长江中游砂山的磁组构特征及古气候环境意义

为了建立长江中游风成砂的磁组构识别标志，讨论磁组构参数与古气候、古环境之间的关系，对新近发现的江西九江新港砂山剖面进行了野外考察和磁化率定向样品的采集和测试，结果表明：①新港风成砂剖面中砂层和古土壤化的亚黏土层的磁组构参数值具有较大的不同，亚黏土层的磁化率（〖WTBX〗κ〖WTBZ〗）在剖面中表现为峰值（均值143.27（10-6SI）），各向异性度（〖WTBX〗P〖WTBZ〗）值却表现为曲线的较低值段。砂层却与亚黏土层相反，〖WTBX〗κ〖WTBZ〗值为谷值（均值为2181（10-6SI）），〖WTBX〗P〖WTBZ〗值总体为高值段，但波动较大。②在新港风成砂剖面中大多数亚黏土层样品的磁面理度（〖WTBX〗F〖WTBZ〗）值大于磁线理度（〖WTBX〗L〖WTBZ〗），而砂层中只有部分样品的〖WTBX〗F值大于L〖WTBZ〗，说明亚黏土层的磁面理发育程度较砂层的要好。③相对于河流和湖泊沉积物（河流沉积物长轴的倾角一般小于10°，短轴的倾角大于80°）而言，新港剖面风成砂的磁化率量值椭球体的长轴的倾角较大（22°～24°），短轴的倾角较小（45°～51°）。④新港剖面风成砂的磁基质颗粒度（〖WTBX〗q〖WTBZ〗）的平均值（0668 8）比长江中游河流沉积物的〖WTBX〗q〖WTBZ〗值（024）偏大，反映风成砂的分选较河流沉积物的要差。上述特点可作为长江中游水成和风成沉积物的磁组构识别标志。⑤在325～262 m的地层剖面中共有9层发育程度不同的古土壤化的亚黏土层，说明在这一期间气候不仅有9次由干冷向暖湿的转变，且每次变化的强度也有所不同。〖

MAGNETIC FABRIC CHARACTERISTICS OF AEOLIAN SAND-DUNE AND ITS PALEOCLIMATIC AND PALEOENVIRONMENTAL SIGNIFICANCE IN MIDDLE REACHES OF THE YANGTZE RIVER

In order to establish the magnetic fabric identification marks of aeolian sands in the middle reaches of the Yangtze River and discuss the relationship between the magnetic fabric parameters，paleoclimate and paleoenvironment，the authors carried out the field investigations and found Xingang sand dune section in Jiujiang City，Jiangxi Province，then collected and measured the magnetic susceptibility oriented samples of the section.The results indicate that (1)The magnetic fabric parameter values of the aeolian sands and clayey layer in the Xingang aeolian sands section are quite different.The magnetic susceptibility 〖WTBX〗(κ)〖WTBZ〗 of clayey layer in this section is at peak and its average is 14327(10-6 SI)，while the anisotropy degree of susceptibility 〖WTBX〗(P)〖WTBZ〗 is in valley.However，aeolian sands are contrary to the clayey layer.〖WTBX〗κ〖WTBZ〗 value is in valley and its average is 21.81(10-6 SI) while 〖WTBX〗P〖WTBZ〗 value is at peak as a whole but with quite large fluctuations.(2)The magnetic foliation〖WTBX〗(F)〖WTBZ〗 of most clayey layer samples are bigger than the magnetic lineation 〖WTBX〗(L)〖WTBZ〗 while only some 〖WTBX〗F〖WTBZ〗 of samples in aeolian sands are bigger than 〖WTBX〗L〖WTBZ〗，which indicates that the magnetic foliation development degree of clayey layer is better than the aeolian sands′.(3) Compared to the fluvial and lake sediments (the inclination of long axis of river sediments is generally smaller than 10°，the inclination of short axis is larger than 80°)，the inclination of long axis of aeolian sands′ magnetic susceptibility ellipsoid in Xingang section is larger (22°～24°) and the short axis′s is smaller (45°～51°).(4)The average value (0668 8) of aeolian sands′ magnetic matrix particle size 〖WTBX〗(q)〖WTBZ〗 is bigger than the 〖WTBX〗q〖WTBZ〗 value (024) of the fluvial sediments in the middle reaches of the Yangtze River，which reveals that the sorting of aeolian sands is worse than fluvial sediments.These characteristics can act as the magnetic fabric identification marks of alluvial and aeolian deposits in the middle reaches of the Yangtze River.(5)There are 9 clayey layers that have different development degree in the 3.25～26.2 m strata section，which points out that the climate in this period not only transferred 9 times from dry cold to warm wet but also had different intensities.〖