第四纪网纹红土的类型与网纹化作用

第四纪网纹红土在中国南方红土发育区内广泛分布，具有特殊的结构，是第四纪红粘土的重要组成部分，按照质地网纹红土可分为均质类网纹红土和非均质类网纹红土。测试结果显示:（1）均质类网纹红土的粒度组成以粉砂（5～50 μm）和粘粒（<5 μm）为优势粒级，砂（>50 μm）含量很小，不足3%；粒度频率曲线多数呈单峰正偏形态，众数峰值位于5.5～6.5之间，部分呈双峰形态，两个峰值分别位于6和8附近；CIA值较高（84.31%～89.26%），ba值较低（0.19～0.30）；稀土元素平均含量和分布模式与风尘沉积相似，87Sr/86Sr值与佳县上新世红粘土接近；εNd（0）介于-10.54～-12.66之间，表明物质源区基本稳定；均质类网纹红土的理化特征体现了风成沉积的特性和风化成土作用的显著影响，可称之为加积型网纹红土。（2）非均质类网纹红土发育自冲洪积相、坡积相沉积物或基岩风化壳红土，土体粉砂含量明显减少，多小于50%，砂含量大大增加，>50 μm和>250 μm粒级的离散系数较高，指示了显著的非均质特征；CIA值多数大于84%，化学元素和风化指标的离散系数多小于10%，显示其经历了较为强烈和稳定的风化作用。第四纪网纹红土的形成先后经历了风化成土过程和网纹化过程，南方红土区在网纹红土发育期内具有整体湿润的气候条件。

The Types and Formation of Quaternary Plinthitic Red Earth

As an important component of the Quaternary red earth, plinthitic red earth has special structures and is distributed over the red earth areas in south China widely. Plinthitic red earth can be divided into homogeneous plinthitic red earth and heterogenous plinthitic red earth according to its texture. Test results show that :(1)Silt(5～50 μm) and clay(<5 μm) are dominant components of homogeneous plinthitic red earth, sand(>50 μm) content is small, less than 3%; grain size frequency curves are mostly single peak and opositive skewed, the peak is between5.5 and 6.5, some samples’ grain size frequency curves are the shape of two peak, peaks are at about 6 and 8 respectively; higher CIA values (84.31%~89.26%) and lower Ba values (0.19～0.30);rare earth element average value and distribution patterns of homogeneous plinthitic red earth are similar to aeolian deposition, 87Sr/86Sr values are close to Jiaxian Pliocene red clay; εNd(0) values are between 10.54 and 12.66, indicate that the genesis of heterogenous plinthitic red earth is stable basically; therefore ,homogeneous plinthitic red earth whose physical and chemical characteristics indicate aeolian genesis and significant impact of weathering could be considered as aggradation plinthitic red earth.(2)Heterogenous plinthitic red earth developed from alluvial flood sediments, slope wash or weathering crust red earth. Silt content of heterogenous plinthitic red earth declines significantly, no more than 50%, sand content increases highly, the coefficients of variation are high in sand (>50 μm) and medium coarse sand (>250 μm) components, all the grain size results indicate obvious heterogenous features; most of heterogenous plinthitic red earth’s CIA values exceed 84% ,coefficients of variation of chemical elements and weathering indicators are less than 10%, all the chemical characteristics suggest a strong and stable weathering process as a whole.The formation of plinthitic red earth went through the process of weathering and formation of plinthitic in turn. The climate may be mainly humid in the formation period of plinthitic red earth in south China.