长江枝江段凤凰滩现代沉积特征

Modern Sedimentary Features of Fenghuang Mid-channel Bar in Zhijiang Reaches, Middle Yangzi River

长江枝江段凤凰滩为一典型的加积型心滩，沉积物自滩头至滩尾为砾、沙、粉沙和泥．滩头发育大中型沙浪、新月形沙垅和舌状沙地；滩中发育大型泥裂；滩尾仅发育小波痕。层理类型以大型槽状和板状交错层为主，层理规模自滩头向滩尾逐渐减小．根据沉积物特征及沉积构造可划分出１６种岩石相类型，它们构成了１０种岩石相组合类型．滩体建筑结构要素分析表明．凤凰滩由纵向砾石坝、纵向前积沙坝、侧向粉沙坝、垂向河漫滩和弧形沙坝５种建筑结构要素组成，其中纵向前积沙坝是心滩的主体．

Fenghuang mid-channel bar is a typical accretion-type bar. The sediment consists of gravel, sand, silt and mud from head to tail of the bar. Large and middle sand waves, crescent and linguloid dunes are developed on the bar head, while large mud cracks are developed on the middle bar. But only samll ripples occur on the bar tail. The beddings are mainly large trough and plannar cross bedding. The size of the beddings decreses gradually from head to tail. There are 16 lithofacies and 10 lithofacies combination in the bar, based on sediment features and sedimentary structures. The midchannel bar consists of five architecture elements. that are longitudinal gravel bar, longitudinal accretion sand bar, lateral accretion silt bar, vertical accretion flood deposit and arc sand bar, of which longitudinal accretion sand bar is the basic part of the bar.