Facies and depositional conditions of a carbonate mound (Tithonian-berriasian, SW-Japan)

Macro- and microlithology of a carbonate mound, at least 300-350 m wide and about 70-80 m thick, of Tithonian (latest Jurassic) to Berriasian (earliest Cretaceous) age was studied in Hitotsubuchi quarry on Shikoku Island, Japan. Quantitative data on limestone composition were obtained by line transect method in the field, and by point counting of thin sections. The mound rests on, and is probably surrounded by, mudstone. The limestone of the mound contains abundant non-skeletal (Bahaman type) particles, such as ooids, superficial ooids, pellets and micritic envelopes, whose formation requires precipitation of calcium carbonate in tropical or subtropical sea water. Calcareous algae are common. In the lower part of the mound, intergranular sparry calcite is a common constituent, suggesting that the sediment was well winnowed. The substrate was unstable and not suitable for large colonial organisms, except in some lenses. In the main, upper part of the mound the rock is predominantly a micritic calcarenite, and colonial organisms increase in importance, especially in some lenses where they occupy 35-45% of the rock volume. No evidence was found for a ripid organic frame, and thus probably no reefs were developed. Deposition of carbonate sediments ceased abruptly, possibly because of some catastrophic depositional event. The deposition of carbonate sediments may have been initiated when low elevation on a shallow sea floor reached into a zone of strong water turbulence. Fine terrigenous sediment particles mostly passed by the site and autochthonously produced carbonate mud was in part winnowed out. The growth of the mound continued by accumulation of "bahamitic" and skeletal grains, in the upper part accelerated by luxuriant growth of large colonial organisms.