湘西吉首地区寒武系碳酸盐岩沉积环境的初步探讨

Preliminary Discussion on Sedimentary Environments of Cambrian Carbonatite in Jishou District of Western Hunan

本文通过岩石组分、组构、沉积构造及岩石共生组合分析,认为湘西吉首地区寒武系碳酸盐岩主要形成在浅水碳酸盐台地.各类灰岩中的内碎屑、藻团粒、生物碎屑及低能鲕粒等颗粒类型广泛发育,白云岩中的残余组分及上述两类岩石普遍含有数量不等的陆源物并具各种流动浅水特征的沉积构造标志等事实支持了这一认识.与上述岩石共生间隔发育的砾屑灰岩(云岩)多属风暴沉积类型,在台地边缘斜坡上部—浅海陆棚环境中形成.

The argument about the sedimentary environment of the carbonatite in Jishou District of Western Hunan focus on that whether the carbonatite deposited in a pelagic continental margin sea or a carbonate tidal flat of shallow water. The authors agree on the latter viewpoint. The main evidence of the carbonatite formed in shallow water is as follows. The dominant associations of the Cambrian carbonatite in the district are laminated microcrystalline limestones, calcareous dolomite, dolomite, dolomitized sparitic grainstone, terrigenous elastics bearing clay-dolomite and calcirudite. The genetic indices of shallow water are the various cross bedding, scour and cutting structure, unsymmetrical ripple-marks and incomplete mud cracks, etc. The materials in the rock fabric, such as oolites, intraclasts, bioclasts, algal lumps, pellets and clays, silts and the well-rounded zircons and tourmalines, etc., are all belong to the deposits of shallow water. The remnant fabric of dolomite and lime-dolomite also indicates that the origin rock formed in very shallow water. Moreover, the components and structures of the calcirudite (dol.) in limestone and dolomite imply that they were storm deposit. According to the above evidence, the authors point out that the sedimentary environment of the Cambrian carbonatite in the district should be a carbonate tidal flat.