Journal of Geography (Chigaku Zasshi)
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
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Showing 1-9 articles out of 9 articles from the selected issue
Cover
  • 2021 Volume 130 Issue 1 Pages Cover01_01-Cover01_02
    Published: February 25, 2021
    Released: March 18, 2021
    JOURNALS FREE ACCESS

     The Upper Carboniferous (Namurian) Central Clare Group is widely exposed on the Atlantic coast of County Clare, western Ireland. The Central Clare Group is composed of clastic sedimentary rocks deposited in fluvio-deltaic systems. The Cliffs of Moher, which are part of the Burren & Cliffs of Moher UNESCO Global Geopark, are well-known vista points for observing the Central Clare Group; nonetheless, the vertical cliffs are not ideal for checking individual beds up close. Along the coast of Spanish Point at low tide, in contrast, we can easily observe interbedded sandstone and mudstone that gently dip 10 to 20 degrees. Due to wave erosion, numerous sedimentary structures, such as cross stratification and ripple marks, appear on exposed bedding surfaces. Many tourists, including non-geologist Japanese, visit the small village of Spanish Point every year, as it is located near Milltown Malbay, the well-known site of the Willie Clancy Summer School (one of Ireland's major summer schools for traditional music and dance).

    (Photograph & Explanation: Tomonori NAYA;

    Photographed on July, 13, 2014)

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Original Articles
  • Shin'ichi MORI, Yujiro OGAWA
    2021 Volume 130 Issue 1 Pages 1-26
    Published: February 25, 2021
    Released: March 18, 2021
    JOURNALS FREE ACCESS
    Supplementary material

     Mapping was carried out of well exposed folded and faulted strata in the Ubajima Islands off Chigasaki City, Kanagawa Prefecture, central Japan of the northernmost part of Sagami Bay, where the present Philippine Sea plate subduction boundary extends beneath the central part of the North America plate. The strata were dated with radiolarian fossils as middle to late Miocene, and correlated with part of the Misaki Formation of the Miura Group of the southern Miura Peninsula. The strata are composed of dominant basaltic pyroclastic or volcaniclastic sedimentary rocks with sporadic felsic tephra layers and rare silty sedimentary rocks, and were defined as the Ubajima Formation. Detailed field mapping of the Ubajima Formation verified that the strata are deformed in a complicated way into duplex structures having two stages. The first stage is characterized by bedding parallel to subparallel thrust faults in the SW direction of the horizontal reference frame. This might be of the antiformal stack type known by many parallel repetitions of the same horizons. At the second stage, the same parts are further duplicated into other antiformal stack-type duplex structures, forming multi-duplex structures. Finally those structures are folded and faulted by a fault-propagation fold system that has strong shear zones at propagation fault zones. The Ubajima Formation is thought to be deposited on the forearc area of the Izu volcanic arc, then accreted to the Honshu side by accretionary prism formation. These structures represent good examples of accretionary prism toes along subduction zones.

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  • Kotaro FUKUI, Toshio SONE, Jorge A. STRELIN, Junko MORI
    2021 Volume 130 Issue 1 Pages 27-41
    Published: February 25, 2021
    Released: March 18, 2021
    JOURNALS FREE ACCESS

     James Ross Island, Antarctic Peninsula, has recently experienced severe climate change: warming from the 1970s until the early 2000s and cooling from the early 2000s until around 2014. Lachman II, a 2-km long debris-covered glacier (ice-cored rock glacier), is located at the northern part of James Ross Island. On Lachman II, the surface elevation lowered at a rate of 1.1 and 0.7 m a−1 at the center of the upper stream of the debris-covered area from 1992 until 1995. However, in response to the cooling trend from the early 2000s until around 2014, the surface lowering rate temporarily slowed to 0.2 and 0.4 m a−1 for at least seven years from 2008 to 2015 at the center of the upper stream of the debris-covered area. In response to extremely high air temperatures in 2016, the glacier surface lowered again, accelerating to 0.5 and 0.6 m a−1 from 2015 to 2017. A ground penetrating radar (GPR) image displayed a reflection plane along the longitudinal section of the glacier, which indicates that a debris-rich ice layer was thrusting up from the glacier bottom. The origin of the upward thrust is supported by the fact that the surface rock debris consists mainly of sandstone and mudstone derived from the bottom of the glacier.

