A landslide occurred on the south slope of the hill, so-called ‘Mt. Maruyama’, Nishime, Tsuruoka, Yamagata Prefecture, on December 31, 2022, destroying 17 houses and causing two deaths. We surveyed the geology of ‘Mt. Maruyama’ and describe its rock facies and minerals that have formed from weathering. The landslide head scarp is composed of weakened basaltic andesite and minor sedimentary rocks. More broadly, the facies at ‘Mt. Maruyama’ consist mainly of sedimentary rocks, including tuffaceous conglomerate, tuffaceous sandstone, lapilli tuff, and pumiceous tuff associated with a lens-shaped body of basaltic andesite. These rocks might belong to the lower to middle Miocene Zenpoji or Oyama Formations. Powder X-ray diffraction (XRD) analyses identified halloysite, kaolinite, and goethite in all facies, suggesting that the igneous and pyroclastic rocks have been strongly weathered by groundwater. The weakened nature of the rocks indicates that ‘Mt. Maruyama’ has been affected by deep weathering.

日本の付加体中に含まれる中古生代の海洋性岩石（主に石灰岩と層状チャートからなる）は，陸源砕屑物の到達しないパンサラッサ海の遠洋域で長期間堆積した記録を持つ．これらの堆積岩類は，微化石研究の進展により詳しい年代決定が可能になり，さらに1990年代からは主要・微量元素や同位体分析による多元素の情報を得ることで，高い時間解像度で古環境の復元が行われてきた．本巡検では，九州東部の津久見地域および高千穂地域に分布する秩父帯のペルム系〜ジュラ系石灰岩・層状チャートを対象として，これらの堆積岩類が記録した中古生代の古環境イベントに焦点を当てた巡検を予定している．特に，（1）ペルム紀キャピタニアンの寒冷化と絶滅イベント，（2）ペルム紀末大量絶滅，（3）前期三畳紀の海洋無酸素事変と中期三畳紀の回復過程，（4）後期三畳紀のカーニアン多雨事象，（5）後期三畳紀ノーリアンの天体衝突，（6）前期ジュラ紀トアルシアンの海洋無酸素事変を記録した堆積岩類を観察する．

Since 2020, a four-year study of the potential geological disposal of nuclear waste has been conducted for the Kamoenai and Suttsu areas of Hokkaido. In the determination of candidate sites for nuclear waste disposal, Quaternary volcanoes must be at least 15 km from such repositories. Potassium–Ar ages of late Cenozoic volcanic rocks in the Kamoenai and Suttsu region were determined to evaluate potential sites for the geological disposal of nuclear waste. We obtained ages of 4.96 ± 0.15 Ma for the Tomaruyama Lava (early Pliocene) and 2.70 ± 0.60 Ma (late Pliocene to early Pleistocene) for the Isoya Lava. This suggests the Suttsu area is an unsuitable site for a nuclear waste repository. The Isoya Lava consists of andesite to dacite with icelandite-like characteristics, high FeO*/MgO ratios, similar to those of the nearby early Pleistocene Raidenmisaki Volcanic Breccia Formation.

九州西端に位置する長崎変成岩は西彼杵（にしそのぎ），野母（のも），天草高浜の3つのユニットから構成され，南北方向の分布を示す．西彼杵ユニットは約90 Maの片岩類と蛇紋岩類からなる単調な同斜構造を示す低温高圧型変成岩である．西彼杵ユニットには，片岩類に挟在されて蛇紋岩メランジュが複数存在する．そのうち最大のものは三重（みえ）メランジュで，厚さ最大350 m，延長約8 kmに達する．三重メランジュはヒスイ輝石岩を産することが特徴的である．また西海市（さいかいし）雪浦（ゆきのうら）には，片岩中に層状に発達する塊状蛇紋岩の一部がメランジュとなっており，メランジュ中の構造岩塊からマイクロダイヤモンドを産する．このような超高圧条件を示す岩石は西彼杵ユニットの他の場所からは発見されていない．また日本でも唯一の産地である．西彼杵ユニットの変成温度条件は泥質片岩に含まれる石墨の顕微ラマン分光法による温度推定により，450–500 °Cと推定される．圧力条件については三重メランジュについてヒスイ輝石＋石英の存在から1.5 GPa程度，雪浦メランジュについてはマイクロダイヤモンドの存在から2.8 GPa程度以上，また片岩類についてはザクロ石藍閃石片岩中のザクロ石，オンファス輝石とフェンジャイトを用いた圧力計より1 GPa程度（ザクロ石コア部）から2.2 GPa程度（ザクロ石リム部）と推定される．

