Researchers say they have found marks left behind as tsunamis produced by the infamous dinosaur-killing asteroid reached shallow waters off North America 66 million years ago. If they're right, these are the largest such formations on Earth.
Dr Gary Kinsland of the University of Louisiana at Lafayette was involved in two seemingly unrelated projects when an unexpected connection was revealed. He was studying the Chixculub crater that marked the end of the Cretaceous Era. Meanwhile, he was also assigning graduate students to map the presence of coalbed methane in northern Louisiana using ground-penetrating radar that reflects off changes in subsurface composition.
One graduate student, Kaare Egegahl, brought Kinsland a stratal image showing ripple marks at a geological “horizon” where a marked change in physical characteristics occurs.
Years later, after further investigation, those ripples are the subject of a paper in Earth and Planetary Science Letters. They look like those left behind by ordinary waves on sandy beaches – but are 16 meters (53 feet) high and 600 (0.4 miles) meters apart.
From observing what happens when you throw a stone into a pond, it's clear that a miles-wide mass crashing into the Atlantic Ocean will make waves. It's to be expected the enormous tsunamis released by an impact as powerful as at Chicxulub would scour distant coastlines, leaving highly visible marks, particularly on soft ground.
Finding those impressions so many years later is more surprising, but these researchers think the ripples were preserved because they were made not on land, but in what was then just the right depth of water.
“These megaripples are preserved as a result of having formed below storm wave base and being buried by Paleocene deep water shales,” they write. The contours on what was once the sea bed have been buried in almost a mile of sediment and soil since then. Nevertheless, their cause can be identified not only from the horizon's timing, but their direction – pointing straight towards Chicxulub.
Calculating the depth of water at the time – around 60 meters (197 feet) – and the ripple size, the authors calculate the waves responsible could have been up to a mile high, matching previous estimates.
Not everyone is convinced. Sciencenews quotes Dr Pedro J.M. Costa, a sedimentologist at the Universidade de Coimbra, Portugal, saying; “It’s hard to see how such a high-energy event could form ripple marks because they are usually associated with much calmer environments.”
Dr Costa does not have an alternative explanation for the shapes found in the recent paper, however, saying that “Maybe [the Chicxulub impact was] such a high-magnitude event that what we see in normal tsunami events don’t apply to this one.”
The finding comes two years after the announcement of fossils of creatures that appear to have been killed the day of the impact by waves formed in North America's inland sea by seismic waves generated by the impact.