Major formation mechanisms and models of marine dolomite in middle and western basins of China

Marine dolomite is mainly developed in Tarim, Sichuan and Ordos basins in middle and western China. Recent study of the dolomite origin in key strata shows that the formation of massive marine penecontemporaneous dolomite and burial dolomite was closely related to the carbonate evaporative platform. The brine in the evaporative platform was not only the dolomitizing fluid of the penecontemporaneous dolomite which often coexisted with the evaporite and was formed by evaporation pump and seepage reflux mechanism, but also the pore water with abundant Mg²⁺ which could induce further water-rock reaction with coarse limestone by lateral seepage, lateral and vertical compact mechanism and vertical thermal convection mechanism in penecontemporaneous to shallow burial and even medium to deep burial environment, resulting in widespread burial dolomitization around, above and below the evaporite. Hydrothermal dolomitization was often the reworking action on previous dolomite which was confined to the fractures, sequence boundaries, beddings and porous strata, with deep mixed hot water, deep circulation water and magmatic solution as the hydrothermal dolomitizing fluid. Besides microbial dolomitization by which microbe could induce dolomite precipitation, the organic matter might also provide Mg²⁺ to dolomitization, as reflected by the dolomitization of the algal limestone. The release of Mg²⁺ from the microorganism and macro-algae was an important factor for the formation of part of the dolomite in the algal limestone. Biotur-bation could facilitate dolomitization by enhancing the porosity and permeability of the rock which could provide space and channel for the dolomitizing fluid.