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  • Sena KONO, Yukio ISOZAKI, Tomohiko SATO, Xingliang ZHANG, Wei LIU
    2021 Volume 130 Issue 1 Pages 43-62
    Published: February 25, 2021
    Released: March 18, 2021
    JOURNALS FREE ACCESS

     Detailed lithostratigraphy of the Lower Cambrian in South China, composed mostly of phosphorite and carbonate, is analyzed at the Xiaolantian section in the Chengjiang area, Yunnan. Six distinct lithostratigraphic units (Units 1 to 6 in ascending order) are recognized in the small shelly fossil (SSF)–bearing Zhongyicun Member, together with one particular terrigenous clastic unit (Unit 3) in the middle. Sedimentary features of these beds indicate that they were deposited in shallow marine settings in off–shore to upper slope of the Kangdian rift-related basin in western South China. The lithostratigraphy of the Xiaolantian section correlates not only with those of the neighboring sections at Hongjiachong and Maotianshan in Chengjiang, but also with those of five other sections (Meishucun, Wangjiawan, Xiaohuangcaoling, Zhujiaqing, and Laolin) in Yunnan. This regional correlation highlights a particular unit composed of terrigenous clastics solely in the middle of the Zhongyicun Member, e.g., Unit 3 of the Xiaolantian section. This unit serves as a useful key bed for a lithostatigraphical correlation within the basin because a similar unit (interval) was recognized at the same horizon in seven other sections in Yunnan. This key bed and its stratigraphic horizon are noteworthy, as this marks a significant horizon of SSF zone boundary; i.e., between the Anabarites trisulcatus–Protohertzina anabarica Zone and the Paragloborilus subglobosus–Purella squamulosa Zone of the Terreneuvian, Lower Cambrian.

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  • Yuto ISHIZAKA, Yukio ISOZAKI, Yukiyasu TSUTSUMI
    2021 Volume 130 Issue 1 Pages 63-83
    Published: February 25, 2021
    Released: March 18, 2021
    JOURNALS FREE ACCESS
    Supplementary material

     To reconstruct the tectono-sedimentary history of the Cretaceous–Paleogene arc-trench system in western Hokkaido, U–Pb ages were measured of detrital zircons in 11 Cretaceous and Paleogene fore-arc sandstones of the Yezo Group in the Oyubari and Mikasa areas. Age spectra of dated zircons demonstrate that Aptian sandstones of the basal Yezo Group are characterized by the dominant occurrence of Early Cretaceous grains with small amounts of Jurassic, Triassic, Permian, and Precambrian grains (Type 1). In contrast, those of Cenomanian to Paleocene sandstones of the middle to upper Yezo Group are dominated almost totally by mid- to Late Cretaceous grains (Type 2). This remarkable change from Type 1 to Type 2 occurred irreversibly during the Albian, indicating that the surface crust in the provenance in western Hokkaido was significantly renewed then. Among nine sandstones of Type 2, a positive correlation exists between zircon peak age and stratigraphic age, suggesting the unidirectional/gradual replacement of exposed magmatic rocks in the provenance, in particular, new volcanics/intrusives (Rebun–Kabato belt to the west). The pre-Cretaceous zircons in Type 1 sandstones were probably recycled from sandstones in the Jurassic accretionary complexes of the Oshima belt to the west. The coeval and identical turnover in zircon age spectra of the Cretaceous fore-arc sandstones recognized in Southwest Japan, from Kyushu to Kanto district, suggests that the fore-arc basin and its provenance with monotonous arc crustal rocks have ubiquitously developed along the Cretaceous East Asian margin over more than 1,500 km before the Miocene opening of the Japan Sea.