The Uchinami pluton, which is distributed upstream of the Uchinami-gawa River in Ono City, Fukui Prefecture, consists mainly of altered quartz diorite, granodiorite, and quartz monzodiorite. A previous study reported a Cretaceous K–Ar whole-rock date (106.5 Ma) of the pluton and interpreted it as one of the Cretaceous (post-Tetori–Pre-Nohi) plutonic rocks in the northern part of central Japan. We performed U–Pb dating of zircons from two rock samples of the Uchinami pluton and obtained weighted average ²⁰⁶Pb/²³⁸U dates of 189.3 ± 1.8 Ma and 189.7 ± 1.6 Ma (2σ). The difference between the U–Pb zircon dates and the K–Ar whole-rock date is ca. 80 million years, which is too long for the duration of cooling of the pluton from the crystallization of the zircons. Instead, the K–Ar whole-rock date presumably indicates the rejuvenation by the Miocene intrusive rocks (e.g. Arashimadake Cauldron). Thus, the Uchinami pluton is reasonably a member of the Jurassic Hida Younger Granite but not a Cretaceous pluton.

The Lower Miocene Yoka Formation in the eastern San’in region, southwest Japan, records volcanism that occurred during the opening of the Japan Sea, and consists mainly of mafic volcanic rocks. There have been no reports of rhyolite in the formation, and Miocene felsic volcanic rocks in the region are considered to unconformably overlie the formation. However, here we report the presence of two hornblende rhyolite bodies in the Yoka Formation in the Hamasaka area, Hyogo Prefecture. Zircon U‒Pb analysis yielded mean ages of 19.36 ± 0.10 Ma (2σ; MSWD = 0.77) and 19.50 ± 0.08 Ma (2σ; MSWD = 1.23) for samples collected from the two rhyolite bodies. Field observations indicate that the rhyolite dated to 19.36 ± 0.10 Ma is probably a lava flow. Our U‒Pb ages are consistent with previously reported radiometric ages for the Yoka Formation (ca. 21.5‒19.0 Ma). These results indicate that Miocene felsic volcanic rocks in the eastern San’in region include those coeval with the mafic volcanic rocks in the Yoka Formation.

A tephra correlated with the So-OT tephra from Shiobara caldera in northern Tochigi Prefecture was identified in the Yabu Formation of the Middle Pleistocene Shimosa Group in the GS2012-OMY core, drilled at Fukasaku, Saitama City, Saitama Prefecture, on the central Kanto Plain, Japan. This tephra and the So-TN tephra immediately below it have been recognized at several locations on the Kanto Plain, indicating they occur between the Yb1 and Yb5 marker tephras in the Yabu Formation. Furthermore, the So-OT tephra is stratigraphically located just above the sea-level highstand of Marine Isotope Stage 9.3. Recently, the importance of the Yabu Formation on the Kanto Plain has been increasing from both applied geological and hydrogeological viewpoints as a foundation for structures and as an aquifer. The identification of additional marker tephras in the Yabu Formation is expected to improve our understanding of its distribution.

1804年7月10日，東北地方の日本海沿岸においてマグニチュード7.3の地震が発生した（1804年象潟地震）．この地震による海岸隆起により象潟湖の湖底が露出し，陸地化した．津波も発生し，象潟での最大浸水深は4～5mと推定されている．しかしながら，この津波に関連するイベント堆積物の詳細な報告はなされていない．そのため本研究では，2 m程度のコア掘削による，津波堆積物調査を行った．調査地点は，秋田県にかほ市象潟町の海岸近くの水田にて，海岸線にほぼ直交する測線上に11点を設定した．層相観察の結果，耕作土のほぼ直下にイベント堆積物と考えられる淘汰の良い黄褐色の細～中粒砂層が見出され，これらは内陸への薄層化が見られ海岸から約1,400 m付近まで追跡できた．粒度分析によるとイベント堆積物の内陸への細粒化傾向が認められる．また，堆積物中の珪藻化石分析では汽水～海水生種が大半を占めた．また，イベント堆積物上層は淡水生種，下層は海水生種が占有した．以上のことから，この砂層は1804年象潟地震による隆起と同時に堆積した津波堆積物である可能性が高いと考えられる．

