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Volume 49, Issue 1
Displaying 1-4 of 4 articles from this issue
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The Paper for the 2008 Japan Association for Quaternary Research Award
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Yoshiaki Matsushima2010 Volume 49 Issue 1 Pages 1-10
Published: December 01, 2010
Released on J-STAGE: March 27, 2012
JOURNAL FREE ACCESSMolluscan assemblages containing warm water species now not living in Hokkaido, such as Trapejium liratum, Meretrix lusoria, Mactra veneriformis, Macoma contabulata, etc., are found in several horizons of Holocene shallow marine sediments and shell mounds in Hokkaido. Based on the detailed datings of the warm water molluscan assemblages in the Okhotsk Sea coast area of Hokkaido, four stages of warming of the Okhotsk Sea coast are recognized. These are 7,200-5,000 years BP, 4,200-3,200 years BP, 2,500-2,300 years BP, and 1,000-900 years BP. Correlative warm stages are also recognized in the offshore sediments of the Japan Sea and the Okhotsk Sea, based on the planktonic diatoms in the core samples. These warm stages indicate the activation of the Tsushima warm current, when mollusks invaded synchronously into the Okhotsk area.View full abstractDownload PDF (1449K)
Articles
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Kuniyoshi Tomita, Yoshihiko Kariya, Go Sato2010 Volume 49 Issue 1 Pages 11-22
Published: December 01, 2010
Released on J-STAGE: March 27, 2012
JOURNAL FREE ACCESSA landform continuum consisting of a cirquelike and a moraine-like topography is present on the eastern side of Mount Chogatake (2,664 m ASL), central Japan. There have been contrary and unresolved theories regarding the final morphogenetic process of these topographies : a Pleistocene glacier theory and a undated landslide theory. We therefore reevaluated the problematic topographies through a detailed redescription of the field geology and a geomorphological interpretation using aerial photos and 10 m DEM-generated shaded images. A thick (>100 m) deposit of brecciated and crackled clasts of sand and mud stones supported with fine matrices (sand to silt size) is evident around the moraine-like topographies. Unlike typical till or outwash, this deposit does not have subangular or subrounded gravels. No sedimentary structure indicating fluvial transport and deposition such as cross-bedding is found. Characteristic landforms such as linear to curved depressions and scarplets as well as valley side bulges indicating gravitational mass rock creeping or toppling are developed on and beside a ridge that encompasses the pseudo cirque-terminal moraine continuum. Judging from these geological and geomorphological characteristics, we conclude that the problematic topographies could be induced by large-scale slope failure (est., 3.2×107 m3). The age of the landsliding is uncertain. Although the direct trigger (s) of failure is also unspecified, it would appear that Holocene active faults and active volcano near Mount Chogatake as well as humid climates in the southern Hida Mountains may create be favorable conditions to generate the movement.View full abstractDownload PDF (1876K) -
Osamu Fujiwara, Hiroshi Machida, Jun-ichi Shiochi2010 Volume 49 Issue 1 Pages 23-33
Published: December 01, 2010
Released on J-STAGE: March 27, 2012
JOURNAL FREE ACCESSA tsunami deposit caused by the great Akahoya eruption that occurred around 7,300 cal BP in Kikai caldera, off southern Kyushu, was found in the Yokoo midden, Oita City, about 300 km north of the source caldera. The Akahoya eruption is one of the largest volcanic eruptions that occurred in the world during the last 10,000 years.
A thick bed (about 65 cm in thickness) of Kikai-Akahoya ash (K-Ah) was observed in some excavation pits of the midden. The ash bed was deposited in a small valley located at the head of a shallow bay. A successive stack of six depositional units, I to VI in ascending order, characterizes the ash bed. The following five characteristics indicate that the succession of Unit I to V (about 35-cm-thick in total) was caused by a tsunami accompanied by the Akahoya eruption : 1) Units I to V have sedimentary structures showing that each of them was deposited from a waning flow. 2) Interruptions of sedimentation were recognized between each depositional unit. For instance, Unit II exhibits many deformation structures caused by liquefaction or fluidization. Unit III eroded the deformed Unit II. 3) Cyclic alternation of flow directions, landwardand seaward-, is displayed by the pair of underlying and overlying depositional units. 4) A progressive finingand thinning-upward trend is shown by Units I to V. 5) Unit VI consists of massive and pure K-Ah ash, and it mantles the ripple laminations on the upper surface of Unit V.
The first four sedimentary features represent the long-period run-ups and backwashes of tsunami waves. Each depositional unit represents deposition from the individual wave in a tsunami wave train. Punctuated sedimentation of the depositional units indicates that the stacking of Unit I to V resulted from a wave train with a long-wave period. Vertical stacking of finer and thinner depositional units in the upper horizon reflects the progressive decrease in power of successive waves. Unit VI is ash fall deposit and evidences the synchronicity of the formation of Units I to V and the Akahoya eruption.View full abstractDownload PDF (1561K)
Short Article
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Masayuki Oishi2010 Volume 49 Issue 1 Pages 35-42
Published: December 01, 2010
Released on J-STAGE: March 27, 2012
JOURNAL FREE ACCESSVolcanic glass is often used to correlate tephras. However, other volcanic products, such as phenocrysts, are also sometimes used for this purpose. It is therefore desirable that many kinds of phenocrysts can be used for correlation of tephras. In particular, plagioclase phenocrysts are abundantly present in volcanic products across Japan. Because the refractive index of plagioclase reflects its chemical composition, it is expected that this index will allow quick and easy correlation of tephras. This study examined whether results in accordance with correlations by earlier studies were provided using refractive indices of plagioclase phenocrysts. Tephras erupted from the Nantai, Haruna, Asama, and Azumaya volcanoes located around the Northern Kanto region are used in this study.
As the result, refractive indices of plagioclase phenocrysts in Imaichi scoria (Nt-I) and Shizu welded scoria flow (NlSs) from Nantai volcano ; Hassaki tephra (Hr-HP), and Shirakawa pyroclastic flow from Haruna volcano ; Itahana Yellow tephra (As-YP) and Komoro 1 pyroclastic flow, and Asama A tephra (As-A) and Onioshidashi clastgenic lava flow from Asama volcano ; and Sugadaira 2 tephra (SgP. 2) and Yokokawa 2 pumice bed (YoP-2) from Azumaya volcano were the same. These results support the correlations of earlier studies.View full abstractDownload PDF (882K)
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