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Short Article
  • Hiroshi MORI, Yosuke TOMOOKA, Tetsuya TOKIWA, Yui KOUKETSU
    2021 Volume 130 Issue 1 Pages 85-98
    Published: February 25, 2021
    Released: March 18, 2021
    JOURNALS FREE ACCESS
    Supplementary material

     Raman spectral analyses were conducted of carbonaceous materials (CM) in meta-sedimentary rocks to estimate the metamorphic thermal structure of the Outer Zone of Southwest Japan in the Akaishi Mountains area, central Japan. Twenty samples of pelitic and psammitic rocks were collected along the Koshibu River and by traversing the Sanbagawa, Chichibu, and Shimanto belts. The datasets of the full width at half maximum of the D1-band (FWHMD1)—which is a sensitive thermal parameter of the Raman spectra of CM—imply that each rock records a single metamorphic thermal event. The peak metamorphic temperatures estimated with Raman CM geothermometry show 286-339°C in the Sanbagawa Belt, 284-350°C in the Chichibu Belt, and 328-347°C in the Shimanto Belt with a continuous spatial change including at the boundaries between belts. From northwest to southeast, a clear temperature decrease is observed in the Sanbagawa Belt, but an increase in the Chichibu Belt with a constant high-T condition—similar to those recognized in southeast part of the Chichibu Belt—in the Shimanto Belt. A comparison of geological structures suggests that the present thermal structures in the Chichibu and Shimanto belts were formed with the formation of a “twist-bend” structure related to the collision of the Izu-Ogasawara (Izu-Bonin) Arc with respect to the Honshu Arc at middle Miocene. In contrast, the thermal structure in the Sanbagawa Belt was mainly formed before about 20 Ma but the above middle Miocene tectonics may involve the formation of the apparent steep geothermal gradient.

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Letters
  • Kunio OMOTO
    2021 Volume 130 Issue 1 Pages 99-105
    Published: February 25, 2021
    Released: March 18, 2021
    JOURNALS FREE ACCESS

     Comprising beachrock, Maibahbama lies southeast of Miyako Island. It is approximately 400 m in length and 3 to 17 m in width. Fossil shell, coral and calcarenite samples were collected from the eastern part of Maibabahbama in order to determine their ages using accelerator mass spectrometry (AMS). 14C age of a Nerita albicilla yielded an age of Modern (100.7 ± 0.3 pMC); however, Tridacna gigas yielded an age of 981 ± 22 yr BP. Coral fragments yielded an age of 1746 ± 23 yr BP, while four calcarenite samples were between 1350 ± 23 and 1567 ± 22 yr BP. The above samples were collected from two outcrops standing 1 m apart and within a 10 cm circle. Five of the seven samples coincided with the estimated ages of past tsunamis within an error range of ±3σ. Consequently, the several samples collected from beachrock at the eastern part of Maibahbama might have been washed ashore by past tsunamis before being cemented. Formation of the eastern part of the beachrock started at 980 yr BP and ended recently. Therefore, they do not coincide with the older ages of other tsunami boulders (Porites sp.) nearby, reported previously.

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  • Yasuo MIYABUCHI, Masayuki TORII
    2021 Volume 130 Issue 1 Pages 107-116
    Published: February 25, 2021
    Released: March 18, 2021
    JOURNALS FREE ACCESS
    Supplementary material

