现代渭河西安段沉积体沉积相与岩相特征

Characteristics of Sedimentary Facies and Lithofacies of Modern Weihe River in Xi'an

以渭河西安段草滩沉积体为例,通过剖面和探槽的详细研究,结合野外密集采样及样品的分析结果,对现代渭河草滩沉积体的粒度特征、沉积相、岩相、地层层序及沉积模式进行了综合研究。结果表明,现代渭河草滩河流沉积体主要由砂质、泥质和少量砾质沉积物组成,是一个由三期沉积旋回组成的曲流河沉积体。在现代渭河草滩沉积体内部,共识别出6种岩相,即平行层理含细砾粗砂岩相、槽状交错层理中细砂岩相、平行层理中细砂岩相、块状层理含砾中砂岩相、交错层理含砾中砂岩和块状层理泥岩相;3种沉积微相,即河床滞留沉积、边滩和泛滥平原,这些沉积微相构成了草滩沉积体内部垂向上三套不同的沉积层序:下部为含砾粗砂和中砂组成的河床亚相层序,砂质沉积厚度较大;中部为一个完整的曲流河沉积层序,由滞留沉积、边滩和泛滥平原沉积构成;上部则主要为边滩和泛滥平原沉积层序。粒度分析结果表明河床滞留沉积物中的砂体粒径多分布在0.2~1.2 mm,边滩沉积物中砂体粒径多分布在0.1~1.0 mm,两者均是良好的砂矿体,是建筑用砂的良好来源。

Taking modern deposit of Weihe River at Caotan,Xi'an,as an example,particle size,sedimentary facies,sedimentary sequences and depositional model were studied based on outcrops and trench observation,and test results of selected samples. The results indicate that modern Weihe River in Xi'an is a meandering river which composed by three sedimentary cycle,and sediments in the river include sandy and argillaceous sediments,and only a little of gravely sediments. There are 6 kinds of lithofacies identified in Caotan deposit of modern Weihe River,and they are coarse sandstone with some fine gravel( parallel bedding) lithofacies,medium to fine( trough cross bedding)sandstone lithofacies,medium to fine sandstone( parallel bedding) lithofacies,medium sandstone( massive bedding)lithofacies,medium sandstone with some gravel( cross bedding) lithofacies and mudstone( massive bedding) lithofacies. Three kinds of sedimentary microfacies developed well,including channel lag deposit,point bar and flood-plain microfacies,and three sedimentary sequences were composed by these sedimentary microfacies. Lower sedimentary sequence with large thickness of sandstone was composed mainly by coarse and medium sandstone. Middle sedimentary sequence is a whole meandering river sedimentary body,and is composed by channel lag deposit,point bar and floodplain deposit. Upper sedimentary sequence is dominantly composed by point bar and flood-plain deposit. Grain size analysis show that diameter of particles in channel lag deposit is 0.2 ~ 1.2 mm,and 0.1 ~ 1.0 mm in point bar,and they are favorable sand orebody which used for building sand.