Upward concentration of heavy minerals in the SK050 volcaniclastic turbidite in the Niigata region, central Japan(Natural Sciences)

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The SK050 Tephra Bed is one of the prominent subaqueous volcaniclastic gravity flow deposits in the Niigata region. It is early Pleistocene of about 1.3Ma in age and intercalated in the Uonuma Group. The SK050 is 11-72cm thick and tracable from the Tsunan area northwards through Tokamachi and Kawanishi areas to the Oguni area in more than 30km. The thickness distribution of the SK050 at 26 localities shows two maxima areas. In the northern Kawanishi area, situated between these two maxima, the SK050 is thin of 12-20cm thick and finer in grain size. This thickness distribution pattern of the SK050 turbidite was interpreted as due to a steeper slope in the northern Kawanishi area. The SK050 is dacitic crystal-vitric tephra showing remarkable grading and upward concentration of heavy minerals such as hornblende and orthopyroxene. Vertical changes of the grain size and components ratio were examined in detail at Shibanomata of 65cm thick by every 5cm divisions. The Mdφ changes from around 0.0φ to 1.6φ to the top. The frequency distribution of the grain size in every 5cm division is bimodal as a whole. The one mode is at around -0.5φ, consisting of the pumice component. The other mode is at +1.0φ to +1.5φ, consisting of crystal component. According to this grain size-component relation, crystal ratio in the components remarkably increases from 40cm from the base to the top, from 22% at the 40cm to 73% at the top. Thus, the upward concentration of heavy minerals in the SK050 was interpreted as mainly due to the smaller settling velocity of heavy minerals in spite of larger density as a result by their smaller grain size than pumice components.
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Upward concentration of heavy minerals in the SK050 volcaniclastic turbidite in the Niigata region, central Japan(Natural Sciences)

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