长江与尼罗河三角洲晚第四纪沉积对比研究

Yangtze and Nile Delta: A Comparative Study on the Late Quaternary Sediment

对比研究显示了长江径流潮流型和尼罗河波浪型三角洲的沉积特征 :1)两地三角洲都始于距今 75 0 0～70 0 0 a;2 )距今 70 0 0 a以来的海平面上升速率趋于缓慢 ;3)晚更新世末期都发育了硬土泥 ,为冲湖积沉积。但长江多为暗绿色 ,含菱、钙质结核 ,反映了阴冷的古气候特点 ;尼罗河呈黄褐色 ,含大量钙、石膏结核和咸水生物 ,指示干燥性气候背景 ;4)两地三角洲全新世沉积相似 ,在早期海侵层以上都发育了中后的前三角洲、三角洲前缘和三角洲平原沉积环境 ;5 )尼罗河三角洲沉积物普遍粗于长江。前者以沙质为主 ,后者以泥质粉砂占优 ,显示了波浪型三角洲强烈的沿岸搬运沉积动力。此外 ,径流潮流型三角洲的特点以潮坪沉积为主 ,泥质含量高 ,常见水平、波状和包卷层理组合类型和芦苇等植物根系。淡水湖沼沉积发育 ;波浪型则显示了沙坝 -泻湖沉积体系 ;沙质含量高。泻湖沉积以泥和泥质砂为主 ,含许多咸水生物和黄铁矿 ;见较多的石膏等蒸发性矿物 ;上覆大型槽状交错层理的滨海沙。

The Yangtze delta of eastern China and the Nile delta of Egypt that occur at the same latitude belong to fluvio-tide dominated and wave-dominated sedimentary patterns, in light of their physio-geographical variables, including local geology, climate, tides, waves, and longshore currents. To make a comparison between the two deltas regarding late Quaternary sediment sequences and sedimentary facies is of great significance in better understanding of the delta formation, in association with sea-level fluctuations, climate changes, sediment transport and river valley morphology. This will be also of great help to comprehend the delta origin of global scale. Our numerous sediment boreholes were collected from the two delta plains. Petrological, microfossil and radiometric analyses reveal that the stiff muds occur widely as basal sediment section below the Holocene delta sediments and are distributed as patches along the paleo-river channels. Many siderite and calcareous nodules found in the Yangtze can indicate the cool and damp climate setting during the Late Pleistocene. In contrast, numerous calcareous and gypsum nodules appeared in the Nile is of dry and hot climate implications. Many 14C dates obtained from the stiff muds section range from 25 000 to 10 000 y B.P. The overlying Holocene transgressive sediment section occurs at the both delta region as Holocene sea level invaded into inland. The Nile delta transgressive section consist primarily of coarser sands, while the Yangtze is basically composed the silt sand. 14C dates derived from the sediment sections range primarily from 11 000 to 7500 y B.P. It is further proved by our study that the Holocene delta sedimentary faces, i.e. prodelta clay, delta-front sand and delta plain muds are commonly developed as the main sediment body at the both delta areas. The sediment thickness of the both delta sediment sections can reach >40 m. The dominant sediment facies in the Yangtze delta is represented by the tidal flat and marsh environments, largely characterized by the fine grained mud deposits with sets of convoluted, horizontal and ripple beddings. Reed root traces prevail throughout. In comparison, the Nile deposits are essentially marked by the barrier-lagoon system, primarily the pooly-sorted clayey sand with abundant brackish water organism. Gypsum occurs widely as evaporates. 14C dates indicate their chronological formation ranging from 7 500 to 7 000 y B.P. Sediment particles formed the various Holocene delta facies in both deltas are greatly different. The sandy sediment is dominant in the Nile and muddy in the Yangtze. This illustrates the stronger winnowing processes and longshore sediment transport in the Nile sandy coast, and the siltation prevails in the Yangtze muddy flat. The Holocene sea-level fluctuations in the both delta region are reconstructed by many 14C dates obtained from the freshwater marshes and lagoonal sections. This is believed that the sea level curve of the Nile on the basis of lagoonal dates is more reliable to reflect the eustatic change, due to lesser sediment compaction than that of the peaty materials. Obviously, the Holocene sea levels were not fluctuated coincidently at both sites with time. Sea level during 7 000 y B.P. was positioned -4 m below the present in the Yangtze coast, and -10 m in the Nile, and was -3 m and -8.5 m in the Yangtze and Nile at about 5 000 y B.P., respectively. The local subsidence due to isostacy and/or sediment compaction can interpret the discrepancy, aside from many other factors, such as un-coevally fluctuated climate-warming trend during Holocene, etc. This is also demonstrated that the Holocene delta construction initiated from 7 500～7 000 y B.P., not only supported by the dates derived from the basal strata of the prodelta clay facies, but also from the chronology of marsh and lagoon facies, formed widely on the leeside of chenier ridges of the Yangtze, and on the sand ridge of the Nile.