ORIGIN OF NODULAR LIMESTONE IN DEVONIAN SYSTEMIN SOUTH CHINA

In South China, the nodular limestone which is a special carbonate type.was distributel in Devonian System. The petrological research, δ13C and δ18O, X-ray diffraction, CaCO3 and organic carbon contents are done and the origin of the nodular limestone is detailedly studied. In Dachang area of Guangxi Province, the nodules of the nodular limestone which is more pure show the lens, nodule and pod. The nodule without or with a few quartz fragments is mainly composed of calcite (CaCO390%). The matrix of the nodular limestone, however, is more complex, which contains some clay minerals, a few quartz fragments and black organic materials as well as a few autogenetic quartz grains.The δ13C values of the nodules and matrixes are more than -2‰, however, the δ13C values of the nodules of same samples is higher than the values of the matrix, in which the average difference between them is about 1‰, and differences are about coincident. CaCO3 content of the nodules is higher than that of the matrixes and the difference is more obvious. In Fankou, the nodular limestone contains more bioclasts, than in Dachang. the nodule and the matrix contain more quartz fragments. The matrix contains more quartz fragments than the nodule, but does not contains autogenetic quartz grains. The nodules being composed of block of the bioclastic limestone do not show internal structure, but show flame and flow structures, disturbance, slip and roll-up beddings. The composition has no more difference between the matrix and the nodule which are mainly composed of calcite, quartz clast and clay minerals, as well as a few dolomite and pyrite. TheδI3C values of the matrix and the nodule are lower than -2‰, and the difference of the δ13C values between the matrix and the nodule is no marked in same sample. Somtimes the value of matrix is higher, somtimes the value of the nodule is higher than that of the matrix. These characters and developmental history of Devonian System in South China indicate that, Dachang area lay on the depression with deep water and far distance from shore, and with or without a few addition of terrigenous materials. The nodule was formed without submarine flow. The submarine flow formed in certain period soluted carbonate which deposited in precedent stage and remained more insoluble clay minerals, so that the matrix was formed, the isotopic exchange between submarine flow and the carbonate decreases the δ13C value of the matrix. During the sedimentation of the nodular limestone in the Middle and Upper Devonian System in Fankou, this area belongs to a lagoon environment with shallow water and offshore. Bioclastic limestone and micrite beds formed by alternational sedimentation formed the nodular limestone under the pressure, in which the soft micrite sediments removed upward and formed the flame and flow structures. Semisolided mass of the bioclastic limestone formed the nodule under the pressure. The δ13C values between the nodule and the matrix do not show any distinctively difference, because they belong to the same sedimentary environment.