A deep dive into the Yukon River’s ancient history could result in a new name for a rock formation

a group of scientists
Yoshitsugu Kobayashi (far left), Gilbert Huntington (second left), Tony Fiorillo (center) and Paul McCarthy (seated) look over a geologic map of Alaska to figure out where the best opportunity to find a package of 100-million-year-old sedimentary rocks may be. (Emily Schwing/KYUK)

A section of sedimentary rocks along the middle section of the Yukon River could get its own name after a research team spent three weeks this summer digging into the fine-grained details of sandstones and siltstones that line the riverbanks.

The team relied on a geologic map and an academic paper published in 1989 to pinpoint the location of the rocks, which date back to the Early Cretaceous period, almost 100 million years ago.

The rocks are exposed in a number of spots along a more than 100-mile stretch of the middle section of the Yukon River, between Galena and Kaltag. They’re layered like a cake and stacked high, visible in exposed bluffs.

a scientist
University of Alaska Fairbanks geology professor Paul McCarthy said that it has been almost four decades since anyone looked deeply at the sedimentary rocks along a section of the Yukon River between Galena and Kaltag. His findings mean that these rocks could get their own formation name. (Emily Schwing/KYUK)

In some places they’re hard to find, because the state’s geologic map isn’t precise.

”And that’s okay because, you know, if you’re mapping a large area at a big scale, you’re not stopping on every beach and you’re not stopping at every outcrop. And sometimes from 100 yards out in a boat going by they look pretty much the same,” said University of Alaska Fairbanks geologist Paul McCarthy.

McCarthy said that it has been almost 40 years since anyone else has nosed up to the rocks in the way he has this summer.

“It’s one of those Alaskan things, where knowing about this being here and actually being able to get money to come all the way out here to do the work is not that easy,” McCarthy said. “There’s no oil and gas or mineral reason to be out here doing the work, so it makes it even harder to justify something out here for just the pure joy of science.”

Geologists back in the 1980s may also have simply flown over the region in a helicopter. They probably didn’t get out and look at every exposed outcrop. That’s why McCarthy was here this summer.

“I won’t say we’ve measured every outcrop and looked at every beach along this 100-mile stretch,” McCarthy said, “but we’ve looked at most of them. And so we have detailed information and a pretty good handle on what’s here, and what makes it up, and what it looks like.”

McCarthy and colleagues traveled more than 100 river miles along the Yukon. The goal was to understand how dinosaurs lived in Alaska’s Interior during the early Cretaceous. For his part, McCarthy was tasked with defining the ancient reptiles’ preferred habitat.

a group of scientists
A three-man research team spent three weeks exploring more than 100 miles of Yukon river banks this summer to find out more about how dinosaurs lived in the region during the Early Cretaceous. (Emily Schwing/KYUK)

When sedimentary rocks form, they lock in the story of the place. By looking at the rocks up close, McCarthy was able to uncover specific details preserved in the rocks and recreate that story. He said that the landscape during the time of the dinosaurs wasn’t unlike the Yukon River landscape of today.

“Yeah, meandering channels, and oxbow lakes, and vegetated floodplain. That’s exactly what it is,” McCarthy said.

This package of rocks is unique enough to this stretch of the Yukon River that the team believes they may be able to name it. And that would be a first for McCarthy, who has traveled Alaska looking at the state’s rocks for four decades.

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