     Intense rainfall related to a stationary front on July 3-4, 2020 (maximum total 513 mm; maximum 4-hour rainfall 260 mm) triggered many landslides and floods in the southern part of Kumamoto Prefecture, Kyushu, southwestern Japan. The distribution of landslides coincided with the highest concentration of rainfall in the region. The July 4, 2020 landslides were concentrated mainly in Ashikita and Tsunagi towns by the Yatsushiro Sea. Most of the landslides occurred on forested slopes steeper than 20°. They occurred in thick weathered soil layers overlying sedimentary and volcanic rocks; the thickness of the slides was generally less than 20 m. Some of those occurring on steep upper slopes along valleys mobilized completely into debris flows, traveling along stream channels and flooding at the lower reaches. The landslides and debris flows transported sediments and woody debris, which damaged houses and caused fatalities at some sites. The July 4, 2020 landslides and associated debris flows occurred mainly in areas underlain by Jurassic accretionary complexes of the Chichibu Belt. Some landslides were affected by fault fracture zones with the formation of thick weathered soil layers. These geologic characteristics of landslides provide important information for preventing or mitigating similar future disasters in accretionary complex zones.

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The 100s: Significant Exposures of the World (No.12)
  • David J. LOWE, Adrian PITTARI
    2021 Volume 130 Issue 1 Pages 117-141
    Published: February 25, 2021
    Released: March 18, 2021
    JOURNALS FREE ACCESS

     The Taupō eruption, also known as eruption Y, occurred in late summer to early autumn (typically late March to early April) in AD 232 ± 10 yr at Taupō volcano, an ‘inverse’ caldera volcano underlying Lake Taupō in the central Taupō Volcanic Zone, North Island, Aotearoa New Zealand. The complex rhyolitic eruption, the most powerful eruption globally in the last 5000 years, lasted between several days and several weeks and generated five markedly contrasting pyroclastic fall deposits (units Y1 to Y5) followed by the extremely violent emplacement of a low-aspect-ratio ignimbrite (unit Y6). The fall deposits include three phreatomagmatic units, Y1, Y3, and Y4, the latter two being the products of archetypal phreatoplinian events; and two magmatic units, Y2 and Y5, the latter being the product of an exceptionally powerful plinian (previously described as ‘ultraplinian’) event with an extreme magma discharge rate around 108 to 1010 kg s−1. The pyroclastic fall-generating eruptions were followed by the climactic emplacement of the entirely non-welded Taupō ignimbrite (Y6). It was generated by the catastrophic collapse of the 35 to 40-km-high plinian eruption column (Y5) that produced a very-fast-moving (600 to 900 km h−1), hot (up to 500°C) pyroclastic flow (density current) that covered about 20,000 km2 of central North Island over a near-circular area ∼160 km in diameter, centred on Lake Taupō, in fewer than about ten to 15 minutes. This violent ground-hugging pyroclastic flow generated its own air lubrication, forming a near-frictionless basal region, and the resultant highly fluidised ignimbrite was spread as a near-continuous but thin sheet over the entire landscape, both infilling valleys and mantling ridges. Caldera collapse formed a new basin in the older Ōruanui caldera in Lake Taupo. The pressure-wave arising from the plinian-column collapse probably generated a global volcano-meteorological tsunami. Studied intensely by extraordinary volcanologists Colin Wilson and George Walker, and others, the exceptionally well-preserved and readily-accessible Taupō eruptives provide a one-in-a-hundred classic sequence that is arguably the most informative in the global world of volcanology with respect to explosive rhyolitic eruptions and their products. The total volume of the Taupō eruptives amounts to ∼35 km3 as magma, equivalent to ∼105 km3 of bulk (loose) pyroclastic material, of which the Taupō ignimbrite comprises ∼30 km3. The impacts and landscape response of the eruption were profound, spatially extensive, and enduring, and the young glassy soils (Vitrands in Soil Taxonomy, Pumice Soils in the New Zealand Soil Classification) developed in the silica-rich pumiceous deposits, although well suited to plantation forestry (especially exotic Pinus radiata), pose unique problems for agriculture and other land uses, including a high susceptibility to gully erosion and an inherent deficiency in cobalt and other trace elements, and require special management.

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