A paleomagnetic study of terrestrial sediments was performed to acquire reliable Cretaceous paleomagnetic data for the inner arc of the southwestern Japanese islands (southwest Japan). Samples of red bed sedimentary rocks from the Early Cretaceous (approximately 110 Ma) Sasayama Group were collected from 28 sites. A high-temperature magnetization component with unblocking temperatures of up to 690°C was isolated from all the sites. The site-mean directions of the component fail the fold test, and incremental unfolding for the tilt yielded an optimum mean direction (D = 77.0°, I = 51.8° with α₉₅ = 2.9°) that passes the fold test, yielding a paleomagnetic pole at 28.0° N, 203.0° E with A₉₅ = 3.5°. Previously reported site-mean directions of the Sasayama basin from six sites were incorporated with those of this study to provide a grand-mean direction for the Sasayama Group, yielding an optimum mean direction through incremental unfolding (D = 75.8°, I = 53.0° with α₉₅ = 2.8°) and a corresponding paleomagnetic pole (29.4° N, 202.2° E with A₉₅ = 3.4°). This pole shows a paleomagnetic rotation of 64.9°±6.1° with respect to the coeval pole for East Asia, with a negligible latitudinal difference of 4.6°±4.9°.

We studied the chronology of Paleogene formations in the northwestern part of Mihara City, Hiroshima Prefecture, Japan. Paleogene rocks in the study area consist primarily of arkosic granular conglomerates interbedded with sandstone, mudstone, and fine tuff. We obtained zircon U–Pb ages from tuff and silicified wood and performed a preliminary investigation of pollen fossils. A weighted mean U–Pb age of 39.2 ± 0.1 Ma was obtained, indicating that the tuff layer is Eocene (Bartonian) in age. This age, if interpreted as the Paleogene depositional age for the study area, is older than previously assumed, suggesting that the chronology of the Paleogene should be revised. The silicified wood is white and poorly preserved; therefore, it could not be comprehensively assessed. However, we determined that the wood belongs to the Taxodioxylon sp. of the family Cupressaceae. Pollen fossils include Fagaceae, which are similar to the evergreen type Quercus sp. and Castanopsis sp. These results provide the first firm evidence for the conventional age estimation of Paleogene formations in the study area and indicate the existence of terrestrial and fluvial depositional environments in the south-central part of the Chugoku region during the Eocene (Bartonian).

Abundant serpentinite cobbles have been found in the Kashima River in the Omachi area, central Japan. Their abundance decreases sharply downstream to the Takase River. Small serpentinite bodies must be exposed in the Kashima River catchment, where Paleogene granitic rocks (i.e., the Ariake Granite) are widely exposed. These small serpentinite bodies have been interpreted as blocks surrounded by granitic magmas and are expected to have experienced intense contact metamorphism. They are part of the serpentinite and metaperidotitic rocks commonly found in the northwestern part of Fossa Magna. The mode of occurrence of the serpentinite bodies has not been investigated in detail because of the steep mountainous terrain of the Kashima River area, and the petrography and petrology of the bodies are only easily examined in river cobbles from the area. The serpentinite cobbles are characterized by olivine–tremolite–antigorite or olivine–diopside–antigorite assemblages with or without a small amount of talc, and most have been serpentinized further at low temperatures. Their mineral assemblages and chemical compositions are similar to those of the metaperidotites exposed at Happo-O’ne and the surrounding area, which record weak contact metamorphism (up to talc zone), although the metaperidotite protoliths had slightly but distinctly different in serpentine and diopside contents. The small serpentinite bodies exposed upstream of the Kashima River record only weak contact metamorphism, even though they occur in a granitic complex. This suggests that they were not trapped by the granitic magmas during intrusion but are tectonic blocks that were possibly emplaced into the granitic rocks by faults. This work provides an example of the value in studying the petrology of river cobbles.

Dark veins occur in a massive outcrop of pelitic–psammitic schists at the mouth of the Akirigami river in western Tokunoshima in the Amami Islands, southwest Japan. The dark veins are 10–20 cm thick and roughly parallel to the schistosity of the host rocks. Irregular branches and stepwise traverses across the schistosity are observed in places. The veins are filled with rock fragments (pelitic schist, psammitic schist, and rare amphibolite) and fine-grained matrix. A power-law size–frequency relationship for the clasts in the dark vein and powder X-ray diffraction profiles of the host pelitic schist and the matrix of the dark vein indicate that the dark veins are filled with crushed pelitic–psammitic schists and hardly include amorphous material. In the matrix of the dark veins, very fine anatase (TiO₂) grains surround calcite (CaCO₃) and titanite (CaTiSiO₅) forming rings. This suggests the decomposition of titanite owing to the infiltration of a CO₂–H₂O fluid. We propose that the dark veins were derived from intensive cataclasis and fluidization enhanced by excess hydrostatic pressure.

