Kitakami Terrane Research Articles

SEDIMENTARY ENVIRONMENT AND REDOX CONDITIONS OF THE LOWER TRIASSIC OSAWA FORMATION IN THE SOUTHERN KITAKAMI TERRANE, JAPAN: INSIGHTS INTO OCEAN REDOX STRATIFICATION AND FAUNAL RECOVERY

ABSTRACT The sedimentary environments and redox conditions of the Lower Triassic Osawa Formation in the Southern Kitakami Terrane were reconstructed based on lithofacies, trace fossils, and other paleontological content. The muddy and sandy lithofacies of the Osawa Formation lack evidence of storm waves despite the presence of storm-induced turbidites, suggesting that the oldest deposits of the Osawa Formation were deposited in the proximal part of the outer shelf. In turn, water depth increased from the lower to upper part of the formation, ultimately recording the distal part of the outer shelf. In addition to sandy lithofacies caused by turbidity and traction currents, multiple sandy layers within the muddy lithofacies would have originated via supply into the prodelta setting from a fan delta system. Collapses of the delta front or river system floods could have generated hyperpycnal flows, resulting in abundant supplies of mud and organic matter. Trace fossil analyses revealed that the degree of bioturbation (ichnofabric indices) dramatically decreased as water-depth increased, indicating a steep oxic-dysoxic gradient along the onshore-offshore transect. Diagenetic pyrite framboids indicative of dysoxic/anoxic benthic conditions are abundant at greater water depths. Abundant pyrite framboids less than 6 μm in diameter suggest intermittent euxinicity. With an increase in global weathering, abundant sediment supply including organic matter from the fan delta system could have contributed to the development of ocean redox stratification, which appears to have impacted on the adaptation of both nektic and benthic animals of this area during the Early Triassic.

The basement geology of Japan from A to Z

AbstractThe Precambrian and lower Paleozoic units of the Japanese basement such as the Hida Oki and South Kitakami terranes have geological affinities with the eastern Asia continent and particularly strong correlation with units of the South China block. There are also indications from units such as the Hitachi metamorphics of the Abukuma terrane and blocks in the Maizuru terrane that some material may have been derived from the North China block. In addition to magmatism, the Japanese region has seen substantial growth due to tectonic accretion. The accreted units dominantly consist of mudstone and sandstone derived from the continental margin with lesser amounts of basaltic rocks associated with siliceous deep ocean sediments and local limestone. Two main phases of accretionary activity and related metamorphism are recorded in the Jurassic Mino–Tanba–Ashio, Chichibu, and North Kitakami terranes and in the Cretaceous to Neogene Shimanto and Sanbagawa terranes. Other accreted material includes ophiolitic sequences, e.g. the Yakuno ophiolite of the Maizuru terrane, the Oeyama ophiolite of the Sangun terrane, and the Hayachine–Miyamori ophiolite of the South Kitakami terrane, and limestone‐capped ocean plateaus such as the Akiyoshi terrane. The ophiolitic units are likely derived from arc and back‐arc basin settings. There has been no continental collision in Japan, meaning the oceanic subduction record is more complete than in convergent orogens seen in intracontinental settings making this a good place to study the geological record of accretion. Hokkaido lacks most of the Paleozoic history recognized in Honshu, Shikoku, Kyushu, and the Ryukyu Islands to the south and its geology reflects the Cenozoic development of two convergent domains with volcanic arcs, their approach, and eventual collision. The Hidaka terrane reveals a cross section through a volcanic arc and the main accretionary complex of the convergent system is represented by the Sorachi–Yezo terrane.

