Fluid migration through unconsolidated layered system-Fluid injection structures in alternated sandstone and mudstone layers of the Nichinan Group, Miyazaki Prefecture, Japan.

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  • 未固結砂泥互層における流体の移動過程　　宮崎県日南層群の砂岩泥岩互層中にみられる流体噴出構造
  • ミコケツ サデイ ゴソウ ニ オケル リュウタイ ノ イドウ カテイ ミヤザキケン ニチナンソウグン ノ サガン デイガン ゴソウ チュウ ニ ミラレル リュウタイ フンシュツ コウゾウ

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A series of fluid injection structures (FIS) occur in the turbidite sequence of the Tertiary Nichinan Group in the Izakibana area, Nichinan City, Miyazaki Prefecture. They are funnel structures, vent structures (newly defined) and small sandstone dikes. Reverse cone-type funnel structures are formed stepwise in a single sandstone bed located from Middle Unit to lower Upper Unit. Vent structures lager than funnel structures penetrate through multiple beds of mainly Middle Unit. Mudstone beds in all sedimentary units contain small dikes tapering upwards or downwards. Tank experiments were carried out on multiple layered system to investigate the deformation processes induced by the upward migration of the fluid. As a result, a lens-shaped water-filled void was formed temporarily beneath a clay layer, which expands with increasing fluid percolating upward through the underlying sand layer. The failure process of the overlying layers varies according to the migration process of the water-filled void. The most distinctive characteristic is a stepwise migration of void with local fluidization within single sand layer. The cracks in clay layer are filled with sand avalanching due to strength reduction induced by incipient fluidization. The fluidized part in multiple layers is vertically expanded with increasing apparent flow velocity.The formation of the FIS at the Izakibana area can be explained as follows ; (1) concentration of fluid with higher flow velocity through mud layer formed vent and some funnel structures, (2) flow velocity decreased due to lower pressure in divided voids, (3) stepwise funnel structures and small scale sand dikes were formed by upward migration of smaller voids, (4) cracks in a split mud layer are filled by avalanched sands into void forming sand dike tapering downward. These FIS are suggestive of genuine of fluid-filled void formed in a multiple layered system.

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