STUDY ON THE GUANZHUANGYU LOESS STRATIGRAPHY IN MANGSHAN, ZHENGZHOU
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摘要: 邙山官庄峪剖面共厚154.9 m, 出露S10及以上黄土-古土壤地层序列, 特征的马兰黄土厚度达70.125 m, 具有明显的粗粒、均匀特点, 且在该段地层中部, 并没有比较明显的弱古土壤层。磁性地层测量表明B/M界线置于深度132.04 m处, 位于L8的下部; 剖面底部未进入Jaramillo正极性亚时。与赵下峪剖面对比结果显示, 以B/M界线为标志, 72 m以下地层两者高度一致, 只是对地层的划分不同而已; 但赵下峪剖面中比较特征的具有较高磁化率值的L1SS1地层单元, 在官庄峪剖面却无明显表现, 其原因可能需进一步深入研究。Abstract: The thickness of the Guanzhuangyu section in Mangshan is 154.9 m, with the loess-paleosol sequence of S10 and above exposed. The typical malan loess there is coarse grained and uniform with the layer thickness up to 70.125 m, and there is no obvious weakly developed paleosol layer in the middle of this strata. Magnetic stratigraphic measurement indicate that the boundary of B/M is located at the bottom of L8 with the depth of 132.04 m, and the bottom of the profile is not in Jaramillo. Compared with Zhaoxiayu section, it is shown that the heights of the strata below 72 m in both are highly consistent marked by the boundary of B/M, just with different stratigraphic division. However, the magnetic susceptibility values of the stratigraphic unit L1SS1 become apparently higher in Zhaoxiayu section, while there is no obvious change in Guangzhuangyu section. The reason for this phenomenon deserves further research.
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Key words:
- magnetostratigraphy /
- Guanzhuangyu in Zhengzhou /
- Mangshan loess
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中原邙山以发育巨厚的马兰黄土而闻名中外[1~4]。由于其地理位置特殊，因而在黄河的形成、演变以及华北平原的形成、演化等研究中，都具有举足轻重的地位。尽管对邙山黄土的研究已取得长足进展[5~9]，但目前在地层划分上，尤其是对S2以上地层的划分[10~11]还存有分歧。在赵下峪西侧1 km处的官峪村东侧，有一条简易公路从邙山源源面通到黄河滩地，沿公路地层出露较好，为了进一步查明邙山黄土的发育情况，本文对该剖面进行了详细的野外观察和室内研究工作。
1. 地层发育
野外观察显示，官峪剖面出露较好。详细的地层测制结果表明，地层剖面共厚154.9 m，出露S10及以上黄土-古土壤地层序列；其中底部的S10出露不完整，仅见其上部少部分地层，其余地层较完整。古土壤层S1和S5地层在野外能非常清楚地辨认出来，分别位于地层深度71.625~83.100 m和117.95~121.30 m层段。另外，从L2层段开始，各黄土层顶部均分布有钙结核层段，L9黄土层位于地层深度135.64~148.00 m层段，厚度达12.36 m。而特征的马兰黄土L1位于地层深度1.500~71.625 m，厚度达70.125 m，具有明显的粗粒、均匀特点，并且在该段地层中部，并没有比较明显的弱古土壤层。
2. 磁性地层
本文对上部128 m以2.5 cm间隔、下部27 m以2.0 cm间隔采集了磁化率样品，用Bartington MS2型磁化率仪进行低频磁化率测试，结果如图 1所示。根据磁化率曲线的峰谷变化可以较明显区分黄土与古土壤，且与野外观察相一致。图 1显示，S1—S10古土壤分层清晰，特别是L9中发育了一层弱古土壤，其磁化率值接近S1古土壤的磁化率值。
