Magnetic Fabric Characteristics and Implications for Its Origin of "Wushan Loess"
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摘要: 为了探讨"巫山黄土"的成因类型, 对新近发现的"巫山黄土"进行了磁组构测试和磁化率主轴特征分析, 并与长江现代沉积物以及长江中游一带分布的风积黄土和"砂山"的磁组构特征进行了对比.结果表明: (1)"巫山黄土"的κ、P、F、L、E值与风积黄土和"砂山"接近, 却明显低于长江现代沉积物的相应值; (2)在磁组构参数F-L关系图上, "巫山黄土"的数据点多集中于坐标原点附近, 与长江中游一带风积黄土和"砂山"的特征也类似, 均指示了沉积动力相对比较弱的沉积环境.而长江现代河流沉积物的数据点却主要分布于F轴附近, 反映长江现代河流沉积物的F较L发育; (3)"巫山黄土"的磁化率最大主轴特点也与长江中游风积黄土和"砂山"的接近, 偏角的方向比较分散, 长轴的倾角为36°~38°, 短轴的倾角为37°~39°, 而长江现代河流沉积物的磁化率最大主轴偏角的方向比较稳定, 并且长轴的倾角一般小于10°, 短轴的倾角大于80°; (4)"巫山黄土"样品的T值介于-1~1之间, 而长江现代河流沉积物的T值却主要以大于1为主.综合看来, "巫山黄土"的磁组构特征与长江中游一带的风积黄土和"砂山"接近, 指示了其风积成因的特点.Abstract: In this paper we present the magnetic fabric and magnetic susceptibility characters of one newly found "Wushan Loess" Section, which was also compared with the modern Yangtze River sediments and aeolian loess and "Sand-Dune" sediments distributed at the Middle Yangtze River. The results are as follows. (1) The κ, P, F, L, E values of the "Wushan Loess" are close to those of the aeolian loess and "Sand-Dune" sediments, however these values are obviously lower than those of the modern Yangtze River sediments. (2) Shown on the F-L graph, data of the "Wushan Loess" converge upon the origin of coordinates, similar to the aeolian loess and "Sand-Dune" sediments distributed at the Middle Yangtze River, which reveals sedimentary environment with relatively lower power. Data of the modern Yangtze River sediments lies around the F-axis, showing more dominance of F rather than L. (3) Maximum magnetic principal axis of the "Wushan Loess" is similar to that of the aeolian loess and "Sand-Dune" sediments distributed at the Middle Yangtze River, with the direction of declination relatively dispersing and the inclination of long axis varying between 36° and 38° and the inclination of the short axis varying between 37° and 39°. However the direction of maximum magnetic principal axis declination of the modern Yangtze River is relatively stable with inclination of the long axis smaller than 10° and short axis bigger than 80°. (4) T values of the "Wushan Loess" vary between -1 and 1 while T values of the modern Yangtze River are mainly higher than 1. Generally, magnetic fabric characters are similar to aeolian loess and "Sand-Dune" sediments distributed at the Middle Yangtze River, revealing its aeolian origin.
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图 1 研究剖面位置
Fig. 1. Map showing the geography and section position of the research area
图 2 长江中游部分沉积物磁组构参数
a.长江现代河漫滩沉积(张玉芬等，2008)；b.武汉青山风成黄土(下蜀黄土)；c.文中“巫山黄土”；图中n是样品数
Fig. 2. Combination diagrams of magnetic fabric parameters of sediments in the Middle reach of Yangtze River
图 3 不同沉积类型的AMS主轴等面积赤平投影
a.武汉长江河漫滩(n=305)；b.武汉青山风成砂(n=313)；c.本文研究的“巫山黄土”(n=146)；●.κmax；▲.κint；+.κmin
Fig. 3. Stereographic projection of anisotropy of magnetic susceptibility ellipsoids of some sediment types
表 1 不同沉积类型的磁组构参数
Table 1. Magnetic fabric analysis parameters of different deposit types
沉积类型与 沉积环境 采样地点 样品数 к(10-6SI） P F L q T E Imax（°） Imin（°） 水成沉积 河流 现代河流 长江宜昌 34 2 009.1 1.027 6 1.022 5 1.005 1 0.225 5 0.608 4 1.017 3 8.50 74.7 长江輝洲湾 305 1 538.7 1.046 0 1.038 1 1.007 6 0.257 0 0.593 0 1.030 3 9.50 74.3 长江武汉 47 1 403.3 1.039 2 1.036 4 1.002 7 0.121 9 0.821 6 1.033 6 5.80 80.5 汉江仙桃 66 708.2 1.072 9 1.062 9 1.009 5 0.167 9 0.716 2 1.052 9 7.70 80.6 髙位碌石层 李家院砾石层 8 1 504.3 1.024 4 1.016 9 1.007 3 0.347 1 0.406 1 1.009 4 9.30 71.4 白洋渡砾石层 26 355.0 1.025 5 1.021 1 1.004 3 0,202 8 0.644 9 1.016 7 6.00 80.1 溃口扇 长江輝洲湾 307 782.2 1.041 2 1.028 1 1.010 3 0.400 1 0.361 6 1.015 3 17.90 68.4 风成沉积 风尘砂 武汉青山 312 657.0 1.016 4 1.010 7 1.005 7 0.535 6 0.214 2 1.005 0 21.30 55.9 岳阳君山 53 106.0 1.006 1 1.003 0 1.003 1 0.739 7 0.012 0 0.999 8 29.00 42.0 江西新港 430 38.5 1.028 6 1.015 5 1.012 9 0.628 5 0.097 8 1.002 7 24.20 45.1 风尘黄土 武汉青山 48 750.0 1.008 6 1.005 0 1.003 5 0.653 0 0.091 1 1.001 6 22.00 54.0 江西新港 116 161.4 1.015 9 1.008 9 1,006 9 0.589 1 0.155 0 1.003 0 16.00 54.2 本文研究的“巫山黄土” 146 361.4 1.007 3 1.004 0 1.003 4 0.672 1 0.049 9 1.000 4 37.34 38.0 注:参数κ.平均磁化率(总磁化率) ;P.各向异性度;F.磁面理;L磁线理;q.磁基质颗粒度;T形状因子;E.扁率;Imax和Imin分别为最大、最小主轴的倾角. 
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