The 14–16 cal ka caldera-forming eruption of Nigorikawa volcano, Southwest Hokkaido, Japan, simultaneously ejected anomalously Ba-rich gray pumice (high-Ba gray pumice), low-Ba gray pumice, and white pumice. We investigated the petrological characteristics of each type of pumice to reveal the genesis of the high-Ba gray pumice. Although the glass compositions and mineral assemblages in the coexisting high- and low-Ba gray pumices are the same, the high-Ba gray pumice typically contains fine-grained, argillaceous material in the vesicles. Chemical analyses and X-ray diffraction patterns indicate that the fine-grained fill is composed of Fe-allophane and kaolinite with high Ba contents formed by hydrothermal alteration (dilute acid–neutral). The freshness of the volcanic glass hosting the filled vesicles and the stratigraphic variation in pumice componentry suggest that the fills were precipitated by trapped hydrothermal water in a relatively rapid process (e.g., the recycling of juvenile pumice) during the caldera-forming eruption. Similar hydrothermal alteration and recycling generating high-Ba gray pumice in the vent might occur frequently during explosive eruptions, and the possibility of in situ alteration should be carefully evaluated, even for young (Holocene) volcanoes.

This review summarizes geochemical studies in tsunami deposit research. The geochemical characteristics and behavior of tsunami deposits are described, and future research issues are identified and discussed. As geochemical approaches have focused on traces of seawater inflow or environmental changes rather than tsunami deposits themselves, such approaches are expected to contribute to (1) identifying tsunami deposits from geological layers, and (2) reconstructing tsunami inundation areas with high accuracy. After recent giant tsunamis, many geochemical characteristics of tsunami deposits have been reported, and the behaviors of various geochemical proxies have been gradually clarified. However, the usefulness of these proxies and their behaviors in different depositional environments remain poorly understood. In the future, it will be necessary to better understand the geochemical characteristics of both modern and paleo tsunami deposits and to develop and apply new proxies and analytical methods. Collaboration with other research fields will prove indispensable for these purposes; therefore, cross-disciplinary research should be promoted.

The Tomari volcanic rocks were products of the Middle Miocene volcanic front of the NE Japan arc. The volcanic activity has been divided into four stages with five lithologies, as assessed from surface geology. Eruptive rocks of Stage I (oldest) are the Kanatsuyama Andesite, the lower and upper units of Stage II are the Gassan Andesite and Tomari Basalt, respectively, Stage III comprises the Takahoko Andesite, and Stage IV is the Fukkoshi Andesite. Coring investigation was conducted on the northwestern side of the exposed Tomari volcanic rocks on central Shimokita Peninsula, Aomori Prefecture, in the northern NE Japan arc during 2014–2016. In the SN-010 core, the Tomari volcanic rocks occur from the bottom of the core at 1484 m below the surface to 308 m below the surface; the lower boundary of the volcanic rocks was not penetrated in the SN-010 borehole. We collected eight samples from the core and described their petrological characteristics to correlate the volcanic stages of the Tomari volcanic rocks between the core and surface geology.

Modal compositions, especially abundant olivine phenocrysts and the occurrence of chromium-spinel, define the Tomari Basalt as a rock with a unique signature within the exposed Tomari volcanic rocks. The Tomari Basalt includes a primitive basalt characterized by high whole-rock contents of MgO, Cr, and Ni compared with the other Tomari volcanic rocks. The SN-010 core samples at a depth of ~1003 m have similar petrological characteristics to those of the Tomari Basalt, except that olivine phenocrysts in the core samples are perfectly altered.

LILE/HFSE ratios, such as Ba/Nb, change below 1197.86 m (deep core samples) and above 1003.35 m (shallow core samples) in the SN-010 core. Ratios for shallow core samples are similar to those for surface samples. HFSE/HFSE ratios, such as Zr/Nb, show similar values among deep, shallow, and surface samples, indicating a cognate source mantle. These findings imply changes in the slab-derived component during magma generation associated with the opening of the Japan Sea.

Cenozoic alkali basalts are distributed from the northern Kyushu to Chugoku regions. I present new K-Ar ages and geological and geochemical data for the basalts on Oronoshima and Eboshijima, northern Kyushu, southwest Japan. The volcanic rocks on Oronoshima comprise basaltic andesite, trachybasalt to basalt, and basaltic andesite in ascending stratigraphic order, intercalated with volcaniclastic and clastic rocks. The K-Ar age of the plagioclase in the lowest basaltic andesite is 10.78±0.25 Ma (Late Miocene). On Eboshijima, the volcanic rocks comprise hornblende basalt to trachybasalt. The K-Ar age of the basalt is 4.27±0.10 Ma (Early Pliocene), which is within the age range of hornblende basalts distributed across northern Kyushu (4.50-3.51 Ma).

Fossil winged ants (family Formicidae) are sometimes found in lacustrine deposits. In the Shiobara Group, however, only a fossil of a large wingless queen ant has been described. Two types of fossil winged ants belonging to the subfamily Formicinae are newly described here and identified as Lasius sp. and Camponotus sp.

Lake and marine sediments are important research targets for reconstructing past environments and ecological studies. When the lake and sea surfaces are frozen, it is possible to move on the ice, making it easy to access a broad area. Therefore, by acquiring basic information such as depth and images with portable instruments, more extensive surveys can be conducted during the freezing season. We have modified the commercially available BlueROV2 to perform a topographic survey under the ice in lakes and shallow water in high mountain regions and Antarctica. In this paper, we report on the operational evaluation in cold regions during the training at Lake Shumarinai, Hokkaido, Japan.

