Frenchman mountain dolostone: 500 million-year-old grand canyon rock
layer finally gets a name
Geologists name ancient rock layer after Las Vegas mountain that contains similar strata

Date:
May 3, 2023
Source:
University of Nevada, Las Vegas
Summary:
A research team outlines how it identified and bestowed a moniker
upon a previously unexplored 500 million-year-old Grand Canyon
formation: The Frenchman Mountain Dolostone. The newly named rock
layer has lain hidden in plain sight throughout the Grand Canyon
for millennia, but -- until now -- geologists had not named it or
studied it in detail.


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FULL STORY ==========================================================================
The Grand Canyon is one of the Seven Wonders of the World, visited by
millions of admirers each year. So, naturally, you'd think that all of
its rock layers had been studied and named. But you'd be wrong.

In a new report published this spring in the Geological Society of America journal Geosphere, a UNLV-led research team outlines how it identified
and bestowed a moniker upon a previously unexplored 500 million-year-old
Grand Canyon formation: The Frenchman Mountain Dolostone.

The newly named rock layer has lain hidden in plain sight throughout
the Grand Canyon for millennia, but -- until now -- geologists had not
named it or studied it in detail.

The UNLV research team named it the Frenchman Mountain Dolostone (FMD)
-- after a similarly named mountain that lies adjacent to Las Vegas,
Nevada. That's where the FMD is thickest, most complete, and most
accessible for study.

Through scientific detective work, the researchers were able to narrow
down the age of this stratigraphic interval and its relationship to
strata in the Grand Canyon.

"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," said lead author Steve Rowland,
an emeritus professor of geology at UNLV and paleontologist at the Las
Vegas Natural History Museum. "Establishing detailed descriptions and
thickness measurements of the strata at Frenchman Mountain and also
in the Grand Canyon has finally allowed us to solve this problem."
The FMD is over 1,200 feet thick at Frenchman Mountain, Rowland said,
but it thins dramatically toward the east. The portions exposed within
the Grand Canyon range in thickness from nearly 400 feet near the "West
Rim" Skywalk to less than 100 feet in Marble Canyon, in the eastern part
of Grand Canyon National Park.

In 1945, geologist Edwin McKee distinguished -- but did not formally
name - - the cliff-forming interval of rocks that occur just above the well-known Muav Formation. The FMD contains no fossils, so McKee was
unsure of its age.

Rowland's team used a relatively new technique to determine the
FMD's age - - subtle differences in the ratio of stable isotopes of
carbon. Fluctuations in the ratios of these isotopes occurred at the same
time all over the Earth as the layers were deposited. The researchers
compared fluctuations in the Frenchman Mountain strata with those
identified in precisely dated rock layers elsewhere in the world. The
results indicate that the newly named formation was deposited over an
interval of 7.3 million years, during the Cambrian Period, between 502.8 million and 495.5 million years ago.

The FMD is the first new formation to be named in the canyon since 1985
when the Surprise Canyon Formation was named. It is also the first rock
layer exposed in the Grand Canyon to be named for a location outside
the Grand Canyon region.

In addition to Rowland, the research team included former UNLV graduate
student Slava Korolev, Denver Museum of Nature and Science geologist
James Hagadorn, and UNLV mathematics professor Kaushik Ghosh.

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========================================================================== Story Source: Materials provided by University_of_Nevada,_Las_Vegas. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Rock_layers_including_Frenchman_Mountain_Dolostone ========================================================================== Journal Reference:
1. Stephen M. Rowland, Slava Korolev, James W. Hagadorn, Kaushik Ghosh.

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 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/05/230503200451.htm

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