Early dolomitisation of the Lower-Middle Ordovician cyclic carbonates in northern Tarim Basin,NW China |
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| Authors: | Chuan Guo DaiZhao Chen ShaoFeng Dong YiXiong Qian CunGe Liu |
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| Institution: | 1.Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing,China;2.Wuxi Research Institute Petroleum Geology,SINOPEC,Wuxi,China;3.College of Petroleum Engineering,Guangdong University of Petrochemical Technology,Maoming,China |
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| Abstract: | High-frequency metre-scale cycles are present within the Lower-Middle Ordovician carbonate successions in northern Tarim Basin, NW China. These metre-scale cycles were variably dolomitised from top to bottom. Three types of replacive dolomites were recognised, including dololaminite (very finely to finely crystalline, planar-s to nonplanar-a dolomite; type-1), patterned dolomite (finely crystalline, planar-s dolomite; type-2), and mottled dolomite (finely to medium crystalline, nonplanar-a(s) dolomite; type-3). Petrographic evidence indicate these dolomites were primarily deposited in supratidal to restricted subtidal environments, and formed in near-surface to shallow burial realms. Geochemically, all types of dolomites have similar δ 13C and 87Sr/86Sr ratios comparable to calcite precipitated in equilibrium with the Early-Middle Ordovician seawater. These geochemical attributes indicate that these dolomites were genetically associated and likely formed from connate seawater-derived brines. Of these, type-1 dolomite has δ 18O values (?4.97‰ to?4.04‰ VPDB) slightly higher than those of normal seawater dolomite of the Early-Middle Ordovician age. Considering the absence of associated evaporites within type-1 dolomite, its parental fluids were likely represented by slightly evaporated (i.e., mesosaline to penesaline) seawater with salinity below that of gypsum precipitation. More depleted δ 18O values (?7.74‰ to?5.20‰ VPDB) of type-2 dolomite and its stratigraphic position below type-1 dolomite indicate the generation of this dolomite from mesosaline to penesaline brines at higher temperatures in near-surface to shallow burial domains. Type-3 dolomite yields the most depleted δ 18O values (–9.30‰ to–7.28‰ VPDB), pointing to that it was most likely formed from coeval seawater-derived brines at highest temperatures in a shallow burial setting. There is a downward decreasing trend in δ 18O values from type-1 through type-2 to type-3 dolomites, and in abundance of dolomites, indicating that the dolomitising fluids probably migrated downward from above and persisted into shallow burial conditions. |
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