South-central Hokkaido Research Articles

High abundances of degraded triterpenoids in Miocene turbiditic sediments of south-central Hokkaido, Japan

Aliphatic and aromatic degraded triterpenoids (TTs), including des-A and des-E TTs, were investigated in turbiditic and hemipelagic mudstones from the Miocene Kawabata Formation (Ishikari basin) and Abetsu and Nibutani formations (Hodaka basin) of south-central Hokkaido, Japan. des-A TTs with carbon skeletons of oleanane, ursane, and lupane are derived from angiosperms, while des-E TTs with hopane carbon skeletons are of bacterial origin. These compounds are thought to form through microbial degradation under dysoxic and anoxic conditions. We found that the concentrations and abundances of total degraded TTs, especially des-A TTs, relative to total organic carbon (TOC) were markedly higher in the Kawabata Formation despite significant levels in the Abetsu and Nibutani formations. These results clearly suggested that huge amounts of des-A TTs may have been transported to, and accumulated in, the Ishikari basin during the late Miocene. Degraded TT/TOC ratios are correlated with aquatic macrophyte n-alkane proxy (Paq) values in the Abetsu Formation. Higher Paq values are interpreted as indicating large contributions of aquatic, submerged or floating macrophytes, and are commonly observed in lake and pond environments. Thus, large amounts of degraded TTs were likely produced through the biodegradation of transported angiospermous TTs in dysoxic or anoxic environments, such as ponds and wetlands. Furthermore, we assumed that organic matter deposited in the Hidaka basin was transported from wetlands and marsh areas, especially floodplain lakes, of paleo-Hidaka Island. The class distributions of degraded TTs varied among samples from these formations. The higher relative abundances of des-A lupanes in the Ishikari basin (Kawabata Formation) suggest that TTs were supplied from mountainous forested areas, where lupenoid-producing woody plant taxa may occur. Meanwhile, less abundant des-A lupanes in the Hidaka basin (Abetsu and Nibutani formations) suggest little or no supply of TTs from mountainous forested areas.

Interspecific social dominance networks reveal mechanisms promoting coexistence in sympatric charr in Hokkaido, Japan.

Coexistence of species requires equalizing mechanisms that minimize fitness differences, which are balanced by stabilizing mechanisms that enhance negative intraspecific interactions versus interspecific ones. Here, we develop a simple theoretical framework that allows measuring the relative strength of intraspecific versus interspecific competition in dominance hierarchies. We use it to evaluate mechanisms promoting coexistence between two congeneric charr that compete for foraging positions, which strongly influence density-dependent growth and survival. Agonistic interactions (n=761) among 71 Dolly Varden Salvelinus malma and whitespotted charr Salvelinus leucomaenis were measured by snorkelling in two pools in the sympatric zone of a Hokkaido stream during two summers. Interspecific dominance hierarchies, analysed using three methods, were closely correlated with fish length but the species treated each other equally. Ranks for the most dominant fish in each pool, determined directly by knockout experiments, were also virtually identical to ranks by length. Similarly, exponential random graph modelling of the social networks provided no evidence that either species was dominant over the other. Instead, larger fish were more likely to win contests, especially over fish of the next lower ranks. These results demonstrated that the two species were nearly ecological equivalents in accessing key resources in this sympatric zone. Nearly identical growth and stable densities over 4years further supported this inference, although Dolly Varden were a minority (29% of the assemblage), a sign of some fitness difference. Detailed foraging observations coupled with two concurrent studies revealed an effective stabilizing mechanism. Dolly Varden shifted to feeding directly from the benthos when drifting invertebrates declined, a behaviour enhanced by morphological character displacement, thereby partitioning food resources and enhancing intraspecific competition while avoiding agonistic encounters with whitespotted charr. The plurality of evidence indicates that fitness differences between these ecologically equivalent species are small in this local assemblage, and balanced by resource partitioning, a modest stabilizing mechanism that promotes coexistence. The theoretical framework presented here is a useful tool to evaluate the strength of interspecific versus intraspecific competition, which combined with information on trade-offs in ecological performance can contribute to a mechanistic understanding of species coexistence.