Biogeographical and Biostratigraphical Significance of a New Middle Devonian Phacopid Trilobite from the Naidaijin Formation, Kurosegawa Terrane, Kyushu, Southwest Japan

Abstract. A newly discovered trilobite from the Naidaijin Formation in the Kurosegawa Terrane of southwest Japan is referred to Toxophacops (Atopophacops) fujiwara sp. nov. The trilobite occurs in strata 2 m below a level with detrital zircons that yield an age of 383.9±4.4 Ma that is likely Givetian (late Mid-Devonian). Elsewhere, Early to Mid-Devonian trilobite species of T. (Atopophacops) and T. (Toxophacops) have been recorded from the Nakazato Formation of the South Kitakami Terrane of northeast Japan, and from the Zhusilenghaierhan region of western Inner Mongolia, North China. The new trilobite species extends the geographical range of the phacopid subfamily Echinophacopinae, and supports the affinities of some Devonian Japanese trilobites with those of north China. The occurrence of Toxophacops in the Lower Member of the Naidaijin Formation, combined with other trilobite evidence, confirms a clear biostratigraphical distinction between the Lower Member (upper Lower and Middle Devonian) and Upper M...

2 南部北上帯西縁部のデボン系鳶ヶ森層と"母体層群"の低角構造

2 南部北上帯西縁部のデボン系鳶ヶ森層と"母体層群"の低角構造

Geochemical indication of provenance of Permian and Mesozoic shales of Southern Kitakami Terrane, Northeast Japan

Geochemical indication of provenance of Permian and Mesozoic shales of Southern Kitakami Terrane, Northeast Japan

Ordovician backarc‐basin metadolerite and metabasalt of the South Kitakami Terrane, Northeast Japan

AbstractBasement rocks that occur along the northern margin of the South Kitakami Terrane in Japan consist of Ordovician ultramafic rocks (Hayachine ultramafic complex), gneissose amphibolite (Kuromoriyama amphibolite), and mafic rocks (Kagura igneous rocks, KIR). The KIR are composed of metagabbro, metadolerite, metabasalt, and minor felsic–intermediate dikes. Although the KIR contain green hornblende due to metamorphism of greenschist to epidote–amphibolite facies, they rarely retain primary brown hornblende. Approximately 30% of the metabasalt shows porphyritic textures, with phenocrysts of saussuritized plagioclase and/or altered mafic minerals. The geochemistry of the common metadolerite and metabasalt of the KIR shows a tholeiite trend, a low TiO2 content, and high Th/Nb and Ti/V ratios. The KIR are therefore indicative of a supra‐subduction zone tectonic setting, which implies a backarc origin (as also indicated by discrimination diagrams). Trace element patterns of the KIR resemble those of the backarc‐basin basalt of the Japan and Yamato basins in the Japan Sea. We propose that the KIR formed during backarc spreading from the Ordovician to Early Silurian. This view is supported by the geochemical data, the tectonic setting of the Hayachine ultramafic rocks, and the provenance of clastics within Silurian sedimentary rocks.

Significance of Permian to Triassic zircon from the boundary zone between the Nedamo Terrane and the North Kitakami Terrane

Significance of Permian to Triassic zircon from the boundary zone between the Nedamo Terrane and the North Kitakami Terrane

Ordovician back-arc-basin basaltic rocks in the South Kitakami Terrane, Northeast Japan

Ordovician back-arc-basin basaltic rocks in the South Kitakami Terrane, Northeast Japan

Coprolites from the upper Osawa Formation (upper Spathian), northeastern Japan: Evidence for predation in a marine ecosystem 5 Myr after the end-Permian mass extinction

