Taishu Group Research Articles

Age of Initial Submarine Volcanism in the Paleo-Tsushima Basin and Implications for Submarine Volcanism in the Opening Stage of the Japan Sea in Northern Kyushu

The Tsushima Lapilli Tuff, the thickest tuff in the Taishu Group on Tsushima Island, underwent a thermal event after deposition, and has not previously yielded a reliable age because various ages have been reported. This study clarifies the eruption age and thermal history of the Tsushima Lapilli Tuff based on fission-track (FT) and U–Pb dating of zircon grains using laser ablation inductively coupled plasma mass spectrometry (ICP-LA-MS) and evaluates submarine volcanism during deposition of the Taishu Group in the southwestern Japan Sea, as well as volcanism change on Tsushima Island. This study revealed that thermal events caused rejuvenation in some single-grain FT ages after deposition in the Tsushima Group, and that the eruption age of the Tsushima Lapilli Tuff was 16.2 ± 0.7 Ma; the age of the largest submarine volcanism event in the Taishu Group in Tsushima Island was thus determined. On the basis of our previous studies, this age and tectonism strongly indicate that felsic submarine volcanism occurred between 18 and 16 Ma, accompanied by rapid subsidence, and the volcanism changed from felsic volcanism originating from melting of old continental crust by asthenospheric upwelling to mafic volcanism originating from small-scale lithospheric mantle upwelling from 13.6 Ma onward.

Styles of mesoscale brittle deformations associated with the Miocene folding of the Taishu Group, Tsushima, between the Japan Sea and East China Sea backarc basins

AbstractThe Taishu Group is a folded, Eocene–Lower Miocene, thick sedimentary package exposed widely on Tsushima Island between the Japan Sea and East China Sea. This location makes the strata important to understand tectonics and paleo‐environments in the Far East, but the timing of the folding is controversial. We studied the styles of brittle deformations of the strata. It was found that flextural‐slip folds were dominant. Mesoscale faults were classified into two groups: NE–SW trending reverse faults and NW–SE trending strike‐slip faults. Members of both the groups showed movements largely perpendicular to the fold axes. The latter group consisted of sinistral and dextral faults. Accordingly, we interpreted that they were transfer faults activated during the folding. Consequently, mesoscale faults and flexural‐slip faults evidence the map‐scale plane strain of the Taishu Group in the plane perpendicular to the NE‐trending fold axes. There were few transpressional deformations in the group. This is inconsistent with the transpression hypothesis for explaining the simultaneous folding and Japan Sea opening. Another hypothesis in which the folds in Tsushima are regarded as an onshore part of the Taiwan‐Shinji fold belt is inconsistent with the timing of folding suggested by mining geologists to be consistent with and contemporaneous with this deformation. On the other hand, we found that dolerite dikes and sills were involved in the folding. Therefore, we conclude that the folding began during the late Early Miocene time and climaxed during the ore mineralization at around 15 Ma. We suggest that the folding in Tsushima was the easternmost manifestation of the compressional regime around the Yellow Sea and East China Sea in the Early to early Middle Miocene, and that the compression was brought about by the arrival of the Philippine Sea plate to initiate buoyant subduction under Kyushu.

Formation, diagenesis and fauna of cold seep carbonates from the Miocene Taishu Group of Tsushima (Japan)

