Optical dating of river terrace sediments from Kanto plains, Japan

Abstract

Optical dating of river terrace sediments has been carried out in a test of the methodology for luminescence age determination of river terrace formation over the last 100 ka. In this study, samples were collected at river terraces, whose ages were estimated by marker tephra geochronology and from a recent riverbed in the Kanto Plain, Japan. Samples from the recent riverbed show a weak OSL signal only and gave young ages. Samples of river terrace sediments for optical dating were collected systematically from the section of about 8 m at the wall of a gravel quarry. The optical ages found corresponded well with those estimated by the age of the marker tephras for samples deeper than 2.5 m. However, the samples from the ground surface to 2.5 m exhibited considerable scatter in the growth curves and gave a wide range of ages, all significantly exceeding expectation. This was attributed to the mixing of different age sediments by inundation and/or human activities. Analyses of OSL from single grains of quartz confirmed that these sediments were composed of grains having a wide range of palaeodoses, consistent with such mixing.