Characteristics of relocated hypocenters of the 2018 M6.7 Hokkaido Eastern Iburi earthquake and its aftershocks with a three-dimensional seismic velocity structure

I relocated the hypocenters of the 2018 M6.7 Hokkaido Eastern Iburi earthquake and its surrounding area, using a three-dimensional seismic structure, the double-difference relocation method, and the JMA earthquake catalog. After relocation, the focal depth of the mainshock became 35.4 km. As previous studies show, in south-central Hokkaido, the Hidaka collision zone is formed, and anomalous deep and thickened forearc crust material is subducting at depths of less than 70 km. The mainshock and its aftershocks are located at depths of approximately 10 to 40 km within the lower crust of the anomalous deep and thickened curst near the uppermost mantle material intrusions in the northwestern edge of this Hidaka collision zone. Like the two previous large events, the aftershocks of this event incline steeply eastward and appear to be distributed in the deeper extension of the Ishikari-teichi-toen fault zone. The highly inclined fault in the present study is consistent with a fault model by a geodetic analysis with InSAR. The aftershocks at depths of 10 to 20 km are located at the western edge of the high-attenuation (low-Qp) zone. These kinds of relationships between hypocenters and materials are the same as the 1970 and 1982 events in the Hidaka collision zone. The anomalous large focal depths of these large events compared with the average depth limit of inland earthquakes in Japan could be caused by the locally lower temperature in south-central Hokkaido. This event is one of the approximately M7 large inland earthquakes that occurred repeatedly at a recurrence interval of approximately 40 years and is important in the collision process in the Hidaka collision zone.

A New Species ofGaudryceras(Ammonoidea, Gaudryceratidae) from the Lowest Maastrichtian of Hokkaido, Japan and Its Biostratigraphic Implications

Gaudryceras hobetsense sp. nov. is described from the Nostoceras hetonaiense Zone (= lowest Maastrichtian) of the Hobetsu area, south-central Hokkaido, Northern Japan. Its shell is characterized by fine lirae on early whorls, distant rounded or flat-topped, narrow band-like ribs on middle and later whorls, and frequent collar-like ribs on later whorls. The occurrence of this new species strongly suggests the presence of lowest Maastrichtian strata in southern Alaska, and sheds light on the age delineation of the chronologically poorly defined beds in northern and eastern Hokkaido.

<p><strong>Crinoid diversity in the Upper Cretaceous Yezo Supergroup, Hokkaido, northern Japan*</strong></p>

New data, principally from ‘local’ encrinites in the Yezo Supergroup of Hokkaido, northern Japan, suggest that many relict ‘Jurassic-type faunas’ identified in Europe and central Japan might have persisted in the Late Cretaceous deep-water (> 100 m) muddy sandstone facies found throughout the island of Hokkaido. For example, Isocrinus and Balanocrinus occur in the outer shelf sandy mudstone facies of the Turonian-Coniacian Saku Formation and the Coniacian- Campanian Haborogawa Formation, of the Mikasa area (central Hokkaido), respectively. Isocrinus is also found in the Turonian-Coniacian deep-water siltstones and sandy siltstones of the Nishi-chirashinai Formation of the Nakagawa area (northern Hokkaido). In contrast, Isselicrinus, which belongs to ‘Late Cretaceous type-faunas’, occurs in the uppermost Creta­ceous (Campanian) outer shelf mudstone facies of the Upper Yezo Supergroup around Hobetsu (south-central Hokkaido). Although collections of crinoids from the chalks of northern Europe are themselves largely free from sampling biases, Cretaceous crinoid diversity data have been markedly influenced by fossils preserved in the relatively homogeneous chalk facies where the faunal differences between the deep-water Danish, English and north German chalks (> 250 m) and the shallow-water facies found around Maastricht, the Netherlands (< 50 m) are slight. The results of this preliminary report on the crinoid faunas of Hokkaido suggest that global crinoid diversity reported from the Upper Cretaceous might be influenced by sampling and facies biases.

