Abstract:

The Xiaweidian outcrop in Beijing is a typical Cambrian stratigraphic profile in North China. Previous studies focused mainly on the depositional environment of the Zhangxia Formation, whereas the oolitic types and their diagenesis of the formation are poorly understood. In this paper, field geological and microscopic petrographic observations and XRD analysis were conducted to identify the oolitic types and elucidate the mechanism of oolitic dolomitization.It comes to a conclusion as follows: (1) Major oolitic types include radial ooids, concentric-radial ooids and monocrystalline/polycrystalline ooids, and minor concentric ooids, compound ooids and algae ooids. (2) Oolites are dolomitized whereas the interstitials are not, and the degree of dolomitization in the radial layer is higher than that in the concentric layer.Dolomite particles in the radial layer are radially distributed around the core, and the concentric rings occur around the core. (3) Dolomite grains in the oolites are mostly mesocrystalline or fine-grained, euhedral to subhedral, and returning infiltration of dolomitization was likely the main cause of the oolitic dolomitization, which resulted in a higher degree of dolomitization of the luminescent oolite limestone than the stucco oolite limestone.

Key words: ooid, Zhangxia Formation, dolomitization, Xiaweidian in Beijing, diagenesis