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.
Reading the stone book
Geologists can distinguish these tsunami deposits from the regular, thin layers of the flysch. A megaturbidite is significantly thicker and more chaotic, representing a single, high-energy event rather than slow, periodic sedimentation. The base of a megaturbidite layer often shows evidence of scouring, where the force of the turbidity current has ripped into the pre-existing seabed.
By studying these five layers, researchers can reconstruct a history of seismic hazards in the Bay of Biscay. The recurrence interval for these major tsunami-generating earthquakes averages around 2,200 years. This geological record provides a much longer-term perspective on tsunami risk than historical records can offer. The layers show that while infrequent, large tsunamis are a natural part of this coastline's geological history. The entire area is now recognized as the Basque Coast UNESCO Global Geopark, protecting its immense scientific value. The flysch layers here are so continuous and well-preserved that they also document other major Earth events, including the asteroid impact that led to the extinction of the dinosaurs 66 million years ago.