The southern Ashikita region in southwestern Kyushu, southwest Japan, is underlain by Jurassic to Lower Cretaceous accretionary units (Chichibu Belt) and the Cretaceous Saiki Subgroup (Shimanto Belt) in its northern and southern parts, respectively. These pre-Paleogene units are widely covered by Neogene and younger formations, and thus are exposed in isolated outcrops over a narrow area. The modal compositions of representative pre-Paleogene sandstones in this region were determined. A ternary Q–F–R plot of the sandstone compositions indicates the sandstones are mostly feldspathic. A Qm–F–Lt plot constructed using the Gazzi–Dickinson method indicates the sandstones can be divided into two groups based on the content of monocrystalline quartz (Qm). Seven samples from the northern part of the study area have high Qm/F values (≥0.69) and 10 samples from the southern part of the study area have low Qm/F values (≤0.50). The boundary between the locations of the high- and low-Qm/F samples coincides with the known position of the Butsuzo Tectonic Line in this region.

九州中部には，北東―南西方向に延びる黒瀬川帯が分布し，シルル紀以降の化石群が産出する．特に，祇園山層は，日本のシルル～デボン系の‘模式地’として有名であり，豊富な化石群を産出する．デボン系山出（やまいで）層は，2000年代に入って命名され，本邦最西端の古鱗木の化石産地でもある．石炭系柿迫（かきさこ）層は，暴風時波浪限界水深より大きい深度で堆積した地層で，生物砕屑物を含む多様な岩相からなる．本巡検では，まず祇園山層の露頭の観察や大型・微化石試料の採集をおこない，堆積環境について考察する．続いて，デボン系山出（やまいで）層にて，古鱗木をはじめとした植物化石の収集と岩相と植物化石の保存・産状を観察する．最後に，石炭系柿迫層にて，様々な岩相の石灰岩を観察し，酸処理によって微化石が抽出できる岩相の一例を紹介する．

The Takada Granodiorite, in the central part of the San’in batholith in southwest Japan, is thought to have undergone prolonged magmatic activity based on zircon U–Pb dating. The pluton consists mainly of hornblende–biotite granodiorite that shows textural evidence for mingling with mafic magma (Ogi Quartz diorite). Oscillatory zoning in minerals (e.g., anorthite content in plagioclase, Zr content in titanite, and Ba content in alkali feldspar) and remelting textures are common in the Takada Granodiorite. Hornblende and titanite geothermobarometers indicate emplacement pressures of 180–200 MPa with temperature fluctuations near the solidus. Moreover, whole-rock Rb–Sr isotopic compositions did not form a well-defined isochron. These observations suggest multiple injections of granitic magma with isotopically heterogeneous source material in addition to mafic magma, which account for the prolonged magmatic activity that produced the Takada Granodiorite.

This study updates the mid-Cretaceous (100 Ma) paleomagnetic pole for the inner arc of the southwestern Japanese islands, referred to as southwest Japan, to investigate potential stagnation in its apparent polar wander path (APWP). The updated pole interpolates the Cretaceous APWP for southwest Japan, providing insights into the geodynamic characteristics of the region during this period. For paleomagnetic analysis, red mudstone samples were collected from four sites in the 100 Ma Hayama Formation in the Hayama area. A high-temperature remanent magnetization component, carried by hematite, was isolated from the four sites, passing the fold test. The primary nature of the magnetization was confirmed by the detrital origin of the magnetic carrier. The primary directions, combined with previously reported results, were used to define a paleomagnetic pole representing the 100 Ma pole for southwest Japan, located within the group of Cretaceous poles. The APWP for southwest Japan between 110 and 70 Ma was revised to evaluate stagnation in polar motion in this region.

