Spherical carbonate concretions are hard, dense, carbonaceous rocks formed in various sedimentary environments worldwide. This study investigates the formation process of concretions found in the Setogawa Group (a Miocene accretionary complex) in Shizuoka, Japan, through geological observations and geochemical analyses. Circular cross-sections of burrows of
Phycosiphon-like ichnofossils and high calcite content suggest that they were formed in the shallow subsurface prior to compaction. Concentrated fish bone fossils at the center of the concretion and negative δ
13C values ranging from -9.7‰ to -6.4‰ indicate that the carbon source was organic matter derived from dead body of fishes and trace makers or fill material of ichnofossils. Elemental compositional analyses reveal that the concretions exhibit a reaction front less than 10 cm in width and likely formed within several tens of years. The concretions in the Setogawa Group demonstrate high mechanical strength, enabling them to withstand deformation caused by the formation of the accretionary complex and the collision of the Izu-Bonin Arc.
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