Lower Triassic marine deposits, which can potentially provide important information on the diversity and trophic structure of marine ecosystems following the end-Permian mass extinction, are rarely exposed in the world. Given the sparse body fossil record from the Lower Triassic, non-body fossil evidences, such as trace fossils for example, may provide additional windows on fossil ecosystems. Herein, the authors recently found dark, amorphous lumps of various sizes, ranging from a few millimetres to approximately 7cm in maximum dimension, from the offshore marine deposit of upper Osawa Formation (Spathian, Lower Triassic) in the South Kitakami Terrane of northeast Japan. Compared with the surrounding siliciclastic sediments, the lumps are rich in phosphate and organic carbon, and poor in silicates. Shape, mode of occurrence, bone inclusion and geochemical signatures of these lumps confirm that these are coprolites, produced not by benthic sediment feeders, but most likely by nektonic animals. Bone inclusion also suggests that carnivorous fish and/or marine reptiles, constituting the higher part of the food chain, produced at least a part of the specimens. Despite the low diversity of the known fossil vertebrates from the Osawa formation, coprolites first shed light on the predator–prey interactions in the marine ecosystem in South Kitakami Terrane 5Myr after the end-Permian mass extinction.

Ambiguous biogeographical patterns mask a more complete understanding of the Ordovician to Devonian evolution of Japan

AbstractFossil assemblages of the Ordovician to Devonian successions of Japan suggest complex temporal, environmental and geographical controls on their biogeographical signature. Thus, limited similarity at the species‐level between the trilobite, brachiopod and ostracod faunas of the South Kitakami, Hida‐Gaien and Kurosegawa terranes in part reflects the sporadic stratigraphic distribution of shelly fauna within these terranes. As a result, and with the exception of corals and pan‐tropical radiolarians, species‐level similarities are greater with other regions of East Asia and Australia than amongst the Japanese terranes. The Silurian faunas of the South Kitakami Terrane have affinities with North America, Europe, Central Asia and Australia, but there is no overriding signature to support proximity either to South China or Gondwana. Notably, brachiopod and trilobite faunas of the Middle Devonian suggest strong connections with North China. Trilobite, coral and ostracod faunas of the Hida‐Gaien Terrane show affinity, including at species level, with Siluro‐Devonian faunas from westerly‐situated palaeocontinents, especially those of Central Asian and European affinity, suggesting a continuation of the Central Asian Orogenic Belt, or of its associated lithofacies. Greater diversity of groups such as ostracods and trilobites in this terrane may signal closer links with continental shelf faunas of East Asia. The dominant biogeographical signature of the Kurosegawa Terrane is from corals and trilobites, suggesting links with the Siluro‐Devonian of Central Asia, Australia and South China. The variable biogeographic signal of the Japanese faunas may reflect the lifestyles of organisms with different physiologies and larval dispersal mechanisms, as well as the relative incompleteness of the Japanese fossil record. The present state of knowledge of the faunas cautions against placing Japan in relative proximity to the North or South China plates, or of presenting the Japanese terranes as a unified island arc to the north of the South China Plate during the Early Palaeozoic.

Tectonic constraints to Cretaceous magmatic arc deduced from detrital heavy minerals in northeastern Japan – evidence from detrital garnets, tourmalines and chromian spinels

Tectonic histories of sedimentary basins in the Cretaceous Japan arc have been assessed to understand the response of the Asian continental margin to the oblique subduction of the Paleo-Pacific (i.e. Kula or Izanagi) Plate beneath the Asian continent during the Early Cretaceous and that which subducted orthogonally in the Late Cretaceous. In the Lower Cretaceous Kuji Group (Santonian–Campanian) of the Kitakami Massif in northern Japan, sandstone petrography and chemistry of detrital heavy mineral grains were performed on sandstones to assess the tectonic environment on the basis of provenance analysis.Sandstone petrography results suggest that the material of the Kuji Group was derived mainly from areas of a Cretaceous volcanic belt (Rebun-Kabato Belt) and from a Jurassic accretionary complex (North Kitakami Terrane), which was intruded by Cretaceous granite, adjacent to the depositional basin. The chemical composition of detrital garnets suggests a North Kitakami Terrane origin, and detrital tourmalines are considered to have been derived mainly from meta-sedimentary rocks. The composition of detrital chromian spinels are compositionally diverse and mainly derived from tholeiitic and intra-plate basalts showing high TiO2 (>about 1.0 wt%) and island arc basalts with moderately low TiO2 (1.0 > TiO2 > 0.5 wt%) and high Cr#. Latter chromian spinels can be considered as a record of island arc activity including high Mg-andesite in Early Cretaceous time. Because adequate source rocks of the spinels are elusive near the basin compared with those of detrital garnets and tourmalines, these rocks are believed to have been disturbed by Cenozoic tectonics and eroded and covered by newly formed volcanic and sedimentary rocks.Comparison of chemical composition of the chromian spinels between Lower and Upper Cretaceous deposits in northern Japan indicates that chromian spinels with very low TiO2 (<0.5 wt%) prevail in the Lower Cretaceous (Aptian–Albian). In contrast, chromian spinels showing moderately low TiO2 predominated in the Upper Cretaceous (Santonian–Campanian). This clear difference suggests the change of oceanic plate motion around Japan arc promoted the change of source rock assemblage and the arc volcanic activity in mid-Cretaceous time. Thus the characteristics of detrital heavy mineral composition of the Kuji Group give the key to clarify the interaction between the swaying of young and hot plate and development of the Cretaceous island arc in eastern Asian margin.