One of the tectonic features that characterizes the Miocene geology of Northeast Japan is crustal block rotation during and after the opening of the back-arc Sea of Japan. However, the spatial and temporal characteristics of this block rotation are not well understood. A better understanding of this block rotation would shed light on the Miocene tectonic evolution of Northeast Japan. We carried out geological mapping and geochronological and paleomagnetic analyses to determine the timing of block rotation on the southern margin of the Asahi Mountains, on the back-arc side of Northeast Japan. We present a geological map and chronostratigraphic model of the lower-middle Miocene strata and redefine two lower Miocene formations. K-Ar dates show that andesite dikes and sills were intruded at ~15 Ma. The crust beneath the study area was rotated counterclockwise relative to the Asian continent, although it was broken into blocks with varying degrees of rotation. In the eastern part of the study area, counterclockwise rotation of ~40°-50° relative to the continent occurred between 17.5 and 15 Ma. Rotation probably did not occur during the earlier development of intra-arc rift basins. The timing of the rotation overlaps with that of the formation of the Myozawa Syncline, which is a growth fold that formed during the deposition of shallow marine sediments during the latest early Miocene. Complex block rotation in a possible dextral transtensional regime in the Asahi Mountains was likely confined to a period of 2 Myr between 17 and 15 Ma. This block rotation probably occurred during the late stage of the opening of the back-arc Sea of Japan.

Abstract: We identified subsolvus syenite (episyenite) veins formed by hydrothermal metasomatism in the Hira granite pluton, west of Lake Biwa, central Japan. The veins comprise four facies with different mineral assemblages and bulk chemical compositions. Feldspars in the syenite include K- and Na-feldspars that are close to end-member compositions. Other silicate minerals include holmquistite, green mica, and chlorite, which are variably associated with sphalerite and Fe-bearing material (magnetite, hematite, and amorphous Fe oxides). The subsolvus syenite in the Hira granite pluton underwent K-feldspathization, albitization, and Zn mineralization. We present petrographic and mineralogical data for the syenite.

Zircon U-Pb dating was carried out on granitic rocks from plutons in the Taiheizan Complex in Akita Prefecture, NE Japan. Two granodiorites from the Main Intrusive Rocks yielded weighted mean ²⁰⁶Pb/²³⁸U dates of 103.4±1.0 Ma and 115.6±1.1 Ma (1SE). Three porphyrygranite from the Young Intrusive Rocks yielded weighted mean ²⁰⁶Pb/²³⁸U dates of 11.4±0.1 Ma, 4.7±Ma, and 4.8±0.1 Ma (1SE). The older and younger dates can be interpreted as the ages of early and later stages of granitic intrusion at the sampling locations. The Pliocene U-Pb dates suggest that the Nibetsu Body in the Young Intrusive Rocks is one of the youngest granitic plutons currently exposed on Earth.

Miocene alkali basalt dikes have been reported in a few locations in the Outer Zone of Southwest Japan. We report a new location where alkali basalt dikes have intruded the Cretaceous Sanbagawa metamorphic rocks in Sakurasanri, Northwestern Shikoku. Middle Miocene igneous rocks of the Ishizuchi Group are also found in this region. K-Ar dating of groundmass feldspar in a Sakurasanri alkali basalt dike gave an age of 15.7±0.5 Ma, which is close to or older than the age of igneous rocks of the Ishizuchi Group. The trace element characteristics of the Sakurasanri alkali basalt are similar to those of oceanic island basalt (OIB), although the former was affected by the assimilation of felsic crustal rocks. The Sakurasanri alkali basalt contains abundant megacrysts and gabbro xenoliths. The megacrysts include crystallization products (Cr-rich diopside, olivine, and Cr-spinel) from a primitive basaltic magma and antecrysts (Ti-rich diopside, less-magnesian olivine, and calcic plagioclase) from earlier pulses of alkali basalt magma. Prolonged OIB-type magmatism could have been the heat source of some of the forearc magmatism in Southwest Japan during the middle Miocene.

The lower-middle Pleistocene Kazusa Group (forearc basin fill) and lower Pleistocene Chikura Group (trench slope basin fill) on Boso Peninsula of central Japan have been studied intensely and are the type sections for the northwestern Pacific region due to the abundant tephra beds and robust chronological framework obtained using biostratigraphy and magnetostratigraphy. We demonstrate a magneto-tephrostratigraphic correlation between the lower part of the Kazusa Group and the Hata Formation of the Chikura Group. We identified two zones of normal polarity, in the uppermost Katsuura Formation and in the upper Ohara Formation through to the lower Kiwada Formation of the Kazusa Group, which can be correlated with the Feni and Olduvai normal polarity subchrons, respectively. The horizons of reversed polarity between these subchronozones and below the Feni belong to the Matuyama reversed polarity chron. Based on the stratigraphic positions, petrographic characteristics, and major and trace element compositions of volcanic glass, the Kmj-3, Kmj-10, Kmj-18, Kmj-29, Kmj-41, Kmj-53, Kmj-68, and Kmj-71 tephra beds in the Hata Formation can be correlated with the Kr31, KRm, KH2, IW2, OFN2, KB, HS C, and HS A tephra beds in the Kazusa Group, respectively. These tephra correlations are consistent with the magnetostratigraphy of both groups. Our results highlight some important marker tephra beds, including IW2 in the Feni normal subchronozone and HS C and HS A, immediately below and above the lower Olduvai reversal boundary, respectively. Although the thickness of the lower Kazusa Group is generally several times that of the corresponding Hata Formation, both forearc and trench slope basins would have developed in parallel with no apparent erosional gap between 2.2 and 1.9 Ma.

