Cone-in-cone structure is observed in the Kosho Alternation Member of the Itsukaichimachi Group (Miocene) exposed along the Akigawa River in Akiruno City, Tokyo. This sedimentary structure has long been studied in Europe and North America but there remains many problems still in debate. Study of this structure, however, has mostly been ignored in Japan, and the present cone-in-cone structure was misinterpreted as stromatolites by the previous authors. We describe this material and briefly introduce studies in foreign countries in this occasion.
One of the well-preserved cone-in-cone structures in the Kosho Alternation Member is covered with thin banded clay and calcareous ironstones of which iron minerals are mostly siderite. The present samples indicate that the structure was formed by crystallization of calcite in free from the stress field and fibrous crystal behaved similar to ice needles in the ground. Microscopic observation of the structure shows that calcite and siderite were almost syngenetically crystallized immediately below the sea-floor of more than -200m depth.