Affiliation:
1. State Key Laboratory of Biogeology and Environmental Geology China University of Geosciences Wuhan 430074 China
2. Hubei Key Laboratory of Marine Geological Resources China University of Geosciences Wuhan 430074 China
3. Department of Earth and Atmospheric Sciences University of Alberta Edmonton T6G 2E3 Canada
4. State Key Laboratory of Marine Geology Tongji University Shanghai 200092 China
5. Zhanjiang Branch of China National Offshore Oil Corporation Guangzhou 524057 China
Abstract
AbstractMiocene 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 δ18O values of +0.3 to +4.7‰ (average +2.0‰, n = 50), δ13C values of +2.2 to +3.2‰ (average +2.5‰, 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 87Sr/86Sr values of 0.70847 to 0.70900 (average 0.70879, n = 32). In all cases, dolomitization post‐dated the subaerial exposure. The 87Sr/86Sr 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.
Funder
National Natural Science Foundation of China
State Key Laboratory of Biogeology and Environmental Geology
State Key Laboratory of Marine Geology
Subject
Stratigraphy,Geology,General Medicine
Cited by
1 articles.
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