Integrated Sedimentology And Sequence Stratigraphy Of The Cambrian Ooids In The North China Platform

The North China Platform is a vast crantonic platform that extended around 1500 km east-west and 1000 km north-south and belongs to the Sino-Korean plate.The various regions(Shanxi,Hebei and Inner Monogila)of the North China Platform are the most favorable sites for the sequence stratigraphic division that reveals development of oolitic-grain bank in the upper part of the Cambrian strata.The ooidal grain bank is the common features of Cambrian Miaolingian series,while Furongian is dominate with storm deposited edgewise conglomerates and bioherms.However,out of them,most of the appropriate section “Kelan,Shanxi” has selected,which is reflected as package of thick depositional sequences of oolitic-grain bank in the Miaolingian and Furongian series.By following recent chronostratigraphic division of Cambrian,the Cambrian strata in the Kelan section can be subdivided into seven third-order sequences(DS1-DS7)based on cyclicity in sedimentary facies stacking patterns.The exposed rock units can be grouped into Xuzhuang,Zhangxia and Gushan formations of the Miaolingian,while Changshan and Fengshan formations of the Furongian age.Xuzhuang Formation at its base possesses the mixed tidal flat facies which graduates into ramp-type carbonate deposits in the Zhangxia,Gushan,Changshan and Fengshan.This shift from “TST+CS+HST” to “CS+HST” indicates the change from the immature to mature phase of the carbonate platform.In addition,the Cambriansuccession indicates an unconformable contact with the underlying Archean metamorphic rocks,and a parallel unconformity with the overlying Ordovician Yeli Formation.The calcareous mudstone in the Cambrian strata of study area exhibit condensed section,micritic limestone comprising deep to middle ramp facies and the oolitic-grain bank facies in Miaolingian and Furongian series represent the basic depositional fabric of Cambrian in north-western part of Shanxi Province,North China Platform.These massive oolitic grainstones demonstrate that the oolitic-grain bank facies constitute the late-highstand systems tract(LHST)or forced-regressive systems tract(FRST).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.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.Furthermore,microscopic and geochemical studied not only give the evidences of the calcite mineralogy of these ooids but also indicate the abundance of microbial fossils(Girvanella)and microbialrelated sediments(pyrite and remnants of EPS and or microbial biofilm)in these ooids that give the solid evidences of microbial genesis of these shallow water ooids similar to the modern 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 palaeogeographical reconstruction of the Cambrian Miaolingian and Furongian series.