FIRST DISCOVERY OF AN OLDEST RENALCIS MOUND FACIES AND ITS GEOLOGICAL SIGNIFICANCE——AN EXAMPLE FROM THE UPPER SINIAN DENGYING FORMATION IN NORTHEASTERN YUNNAN

The Dengyingian(Late Sinian)sedimentary environment in northeastern Yunnan is a rimmed peritidal epeiric platform. From SW through NE to SE, four sedimentary facies belts have been distinguished, including tidal flat, shallow subtidal flat, platform-margin shoal and basin facies belts.
For several years, numerous studies have been conducted on the Dengyingian sedimentary facies and palaeogeography in the study area, and for the first time Renalcis and Eiphyton, some of which have developed into the mound faeies, have been recognized in large amounts in the Dengying Formation, especially in the second member of the formation. So far, these Renalcis have been the oldest and their growth horizons are the lowest. The environments of Renalcis and Epiphyton consist mainly of tidal flat lagoon and low-energy shallow subtidal flat.
The original mineral compositions of Renalcis dolostones are high-Mg catcite(cell wall)aragonite(cell centre). These minerals have undergone a series of diagenesis, including neomorphism, dolomitization, silicification and recrystallization. The irregular and stratoid fenestral and stromatactoid structures are common in Renalcis dolostones and mound facies in the Dengying Formation, which are bound up with the Pb-Zn sulfide ore deposits and oil-gas reservoir traps. Therefore, the discovery of Renalcis dolostones and mound facies may provide one useful approach to prospecting for the concealed Pb-Zn deposits and oil-gas resources in the study area.