All issues
Volume 122, Issue 1
Displaying 1-3 of 3 articles from this issue
- |<
- <
- 1
- >
- >|
Articles
-
an examination of the Jurassic accretionary complex in the west Akka areaSatoshi Takahashi, Masayuki Ehiro, Noritoshi Suzuki, Satoshi Yamakita2016 Volume 122 Issue 1 Pages 1-22
Published: January 15, 2016
Released on J-STAGE: April 15, 2016
JOURNAL FREE ACCESSThe North Kitakami Belt of Northeast Japan, which consists of a Jurassic accretionary complex, is divided by the Iwaizumi Tectonic Line into two sub-belts, the Kuzumaki-Kamaishi and Akka-Tanohata sub-belts, which differ in terms of the dominant feldspar type in sandstone and the presence or absence of Paleozoic oceanic sedimentary rocks. We carried out a detailed study of the geology of the boundary area between these two sub-belts in the west Akka area in Iwaizumi Town, Iwate Prefecture.
Our survey resulted in the identification of three tectonostratigraphic units. From lower to upper (east to west), these are the Takayashiki (mixed facies), Seki (coherent facies of chert-clastic sequences associated with thick sandstone layers), and Otori (coherent facies of chert-clastic sequences and dominant mudstone) units. Paleozoic fossils occur only in the Seki and Otori units, supporting the criterion proposed previously for identifying the location of the Iwaizumi Tectonic Line. However, the dominant feldspar composition of sandstone changes gradually from plagioclase-dominant in the Otori Unit, through equal amounts of both plagioclase and K-feldspar in the Seki Unit, to K-feldspar-dominant in the Takayashiki Unit. Thus, the feldspar compositions from west to east are not consistent with the proposed location of the tectonic line. The tectonostratigraphic units and sandstone compositions of the North Kitakami Belt are well correlated with those of both the Oshima and South Chichibu belts, which are the northern and southern tectonic extensions of the North Kitakami Belt, respectively. However, the oldest age of each unit is different between these three geologic belts. Paleozoic fossils have not been reported from the Shimonosawa Unit of the Oshima Belt, which corresponds to the Seki Unit, whereas the Sambosan Unit of the South Chichibu Belt, which can be correlated with the Akka-Tanohata sub-belt, yields Paleozoic fossils.View full abstractDownload PDF (8137K) -
Jun Tajika, Sunao Ohtsu, Tetsuya Inui2016 Volume 122 Issue 1 Pages 23-35
Published: January 15, 2016
Released on J-STAGE: April 15, 2016
JOURNAL FREE ACCESSA small mound excavated at the Apporo 1 archaeological site, at the southeastern margin of the Ishikari Lowland, is inferred to be a landslide body originating from a modern pyroclastic fall deposit. Its generative process, inferred from observations of its interior structure, are discussed. The mound mainly comprises pumice and rock fragments originating from the Tarumai-d pyroclastic deposits (Ta-d: 8-9 ka). It rests upon humus overlying the Ta-d deposits on a river terrace, and is covered by the Tarumai-c pyroclastic deposits (Ta-c: 2.5-3 ka). On a longitudinal trench wall, we found an asymmetric extensional structure resembling a normal listric fault in the mound deposit, and an asymmetric duplex-like structure composed of a mixture of humic soil and loam at its base. On the trench wall, in the transverse direction, the observation of horst and graben structures in the mound deposit indicates a lateral tensile stress field. These observations suggest that the mound was a landslide deposit derived from a hill slope and emplaced on a terrace surface. The landslide, which developed within an unconsolidated, stratified pyroclastic deposit, was probably triggered by an earthquake. Conventional radiocarbon dating of humus, and dates based on the relationships of the strata, indicate that the slide occurred between ca. 4,600 and 2,500 years BP. This age may correspond the most recent seismic event on the Umaoi Fault in the Ishikari-teichi-toen fault zone.View full abstractDownload PDF (8762K)
Report
-
Toshifumi Katsumura, Hiroshi Yamamoto2016 Volume 122 Issue 1 Pages 37-42
Published: January 15, 2016
Released on J-STAGE: April 15, 2016
JOURNAL FREE ACCESSThe Nishime area of Hokusatsu District, southwest Kyushu, Japan, is underlain by a chert-clastic complex comprising the Nitao and Momigi Units of Chichibu Belt; a mélange of sandstone, chert, and limestone clasts in an argillaceous matrix, corresponding to the Omae Unit of Chichibu Belt; and coherent sandstone-mudstone formations that belong to the Saiki Subgroup of the Shimanto Belt. These pre-Paleogene bedrock formations are mostly covered by Neogene and younger formations. On the basis of detailed geologic mapping of this area, we propose two precise locations for the boundary between the Omae Unit and the Saiki Subgroup. This boundary is traditionally called the Butsuzo Tectonic Line (BTL). The first proposed location is on the Ushinohama coast, where a brittle shear zone separates the Saiki Subgroup from the mélange of the Omae Unit. Here, deformation structures in the shear zone indicate northeastward thrust of the hanging wall of the Omae Unit. The second location is at Chugenda. Although direct contact is not exposed at the Chugenda location, a mélange similar to the hanging wall of the Ushinohama coast shear zone is juxtaposed against coherent beds of sandstone and mudstone. We suggest that a geotectonic boundary corresponding to the BTL runs through the Ushinohama coast and Chugenda.View full abstractDownload PDF (5662K)
- |<
- <
- 1
- >
- >|