Sedimentary Characteristics and Geneses of Pebbly Meandering River: A Case from Dashihe River in Qinghuangdao~Area
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摘要: 前人对河流沉积特征的总结主要针对砂质曲流河和砾质辫状河,但几乎未涉及砾质曲流河类型.以秦皇岛地区大石河北庄坨村-上庄沱镇河段为例,通过现代沉积学考察,描述了砾质曲流河的地貌单元和沉积特点,总结了沉积序列和沉积模式,探讨了该类河流的成因机制.其地貌单元包括高弯曲的单河道、交替分布于河道两侧的凹岸-凸岸、流槽和河漫滩.受两侧山地的近物源供给,自下而上发育河床底部滞留沉积和砾质曲流沙坝、砂砾质流槽、砂质河漫滩沉积,具叠瓦状构造和大型板状-楔状-槽状交错层理,总体呈现正旋回序列和不典型的二元结构.砾质曲流河在地貌形态上与砂质曲流河类似,但在水动力条件、沉积物岩性组成和沉积序列方面与砾质辫状河类似,表现为双重性质.依据研究区3级河流阶地,结合该类河型在现今山地普遍分布的特点,分析指出砾质曲流河的成因在于区域构造抬升和河流强烈侵蚀下切.本文研究对河型分类、分布及控制因素的认识带来了挑战.鉴于砾质曲流河响应于区域构造抬升的特点,地史时期该类河型的识别必然对造山运动具有重要指示意义.Abstract: The patterns of sandy meandering river and pebbly braided river had been well documented on their sedimentary characteristics, but the pattern of pebbly meandering river had been rarely previously reported. Based on modern sedimentary investigations on the Xiaobangshuiya-Shangzhuangtuo stretch of Dashihe River in Qinghuangdao Area taken as a typical example of pebbly meandering river, the geomorphology units and sedimentary characteristics are described, and then the depositional succession and model are summarized to insight into its geneses in this paper. The geomorphology units include a single highly-sinuous channel, convex and concave banks alternately locating at the two sides of the channel, launder and small-scale flood plains. Sourced by the nearby mountainous provenance, the sediments are identified as lags deposits, pebbly point-bar deposits, gravelly launder deposits and sandy flood plain deposits with imbricate structure, and large-scale tabular, wedge-shaped and trough cross-bedding from bottom to top, presenting a fining-upward succession called as an atypical dual-structure that is constructed by coarse sediments in the lower and fine sediments in the upper. Accordingly, the pebbly meandering river is featured with geomorphology units similar to a sandy meandering river, and with hydrodynamic conditions, lithologic compositions and depositional succession similar to a pebbly braided river, so are called as dual properties. Integrating with the pebbly meandering river widely occurring in modern mountainous regions, three-graded river terraces in the study area indicated that the river pattern of pebbly meandering river is attributed to regional tectonic uplifts and river’s intense erosional actions. The study in this paper brings a challenge on understanding the classification, distribution and control factors of river patterns. Identifications of geo-historical pebbly meandering rivers should be significant to determine orogenic movements due to their close response to regional tectonic~uplift.
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Key words:
- river pattern /
- meandering river /
- braided river /
- depositional succession /
- sedimentary model
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