图 1 郑州邙山官庄峪黄土-古土壤剖面磁性地层序列Figure 1. Magnetic stratigraphic sequence of Guanzhuangyu loess-paleosol profile in Mangshan, Zhengzhou结合前人的研究成果，重点对剖面上部S1以上地层(0~72 m)以50 cm间隔，B/M界线附近地层(128.0~135.0 m)以及剖面底部(151.5~155.0 m)以25 cm间隔采集了古地磁样品，全部使用超导磁力仪进行测试，测试结果见图 1。
古地磁测试结果显示，上部72 m地层包括S0、L1和S1顶部，其磁性地层测试结果全部为正极性，未见Laschamp事件。剖面深度151.5~155.0 m区间发育S9、L10和S10地层单元，磁性测试结果显示全为负极性，未进入Jaramillo正极性亚时(见图 1)
深度128.0~135.0 m剖面区间发育L7、S7、L8和S8地层单元，磁性测试结果表明，在L8下部，极性转换开始于132.86 m，结束于132.04 m。以全部倒转为标志，则B/M界线置于深度132.04 m处，位于L8的下部(见图 2)。
图 2 邙山官庄峪黄土-古土壤剖面下部B/M磁性地层序列Figure 2. The B/M magnetic stratigraphic sequence below Guangzhuangyu loess-paleosol profile3. 地层对比
将邙山官庄峪剖面与赵下峪剖面进行对比(见图 3)，可以看出如下关系：
图 3 邙山官庄峪黄土剖面与赵下峪黄土剖面地层对比Figure 3. Stratigraphy comparison between Guanzhuangyu loess and Zhaoxiayu loess profiles in Mangshan① L1SS1层段，赵下峪剖面磁化率值较高，而官庄峪剖面却无明显表现。也就是说，赵下峪剖面将该段划分为L1SS1地层单元是对的，而不能划分为S1地层单元。
② S1层段，官庄峪剖面磁化率曲线与赵下峪剖面磁化率曲线基本相似，只是赵下峪剖面顶部的弱发育古土壤层厚度比官庄峪剖面大。
③ S1—L5层段，官庄峪剖面磁化率曲线与赵下峪剖面磁化率曲线基本相似。
④ S5—L9层段，两者差别较大：官庄峪剖面S5、L6、S6与赵下峪剖面的S5对应，官庄峪剖面L7与赵下峪剖面的L6对应，官庄峪剖面S7与赵下峪剖面的S6对应，官庄峪剖面S8与赵下峪剖面的S7对应，官庄峪剖面L9与赵下峪剖面的L8、S8、L9对应。这种对应关系还表现在磁性地层上，将赵下峪剖面的B/M界线上移到149.2 m处，与官庄峪剖面B/M界线能较好地对应(见图 4)。
图 4 邙山官庄峪黄土剖面磁性地层与赵下峪黄土剖面磁性地层对比Figure 4. Magnetostratigraphy comparison between Guanzhuangyu loess and Zhaoxiayu loess profiles in Mangshan⑤ S9—S10层段，官庄峪剖面磁化率曲线与赵下峪剖面磁化率曲线基本相似。
4. 结论
邙山官庄峪剖面共厚154.9 m，出露S10及以上黄土-古土壤地层序列，特征的马兰黄土L1厚度达70.125 m，但却未发育较明显的弱古土壤层。B/M界线位于L8的下部剖面深度132.04 m处；上部72 m地层包括S0、L1和S1顶部，全部为正极性，未见Laschamp事件；剖面深度151.5~155.0 m区段发育S9、L10和S10地层单元，全部为负极性，未进入Jaramillo正极性亚时。与赵下峪剖面对比表明，72 m以下地层两者高度一致，只是对地层的划分不同而已；但赵下峪剖面中比较特征的具有较高磁化率值的L1SS1地层单元，在官庄峪剖面却无明显表现。两个剖面相距只有几千米，出现这么大的差别，尚需进一步深入研究。
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图 1 郑州邙山官庄峪黄土-古土壤剖面磁性地层序列
Figure 1. Magnetic stratigraphic sequence of Guanzhuangyu loess-paleosol profile in Mangshan, Zhengzhou
图 2 邙山官庄峪黄土-古土壤剖面下部B/M磁性地层序列
Figure 2. The B/M magnetic stratigraphic sequence below Guangzhuangyu loess-paleosol profile
图 3 邙山官庄峪黄土剖面与赵下峪黄土剖面地层对比
Figure 3. Stratigraphy comparison between Guanzhuangyu loess and Zhaoxiayu loess profiles in Mangshan
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