Late Miocene-Pleistocene Calcareous Nannofossil Biostratigraphy of Shenhu Gas Hydrate Drilling Area in the South China Sea and Variations in Sedimentation Rates
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摘要: 2007年我国首次在南海北部陆坡神狐海域实施了天然气水合物钻探, 并钻取水合物实物样品.为了解钻区地层、水合物产出带(the zone of gas hydrate occurrence)或水合物储层的地层时代以及沉积速率特征, 对其中4口钻孔(SH1B、SH2B、SH5C和SH7B)岩心沉积物进行钙质超微化石年代地层学和沉积速率变化的研究.本次工作识别出17个新近纪钙质超微化石事件, 确定了神狐钻探所钻达最老地层为新近系上中新统; 水合物产出带的地层为上中新统-上新统.这4个钻井地层沉积速率的变化特征因站位和时期而异.中新世以来各地质时期沉积速率差异较大, 全新世最高(20~34.16 cm/ka之间), 其次为更新世和晚中新世(3.14~5.74 cm/ka), 上新世最低(1.88~3.27 cm/ka).此外, 水合物产出带地层的沉积速率在各钻孔也有差异, SH2B孔为4.18 cm/ka, SH7B孔为1.88 m/ka.表明南海水合物产出层位沉积速率差异较大, 沉积速率与水合物成藏的关系可能比前期的认识更为复杂.Abstract: The first Chinese gas hydrate drilling program was carried out on the Shenhu area in 2007 and obtained gas hydrate samples. In order to understand the strata, age of the zone of gas hydrate occurrence and features of sedimentation rates, Late Miocene to Pleistocene calcareous nanofossil biostratigraphy in 4 holes (SH1B, SH2B, SH5C and SH7B) from the Shenhu gas hydrate drilling area of the northern South China Sea have been studied. A total of 17 Late Miocene to Pleistocene nanofossil events are recognized. The oldest sediments recovered are in an age of late Miocene, younger than 7.362 Ma, and the age assigned for gas hydrate occurrence zone is late Miocene in Hole SH2B and late Miocene to early Pliocene in Hole SH7B. Sedimentation rates varied with holes and times, increasing from Pliocene, and reaching the highest values in Holocene, for example, 1.88-3.27 cm/ka during Pliocene and 20-34.16 cm/ka in Holocene, respectively. Sedimentation rates of 1.88 cm/ka to 4.18 cm/ka were estimated for sediment sequences in gas hydrate occurrence zone. The results suggest that the relationship between sedimentation rates and occurrence of gas hydrates should be more complicated than our earlier knowledge.
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图 1 南海北部神狐水合物钻探区构造位置(a)及地形地貌图(b)(据匡增桂和郭依群，2011修改)
Fig. 1. The location (a) and geography and physiognomy of the gas hydrate sample area in the Shenhu waters, the northern SCS (b)
图 2 南海神狐海域水合物钻探区4个钻孔钙质超微化石带的划分与对比(虚线代表推测界线)
Fig. 2. Correlation of calcareous nannofossil zones and events between holes SH1B, SH2B, SH7B and SH5C in the Shenhu Area, South China Sea
表 1 晚中新世钙质超微化石事件及年代(据Raffi et al., 2006)
Table 1. Calcareous nannofossil event chronology scheme(from Raffi et al., 2006)
事件 属种(Taxon) 化石带 年龄(Ma) X Gephyrocapsa spp. (＞3.5 μm)-Emiliania huxleyi 0.063 FO Emiliania huxleyi NN21 0.291 LO Pseudoemiliania lacunosa NN20 0.436 LCO Reticulofenestra asanoi 0.9 RE Gephyrocapsa spp. (≥4 μm) 1.04 FCO Reticulofenestra asanoi 1.078* LO Gephyrocapsa spp. (＞5.5 μm) 1.24 LO Helicosphaera sellii 1.34 FCO Gephyrocapsa spp. (＞5.5 μm) 1.46 LO Calcidiscus macintyrei 1.60 FO Gephyrocapsa spp. (＞5.5 μm) 1.560~1.617* FO Gephyrocapsa spp. (＞3.5 μm) NN19 1.67 LO Discoaster brouweri NN18 2.06 AB Discoaster triradiatus 2.135~2.216* LO Discoaster pentaradiatus NN17 2.393~2.512* LO Discoaster surculus NN16 2.52 LO Discoaster tamalis 2.87 LO Sphenolithus spp. 3.65 LO Reticulofenestra pseudoumbilicus (＞7 μm) NN15-14 3.79 FCO Discoaster asymmetricus 4.13 LO Amaurolithus primus 4.50 LO Ceratolithus acutus 5.04 FO Ceratolithus rugosus NN13 5.12 FO Ceratolithus acutus 5.32 LO Discoaster quinqueramus NN12 5.59 FO Amaurolithus spp.=FO Amaurolithus primus NN11b 7.362~7.424* 注：X.属种丰度交叉变化转换界面(cross-over in abundance)；FO.初现面(first occurrence)；LO.末现面(last occurrence)；LCO.连续出现沿限的末现面(last consistent occurrence)；RE.再次进入出现的初现面；AB.顶锋带开始界面(Acme Beginning)；FCO.连续出现初现面(first consistent occurrence)；*.数据来自大西洋或地中海记录(datum based on Atlantic or Mediterranean records). 
