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

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.