Sedimentary and petrophysical characteristics of various turbidites at IODP Sites U1499 and U1500 in the northern South China Sea
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摘要: 深海浊流沉积是重要的油气和天然气水合物勘探目标，对古海洋、古环境、古构造乃至古气候等方面的研究具有重要的科学意义。前人对于浊积岩的研究多从沉积学角度进行，从岩石物理角度开展的相关研究很少。本文将岩芯宏观沉积学分析、薄片（或涂片）分析与岩石物性分析相结合，研究南海海盆北部IODP 367航次U1499、U1500站位浊流沉积的沉积特征和岩石物理响应。共识别出了3类浊积岩：钙质、陆源碎屑和陆源碎屑—钙质混积浊积岩，以陆源碎屑和混积浊积岩最发育。不同类型的浊积岩表现出不同的岩石物性特征：钙质浊积岩表现为低磁化率、高颜色反射率亮度、密度变化较大和低自然伽马；陆源碎屑浊积岩表现为低磁化率、中—低亮度、中—高密度、中—低自然伽马；混积浊积岩的磁化率、亮度和自然伽马变化大，密度中—高。南海海盆浊积岩的丰度在发育时间上以晚中新世和中—晚更新世最高，然后依次为早更新世、上新世和早—中中新世。晚中新世和中—晚更新世全球海平面总体处于低位时期，有利于浊积岩的发育。自晚中新世以来，南海海盆钙质浊积岩的丰度总体呈逐渐减少趋势，可能与南海海盆水深逐渐加大、碳酸盐沉积逐渐萎缩等因素有关。
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关键词:
- 浊流沉积 /
- 岩石物性 /
- IODP 367航次 /
- 海盆 /
- 南海
Abstract: The deep-sea turbidite is an important target for petroleum and gas hydrate exploration, and is of significance to paleoceanographic, paleoenvironmental, paleotectonic and paleoclimatic researches. Turbidites have been investigated mostly in a sedimentological and less in a petrophysical perspective. In this paper, cores and thin sections (and /or smear slides) are used to study the sedimentary characteristics and petrophysical responses of the turbidites at the IODP Sites of U1499 and U1500 in the northern part of the South China Sea (SCS). Three types of turbidites are identified, i.e. the calcareous, terrigenous, and terrigenous-calcareous mixed turbidites. The later two types dominate. Each type of turbidites has its own petrophysical characteristics. The calcareous turbidites are characterized by low magnetic susceptibility, high brightness in color reflectance spectrometry, variable density, and low natural gamma radiation; the terrigenous turbidites characterized by low magnetic susceptibility, low to medium brightness, medium to high density, and medium to low natural gamma radiation; and the mixed turbidites characterized by variable magnetic susceptibility, brightness and natural gamma radiation and medium to high density. The turbidites mainly occur in the late Miocene and the middle-late Pleistocene sequences, followed by the early Pleistocene, the Pliocene, and the early-middle Miocene. The euatatic lowstand periods in the late Miocene and middle-late Pleistocene could be favorable for the deposition of the turbidites in the SCS. The gradual decrease in the abundance of calcarous turbidites since late Miocene could be associated with the gradual deepening of the oceanic basin and the shrinking of carbonate depositional area in the SCS.-
Keywords:
- turbidites /
- petrophysical properties /
- IODP Expedition 367 /
- oceanic basin /
- South China Sea
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图 2 钙质浊积岩的岩心、薄片（左侧为单偏光，右侧为正交光）及物性特征
a. U1500B孔5R-1岩芯（874.93～875.01 mbsf）；b. U1499B孔17R-4岩芯（804.98～805.04 mbsf）。eb，底界侵蚀面；Ta-Te，鲍马序列a-e段；bs，远洋-半远洋泥质背景沉积。
Figure 2. Characteristics of calcareous turbidites in cores, thin sections （right panel, plane polarized light on the left and perpendicular polarized light on the right）, and physical properties
（a） Core 5R-1 （874.93～875.01 mbsf）, Hole U1500B; （b） Core 17R-4 （804.98～805.04 mbsf）, Hole U1499B. eb, erosional base; Ta-Te, Ta to Te intervals in Bouma sequence; bs, background sediments.
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图 3 陆源碎屑浊积岩的岩心、薄片（左侧为单偏光，右侧为正交光）及物性特征
a. U1500B孔23R-1岩芯（1 049.55～1 049.72 mbsf）；b. U1500A孔29R-1岩芯（777.00～777.26 mbsf）。eb，底界侵蚀面；Ta-Te，鲍马序列a-e段；bs，远洋-半远洋泥质背景沉积。
Figure 3. Characteristics of terrigenous clastic turbidites in cores, thin sections （right panel, plane polarized light on the left and perpendicular polarized light on the right）, and physical properties
（a） Core 23R-1 （1 049.55～1 049.72 mbsf）, Hole U1500B; （b） Core 29R-1 （777.00～777.26 mbsf）, Hole U1500A. eb, erosional base; Ta-Te, Ta to Te intervals in Bouma sequence; bs, background sediments.
