尼罗河与长江三角洲晚更新世末期硬土层特征及其成因对比研究

COMPARATIVE STUDY OF THE LATE PLEISTOCENE NILE AND YANGTZE STIFF MUDS:SEDIMENT AND ORIGIN

作者根据多年来在尼罗河与长江三角洲工作中所获大量实测资料，探讨两个三角洲晚更新世末期硬土层沉积特征、时空分布及其成因；剖析干旱性气候和湿冷性气候对三角洲硬土层成因的影响；指出两个三角洲晚更新世末期的硬土层主要为洪水泛滥堆积物，但在枯水期受到了风的改造。

In the Nile and Yangtze delta regions, there are several layers of stiff muds of the Late Pleistocene age. Recently, extensive investigations have been carried out by corings to study the relationship between the stiff muds and its sedimentation, distribution, chronology and origin, and its connection to paleochannels and floods. This study is based on the extensive petrological analyses,including grain-size, bedding structure, mineral composition, clay minerals, 14Cdating, trace elements, and fauna species.In general, the Nile stiff muds can be divided into 4 subfacies: 1) calcareous nodule-bearing stiff muds, consisting of yellow and gray clayey silt. Quartz in the sediment is highly iron-stained, and unidirectional beddings (wavy and horizontal)are locally present. In this subfacies; Fe and Mn oxides prevail. Calcareous nodules occur locally as thin layers, below which calithe is often observed; 2)gypsum nodule-bearing stiff muds, composed of gray silty clay. Gypsum crystal can reach 3%～40%, and gypsum nodules in sporadic and thin layer statues are obvious. It is surprising that Jarosite characteristic of hot dry climate and lamination rich in organic matter are seen, 3) shell debris-bearing stiff muds,made up of yellowish gray silty clay. Shell both in fragment and whole, and mica are seemingly increased in this subfacies. It is confirmed that these shells are of the brackish water origin, living in an interior salt lake formed by hot dry climate condition; and 4) plant debris-bearing stiff muds, consisting of dark peat and silty clay. Plant debris is up to 20%～80% in the sediment, calcareous nodules are sporadically distributed. Meanwhile, it is worth to mention that eolian sands are all observed at the top of the profiles containing the above stiff muds subfacies.The stiff muds in the Nile is buried-5m below the present surface in thesouthern delta plain, and progressively increased in depth to 30～40m to the northern plain. This muds are distributed mainly on the both sides of the former Nile myer, where the calcareous nodule-bearing stiff muds are dominant, with > 60% of the total distribution area of 4 subfacies, gypsum nodule-bearing stiff muds are > 30%, and shell-and plant-debris stiff muds occur rarely. In addition,radio-carbon dating reveals that the stiff muds were primarily accumulated in 2 phases: 16 000～> 10 000aB. P. and 28 000～22 000aB. P., suggesting relatively low sea-levels in the two time intervals. The stiff muds, afterward, were greatly suffered from hot dry climate impact of the Mediterranean and the seasonal wet-dry environmental change due to periodic Nile fluvial flooding, and also from the compaction due to upper Holocene sediments.The Yangtze delta stiff muds of Late Pleistocene age are largely composed of clayey silt. Quartz is clean with no iron-stained, and semi-angled in roundness.Nascent nodules reach 8%～63%, plant debris and mica are often observed.Clay minerals are dominated by illite (50%～60% ), indicating an aqueous envi ronment. Moreover, pollen species, such as Betula and Picea in the sediments occurred as indication of cool climate. At the top of the Yangtze stiff muds, many tree roots with phosph-cemented occur.The Yangtze stiff muds can be generally divided into 2 layers, the upper one is distinctive of dark green color, without beddings. Phosph and calcareous nodules, and root traces are present, in contract, the bottom one is characterized by yellowish green color, relatively coarse particles, horizontal bedding and brackish water fauna species of tidal flat environments. Nodules in the bottom stiff muds are greatly reduced. The sediment changes between these layers are gradual. In ad dition, trace elements (Sr, Ba, B, Ga, V, Sr/ Ba and B / Ga) are all of terrigenous range. There is no obvious evidence for eolian processes found at the top of the stiff muds profiles.The Yangtze stiff muds are buried 25～26m below the present surface in the eastern delta plain, progressively become shallow (1～5m) westward, and eventually exposed west of Taihu Lake area in the south