Aspects of the Sedimentologic and Structural Development of the Eastern Hokuroku District, Japan
Albert L. Guber, Geoffrey R. Green, 1983. "Aspects of the Sedimentologic and Structural Development of the Eastern Hokuroku District, Japan", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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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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This paper consists of three parts. The first is an overview of the geologic history of the Green Tuff region where all Kuroko deposits occur. The second part presents a description of the stratigraphy and an interpretation of the structural and igneous history of the Hokuroku district, the most important Kuroko mining district. The third part is an analysis of the role of submarine calderas in Kuroko genesis.
The sequence and causes of the major geologic events that have occurred in Japan and its vicinity since the Cretaceous are interpreted as follows: (1) an active but shallow-dipping north-northwestward subduction of the Pacific plate under the Asian continent during a period from approximately 130 to 65 m.y. ago resulted in ilmenite series magmatism in the outer zone of Japan, then still a part of mainland Asia; (2) about 65 to 40 m.y. ago, the direction of the subducted Pacific plate changed to westward and the angle of subduction steepened, initiating back-arc spreading in the Japan basin province and migration of Japan away from the Asian mainland until about 30 m.y. ago; (3) during the period 65 to 30 m.y. ago, the basaltic crust created in the Japan basin province was subducted eastward under the Yamato Ridge province, resulting in calc-alkaline and magnetite series igneous activity in the inner zone of Japan; (4) about 25 m.y. ago, the first sea (proto-Japan Sea) was formed in the Japan basin province as a result of the eustatic rise of the sea following cessation of spreading there about 30 m.y. ago; (5) back-arc spreading was active in the Yamato basin province during the period between 25 and 5 m.y. ago, cansing bimodal volcanism and subsidence in the flanking Inner Honshu and Yamato Ridge provinces [the Hokuroku basin (i.e., a Kuroko-bearing basin), Niigata oil field basin, and Akita oil field basin were all fault-bounded, deep (>2,500 m) marine basins created by rapid subsidence of crustal blocks within a few million years around 17 m.y. ago, although Kuroko mineralization and the accumulation of organic matter were not synchronous]; and (6) the dip of the subducted Pacific plate returned to a shallow angle about 5 m.y. ago, causing the cessation of back-arc spreading and the initiation of subsidence of the Yamato basin province and uplift of the flanking Inner Japan and Yamato Ridge provinces. The Green Tuff activity is, therefore, synonymous with the tectonic and igneous activity that accompanied the formation of the Japan Sea and the Japanese islands during the period from ~65 m.y. ago to the present.