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Oral presentation

Mineral assemblage and geochemistry of sandstones of the miocene Mizunami group, central Japan; A Key to reconstructing the miocene volcanic activities

Sasao, Eiji

no journal, , 

The Miocene Mizunami Group is located in the eastern part of Setouchi Province, in the Inner zone of Southwest Japan. One of the geological characteristics of the Setouchi Province is the weak and locally limited volcanic activity. Recently pyroclastic flow deposits were found in the Mizunami Group. If there had been volcanic activity in the Setouchi Province, reconsideration of Miocene tectonics would be needed. This study aims to reconstruct the volcanic history recorded in sandstones of the Mizunami Group by describing vertical mineralogical and chemical changes using core from a single borehole. As a result, all sandstones studied contain volcanic glass and/or their alteration products. Distinctively, biotite is dominant in some sandstone in the Toki Lb and Hongo Formations. The second dominant heavy mineral in the Hongo and Toki Lb Formations is amphibole. Pyroxene is the dominant heavy mineral in the uppermost part of the Hongo Formation and in the Akeyo Formation. These facts imply that these heavy minerals were derived from volcanic ash. If so, the change in chemistry and heavy mineral compositions indicates an extended, though not necessarily continuous, history of volcanic activity that supplied volcanic ash to the Mizunami sedimentary basin.

Oral presentation

Pristene, phytene, and sterenes in immature Horonobe diatomaceous sediments as potential indicators of paleo-temperature and depositional environment

Amo, Miki*; Suzuki, Noriyuki*; Shinoda, Toshio*; Ratnayake, N. P.*; Takahashi, Kazuharu

no journal, , 

Diagenetic degradation and transformation of steroid and acyclic isoprenoid hydrocarbons in immature Horonobe diatomaceous mudstones of Wakkanai and Koetoi Formations were studied using the borehole core samples from HDB-3 and HDB-4. Late Miocene to Pliocene marine diatomaceous mudstones in Horonobe, Hokkaido, Japan are rich in steroid hydrocarbon such as sterenes, steradienes and sterane and isoprenoid hydrocarbons such as pristenes, phytenes, pristane and phytane. The present study attempts to constrain the diagenetic pathway of these compounds in a thick sedimentary sequence of immature mudstones. Detailed geochemical analyses for Horonobe diatomaceous mudstones revealed the timing of transformation and the compositional change of isoprenoid hydrocarbons during diagenesis. Potential maturity parameters based on ratios of sterenes to steranes, pristene to pristane, and phytene to phytane are proposed in the present study. Possible potentials of these compounds as indicators of depositional environment for immature sediments are also investigated in the present study.

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