Tosa-suzuri inkstone of Gendani, a traditional craft product of Kochi Prefecture, Japan, has been produced in Mihara Village, Hata District, using the Miharaishi slate that occurs in the Shimanto Belt. The slate-bearing sequence was thought to be Cretaceous in age. The slate is known for its fine texture that is ideal for grinding inksticks, and contains a copper-colored powder called “golden stars” that enhances the beauty of the inkstone. However, the geological characteristics of the slate have received little scientific attention. Based on a literature survey, the slate-bearing sequence is estimated to be middle Eocene-early Miocene in age. X-ray diffraction analyses show that the main minerals in the slate are muscovite and quartz, and X-ray spectroscopy identified pyrite as the main mineral in the golden stars. Microtomographic and electron microscopic observations show step-like microstructures on the surface of the inkstone, particularly at the edges of clay minerals, that may be favorable structures for grinding inksticks. The geological characteristics of the Miharaishi slate are similar to those of the source rocks of high-quality inkstones in other areas of Japan and China.

The Hakobuchi Formation in Hokkaido, Northeast Japan, consists of marine and terrestrial clastic rocks of the uppermost part of the Yezo Group. The formation was previously interpreted to range from the early Campanian to the late Paleocene, with a hiatus near the K/Pg boundary. To date, no definitive evidence for the presence of the Lower Paleocene (Danian) has been identified in the formation. In this study, we present the LA-ICPMS zircon U-Pb dates of a tuff layer in the Hakobuchi Formation found in the Yubari area. Although the tuff contains Late Cretaceous to Paleocene zircon grains with ²⁰⁶Pb/²³⁸U concordant dates of 80.8-61.7 Ma, the weighted average age of the youngest date cluster was 64.1±1.1 Ma (2σ), suggesting that the depositional age of the tuff falls within the Danian. The results provide the first radiometric evidence indicating that the Hakobuchi Formation is at least partly correlated with the Danian, and are essential for discussing the depositional age of the Hakobuchi Formation and the geochronology of the Yezo Group.

A thick deposit of the muddy Shimajiri Group (Upper Miocene-Early Pleistocene) is overlain by the calcareous sandy Chinen Formation (Pleistocene) in southern Okinawa-Jima, Southwest Japan. The abrupt facies change has been explained by a significant unconformity resulted from subaerial erosion (the Shimajiri disturbance) or the consequence of rapid and regional uplift leading to an abrupt environmental change (the Chinen disturbance). We identified sandy mass transfer deposits (MTD) directly overlying the Shinzato Formation-the uppermost part of the Shimajiri Group-in the large Urizun outcrop. The Shinzato-Chinen transition zone yields numerous dolomite concretions. The dolomite is strongly depleted in ¹³C, with δ¹³C values of −23.77‰ to −31.50‰, suggesting that the concretions were derived from anaerobic oxidation of methane (AOM). Zonation in δ¹⁸O values across the concretions, from 3.89‰ in the centers to 2.25‰ at the rims, suggest that the precipitation temperature increased from 12.3±2.0 to 19.5±2.0°C, corresponding to a shallowing of the water depth from 440±100 to 190±60 m. The estimate is consistent with the analysis of benthic foraminiferal assemblage. Given the wide distribution of the Bottom Simulating Reflector (BSR) offshore of Okinawa-Jima, methane hydrates are assumed to have persisted in the Shimajiri Group throughout the Pliocene-Pleistocene. Abrupt uplift and relative sea-level fall during the Chinen disturbance at 2 Ma must have triggered abrupt changes in facies and a massive methane hydrate dissociation. This disturbance resulted in a high methane flux, increased AOM, and the formation of the dolomite concretions in the Shinzato-Chinen transition zone. Methane-derived dolomite concretions are thus a geochemical record of an abrupt tectonic disturbance and hydrate dissociation offshore of Okinawa-Jima.

The Mukaiyama Formation is a Pliocene non-marine sequence exposed widely in the Sendai area of Japan. This formation is thought to provide a detailed record of Pliocene caldera activity in the Tohoku area as it is intercalated with numerous thick felsic tuffs. However, the sources of and correlations between most of the tuffs in the Mukaiyama Formation have not been well studied. We examined the lithostratigraphy of the Mukaiyama Formation in the Sendai area and identified six traceable tuff beds: the Otsutsumizawa, Hirosegawa, Kabasawa, Shishiochizaka I, Shishiochizaka II, and Shionosawa tuffs, in ascending order. These tuffs can be distinguished based on lithology, heavy mineral assemblages, and apatite trace element compositions, and each tuff can be correlated between sections. Our correlation of the tuffs based on apatite trace element compositions suggests that one of the tuffs was derived from the Shiroishi caldera and another from the Nanatsumori caldera. The U-Pb zircon ages of the Hirosegawa and Shionosawa tuffs show that the depositional age of the Mukaiyama Formation is 3.7-3.5 Ma.

We report the petrographic characteristics, major and trace element compositions, and K-Ar ages of Miocene intrusive rocks from western Fukui, central Japan. The intrusive rock samples were basaltic to dacitic (SiO₂ = 51.1-69.6 wt.%) and were divided into four different rock types based on their modal and major element compositions: basalt and basaltic andesite, andesite, pyroxene dacite, and hornblende dacite. The compositions of the samples showed a tholeiitic trend and were plotted in the medium-K field. The MORB-normalized trace element patterns of the intrusive rock samples were similar to those of the Kunimidake Volcanic Rocks and Ichinose Rhyolite, which are located in the same area. The K-Ar ages of the plagioclase in the samples from the NNW-SSE-trending dike (pyroxene dacite) and stock (andesite) were 14-12 Ma and were slightly younger than those of the Ichinose Rhyolite and Kunimidake Volcanic Rocks (15-14 Ma). These observations indicate that multiple supplies of basaltic to dacitic magmas occurred subsequent to the activities of the Ichinose Rhyolite and Kunimidake Volcanic Rocks after the opening of the Japan Sea.

