Southern Kitakami Research Articles

Exploring the Eastern Abukuma Mountains: A field guide to high-pressure metamorphic rocks and strike-slip crustal shear zones in the Southern Kitakami Belt, NE Japan

Exploring the Eastern Abukuma Mountains: A field guide to high-pressure metamorphic rocks and strike-slip crustal shear zones in the Southern Kitakami Belt, NE Japan

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.

Death or Living Assemblage? The Middle Permian Discinid Brachiopods in the Kamiyasse Area, Southern Kitakami Mountains, Northeastern Japan

Linguliformean brachiopods often exhibit a clustered occurrence. Because there has been no detailed analysis of whether these clusters represent death or living assemblages, we exemplified the burial processes of the Middle Permian discinid brachiopods from the Hoso-o Formation with the systematic descriptions of Orbiculoidea verum sp. nov. and Discinidae gen. et sp. indet. There were three types of discinid occurrences: 1) articulated Orbiculoidea in a ventral-side-down orientation, 2) disarticulated valves of both species in a convex-up orientation, and 3) disarticulated ventral valves of the Orbiculoidea in a convex-down orientation. The articulated specimens occurred in laminated mudstone and must have been autochthonous, while the disarticulated specimens with a convex-up orientation were para-autochthonous, resulting from turbiditic or storm flow. The specimens with convex-down ventral valves always occurred at the base of the sandstone beds with their ventral apexes slightly penetrating the underlying mudstone. This is the argument for an autochthonous ventral valve, whereby only the dorsal valve was transported after death. The distribution of epibionts suggests that the life posture of the Orbiculoidea was a ventral-side-down orientation, with its anterior half slightly higher than the sediment-water interface, which was advantageous for the feeding flow and the clearance of sediments around the posterior margin. Although dense monospecific assemblages of Orbiculoidea occur only as disarticulated valves, the exclusive occurrence of articulated individuals in the underlying mudstone suggest that the animal inhabited the specific environments opportunistically, which has never been explored in other fossilised organisms.

Early Permian (Artinskian and Kungurian) Fusulines from the Nakadaira Formation in the Kamiyasse Area, Southern Kitakami Mountains, Japan

Fourteen species of early Permian (Artinskian and Kungurian) fusulines are described from the upper part of the Nakadaira Formation in the Kamiyasse area, South Kitakami, Japan, correlatable to the upper part of the Sakamotozawa Formation in the type area. Among them, Praeparafusulina pseudojaponica, Juresanella juresanensis, and Juresanella uralensis diagnostic in the Nakadaira Formation are uncommon or absent in seamount-originated limestones of the Permian and Jurassic terranes of Japan, and important paleobiogeographically. Darvasella kitakamiensis sp. nov. is distinguished from the known species of the genus by its more regularly folded septa and the presence of axial fillings. Based on these fusulines, the uppermost part of the Nakadaira Formation is correlated to the Artinskian/Kungurian of Cisuralian, suggesting that the carbonate facies having early and most middle Guadalupian fusulines is not developed in the Kamiyasse area, as well as probably throughout the southern Kitakami.

Trace elemental and Sr-Nd-Hf isotopic compositions, and U-Pb ages for the Kitakami adakitic plutons: Insights into interactions with the early Cretaceous TRT triple junction offshore Japan

We conducted major, trace element, Sr-Nd-Hf isotopic, and U-Pb geochronologic analyses of early Cretaceous Kitakami granitic plutons, northeast Japan. We suggest that these plutons include rocks of adakitic affinity, which indicate partial melting of an eclogitic slab. The Kitakami adakites were mostly derived from juvenile oceanic crustal sources, but include inherited zircons with Archean to Neoproterozoic Hf model ages. Rather than being directly derived from crustal rocks of these ages, zircons may have been detrital in the voluminous trench fill sandstone within the Jurassic northern Kitakami accretionary prism. The exhumation of older basement that served as the detrital zircon source may have been triggered by ridge subduction. Although the plutons show a general southward younging from northern Kitakami (120–130 Ma), southern Kitakami (115–125 Ma), and Abukuma (100–115 Ma), restoration of post-crystallization left-slip faults inverts this spatial pattern and results in a northward-younging pattern. This suggests northward migration of a trench-ridge-transform triple junction that included the Izanagi–Farallon ridge. The Sambagawa eclogite may be genetically related to the Kitakami adakite in time and space if the transform duplexing is restored.

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

Antitropical brachiopods from the middle Permian (Wordian) of the southern Kitakami Mountains, northeast Japan

Antitropical brachiopods from the middle Permian (Wordian) of the southern Kitakami Mountains, northeast Japan

Petrology and geochronology of late Cambrian granitic rocks from southern Kitakami Mountains, Japan

Petrology and geochronology of late Cambrian granitic rocks from southern Kitakami Mountains, Japan

Fusulines from the Lower Permian Sakamotozawa Formation of the Southern Kitakami Belt

Fusulines from the Lower Permian Sakamotozawa Formation of the Southern Kitakami Belt

南部北上山地，栗橋・蟹岳花崗岩体のK-Ar年代

南部北上山地，栗橋・蟹岳花崗岩体のK-Ar年代

Folding of granite and Cretaceous exhumation associated with regional-scale flexural slip folding and ridge subduction, Kitakami zone, northeast Japan

