GRAIN-SIZE CHARACTERISTICS AND HYDRODYNAMIC CONDITIONS OF THE CHANGJIANG ESTUARINE DEPOSITS SINCE LAST GLACIAL

A drilling hole named The Bohole H1 was drilled at the Changjiang estuary for study of grain-size features of the deposits since last glacial and for interpretation of their dynamic conditions. Grain-size parameters are calculated and diagrams, such as So-Md diagram, C-M diagram, probability accumulation curves, frequency distribution curves and environmentally sensitive grain-size components used for interpretation of hydrodynamic conditions. The study suggests that the area experienced five types of sedimentary environment, i.e. fluvial channel, flood plain, estuary, shallow marine and delta since last glacial age. Fluvial channel sediments mainly consist of sandy deposits with coarser mean and unstable sorting coefficient. There are two segments in the probability accumulation curves dominated by saltation population and two peaks in the frequency distribution curves. The environmentally sensitive grain-size components are mainly composed of coarse deposits. The flood plain sediments have more silt and less sand which are, in general, fine and well sorted. In probability accumulation curves there are two segments. Suspension population is high. And the frequency distribution curves sometimes show two peaks. Environmentally sensitive grain-size is rather fine. The estuary sediments consist of sand and silt with fluctuated sorting coefficients. Saltation population dominates the probability accumulation curves with some transitional population as the subordinate. Frequency distribution curves show two peaks. Shallow marine sediments are mainly composed of muddy clay with little sand characterized by fine deposits of bad sorting. It mainly consists of suspension population and thus only one peak is observed in frequency distribution curves. The environmentally sensitive grain-size remains fine and distributed in a wide spectrum. Deltaic sediments, which have a coarser mean and good sorting coefficient, are mainly made of fine and silt sand, showing two segments in probability accumulation curves dominated by saltation population. The environmentally sensitive grain-size components are similar to those of the floodplain sediments. In conclusion, sediments grain-size characteristics of the Borehole H1 sediments taken from the Changjiang estuary since glacial age can well reflect the hydrodynamic condition variations.