Formation of gigantic spherical carbonate concretion in early diagenesis

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  • 初期続成過程における巨大球状炭酸塩コンクリーション形成
  • ショキ ゾクセイカテイ ニ オケル キョダイ キュウジョウ タンサンエン コンクリーション ケイセイ

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Abstract

<p>Many types of spherical carbonate concretions are common in sedimentary strata of various ages worldwide. However, the process by which these concretions form is not completely understood. This study investigated a gigantic spherical carbonate concretion, with a diameter of ~1.5 m, identified in Miocene fine tuffaceous sandstone in the Lower Toyohama Formation (Morozaki Group) on Chita Peninsula, Aichi prefecture, Japan. Detailed field and microscopic observations, porosity measurements, mineralogical examination, and geochemical analyses were carried out to understand the formation of such a large concretion in a marine sediment. The field exposure shows that the concretion formed during early diagenesis, before compaction due to subsequent sedimentation. Thin section observations and geochemical, isotopic, and XRD analyses revealed that the concretion formed as calcite and was later replaced by dolomite. By combining the porosity and dolomite composition with a porosity-burial depth relationship, it is estimated that the concretion formed at a depth below the seafloor of up to a few hundreds of meters, and was buried to about 2400 to 5500 m at the deepest. This depth is consistent with the depth of zeolite formation in the rock matrix, as estimated from the geothermal gradient. Interpretation of geochemical analyses suggests that the gigantic concretion was formed over several decades. Our results indicate that even gigantic carbonate concretions form quite rapidly after marine sediment deposition during early diagenesis.</p>

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