Sedimentary Environment and Paleoenvironment Evolution of Early Middle Pleistocene Stage in Mianning Area
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摘要: 昔格达组地层是我国西南地区晚新生代典型的沉积地层之一，但针对其沉积环境和古环境的研究记录较少。为此以冕宁县城西北方向钻孔揭示的昔格达组沉积物为研究对象，通过沉积物年代、粒度和孢粉测试分析，结合沉积序列、岩性岩相、沉积构造特征等，可知736～719 ka期间沉积物以滨湖亚相的沉积环境为主，植被分布存在显著的垂直地带性，气候整体温暖湿润；719～563 ka期间沉积物以深湖-滨湖亚相的沉积环境为主，此阶段针叶类植被花粉含量增加，禾本花粉含量稍有下降，气候较上一阶段变冷干；563～455 ka期间沉积物以半深湖-浅湖亚相沉积环境为主，此阶段针叶类植被花粉含量增高，阔叶类植被花粉含量稍有减少，指示该时期古环境较为冷湿。Abstract: The Xigeda Formation is a typical late Cenozoic sedimentary formation in southwest China, but there are few records of its sedimentary environment and paleoenvironment. This research mainly focuses on the sediments of the Xigeda Formation revealed by drilling in the northwest direction of Mianning county. Through the analysis of sediment chronology, particle size and palyore, the comprehensive sedimentary sequence, lithology and sedimentary structure characteristics, results show that the sedimentary environment of Binhu subfacies was dominant during 736～719 ka, and the vegetation distribution showed significant vertical zonality, and the overall climate was warm and humid; during 719～563 ka, the sedimentary environment of deep lake-Binhu subfacies increased the content of conifer, decreased the content of grass, and the climate became cold and dry; during 563～455 ka, the sedimentary environment was dominated by semi-deep lacustrine and shallow lacustrine subfacies. The content of coniferous vegetation increased while that of broad-leaved vegetation decreased slightly, indicating that the paleoenvironment in this period was relatively cold and wet.
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Key words:
- Mianning /
- Xigeda formation /
- Sediments /
- Grain-size /
- Pollen
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图 3 冕宁ZK1沉积物平均粒径、中值粒径、黏土、粉砂、细砂、粗砂含量随深度的变化
Figure 3. Variation of mean particle size, median particle size, clay, silt, fine sand and coarse sand contents with depth in the ZK1 sediment of Mianning region
图 4 冕宁ZK1昔格达组沉积物孢粉测试结果
Figure 4. Pollen results of the Xigeda group sediments in ZK1 borehole
图 5 冕宁ZK1钻孔沉积物3个变化带频率分布曲线
Figure 5. Frequency distribution curves of sediments in ZK1 borehole
图 6 冕宁ZK1钻孔沉积物3个变化带概率累积曲线
Figure 6. Probability accumulation curve of three variation zones in the sediments of ZK1 borehole
图 7 ZK1钻孔沉积物粒度和孢粉综合对比
Figure 7. Comprehensive comparison of sediment particle size and palyron in ZK1 borehole
表 1 ZK1钻孔昔格达组岩性描述与地层层位序列