Zircon double-dating (U–Pb and (U–Th)/He) was performed on a tephra in the Pleistocene Chiraigawa Formation, Oshamambe district (Kuromatsunai lowland), southwestern Hokkaido. The obtained ages were 1.035 ± 0.048 Ma and 1.030 ± 0.054 Ma, respectively, which are mutually consistent. These results indicate that the zircons from this tephra crystallized in a magma at ca. 1.0 Ma and an eruption producing the tephra happened instantly. The applied methodology appears to be robust for dating Quaternary volcanic activity, similarly to another zircon double-dating (U–Pb and fission-track) approach. Furthermore, U–Pb dating of a tephra in the Pleistocene Setana Formation yielded an age of 1.063 ± 0.037 Ma. These results indicate that the depositional age of the Setana Formation is in close agreement with previous estimates, but that the Chiraigawa Formation, which was previously assumed to be Middle Pleistocene, is Early Pleistocene. The ages also indicate that the Setana and Chiraigawa formations were deposited in a very short time interval.

サハリン南西部のモネロン島から得られた深部ボーリングコア中の白亜紀火山岩類は，玄武岩質から安山岩質の島弧マグマ組成を有し，礼文－樺戸帯に属する．モネロン島火山岩のHFS元素組成はN-MORBに類似し，イザナギプレートの沈み込み時にスラブ堆積物メルトの寄与量が減少していたことが示唆される．

西太平洋周辺のテクトニクスは，主に花崗岩質岩の研究により，約127 Maごろのファラロン－イザナギ海嶺の沈み込みと，56–46 Maのイザナギ－太平洋海嶺の沈み込みに伴う火成活動の停止，およびマントル組成の置換が議論されている．一方で，モネロン島火山岩に関する本研究の同位体データは，本地域周辺におけるイザナギ－太平洋海嶺の沈み込み前後でのマントル置換を否定する．花崗岩質マグマは地殻成分の影響を強く受けていることが多いため，スラブ－マントル相互作用の解明には，玄武岩質の岩石を用いることが重要である．

鳥取県中南部に分布する“上部中新統”三徳層は日本の温帯林の起源とされる三徳型植物群の模式層である．本研究では，本層の年代を決定するため，かつて三徳型の三朝成植物群が報告された三朝町成地域で本層上部のジルコンU–Pb年代を求め，花粉層序からその妥当性を検証した．大型植物化石産出層に挟在する浮石質凝灰岩層および凝灰岩層の年代は約4．7 Ma（前期鮮新世中頃）であった．同層の花粉群集は冷温帯要素と暖温帯要素が混在し，マツ科植物のCathaya属が継続的かつ普通に産した．この特徴は新第三紀花粉層序のNP-5（5.5–3.0 Ma）に一致し放射年代と矛盾しない．三朝成植物群の温暖要素はかつて中新世温暖期の要素が残存したと捉えられたが，本研究から，同植物群が前期鮮新世の暖温帯気候を代表することがわかった．これは同地における三徳型植物群の層位変化の見直しのみならず層序体系の見直しにも貢献すると考えられる．

Despite the recent diversification of Tenthredinid sawflies in Japan, fossil records of this family are notably lacking. Here, we describe specimens belonging to this family from the Chibanian (Middle Pleistocene) lacustrine deposits of the Shiobara Group. These specimens are identified as Tenthredinidae gen. et sp. indet A and B and Nematinae gen. et sp. indet. This study represents the first report of fossil tenthredinids in the Shiobara Group and also an incontrovertible Cenozoic fossil record of this family from Japan.

前期–中期中新世の西南日本前弧域の応力史解明には前弧海盆堆積物の年代情報が必要である．紀伊半島南西部に分布する下部–中部中新統田辺層群の堆積時期制約のため，砕屑性ジルコンのU–Pb年代を測定した．221粒子のコンコーダントな年代値のうち，最若粒子年代値は約19.4 Maであり，田辺層群の堆積年代の上限はこれより若いと考えられる．浮遊性有孔虫化石が示唆する堆積時期（約16.3 Ma以降）より若い粒子は含まれなかった．紀伊半島周辺で噴出物を伴う火山活動が広範囲で活発化した約15 Maより若いジルコンが含まれないことから，田辺層群はそれ以前の堆積物であると考えられる．田辺層群の堆積時期には後背地周辺の火山活動が低調だったために，砕屑性ジルコンの最若年代値と実際の堆積時期が乖離した可能性がある．以上の結果は，田辺層群中の砕屑岩脈が記録する東西引張応力が約15 Ma以前に働いたことを示唆する．

