Nemuro Group Research Articles

An intraoceanic juvenile arc of shoshonite and adakitic andesite in the Nemuro Belt, the Lesser Kuril Arc, across the K/Pg boundary

In situ eruptions and accreted blocks of Mesozoic basalts exhumed in present-day forearc settings, around the Pacific Rim record paleovolcanic events in intraplate oceanic settings since the closure of the Panthalassa Ocean. Basaltic and andesitic volcanic and intrusive rocks occur in the present-day forearc area of the Late Cretaceous Nemuro Belt in northeastern Japan. Most basalts in this area are K-rich (i.e. shoshonites), similar to rocks erupted in oceanic back arc or petit-spot settings. In this study, we investigated the occurrence, geochemistry, and age of these shoshonitic rocks (ca. 70 Ma), along with associated adakitic andesites (ca. 62 Ma). The petrogenesis of these igneous rocks and petrography of the associated Nemuro sandstones were used to constrain the nature of the Nemuro paleo-arc during the Late Cretaceous to early Paleogene. The shoshonitic, adakitic, and basaltic rocks in the Nemuro sandstones formed in an intraoceanic juvenile arc setting. Previous paleomagnetic studies have suggested that the Nemuro and Lesser Kuril paleo-arcs were approximately ENE–WSW-trending, located in the central paleo-Pacific, and extensions of the Olyutorsky (eastern Kamchatka) and Terpenia (eastern Sakhalin) paleo-arcs near the mid-ocean ridges of the Kula/Izanagi and Pacific plates in the central Pacific. Our findings are consistent with these previous results, although the shoshonites in the lower part of the Nemuro Group indicate that the Nemuro paleo-arc drifted rapidly northward or northwestward after ca. 65 Ma.

Collisional bending of the western Paleo‐Kuril Arc deduced from paleomagnetic analysis and U–Pb age determination

AbstractThe Paleo‐Kuril Arc in the eastern Hokkaido region of Japan, the westernmost part of the Kuril Arc in the northwestern Pacific region, shows a tectonic bent structure. This has been interpreted, using paleomagnetic data, to be the result of block rotations in the Paleo‐Kuril Arc. To understand the timing and origin of this tectonic bent structure in the Paleo‐Kuril arc‐trench system, paleomagnetic surveys and U–Pb radiometric dating were conducted in the Paleogene Urahoro Group, which is distributed in the Shiranuka‐hill region, eastern Hokkaido. The U–Pb radiometric dating indicated that the Urahoro Group was deposited at approximately 39 Ma. Paleomagnetic analysis of the Urahoro Group suggested that the Shiranuka‐hill region experienced a 28° clockwise rotation with respect to East Asia. The degree of clockwise rotation implied from the Urahoro Group is smaller than that of the underlying Lower Eocene Nemuro Group (62°) but larger than that of the overlying Onbetsu Group (−9°). It is thus suggested that the Shiranuka‐hill region experienced a clockwise rotation of approximately 34° between the deposition of the Nemuro and Urahoro Groups (50–39 Ma), and a 38° clockwise rotation between the deposition of the Urahoro and Onbetsu Groups (39–34 Ma). The origin of the curved tectonic belt of the Paleo‐Kuril Arc was previously explained by the opening of the Kuril Basin after 34 Ma. The age constraint for the rotational motion of the Shiranuka‐hill region in this study contradicts this hypothesis. Consequently, it is suggested that the process of arc–arc collision induced the bent structure of the western Paleo‐Kuril Arc.

Hydrocarbon generation potential and thermal maturity of coal and coaly mudstones from the Eocene Urahoro Group in the Kushiro Coalfield, eastern Hokkaido, Japan