A microtremor array survey is a tool for modeling subsurface S-wave velocity structure using small ground oscillations, or microtremors, which can be recorded by multiple seismometers at the surface. This paper presents an overview of the method, including the principles, some warnings, and an example survey. In some parts, we focus on the application of the method to shallow geological surveys, to depths of several tens of meters. For further interested readers, we finally list the pieces of literature for review.

The Lower Tuff of the Utaosa Rhyolite, the Teragi Group, is a large-scale pyroclastic flow deposit with a volume of ~100 km³. Its eruption formed a volcanic-collapse structure called Teragi Cauldron. In this study, we present new uranium‒lead (U‒Pb) ages of zircons from a sample of welded pumice lapilli tuff from the Lower Tuff to determine the age of the eruption. U‒Pb isotopic data were collected using multiple-collector ICP-mass spectrometry coupled to a femtosecond laser ablation system for precise age determinations. Modified ²⁰⁷Pb method was used to calculate accurate U‒Pb ages with the correction of deviation derived from non-radiogenic lead and initial radioactive disequilibria. The resulting ²³⁸U‒²⁰⁶Pb ages of each spot analysis are ranging from 3.3 to 2.7 Ma, with a weighted mean age of 3.03±0.01 Ma (coverage factor k = 2, n = 70, MSWD = 2.6). The mean age overlaps within analytical uncertainties with a previously reported biotite K‒Ar age for the Lower Tuff. Our new U‒Pb age confirms that the caldera-forming eruption occurred at 3.1‒3.0 Ma.

Early Miocene sedimentary sequences in the Chubu region of central Japan contain numerous tuff beds, and comparisons and correlations of these beds between sedimentary basins are expected to lead to a better understanding of the stratigraphic architecture of the sequences. In this study, we discuss the correlation of pumiceous tuff beds between the Himaka and lower Yamami Formations of the Morozaki Group in Aichi Prefecture and the Hazama Member of the Akeyo Formation of the Mizunami Group in Gifu Prefecture. We obtained zircon U-Pb ages of 17.87±0.75 Ma and 17.36±0.40 Ma for pumiceous tuff beds from the Himaka and lower Yamami Formations, respectively, and these ages are consistent with the published U-Pb age for the Hazama Member. Paleomagnetic analyses reveal reverse polarity magnetizations in the Himaka and lower Yamami Formations and the Hazama Member. These results suggest that these three units correspond to Chron C5Dr of the paleomagnetic chronology. Chemical analyses of volcanic glasses from pumiceous tuffs in the lower Yamami Formation and the Hazama Member display a single trend in chemical variation diagrams. Plagioclase compositions of pumiceous tuffs in the three units are also consistent. These chemical characteristics indicate that the volcanic glasses and plagioclase of the three units were generated from the same magma plumbing system. The volcaniclastic products of the Morozaki Group examined in this study are considered to have been supplied from the Hazama Member.

Futamatayama volcano is a Quaternary stratovolcano located 6 km north of the Nasu volcano group in southern Fukushima Prefecture, NE Japan. We investigated the eruption history and magmatic processes of the volcano using geological, petrological, and geochronological [thermoluminescence (TL) dating] analyses. The eruptive activity of the volcano can be divided into two stages. The lava flow stage (Stage 1: 3.56 km³ DRE) included at least seven lava flows, and the lava dome stage (Stage 2: 0.09 km³ DRE) involved the formation of two lava domes and a small pyroclastic flow. We obtained TL ages of 163±7 ka, 93±3 ka, and 79±3 ka from the lava flows (Stage 1) and 56±4 ka from a lava dome (Stage 2), which indicate that the volcano formed over a period of >100 ky. The eruption products of Futamatayama volcano commonly consists of felsic rocks (SiO₂ = 56.2-68.4 wt.%) that contain mafic enclaves (SiO₂ = 50.6-59.3 wt.%), indicating magma mixing. On an FeO*/MgO-SiO₂ (whole-rock) diagram, data from Stage 1 and Stage 2 form subparallel linear trends (Stage 1: FeO*/MgO = 1.9; Stage 2: FeO*/MgO = 2.2; at SiO₂ = 64.1 wt.%). These trends suggest that the magmatic system changed between Stage 1 and Stage 2, with different end-member magmas in each stage. Compositional variations between the mafic end-member magmas can be explained by olivine and pyroxene fractionation from a common basaltic magma. Variations in Rb/Ba ratios between the mafic and felsic end-members suggest that the latter could not have been derived from the former by simple fractional crystallization. The felsic end-member magmas of Stage 1 and Stage 2 were produced by different degrees of partial melting of crustal materials in response to heating by mafic end-member magmas.

