The Changjiang Incised-valley System And Estuary Evolution During The Postglacial Period

The postglacial Changjiang incised-valley system consists of the earlier valley fills and consequent estuarine and deltaic deposits. In this paper, the author attempted to discuss a series of problems evolving in the infilling of incised-valley, the evolution of estuary and the development of delta during the postglacial period, through analyzing several typical cores in the key positions of the postglacial Changjiang incised-valley.There are 4 types of sedimentary facies found in the postglacial Changjiang incised-valley system: fluvial facies, estuarine facies, shallow marine facies and detaic facies. The stacking patterns of these facies varies in different place the incised-valley. In the downstream area of the incised-valley, the fluvial facies, estuarine facies, shallow marine facies and detaic facies occur in tunas in the cores from bottom to up. As the cores moving upstream the incised-valley, the shallow marine and the estuarine facies become thinner and thinner, and finally the shallow marine pinch out at first, and so do the estuarine facies. In contrast, the deltaic facies become thicker upstream the incised-valley and changes into mouth bar facies, one of sub-facies of the estuarine facies. The 14C dating data indicates that the postglacial transgression reach the maximum before 6000 years. This time, as well as the developing phase of the mouth bar of Huangqiao, Jinsha and Chongrning, is similar to that estimated by earlier researchers.It is inferred by the element geochemical data analyzing of core sediments that the provenance of the sediments in the Changjiang delta has changed to a certain degree during postglacial period. The content of sediments from nearby provenance is relatively higher in the fluvial facies formed in early postglacial period. And the deltaic deposits formed in late postglacial period are probably mixed with Sediments from the Huanghe river.The vertical changing curves of some chemical weathering indicators, such as the element content ratios of K/Na, Al/Na, Al/Ca, and the chemical index of alteration (CIA) in the core HM03 are very similar to that in the core CX03, which indicates that the Changjiang delta area has undergone two climate changing course of "warm and wet to cold and dry" during postglacial period. The ¹⁴C dating of the core CX03 shows that the first cold and dry climate phase may correspond to the Younger Dryas period, and the second warm and wet phase may correspond to the Holocene Mega-thermal period. This result is very close to that from spore and pollen data of the Changjiang delta, which indicates that the these chemical weathering indicators can serve as the alternative indices of climate changes in the drainage basin.The organic carbon isotope ratios of the sediments from the core HM03 and CX03 are obviously controlled by sedimentary environments. The very lower value ofδ¹³C in the estuarine facies of the core HM03 may result from the influence of the Holocene Mega-thermal event. As the organic carbon isotope ratios of the sediments in river mouth area seam to be influenced by several factors, it should be very careful if we want to study the climate changes by using the value ofδ¹³C.After comparing the characters between the sand bar sediments from the core HQ03, HM03 and CX03, it is inferred that the Huangqiao sand bar was formed in an environment with much higher tidal energy than in which the Changxing sand bar was formed. Thus, it is sedimentologicaly proved that there was once a macro-tidal estuary of the Changjiang river in postglacial period. The genetic analysis of macro-tides in the Qiantangjiang river mouth suggests that the horn-like topography is the key factor causing the tidal bores. The fundamental reason of the transition from the macro-tidal estuary to the meso-tidal river mouth is the disappear of the horn-like topography. According to the historical changes data of ancient coast line in the Changjiang area, the horn-like estuary came to its end after the Jinsha sand bar grew higher than the sea-level which took place around 2000 years before. Therefore, the transition from the macro-tidal estuary to the meso-tidal river mouth should occur at that time.The distributing pattern of sediments of the ancient Changjiang estuary during the postglacial transgression maximum are very similar to that of the modern Qiantangjiang estuary, but very different from that of the popular model of estuary. It is possible that the ancient Changjiang estuary and the modem Qiantangjiang estuary represent a special type of estuaries that differ from others. According to the distributing patterns of energy and sediments in the Qiantangjiang estuary, a realized model of estuary is established and then is generalized to a model of incised valley. The predicting succession of facies in the subaquatic Changjiang delta coincide with the real succession, which suggests this model is useful.The sequence boundary and maximum flooding surface of the postglacial transgressive cycle in the Changjiang delta have been identified by 265 cores. Based on these data, the sediment amounts and the thickness-isopach maps of postglacial sedimentary cycle, transgressive and regressive successions in the Changjiang delta have been worked out. The results show that the sediment amounts of the postglacial cycle, transgressive succession and regressive succession are 114415.5×10⁸ tons, 7955.9×10⁸ tons and 6459.6×10⁸ tons, respectively. The postglacial sediments deposited in the incised valley are more than those in the two flanks, and the sediments contained in the transgressive succession are more than those in the regressive succession. The postglacial sediments deposited in the southern flank are more than those in the northern flank, and the sediments in the area seaward from the postglacial transgression maximum (PTM) of each flank are more than those in the area landward from the PTM. Considering both the possible changes of the ratio between the sediment amount remained in the modern Changjiang delta and the sediment discharge of the Changjiang river in the postglacial period, and the changes of the sediment discharge, the authors believe that in the postglacial period, the sediment discharge of the Changjiang river is 1.82×10⁸t tons/a on average, totaling to 27244.5×10⁸ tons.