The asteroid impact that slew the dinosaurs may have also indirectly sculpted the most important ripple marks ever found on Earth.
A series of ridgelike structures quite three stories high and spaced nearly two Eiffel Towers apart appear to be buried about 1,500 meters beneath central Louisiana. The oversized features are mega ripples shaped by a huge tsunami generated by the Chicxulub asteroid impact, researchers report within the Sept. 15 Earth and Planetary Science Letters.
“It’s just interesting that something that happened 66 million years ago might be so well preserved, buried 5,000 feet down within the sediments of Louisiana,” says geologist Gary Kinsland of the University of Louisiana at Lafayette.
Ripple marks are repeating sequences of ridges typically found on sandy beaches or stream bottoms that form as wind or water flows over and moves loose sediment. But ripple marks on the beach are often centimeters tall, while the structures found by Kinsland’s team have a mean height of 16 meters and are spaced about 600 meters apart.
The marks’ shape, size, orientation, and site suggest that they formed after the Chicxulub asteroid crashed into what’s today’s Yucatan in Mexico, generating a tsunami that washed across the sediments of the Gulf of Mexico and over what’s now Louisiana, which was underwater at the time. Despite the tsunami’s width, nobody has ever found ripple marks formed by the wave before.
Geologist Kaare Egedahl initially discovered the newly described ripples while checking out coal deposits. Studying at the University of Louisiana at Lafayette at the time, Egedahl had been combing through seismic reflection data – 3-D images of buried rock and soil generated by underground sound waves — provided by the Devon Energy company. Egedahl, now at the oil and gas service Cantium, found the ripples atop a layer of rock thought to possess formed from debris surprised by the Chicxulub asteroid impact. He then shared his finding with Kinsland.
“I knew where that layer was from in geological time, and that I knew what happened there,” Kinsland says. “I knew there should be a tsunami.”
The supposed ripple marks were preserved all this point because of the depth at which they formed underwater, Kinsland says. Other studies suggest that the region of present-day Louisiana during which the ripples took shape was 60 meters below the ocean surface at the time. At that depth, the ripples would are beyond the reach of tumultuous storm activity that would have erased them. Then, over many years, the marks were slowly buried by other sediments.
A smaller, analogous set of structures may exist off the East Coast of Japan. There, a repeating sequence of underwater dunes was reported to possess appeared after the 2011 Tohoku earthquake and tsunami. Those dunes look nearly just like the ripple marks buried beneath Louisiana, apart from their size, Kinsland says, supporting the notion that the taller structures were also produced by a tsunami, though one among a way larger magnitude.
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Still, there’s contention over whether the features beneath Louisiana really are mega ripples formed by the Chicxulub tsunami.
“It’s hard to ascertain how such a high-energy event could form ripple marks because they’re usually related to much calmer environments,” says sedimentologist Pedro J.M. Costa of the Universidade de Coimbra in Portugal. And ripple marks typically form from frequent and recurring wave motion, while tsunamis don’t have many waves, he explains. Costa, who is an expert on tsunami deposits, says that reconstructing the lay of the seafloor at that point of the impact and conducting experiments could help unravel the origins of the structures found by Kinsland’s team.
This new work is vital because it opens a discussion, Costa says. “Maybe [the Chicxulub impact was] such a high-magnitude event that what we see in normal tsunami events don’t apply to the present one.”