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图 4 混积浊积岩的岩心、薄片（或涂片）（左侧为单偏光，右侧为正交光）及物性特征
a. U1499B孔15R-1岩芯（781.20～781.35 mbsf）；b. U1499A孔5H-6岩芯（43.57～43.68 mbsf）。eb，底界侵蚀面；Ta-Te，鲍马序列a-e段；bs，远洋-半远洋泥质背景沉积；S，滑塌沉积。
Figure 4. Characteristics of mixed turbidites in cores, thin sections （or smear slide）（right panel, plane polarized light on the left and perpendicular polarized light on the right）, and physical properties
（a） Core 15R-1 （781.20～781.35 mbsf）, Hole U1499B; （b） Core 5H-6 （43.57～43.68 mbsf）, Hole U1499A. eb, erosional base; Ta-Te, Ta to Te intervals in Bouma sequence; bs, background sediments; S, slump deposits.
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图 5 典型浊积岩岩芯照片
a-d. 钙质浊积岩；e-h. 陆源碎屑浊积岩；i-l. 混积浊积岩。a. U1499A孔5H-1岩芯（36.85～37.00 mbsf）；b. U1499A孔36X-2岩芯（329.67～329.77 mbsf）；c. U1499A孔48X-5岩芯（449.85～450.24 mbsf）；d. U1500A孔26R-6岩芯（754.85～755.07 mbsf）；e. U1500A孔33R-1岩芯（816.58～817.03 mbsf）；f. U1500B孔31R-1岩芯（1 127.72～1 127.90 mbsf）；g. U1500B孔8R-1岩芯（904.43～904.57 mbsf）；h. U1500B孔37R-1岩芯（1 184.80～1 185.03 mbsf）；i. U1499A孔70X-CC岩芯（650.27～650.32 mbsf）；j. U1500A孔27R-1岩芯（757.65～757.71 mbsf）；k. U1499A孔6H-1岩芯（46.37～46.41 mbsf）；l. U1499A孔6H-2岩芯（47.65～47.86 mbsf）。eb，底界侵蚀面；Ta-Te，鲍马序列a-e段；bs，远洋-半远洋泥质背景沉积；D，碎屑流沉积。
Figure 5. Core photos of typical turbidites
（a-d） calcareous turbidites; （e-h） terrigenous clastic turbidites; （i-l）mixed turbidites. （a） Core 5H-1 （36.85～37.00 mbsf）, Hole U1499A; （b） Core 36X-2 （329.67～329.77 mbsf）, Hole U1499A; （c） Core 48X-5 （449.85～450.24 mbsf）, Hole U1499A; （d） Core 26R-6 （754.85～755.07 mbsf）, Hole U1500A; （e） Core 33R-1 （816.58～817.03 mbsf）, Hole U1500A; （f） Core 31R-1 （1 127.72～1 127.90 mbsf）, Hole U1500B; （g） Core 8R-1 （904.43～904.57 mbsf）, Hole U1500B; （h） Core 37R-1 （1 184.80～1 185.03 mbsf）, Hole U1500B; （i） Core 70X-CC （650.27～650.32 mbsf）, Hole U1499A; （j） Core 27R-1 （757.65～757.71 mbsf）, Hole U1500A; （k） Core 6H-1 （46.37～46.41 mbsf）, Hole U1499A; （l） Core 6H-2 （47.65～47.86 mbsf）, Hole U1499A. eb, erosional base; Ta-Te, Ta to Te intervals in Bouma sequence; bs, pelagic to semipelagic background sediments; D, debrites.
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图 8 IODP U1499与U1500站位不同地质年代钙质、陆源碎屑及混积浊积岩的发育丰度直方图（a-b）及全球海平面曲线(c)
a. 单位地质时间内浊积岩的层数（层/Ma），b. 单位地层厚度内浊积岩的厚度（cm/m），c. Haq全球海平面曲线[13]。
Figure 8. Histograms showing the abundance of calcareous, terrigenous clastic, and mixed turbidites in different geological ages at IODP Sites U1499 and U1500 (a-b), and Haq et al. global eustatic curve
a. histogram of the layer number of the turbidites per unit time（layers/Ma）, b. histogram of the layer thickness of the turbidites per stratigraphic thickness（cm/m）, c. global eustatic curve[13].
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图 9 U1499与U1500站位各类浊积岩的岩石物性参数分布直方图
图中的曲线为不同类别浊积岩、不同岩性物性数据的正态拟合曲线。a1-a4，中—晚更新世；b1-b4，早更新世；c1-c4，上新世；d1-d4，晚中新世；e1-e4，早—中中新世。
Figure 9. Histograms of the petrophysical parameters for different types of turbidites at U1499 and U1500 sites
The curves in the figure represent normal distribution curves by fitting the physical property data of different turbidites. a1-a4, middle-late Pleistocene; b1-b4, early Pleistocene; c1-c4, Pliocene; d1-d4, late Miocene; e1-e4, early-middle Miocene.
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图 10 U1499与U1500站位各类浊积岩的岩石物性参数交会图
a1-a3，中—晚更新世；b1-b3，早更新世；c1-c3，上新世；d1-d3，晚中新世；e1-e3，早—中中新世。
Figure 10. Crossplots of the petrophysical parameters for different types of turbidites at U1499 and U1500 sites
a1-a3, middle-late Pleistocene; b1-b3, early Pleistocene; c1-c3, Pliocene; d1-d3, late Miocene; e1-e3, early-middle Miocene.
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