摘要
沉积物微量元素特征对指示沉积盆地演化历史、沉积环境及沉积物的物质来源具有十分重要的示踪作用。宝应沉积物的微量和稀土元素地球化学特征印证了苏北平原物质来源具有多源性,与黄河、长江具有类似的物源,并可能受基底碱性火山岩和黄土高原粉尘物质的影响。钻孔沉积物具有轻稀土适度富集、缓右倾斜型的稀土元素配分模式,呈现Ce正异常的总体特征;海侵段的稀土含量比陆相沉积段略低。δEu在陆相沉积中为1.16,呈微小的正异常;在海侵沉积中为0.99,基本不显示异常。稀土元素含量与沉积物粒度之间存在一定相关性:随着粒度由粗变细,稀土元素含量增加;ΣREE的高值段对应粘土含量较高,反之亦然。沉积物REE及Rb/Sr比值的纵向变化反映了海陆交互过渡带古气候环境的波动:ΣREE的高值段与δEu和δCe的低值段代表了冷湿的气候环境;ΣREE和δCe的低值与δEu的高值段反映了暖湿润的环境信息。结合地貌、岩性及微体古生物,综合分析表明苏北平原在第四纪曾遭受了多次海水淹覆,海-陆交互堆积作用明显,从古海湾逐渐发育堆积最后由河流冲积物覆盖成陆。
The Northern Jiangsu Basin which has been developed from the Changjiang Plate since Late Mesozoic, is a large sedimentary basin composed of several small sub-basins shaped as a ＂dustpan＂. The basin is dominated by deep faults which have controlled the sedimentation since Late Tertiary. The west-tilting topography of East Asia was reversed with the uplift of the Tibetan Plateau and the opening of marginal seas, resulting in an Asian fluvial system radiating from the uplifted center of the continent. The Northern Jiangsu Plain, which is above the sea level, comprises the continental component of the Southern Yellow Sea Basin. The Baoying borehole （ BY1 ） penetrating 144.98m depth was drilled at Wangzhigang Town of Baoying County （33°14′21″N, 119°22′41″E）, and 96.81m of clay and silty sediment samples was recovered. Sedimentary facies of the core show a series of environmental changes from coastal bay of a shallow sea to a wide alluvial plain during Pleistocene time. Comprehensive studies of geomorphology, structure, sedimentary facies, minerals, as well as chemical element analysis, paleomagnetic analysis, and microfossil analysis have indicated a fluvial-marine interaction during the evolution of the plain since 2.58Ma ago.
The geochemical characteristics of trace elements in the sediments can be used to infer the evolutionary history of sedimentary basins, depositional environments, and the source of sediments. In this study, we collected 139 sediment samples from BY1 in the central lowland area of the Northern Jiangsu Plain, and analyzed their trace and rare earth element concentrations using ICP-MS method. Our data show higher REE, Cr and Co, and lower Sr, K, Rb, Ta, and Nb concentrations when compared to the average upper crust value. Trace elements Th, Ta, Nb, and Cr are relatively enriched in sediments from marine facies, whereas trace elements Ba, REE, and Co are enriched in sediments from fluvial facies. The geochemical data suggests that the source material of the north Jiangsu Plain is similar to that from the Changjiang and Huanghe rivers, and influenced both by the basement alkali volcanic rocks in the basin and the dust powder from the Loess Plateau. The Baoying sediments show high ∑REE concentrations with a LREE enriched, slightly right-tilting REE distribution pattern. A slightly positive Ce anomaly is observed in the sediment samples. The sediments from marine facies show slightly lower REE concentrations without Eu anomay （δEu = 0.99） than the sediments from fluvial facies which have small positive Eu anomaly （δEu = 1.16）. A general correlation between ∑REE concentration and sediment grain size is observed, with finer clay-rich sediment commonly having higher ∑REE concentrations. The variations of a number of parameters of REE and Rb/Sr ratios with depth indicate that the trace element records in the land-sea transition sediment columns could be useful proxies for variations of climate shifts in the East Asian monsoon system over the past 2.58Ma. It is suggested that the sediment sections with high ∑REE combined with low δEu and δCe represent a cold-wet climate, whereas low ∑REE and δCe with high δreflect a warm-humid climate.
In summary, it is clear from multiple lines of evidence that the evolutionary history of the inner part of the northern Jiangsu Plain is characterized by multiple fluvial-marine interactive events during Pleistocene and Holocene.
出处
《第四纪研究》
CAS
CSCD
北大核心
2007年第5期735-749,共15页
Quaternary Sciences
基金
国家自然科学基金项目(批准号:40271004)资助
关键词
微量元素
稀土元素
地球化学
沉积环境
苏北平原
trace elements, REE, geochemistry,depositional environment, Northern Jiangsu Plain
