Photo/IllutrationOverseas researchers study strata of rock called the "Chiba section" in Ichihara, Chiba Prefecture, in August 2015. (Asahi Shimbun file photo)

  • Photo/Illustraion

Strata of rock discovered in Chiba Prefecture could be recognized as a boundary on the international geologic time scale, which would be a first for Japan.

Japanese scientists have suggested calling the geologic age “Chibanian” after the strata or rock in question found in the city of Ichihara.

The Chibanian age ran between about 126,000 years and 770,000 years ago.

The Chiba strata and two candidates from Italy were in the running to be chosen as the Global Boundary Stratotype Section and Point (GSSP) for the age.

The trio were assessed by a lower committee of the International Union of Geological Sciences (IUGS), consisting of 16 experts from nine countries, and by Nov. 13, the panel had voted for Chiba over its Italian rivals as it clearly shows evidence of geomagnetic reversal.

In geology, Earth’s geological history is classified into 115 stages, which are based on the age of rocks and of fauna found as fossils.

To define a standard of boundaries between ages, IUGS has been using one GSSP for the lower boundary of each age. Most of the age names have already been selected, but there are some that remain without a name. Many were selected from strata in Europe and none in Japan until now.

The strata, called the "Chiba section," was found along the Yorogawa river in the Tabuchi district of Ichihara. It consists of volcanic ash that is 770,000 years old.

The Chiba section has already attracted international academic interest as it is in a rare location where strata from 2.4 million years to 500,000 years ago can be readily observed and evidence of the last geomagnetic reversal can be examined.

The magnetic poles of Earth have repeatedly switched throughout the planet’s history and rocks with magnetic property can reveal the orientation of the magnetic poles at the time they were formed.

The last geomagnetic reversal is now considered to have occurred 770,000 years ago, a finding made two years ago by a research team led by Yusuke Suganuma, associate professor of ancient geology at the National Institute of Polar Research. The team analyzed the sediments in the Chiba section and found clear indications as to when the phenomenon occurred.

The final decision for the GSSP, as well as the naming of the geologic age, will be made after three more assessments by upper committees of IUGS, and is expected to take another one or two years.