Whereabouts of the collision belt between the Sino-Korean and South China blocks in the northeast Asian margin

This study delineates the collision belt (late Permian-Triassic) between the Sino-Korean and South China blocks in the northeast Asian margin, based on correlation of endemic fossil components (corals) in shallow marine deposits of the Silurian and Devonian, identified in both south China and southwest and northeast Japan. The Qinling-Dabie Belt in China was offset by the left-lateral Tanlu Fault and contiguous to the Sulu Belt. The Imjingang Belt in the Korean peninsula was, in turn, offset by the right-lateral South Korean Tectonic Line, and extended eastward to the Namhae Belt (newly named). Further east, it extended to the Higo Belt and the Kitakami Terrane in Japan, prior to the opening of the East Sea in the Miocene. The collision in the northeastern Asian margin in the Triassic was superposed by contemporaneous subduction of the paleo-Pacific Plate.

Sinistral tectonics of the boundary zone between the South Kitakami Terrane and the Nedamo Terrane in the northern part of Mt. Hayachinesan, Northeast Japan

Sinistral tectonics of the boundary zone between the South Kitakami Terrane and the Nedamo Terrane in the northern part of Mt. Hayachinesan, Northeast Japan

Tectonics of an Early Carboniferous forearc inferred from a high‐P/T schist‐bearing conglomerate in the Nedamo Terrane, Northeast Japan

AbstractThe Nedamo Terrane, an Early Carboniferous accretionary complex, is the oldest biostratigraphically dated accretionary complex in Japan. The purpose of this study is to describe and interpret a conglomerate from the Nedamo Terrane that contains clasts of high‐pressure/low‐temperature (high‐P/T) schist (mainly garnet‐bearing phengite schist) and ultramafic rock, and to infer the tectonics of an Early Carboniferous arc–trench system at the eastern margin of the paleo‐Asian continent. Clasts of high‐P/T schist and ultramafic rock within the conglomerate make up 8.4 and 6.7% of the total clasts, respectively, based on modal counts. These clasts are subangular to subrounded, whereas volcanic clasts are well rounded. The source of the schist clasts, which yield a radiometric age of 347–317 Ma, is considered to be the Renge Metamorphic Rocks of Southwest Japan or equivalent rocks. Based on the chemical composition of chromian spinel, the source of ultramafic clasts is inferred to be the island‐arc‐type Ordovician Miyamori and Hayachine ultramafic complexes in the Kitakami Massif. The conglomerate records multiple provenance regions, including an island arc (South Kitakami Terrane) and a forearc ridge; the high P/T schist and ultramafic rocks were exhumed in the forearc region. The duration of the interval from the early stages of exhumation of the schist to its deposition in the trench as clasts is estimated to have been less than 30 my.

