SEDIMENTARY CARBONATE ROCKS WITH DOLOMITE / CALCITE MICRO-LAMINAE: POTENTIAL INDICATOR FOR SEASONAL CHANGE

Using in-situ X-ray diffraction (XRD) with support of optical imaging, scanning electron microscope (SEM) and transmission electron microscope (TEM), a micro-laminated carbonate rock from west Maryland, which formed during mid-Ordovician, is examined. Through mineralogical examinations, we discovered that this sample contains two distinctly different micro-laminae, darker layer with numerous euhedral dolomite micro-crystals, and brighter with mostly microcrystalline matrix. In-situ XRD results show that darker layer is predominated by ordered-dolomite and calcite along with small amount of detrital minerals, which are mostly quartz and potassium feldspars, while the brighter layer is dominated by calcite with fewer detrital material and significantly less dolomite.

Our study shows that since each laminae has an average width of ~0.2mm. Moreover, repetition between dolomite-dominated layer and calcite-dominated layer suggest a possible annual cyclic, that seasonal change strongly influences the formation of dolomite. Since darker layer has higher porosity, residual organic matters, and small amount of terrestrial minerals, the darker layer might indicate the summer season with more microbial activity and larger discharge of surface runoff. Vice versa, the brighter layer will therefore imply winter with an environment with less organic materials associated with the carbonate sediments. Dolomitization (i.e., transformation from calcite and Mg-calcite into dolomite in sediment basin in ambient environment) requires catalysts like polysaccharides and dissolved hydrogen sulfide to weaken surface Mg-water bonding and promote surface water removal. The layers rich in microbial mass promote the dolomite crystallization at low temperature through reaction with ambient seawater. Considering that summer with frequent storm events, higher temperature and greater density of microbial mass, dolomitization should be easier to occur in buried layers through sulfate-reduction and fermentation. Consequently, appearance of dolomite in micro-laminated carbonates could be used as a signal to interpreting ancient seasons.