Seabed offers evidence of meteorite impact during dinosaur era

The Mesozoic Era, often referred to as the “Age of the Dinosaurs,” ended 66 million years ago when a 6-mile-wide meteorite crashed into the Gulf of Mexico. The impact created a 120-mile-wide crater called Chicxulub, after a small village on the edge of the crater on the Yucatan Peninsula.

The impact produced magnitude 11 earthquakes that caused huge underwater landslides all around the northern edge of the Gulf. It also produced tsunami waves that would have reached 5,000 feet high, or nearly a mile.

Tsunami deposits, made up of thick layers of coarse sand and chipped rock, have been found at more than 10 sites around the Gulf from Florida to Texas. Some of them were recognized as tsunami deposits even before the theory of impact was proposed.

The tsunamis must also have created large ripples on the seabed. A recent research paper documents the early findings. They were found by analyzing 3D seismic data obtained by companies looking for oil. In seismic oil exploration, sound waves are sent into the earth and they bounce off the layers below. It’s a bit like taking an ultrasound of an unborn baby.

The ripples were discovered buried deep under 5,000 feet of rock in north-central Louisiana. The sea level was much higher at the time of impact, so the Gulf extended much further north, and this area was under about 200 feet of water. The ripples occur where there was a sudden deepening of the old gulf.

The tsunamis arrived about an hour after the earthquake and continued for hours, if not days, as water moved back and forth across the gulf – like water in a bathtub – creating giant ripples across the sea. seabed. They lasted because they formed under the reach of storm waves, which in the Gulf are about 160 feet.

Once things were sorted out, the ripples were covered with a thin layer of dust raised by the impact – dust containing iridium, the chemical signature of a meteorite, then buried by deep water shales. .

Where the sediments deposited before impact rested on those deposited just after, it was a surface that reflected sound waves very strongly. This limit, called the K-Pg limit, marks the end of the Cretaceous.

It was once called the KT boundary, separating the Cretaceous period (whose geological symbol is K after the German word for chalk) from the Tertiary period. Now it is called the K-Pg limit because the long Tertiary period has been split into two shorter periods, an earlier Paleogene (Pg) and a later Neogene.

The K-Pg ripples are asymmetrical, steeper to the north, showing that the waves came from the south where the meteorite struck. They are very similar to those found in Kesennuma Bay, Japan, discovered after the Tohoku earthquake on March 11, 2011, ripples that were not present in a survey taken a few years earlier.

They are also much taller, with an average height of 52 feet and an average distance between their ridges of nearly 2,000 feet, the largest ripples ever found on Earth. Originally, they must have been even taller, having been compacted by the overlying sediment for 66 million years.

Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University.

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