STUDY ON AN OSCILLATORY MICRO-LAMINATED DOLOMITE/LIMESTONE ROCK AND ITS IMPLICATION ON SEDIMENTARY DOLOMITE FORMATION

Forming mechanism of dolomite has been a long lasting question known as the “dolomite problem” (Hardie 1987; Machel and Mountjoy 1986; Mazzullo 2000; Warren 2000; Zenger et al. 1980). Attempts to synthesize dolomite in surface condition using oversaturated solution have been proven to be unsuccessful. Results from our lab have shown that polysaccharide be used as catalyst to synthesize disordered dolomite in room temperature (Zhang et al., 2012). Zhang et al. (2015) also point out that extracellular polymeric substances (EPS) that are dominated by polysaccharides from microorganism have catalytic effect on promoting disordered dolomite growth. The key step to form dolomite is incorporation of Mg²⁺ ions into crystal surface. But this process is inhibited by the strong bonds between water and Mg²⁺ on the crystal surface. Therefore, dehydration of Mg²⁺is essential for dolomite growth.

Ordovician micro-laminate carbonate rock from Southern Appalachian was selected for understanding the relationship between microbial mat and dolomite. In-situXRD, SEM and TEM results indicate that, the sample contains two distinctly different micro-laminae, dark layers with euhedral dolomite crystals, and bright layers dominated by calcite. Diffraction pattern from TEM and XRD shown clear (015) peak for dolomite within this sample, therefore the euhedral dolomite in this sample is ordered.

Our study show that since each laminae has an average width of ~0.2mm and darker layers are generally thinner than brighter layer. Higher porosity and less thickness indicate the dark layers were enrich in organic materials. Since the organic materials, which probably contains fair amount of microbial biomass during deposition, are concentrated in dark layers which catalyze dolomitization. The observed evidence in the natural sample supports preferred dolomitization and the relationship between organic material/ microbial biomass and dolomite micro-laminae.