Sedimentary strata and clay mineralogy of continental shelf mud deposits in the East China Sea

Six cores from the East China Sea have been studied using the isotope Pb-210 and clay mineralogy in order to evaluate the formation of sedimentary strata and to decipher the provenance of shelf sediments. There are two recent mud deposits in the East China Sea. The one extends from the Changjiang River mouth to the southward along the coast of China, and the other is in the outer shelf depression southwest of Cheju Island. These two recent mud deposits are different with respect to the material composition, sedimentation rate and other characteristics. The sediment in the inner shelf area is characterized by abundant silt and more coarser grain-size, but the outer-shelf mud deposits are characterized by rich clay with higher contents of water and organic carbon. At the boundary of the inner-shelf and outer-shelf mud deposits, bimodal sediment are forming the mixture of modern mud and ancient sand. The sediment accumulation rate in the inner shelf mud deposit is 1.70cm/yr or 1.63g/cm² yr and the sediment is characterized by physical stratified mud. The sedimentation rate in the outer-shelf mud deposit shows 0.28cm/yr or 0.18g/cm² yr, and the sediment is characterized by homogeneous mud. The difference in fine-scale stratigraphy is explained by the ratio of mixing rate to accumulation rate, which is much larger for the outer-shelf mud deposit (27.34) than for the inner shelf mud deposit (1.65). The larger ratio suggests biological mixing to destroy physical stratification. The clay minerals in the outer-shelf mud deposit are characterized by relatively higher smectite, a high ratio of chlorite/kaolinite and the existence of calcite concretion, which is similar to the sediment of the central Yellow Sea.