A 500 million year old rock layer in the Grand Canyon has finally been given a name: Frenchman Mountain Dolostone.

The stupendous gorge is visited by a huge number of admirers every year. Therefore, it would stand to reason that all of its rock layers had been investigated and given names. However, you’d be wrong.

A UNLV-led research team describes how they identified and gave a 500 million-year-old Grand Canyon formation a name in a new report that was published this spring in the journal Geosphere: Dolostone Mountain’s Frenchman

For millennia, geologists had not given the newly designated rock layer a name or conducted in-depth research on it, despite the fact that it had been plainly visible throughout the Grand Canyon.

The UNLV research group named it the Frenchman Mountain Dolostone (FMD), after a similarly named mountain that lies contiguous to Las Vegas, Nevada. The FMD is the thickest, most comprehensive, and easiest to study there. Through scientific investigation, the researchers were able to narrow down the age of this stratigraphic interval and its connection to Grand Canyon strata.

“We have finally been able to resolve this issue thanks to the establishment of thorough descriptions and thickness measurements of the strata at Frenchman Mountain and also in the Grand Canyon.”

Lead author Steve Rowland, an emeritus professor of geology at UNLV

Lead author Steve Rowland, an emeritus professor of geology at UNLV and paleontologist at the Las Vegas Natural History Museum, stated, “For decades, geologists were unable to precisely correlate the succession of strata at Frenchman Mountain with those in the Grand Canyon, in part because Frenchman Mountain was tectonically displaced about 40 miles to the west since the rocks were deposited.” We have finally been able to solve this issue thanks to precise descriptions and thickness measurements of the strata at Frenchman Mountain as well as in the Grand Canyon.

Officer Ravine’s (Arizona) openness, with stratigraphic units named Frenchman Mountain Dolostone here is 117m thick. Credit: Stephen Rowland/UNLV

According to Rowland, the FMD is over 1,200 feet thick at Frenchman Mountain, but it dramatically thins to the east. The bits uncovered inside Fantastic Gully range in thickness from almost 400 feet close to the “West Edge” Skywalk to under 100 feet in Marble Gorge, in the eastern piece of Amazing Gulch Public Park.

Edwin McKee, a geologist, recognized the cliff-forming interval of rocks just above the well-known Muav Formation in 1945, but he did not officially name it. Since there are no fossils in the FMD, McKee was unsure of its age. The ratio of stable carbon isotopes was used by Rowland’s team to determine the FMD’s age—subtle differences in this ratio As the layers were deposited, there were simultaneous shifts in these isotope ratios everywhere on Earth.

The researchers compared the changes found in precisely dated rock layers from other parts of the world to those found in the Frenchman Mountain strata. According to the findings, the newly given name formation was formed between 502.8 million and 495.5 million years ago, during the Cambrian Period.

The FMD is the primary new development to be named in the gorge, starting around 1985, when the Unexpected Gulch Arrangement was named. It is likewise the principal rock layer uncovered in the Fantastic Ravine to be named for an area outside the Great Gorge locale.

Slava Korolev, a former graduate student at UNLV, Denver Museum of Nature and Science geologist James Hagadorn, and UNLV mathematics professor Kaushik Ghosh were also on the research team.

More information: Stephen M. Rowland et al, Frenchman Mountain Dolostone: A new formation of the Cambrian Tonto Group, Grand Canyon and Basin and Range, USA, Geosphere (2023). DOI: 10.1130/GES02514.1

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