Rock formations are significant for geologists, and formation names help keep track of time and events in Earth history. A new name – Frenchman Mountain Dolostone – has been added to a key interval of rocks in the Grand Canyon.

Frenchman Mountain is a large mountain on the east side of the Las Vegas Valley. Many locals mistakenly call it Sunrise Mountain (which is actually the name of the lower peak behind nearby Nellis Air Force Base). Even though Frenchman Mountain is 60 miles from the mouth of the Grand Canyon, an important connection exists between these two geologic features, and geologists have now formally linked them together.

In a study published in the May 2020 edition of the journal Geology, a team of geologists from several western universities and museums, including a researcher from UNLV, have redefined a famous interval of rocks in the Grand Canyon – the Tonto Group – by adding to it the name Frenchman Mountain Dolostone.

Steve Rowland, an emeritus professor of geology at UNLV and a co-author of the paper, and former graduate student Slava Korolev proposed the name Frenchman Mountain Dolostone several years ago. They traced this Cambrian-age layer of rocks from one mountain range to another across the Lake Mead region and deep into the Grand Canyon.

“We’ve known for a long time that Frenchman Mountain was dragged westward several tens of miles from the Grand Canyon region when this part of North America was pulled apart about 15 million years ago,” Rowland said. “So we weren’t surprised to discover that our dolostone layer is correlative with part of the Tonto Group. The surprise is that the interval within the Tonto Group where it belongs has never been formally named, so now it has a proper name."

The research paper now redefines the Tonto Group to include layers above and below that were poorly understood. The Sixtymile Formation is added to the bottom and the Frenchman Mountain Dolostone to the top, and it also adds new radiometric dating.

According to Rowland, the new dates have global ramifications because the new age of 508 to 497 million years old for most of the Tonto Group is much younger than previously thought. This indicates that seas flooded North America very quickly in the Cambrian Period. The new Tonto Group dating has also recalibrated the international Cambrian timescale by adding a key location where the time of extinctions and appearances of fossil groups is precisely and accurately dated.

“It’s a rare privilege to be able to add a new formation name to the geology of the Grand Canyon. That hasn’t happened for a long time,” Rowland said.

The research team includes investigators from Boise State University, University of New Mexico, Utah State University, UNLV, Denver Museum of Nature and Science, and the Museum of Northern Arizona.

The team of scientists is now looking to new technologies to more precisely date the rock layers, including a meter-by-meter study of how carbon-isotope values changed over time. The team is working to characterize and date layers above and below key fossil layers throughout the succession in the Grand Canyon and all across the western United States.

Rowland says this work will help to refine the precise timing of unconformities and depositional episodes, and test hypotheses about how marine flooding episodes relate to global biologic, tectonic, and ocean/atmosphere changes.

Publication Details

“Redefining the Tonto Group of Grand Canyon and recalibrating the Cambrian time scale” appeared in the May 2020 issue of Geology.