Hundreds of millions of years ago, a massive earthquake rattled the earth, sending towering waves crashing across an ancient tropical sea that covered part of present-day Canada and the United States. This startling conclusion comes from new research by two University of Saskatchewan scientists, who uncovered startling evidence of ancient tsunamis wreaking havoc in this long-lost seaway.

Today, the Canadian prairie provinces of Saskatchewan and Manitoba are about as far from the ocean as you can get. The thought of a tsunami in this landlocked region might seem absurd. But 445 million years ago, during the Ordovician period, the geography was entirely different. Much of modern-day Saskatchewan, Manitoba, Montana and the Dakotas lay beneath a shallow inland sea known as the Williston Basin.

It was a totally alien landscape, a completely different geography, explains study co-author Mark Sproat, an adjunct professor in the USask Department of Geological Sciences. We were much closer to the equator back then, and sea levels were higher, so we would have been in a tropical, shallow inland sea instead of the temperate grasslands you see today.

Sproat and lead author Brian Pratt visited three sites north of The Pas, Manitoba, where they uncovered evidence of a brief, high-energy disturbance in this ancient seaway that had gone unnoticed by geologists until now.

At the sites, certain sediment beds had been ripped up into pebbles and mixed with clay that must have come from land. The deeper floor of the basin contained no clay at all, meaning it could only have come from shore. We realized we needed a phenomenon that would rip up the seafloor and then somehow bring all this clay back out again, and do it multiple times, Pratt said.

There was no animal or even plant life on land to witness the dramatic events of that day nearly 500 million years ago. But if an observer had been present, they would have seen a harrowing sight unfold.

Ancient faults lurking deep below the Williston Basin suddenly slipped, sending violent shockwaves through the sea. As the shoreline briefly retreated, a monstrous wave gathered force and came crashing back with tremendous power. This megatsunami likely plowed nearly a mile inland across the gently sloping terrain, scouring and eroding the rocky surface. When it finally receded, it dragged tons of clay back into the sea. More waves followed.

Pratt acknowledges that attributing the strange deposits to ancient tsunamis is a “radical interpretation” of the evidence. However, the USask researchers had one advantage over earlier geologists. The sedimentary layers of the Williston Basin in Canada are almost completely obscured beneath the flat plains, limiting past study to occasional natural outcrops, drill cores and highway cuts.

But several new quarries have recently been excavated in northern Manitoba, unlocking more of the basin’s secrets. It was getting into those quarries that opened our eyes, Pratt said. We could see the layering extending laterally over 100 meters or more, and find the same bed in more than one spot. That gave us a three-dimensional perspective that no one had before.

Big storms can produce similar deposits, but Sproat and Pratt ruled that out due to a lack of other telltale signs of regular storm activity. What’s more, the region was too close ancient equator to experience hurricanes.

The new work provides a clearer picture of the forces that shaped a long-lost world – one where early marine life flourished and diversified. The Williston Basin was covered by a really unusual sea at the top of the continent, an environment we don’t have a good modern analogue for, Sproat said. So we have a unique chance here to study geological processes and their impact on ancient ecosystems in a setting that doesn’t exist anywhere else on the planet.

The USask researchers now plan to survey other sites across Canada for signs seismic wave damage to see if tsunamis played an even greater role in Earth’s history than presumed. It’s not a topic that shows up in geology textbooks, Pratt said. I think it’s time for a paradigm shift.


Sources

University of Saskatchewan | Brian R. Pratt, Colin D. Sproat, A tsunami deposit in the Stonewall Formation (Upper Ordovician), northeastern margin of the Williston Basin, Canada, and its tectonic and stratigraphic implications. Sedimentary Geology, vol.457, doi.org/10.1016/j.sedgeo.2023.106518


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