Sequence Stratigraphic And Sedimentological Features Of The Oolitic-Grain Bank In The North China Platform

The studied section is located in northeast part of Beijing,and comprises a part of the North China Platform that extends 1000 km north-south and 1500 km east–west,which belongs to the Sino-Korean plate.The North China Platform is the most convenient site for the sequence stratigraphic division that reveals development of oolitic-grain bank in the upper part of the Cambrian Miaolingian strata.The ooidal grain banks in the Miaolingian Xuzhuang,Zhangxia and Gushan formations in the Xiaweidian section represent the unique depositional fabric of Cambrian in the North China Platform.Based on the facies stacking patterns and cyclicity in the chronostratigraphic context,the overall Cambrian strata in the Xiaweidian section can be subdivided into seven third-order depositional sequences?DS₁-DS₇?.The Miaolingian strata deposited into four third-order sequences?DS₁-DS₄?based on cyclicity in sedimentary facies stacking patterns i.e,Maozhuang,Xuzhuang,Zhangxia and Gushan formations,among them the Maozhuang Formation is devoid of oolitic grain bank.Xuzhuang Formation at its base possesses the mixed tidal flat facies which graduates into ramp-type carbonate deposits in the Zhangxia and Gushan formations.This shift from"TST+CS+HST"to"CS+HST"indicates the change from the immature to mature phase of the carbonate platform.Micritic limestone comprising deep to middle ramp facies and the oolitic-grain bank facies in Miaolingian series represent the basic depositional fabric of Cambrian in the 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?in response to relative sea-level fall.The grain banks occupy upper parts of the third-order depositional sequences.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,different characteristics of ooids including size distribution,composition,morphology and internal and external cortical architecture were taken into consideration.Radial-concentric,micritic,superficial,composite,pseudo-ooids,neomorphosed and geopetal ooids are properly studied under microscope.The principal carbonate minerals are calcite and its unstable polymorph aragonite and dolomite.The dark laminae in several ooids most probably show the remains of filamentous cyanobacteria taking part in the construction of ooids.They destroy the cortex through boring,which is then subsequently filled by aragonite.The morphology of ooids from Cambrian Miaolingian strata characterize their development in a high energy shallow water environment with an active role of microbe.The forming pattern of oolitic-grain bank controlled by their sequence stratigraphic position in the Xiaweidian section of the western suburb of Beijing provides an important clue and a research direction for the regional correlation,as well as the palaeogeographical reconstruction of the Cambrian Miaolingian series in the North China Platform.