Sea floor offers evidence of meteorite fall in dinosaur age
The Mesozoic Era, often called “the Age of Dinosaurs”, ended 66 million years ago when a 6-mile-wide meteorite crashed in the Gulf of Mexico. The impact created a 120-mile-wide crater called Chicxulub, after a small village on the crater’s edge 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 risen 5,000 feet high, almost a mile.
Tsunami deposits, consisting of thick layers of coarse sand and rippedup 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 impact theory was proposed.
The tsunamis must have also created large ripples on the sea floor. A recent research paper documents the first ones discovered. They were found by analyzing 3D seismic data obtained by companies looking for oil. In seismic oil exploration, sound waves are sent down into the earth and they bounce back from the layers below. It is sort of like taking a sonogram of an unborn baby.
The ripples were discovered deeply buried beneath 5,000 feet of rock in north-central Louisiana. Sea level was much higher at the time of the impact, so the Gulf extended much farther north, and that area was under about 200 feet of water. The ripples occur where there was an abrupt shallowing of the ancient Gulf.
The tsunamis arrived about an hour after the earthquake and continued for hours to days as water sloshed back and forth in the Gulf — like water in a bathtub — creating the giant ripples on the sea floor. They lasted because they formed below the reach of storm waves, which in the Gulf is about 160 feet.
After things settled, the ripples were covered by a thin layer of dust thrown up by the impact — dust containing iridium, the chemical signature of a meteorite, then buried by deep water shales.
Where sediments deposited before the impact rested on those deposited right after it was a surface that reflected the sound waves very sharply. That boundary, called the K-PG boundary, marks the end of the Cretaceous Period.
It used to be called the K-T 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 boundary because the long Tertiary Period has been divided into two shorter periods, an earlier Paleogene (Pg) and a later Neogene.
The K-PG ripples are asymmetrical, steeper on the north, showing the waves came from the south where the meteorite hit. They are very similar to ones found in Kesennuma Bay, Japan, discovered after the March 11, 2011, Tohoku earthquake, ripples that were not there in a survey taken a few years earlier.
They are also much larger, with an average height of 52 feet and an average distance between their crests of almost 2,000 feet, the largest ripples ever found on Earth. Originally they must have been even taller, having been compacted by overlying sediments for 66 million years.
Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University.
gnidovec.1@osu.edu
They were found by analyzing 3D seismic data obtained by companies looking for oil. In seismic oil exploration, sound waves are sent down into the earth and they bounce back from the layers below. It is sort of like taking a sonogram of an unborn baby.