought

Updated: Oct. 1, 2020 at 4:32 PM CDT
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CARBONDALE, Ill. (KFVS) - A team of SIU researchers led by Sally Potter-McIntyre, an associate professor in SIU’s School of Earth Systems and Sustainability, have discovered that the Mississippi may be more than three times older than previously thought.

In 2014, geologists agreed that the Mississippi River began flowing as early as 20 million years ago, but Potter-McIntyre’s team found evidence the river began flowing about 70 million years ago.

The key to the discovery was the age of zircon fragments in sandstone found in Southern Illinois.

Zircons are very hard and don’t weather away like other minerals, said Potter-McIntyre.

They also contain uranium that functions as a sort of “geological clock.”

“The uranium decays to lead at a steady rate that is not affected by chemical reactions, temperature or heat. So, we can measure the amount of uranium and the amount of the daughter product – lead – and determine when that grain was formed,” she said. “If you have zircons that formed recently from, for example, volcanic eruptions, then you can tell the maximum age of the sample. That is what we were originally hoping for.”

That test can also reveal a range of ages of zircons in the sample.

Sand and sandstones are formed from grains sourced from many different places and if you know the age of those places, you can tell which mountains were feeding sand into the basin, Potter-McIntyre said.

“In the case of these samples, they were statistically most similar to the older rocks in the Illinois Basin, suggesting that the sediments were eroding from Illinois, rather than the Appalachian Mountains,” as geologists more recently had believed.

That meant the area known as Mississippi Delta was already a low spot by the late Cretaceous period, and water was running through to the Gulf of Mexico.

The whole thing started when a graduate student, Jeremy Breeden, became interested in the Cretaceous deposits found at the most southern tip of Illinois.

The deposits were at least 70 million years old and were unusual because the sediments have never been buried and turned into rocks like other sediments that age.

“He was hoping to find some way to refine the age of the deposits in order to better understand the history of the Illinois Basin,” Potter-McIntyre said.

Breeden, along with researchers from the Illinois State Geological Survey, visited outcrops of the sediments, collecting gallon bags of sand.

Breeden then separated the zircons out using a dense liquid that only allowed those denser grains to sink to the bottom.

The team then sent the zircons to the University of Arizona, where there were mounted in epoxy and polished.

Team members then used high-tech scanning equipment to measure the isotope decay in the uranium contained in the samples.

“We then compared these results with both published results and some data given to us by co-author Dave Malone from Illinois State University,” Potter-McIntyre said.

The research findings have led to a vastly new understanding of how our present-day world was formed in North America.

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