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表 2 南海神狐海域水合物钻探区各钻孔钙质超微化石事件的深度分布
Table 2. The depths of calcareous nannofossil events at holes SH1B, SH2B, SH5C and SH7B
生物事件 化石带 年龄(Ma) 深度(m) SH1B SH2B SH7B SH5C FO E. huxleyi Acme NN21 更新世 0.063 8.00 8.10 7.40 FO E. huxleyi NN21 0.291 18.55 15.00 10.55 LO P. lacunosa NN20 0.436 25.70 25.95 LCO R. asanoi NN19 0.90 29.96 FCO R. asanoi NN19 1.078 31.64 96.3 FO Gephyrocapsa spp. (＞5.5 μm) NN19 1.617 104.85 FO Gephyrocapsa (medium) spp. NN19 1.67 96.73 LO D. brouweri NN18 2.06 < 75.00 < 75.00 99.80 LO D. pentaradiatus NN17 上新世 2.393 123.85 LO D. surculus NN16 2.52 133 LO D. tamalis 2.87 131.50 140.50 LO S. abies/neoabies 3.65 145.00 LO R. pseudoumbilicus NN15 3.79 155.54 ＞175.17 LO Amaurolithus spp. NN14 4.50 159.52 LO C. acutus NN13 5.04 161.20 FO C. acutus NN12 5.32 143.00 150.85 171.2 LO D. quinqueramus NN12 晚中 5.59 185.00 173.20 FO A. amplificus NN11b 新世 7.362 ＞261.86 ＞235.86 ＞194.18
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表 3 神狐水合物钻探区钻孔样品Emiliania huxleyi扫描电镜下数量统计结果
Table 3. The abundance of Emiliania huxleyi at holes SH1B, SH5C and SH7B observed under scan electric microscope
SH1B孔 SH5C孔 SH7B孔 深度(cm) 超微化石总个数(6 000倍，10个视域) Emiliania huxleyi个数(6 000倍，10个视域) Emiliania huxleyi百分含量(%) 深度(cm) 超微化石总个数(6 000倍，10个视域) Emiliania huxleyi个数(6 000倍，10个视域) Emiliania huxleyi百分含量(%) 深度(cm) 超微化石总个(数6 000倍，10个视域) Emiliania huxleyi个数(6 000倍，10个视域) Emiliania huxleyi百分含量(%) 510 15 0 0 510 46 4 9 530 35 0 0 595 44 11 25 595 55 5 9 650 55 11 20 655 88 31 35 660 55 2 4 700 27 10 37 700 61 21 34 700 25 2 8 810 43 15 35 757 60 13 22 740 88 7 8 910 20 2 10 800 89 31 35 800 62 2 3 1 015 33 5 15 840 91 36 40 840 63 2 3 1 095 41 3 7 905 45 5 11 905 87 1 1 1 160 52 3 6 945 86 22 26 970 61 2 3 1 200 27 2 7 1 015 68 2 3 1 055 75 3 4 1 265 45 2 4 1 055 110 14 13 1 095 31 0 0 1 310 26 3 12 1 095 85 9 11 1 140 84 0 0 1 365 13 4 31 1 140 68 8 12 1 200 - - - 1 440 27 2 7 1 200 61 5 8 1 245 130 0 0 1 500 20 4 20 1 310 25 0 0 1 285 113 0 0 1 600 115 0 0 1 415 26 2 8 1 370 85 0 0 1 650 132 0 0 1 500 23 0 0 1 440 39 0 0 1 705 153 0 0 1 600 26 2 8 1 500 55 0 0 1 750 160 0 0 1 705 61 1 2 1 550 63 0 0 1 815 290 0 0 1 815 152 3 2 1 600 40 0 0 1 855 18 0 0 1 915 57 0 2 1 705 23 0 0 1 915 67 0 0 1 960 85 0 1 1 810 70 0 0 2 000 23 0 0 2 000 67 0 4 1 915 55 0 0 2 050 21 0 0 2 045 41 0 0 2 000 65 0 0 2 090 17 0 0 2 110 65 0 2 2 085 33 0 0 2 150 55 0 0 2 150 47 0 0 2 150 60 0 0 2 195 75 0 0 2 195 45 0 0 2 215 30 0 0 2 240 100 0 0 2 235 54 0 0 2 255 65 0 0 2 280 30 0 0 2 280 42 0 0 2 300 45 0 0 2 300 90 0 0 2 320 93 0 0 2 358 70 0 0 2 325 210 0 0 2 365 87 0 0 2 400 200 0 0 2 505 150 0 0 2 615 120 0 0 2 715 58 0 0
