The impact of the asteroids that killed the dinosaurs may also have indirectly sculpted the largest ripple marks ever found on Earth.
A series of rope-shaped structures more than three stories high and spaced nearly two Eiffel Towers apart appear to be buried about 1,500 feet below downtown Louisiana. The oversized features are megaparks shaped by a massive tsunami generated by the impact of the asteroid Chicxulub, researchers report in the Earth and Planetary Science Charts on September 15th.
“It’s interesting that something that happened 66 million years ago can be preserved so well, buried 5,000 feet in Louisiana sediments,” says geologist Gary Kinsland of the University of Louisiana at Lafayette.
Ripple marks are repetitive sequences of ridges that are typically found on sandy beaches or stream bottoms that form as wind or water flows and moves loose sediments. But ripple marks on the beach usually measure inches high, while the structures found by the Kinsland team have an average height of 16 meters and are spaced about 600 meters apart.
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The shape, size, orientation and location of the marks suggest that they formed after the asteroid Chicxulub collided with what is now the Yucatan Peninsula in Mexico, generating a tsunami that crossed the sediments of the Gulf of Mexico and on which today is Louisiana. underwater at the time (SN: 02/11/17). Despite the width of the tsunami, no one has ever found ripple marks formed by the wave.
Geologist Kaare Egedahl initially discovered the newly described waves while searching for coal deposits. Studying at the University of Louisiana at Lafayette at the time, Egedahl was combing seismic reflection data (3D images of buried rocks and soil generated by underground sound waves) provided by the company Devon Energy. Egedahl, now in the oil and gas company Cantium, found the waves on top of a layer of rock that is believed to have formed from debris shaken by the impact of the asteroid Chicxulub. He later shared his finding with Kinsland.
“I knew where that layer was in geological time and I knew what was going on there,” Kinsland says. "I knew there should be a tsunami."
Kinsland says the alleged ripple marks have been preserved all this time thanks to the depth at which they formed underwater. Other studies suggest that the region of present-day Louisiana in which the waves took shape was 60 meters below sea level at that time. At that depth, the waves would be out of reach of a tumultuous storm activity that could erase them. Then, over millions of years, the marks were slowly buried by other sediments.
On the east coast of Japan there may be a smaller, analogous set of structures. There, it was reported that a repeated sequence of underwater dunes appeared after the Tohoku earthquake and tsunami in 2011. Those dunes look almost identical to the ripple marks buried under Louisiana, except for their size, says Kinsland, which supports the notion that Higher structures were also produced by a tsunami, although one of much larger magnitude.
Still, it is debated whether the features beneath Louisiana are actually megapairs formed by the Chicxulub tsunami.
“It’s hard to see how such a high-energy event can form ripple marks because they’re usually associated with much quieter environments,” says sedimentologist Pedro J.M. Coast of the University of Coimbra in Portugal. And ripple marks usually form from a frequent, recurring wave motion, while tsunamis don’t have many waves, he explains. Costa, who is an expert on tsunami sites, says rebuilding the seabed at that time of impact and conducting experiments could help unravel the origins of the structures found by the Kinsland team.
This new work is important because it opens a discussion, Costa says. "Perhaps (the impact of Chicxulub was) an event of great magnitude that what we see in normal tsunami events does not apply to this one."