Sequence-Stratigraphic Framework And Its Palaeogeographical Background For The Middle-Upper Cambrian Series Of Guizhou And Its Adjacent Areas

The study area includes Guizhou and its adjacent areas. Middle-upper Cambrian series is composed of changeful rock strata sequence. The whole strata sequence groups a second-order sequence of Cambrian that can be subdivided into seven third-order sequences (CS6-CS12). In the shallow water platform facies area of northwest, there is a set of thick dolomite of Loushanguan Group (several hundred metres to more than 1000 metres), almost without fossils. The strata, in the bottom of dolomite of Loushanguan Group, is restricted shelf facies or lagoon facies, thin to middle layers dolomite, whereas which can be often found more or less trilobite fossils. It is called Douposi Formation, which not only becomes the most important symbol of deducing the time of Loushanguan Group in the northwest, but also is the product of environmental deepening caused by the second-order transgression. In the central of Guizhou, the stratigraphic horizon, comparatively sandy-muddy dolomite of Douposi Formation, is a little higher, which can reach the top of middle Cambrian series. This set of strata is called Shilengshui Formation and Gaotai Formation, which is covered with dolomite of Loushanguan Group. In the palaeoslope background of eastern area from Yuping to Tongren, the strata that is only equal to the dolomite of Loushanguan Group of upper Cambrian series Fengshan stage, is not thick. According to fossils information, the lower strata composed of grainstone, packstone, brecciola and a little dolomite is called Aoxi Formation, Chefu Formation and Bitiao Formation. From Danzhai to Taijiang area in Guizhou, middle Cambrian series is made up of slope facies brecciola, micrite, shale etc, named Kaili Formation and Jialao Formation. In the deeper background of Sandu area in Guizhou, middle-upper Cambrian series is composed of shelf facies to basin facies shales, marlite, micrite and brecciola. The lithology is relatively simple, which is divided to Duliujiang Formation, Yangjiawan Formation and Sandu Formation. From northwest to southeast, the dolomite of Loushanguan Group pinches out as a result of progradation, which representated the process of environmental shallowing caused by second-order regression. In middle-late Cambrian epoch, second-order sea level rises firstly and then descends. In the process of rising, Kailli Fauna develops, and in the process of descending, there is a large scale of extinct event of trilobites. We can deduce that there are some relations between evolvement of biology and the changing of sea level. Second-order sequence is a sedimentary-facies succession of upward-shallowing. Sequence-stratigraphic framework and sequence-stratigraphic succession is controlled by the process of transgression deepening and regression shallowing, as well as the process of palaeogeographic evolution of environmental deepening spatial variation, which reflects the regular of the evolutionary succession of sediments and the distributional pattern of sedimentary facies.