Crustal evolution of the southwestern Kuril Arc, Hokkaido Japan, deduced from seismic velocity and geochemical structure

The southwestern Kuril Arc, which is not affected by a series of accretion and collision processes, is one of the most important experimental sites among the world arc–trench systems for investigating the evolution of arc crust. Published petrological and seismic studies have shown that, in south-central Hokkaido, the upper part of the Kuril Arc crust is thrusting upward toward the west while the lower part is descending downward in the same direction. Among several hypotheses for formation of continental crust, we discuss the role of delamination of the island arc lower crust based on geochemical and seismic velocity structure models. Seismic velocity structure is obtained from an onshore–offshore seismic survey across the southwestern Kuril Arc–Trench system. The structure in the back-arc area shows a thick accretionary complex formed by Paleogene accretion and collision. Based on published seismic results, this accretionary complex is considered to continue on land to the Cretaceous accretionary complex observed in the Hidaka Belt. The structure of the Kuril Arc appears characteristic of upper crustal andesitic rocks (6.0–6.2 km/s) and thick middle to lower crust (6.5–7.3 km/s). Based on published geochemical study, we consider a geochemical model of simple crustal evolution explaining the mechanism of generation of andesitic rocks on the Kuril Arc. Each volume of andesitic, basaltic, and mafic layers estimated from the geochemical model is consistent with those obtained from the seismic velocity model. Crustal composition estimated from the geochemical model and major-element chemistry of the volcanic rocks in the Kuril Arc indicate that mechanical delamination is insufficient to cause the oceanic island arc crust to evolve into continental crust.

Recursive travel-time inversion: A tool for real-time seismic tomography

Abstract A new recursive inverse scheme is applied to a currently popular problem named seismic travel-time tomography, in order to enhance the efficiency and reliability in obtaining a new velocity model if a small number of new data are added to a large data set in the past. In comparison with conventional inverse schemes in seismic tomography, either least-squares or iterative types, this scheme does not require large amounts of matrix-type computations but utilizes the amount of modification in model parameters responsible for each new data set. We also introduce the computation of a collocation travel time (i.e., from a given station to every grid point) for the reference velocity model inverted by the data for all the past events, using a ray tracing scheme called the Huygens’ method (Saito, 2001), suitable to computations prior to a new event. Combining the above information already stored with the recursive inverse scheme, we can obtain a new or updated velocity model immediately after a new event takes place, because a temporal interval between two events is usually very long in a given local area. Since the model is revised at each recursive step, we perform ray tracings with the updated reference model to get more accurate ray paths and travel times than the conventional inversion schemes that use all the ray tracings for the same reference model. We first showed the validity and stability of the proposed method with synthetic data. We then applied the new approach to the P-wave travel-time data recorded in the Hidaka, south-central Hokkaido, Japan, region, and compared our result with other previous results. Our result shares the overall feature with the previous ones. In addition, a new low-velocity zone is detected in the east of the Hidaka mountains at the depth of 10 km, corresponding to the collision zone of two arcs, due to the use of the updated reference velocity model at each recursive step. We also confirmed that the order of data does not affect the final result, so that the present approach is shown as an appropriate tool for so-called real-time seismic tomography: a updated velocity model is immediately obtained at each time that a new event takes place, in order to monitor temporal variations of model parameters such as velocity structure on the real-time basis.

Blueschist-bearing fluvial conglomerate and unconformity in the Cretaceous forearc sequence, south central Hokkaido, northern Japan: Rapid exhumation of a high-P/T metamorphic accretionary complex.

