Sedimentary facies of Lower and Upper Carboniferous boundary section in Daihua region, Changshun County, Guizhou
The boundary between the marine Lower and Upper Carboniferous in Daihua region, Changshun County, Guizhou can be determinated by conodont and foraminifer fossils. The section ranges from the Dawuba Formation of Lower Carboniferous age to the Huashiban Formation of Upper Carboniferous age. On the basis of detailed studies of carbonate microfacies, five sedimentary factes from the Daihua section can be distinguished. Their features are briefly described respectively as follows (in ascending order):(1) The basin factes is composed of silicilith and mudstone interlayers which contain a little silt-sized brachiopod and molluscan debris. The petrological and paleontological features indicate that the deposits were formed in a restricted and reducing environment; (2) The open-shelf factes with better water circulation has more abundant bioclasts than the basin factes. They include foraminifers, echinoderms,brachiopods, mollusca and a small quantity of bryozoans. The sediments mainly consist of wackestone and siliceous bands; (3) The lower slope factes near basin margin was located at the lower part of slope. The predominant bioclasts are sponge spicule, thin-shelled bivalves, brachiopods and echinoderms. The whole gastropods also can be found in the limestone. Silicification is the most characteristic diagenesis. The calcite in molluscan and trilobite shells was replaced by the self-grown quartz. The packstone containing blue-green algal debris, foraminifers and echinoderms derived from shallow water environments interbeds with the wackestone bearing sponge spicule derived from deep water environments because of the action of turbidite. The latter contains graded beddings; (4) The middle slope factes with an open environment between the upper slope factes and the lower slope factes was tens of meters deep. The packstone is mainly composed of fine sand-sized bioclasts such as echinoderms, brachiopods, foraminifers, blue-green algae, bryozoans and mollusca; (5) The middle slope factes continuously grades up into the upper slope factes which was adjacent to the organic bank of platform margin. The limestone consists of coarse sand-sized to gravel-sized intraclasts and bioclasts whose size decreases with the increasing water depth. The organisms including gastropods, echinoderms, brachiopods, foraminifers, ammonoids, algae and corals were derived from shallow water environments. The fine, thin graded beddings found in the limestone were generated by storm. The diagenesis is characterized by the strong solution and recrystalization which result in the development of fine-crystalline or coarse-crystalline limestone.The vertical changes of sedimentary factes show the depositional history from the basin factes to the upper slope factes from the late Early Carboniferous to the middle Late Carboniferous. The Lower-Upper Carboniferous boundary lies within the middle slope factes representing continuous deposition in this section.
【CateGory Index】： P534
【CateGory Index】： P534