浅水三角洲研究进展与陆相湖盆实例分析

Progress of shallow-water delta research and a case study of continental lake basin

通过总结浅水三角洲的研究进展,结合鄂尔多斯盆地延长组浅水三角洲的研究,对浅水三角洲沉积特征进行讨论。结果表明:稳定的构造背景、盆地面积大、古地形坡度缓、湖平面频繁升降变化、水体浅及物源供给充分是浅水三角洲发育的基本条件;总体而言,浅水三角洲沉积缺少传统三角洲三层式结构特征,不同类型浅水三角洲沉积体存在差异,如浅水辫状河三角洲沉积物粒度粗,水体能量强,垂向上相序不完整,河口坝微相难于保存,而在浅水曲流河三角洲沉积体系中则河口坝砂较为常见。实例分析表明:鄂尔多斯盆地延长组下组合发育浅水辫状河三角洲和浅水曲流河三角洲两种类型,具有三角洲平原分布范围小、三角洲前缘分布广的特征。河道宽深比大,沉积物相对较粗,平原分流河道沉积中可见多次叠置的冲刷面构造和大型槽状、楔状交错层理等。三角洲前缘延伸远,长度可达一百多千米,水下分流河道在平面上多分支和分叉,垂向上多为间断正韵律。最后,讨论了研究区浅水三角洲沉积体系时空演化规律。

By summarizing the research progress of shallow water delta and combining with the study of shallow delta of the Yanchang Formation in Ordos Basin,this paper discussed the sedimentary characteristics of the shallow water delta. The results indicate that the basic conditions for development of shallow water delta include stable tectonic setting,large basin area,low relief of paleotopography,frequent changes of lake level,shallow water and abundant provenance supply. In general,shallow water delta deposition lacks the traditional characteristics of three-tier structure.There are differences between different types of shallow water delta sedimentary bodies.For example, shallow braided river delta features in coarse grained sediments,strong water energy,incomplete vertical facies sequence,and poor preservation of mouth bar.In contrast,mouth bar is common in shallow water meandering river delta.The case study of continental lake basin shows that the Yanchang Formation of Ordos Basin developed shallow water braided delta and shallow water meandering delta, which are characterized by small area of delta plain and widespread delta front zone.Its width-depth ratio of channel is large,the sediments are coarse,and it is common to find multiple superimposed erosion surfaces and large trough-shaped, wedge-shaped cross-bedding in plain distributary channel deposits.Delta front can extend more than one hundred kilome-ters and underwater distributary channels are characterized by lateral branches and bifurcate and vertical intermittent fining upwards rhythm.Moreover,the space-time evolution of shallow water delta sedimentary system in the study area is also discussed.