Cretaceous forearc basin deposits of the Nitarachi-Yezo Sequence overlie the high-P/T metamorphosed Iwashimizu accretionary complex in the Mitsuishi River area of the southern Kamuikotan Zone. In the sequence, the Albian Utafue Formation (Middle Yezo Group) unconformably overlies the Valanginian-Barremian Iwashimizu Complex. The basal part of the Utafue Formation comprises a fluvial sequence, which contains greenstone clasts with high-P/T metamorphic minerals such as Na-amphibole and Na-pyroxene. The conglomerate also contains Valanginian-Barremian mudstone clasts derived from the Nitarachi-Yezo Sequence. The fluvial conglomerate clearly shows that the Iwashimizu Complex and lower part of the Nitarachi-Yezo Sequence were exposed subaerially in the Albian and were supplying clasts to the forearc. The Iwashimizu Complex, the high-grade part of which was metamorphosed at a depth of over 20km, is thus concluded to have been rapidly exhumed within ca. 20Ma after peak metamorphism. The complex was exhumed as a coherent mass, not as a serpentinite diapir. The exhumation is inferred to have resulted from lifting by underplating of abundant seamount volcanics, and by increased buoyancy of subducted oceanic lithosphere, which became younger during the Cretaceous.

Integrated megafossil-foraminiferal biostratigraphy of the Santonian to lower Campanian (Upper Cretaceous) succession in northwestern Hokkaido, Japan

The Upper Yezo Group (Coniacian to lower Campanian) in the area of Haboro to Kotanbetsu in northwestern Hokkaido, Japan, yields abundant mega- and microfossils at many levels. In this paper, an integrated analysis of the ammonites, inoceramids, and Foraminifera has been carried out to determine the detailed biostratigraphy of the Santonian to the lower Campanian succession of northern Pacific, with reference to the proposals of the Subcommission on Cretaceous Stratigraphy (SCS) of the International Union of Geological Sciences in 1983 and 1995.The Santonian/Campanian stage boundary in the Japanese stratigraphic scale is defined on the basis of the first occurrence ofMenabites mazenotiorSubmortonicerascf.condamyi. This horizon is located very close to the boundary between theInoceramus amakusensisandI.(Platyceramus)japonicusZones, which is also situated between the last occurrence ofMarginotruncana pseudolinneianaand the first occurrence of the typical form ofGlobotruncana arca. On the other hand, the first occurrence ofAspidolithus parcus parcusis above that ofG. arca, and both horizons are located in theI.(P.)japonicusZone in south-central Hokkaido. A K-Ar age of 82.2±0.6Ma, reported by Shibata & Uchiumi (1995), is situated in the lower part of theI.(P.)japonicusZone, which agrees well with the Santonian/Campanian boundary age of 83.5Ma. Unfortunately, the marker species selected in 1995 by the SCS to definine the base of the Campanian Stage, such asPlacenticeras bidorsatum,Gonioteuthis granulataquadrataandMarsupites testudinarius, have not yet been found in the Cretaceous deposits of Japan.