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表 5 南海水合物钻探区与布莱克海脊晚中新世以来沉积速率的对比
Table 5. Sedimentation rates of the Blake Ridge region and Shenhu gas hydrate-drilling area
航次 站位 岩心长(m) 沉积速率(cm/ka) 钻井底部年代(Ma) 钻达地层 水合物层位 全新世 更新世 上新世 中新世 DSDP76 ★533 399 NR 8.3 0.5~21.3 NP -3.1 上新统 ODP164 991 56 NR 0.4~2.1 1.6 NP -5.9 中新统 992 50 NR 0.7 - 9.6 -8.2 中新统 993 52 NR - - 21 -9.8 中新统 ★994 704 NR 3~9.8 3-14 30.3 -6.0 中新统 上新统 995 704 NR 2.9~8 8.9~14 30.3 -6.1 中新统 ★996 63 NR 4.8 NP NP -1.1 更新统 上新统 ★997 750 NR 2.7~4 2.7~20.5 25.6 -6.4 中新统 上新统 南海水合物钻探 SH1B 261.86 26.25 2.02 2.94 ＞5.85 < 7.362 中新统 ★SH2B 238.85 - 1.96 3.27 ＞4.18 < 7.362 中新统 中新统 SH5C 175.17 34.16 5.16 - < 3.790 上新统 ★SH7B 194.18 20 5.71 1.88 < 7.362 中新统 中新统-上新统 注：★533代表钻取水合物的站位.
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表 4 神狐水合物钻探区钻孔中新世-全新世各时期的沉积速率(cm/ka)
Table 4. Variation in sedimentation rates during Miocene to Holocene in Shenhu gas hydrate-drilling area
站位 全新世(12 ka) 更新世(1 806 ka) 上新世(5 332 ka) 晚中新世晚期(7 362 ka) 底界线(cm) 厚度(cm) 沉积速率(cm/ka) 底界线(cm) 厚度(cm) 沉积速率(cm/ka) 底界线(cm) 厚度(cm) 沉积速率(cm/ka) 底界线(cm) 厚度(cm) 沉积速率(cm/ka) SH1B 315 315 26.25 6 000* 5 685 3.14 14 300 10 359 1.94 ＞26 186 ＞11 886 ＞5.85 SH2B - - - 6 000* - - 15 085 11 543 3.27 ＞23 586 ＞8 501 ＞4.18 SH5C 410 410 34.16 9 673 9 263 5.13 - - - SH7B 240 240 20 10 600* 10 360 5.74 17 120 6 635 1.88 注：带*标号为估算深度.
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表 5 南海水合物钻探区与布莱克海脊晚中新世以来沉积速率的对比
Table 5. Sedimentation rates of the Blake Ridge region and Shenhu gas hydrate-drilling area
航次 站位 岩心长(m) 沉积速率(cm/ka) 钻井底部年代(Ma) 钻达地层 水合物层位 全新世 更新世 上新世 中新世 DSDP76 ★533 399 NR 8.3 0.5~21.3 NP -3.1 上新统 ODP164 991 56 NR 0.4~2.1 1.6 NP -5.9 中新统 992 50 NR 0.7 - 9.6 -8.2 中新统 993 52 NR - - 21 -9.8 中新统 ★994 704 NR 3~9.8 3-14 30.3 -6.0 中新统 上新统 995 704 NR 2.9~8 8.9~14 30.3 -6.1 中新统 ★996 63 NR 4.8 NP NP -1.1 更新统 上新统 ★997 750 NR 2.7~4 2.7~20.5 25.6 -6.4 中新统 上新统 SH1B 261.86 26.25 2.02 2.94 ＞5.85 < 7.362 中新统 南海水合物钻探 ★SH2B 238.85 - 1.96 3.27 ＞4.18 < 7.362 中新统 中新统 SH5C 175.17 34.16 5.16 - < 3.790 上新统 注：★533代表钻取水合物的站位.
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