Table 1. Lithologic description of Xigeda Formation in ZK1 borehole and the stratigraphic position sequence
深度/m 岩性 岩性特征 层位 83.2～84 泥岩 黄褐色，泥质，岩芯较完整，质地较软，手捏易碎，块状、短柱状，少量碎块状。水平接触，接触面不明显。 19 84～84.2 粗砂 黄褐色，湿，松散。粒级组成：砂砾80%～85%，粉黏粒10%～15%。 20 84.2～85.9 泥岩 黄褐色，泥质，岩芯较完整，质地较软，手捏易碎，块状、短柱状，少量碎块状。近水平接触。84.4 m处轴夹角75°。 21 85.9～87.1 细砂岩 黄褐色，岩芯较完整，质地较软，手捏易碎，半成岩，岩状不明显。成分以石英、长石为主。近水平接触，86.4 m处轴夹角76°。 22 87.1～87.3 泥岩 黄褐色，泥质，岩芯较完整，质地较软，手捏易碎，块状、短柱状，少量碎块状。水平接触，接触面不明显。 23 87.3～87.6 中粗砂 浅黄色，湿，松散。成分以石英、长石为主。粒级组成：砂砾85%～90%，粉黏粒10%～15%。 24 87.6～88.4 泥岩 黄褐色，泥质，岩芯较完整，质地较软，手捏易碎，块状、短柱状，少量碎块状。水平接触，接触面不明显，87.7 m处轴夹角81°。 25 88.4～88.7 砂岩 黄褐色，土状，岩芯呈破碎状，手捏易碎，呈碎块状。成分以石英、长石为主。 26 88.7～94.8 泥岩 黄褐色、浅黄色，泥质，半土状，岩芯较完整，质地较软，手捏易碎，块状、短柱状，少量碎块状，偶夹2～3 cm棕红色中细砂，约4～5层。近水平接触，91.8 m处轴夹角84°。 27 94.8～96.5 黏土岩 灰色，泥质、黏粒，土状，半成岩，岩状不明显。泥质、黏粒。近水平接触，94.9 m处轴夹角80°。 28 96.5～101.5 中细砂 灰色，湿，松散。成分以石英、长石矿物为主，局部粉黏粒含量较多。粒级组成：砂砾80%～85%，粉黏粒15%～20%。 29 101.5～128.4 黏土岩 灰色、深灰色，泥质、黏粒，大部分为半成岩土状，少量为岩状，质地较软，手捏易碎，块状、短柱状，少量碎块状。近水平接触，117 m处轴夹角83°，123.1 m处轴夹角79°，127.7 m处轴夹角85°。 30 128.4～131.4 细砂 深灰色，湿，松散，局部含粉黏粒较多。成分以石英、长石为主。粒级组成：砂砾75%～80%，粉黏粒20%～25%。 31 131.4～139.4 黏土岩 灰色、深灰色，泥质，土状、软土状，湿，块状、短柱状，少量碎块状，局部较松散。近水平接触，135.3 m处轴夹角86° 32 139.4～143.5 细砂 深灰色，湿，松散，局部含粉黏粒较多。成分以石英、长石为主。粒级组成：砂砾70%～75%，粉粒15%～20%，黏粒5%～10%。 33 143.5～146.2 中粗砂 中粗砂，灰色、灰黄色，湿，松散。偶含圆砾：石英岩、花岗岩，磨圆度较好，主要为次圆状，少量为次棱角状。粒级组成：圆砾5%，砂砾80%～85%，粉粒5%～10%，黏粒＜5%。 34 146.2～153.2 中细砂 灰色、灰绿色，湿，松散，局部（下段）含粉黏粒较多。成分以石英、长石为主。粒级组成：砂砾75%～80%，粉粒15%～20%，黏粒5%～10%。 35 
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表 2 ZK1钻孔年代测试结果
Table 2. The ZK1 borehole dating results
样品物质 深度/m 铀U/
（ug·g−1）钍Th/
（ug·g−1）K2O/% 含水量/% 古剂量/Gy 年剂量/
（Gy·ka−1）年龄/ka 黄褐色泥岩 84.5 6.71 21.8 2.93 31.00 1 896±250 4.10 462±62 灰色黏土岩 125.5 2.95 13.5 2.81 16.22 2 608±456 3.74 697±120 灰色细砂 139.9 3.68 12.1 2.08 23.50 2 110±270 2.94 718±92
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表 3 冕宁ZK1钻孔不同粒径、平均粒径和中值粒径在3个带的变化特征
Table 3. Variation characteristics of different particle size, average particle size and median particle size in three zones of the ZK1 sediment in Mianning region
分带 类别 黏土 粉砂 细砂 粗砂 中值粒径 平均粒径 I带 范围 1.6%～9.7% 16.1%～51.7% 4.7%～30% 14.5%～77.6% 45.9～432 μm 63.1～450 μm 平均值 5.12% 34.7% 12.5% 47.6% 148.6 μm 193.2 μm II带 范围 2.8%～27.9% 17.4%～88.8% 0.3%～31.8% 0～75.8% 8～210.4 μm 6.2～226 μm 平均值 11.5% 72.7% 7.12% 8.5% 40.6 μm 32.7 μm III带 范围 3.5%～18.7% 38.8%～88% 3.2%～26.3% 0.2%～43.6% 9.2～85.4 μm 13.9～161.8 μm 平均值 7.76% 65.4% 10.2% 16.5% 36.8 μm 63.5 μm
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