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于美娜,郭进京等——西秦岭北缘两套新生代砾岩沉积特征比较及地质意义

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西秦岭北缘两套新生代砾岩沉积特征比较及地质意义

于美娜 郭进京 沈 迪 宫恩麟 常西玲 常璐璐

(天津城建大学地质与测绘学院 天津 300384

摘 要 西秦岭北缘构造带是现今青藏高原东北缘重要构造边界带。该构造带内新生代沉积地层出露完整,包括渐新统—中新统巨厚的河流相—洪泛相—三角洲相—浅湖相—深湖相多旋回沉积地层和角度不整合其上的上新统冲积扇砾岩层,其中漳县含盐盆地沉积序列底部渐新统砾岩层(孙家峡砾岩)和上新统砾岩层(韩家沟砾岩)是两套代表性砾岩沉积。虽然两套砾岩都为粗砾岩,但沉积特征却存在显著差异。本文通过对两套砾岩的地貌特征、沉积岩石组合、砾岩的砾石结构和成分及古流向等特征的比较,提出了孙家峡砾岩是一套典型的河流加积砾岩层,而韩家沟砾岩则是一套典型的冲洪积扇粗砾岩的认识。结合对两套砾岩空间分布、地层产状和其上覆、下伏地层的接触关系以及其与区域断层之间的构造配置关系等分析,认为渐新统孙家峡砾岩是伸展正断层控制的断陷河谷沉积,而上新统韩家沟砾岩则是逆冲断层控制的冲洪积扇沉积。这为西秦岭北缘新生代不同阶段的构造属性、构造转换和青藏高原东北缘隆升过程等提供了新的地质约束。

关键词 西秦岭北缘 新生代 上新统砾岩 渐新统砾岩 砾石特征

 doi: 10.12017/dzkx.2021.040 

Comparison of characteristics of two sets of Cenozoic conglomerates and its geological significance in Zhangxian area, northern margin of western Qinling

Abstract:The northern margin tectonic belt of the West  Qinling is an important tectonic boundary zone in the northeastern margin of Qinghai-Tibet Plateau. There are exposed the complete Cenozoic sedimentary stratigraphic sequences in this tectonic belt which include Oligocene-Miocene thickened fluvial-lacustrine-delta facies and salt bearing sedimentary sequence and Pliocene alluvial fan conglomerate unconformable covering on the Oligocene-Miocene sequence. The Oligocene Sunjiaxia conglomerate developed at the bottom of the Oligocene-Miocene sequence and Pliocene Hanjiagou conglomerate are two typical conglomerates. Although the two sets of conglomerates are all coarse conglomerates, there are significant differences in sedimentary characteristics. By comparing the geomorphic features, sedimentary rock association, gravel size, roundness, sorting and composition of the two sets of conglomerates, it is proposed that the Sunjiaxia conglomerate is a typical fluvial aggradational conglomerate, while the Hanjiagou conglomerate is a typical alluvial fan coarse conglomerate. Furthermore, based on the analysis for the spatial distribution, occurrence, contact relationship with their overlying and underlying strata and as well as the tectonic configuration relationship with the regional faults, it is suggested that the Oligocene Sunjiaxia conglomerate is the fault valley deposits controlled by the extensional normal fault, while the Pliocene Hanjiagou conglomerate is the alluvial fan deposit controlled by thrust fault. The above understandings provide new geological constraints for the tectonic attribute, tectonic transformation and uplift process of the northeastern margin of the Qinghai Tibet Plateau in different stages of Cenozoic.

Keywords The northern margin of West Qinling, Cenozoic era, The Oligocene Sunjiaxia conglomerate, Pliocene Hanjiagou conglomerate, Gravel characteristics

西秦岭北缘构造带是现今青藏高原东北缘重要构造边界带(王志才等,2006),该构造带漳县段断裂构造发育,新生代沉积地层出露完整,包括近4 000 m厚的渐新统—中新统河流相—洪泛相—三角洲相—浅湖相—深湖相多旋回沉积地层和角度不整合其上的上新统冲积扇砾岩层(郭进京等,20162017),是研究西秦岭北缘构造带新生代构造演化和青藏高原东北缘新生代变形和构造隆升的理想场所。其中孙家峡一带出露的渐新统—中新统含盐盆地沉积底部渐新统砾岩层和漳县北部方家沟—韩家沟—裴家沟—红沟一带分布的上新统砾岩层是两套特征迥异的砾岩沉积,前者在孙家峡出露典型,因此我们称之为孙家峡砾岩,后者在韩家沟最具代表性,因此我们称之为韩家沟砾岩。这两套砾岩代表西秦岭北缘地区不同阶段、不同构造环境的粗碎屑沉积,对其特征及形成环境的对比研究,可以为西秦岭北缘新生代构造演化过程和青藏高原东北缘隆升提供重要科学依据和地质约束。本文通过对这两套砾岩空间分布、岩石组合、沉积环境等研究,特别是对两套砾岩的砾石大小、砾石磨圆度、砾石成分和砾石排列等较详细的研究对比,提出了两套砾岩分别代表渐新世伸展正断层控制的断陷河谷沉积和上新世逆冲断层控制的冲洪积扇堆积,为西秦岭北缘构造带新生代构造演化提供了新的地质依据。

image1.png

1 西秦岭北缘漳县地区地质图

Fig. 1 Geological sketch map of Zhangxian area in northern margin of West Qinling

image2.jpeg

2 西秦岭北缘漳县地区渐新统孙家峡砾岩地层剖面(位置见图1A-B)

1. 含砾砂岩;2. 灰岩;3. 砂岩;4. 泥岩;5. 粉砂质黏土岩;6. 石英砂岩;7. 断层;8. 砾岩砾石统计点;

E3N1. 渐新统—中新统红色砂质黏土岩、黏土岩夹灰色砂砾岩层;C. 石炭系灰岩层

Fig. 2 Stratigraphic section of Oligocene Sunjiaxia conglomerate in Zhangxian area, northern margin of West  Qinling

(location of A-B in Fig. 1)

image3.png

3 西秦岭北缘漳县地区上新统韩家沟砾岩地层剖面(位置见图1 A’-B’,据许何宏昕等,2020修改)

1. 粗砾岩;2. 中砾岩;3. 细砾岩;4. 细砂质砾岩;5. 砂砾岩;6. 红色粉砂质黏土岩;

7. 红色黏土岩;8. 断层;9. 砾岩岩性分段编号;10.砾岩砾石统计点

E3-N1. 渐新统—中新统红色砂质黏土岩、黏土岩夹灰色砂砾岩层;N2. 上新统粗砾岩层

Fig. 3 Stratigraphic section of Pliocene Hanjiagou conglomerate in Zhangxian area, northern margin of West  Qinling

 (location of A’-B’ in Fig. 1, modified from Xuhe et al., 2020)

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