北海道の古第三系堆積盆の変遷

The Paleogene of Hokkaido is dominated in coal-bearing formations. Four stages are discriminated in evolution of the Paleogene sedimentary basins, above all, coal basins. A subsiding deep trough extended in the axial part of Hokkaido and southeastern Sakhalin between the East Asian continent and the Okhotsk microcontinent, and was filled with a 5 km-thick, clastic turbidite facies in the Paleocene. At the stage I in the latest Paleocene to earliest Eocene, the Haboro coal basin formed as a fore-arc basin in north-central Hokkaido on the west side of the trough. By contrast, the Nemuro shelfal clastic basin existed on the east side of the trough and encircled the southwest margin of the microcontinent. A drastic paleogeographic change occurred in the Early Eocene. The axial part of Hokkaido, which had been occupied by the deep trough, turned to upheaving associated with the Hidaka metamorphism, probably due to collision of the Okhotsk microcontinent against the East Asian continent. As a result, the trough and the sedimentary basins on its both sides were converted into a land. At the stage II in the middle Middle Eocene, the Ishikari-Uryu coal basin formed as an inter-arc basin in south-central Hokkaido where the Late Cretaceous continental fore-arc basin preexisted. The coal basin was first restricted to the Ishikari coalfield in the south, and later it enlarged to the Uryu coalfield in the north by the northward Wakanabe-Shiraki transgression. At the stage III in the late Middle Eocene, the Ishikari-Uryu coal basin extended to the west due to the subsidence of the Kabato-Rebun Cretaceous volcanic terrane. Basal fanglomerates, a few hundreds meters thick, overlapped the Cretaceous basement rocks in the west part of the coal basin. At the same stage, the Kushiro coal basin of the Kushiro coalfield formed incisely within the Nemuro shelfal basin in southeast Hokkaido, although it extended to the west and overlapped the Cretaceous basement rocks of the Tokoro terrane with conspicuous fanglomerates. At the stage IV in the Late Eocene to Early Oligocene, all coal basins of the previous stages subsided to the site of the deposition of shallow-marine to upper bathyal muddy sediments. In central Hokkaido, the Late Eocene marine basin of the Poronai Formation and its equivalents extended northwards from the Ishikari-Uryu coal basin to the Haboro coal basin of the stage I, and further to the north, to Sakhalin. However, the Early Oligocene mudrocks of the Momijiyama Formation was recognized only in the southern part of the Ishikari coalfield, and showed a restricted embayment environment. In east Hokkaido, the Early Oligocene continental sediments of the Wakamatsuzawa Formation overlapped the Cretaceous basement rocks of the Tokoro terrane on the northwest of the Late Eocene to Early Oligocene marine basin of the Onbetsu Group.The Paleogene of Hokkaido has a total thickness of two to five kilometers. The Paleogene strata of central Hokkaido are commonly extensively folded and overthrusted. The thick and folded Paleogene in central Hokkaido is undoubtedly traceable to Sakhalin, and possibly to the west side of central and northern Kamchatska.The third-ordered eustatic curves of Haq et al. (1987) indicate that, as a whole, Late Eocene is low stand while Early Oligocene is high stand. In Hokkaido, however, marine basins are most widespread in the Late Eocene, whereas they are restricted in the Early Oligocene. Local tectonic movements are therefore essential for the evolution of the Paleogene sedimentary basins in Hokkaido.

A Chert-clastic sequence spanning the late Triassic-early Cretaceous period of the Kamuikotan Complex in the Shizunai area, south-central Hokkaido, Japan

A Chert-clastic sequence spanning the late Triassic-early Cretaceous period of the Kamuikotan Complex in the Shizunai area, south-central Hokkaido, Japan

Application of Plate Tectonics Concepts in Hydrocarbon Exploration: Hokkaido Corner: ABSTRACT

An area prospective for hydrocarbon production is located offshore from south-central Hokkaido in northern Japan. The sediments there are contained in a forearc basin associated with the Japan-Kurile subduction system to the east. At the juncture of the Japan and Kurile Trenches, a major change in direction, associated with a transverse fault, is referred to as the Hokkaido Corner. The transverse fault is known to disrupt the subducting slab from measurements and studies of earthquake foci and focal mechanisms. The prospective forearc basin overlies the disrupted subducting slab, establishing a probable wrench-fault system. The history of deformation in the basin has been related to the northwesterly movement of the Pacific oceanic plate and its subduction under the Japanese Island arc. Seaward-dipping, stacked thrust sheets were found in wells drilled in the forearc basin. These sheets have been explained as resulting as resulting from gravity sliding of mountains at the eastern basin margin. The mountains formed as an uplifted belt above the toe of the continental mass overlying the subducting slab west of the Japan-Kurile Trenches. Further complications involve a history of submarine canyon erosion, wrench faulting, volcanic activity, and rapid uplift and subsidence with consequent complex sedimentation. Gravity, magnetics, seismic,more » well, and petrophysics information were integrated in order to interpret this area. Several localities in and near Australia are described where an interpretation using plate tectonics concepts is indicated.« less

A Study of the Population Ecology of the Brown Dipper Cinclus pallasii at the Tokachi River System, Hokkaido