AbstractThe studied seep carbonates from Tsushima, Japan, are embedded within marine siliciclastics of the lower Miocene Taishu Group and represent the earliest evidence of hydrocarbon seepage in the Sea of Japan. In contrast to Miocene and Pliocene examples from Honshu, which are often found above anticlines, the seeps from Tsushima formed within a pull-apart basin before major anticlines had formed. The three carbonates from Fukuzaki, Kanoura and Tanohama are composed chiefly of calcite, with significant admixture of ankerite only at Kanoura. The stable carbon isotope composition of calcites (δ13C as low as −40.2 ‰ VPDB for Fukuzaki, −41.8 ‰ VPDB for Kanoura, and −52.8 ‰ VPDB for Tanohama) indicate methanogenic origin of the carbonates. Textures of these deposits, including radiaxial and yellow cements, are indicative of formation at a methane seep. The stable oxygen isotope composition of calcites (δ18O values as low as −14.4 ‰ VPDB for Fukuzaki, −14.5 ‰ VPDB for Kanoura and −13.9 ‰ VPDB for Tanohama) indicate that they were influenced by burial fluids. Burial diagenesis is also indicated by the stable isotopic compositions of ankerite (δ13C ranging from −19.1 ‰ to −7.1 ‰ VPDB, δ18O from −11.1 ‰ to −9.7 ‰ VPDB). Molecular fossils from Tanohama comprise n-alkanes with short-chain predominance, interpreted to have formed due to thermal cracking of organic matter. The carbonates yield a chemosynthesis-based community comprising vesicomyids Pleurophopsis chitanii, P. cf. hamuroi, the bathymodiolin ‘Bathymodiolus’ akanudaensis, the lucinid Lucinoma sp. and the provannid Provanna? sp., which have never been hitherto identified. ‘Bathymodiolus’ akanudaensis, Lucinoma sp. and Provanna? sp. are the oldest records of these taxa in the Sea of Japan.

Origin and water depth of a newly identified seep carbonate and paleoecology of Bathymodiolus in the Miocene Taishu Group, southwestern Japan

The Fukuzaki Limestone is a small limestone lens embedded in siliciclastic Taishu Group rocks on Tsushima Island in the southwestern Japan Sea. A fossil assemblage in this limestone consisting of the bivalve mollusc Bathymodiolus sp. (Bathymodiolinae) was previously inferred, on the basis of the limestone's isolated lenticular occurrence and the chemosynthetic nature of modern Bathymodiolinae, to rely on chemosynthetic production, but conclusive evidence was not provided. Carbon, nitrogen, oxygen, and sulfur isotope ratios in the limestone and its host rock mudstone were measured to clarify the paleoecology of the fossil assemblage and the formation environment of the limestone. The low δ13C (−39.8‰ to −31.7‰) and δ18O (−13.8‰ to −11.9‰) values of carbonate of the limestone lens strongly suggest that the limestone is composed of authigenic carbonate rock precipitated from bicarbonate derived from anaerobic oxidation of biogenic or thermogenic methane. The fossil assemblage in the limestone lens is consistent with a chemosynthesis-based animal assemblage harboring thioautotrophic (sulfur-oxidizing) symbionts, and the low δ13C and δ34S values of total organic carbon and sulfide sulfur, respectively, in the limestone lens and mud layers suggest that they reflect chemosynthetic production using hydrogen sulfide derived from anaerobic methane oxidation by microbes. Furthermore, the stratigraphic position of the lens in the upper part of the Lower Unit of the Taishu Group indicates that it was precipitated at a water depth >800 m (deeper than the lower middle bathyal zone). These findings indicate that a deep-sea basin locally supplied with methane had already formed in the southwestern Japan Sea by rapid subsidence during the opening stage of the Japan Sea (16–17 Ma).

Miocene dislocations during the formation of the Sea of Japan basin: Case study of Tsushima Island

Lower Miocene rocks of the Taishu Group accumulated in the Tsushima pull-apart graben, which downwarped and was filled with sediments at a particularly high rate (about 2700 m/Ma), in the background of northeastern regional shortening. A considerable part of the sedimentary prism is composed of material supplied by landslide blocks from the shallow shelf. Folding and penetration of granite intrusions on Tsushima Island occurred ca. 15 Ma ago, simultaneously with the main phase of opening of the Sea of Japan, in the field of different, northwestern shortening, which had a local character and was related to clockwise rotation of the Southwestern Japan block. These rotations in turn could have been the result of an intensive rifting episode in the Central and Honshu basins of the Sea of Japan, which are located north of Tsushima Island.

