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Based on the studies of paleontologic and sedimentologic characteristics of over 500 drill core logs and samples, four aspects of the Miocene-Pliocene history were developed for the eastern half of the Hokuroku district of Japan. Initial subsidence lowered the district from subaerial to submarine water depths of about 3,500 m. The lithic record of this subsidence is found in the Sasahata Formation, whose composition, texture, and bed forms suggest very short transport from local sources. The thickness distribution of the Sasahata suggests that it was. derived from high-angle fault blocks which were active during subsidence.

After initial subsidence, considerable inversion of the deep sea-floor topography took place as the sea floor was reorganized into a series of north west-soutbeast-trending highs and lows. The highs were subjected to little or no muddy turbidite sedimentation and probably never rose more than 500 m above the adjacent sea floor. The lows were to become the sites of accumulation of thick volcanic sequences.

After the deep sea-floor topography was reorganized, dacitic volcanism commenced and at least two volcanic centers developed—the Kosaka and Sasamori centers. During the early part of the history of the Sasamori center (8 km in diameter), extrusion of dacitic lavas and associated pyroclastics of the Uwamuki Formation took place around the periphery of the volcanic center, producing a doughnut-shaped distribution pattern for these rocks. Subsequent peripheral venting produced the Akamori-Harukisawa pyroclastics and added to the doughnut-shaped mass of Uwamuki volcanics. Andesitic and basaltic lavas were produced on the outer margins of the Sasamori center, and the emplacement of dacitic and quartz dioritic intrusions in the central areas concluded the history of the Sasamori center.

During uplift of the area to subaerial conditions, sedimentary basins, such as the Noguchi and Hanawa depressions, developed and became the sites of thick (1,000-3,000 m), marine to nonmarine sedimentary fill.

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