The genesis of phosphatic and carbonate rocks in the Toyoma Formation, Northeastern Japan

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概要

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• Various phosphatic and carbonate rocks occur in the Upper Permian Toyoma Formation. They are classified into phosphatic nodules, A-, B-and C-type rocks.The isotopic ratios of whole-rock calcite,pyrite and apatite in the rocks and the results of pyrite microanalysis using a high-resolution secondary ion mass spectrometer are presented. The carbon and sulfur isotopic compositions indicate that the rocks were formed through microbial sulfate reduction. Three distinct zones have been recognized within the Toyoma Formation that define its early diagenesis. The three zones’compositions differ in their mole fractions of authigenic material and the δ13C of their calcite deposits. Zone 1 is the region of oxic surface sediment in which sulfur-oxidizing bacteria grew.Because sulfuric acid is generated by the chemosynthesis of the bacteria, biogenic phosphate debris such as fish bones and scales were dissolved.The phosphate ions thus formed were the source of the phosphatic rocks of the Toyoma Formation.Zone 2 is the anoxic zone that underlay Zone 1. Anaerobic sulfate-reducing bacteria grew in this zone, resulting in increased pore water alkalinity and the precipitation of pyrite and apatite. Phosphatic nodules and A-and B-type rocks began to grow in this zone.Zone 3 is characterized by calcite precipitation. The pore water, saturated with calcite components, segregated from the sediments into rocks allowing the rocks to be calcareous. As sediments became buried deeper and deeper, they experienced the conditions of each successive zone. However, the period of final consolidation of the rocks differed from one rock type to another as evidenced by the varying δ1C values for calcite contained in the different rock types. Although the rocks are mixtures of authigenic minerals originating in different zones, it was possible to estimate the range of diagenetic zones in which each rock type was formed.

• 2012-01-30

著者

• Morikiyo T

  Department Of Geology Faculty Of Science Shinshu University

• Iwamasa Kazuo

  Department Of Geology Faculty Of Science Shinshu University

• Matsunaga Kinuko

  Department Of Geology Faculty Of Science Shinshu University

関連論文

• The sources of phosphorus for the phosphatic rocks in the Toyoma Formation, Northeastern Japan
• Variations of divalent cation concentrations in pore water and the precipitation of Mg-rich authigenic mineral during early diagenesis of Toyoma Formation
• The genesis of phosphatic and carbonate rocks in the Toyoma Formation, Northeastern Japan

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