A SEDIMENTARY MODAL OF THE TIDAL FLAT OF EPICONTINENTAL SEA

Late-Precambrian sedimentary carbonate rocks are well developed in Jixian County, Tianjin, especially those in the Wumishan Formation. They are about 3000m thick and are dominanted by sedimentary dolostone. In terms of the microscopical features and macroscopical occurrences, the transgressive sediments of the Wumishan Formation can be divided into three sedimentary facies zones. 1. Supratidal zone This is a uppertidal environment in open flat. Sediments are dolarenite and dolrudite which contain terrigenous grains. They are deposited in the environment of low grade momentum of sea water. 2. Intertidal zone The sedimentary area of this zone is from an extension of the supratidal zone flat mentioned above towards the sea. The sediments of this zone are mainly finegrained carbonates which contain silicolites and algites. Overall, this zone belongs to a tidal flat environment which is shelf-beachy facies of epicontinental sea. Sedimentation happens under a rather strong hydrodynamic environment. 3. Subtidal zone It has a sedimentary region of underwater flat being dominanted by relatively open high energy zone and a slope depression of relatively high energy zone on the flat. Overall, this zone belongs to an environment of lower tidal flat of shelf-beachy facies of epicontinental sea. The transgressive sedimentary model of the Wumishan Formation mentioned above can play similar role as Bouma's turbidite sequences in providing g'uidance and prognosis for further researches and offering bases for genetic explanation.