Geohistory 2. Sea of Japan Formation Period
26–6 million years ago (Paleogene–Neogene of the Cenozoic era)
Due to volcanic activity that began around 26 million years ago, the Oki Islands and the Japanese archipelago gradually began to separate from the Eurasian continent. The ground that was stretched out became a depression, which filled with water, creating a large lake. At that time, the land that would later form the Oki Islands was at the bottom of that lake.
Afterwards, the Japanese archipelago continued to move apart, and seawater entered the place where the lake was, resulting in the birth of the current Sea of Japan. At that time, the land that would later form the Oki Islands was at the bottom of the deep sea.
A crocodilian fossil found in green tuff
Widely distributed along the coast of the Sea of Japan, green tuff is a stratum that formed in rivers and lakes about 20 to 15 million years ago. It is called green tuff because it is often green in color due to the volcanic ash contained in it.
In the early summer of 2008, a university student who visited the Oki Islands for a geological survey discovered a crocodilian tooth fossil in this stratum. Fossils such as snails and freshwater bivalves had already been found at the site, but this was the first time the fossil of a crocodilian's tooth was found.
A shark's tooth fossil found in diatomaceous earth
Diatomaceous earth is formed by the hard shells of diatoms, a type of microscopic algae, which accumulate at the bottom of bodies of water. Diatoms inhabit most water environments on Earth.
This stratum can be found in the Oki Islands overlaying the green tuff stratum.
A fossil of a shark's tooth has been discovered from this stratum from about 16 million years ago. Based on the fact that sharks, which are deep-sea creatures, lived there, it is thought that as the Japanese archipelago continued to separate from Eurasian continent, seawater flowed into the depression between them. At that time, the land that would later form the Oki Islands was at the bottom of the deep sea.