Two chronologically distinct plant macrofossil assemblages, the Late Miocene Tokachihoroka and the Early Pleistocene Taushubetsu floras, are recognized in the Lake Nukabira area of eastern central Hokkaido on the northernmost island of the Japanese Archipelago. The Tokachihoroka flora from the lacustrine Tokachihoroka Formation comprises 47 taxa in 18 families and 27 genera. The Taushubetsu flora from the lacustrine Taushubetsu Formation comprises 42 taxa in 20 families and 26 genera. The dominant arboreal taxa of these two floras is Betula maximowicziana-relative birch, which is associated with other deciduous broadleaf Betulaceae species, as well as Acer spp. and an evergreen conifer, Picea sp. The compositions of these floras, ecologies of their contemporary counterparts, and results of quantitative climate analysis based on CLAMP (the Climate Leaf Analysis Multivariate Program) indicate that the floras represent a combination of lakeside, slope, and subalpine vegetation types under humid and cool temperate climate conditions. Although the Taushubetsu flora represents a more modern species composition than the Tokachihoroka flora, the physiognomies of both floras reflect volcanic influences under similar humid and cool temperate climatic conditions.

To better understand how ground motion in the Tokyo Lowland is affected by the geological conditions in the shallow subsurface, we analyzed borehole logs and conducted microtremor observations along the Ueno-Koiwa survey line, which crosses the buried valley beneath the Tokyo Lowland. Remarkable peaks in the H/V spectra at ~1 Hz were found in the area of the lower buried terrace (buried flat surface 2), whereas the peaks at ~1 Hz are small in the area of the buried valley bottom with the thickest valley-filling post-Last Glacial Maximum (LGM) deposits. The sharp peaks around buried terrace 2 are generated by the large contrast in the physical properties of the soft, muddy post-LGM deposits and the underlying gravel bed, which suggests larger ground-motion amplification during an earthquake. The variation of the ground motion characteristics along the survey line obtained by microtremor observation is consistent with the distribution of seismic intensity during the 1923 Kanto Earthquake. It is, therefore, important to consider not only the thickness and physical properties of the post-LGM deposits during seismic hazard assessment, but also the total geological composition, including the Pleistocene strata below the post-LGM deposits.

Spherical Fe- oxide concretions are found in the Navajo Sandstone Formation in Utah, USA, which range in size from a few millimeters to a few centimeters. The interior of the concretions is filled with sand and the surface is covered with an Fe oxide rind. The process that formed the Fe rinds, the timescales of formation, and the effect of pores in the concretions and the dissolved oxygen concentration on the rind formation were studied experimentally.

Fe oxide concretions were artificially prepared in a petri dish. An experimental system was established to capture images of the forming Fe rind (brown band). The chemical distribution was investigated by X-ray fluorescence analysis. The results suggest the following two points about the formation of natural Fe oxide concretions. (1) In the early stages of Fe rind formation, there is a rapid increase in the width of the rind. The Fe rind grows rapidly there are many pores in the concretion. (2) Fe oxide concretions with clear rinds were formed at lower dissolved oxygen concentrations.

We propose a new stratigraphic unit, the Nagasaki Kitaura Formation, for the Upper Cretaceous sedimentary rocks on the coast of Kitaura, Nagasaki City, Nagasaki Peninsula, northwestern Kyushu, Japan, to replace the previously named Kitaura Formation. The Nagasaki Kitaura Formation is divided into two lithostratigraphic units that are in fault contact: the Akasakino-hana Sandstone and Mudstone Member (ASMM), the lower, thin unit (>13 m thick), and the Zatobama Gravelly Sandstone and Mudstone Member (ZGSMM), the upper, thicker unit (>140 m). The ASMM consists of shallow marine deposits yielding ammonoid (Polyptychoceras obatai and cf. Phylloceras sp.) and bivalve fossils including an inoceramid (Platyceramus japonicus), and an incomplete femur of a hadrosauroid dinosaur was unearthed from the fluvial ZGSMM. Based on the biostratigraphic range of Platyceramus japonicus (late Santonian? to early Campanian), Polyptychoceras obatai (late Santonian), and the ²⁰⁶Pb/²³⁸U dates of the detrital zircon from the ASMM (the youngest concordant date = 83.6±5.0 Ma; the weighted mean age of the youngest date cluster = 85.74±0.75 Ma; 95% confidence level), the Nagasaki Kitaura Formation is no older than the late Santonian and potentially extends to the Campanian in age. The main body of the formation, the ZGSMM, is correlated with the lower part of the Mitsuse Formation (middle Campanian) on the western Nagasaki Peninsula. The stratigraphy of the Nagasaki Kitaura Formation can be assessed in association with the depositional environments of the lower half of the Upper Cretaceous Himenoura Group in western Kyushu.

The lower Miocene strata in the San’in region were presumed to be a graben-fill deposit based on their distribution and the extensional setting. However, previous research has presented little evidence for grabens; only a few syndepositional faults have been found, and the regional stress has been called into question. To collect more basic data that can improve our understanding of the origin of the Miocene basins, we present geological data for the Tajima Mihonoura area, eastern San’in region, Southwest Japan, including the structure of the base of the lower Miocene Yoka Formation, the zircon U‒Pb age of the formation, and the paleostress during its deposition. There is talus or basal breccia at the contact between the basement and the Yoka Formation, and no faults were found along the contact. These observations indicate that the Yoka Formation unconformably overlies the basement. Although the thickness of the lower Miocene strata was used to estimate the shape of the grabens in the San’in region, we show that the thickness in the study area was dependent on the paleogeography during the early Miocene. Zircon from a felsic pumice tuff from the Yoka Formation was dated, yielding a weighted mean ²³⁸U‒²⁰⁶Pb age of 19.6±0.15 Ma (2σ) from the grains with concordant Miocene ages. This mean age is consistent with other chronological constraints on the formation obtained in the Tango Peninsula and the Tajima-Myokensan area, which form its eastern and southern margins, respectively. Paleostress inversion from the orientation of dikes related to the Yoka Formation suggests that the study area was subject to NE-SW extension with a low stress ratio. This stress probably represents the regional stress in the eastern San’in region, given the similarity of paleostresses reported recently from other areas of Hokutan.

