Carbonate concretions and lensy and banded carbonate rocks are distributed in the Cretaceous, Paleogene and Neogene strata of the Joban area. These carbonate rocks were studied from the stratigraphic, chemical and mineralogical points of view to clear their origin. The results are summarized as follows:
1. These carbonate rocks are composed of detrital materials and carbonate minerals, the latter minerals are filled in the pore spaces of the former particles.
2. Only calcite and dolomite are found in these rocks.
3. The carbonate content of these rocks ranges from 25 to 85 per cent in weight. The variation originates from their stratigraphic position and their host rocks. In a single carbonate rock body, however, the content shows only a little difference between the inner and outer parts.
It is considered that calcium and magnecium in these rocks have been supplied from porewater and sedimentary materials. CO2 must be derived from organic materials, because these carbonate rocks have high content of P2O5 compared with low content of their host rocks and contain rich molluscan fossils.
These carbonate rocks have low ratio of SiO2/Al2O3 as compared with high ratio of their host rocks. This may indicate that the concentration of carbonate minerals and the release of SiO2 in these rocks have been occured at the same time. The action separating SiO2 and carbonate in sediments might promote the concentration of carbonate minerals.
These transformations must be occured during early diagenesis when the pore-water in sediments have been rich considerably and the flexibility of sediments have been still good.