This study reveals the hydrocarbon generation potential and thermal maturity of coal and coaly mudstones from the Eocene Urahoro Group in the Kushiro coal basin (Kushiro Basin) of eastern Hokkaido (Japan). The coal is classified as subbituminous or high volatile C bituminous, and characterized by high hydrogen content (6.1 ± 0.4 wt%, 1σ, dry ash free basis). The hydrogen index (HI = 180–409 mg hydrocarbon/g total organic carbon) and random mean vitrinite reflectance (VRr = 0.42–0.56%) indicate that the coal and coaly mudstones in the Urahoro Group are gas- and oil-prone or oil-prone. Based on the HI and VRr values, the effective oil window starts at vitrinite reflectance ranges of 0.55 to 0.80%. The vitrinite reflectance of coal samples from the Kushiro Coal Mine is inversely proportional to the HI value, although each coal seam has the identical thermal history. This result suggests that the vitrinite reflectance of the samples in the Kushiro Coal Mine is suppressed. Based on the corrected vitrinite reflectance (non-suppressed vitrinite reflectance), the maturity of the coal in the Kushiro Basin increases towards the north. The non-suppressed vitrinite reflectance of the samples from the northernmost part of the basin (Hokuyo Area) was approximately 0.8%, which is within the start of the oil window. The maturity of the coal-bearing strata in the northern area of the basin could be affected by volcanic activity along the volcanic front. In the Hokuyo Area, the Urahoro Group overlies a sequence of Cretaceous–Eocene marine clastic rocks (Nemuro Group), which are about 3000–4000 m in total thickness, suggesting that the organic matter in the marine clastic rocks should reach higher maturity (VRr > 0.8%).

Late Cretaceous to early Paleogene forearc magmatism and subduction initiation in the Paleo-Kuril arc-trench system, eastern Hokkaido, Japan

New major and trace elemental data presented here imply a temporal and spatial change in the magmatic evolution and mode of subduction initiation in the Paleo-Kuril arc-trench system, eastern Hokkaido, Japan. Late Cretaceous to early Paleogene igneous rocks from the Nemuro Group (Nokkamappu, Hamanaka, and Kiritappu Formations) in the forearc basin of the Paleo-Kuril arc consist of the tholeiitic, alkaline and calc-alkaline rock series, based on petrographic and major-element (K2O, total alkalies [K2O + Na2O], and SiO2 contents) data. Immobile-element features on the Zr/TiO2 vs. Nb/Y diagram indicate that a majority of the alkaline rocks are classified as non-alkaline rock. The low TiO2 (< 1%), high V/Ti, and low Nb/Yb ratios of the igneous rocks show that the original mantle compositions of alkaline and non-alkaline rocks were highly refractory in nature. The magmas were produced with a 10–20% partial melting of the depleted mantle, which is common in arc-related non-alkaline magma. The high Th/Yb, Ba/La, Rb/La, and Ba/Th ratios and low La/Sm ratios of the alkaline and tholeiitic rocks from the Nokkamappu and Hamanaka Formations strongly reflect the imprint of substantial fluid components from the subducting slab. The enriched K and Rb in the alkaline magma source are indicative of fluid derived from mica breakdown. Cs fractionation in the slab-derived fluids is indicated by variations in Y/Cs and Nb/Cs, and the fluids are controlled by fluid derived from the breakdown of mica such as phengite. Tholeiitic and calc-alkaline rocks from the Kiritappu Formation show less aqueous fluids. Late Cretaceous magmatism, with decompression melting and formation of the forearc basin, resulted from rollback during subducting initiation, which was associated with a rift system created by the upwelling of depleted mantle following the onset of the sinking of the slab into the mantle. Subsequently, the rollback of the subducting slab slowed down and stabilized, and consequently, normal tholeiitic and calc-alkaline magmas were produced in the early Paleogene.

Temporal changes in the internal stresses and pore pressures in a large-scale submarine mass transport deposit

We examined the temporal changes in the internal stresses and pore fluid pressures of a submarine mass transport deposit (MTD) in the Akkeshi Formation of the Upper Cretaceous–Paleocene Nemuro Group, eastern Hokkaido Island, Japan. We first analyzed previous paleostress field results from meso-scale faults in the MTD blocks, which indicated two phases during the evolution of the debris flow: phase I, radial spreading of the flow body during downslope movement; phase II, the flow body underwent compression during deposition on the basin plain. We also estimated the pore fluid pressure ratio from the fault orientation distribution. There was a large increase in the pore fluid pressure ratio during the transition from phase I to phase II that continued to rise during the initial stage of phase II and then decreased in its latter stages, whereas the maximum horizontal compressive stress increased throughout phase II. This variation in pore fluid pressure relates to the dynamics and evolution of the debris flow, where the clasts in the central part of the flow were supported by the excess pore pressure due to the compression of the debris flow as the flow head decelerated. Although pore fluid pressure plays a critical role in the dynamics of debris flows, there was no previous methodology to quantify both the stress fields and pore fluid pressures in large debris flows and their resultant MTDs. Our results implemented for outcrop studies imply that meso-scale faults in MTDs can provide clues to better understand these paleoflow mechanisms.

