A Pacific Coral Atoll Embedded in Japan

The Akiyoshidai Karst Landscape of Western Honshu

Aug 26, 2009 Gina Barnes

A limestone reef from the Pacific is stranded in western Honshu's geological make-up. How did it get there?

Japanese geology is complex: part volcanic, mostly sedimentary. Among those sediments are oceanic rocks and seafloor deposits, including atoll coral reefs. One coral atoll has been incorporated into the body of Honshu, Japan’s main island, through ancient accretionary tectonics.

The Limestones of Akiyoshidai National Park

Yamaguchi Prefecture, in far western Honshu, hosts a broad area of exposed limestone karst, 130 km² in extent. This is Akiyoshi plateau (Akiyoshi-dai), designated as a Natural Monument by the Japanese government and showcased as a geological park and site museum.

The biggest attraction is the Shuhodo, or Akiyoshi-do, cavern. Caves are a typical feature of karst landscapes: water percolating down from the surface dissolves limestone, and subsurface streams gradually carve out underground hollows. About 1 km of the 10 km of the surveyed system of 440 caves is open to the public, where a blue-ceiling cavern and a series of travertine stepped pools can be seen.

On the surface, the limestone plateau is pitted with conical erosion sinkholes, more formally called dolines. These are exposed because the area of the nature reserve is fired every year to remove tree growth. The vista over the plateau is thus eerily barren compared with the surrounding forested mountains.

To reach Akiyoshidai, take a local bus from the Shin-Yamaguchi station of the western Bullet train line into the mountains of Yamaguchi Prefecture, 300 km from the Pacific Ocean. So why is an oceanic limestone located so far inland?

A Pacific Coral Atoll in the Japanese Islands

The Akiyoshi limestones date back to the Carboniferous-Permian periods (350–250 million years ago) when coral reefs formed atop a subsiding volcanic seamount chain in the far southern Pacific. This chain of dead volcanoes was transported westwards on the oceanic plate towards the area later to become Japan along the Eurasian coast.

About 250 million years ago at the end of the Permian, this palaeo-seamount chain reached the subduction trench in the western Pacific. Typically, such cold volcanic rocks are tipped into the trench and subducted down with the rest of the oceanic plate. But sometimes, the coral reefs that formed atolls around the volcanoes’ summits detach and are bulldozed by the down-going plate into the continental shelf. This is the process of accretionary tectonics, where the continental landmass grows through acquiring new oceanic materials.

The atoll forming Akiyoshidai is only one of several such limestone accretions from the palaeo-seamount chain that are scattered throughout Japan. It is, however, the largest and most intact. The Akiyoshi Accretionary Complex, making up part of Honshu's Permian geological belts, derives its name from this plateau but incorporates chert, shale, sandstone and greenstone as well as limestone.

The Atoll, Folded by a Meteorite Strike?

Surprisingly, the Akiyoshi atoll is folded in half! Following the stratigraphy of coral development, the surface layers are oldest, increase in youth downwards, then begin to age again. This is consistent with the ring-shaped atoll folded over on itself, like a doughnut folded in half.

Scientists have recently ascertained that the folding might have happened out in the ocean, before the atoll reached land. The presence of shocked minerals suggests a meteorite strike, but Y. Miura of Yamaguchi University has proposed in Lunar and Planetary Science XXXV (2004) that the actual folding was caused by a shock wave in the sea. This hypothesis has yet to be confirmed, but the fact that half the limestone layers at Akiyoshi are inverted is incontrovertible.

Limestones in Japan

In other parts of the world, limestones on land are usually a product of uplift of a continental shelf region where warm shallow seas allowed off-shore reef formation. However, almost all limestones in mainland Japan arrived there through accretion, not simple uplift, and they form part of the jumbled sediments that are called Accretionary Complexes. Akiyoshi is unusual in that it is a virtually intact, if deformed, atoll reef. Another pair of atolls, the Daito Islands, are approaching Japan from the southeast. Only about 200 km from the Japan Trench, will they be the next additions to the islands’ geology?

Reference:

Sano, Hiroyoshi and Kanmera, Kametoshi (1988) “Paleogeographic reconstruction of accreted oceanic rocks, Akiyoshi, southwest Japan.” Geology 16.7:600-603.

The copyright of the article A Pacific Coral Atoll Embedded in Japan in Geography is owned by Gina Barnes. Permission to republish A Pacific Coral Atoll Embedded in Japan in print or online must be granted by the author in writing.

Akiyoshi-do Cave Entrance	Karst Landscape at Akiyoshi-dai, Western Japan	Akiyoshi-dai National Park Stele, Yamaguchi, Japan
Dolines and Site Museum at Akiyoshi-dai Park