5:15 PM - 6:30 PM
[SGL22-P05] Stratigraphy and age of Goto Islands, Nagasaki Prefecture
Keywords:Goto Islands, Nagasaki Prefecture, Stratigraphy, age
The Goto Islands are located about 100km west of the mainland of Nagasaki Prefecture, and consist of five main islands (Fukuejima, Hisakajima, Narushima, Wakamatsujima, and Nakadorijima islands). The Goto Group, which consists of Miocene sedimentary rocks, rhyolites and granites are distributed throughout the Goto Islands, and Quaternary basalt volcanoes are distributed along the coastline. We created a geological map of the above five islands and restored the stratigraphy in order to obtain a detailed stratigraphy for the Goto Group. As a result, the Goto Group can be divided into three units: lower unit (thickness 300 m +) is greenish volcanic clastic rock: middle unit (thickness is about 1000 m) consists of alternating layers of sandstone and mudstone, which is predominantly mudstone containing ripple lamina: upper unit (thickness is about 1500 m) consists of alternating layers of sandstone and mudstone, which is mainly thick sandstone containing a large amount of cross-bedding. The Goto Group is unconformably overlain by rhyolitic volcaniclastic rocks (Goto felsic volcanic rocks), which consists of Fukue rhyolites (Matsui and Kawada, 1986) in Fukuejima Island and Narao rhyolite (new name) in Nakadorijima Island.
The K-Ar age of the Fukue rhyolites was reported 15.5 ± 0.4 Ma (NEDO, 1990), and of the granites 13.2 ± 1.0 Ma (Kawada et al., 1994). There are some FT age data of the Goto Group (Ishikawa and Tagami, 1991) as 19-15 Ma, but these data still have wide error.
In this work, in order to clarify the sedimentary ages of the Goto Group and the Goto felsic volcanic rocks, we performed U-Pb dating of zircon and obtained accurate age values for each rock.
The green volcaniclastic rocks at Kayabaura,-which identified at the bottom of the lower part of the Goto Group contain many rounded zircon grains, many of which age from 1.8Ga to100 Ma, but among them, we found younger small angular zircons, which shows 21.7 ± 0.5 Ma and 22.6 ± 0.5 Ma. This is the oldest reported age of the Goto Group to date. In Narushima Island, felsic tuff within middle unit of the Goto Group at the Kabutozaki shows 17.66 ± 0.19 Ma. We also identified Fukue rhyolites at Daiho and Kannondaira of the Fukuejima Island. The age shows 16.36Ma ± 0.09Ma and 16.80 ± 0.36Ma, respectively.
The following geological ages were clarified by this dating. It is thought that the volcaniclastic sequence in the lower unit of the Goto Group were deposited around 20-18 Ma, the sandstone and mudstone alternating sequence in the middle unit were deposited at 17.5-17 Ma, and the upper unit was deposited at 17 Ma-16.5 Ma when the clockwise rotation period of SW Japan (Hoshi, 2018). The Fukue rhyolites, the age of 16.5 Ma is obtained, and it is thought to have been formed by volcanic activity after the sandstone deposition of the Goto Group. However, the geological relationship between the Goto Group and the Fukue rhyolites has not been found on Fukuejima Island so far, and it is considered that the Fukue rhyolites are distributed in the central part of Fukuejima Island as depression due to the normal fault.
The K-Ar age of the Fukue rhyolites was reported 15.5 ± 0.4 Ma (NEDO, 1990), and of the granites 13.2 ± 1.0 Ma (Kawada et al., 1994). There are some FT age data of the Goto Group (Ishikawa and Tagami, 1991) as 19-15 Ma, but these data still have wide error.
In this work, in order to clarify the sedimentary ages of the Goto Group and the Goto felsic volcanic rocks, we performed U-Pb dating of zircon and obtained accurate age values for each rock.
The green volcaniclastic rocks at Kayabaura,-which identified at the bottom of the lower part of the Goto Group contain many rounded zircon grains, many of which age from 1.8Ga to100 Ma, but among them, we found younger small angular zircons, which shows 21.7 ± 0.5 Ma and 22.6 ± 0.5 Ma. This is the oldest reported age of the Goto Group to date. In Narushima Island, felsic tuff within middle unit of the Goto Group at the Kabutozaki shows 17.66 ± 0.19 Ma. We also identified Fukue rhyolites at Daiho and Kannondaira of the Fukuejima Island. The age shows 16.36Ma ± 0.09Ma and 16.80 ± 0.36Ma, respectively.
The following geological ages were clarified by this dating. It is thought that the volcaniclastic sequence in the lower unit of the Goto Group were deposited around 20-18 Ma, the sandstone and mudstone alternating sequence in the middle unit were deposited at 17.5-17 Ma, and the upper unit was deposited at 17 Ma-16.5 Ma when the clockwise rotation period of SW Japan (Hoshi, 2018). The Fukue rhyolites, the age of 16.5 Ma is obtained, and it is thought to have been formed by volcanic activity after the sandstone deposition of the Goto Group. However, the geological relationship between the Goto Group and the Fukue rhyolites has not been found on Fukuejima Island so far, and it is considered that the Fukue rhyolites are distributed in the central part of Fukuejima Island as depression due to the normal fault.