A Record of Ancient Upheaval
The striking coastal cliffs of the Miura Peninsula contain a dramatic story of ancient geological turmoil. The contorted rock layers visible here are part of the Hayama Group, the oldest geological formation on the peninsula. Dating back to the Early to Middle Miocene epoch, between 15 and 18 million years ago, these rocks were once soft sediments on the deep ocean floor. The patterns are slump folds, the result of massive submarine landslides.
At that time, this region was part of a deep sea basin associated with a subduction zone, where the Philippine Sea Plate was beginning to dive beneath the Eurasian Plate. The Hayama Group is an accretionary complex, a wedge of sediment and rock scraped off the descending oceanic plate and plastered onto the continental plate. Tectonic activity created unstable slopes on the seafloor. Earthquakes, common in such an environment, likely acted as triggers, shaking the water-saturated layers of sand and mud and sending them sliding downhill. As the mass of sediment moved, the layers within it folded and buckled, creating the complex shapes now frozen in stone. These are not the folds of hard rock bent by immense pressure over millions of years; they are the instantaneous contortions of a soft, slumping mass.
Reading the Slump
Geologists study these slump folds as indicators of the past environment. The rock here is primarily an alternating sequence of sandstone and mudstone, with interbedded layers of tuff (volcanic ash). The direction in which the folds are overturned, a feature known as vergence, points directly down the original slope of the seafloor, or paleoslope. By measuring the orientation of these folds along the coastline, scientists can reconstruct the underwater topography from millions of years ago.
The layers themselves tell a story of rapid deposition. The alternating sandstone and mudstone beds are characteristic of turbidites—sediments deposited by dense, fast-moving underwater currents that cascade down submarine slopes. The presence of tuff layers indicates nearby volcanic activity, a common feature of the Izu-Bonin-Mariana Arc system with which this region is associated. Excellent examples of these slump folds can be seen along the Akiya coast, particularly around Tateishi. Here, the hardened layers of the Hayama Group have been eroded by the waves, exposing clear cross-sections of the ancient submarine landslides. Fossils of deep-sea chemosynthetic communities, including solemyid bivalves, have also been found within the Hayama Group, showing this deep-marine environment.