Dolomitisation of the Lower-Middle Ordovician Yingshan Formation in the Tazhong area, Tarim Basin, western China

Abstract

The carbonate unit in the Lower-Middle Ordovician Yingshan Formation in the Tazhong region of the Tarim Basin in western China is partially to completely dolomitised. Three types of dolomite were identified in this unit: microcrystalline (<20 μm) to finely crystalline (20–50 μm) subhedral dolomite rhombs (Rd1), finely crystalline (50–250 μm) anhedral to subhedral mosaic dolomites (Rd2), and medium to finely crystalline (200–500 μm) euhedral to subhedral sucrosic dolomites (Rd3). Three types of calcite cement were also identified: coarsely crystalline mosaic calcite cement in mouldic pores (Cc1), coarsely crystalline intergrown calcite cement (Cc2), and coarsely crystalline sparry and veined calcite cement in the fractures (based on hand sample observation) (Cc3). The δ13C values and 87Sr/86Sr ratios of the Rd1 dolomite in the Yingshan Formation are consistent with those of Lower to Middle Ordovician marine limestone, suggesting that the isotopic ratios were inherited from the precursor limestone. Stratigraphic, petrographic and geochemical data constrain the formation of the Rd1 dolomite to a shallowly buried environment involving seawater with elevated salinity. The Rd2 dolomite yields lower δ18O values (−7.5 to −4.7 ‰) and contains less Sr and Na and more Fe and Mn than the Rd1 dolomite (−5.3 to −3.2 ‰). Therefore, the dolomitising fluids of the Rd2 dolomite were likely derived from the infiltration and diffusion of residual evaporitic water resulting from intense compaction at elevated temperatures. The Rd3 dolomite generally features lower δ18O values (−9.0 to −7.0 ‰) than the Rd2 dolomite, but the δ18O values of portions of the Rd2 and Rd3 dolomites overlap. Thus, the Rd3 dolomite may represent recrystallisation of the Rd2 dolomite. The Rd3 dolomite contains significantly lower Sr and Na contents and significantly higher Fe and Mn contents than the Rd1 and Rd2 dolomites, suggesting that the dolomitic fluids precipitated under reducing conditions during burial. The late-stage, medium-coarse to very coarse calcite postdates all the dolomites and is present as cement in fractures and pores. The δ18O values of the Rd3 dolomite and Cc2 calcite cements (−12.6 to −7.9 ‰) are similar, suggesting that the calcite cement in the pores may represent calcite supersaturation associated with burial dolomitisation. The Cc3 calcite cements feature slightly higher δ13C values (+0.2 to +0.4 ‰) and higher radioactive 87Sr/86Sr ratios (0.709280 to 0.709483) than those of the limestone (0.707955 to 0.708231) and Rd3 dolomite (0.707723 to 0.708345) and, in places, lower δ18O values (−11.6 to −6.3 ‰) than the latter. Consequently, the Cc3 calcite cement either formed from the upward migration of basinal fluids in a burial environment or was impacted by hydrothermal activity.