Middle Permian (Midian) Foraminifers of the Kamiyasse Formation in the Southern Kitakami Terrane, NE Japan

A refined foraminiferal biostratigraphy of the Middle Permian Kamiyasse Formation in the Kamiyasse area north of Kesennuma, Southern Kitakami Terrane, NE Japan, is presented. Based on the stratigraphic distribution of schwagerinid and neoschwagerinid fusulinoideans, the Kamiyasse Formation, equivalent to the Kanokura Formation in the type area (Setamai-Yahagi area), is subdivided into the Monodiexodina sutchanica, Parafusulina motoyoshiensis and Lepidolina shiraiwensis Zone in ascending order, all of which appear to correspond to the Midian (latest Wordian to Capitanian) in age. The M. sutchanica Zone is here defined as integrated with cross-stratified sandstone beds in the bottom of the formation, and this zone is no longer equivalent to the previously defined M. matsubaishi (junior synonym of M. sutchanica) Zone in the Iwaizaki Limestone and the Kanokura Formation of the type area. The other two zones are defined with the first occurrences of the zonal species. Foraminifers are highly diverse along the stratigraphic interval between the Parafusulina motoyoshiensis and Lepidolina shiraiwensis Zones. Fifty-seven species belonging to 42 genera of foraminifers are identified. Nineteen species among them are systematically described and discussed, including the following three species: Baisalina rikuzenensis sp. nov., whose middle and later whorls are subdivided into more than fifteen chamberlets by septal protrusions; Wutuella sp., previously misidentified as “Cancellina” sp.; and Septagathammina sp., exclusively known from South China.

Cadmium contents of Paleozoic limestones in South Kitakami Terrane, northeast Japan

Cadmium contents of Paleozoic limestones in South Kitakami Terrane, northeast Japan

Lithology of the Nedamo Terrane, an Early Carboniferous accretionary complex, and its southern boundary with the South Kitakami Terrane

本見学コースでは，北上山地に分布する前期石炭紀付加体を含む根田茂（ねだも）帯と南部北上帯北縁部のオルドビス～シルル紀島弧地質体を観察する．根田茂帯の綱取ユニットは化石によって時代が決定された付加体としては日本最古であり，これに相当する地質体は現在のところ日本列島の他地域にはない．根田茂帯は泥岩／珪長質凝灰岩互層（MS互層）と緑色岩類の卓越で特徴づけられ，日本列島各地のジュラ紀付加体や白亜紀付加体とは異なる岩相を示す．また南部北上帯北縁部には，シルル紀砕屑岩類の基盤となる島弧型緑色岩類や角閃岩・超苦鉄質岩が分布している．本見学では，これらの地質体を観察し，古アジア縁辺部における古生代沈み込み帯のテクトニクスを議論したい．

SHRIMP zircon and EPMA monazite dating of granitic rocks from the Maizuru terrane, southwest Japan: Correlation with East Asian Paleozoic terranes and geological implications

Abstract The Maizuru terrane, distributed in the Inner Zone of southwest Japan, is divided into three subzones (Northern, Central and Southern), each with distinct lithological associations. In clear contrast with the Southern zone consisting of the Yakuno ophiolite, the Northern zone is subdivided into the western and eastern bodies by a high‐angle fault, recognized mainly by the presence of deformed granitic rocks and pelitic gneiss. This association suggests an affinity with a mature continental block; this is supported by the mode of occurrence, and petrological and isotopic data. Newly obtained sensitive high mass‐resolution ion microprobe (SHRIMP) zircon U–Pb ages reveal the intrusion ages of 424 ± 16 and 405 ± 18 Ma (Siluro–Devonian) for the granites from the western body, and 249 ± 10 and 243 ± 19 Ma (Permo–Triassic) for the granodiorites from the eastern body. The granites in the western body also show inherited zircon ages of around 580 and 765 Ma. In addition, electron probe microanalysis (EPMA) monazite U–Th–total Pb dating gives around 475–460 Ma. The age of intrusion, inherited ages, mode of occurrence, and geological setting of the Siluro–Devonian granites of the Northern zone all show similarities with those of the Khanka Massif, southern Primoye, Russia, and the Hikami granitic rocks of the South Kitakami terrane, Northeast Japan. We propose that both the Siluro–Devonian and Permo–Triassic granitic rocks of the Northern zone are likely to have been juxtaposed through the Triassic–Late Jurassic dextral strike‐slip movement, and to have originated from the Khanka Massif and the Hida terrane, respectively. This study strongly supports the importance of the strike‐slip movement as a mechanism causing the structural rearrangement of the Paleozoic–Mesozoic terranes in the Japanese Islands, as well as in East Asia.