(1) Breeding habit, home range, territory and seasonal changes of a population of the Brown Dipper Cinclus pallasii were investigated from March 1980 to August 1982, along the upper reaches of the Tokachi river system, in south-central Hokkaido.(2) Dippers breed once a year. Pairing starts in early February. Home ranges are established first, then territories are formed within the home ranges. Nest construction begins from the middle of April. Clutch size is 4-5 (mean 4.7), and eggs start hatching from the middle of May. The incubation period was not ascertained. The fledgling period was of 21 days. Breeding success was 25%, and seemed to be dependent on predation.(3) The young remain with their parents after fledgling, unitl the parent birds start molting in July when the family disperses.(4) Dippers are strongly attached to their breeding area and 58.8% returned to the same places.17.7% of individuals birds came back every year for three years, but none were observed to come back for four successive years.(5) The average home range length was 904±88 meters.(6) Territories formed on average 81% of the home ranges.(7) Estimations of Brown Dipper population by the line census and the simultaneous surveys gave similar results.(8) The environmental assessment was made on the basis of feeding types, feeding frequencies, and type of river-bed. Home range length (size) tended to be shorter where the environmental index was higher.(9) The Brown Dipper numbers showed seasonal changes. A maximum population was usually reached in the breeding season, and the number of birds decreased with fledgling of the young due to the post-breeding movement of the population. However, there was an increase of birds from summer to autumn. Winter numbers depended on the severity of the winter, in some years birds left the area completely. Numbers increased again towards the pairing season in February.

Geometrical Structural Analysis of Cretaceous System in South Central Hokkaido

The writer presents here the result of his geometrical structural analysis of Cretaceous System which crops out in south central part of Hokkaido. Cretaceous geology is megascopically compiled on the basis of photo-geologic interpretation (Fig. 1).Of two commonly used stereographic methods (Wulff's net, lower hemisphere projection), π-diagram (S-pole diagram) is adopted for finding the direction of fold axis. Measurements of 1598 bedding planes observed in the surveys conducted by his many colleagues before 1961, have been used as data for construction. Some typical projections are figured as several kinds of stereogram (Fig. 2).Structural homogeneity decided from regular orientation (acute distribution of S-poles on great circle on the net) of π-diagram, isdiscernible in the intrasub-area. Orientation of axial surfaces and fold axes are shown (Fig. 3 and 4), from which fold forms are inferred. Taking into consideration the subareal S-poles counting densities (Fig. 5), structurally homogeneous domains have been studid (Fig. 6).

Geospans and Quantitative Approach to Tectonic Analysis in the Upper Cretaceous System, South Central Hokkaido

This paper deals with the structural analysis of the Upper Cretaceous System in SouthCentral Hokkaido, on Lie basis of grasping quantitatively the change in velocity of downwarping during the entire period.The writer attempts to measure quantitatively the movement of subsidence on the assumption that the rate of subsidence is equivalent to the velocity of downwarping, because the subsidence during sedimentation is misty accepted as common or/and true evidences in the ralation of upheaval of source areas and sinking of sedimentary basins.Zonal succession, composed of the species of genus Inoceramus, is represented as widest common divisor by six zones, namely; shikotanensis•orientalis•japonicus•uwajimensis•hobetsensis and concentricus nipponicus, in descending order. Each zone is extended over the whole areas under discussion without any breaks throughout the time. These zones roughly correspond to the international stages. (Fig. 1.)On the otherhand "Holmes's symposium" has presented the Time Scale in the worldwide grade, in which the Cretaceous dating from 136 million years to 65 million years subdivided into eleven stages of apploximately equal duration.Recently the laboratory research of University of Alberta and Geological Survey of Canada have supplied some noticeable results concerning the dating of the Cretaceous System mainly in South Western Alberta. However, assumption of equal duration of each stage in Upper Crtaceous Epoch is not entirely ascertained even by so many materials on the western part of the country.Upper Cretaceous Zoning (space) and corresponding Duration (time) present the factors to resolve the movement on the subjects quantitatively. Velocities of downwarping in six stages are quite interesting and distinguishable especially in the middle stage ranging from Turonian to Santonion, instead of clear coincidence of upper and lower stages in velocity gradient. (Fig. 5). Figures (in Fig. 4) and subsequent plotts (in Fig. 5A) may be changed according to true determined ages in future, however, characteristic differences and its gradients in velocities among stages remain unaltered.Upper Cretaceous sedimentary basin might be divided into four subbasins, representing different tendency of subsidence which controlls the sedimentation during the Epoch. (Fig. 7)