Classification of Seafloor Geological Types of Qianyu Seamount from Mid-Pacific Seamounts Using Multibeam Backscatter Intensity Data
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摘要: 中、西太平洋海山区是富钴结壳的重要富集区，钙质远洋沉积、碳酸盐岩沉积及重力作用引起的滑塌沉积是海山山顶和斜坡的主要沉积类型，它们的空间分布规律对于富钴结壳的分布至关重要.在国内首次利用EM122多波束回波强度资料对中太平洋潜鱼海山进行了底质类型研究，对回波强度资料进行处理和统计分类，并与浅地层剖面和地质取样结果对比，分析得出了4种底质类型，即富钴结壳、钙质远洋沉积、碳酸盐岩基底及碎屑流沉积.这几种底质类型具有不同的回波强度特征，其中富钴结壳区表现为均一的回波强度高值特征；钙质远洋沉积区表现为均一的回波强度低值特征，二者回波强度值相差约20 dB.结果显示潜鱼海山山顶仅局部发育钙质远洋沉积，大部分为碳酸盐岩基底出露区，山顶边缘及侧翼山脊处为主要的富钴结壳分布区.此外，该海山存在3处较大规模的重力滑塌沉积区，主要为碎屑流沉积.Abstract: Cobalt-rich ferromanganese crusts are mainly distributed around the top brim and on the slope of Mid-Pacific seamounts and West Pacific seamounts. Calcium-pelagic sediment, carbonate rock and debris flow driven by gravity process are main seafloor sediment types of seamounts, which control the distribution of cobalt-rich ferromanganese crusts. In this paper, multibeam backscatter intensity data from EM122 multibeam system are first used in China to recognize seafloor geological types of Qianyu seamount on Mid-Pacific seamounts. Backscatter data are processed and classified statistically with verification of sub-bottom profile and ground-truthing data to identify four seafloor geological types including cobalt-rich crust, calcium-pelagic sediment, carbonate rock and debris flow. Different backscatter intensities are represented by different seafloor geological types, cobalt-rich crust area shows as uniform strong backscatter intensity representing, and calcium-pelagic sediment area shows as uniform weak backscatter, their backscatter difference is about 20 dB. The top of Qianyu seamount is mostly outcropped by carbonate rock, and locally covered by calcium-pelagic sediment, and cobalt-rich crust distribute around the top brim and southern and eastern slope ridges. Furthermore, three gravity slide areas are recognized on the Qianyu seamount, which are of debris flows.
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图 3 海山回波强度统计分类结果
Fig. 3. Results of backscatter intensity statistical classification of Qianyu seamount
图 4 海山综合地质解释结果
Fig. 4. Results of synthetic geological interpretation of Qianyu seamount
图 5 海山浅钻样品
a.SD01:角砾岩，角砾为黑色结壳碎块，基质为钙质；b.SD02:凝灰岩，浅黄色，含火山碎屑；c.SD03:生物碎屑角砾岩，上层为风化结壳；d.SD04:生物碎屑礁灰岩，灰白色，含生物碎屑；e.SD05:生物碎屑礁灰岩，浅黄色，轻微磷酸盐化；f.SD06:生物碎屑礁灰岩，可见大量生物壳体；g.SD07:灰岩，白色致密，含少量黑色微结核颗粒；h.SD08:泥灰岩，灰白色，可见硅质条带；浅钻位置见图 1
Fig. 5. Shallow-drilling core sample of Qianyu seamount
图 6 回波强度与4 kHz浅地层剖面对比
剖面位置见图 1
Fig. 6. Synthetic profile compared backscatter intensity with 4 kHz sub bottom profile
表 1 不同回波强度特征类型及其地质解释
Table 1. Different types of backscatter intensity and their geological interpretation
类型 编号 位置 回波强度特征 地质解释 ① 山顶边缘 整体高、强度均一，-20 dB左右 富钴结壳 Ⅰ ② 斜坡底部 整体高、强度均一，-16 dB左右 滑塌岩崩体，落差为320 m ③ 隆起带 整体高值，-20 dB左右，局部低值，-35 dB左右 小海山，落差为400 m，局部发育沉积 ④ 山顶中部 整体低、强度均一，-40 dB左右 钙质远洋沉积 Ⅱ ⑤ 坡底海盆 整体低、强度均一，-38 dB左右 深海软泥沉积 ⑥ 斜坡山脊 整体高值，局部高、低值相间分布，-17~-35 dB 富钴结壳，局部少量沉积分布 Ⅲ ⑦ 山顶中部 整体较低，且高、低值相间分布，-25~-32 dB 碳酸盐岩基底出露，局部少量沉积分布 ⑧ 斜坡底部 高、低相间分布，高值区范围大小不一，-21~-34 dB 滑塌碎屑沉积，碎屑大小不一，局部为来自海山的钙质远洋沉积 
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