Fundamental study on the mode of transportation and sedimentation of subaqueous tephras
| Project/Area Number |
63540607
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| Research Category |
Grant-in-Aid for General Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Research Field |
地質学一般
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| Research Institution | Niigata University |
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Principal Investigator |
KUROKAWA Katsuki Niigata University, Faculty of Education, Professor., 教育学部, 教授 (50089835)
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| Project Period (FY) |
1988 – 1989
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| Project Status |
Completed (Fiscal Year 1989)
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| Budget Amount *help |
¥1,700,000 (Direct Cost: ¥1,700,000)
Fiscal Year 1989: ¥300,000 (Direct Cost: ¥300,000)
Fiscal Year 1988: ¥1,400,000 (Direct Cost: ¥1,400,000)
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| Keywords | Subaqueous tephra / Ash flow turbidite / The Joetsu Ash / The SK020 Ash / The Sug-a Ash / Facies model / Osumilite / Andalusite / Ash How turbidite / Sug-a火山灰 / 層相モデル / テフラの堆積構造 / テフラの粒度特性 / 中央油帯 / 浜忠層 / 西山層 |
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| Research Abstract |
Mode of transportation and sedimentation of subaqueous tephras was investigated especially on those intercalated in the upper Miocene to lower Pleistocene in the Niigata backarc basin, central Japan. They were classified in four categories; (1)subaqueous fallout, (2)ash cloud, (3)subaqueous flow and (4)their epiclastic reworks. Fourteen facies models of these tephras were established in respect to grain-size populations; TypeA1-A5 for silt to medium sand-sized tephras (Population A), TypeB1-B5 for medium sand to granule-sized tephras(Population B) and Type C1-C4 for those containing considerable amounts of pumice clasts larger than granule size(Population C).
It is noteworthy that among these subaqueous tephras, subaqueous flows which were directly transformed from subareal ash flows or subaqueous dense ash clouds are not uncommon regardless of their grain-size population; fine-grained ones(TypeA3), medium-grained ones(TypeB4) and coarse- grained-ones(TypeC3). They show inverse-to-normal grading, containing mud balls or rip-ups, and flowed more than 50km with little decrease of thickness. In this research, they are named ash flow turbidites. The Joetsu Ash (Kurokawa and Hirata,1986) is a representative of the ash flow turbidites of TypeB3. It is cordierite-cummingtonite bearing ash of subareal ash flow origin, flowed into water.
The basic sequence containing the ash flow turbidites was clarified; the ash flow turbidite unit occupies the base of the sequence (Unit I), overlaid by thin alternations of fine-grained ashes (Unit II) and pumice-rich ashes (Unit III). This sequence suggests the flow differentiation from subareal ash flow or subaqueous ash cloud.
Besides, osumilite and magmatic andalusite were found from Pliocene subaqueous tephra (the Sug-a Ash) in the course of this study.
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Report
(3 results)
Research Products
(20 results)
