Cite this paper:
XU Hong, ZHANG Weiwei, WEI Kai, HE Qingkun, JIANG Yunshui, XU Tingting, JIANG Xuejun, YAN Guijing, SONG Hongying, WANG Jianghai. Ferroan dolomites in Miocene sediments of the Xisha Islands and their genetic model[J]. Journal of Oceanology and Limnology, 2018, 36(1): 165-180

Ferroan dolomites in Miocene sediments of the Xisha Islands and their genetic model

XU Hong1,2, ZHANG Weiwei1,3, WEI Kai1,3, HE Qingkun4, JIANG Yunshui1,2, XU Tingting1,2, JIANG Xuejun1,2, YAN Guijing1,2, SONG Hongying4, WANG Jianghai5
1 Key Laboratory of Marine Hydrocarbon Resources and Environment Geology, Qingdao Institute of Marine Geology, Ministry of Land and Resources, Qingdao 266071, China;
2 Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China;
3 China University of Petroleum(East China), Qingdao 266555, China;
4 Shandong University of Science and Technology, Qingdao 266590, China;
5 Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering/South China Sea Bioresource Exploitation and Utilization Collaborative Innovation Center, School of Marine Sciences, Sun Yat-Sen University, Guangzhou 510006, China
Carbonate rocks are important reservoirs for global petroleum exploration. The largest oilfield in the South China Sea, Liuhua 11-1, is distributed in the massive carbonate reef area of the Zhujiang (Pearl) River Mouth Basin. Previous studies showed that one 802.17-m-long core from well Xichen-1 in the South China Sea mainly consisted of white and light gray-white organic reefs. Recently, a Miocene whole core (161.9 m long) of well Xiyong-2, near well Xichen-1, was found to contain six layers of yellowish brown, light yellowish gray, iron black, or light yellowish gray-white organic reefs. Scanning electron microscope images of these layers reveal a typical ferroan dolomite rich in Fe (up to 29%), with the high concentrations of Mn, Cu, W, Zn, Cr, Ni, and Co. Systematic X-ray powder diffraction analysis yields a 1.9-6.1 match in phase ratio with ankerite, 5.4-26.9 with dolomite, and zero with calcite, which indicate that the samples can be classified as ferroan dolomite. The iron and heavy metals are inferred to be originated from multiple volcanic eruptions of Gaojianshi Island in the Dongdao Atoll during the middle-late Miocene. These elements were dissolved in seawater, likely as a sol, and carried to Yongxing Island in the Xuande Atoll by sea currents and tides enhanced by prevailing winds, and deposited as a part of the sedimentation process in the study area. The ferroan dolomite has Sr content of (125-285)×10-6, which is lower than the accepted Sr boundary value of dolomite. This finding suggests that dolomitization occurred during large-scale global glacial regression in the late Miocene. The isolated Xisha carbonate platform, exposed to air, underwent freshwater leaching and dolomitization induced by mixed water, and caused the extensive Fe-Mg exchange along the organic reef profile to form ankerite and ferroan dolomite. These results may help to understand paleoceanographic environmental changes in the South China Sea during the Miocene.
Key words:    ferroan dolomite|X-ray powder diffraction|sedimentation mechanism|Miocene|Xisha Islands   
Received: 2017-05-03   Revised: 2017-06-21
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