Optically Stimulated Luminescence Dating of Gravel Layers in the Western Musashino Uplands, Tokyo, Japan

Abstract

Optically Stimulated Luminescence (OSL) dating from minerals (mostly quartz and feldspar) can determine the burial duration since the last reset (bleaching) due to sufficient sunlight exposure. Although OSL dating is based on the fact that minerals are bleached at the time of deposition, complete bleaching does not always occur. Incomplete bleaching of minerals before deposition can therefore cause age overestimation.

OSL dating has the potential to determine the depositional age of fluvial gravel layers, which is important for validating previous dating exercises and accurate age estimation. However, fluvial gravel layers undergo sunlight exposure during transport and deposition by strong currents such as floods and debris flows. Fluvial gravel layers are therefore considered to undergo poor bleaching due to the short transport times and water turbidity. In addition, fluvial settings in Japan can cause problems in OSL dating of minerals due to short transport distances from their sources. This study aimed to evaluate the applicability of OSL dating using postinfrared infrared-stimulated luminescence (pIRIR) of alkali-feldspar from fluvial gravel layers in Japan. We conducted K-feldspar pIRIR dating of fluvial gravel layers from the Enoki Trench, western Musashino Uplands, Tokyo, Japan.

Our pIRIR dating results showed wide scatter in equivalent doses due to partial bleaching. Therefore, mean equivalent doses caused age overestimation of fluvial gravel layers. However, pIRIR dating using the Minimum age model (MAM) provided consistent age estimates of the fluvial gravel layers (Tachikawa gravels), which had previously been dated using tephra and radiocarbon dating. The pIRIR dating provided the first age results for the buried fluvial gravel layers (Ome formation) under the Tachikawa gravels in the Musashino Uplands. Our data suggested that the Ome formation had been deposited at least some 241 ka ago. The results of pIRIR dating using the MAM indicated the usefulness of estimating the depositional age of fluvial gravel layers.