摘要:
根据长江口外的三口钻井(Ch1,Ch2和Ch3)的沉积岩心资料研究了冰后期长江水下三角洲的沉积构造;它们由前积层,底积层和边缘沉积三部分组成。并根据14C,热发光测年和生物地层的研究探讨了该区自晚更新世以来的环境变迁,在中更新世末,本区为陆相沉积环境,随着玉木冰期中亚间冰期的到来,研究区发生了海侵,当玉木冰期最盛时期,这里又成为湖泊环境,又随着全新世的到来,再度出现浅海环境,随着河口三角洲的逐渐形成,构成了今日长江水下三角洲的沉积体系。
Abstract:
According to the analysis of cores Ch1(99 m), Ch2(63 m) and Ch3 ( 39 m), which were drilled in the outside of Changjiang River estary by the Institute of oceanology, Academia Sineca, the submerged delta of the Changjiang River formed in post glacial period consists of foreset, bottomset and marginal deposit. The fore set usually is over 10-14m in depth, divided into two parts: the upper part is generally composed of yellowish silts which contain many marine mollusk larva shells, and the lower part is the interbeds of thin silt and clay layers, both indicate the variablity of sedimentary environments. The lower boundary of the bottomset is at the depth of 22.07-46.5m, it is about 20-30 m thick and consists of grey clay sediments showing a stable and fine grain depositional period. Most of the cores contain thin silt layers in the bottomset by which the bottomset also can be divided into two parts, the upper softer part and the lower harder part. The marinal deposit is mainly composed of thin unsorted silt-fine sands it contains a lot of well preserved marine mollusk fossils, representing a typical littoral environment. Under the marginal deposit is the silt sediment formed in Wurm glacial term during the low sea level, in which developed thin peat sediments e.g. at the depth of 36.14-36.16 m in core Ch1 , the dark pure peats with 14C age of 10790 ± 120 years B.P., and at 48.4m, with 14C age 18740 ± 650 years B.P.. In the middle Late Pleistocene, a set of graval sediments in Ch1 deposited at 76.03-76.10 m. There are the preserved truncks with 14C age36000 years B. P. We found an individual of Pseudorotalia gaimardii at 62 m and 70m respectively, and some fresh water Lamprotula sp. at 76 m and 84 m. Beneath 84.24m there is a discontinuous surface. The middle Pleistocene sediment of Ch1 consists of clay layer intercalated with fine sand. The thermoluminescent dating at 88.2 m, 88.3m and 91.8m are (215 ± 10.7)×103, (233±11.6)×103 and (255± 12.3)×103 years respectively. There is a discontinuous surface at 91.96 m. From the above-mentioned data, the studied area was a land deposit environment in the late Middle Plesitocene. During the subinterglacial of Wurm glaciation a transgression occurred, later, in the Wiirm glaciation maximum it became a lake environment. Finally, when the climate was getting warmer in Holocene, the area changed to a neritic environment again and with the growth of the river estary delta, the mordern submerged delta system of the Changjiang River has formed.