日本の沿岸低地の多くは最終氷期以降に堆積した沖積層で覆われている．集水域が小さい小規模な開析谷は，上流からの堆積物供給量が少ないために特徴的な堆積環境と物性の沖積層が堆積する．このような沖積層の堆積相および物性を調べることを目的に，埼玉県の芝川低地で2本のボーリング調査（GS-SUM-1, GS-SMS-1）を行った．芝川低地の最終氷期以降の堆積環境は，9 ka以降に海水が入り内湾化し，6 ka頃内湾がもっとも拡大し，4 kaには淡水の湿地環境となったことがわかった．内湾の堆積物は砂分をほとんど含まない泥を主体とし，100 m/s前後と遅いS波速度を示した．堆積物供給が少ないことに加え，内湾が拡大した頃に谷の出口が閉塞されていたために，泥質で軟弱な堆積物が形成されたと考えられる．台地の開析谷の下流部には，このような軟弱な沖積層が存在する可能性が高く，地盤強度の面から注意が必要である．

We propose the name ‘Obaneo Lava’ for Pliocene mafic volcanic rocks at Cape Haneobana, northeastern Tottori Prefecture. We undertook a geological survey of these rocks, measured their whole-rock major element compositions, and subjected them to plagioclase K‒Ar dating. The mafic volcanic rocks comprise at least three lava flows of clinopyroxene–olivine basalt and olivine basalt. Their chemical composition indicates they are alkali basalt. Their uniform compositions and the lack of evidence for dormancy indicate that they were derived from continuous volcanic activity. The plagioclase K‒Ar age of 4.28±0.11 Ma for the upper lava is older than the ages of surrounding Pliocene volcanic units (i.e., the Inabayama Basalt and rocks of Hamasaka Volcano). The Obaneo Lava is distinguished from these surrounding rocks on the basis of chemical composition and radiometric age.

To increase the number of radiometric dates of Jurassic accretionary complexes in Japan, we conducted zircon U–Pb dating of a sandstone sample from the Sawando Complex in the eastern Mino Terrane using a multi-collector inductively coupled plasma mass spectrometer coupled with a multi-spot femtosecond laser ablation system (msfsLA-MC-ICP-MS). Thirty two concordant dates from 72 analyzed grains are divided into seven groups: 1920–1850 Ma (Paleoproterozoic), 1110 Ma (Mesoproterozoic), 492 Ma (Cambrian), 442 Ma (Silurian), 262–260 Ma (Permian), 250–218 Ma (Triassic), and 186–168 Ma (Jurassic). The youngest single grain date (YSG) is 168.0±6.9 Ma (±2σ). The YSG is within uncertainty of radiolarian ages reported in previous studies (late Callovian to middle Oxfordian).

徳島平野地下に分布する更新統は，堆積盆の発達史を解明する上で重要な鍵層となるが，その年代や堆積環境については明らかになっていない．本稿では，テフラ，泥質堆積物の懸濁液の電気伝導度（EC）およびpH分析，珪藻化石，花粉化石を用いることで，徳島平野南東部沿岸地域における地下更新統の汽水～海成層を認定し，年代層序を構築した．既存ボーリングコア試料から見出した4枚のテフラをG9L，鳴尾浜IV（Nh-IV），加久藤（Kkt），阿多鳥浜（Ata-Th）テフラに対比した．また，EC，珪藻化石，花粉化石層序に基づき，海洋酸素同位体ステージ（MIS）11～5eの高海水準期に相当する5枚の汽水～海成層を認定した．MIS 11，9，5eに相当する海成層は平野北部からも報告されており，徳島平野で広く追跡できると考えられる．

Sub-bottom profilers (SBPs) are essential tools for investigating geological structures beneath sea or lake floors. SBPs are widely deployed because they do not interfere with other onboard survey methods such as sediment sampling and seismic profiling. However, SBP data are underutilized because they are stored in multiple different proprietary formats and commercial software is expensive. To improve this situation, we initially targeted a SyQwest Bathy-2010 installed on the R/V Hakuho Maru (JAMSTEC) and developed a procedure for SBP data visualization. This note aims to be a practical manual, with step-by-step guidance and screenshots, allowing interested users to easily process SBP data. The procedure involves three key steps: onboard data conversion, running a Python script, and creation of an index map. To ensure accessibility, the workflow has been tailored for free platforms, including Google Colab, enabling users with limited technical resources or programming experience to process SBP data effectively. We hope this effort democratizes the use of SBP data and facilitates its broader application in marine geological research.