The Last Surviving Ammonoid at the end of the Cretaceous in the North Pacific Region

Abstract. This paper describes the first record of the heteromorph ammonoid Diplomoceras cylindraceum from the uppermost Maastrichtian Kawaruppu Formation of the Nemuro Group, southeastern Hokkaido, Japan. D. cylindraceum was found in a siltstone 15 m below the Cretaceous/Paleogene (K/Pg) boundary. The stratigraphical level of the fossil is estimated to be ∼66.8 Ma, based on the time scale for the geomagnetical polarity and the K/Pg boundary. D. cylindraceum, therefore, was living at least ∼0.8 m.y. before the K/Pg boundary event in the north Pacific region.

The alkaline magmas intruded into the Nemuro Group

The alkaline magmas intruded into the Nemuro Group

The dinoflagellate cyst stratigraphy and the history of paleo-productivity in the Paleogene Nemuro Group.

The dinoflagellate cyst stratigraphy and the history of paleo-productivity in the Paleogene Nemuro Group.

Cretaceous to Paleocene depositional history of North-Pacific subduction zone: reconstruction from the Nemuro Group, eastern Hokkaido, northern Japan

The Campanian–Paleocene Nemuro Group comparing the oldest strata in the Kuril Arc, is distributed in the east of Hokkaido Island, northern Japan. Strata of the group in this region are sedimentologically classified into eight depositional facies, most of which are interpreted as sediment gravity flow deposits. These depositional facies comprise four facies associations. The distal and proximal basin plain facies associations are composed mainly of hemipelagic mudstones and sandstones that are interpreted as gravity flow deposits, and were deposited in a topographically flat environment inferred by good lateral continuity of lithofacies. The channel-levee complex and submarine slope facies associations, in contrast, are composed of hemipelagic mudstone, turbidites and debris flow deposits. These four facies associations stack in ascending order, and represent a regressive succession. Palaeocurrent data indicate that the deposits of the Nemuro Group were transported from the northwest. Hence, the group in the study area is concluded to record slope progradation away from the northern source area. Hiertherto, it has been known that the sea regressed from the Kuril Arc during the Eocene. This was attributed to ridge subduction beneath the Kuril Arc. My study has revealed that regression began as early as Maastrichtian. It may have been induced by volcanic activity in the Kuril Arc or by a significant eustatic fall in sea level.

北海道東部根室層群から新たに産出した後期白亜紀軟体動物化石とＫ／Ｔ境界の層準について

Pachydiscus flexuosus, Tetragonites popetensis, Spheonoceramus hetonaianus, and other molluscan fossils are newly discovered from the Senpohshi, Hamanaka and middle Akkeshi formations, Nemuro Group, eastern Hokkaido, Japan. They are stage index fossils of Lower Maastrichtian. According to calcareous nannofossil biostratigraphy suggesting that the upper Akkeshi Formation is Paleocene in age, the K/T boundary in the Nemuro Group is possibly intercalated between the middle and upper Akkeshi Formation.

Isotopic ages of alkali rocks from the nemuro group in Hokkaido, Japan: Late cretaceous time-scale points

KAr and RbSr age determinations were carried out on igneous rocks from five horizons in the Late Cretaceous Nemuro Group in easternmost Hokkaido, Japan. The results are as follows: dolerite and basalt from the Middle-Upper Campanian Nokkamappu Formation, 75.0–76.2 and 72.0–73.9 Ma, respectively; monzonite from the uppermost Campanian-lowermost Maastrichtian Otamura Formation, biotite KAr and RbSr ages of 70.3 and 69.5 Ma, respectively; dolerite from the Middle Maastrichtian Oborogawa Formation, 69.8 Ma (monzonite from this formation: biotite KAr and RbSr ages of 70.4 and 69.8 Ma, respectively); dolerite from the Late Maastrichtian Hamanaka Formation, 66.0 and 67.6 Ma. These age results are generally consistent with the biostratigraphy, and provide valuable calibration points for the Late Cretaceous time scale. Des mesures d'âge KAr et RbSr ont été réalisées sur des roches volcaniques de cinq horizons de la partie extrême orientale de Hokkaido au Japon dans le Groupe de Nemuro daté du Crétacé supérieur. Les âges obtenus sont: 75,0–76,2 Ma (dolérite) et 72,0–73,9 Ma (basalte) pour la formation de Nokkamappu du Campanien moyen supérieur; 70,3 Ma (biotite, KAr) et 69,5 Ma (RbSr) pour la monzonite de la Formation de Otamura d'âge Campanien terminal-Maastrichtian basal; 69.8 Ma (dolérite) 70,4 Ma (biotite, KAr, monzonite) et 69,8 Ma (RbSr, monzonite) pour la Formation Oborogawa d'âge Maastrichtien moyen; 66,0 et 67,6 Ma pour une dolérite de la Formation de Hamanaka d'âge Maastrichtien supérieur. Ces âges sont, dans leur ensemble, cohérents avec leur position biostratigraphique et fournissent des repères valides pour l'âge numérique du Crétacé supérieur.

