Characteristics and mechanism of dolomitization in the ooids of the Cambrian Zhangxia Formation, Xiaweidian, China

The diagenesis of ooids can be a valuable source of paleodepositional information. This study provides an example of ooids dolomitization in the Cambrian Zhangxia Formation, Xiaweidian, China. The ooids were extensively to completely replaced by euhedral dolomite crystals, but the interstitial material of the oolitic limestone was completely free of dolomitization. The selective dolomitization of the oolitic limestone is poorly understood. Stratigraphic, petrographic and X-ray diffraction data suggested that seepage reflux of the high-magnesium brine played a vital role in the early dolomite precipitation in the cortical laminae of the ooids before they were filled with interstitial materials. The sudden increased in the dolomite content of the oolitic limestone in carbonate containing gypsum provided key evidence for seepage-reflux dolomitization of the ooids. The higher porosity and permeability of the high-magnesium fluid in radial ooids demonstrates that the degree of dolomitization was higher than in the concentric ooids. The distribution of the dolomite was strongly influenced by the original cortical structure of the ooids. These data have significant implications for paleoenvironmental studies in this area.