|Place of Conferral||中科院海洋研究所|
|Keyword||滦河三角洲 七里海沙坝 沉积特征 全新世 演化|
现代滦河三角洲地区全新世早期的海侵沉积和全新世中期的正常滨浅海沉积发育较差。 前三角洲H2孔详细的岩性、测年、粒度、有孔虫、孢粉资料分析表明，晚更新世-全新世初期该地区为发育草原植被的开阔平原环境，全新世中晚期发育正常的滨浅海沉积，但来自物源供应的沉积物较少，孢粉组合与海岸砂质地区和秦皇岛浅海地区相似。之后，发育厚层的混合沉积，陆相性自下而上减弱，混合层可能是1978年唐山地震所引起。混合层之上沉积有现代滦河的前三角洲沉积，具有包卷层理和部分陆相的薄层。浅地层识别了滦河三角洲地区发育的宽度在1~2 km、高度在3~4m、间距在1.5~2.2 km的多道沙堤。
七里海地层下伏晚更新世河流相和湖泊相沉积，部分钻孔可见潮汐通道和涨潮流三角洲沉积，并发育快速沉积的潮滩相和盐沼相，上覆海岸沙丘沉积。沙坝形成于约7 cal kyr BP，之后保持稳定，并在~3.6 cal kyr BP进入快速发育时期，2.10~1.65 cal kyr BP，潟湖快速充填，沙坝规模快速扩大。616 cal yr BP之后，海滩砂在风力作用下向沙坝快速堆积，形成了海岸沙丘。~3.6 cal kyr BP，燕山地区气候变干冷，植被类型由栎属-松属-蒿属退化为松属-栎属-蒿属，降水减少和植被退化共同导致滦河更加容易改道，滦河的向东迁移造成物源供应增加，给七里海沙坝的发育提供了物质基础。小冰期增强的东亚冬季风导致气候更加干冷，七里海沙坝植被退化为荒漠草原型，强大的风力将海滩砂吹扬至沙坝并直接掩埋了沙坝后侧的盐沼。
现代滦河以北沉积厚层的晚更新世-全新世初期河流相沉积，属滦河在13.2~11.2 cal kyr BP形成的冲积扇，曹妃甸三角洲体可能形成于8~7 cal kyr BP时期，南堡地区发育全新世高海平面以来形成的海相突出的三角洲体，王滩三角洲形成于~3.6 cal kyr BP以来，与七里海沙坝的快速发育时期相吻合。历史资料表明，滦河在历史时期长期以至少两条较大分流河道的形式入海，东侧河道在现今河道以西的老米沟附近，西侧河道基本位于大清河-小清河之间。历史时期改道频繁，流路范围基本位于现今滦河-溯河之间，主要由于洪水泛滥而引起。其中，海平面变化、气候变化、新构造运动和人类活动均对全新世滦河三角洲演化及历史时期滦河河道变迁具有重要影响。
Sediments in deltas that connect mainlands and oceans archive signials of sea level, climate, neotectonics, dynamics, and human activities. In this paper, four minor studies have been focused. First, we studied the grain size trends and sedimentary dynamics in the modern subaqueous Luanhe River Delta (MSLRD). Afterwards, the Holocene sedimentary characteristics in the MSLRD and surrounding areas were analyzed. Also, the Holoence sedimentary environments were reconstructed and analysised. Thereafter, we establish the evolution process of the Qilihai dune barrier based on the reconstruction of sedimentary sequence. Moreover, the influences of sediment supply due to the eastward shift of Luanhe River and climate changes due to the intensified East Asian winter monnoon (EAWM) on the development of dune barrier are analyzed. At last, we divided the Holocene Luanhe River Delta (LRD) into three sub-deltas and one alluvial-delta using additional 14C ages of sediment cores and geological profiles collected from previous reports. Influences of sea level changes, climate changes, neotectonics, and human impacts on the evolution of the LRD during the Holocene and historical period are interpreted.