Isotopic ages of alkali rocks from the Nemuro Group in Hokkaido, Japan: Late Cretaceous time-scale points

KAr and RbSr age determinations were carried out on igneous rocks from five horizons in the Late Cretaceous Nemuro Group in easternmost Hokkaido, Japan. The results are as follows: dolerite and basalt from the Middle-Upper Campanian Nokkamappu Formation, 75.0–76.2 and 72.0–73.9 Ma, respectively; monzonite from the uppermost Campanian-lowermost Maastrichtian Otamura Formation, biotite KAr and RbSr ages of 70.3 and 69.5 Ma, respectively; dolerite from the Middle Maastrichtian Oborogawa Formation, 69.8 Ma (monzonite from this formation: biotite KAr and RbSr ages of 70.4 and 69.8 Ma, respectively); dolerite from the Late Maastrichtian Hamanaka Formation, 66.0 and 67.6 Ma. These age results are generally consistent with the biostratigraphy, and provide valuable calibration points for the Late Cretaceous time scale.

Sedimentary history of the late Cretaceous-Paleocene Nemuro Group, Hokkaido, Japan: A forearc basin of the Paleo-Kuril arc-trench system.

Sedimentary history of the late Cretaceous-Paleocene Nemuro Group, Hokkaido, Japan: A forearc basin of the Paleo-Kuril arc-trench system.

Coordinated Mega- and Micro-Hiostratigraphy of the Upper Cretaceous in the Tombetsu Valley, Hokkaido

The Upper Cretaceous(Campanian through Maastrichtian) deposits in the Tombetsu Valley, northern Hokkaido (Fig, 1) are much thicker and more fine-grained than the contemporary sandy and deltaic deposits of the Hakobuchi Group in central Hokkaido. The Tombetsu sequences are subdivided into five formations, from A to E in asending order (Fig. 2), in each of which sandstone demarcates its basal part and mudstone, fine-sandy siltstone and silty very fine-sandstone predominate in the main part. Formations A and B contain (Sphenoceramus) orientalis-Anapachydiscus(Neopachydiscus)naumanni and I. (S.) schmidti-Canadoceras sp, respectively and are referred to the Campanian. This is supported by certain species of planktonic foraminifera from A besides benthic species from A and B. Formations C, D, and E, which are characterized respectively by I. (Endocostea) shikotanensis, I. (S.) hetonaianusPachydiscus (Pachydiscus) flexuosus, and Inoceramus sp, nov.-Gaudryceras sp, nov. contain numerous benthic species of foraminifera, which in turn are mostly common with those from the upper part of the Nemuro Group in eastern Hokkaido. Spiroplectammina grzybowskii from D and E is noteworthy among others. On these lines of evidence Formations C, D, and E are referred to the Maastrichtian for the most part. Table 2 shows the full list of the foraminiferal species based on the samples from selected routes (Figs. 3 and 4) in this area. The Utsunai Formation which overlies unconformably E, contains planktonic foraminifera species (Table 3) of a late Eocene-early Oligocene age.

Sedimentary Petrography of the Nemuro Group

Sedimentary Petrography of the Nemuro Group

Stratigraphic Re-examination of the Nemuro Group

Stratigraphic Re-examination of the Nemuro Group

Sedimentology of the Nemuro Group (Part IV):On the change of the source areas in the Akkeshi Formation

Sedimentology of the Nemuro Group (Part IV):On the change of the source areas in the Akkeshi Formation

Sedimentology of the Nemuro group (Part II):Radiographic investigation of flysch-type sandstones in the Akkeshi Formation of the Nemuro Group

Sedimentology of the Nemuro group (Part II):Radiographic investigation of flysch-type sandstones in the Akkeshi Formation of the Nemuro Group

Sedimentology of the Nemuro Group (Part III):Sedimentation of the lower Akkeshi Member

Sedimentology of the Nemuro Group (Part III):Sedimentation of the lower Akkeshi Member