Ooid distribution and fabric in the Miaolingian of Xiaweidian Section, Beijing (North China Platform)

Abstract

This study is an attempt to describe the distribution and fabric of the Cambrian ooids from the Xuzhuang, Zhangxia and Gushan formations at the Xiaweidian section. The oolitic banks occupy the upper parts of the 3rd order depositional sequences recognized for these formations. Petrographical techniques were applied to describe the sedimentary features of ooid grains. Different characteristics of ooids including size distribution, composition, morphology and the internal and external cortical architecture were taken into consideration. Radial-concentric, micritic, superficial, composite, pseudo-oids, neomorphosed and geopetal ooids are properly studied under the microscope. Different fabrics of ooids have been linked to their different depositional settings, and a variety of sub-environments has been established. The oolitic grain banks are composed mainly of calcite, with noteworthy presence of aragonite and dolomite. The two-fold role of microorganisms during and after the formation of ooids can be recognized under the microscope. The mechanism of ooids fabric modification has been elaborated in detail. Firstly, the dark laminae in several ooids most probably show the remains of filamentous cyanobacteria taking part in the construction of ooids. Secondly, they destroy the cortex through boring, which is then subsequently filled by aragonite. In order to apprehend the sedimentological features of the Miaolingian strata in the Xiaweidian section, this research highlights the distribution of oolites and their resultant fabric in response to relative sea-level variations. The Miaolingian ooids in the Xiaweidian section provide a good reference example of the depositional pattern of oolitic grain banks.