5:15 PM - 6:30 PM
[HQR04-P05] Rock surface dating of granodiorite cobbles obtained from fluvial terrace deposits in the Chichibu Basin, central Japan
Keywords:OSL dating, rock surface dating, fluvial terrace, Pleistocene
Fluvial fill terraces are common in middle reach of Japanese rivers, and they were formed in the last glacial period. Ages of terraces have been investigated mainly based on the tephrochronology, but it may have some problems; abandonment ages of terraces may be underestimated because some part of the tephra deposits might have been eroded, or conversely, may be overestimated because tephra layers overlying terrace deposits only indicates the minimum age of the terraces. These problems should be overcome to correctly estimate the uplift rate in the middle reach of rivers as well as to understand the development of fluvial terraces. Optically-stimulated luminescence (OSL) dating could provide depositional ages of fluvial terraces, which may improve the chronology of fluvial terraces. We need to examine the validity of OSL dating of fluvial terrace deposits based on the comparison of tephra chronology.
Sand-sized grains or silt grains are usually used for OSL dating, but recently, cobble dating method have been developed and applied. Fluvial terrace deposits in the middle reach of Japanese rivers are mainly composed of gravels, and sand layers are sometimes difficult to find. Furthermore, cobbles might be better bleached compared to sand-sized grains because of long residence time in river channels. This study examines the luminescence dating of granodiorite cobbles obtained from the uppermost part of terrace deposits formed in the last glacial period in the Chichibu Basin, central Japan.
We obtained three granodiorite cobbles from an outcrop and treated them in a darkroom. We obtained three cores (with 10 mm diameters and 20–30 mm depths) for each cobble and sliced them using water-cooled, diamond-edged wafering blade, and rock slices of 1.2 or 0.65 mm thickness were produced. Measurements were conducted using Risø TL/OSL DA-20 and a post-IR IRSL50/225 SAR protocol was used to estimate the depositional ages of cobbles. We also obtained samples from a silty sand layer and obtained post-IR IRSL50/225 age of sand-sized K-rich feldspars (180–250 μm) and OSL age of fine-grained quartz grains (4–11 μm).
The OSL age of fine-grained quartz was ca. 9.4 ka, which is 5 ka younger than the UG tephra (16–15 ka) found in the lowest part of the loess deposits overlying the terrace. The decay curve of the quartz OSL signal suggests significant contribution of the medium component, which is thermally unstable, and led to age underestimate. The IR50 age of K-rich feldspars is ca. 46 ka and we infer that the luminescence signal was not completely bleached. Three cobbles yielded IR50 ages of 19–17 ka, which is consistent with the tephra age. The post-IR IRSL50/225 ages of cobbles are considerably different between cobbles (125–34 ka). We introduce and discuss the calculation of dose rate and fading correction methods in the presentation.
Sand-sized grains or silt grains are usually used for OSL dating, but recently, cobble dating method have been developed and applied. Fluvial terrace deposits in the middle reach of Japanese rivers are mainly composed of gravels, and sand layers are sometimes difficult to find. Furthermore, cobbles might be better bleached compared to sand-sized grains because of long residence time in river channels. This study examines the luminescence dating of granodiorite cobbles obtained from the uppermost part of terrace deposits formed in the last glacial period in the Chichibu Basin, central Japan.
We obtained three granodiorite cobbles from an outcrop and treated them in a darkroom. We obtained three cores (with 10 mm diameters and 20–30 mm depths) for each cobble and sliced them using water-cooled, diamond-edged wafering blade, and rock slices of 1.2 or 0.65 mm thickness were produced. Measurements were conducted using Risø TL/OSL DA-20 and a post-IR IRSL50/225 SAR protocol was used to estimate the depositional ages of cobbles. We also obtained samples from a silty sand layer and obtained post-IR IRSL50/225 age of sand-sized K-rich feldspars (180–250 μm) and OSL age of fine-grained quartz grains (4–11 μm).
The OSL age of fine-grained quartz was ca. 9.4 ka, which is 5 ka younger than the UG tephra (16–15 ka) found in the lowest part of the loess deposits overlying the terrace. The decay curve of the quartz OSL signal suggests significant contribution of the medium component, which is thermally unstable, and led to age underestimate. The IR50 age of K-rich feldspars is ca. 46 ka and we infer that the luminescence signal was not completely bleached. Three cobbles yielded IR50 ages of 19–17 ka, which is consistent with the tephra age. The post-IR IRSL50/225 ages of cobbles are considerably different between cobbles (125–34 ka). We introduce and discuss the calculation of dose rate and fading correction methods in the presentation.