Age of the Taishu Group, southwestern Japan

Age of the Taishu Group, southwestern Japan

Age of the Taishu Group, southwestern Japan, and implications for the origin and evolution of the Japan Sea

AbstractThe Taishu Group, a marine formation with a thickness of >5400 m, crops out on Tsushima Island, located in the southwestern Japan Sea. The group, which is generally regarded as early Eocene to early Miocene in age, provides important information about the tectonic setting of the Japan Sea. In this study, we present new SHRIMP U–Pb dates for igneous zircons from the Kunehama Tuff, which is in the basal part of the Taishu Group, and the Oobaura Tuff, which is in the uppermost part of the group. Results show that the Taishu Group was deposited rapidly, during the short interval of 17.9–15.9 Ma (early–middle Miocene), and is equivalent to other early–middle Miocene strata found in the Japan Sea region. Our results provide new constraints on the geological history of the Japan Sea and its islands.

Chemosynthetic fossil molluscan faunas from the Neogene Taishu Group, distributed in Tsushima Islands, Nagasaki Prefecture, the southwest Japan

Chemosynthetic fossil molluscan faunas from the Neogene Taishu Group, distributed in Tsushima Islands, Nagasaki Prefecture, the southwest Japan

Temporal variations in major depositional processes of prograding deltaic systems in the Tertiary Taishu Group, Tsushima Islands, southwestern Japan

The Tertiary Taishu Group in the Tsushima Islands, southwestern Japan is characterized by deltaic systems. The group is subdivided into the Lower, Middle, and Upper Formations. The Lower Formation (Early Eocene) represents a tide-influenced, fluvial-dominated deltaic system, and the Upper Formation (Early Miocene) represents a wave-dominated deltaic system. During the deposition of the Upper Formation, the Japan Sea is interpreted to have started opening and a shallow marine environment appears to have expanded around the Tsushima Islands area. This paleogeographic change is interpreted to have resulted in changes in major depositional processes for the delta formation.

Thermochronology for the Granitic Pluton Related to Lead‐Zinc Mineralization in Tsushima, Japan

Abstract: K–Ar datings and oxygen isotope analyses revealed a cooling history of the Uchiyama granitic pluton, which is genetically related to the Pb‐Zn deposits (Taishu mine) in the Tsushima Islands, Japan. The pluton intrudes into the Paleogene Taishu Group to form the biotite‐hornfels zone, while the Taishu vein‐type Pb–Zn deposits occur in fissures developed in the non‐hornfels zone about 1 to 3 km westward from the contact. Amphibole and biotite K‐Ar ages of the pluton have a wide range from 19 to 13 Ma. Oxygen isotopes of the biotite and coexisting quartz grains indicate that isotopic exchange reactions have occurred under subsolidus conditions, and that the K‐Ar ages are affected by various cooling rates in the pluton. The mineralization age of the Taishu ore deposits is obtained for the first time to be 15.40.8 Ma by a K‐Ar age of 2M1–muscovite in a calcite–quartz–muscovite–chlorite veinlet of the Shintomi orebody. Whole rock K‐Ar ages of biotite‐hornfels near the pluton represent similar ages to the ore deposits. Using blocking temperature calculated from reported diffusivity for argon, the pluton was cooled from 560 to 350C in the period of 17 to 14 Ma. The vein formation took place after the time when temperature in wall rocks of the pluton had dropped below the brittle‐plastic transition (about 400C). These results imply that the cooling of the pluton has caused injection of magmatic fluids into meteoric hydrothermal systems, and the Pb‐Zn mineralization has occurred due to this mixing at the age of about 15 Ma.

