Monday, 26 May, 2025/ Edition 60

Top-notch superlatives like the first and the oldest attract our attention because they signify a breakthrough or new record. This edition of Core Elements looks at some recent studies examining the “oldest geology” in the United States.

Rasoul Sorkhabi

Editor, Core Elements

America’s Oldest Rocks and Xenoliths

Basement map of the contiguous United States, showing locations of candidate oldest rocks with Wyoming Province boundaries. GLTZ— Great Lakes tectonic zone; MN R.—Minnesota River subprovince. From Frost et. al 2025, GSA Today/ The Geological Society of America

What and where are the oldest rocks in the United States? An article in GSA Today answers these questions.

Study design: The researchers focused on three potential areas:

1. The Sacawee gneiss in central Wyoming

2. The Morton gneiss in the Minnesota River Valley

3. The Watersmeet gneiss dome in the upper peninsula of Michigan

They applied U-Pb zircon dating systematics to samples from these areas.

Findings:

• Age data for Wyoming includes a concordia age of 3,452±3 Ma for the Sacawee gneiss; however, data for a nearby tonalite gneiss points to 3,822±4, which researchers interpret as zircon xenocryst.

• The Morton gneiss yielded three age clusters of 3,516 Ma, 3,360 Ma, and 2,595 Ma (younger zircon rim growth).

• The Watersmeet gneiss was dated as 3,623±4 Ma; however, dating a nearby tonalite gneiss gave an older age of 3.8 billion years.

Conclusion: The researchers selected the Watersmeet gneiss at 3.6 billion years as the oldest known rock in the United States, but they also acknowledged that there were zircon xenocryst indications of an older continental crust at 3.8 billion years.

Three-Billion-Year Geologic Record in Montana Xenoliths

Another study describes geological activities spanning three billion years in xenoliths collected from Montana.

Geological location: Researchers from the University of California, Santa Barbara collected samples from two volcanic centers—Big Slide and Robinson Ranch—within the Great Falls tectonic zone.

• The Wyoming Craton (older than 2.5 billion years old) and Medicine Hat block (older than 2.6 billion years) are Archean blocks in central Montana separated by the Great Falls Tectonic Zone.

• The entire region experienced magmatic activity during the Laramide orogeny.

Study design:

• Samples were garnet- and aluminosilicate-bearing xenoliths, including metaigneous rocks and metapelites (metamorphosed fine-grained sediments).

• The researchers utilized U-Pb dating, geochemical analysis, petrological observations, and electron probe microscopy.

Findings:

• U-Pb dating of rutile indicated that the host rock erupted at 46 Ma for Big Slide and 51 Ma for Robinson Ranch by alkalic volcanism during the Laramide orogeny.

• Mineral thermobarometers calculated for mafic granulites yielded peak pressure and temperature conditions of 1.8–2.3 gigapascal and 890–1,000 degrees Celsius. Their composition indicates they originated as arc magmatic cumulates.

• Metapelites showed peak metamorphic conditions of 1.3 gigapascal and 775 degrees Celsius.

• U-Pb dating of igneous core of detrital zircon crystals showed Archean ages as old as 3,300 Ma and 2,700 Ma.

• U-Pb zircon and monazite ages from a metapelite from Robinson Ranch document a metamorphic event at 2,200–2,000 Ma.

• Metapelites from the Big Slide show a transition from detrital igneous zircons to metamorphic zircon growth at 1,800 Ma.

  • Researchers attributed this transition from arc magmatism to continent-continent collision to the collision of the Wyoming and Medicine Hat blocks at 1,800 Ma along the Great Falls suture zone.

Why it matters: The study shows how modern analytical techniques can detect events in lower crustal xenoliths dating back to the Archean.

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Earliest Mayfly and Dinosaur Fossils in the United States

Wikimedia Commons 

All fossils are precious gifts from deep geologic time, but my favorite fossils are those preserved in amber.

Cretaceous Mayflies in North Carolina 

A recent article in Fossil Records describes a baetid fossil from an amber collected in North Carolina.

Rare fossils: Baetidae are a family of mayflies with about 1,000 known species today. They are found on all continents except Antarctica; however, finding these fossils preserved in amber is very rare.

• Baetidae fossils were first discovered in 1856 from Eocene-age Baltic amber.

• The oldest baetidae fossils of Cretaceous age were reported in 1997 from Lebanese amber (129–125 Ma) and Burmese amber (100–94 Ma).

• Miocene-age baetidae fossils were reported from Mexican amber (23–16 Ma) in 2022.

What’s new: V. E. Krynicki collected the new fossil from an outcrop of the Tar Heel Formation (about 83 Ma). It is archived at the American Museum of Natural History.

Researchers who examined the fossil included it in the genus Petracloeon carolinensis. It is a female with a body length of 1.9 millimeters.

Why it matters: Baetidae have a short life span and depend on freshwater habitats. For this reason, the authors hypothesize that baetidae probably originated prior to the Jurassic fragmentation of Panagea and that transoceanic dispersal played a minor role.

The Oldest American Dinosaur

The dinosaurs appeared during the Carnian epoch (237–227 Ma) in the Late Triassic period. Most of the dinosaur fossils from this epoch have been found in the southern Gondwana supercontinent: Brazil, Argentina, Zimbabwe, and India.

In a new study, researchers uncovered fossils of the dinosaur species Ahvaytum bahandooiveche (named from the Native American Shoshone word meaning “long ago–water’s young handsome man”) from the Carnian-age Popo Agie Formation in western Wyoming.

Key findings:

• U-Pb dating of detrital zircons in the formation yielded a maximum depositional age of 230 Ma.

• Wyoming during the Late Triassic was positioned close to the equator.

• The Popo Agie Formation was deposited in a fluvial to lacustrine environment.

• The formation has also yielded important fossils of reptiles and amphibians.

Why it matters:

• The only dinosaur fossil of the Carnian epoch (about 221 Ma) in North America comes from the Dockum Group in west Texas.

• This new find indicates that the dinosaurs did not necessarily originate in Gondwana.

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✉️ To get in touch with Rasoul, send an email to editorial@aapg.org.