Spherical Fe-oxide concretions have been recognized in both terrestrial and Martian strata and are thought to record a variety of past environmental and alteration conditions. This paper presents a comprehensive review of the occurrence and elemental composition of spherical Fe-oxide concretions on Earth and Mars and their proposed mechanisms of formation. On the basis of geological evidence from Utah and Mongolia, Fe-oxide concretions are considered to form by pH-buffering reactions between Fe-rich acidic water and precursor calcite concretions. By comparing the characteristics of hematite spherules in Meridiani Planum and spherical nodules in Gale Crater, we propose that Martian concretions may also have been formed by interaction between pre-existing carbonates and sulfuric acidic water that infiltrated the rocks early in Martian history. The abundant hematite spherules in Meridiani Planum and spherical nodules in Gale Crater can be considered as relicts of the widespread deposition of carbonate that occurred during the late Noachian-early Hesperian (c. 3.8 to 3.7 Ga) and its dissolution during the late Hesperian (c. 3.5 to 3.2 Ga).

Ocean acidification (OA) is a decrease in seawater pH caused by an increase in the dissolved carbon dioxide concentration in seawater and is an ongoing environmental issue. It is a threat to marine organisms with a calcified skeleton. Decreases in the populations and diversity of these marine organisms may change the carbon and nutrient cycles and cause the depletion of marine biological resources. Predicting the long-term (over a decadal scale) change in seawater pH is difficult. Long-term OA has been identified in geological records. The Paleocene-Eocene thermal maximum (PETM; ~56 Ma) was accompanied by benthic foraminifer and ostracod extinction events and is an ideal target for studying OA over geological time. We review previous studies on OA during the PETM. Many studies report a decrease in the carbonate content of deep-sea sediments, suggesting a shallowing of the carbonate compensation depth (CCD) and lysocline during the PETM. Carbonate ion concentrations on the Pacific seafloor were greater than those in the Atlantic, in contrast to the Holocene. Studies using boron isotopic compositions of planktic foraminifer shells estimate a decrease in sea-surface water pH of 0.2-0.5 within 60 ky of the onset of the PETM. The estimated pH values are associated with measurement and calculation uncertainties. The uptake of CO₂ into the ocean and its emission into the atmosphere and ocean circulation affect the geographic pattern of seawater pH and are still debated.

During the back-arc opening of the Japan Sea between 21 and 15 Ma, volcanic activity increased in range to the oceanic side, erupting a variety of magma types with different origins. We obtained a new K‒Ar age of 16.40±0.10 Ma for an aphyric volcanic rock intruding Jurassic sedimentary rocks in Shirosato Town, Ibaraki Prefecture, which indicates that this rock is a product of Early Miocene fore-arc volcanism. Samples of this volcanic rock show intermediate whole-rock geochemical compositions with high Fe and low Al. Trace-element compositions and Sr‒Nd isotopic ratios indicate that the volcanic rocks were derived from a residual melt generated by crystal fractionation of a mafic magma similar in composition to the nearby Motokosawa Basalt. This basalt may have been derived from lithospheric mantle melted by the injection of upwelling high-temperature asthenospheric material involved in the opening of the Japan Sea.

The Upper Miocene to Lower Pleistocene Shimajiri Group, composed of muddy sediments, and the Lower to Upper Pleistocene Ryukyu Group, consisting of reef-complex deposits, are widely distributed throughout the Ryukyu Islands. The Lower Pleistocene Chinen Formation, which has an intermediate lithofacies, is sporadically distributed between both groups in the central and southern parts of Okinawa-jima. The Chinen Formation was previously believed to be limited to Okinawa-jima, but correlated outcrops, aged between 1.71 and 1.39 Ma, have recently been discovered in Kikai-jima, Kagoshima Prefecture. The outcrops consist of calcareous mudstone and sandstone, or sandy limestone rich in bryozoan fragments, with clear contacts between the underlying and overlying groups. The Chinen Formation overlies the So-machi Formation of the Shimajiri Group across a clearly defined angular unconformity, indicating post-depositional tilting and erosion of the Shimajiri Group. In contrast, there are no structural differences to indicate tilting during the period between the deposition of the Chinen Formation and that of the overlying Ryukyu Group. Therefore, Kikai-jima emerged as a result of relative sea-level fall after the deposition of the Chinen Formation, following which the supply of terrigenous sediment decreased.

Carbonate concretions occur in sedimentary rocks of widely varying geological ages throughout the world. Recently, more than 100 gigantic carbonate concretions with diameters ranging from 1 to 9 m have been identified along the Unosaki coast of Oga Peninsula, Akita Prefecture, Japan. The formation process of such gigantic concretions, some of which along the Unosaki coast contain whale bones, remains uncertain. A mineral composition analysis reveals that the major mineral of the concretions is dolomite. Considering the location of dolomite precipitation, their composition implies that the concretions were formed in a reducing environment in which sulfate ions were removed. Stable carbon and oxygen isotopic analysis reveals that the CaCO₃ of whale bone and concretions contains light δ¹³C and heavy δ¹⁸O, suggesting that whale organic matter contributed to the formation of the concretions. The gigantic carbonate concretions were presumably formed by the accumulation and burial of whale carcasses with high sedimentation rates, and subsequent reaction of carbon decomposed by benthic and microbial activity with seawater.