Characterization of Magnetically Zoned Pluton, Tsushima, Japan

Abstract: Systematic measurements of magnetic susceptibility were carried out at the outcrops of the Uchiyama granitic pluton, Tsushima Islands. The pluton consists of hornblende‐biotite granodiorite and biotite granite. It intrudes concordantly along an anticline axis of the Taishu Group in middle Miocene (16. 10. 5 Ma), and crops out in five areas. The results show that the low magnetic susceptibility facies (LM–facies, <100A‐10‐6 emu/g) always occurs in the peripheral part of the pluton, while the high magnetic susceptibility facies (HM–facies, > 250 A 10‐6 emu/g) occupies the core through the medium magnetic susceptibility zone (MM–facies). It is pointed out that the pluton forms a magnetic zoning within a single plutonic body.Chemical compositions of the granitoids show no conspicuous differences in major elements among the LM–, MM–, and HM–facies. Biotites from the LM–, MM–, and HM‐facies also indicate uniform compositions with Fe/(Fe+Mg)=0. 6.The pluton is principally surrounded by black mudstones of the Lower Formation of the Taishu Group, which is deltaic to shallow sea sediments of the Eocene to early Miocene age, and the carbon contents in them were analyzed. The analyses show that carbon was mostly decomposed and lost in the hornfels zone, while mudstones in the non‐hornfels zone usually contain 0. 5 to 0. 7 % C.From these facts, it is concluded that the magnetically zoned pluton at Tsushima was formed by an external volatile buffer system such as CH4–CO2 during the solidifying magma process.

Morpho-dynamic development and facies organization of the Tertiary delta system in the Taishu Group, Tsushima Islands, southwestern Japan.

Morpho-dynamic development and facies organization of the Tertiary delta system in the Taishu Group, Tsushima Islands, southwestern Japan.

長崎県対馬の対州層群下部層から産出した始新世放散虫化石

長崎県対馬の対州層群下部層から産出した始新世放散虫化石

Geologic ages of the Taishu Group and the Katsumoto Formation in the Tsushima and Iki Islands, off Northwest Kyushu on the basis of planktonic foraminifers.

Geologic ages of the Taishu Group and the Katsumoto Formation in the Tsushima and Iki Islands, off Northwest Kyushu on the basis of planktonic foraminifers.

Paleomagnetic study of the Tsushima Islands, southern margin of the Japan Sea.

Paleomagnetic study of Oligocene and Miocene rocks from the Tsushima Islands, the southern margin of the Japan Sea, has revealed considerable deflection of the mean paleomagnetic direction compared with the Eurasian mean pole. Samples were collected from 48 sites of the late Oligocene to Early Miocene Taishu Group and Middle Miocene igneous intrusions with radiometric dates around 15Ma. After demagnetization experiments, 14 sites from six stratigraphic units finally yielded resolvable stable magnetization components. An overall-mean direction of D: -22.4°, I: 44.7°, and α95: 11.7° was obtained. The mean declination was significantly deflected from that of the expected direction calculated from the 20Ma Eurasian mean paleopole. The deflection, -28.1°±14.0°, is explained by a counterclockwise rotation of the Tsushima Islands relative to Eurasia after the igneous activity around 15Ma. This result suggests that the Tsushima Islands were not included in the tectonic regime of the Southwest Japan block, which rotated clockwise during the formation of the Japan Sea at about 15Ma.

Unconformity between the Taishu Group and the Pliocene Ebishima Formation in the Northernmost Part of the Tsushima Islands, west Japan:A Contribution to the Age Problem of the Taishu Group

Unconformity between the Taishu Group and the Pliocene Ebishima Formation in the Northernmost Part of the Tsushima Islands, west Japan:A Contribution to the Age Problem of the Taishu Group

DummyTemporal variations in major depositional processes of prograding deltaic systems in the Tertiary Taishu Group, Tsushima Islands, southwestern Japan

dummyTemporal variations in major depositional processes of prograding deltaic systems in the Tertiary Taishu Group, Tsushima Islands, southwestern Japan