View of a wave-cut platform showing the distribution of gigantic concretions, and photomicrographs and XRF images of concretion in the Unosaki coast, Oga peninsula, Japan.

Fig. S1. (A and B) View of a wave-cut platform on the Unosaki Coast showing the distribution of gigantic concretions. The concretion on the left in (B) is cracked by erosion, and the interior can be observed. The photograph of (A) was provided by the Oga Peninsula–Ogata Geopark Promotion Council and attendees at an Oga Peninsula–Ogata Geopark guide meeting.

Fig. S2. (A and B) Photomicrographs (crossed nicols) of a concretion. Sponge spicules and diatoms are observed in (A), and pyrites are found in (B). (C) Fluorescent photomicrograph of a concretion. The brown and yellow areas signify different degrees of weathering. Trace fossils are clearly observed in the area indicated by the dashed ellipse. (D) Fluorescent photomicrograph of the boundary between whale bone and concretion, showing an irregular boundary.

Fig. S3. (A) Photomicrograph and XRF images of concretion. Color bars show the X-ray intensity of measured elements. (B) Detailed SEM photograph and elemental distribution of concretions. Ca and Mg are distributed throughout, indicating that this entire surface is dolomite, and both micritic and euhedral dolomite are observed (black arrows). Color bars show the relative intensity count of measured elements.