Finding of the Earliest Jomon Site from O-shima Island, Izu Islands, and Its Significance as a Time Marker of the Volcanic Activity

Abstract

Fragments of earthenwares of the Earliest Jomon age are found on a sea-cliff at Shimotakabora on the west coast of O-shima Island, Izu Islands (34°44.3′N., 139°21.6′E.). They are unearthed from the upper brown weathered ash of a unit layer constituting pyroclastic fall deposits exposed there. Associated with them, are found obsidian flakes, an angular block of biotite rhyolite pumice, several round pebbles of compact basalt, and a small amount of charred wood and bone fragments.All the earthenware fragments are identified to be of the Hirasaka type of the Earliest Jomon age whose type locality is in the Miura Peninsula, South Kanto. Two samples of the obsidian flakes have been determined by fission track method to have the same uranium content and age as obsidian exposed on Kozu-shima Island about 60km south-southwest of O-shima. The biotite rhyolite pumice block is, in petrographic characters, similar to the biotite rhyolite from any one of Nii-jima, Shikine-jima or Kozu-shima Islands, all of which lie to the south-southwest. These obsidian and rhyolite pumice were brought by the Earliest Jomon (Hirasaka) men to O-shima Island for making living tools.On the sea-cliffs at Onoue and Tatsunokuchi to the south of Shimotakabora, earthenware fragments of the Kayama, Kijima plus Sekiyama, Moroiso (?) and Odoriba types have been unearthed from the particular stratigraphic levels of superimposed pyroclastic deposits. The earthenware fragments of the Hirasaka type are found at the layer lying nine fall units below the level which contains the earthenware fragment of the Kayama type about seven thousand years old. As the time represented by a unit pyroclastic layer is thought to be a hundred and several tens of years, the earthenwares of the Hirasaka type may be brought to O-shima eight to nine thousands of years ago. This estimated age well coincides with that given so far to the type.