The early Cretaceous granitic plutons intrude the Kitakami zone, northeast Japan, whose southern and northern regions consist of forearc basin and accretionary complex rocks, respectively. All country rock of the Kitakami zone exhibit prominent pressure-solution cleavage and associated folds formed during shortening with a small component of sinistral shear, whereas most plutons show only igneous textures. The Kesengawa granite and some other plutons, however, have foliations that cut the pluton boundaries and are continuous with those observed in surrounding sedimentary rocks. We document a tectonic fold with axial planar foliation in part of the Kesengawa granite. The metamorphic minerals associated with the contact aureole and country rocks of the Kesengawa and other deformed plutons indicate an increase in metamorphic grade toward the pluton showing that deformation of the pluton and country rock took place as the plutons cooled. The Kesengawa pluton and country rocks of the southern Kitakami zone are deformed into regional scale upright folds with parasitic asymmetric folds that verge toward regional anticlinal axes whereas regional scale folds in the northern Kitakami zone are overturned and verge to the east. The Kitakami basement is not bounded by normal or reverse faults, so the style of regional exhumation does not resemble the upright or inclined extrusion noted in other regions, nor is the exhumation associated with extensional doming. Instead, the vergence of parasitic folds toward the regional fold hinges indicates flexural slip deformation at least in the late stages of exhumation and exhumation occurred in the cores of regional scale anticlines. The regional shortening of accretionary prism, forearc basin, and older forearc basement was associated with intrusion of adakitic plutons that thermally weakened the forearc basin and enhanced the deformation and exhumation. The adakitic magmatism and forearc shortening resulted from subduction of the buoyant Izanagi–Kula ridge, a regional event known as the Oshima orogeny.

花崗岩冷却のP/T条件と鉱化作用の関係(2) -南部北上地域のいくつかの岩体について-

花崗岩冷却のP/T条件と鉱化作用の関係(2) -南部北上地域のいくつかの岩体について-

115. 北上山地南部井内統 (?) 産のPosidonia 1新種

115. 北上山地南部井内統 (?) 産のPosidonia 1新種

Sedimentary history with biotic reaction in the Middle Permian shelly sequence of the Southern Kitakami Massif, Japan

AbstractThe transition of sedimentary environments and their organism‐related responses is exemplified in the active margin setting of the Middle Permian in the Southern Kitakami Massif. The transition in the sedimentary environment began with a shallowing‐upward sequence at an upper slope to an outer shelf depositional setting that was associated with a delta system which transported abundant botanic remains in the Hoso‐o Formation. By contrast, the sediments of the overlying Kamiyasse Formation, the base of which is roughly equivalent to the lowermost Capitanian, were deposited at a lower shoreface to the outer shelf setting, which originated from complex depositional sources, including beach, near shore reef mounds and a nearby independent area of shallow and hard substratum with a considerable difference of depth. This unique depositional setting resulted in the seemingly ‘mixed’ fauna associated with this formation. The depositional environment of the overlying Kurosawa Formation is similar to that of the Hoso‐o Formation, but with far fewer monotonous biotic remains. A comparison of lithological characteristics and fossil remains with those of neighboring regions suggests that the independent mounds were generated as if the talus formed elsewhere so as to provide a large amount of skeletal elements to a deeper basin in the earliest Capitanian. Accordingly, the complex sedimentary setting observed in the Kamiyasse Formation occurs widely throughout the South Kitakami Massif, making possible the ‘mixed’ and seemingly diverse fauna from a mixture of multiple allochthonous origins.

1021 Latest Permian ammonoid Paratirolites from the Ofunato district, Southern Kitakami Massif, Northeast Japan

1021 Latest Permian ammonoid Paratirolites from the Ofunato district, Southern Kitakami Massif, Northeast Japan

New Genus Kanokurachonetes (Brachiopoda, Chonetida) from the Middle Permian of the southern Kitakami Mountains, Northeast Japan

A new chonetid genus and species, Kanokurachonetes kanokurensis (subfamily Rugosochonetinae, family Rugosochonetidae), from the Middle Permian (Wordian, Kanokura Formation) of the southern Kitakami Mountains (Northeast Japan) are described.

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.

Middle Permian fusulines from the Hoso-o Formation in Kamiyasse area, southern Kitakami Mountains, Northeast Japan: Their biostratigraphic implications.

南部北上山地宮城県気仙沼市上八瀬地域の中部ペルム系細尾層中の石灰質基質支持礫岩から，新たにSumatrina cf. annae, Pseudodoliolina pseudolepida, Chusenella sinensisおよびその他の有孔虫類を含むフズリナ化石群を発見した．この群集には，テチス域標準年代区分の後期MurgabianからMidianを示すS. cf. annaeが含まれる一方で，前・中期Murgabianの指標となる原始的なNeoschwagerinaやAfghanella，および中・後期Midianを指示する進化型のLepidolinaやYabeinaが共産しない．このことは，この石灰質礫岩層が前期Midian（ペルム系標準年代層序区分の上部Wordian階）に対比されることを示唆する．この解釈は先行研究によるアンモノイド化石を用いた細尾層の年代論とも整合的である．

Early Carboniferous miospores from the southern Kitakami Mountains, northeast Japan

Early Carboniferous miospores from the southern Kitakami Mountains, northeast Japan

2. Ethological interpretation of the trace fossil Zoophycos in the Hikoroichi Formation (Lower Carboniferous), southern Kitakami Mountains, Northeast Japan

2. Ethological interpretation of the trace fossil Zoophycos in the Hikoroichi Formation (Lower Carboniferous), southern Kitakami Mountains, Northeast Japan