Accreted mesozoic pelagic seamount in Chirai quarry, Hokkaido, Japan: petrographic evidence of flint-limestone cycles
Abstract The Jurassic to Cretaceous cyclic sequences of nodular flints and micritic limestones of the Nikoro Group in Chirai quarry, eastern Hokkaido, were studied petrographically and geochemically. They were compared with the Cretaceous to Eocene cycles of nodular flints and pelagic carbonates on the crests of three pelagic seamounts in the North Pacific. The nodular flints and micritic limestones in Chirai quarry were shown to be deposited in a pelagic environment from the Fe-Mg plots of the non-calcareous fractions. The flint nodules were formed due to the dissolution of pelagic carbonates, and three kinds of dissolution are possible: (1) dissolution by deep bottom waters; (2) dissolution by organisms to form burrows; and (3) dissolution by meteoric groundwaters. Although nodules formed due to (2) and (3) can be recognized at some horizons in the quarry, most of the nodules are interpreted to be formed by dissolution due to upwelling bottom waters on the crest of the seamount. The nodules on the crests of modern pelagic seamounts in the Pacific are restricted in distribution on the seamount to places where the water depth on the crest at the time of deposition was in the range 2–4 km. This may suggest that the upwelling bottom water undersaturated with calcium carbonate, is not found at depths shallower than 2 km. The cycles of flints and pelagic carbonates are interpreted to denote the facies on the crest of the pelagic seamount; therefore, the Jurassic to Cretaceous cycles found in Chirai quarry can be recognized as an accreted pelagic seamount.