【作者】 李保华；

【导师】 李从先； 王强；

【作者基本信息】 同济大学 ， 海洋地质， 2005， 博士

【摘要】 冰后期长江下切河谷体系包括早期的河谷充填沉积、中期的河口湾沉积以及晚期的三角洲沉积。本文试图通过对下切河谷典型部位钻孔地层的分析，建立地层格架，并探讨冰后期长江下切河谷充填、河口湾演变及三角洲发育过程中涉及的一些问题。冰后期长江三角洲下切河谷体系包括河流相、河口湾相、浅海相和三角洲相等4种沉积相类型。这些沉积相在古河谷的不同部位组合关系不同，形成下切河谷充填层序在横向上的差异。在古河谷下游地区，下切河谷充填层序中4种沉积相发育齐全，自下而上由河流相、河口湾相、浅海相和三角洲相组成。在上游地区，浅海相和河口湾相相继变薄直至尖灭，三角洲相则发生相变，过渡为河口湾顶砂坝相。钻孔测年资料揭示的长江三角洲冰后期最大海侵发生时间，以及黄桥、金沙、崇明等砂体的发育时间，都与前人研究结果比较吻合。钻孔元素地球化学数据分析表明，冰后期长江三角洲沉积物源发生一定程度的变化，早期河流相中近源成分较多，晚期三角洲相中可能混有黄河源的成分。HM03孔和CX03孔沉积物中的化学风化指标在钻孔垂向上的变化相当一致，反映了冰后期长江流域的气候经历了两个“温湿——冷干”的变化过程，其中第一个冷干期可能对应着新仙女木事件，第二个温湿期可能对应着全新世大暖期。这个结论与其它学者的结论十分相近，表明沉积物的化学风化参数是较好的指示流域气候变化的替代指标。有机碳同位素比值在HM03孔和CX03孔中的变化受沉积环境控制明显。δ¹³C值在HM03孔河口湾相中的变化可能指示了全新世大暖期的影响。由于影响沉积物有机碳同位素比值的因素比较复杂，利用它在河口地层中研究气候变化必须格外小心。对HQ03、HM03和CX03孔上部河口砂坝相沉积物特征的对比表明，黄桥期沉积物所反映的水动力条件最强，海门期次之，长兴期最弱。样品平均起动流速的比较证明，冰后期长江河口湾的为一个强潮河口湾。通过与钱塘江河口湾的比较，认为喇叭状的河口湾和水下沙坎的存在，是强潮形成的必要条件。由此推断，强潮形成条件的消失是冰后期长江强潮型河口湾演变为中潮型河口的主要原因。根据长江三角洲古海岸线变化资料分析表明，喇叭状的河口湾地形在金沙期砂体出露水面后彻底消失。据此推测冰后期长江河口潮汐类型转变的时间应当在金沙期砂体出露地表之时，大约距今2000年前后。冰后期最大海侵时期长江古河口湾的沉积物分布模式与钱塘江现代河口湾的沉积物分布模式十分相似，但是与目前国际流行的河口湾模式相去甚远。因此，长江古河口湾和钱塘江现代河口湾可能代表了某种尚不为人知的河口湾模式。借鉴钱塘江现代河口湾的沉积动力空间变化模式和沉积物分布模式，建立了长江古河口湾的动力和沉积模式，并将其推广为一种下切河谷沉积模式。通过此模式推断的长江水下三角洲垂向沉积相序列，与实际钻孔资料比较吻合。通过对长江三角洲冰后期不同时期地层体积的计算，估算出了冰后期不同时间段长江入海输沙总量的变化，并且对变化原因作了探讨。长江三角洲在整个冰后期、冰后期海侵阶段和海退阶段的沉积物数量分别为14415.5×10⁸t、7955.9×10⁸t和6459.6×10⁸t。。其中，下切河谷沉积量超过三角洲两翼，海侵期沉积量大于海退期，南翼沉积量大于北翼，两翼前缘沉积量大于后缘。通过分析冰后期滞留于现今三角洲地区的沉积物数量与长江输沙量之间比值的变化，估算长江总输沙量在冰后期海侵阶段为15911.9×10⁸t，在海退阶段为11332.6×10⁸t，年均分别为1.92×10⁸t和1.74×10⁸t。在整个冰后期，长江总输沙量为27244.5×10⁸t，年均1.82×10⁸t。更多还原

【Abstract】 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.更多还原