The Discovery of Late Devonian Renalcis and Renalcis Mounds in Guangxi and Their Environmental Significance

Renalcis and Renalcis mounds have been reported from Europe,America, Australia and other places in the world. Famennian stage (the Rongxian Formation) of Late Devonian in Guangxi is an important period when Renalcis flourished. The distribution of the algae is governed by the trenchand platform-crossed palaeogeographic framework. According to the forms, Renalcis may be classified into five types which may be transitional mutually and symbiotic together: clotted, diaphragmshaped, chamber-like, tabular and pockety. The Renalcis mounds consist of hummocky, lenticular or domal sedimentary bodies built up by various types of Renalces assemblages and organic matterrich carbonate mud. Renalcis in the mounds generally take the shapes of nest and shrub, and sometimes have "framework" structure which consist mainly of lime matrix with small amounts of algal pellets, algal ooids and sand. "Stromatactis" structures in the forms of zebra and teeth are usually recognized on each flank of the mounds. Morever, it can be sen from the rocks in the mounds which are characterized by compact structures, a porcelain-white colour, cottony, granophyric and "framework"structures that there are poorly-defined gradations between the mounds and the wall rocks. Renalcis occurs commonly on the back flank of periplatformal beaches, periplatformal slopes and organic reef complexes, whereas the Renalcis mounds chiefly on the periplatformal slopes. The environments in which they were formed are open on the whole but locally restricted. The study of Renalcis, therefore, is of great importance to the determination of palaeogeographic settings of the margins of the platforms and to the prospecting for the mineral resources enriched in slope facies such as manganese and uranium deposits.