1993 Fiscal Year Final Research Report Summary
Magnetostratigraphic study of the Japanese Cretaceous system
| Project/Area Number |
04452080
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| Research Category |
Grant-in-Aid for General Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Research Field |
Stratigraphy/Paleontology
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| Research Institution | Kochi University |
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Principal Investigator |
TASHIRO Masayuki Kochi Univ., Dept. of Geology, Professor, 理学部, 教授 (30117014)
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| Co-Investigator(Kenkyū-buntansha) |
KODAMA Kazuto Kochi Univ., Dept. of Geology, Assoc. Professor, 理学部, 助教授 (00153560)
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| Project Period (FY) |
1992 – 1993
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| Keywords | Paleomagnetism / Cretaceous / Kyushu / Hokkaido / Tectonic rotation |
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| Research Abstract |
Samples for paleomagnetic studies were collected in 1992 from the upper Cretaceous Yezo Group in central Hokkaido. Layrs of reversed polarity were found only from the uppermost horizons of Tan-no-sawa section in Soya-Otoineppu area. Combined with biostratigraphic age analyzes, the reversed polarity can be assigned to the polarity chron 33r in the Campanian stage of late Cretaceous. Most of the other strata vielded single normal polarity correlatable with the Cretaceous normal polarity superchron. Detailed magneto-biostratigraphic examination disclosed that Inoceramus schmidti appeared from the lower Campanian, that is significantly earlier than previously expected. From tectonic viewpoint, most of the strata are characterized by around 20ﾟ eastward deflection in declination, while the upper Tan-no-sawa section is unique in its westward declination shift by approximately 25ﾟ. The dominant eastward declination anomaly agrees well with that observed from the Tertiary sedimentary strata in central Hokkaido. This means that central Hokkaido underwent an around 45ﾟ counter-clockwise tectonic rotation around vertical axis in the late Cretaceous, followed by an about 20ﾟ clockwise rotation in late Tertiary.
In addition to measurements of those Cretaceous samples, a paleomagnetic study was conducted in 1993 for the Pliocene sedimentary rocks in Miyazaki prefecture of south Kyushu. Reliable site mean directions were obtained from 13 out of 37 sites. The overall mean direction after bedding-tilt correction exhibited an around 30ﾟ westward declination shift, which is consistent with previous results from the Miocene sedimentary rocks in Tanegashima Island and Miyazaki area. These consistent declination anomalies suggest that south Kyushu experienced an approximately 30ﾟ counter-clockwise tectonic rotation after the latest Pliocene, or during the last two m.y.b.p.
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