矿床地球化学国家重点实验室知识库

According to the recent chronostratigraphic division of Cambrian, the Cambrian strata in the Kelan section can be subdivided into seven third-order sequences (DS1−DS7)DS1−DS7) based on cyclicity in sedimentary facies stacking patterns. The calcareous mudstone forming condensed section, micritic limestone comprising deep to middle ramp facies and the oolitic-grain bank facies in Series 3 and Furongian series represent the basic depositional fabric of Cambrian in northwestern part of Shanxi Province, North China Platform. These massive oolitic grainstones demonstrate that the oolitic-grain bank facies constitute the late-highstand systems tract or forced-regressive systems tract. The grains occupy upper parts of the third-order depositional sequences in response to relative sea-level fall. Furthermore, this forming pattern of oolitic-grain bank facies does not follow the standard model of sequence stratigraphy, in which deposition is believed to occur principally during sea-level rise, rather a continuous erosional unconformity develops during sea-level fall. Moreover, the microscopic analysis of oolitic grainstone shows the development of concentric and radial, rounded or elliptical, with or without nuclei, monocrystalline or polycrystalline, Girvanella or micritic ooids. The occurrence of diversified ooids in varying proportions provides a new dimension for studying evolution of the oolitic-grain bank in the North China Platform. The forming pattern of oolitic-grain bank controlled by their sequence-stratigraphic position in the Kelan section in the Shanxi province provides an important clue and a research direction for the regional correlation, as well as the paleogeographical reconstruction of the Cambrian Series 3 and Furongian series.