Directions of grain size trends in the southern area with water depth <5m are SW, but those of the northern area with water depths <5m are NW and indicating a sediment supply from the delta to the northern sandy coast. Seaward grain size trends were observed facing directly the river mouth. Directions of grain size trends between water depths 5–15m are SW and the intension are stronger than the others. Grain size trends in the southern area with water depths >15m are SW, but those of the northern area with water depths >15m are NE. Tidal current is the dominating dynamic in the MSLRD and fluvial dynamic effects where water depths <5m. However, the grain size trend analysis method can not explain influences of wave dynamics, topography, lag deposits and human impacts except the tidal current and partly river due to the method itself and our design.
Transgression and offshore deposits are poorly developed during the Early- to Middle Holocene which are characterized by a thin (<1m) abundant shell fragment silty sand deposits. Fluvial environment formed at open lowland with grass vegetation during the Late Pleistocene to Early Holocene. Pollen assemblages in the Middle Holocene marine sediments cue no river entern the surrounding area. Moreover, a mixing deposit as thick as 9.50m with decreasing of marine facies upwards is observed in the L1. The overlying prodelta facies of MLRD contains convolute bedding and thin very fine sand layer. Seismic profiles illustrate subaquatic sand banks with 1–2 km-width and 3–4m-hight, and 1.5-2.2 km-width swales. Delta prograding body is identified with thichness <2m and partly ~5m in the nearshore and swales between sand banks.
Qilihai dune barrier contains underlying fluvial and lucusrine facies, inlet facies, tidal flat facies, saltwater marsh facies, and sand dune facies towards the upper boundary. The Qinlihai dune barrier first appears at ~7 cal kyr BP and grow little until ~3.6 cal kyr BP. During 2.10-1.65 cal kyr BP, the back-barrier was rapidly infilled with the barrier grew up. After 616 cal yr BP, sands were blown from the beach to the barrier and formed the coastal dune overlying the back-barrier sediments. At ~3.6 cal kyr BP, Palynological assemblages changed from Quercus-Pinus-Artemisia to Pinus-Quercus-Artemisia and indicating a cool event. This cool event associated with EAWM strenghthing lead to less rainfall, together with vegetation degradation, would increase the probability of Luanhe River migration; thus, the sediment supply to the Qilihai dune barrier increases. Itensified EAWM blow sand from the beach to the back-barrier during the Little Ice Age with vegetation of grassland. In addition, the initial timing of the Little Ice Age in the Qilihai dune barrier and southern Yanshan Mountains is earlier 100 years than most areas in China.
Fluvial facies dominated the sediments in the north of Luanhe River. Peat layers are found partly upon the fluvial facies, but no marine sediments are observed. The radiocarbon ages of the underlying fluvial facies and overlying peat layers constraint the fluvial sediment deposited during 13.2–11.2 cal kyr BP, and suggesting Luanhe River flowed through this place. As thick as 10m transgressive marine sediments in the Caofeidian area indicate Luanhe River flowed this area probably during 8~7 cal kyr BP. Then, the LR flowed into the Bohai Sea at Nanbao area, and left a delta which was part upon the old Liuzan-caofeidian delta during 7~3.6 cal kyr BP. Finally, the LR flowed to the Wangtan lobe and wandered between Su River and moder LR since 3.6 cal kyr BP. According to the historial documents, the LR has entered the Bohai Sea with at least two distributary channels. These two channels delvier sediments into the Bohai Sea alternately or simultaneous. The west channel was located between the present Daqing River and Xiaoqing River area. The east channel flowed into the Laomigou. The river channels changed frequently during to floods, under the influences of sea level, climate, human impacts, and neotectonics during the historial period. However, the channels were restricted between the present Su River and LR.
|MOST Discipline Catalogue||理学::海洋科学|
|Table of Contents|
|于晓晓. 滦河三角洲全新世沉积特征与演化[D]. 中科院海洋研究所. 中科院海洋研究所,2019.|
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