Carbonate Rocks of the Tosabae Bank off the Kii Strait, Southwest Japan

Abstract

Carbonate rocks consisting of calcite, magnesian calcite and dolomite in various proportions, were dredged from the Tosabae bank off the Kii strait. The petrology of these sediments was investigated by optical, X-ray diffraction, differential thermometric and electron microprobe techniques in order to study the lithification of the carbonate sediments in the Tosabae bank. Magnesian calcite is considered to be a post-depositional precipitate, which occurred at water-sediment interface during cessation of sedimentation. Dolomite is non-ideal, i.e., protodolomite, which is restricted to the superficial zone of the carbonate sediments. Dolomite may be formed by replacement of the pre-existing carbonate minerals in very shallow water or hypersaline lagoon environment. If pre-existing carbonate is magnesian calcite, formation of dolomite seems to occur more easily. From these occurrences, the change in the environment of the Tosabae bank is summarized with the related mineralogy of carbonates as follows: 1) Accumulation of hemipelagic sediments on the Tosabae bank during middle or late Pliocene time (calcite) 2) Cessation of accumulation (precipitation of magnesian calcite at the surface layer of the sediments) 3) Regression and development of very shallow-water condition and/or hypersaline lagoon environment (formation of dolomite at the surface layer of the top sediments of the bank) 4) Subsidence of more than 180 m under sea-level.