New age constraints on counter-clockwise rotation of NE Japan

SUMMARY The Miocene basaltic‐andesitic lavas and Cretaceous granites were sampled in the Abukuma Terrane of northeast Honshu for geochronological and palaeomagnetic investigations to determine precise timing of counter-clockwise rotation of NE Japan. Characteristic component with unblocking temperature of 560‐590 ◦ C is isolated from 26 sites of the Takadate and Ryozen Formations and 12 sites of the Cretaceous granites. Concordant northwesterly palaeomagnetic directions (D = 265 ◦ ‐331 ◦ ) are observed in the Cretaceous granites of the Abukuma Terrane. The Cretaceous mean-palaeomagnetic direction from this terrane (D = 313.5 ◦ , I = 54.6 ◦ , α 95 = 6.3 ◦ and N = 20) is almost identical to Oligo-Miocene (33‐20 Ma) palaeomagnetic direction reported from the Japan Sea side of NE Japan as well as to the Cretaceous palaeomagnetic direction reported from the Kitakami Terrane. This type of behaviour clearly suggests that the Abukuma Terrane underwent a coherent counter-clockwise rotation as a part of NE Japan after 20 Ma. Conventional potassium (K)-Argon (Ar) dating method performed on the plagioclase grains revealed distinctive ages of 14.44 ± 0.88 and 16.52 ± 0.82 Ma for the Takadate Formation (11 sites) and the Ryozen Formation (eight sites), respectively. This type of behaviour indicates that two independent episodes of volcanic activities were probably responsible for the production of igneous rocks in the Takadate and Ryozen areas. Northerly declination is obtained from the Takadate Formation (D = 355.3 ◦ , I = 39.4 ◦ , α 95 = 11.6 ◦ ), while slightly westerly deflected declination appears in the Ryozen Formation (D = 333.5 ◦ , I = 58.0 ◦ and α 95 = 9.6 ◦ ). The here presented results, therefore, suggest that the province of NE Japan was still experiencing a counter-clockwise rotation at 16.5 Ma, however by 14.4 Ma the rotational motion has completely ceased. Compared with the available palaeomagnetic and geochronological data from SW Japan, rotational motion in both provinces of NE and SE Japan has already been ceased by 14 Ma. Thus any additional evidence regarding the complete cessation of rotational motions in NE Japan will provide a key to understand the timing of opening in the Japan Sea.

Chemical composition and ichnofacies of mudstones of the Lower Triassic Osawa Formation in the Ogatsu area, South Kitakami Terrane, northeast Japan

This study aims to reconstruct shelf environment of the Lower Triassic Osawa Formation in the Inai Group, the South Kitakami Terrane (SKT), on the basis of the vertical variations of chemical composition and ichnofacies. The Osawa Formation in the Ogatsu area is divided into the lower part (OL ; intensely bioturbated calcareous siltstones) and upper part (OU ; parallel laminated mudstones). The redox proxies (Zn/TiO2 and Ni/TiO2 ratios) and the benthic activity proxies (ichnofabric index and burrow diameter) suggest that the OL was deposited in a shelf where benthic activity was very high, whereas the OU was deposited in a relatively reductive environment that prevented benthic activity.