Genesis of island dolostones during rapid island subsidence: Example from the Xisha (Paracel) islands, South China Sea

Miocene carbonate successions found on many isolated oceanic islands throughout south-east Asia commonly include unconformity capped 'island dolostones' units. On Shidao, located in the Xisha Islands in the South China Sea, well XK-1 (1268.2 m deep) penetrated a thick succession of carbonates before being terminated in the Jurassic metamorphic basement. The lower part of the succession is composed of the Sanya Formation (Early Miocene) and the overlying Meishan Formation (Middle Miocene), ca 700 m thick, that are formed of limestones and dolostones. An unconformity divides the Sanya Formation into a Lower Member formed largely of limestones, and an Upper Member formed of dolostones (DI-8). DI-8 is separated by a major unconformity into lower (DI-8L) and upper (DI-8U) units, that both include numerous subaerial unconformities. The overlying Meishan Formation is a limestone succession that includes three unconformity-capped dolostones intervals (DI-5, DI-6 and DI-7). Strata below the numerous exposure surfaces throughout the Sanya and Meishan formations are highlighted by Fe-staining, biomoulds, cavities, caves and solution breccia development. The weakly fabric-retentive dolostones in DI-5 to DI-8 are formed of dolomite crystals that have a dirty core encased by a clear rim, with delta O-18 values of +0.3 to +4.7 parts per thousand (average +2.0 parts per thousand, n = 50), delta C-13 values of +2.2 to +3.2 parts per thousand (average +2.5 parts per thousand, n = 50), Fe concentrations of 44 to 442 ppm (average 149 ppm, n = 50), Mn concentrations of 5 to 197 ppm (average 27 ppm, n = 50), Sr concentrations of 169 to 257 ppm (average 218 ppm, n = 50) and Sr-87/Sr-86 values of 0.70847 to 0.70900 (average 0.70879, n = 32). In all cases, dolomitization post-dated the subaerial exposure. The Sr-87/Sr-86 values suggest that the dolostones in DI-8L formed during the late Early Miocene, whereas the dolostones in DI-8U, DI-7, DI-6 and DI-5 probably formed simultaneously during the Late Miocene even though these units are stratigraphically hundreds of metres apart and were located well below sea-level. Dolomitization was preferentially focused in those strata with high porosities and permeabilities that allowed easy circulation of the seawater through the rock. This model suggests that dolomitization was not restricted to strata that were at or close to sea-level. Although dolomitization may have been related to the new oceanographic circulation regime established during that time, the exact properties of the seawater that promoted dolomitization are open to debate. This model of dolomitization may be applicable to many of the 'island dolostones' that are found in the oceans throughout the world.