There should be a lot of supernova explosions near the Solar System since its formation. They have affected the Earth through strong electromagnetic waves, cosmic rays, and blasts. The ionization of the atmosphere by the strong cosmic rays varies the concentrations of ozone and oxides of nitrogen, probably resulting into climate changes. The strong cosmic rays may introduce the evolution and the extinctions of life on the earth. The cosmic rays also generate various radioactive elements through the collisions with the atoms in the atmosphere. Other radioactive elements such as 60Fe may be delivered directly from the supernovae. These elements likely preserved in geological samples of sediments and ice cores.

A bed containing large ammonoids (mostly Mesopuzosia yubarensis, 40-60 cm in diameter) and an underlying bed containing huge calcareous concretions in the middle part of the Obisagawa Member (lower Coniacian), Ashizawa Formation, Futaba Group, are exposed in the Iwaki City Ammonite Center. This study investigated these beds to reconstruct their formation on the basis of sedimentary facies and taphonomy, as well as geochemical analyses of the concretions, including major elements, mineral components, and carbon and oxygen isotopic compositions. Numerous shells of M. yubarensis lacking soft tissues may have been transported into the littoral region through postmortem drift from the initial offshore habitat. Subsequently, these shells were partially destroyed (especially body chambers and umbilical whorls), sorted in size, accumulated, and transported offshore by a series of storm waves and storm-induced currents. Finally, the remains were scattered on mounds of hummocky cross-stratification and rapidly buried within amalgamated hummocky cross-stratified very fine sandstone on the offshore side of a lower-shoreface sedimentary environment. Abundant calcareous concretions of 15-194 cm in diameter and with prolate to oblate spheroid shapes are densely and uniformly distributed under the ammonoid bed. Considering the spatial positioning of the ammonoid shells and concretions within the beds, and isotopic values of δ¹³C = −6‰ to −1‰ and δ¹⁸O = −11‰ to −6‰ for 21 concretion samples, the concretions are interpreted to have begun to form in association with the decomposition of organic matter that had accumulated under the influence of storm waves and storm-induced currents, and carcasses of prolific meiobenthos organisms within shallow substrata. Subsequently, the concretions were buried more deeply and enlarged through further filling of calcium carbonate involving bicarbonate ions generated by the methanogenetic decomposition of organic matter.

This study investigated the mode of occurrence, petrography and geochemistry of basaltic rocks of the Upper Cretaceous accretionary complex (Matoya Group) in the Shimanto Belt in the Kii Naga-shima-Taiki area, eastern part of the Kii Peninsula (Mie Prefecture) to understand their genesis. The Matoya Group in this study area is classified into several stratigraphic units that are distinguished by lithologic assemblage and age. Basaltic rocks included in two of these units; most of which are stratigraphically and closely associated with chert and/or siliceous mudstone, but the remainder occurs as an isolated block in fault contact with the host clastic rocks. Major and trace elements of the basaltic rocks (one sample from the lower unit and five from the upper unit) were analyzed by X-ray fluorescence spectrometry. On the basis of geochemical discrimination diagrams and primitive mantle-normalized incompatible element plots, it is inferred that most of the basaltic rocks have a geochemical affinity to mid-oceanic ridge basalt (MORB) and one sample possibly to oceanic island basalt (OIB). The field observations and petrogenetic pro-perties indicate that the basaltic rocks in the study area formed at depths greater than the carbonate compensation depth in a pelagic environment.

The Gomura fault zone in the Tango Peninsula, southwestern Japan, comprises the Chuzenji (CZJ) and Gomura (GMR) faults and also includes the Go-seihou (GS) fault, which is shorter but has a similar strike to the CZJ and GMR faults. We conducted an audio-frequency magnetotelluric survey at 27 stations along a survey line crossing all of these faults and constructed a two-dimensional resistivity model (GMR2019 model) extending to a depth of 1.5 km. The GMR2019 model can be divided into three areas (Areas 1 to 3). Area 1 is located in the northeastern part of the survey line and shows a typical resistivity structure of the Miyazu granite body, which is almost unaffected by active faulting, and represents the background resistivity structure of the survey area. Area 2 is located in the central part of the survey line and contains the CZJ and GMR faults at its northeastern and southwestern ends, respectively. A highly conductive and subvertical zone is identified just beneath the surface trace of the GMR fault up to ~1 km depth and is interpreted as a fault zone conductor (FZC) formed by fault activity. In contrast, no pronounced FZC is found below the surface trace of the CZJ fault. As both faults have developed in the same granite body under the same tectonic conditions, this difference in FZC can be ascribed to the difference in elapsed time since the last earthquake along each fault; namely, ~100 yr for the GMR fault and 12,000-18,000 yr for the CZJ fault. This shows the possible temporal change in an FZC over a prolonged period of ~10,000 yr. Area 3 is located in the southwestern part of the survey line and contains the GS fault. The resistivity structure and surface displacement related to fault movement of the GS fault are both obscure, suggesting that this fault is likely a secondary fault associated with the adjacent GMR fault.