Carbonate Rock Boulder with a Cone-in-cone Structure Discovered in the Akabira-kawa Riverbed, Ogano-machi, Saitama Prefecture

A carbonate rock boulder with a cone-in-cone structure was discovered in the Youbake riverbed of Akabira-kawa at Oganomachi. This sample is the second example of a cone-in-cone structure found in Japan, following a sample from the Itsukaichi-machi Group. The cone-in-cone comprises fibrous calcite and fine quartz grains. A thin clay band covering the cone has a stepped profile on one side and is rich in chlorite and illite crystals. The carbonate has low δ13C values of −12.1‰ to −13.4‰ and low δ18O values of −6.8‰ to −7.4‰. These isotope data agree well with those of previous studies.