A library of ancient disasters
The cliffs between Zumaia and Deba on the Basque Coast are a geological record, and some of its most dramatic books tell stories of ancient tsunamis. These cliffs are composed of a formation called flysch—a rhythmic, alternating sequence of hard sandstone or limestone and soft marl or clay layers. This entire sequence, more than 5,000 meters thick in total, was deposited on a deep sea floor between 110 and 50 million years ago. Tectonic forces associated with the formation of the Pyrenees later tilted these horizontal layers, lifting them out of the sea and exposing them to the elements.
Within the younger layers of sediment from the Holocene epoch, scientists have identified five distinct, unusually thick layers known as megaturbidites. These are not the normal, thin flysch layers. They are the deposits left by massive, violent underwater landslides. Evidence suggests these submarine slope failures were triggered by powerful earthquakes on the Cantabrian continental slope. Each landslide generated a huge turbidity current—a dense and fast-moving slurry of sediment and water—that spread across the seabed. The resulting deposit also shows the generation of a tsunami at the sea surface. Radiocarbon dating of materials within these layers confirms that five such tsunami-generating events have occurred in the last 11,000 years.