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现代地质 ›› 2020, Vol. 34 ›› Issue (03): 545-553.DOI: 10.19657/j.geoscience.1000-8527.2020.012

• 岩石学 • 上一篇    下一篇

鄂尔多斯盆地西缘中奥陶统地球化学特征及古环境意义

蒋苏扬1,2,3,4(), 黄文辉1,3,4, 张永生2()   

  1. 1.中国地质大学(北京) 能源学院,北京 100083
    2.中国地质科学院 矿产资源研究所 自然资源部盐湖资源与环境重点实验室,北京 100037
    3.中国地质大学(北京) 海相储层演化与油气富集机理教育部重点实验室,北京 100083
    4.中国地质大学(北京) 非常规天然气地质评价与开发工程北京市重点实验室,北京 100083
  • 收稿日期:2018-03-01 修回日期:2019-12-16 出版日期:2020-07-04 发布日期:2020-07-05
  • 通讯作者: 张永生
  • 作者简介:张永生,男,研究员,博士,1963年出生,沉积学专业,主要从事盐类矿床与油钾(锂)兼探研究。Email: zys_601@126.com
    蒋苏扬,男,博士研究生,1990年出生,矿产普查与勘探专业,主要从事沉积学与地球化学研究。Email: 490095071@qq.com
  • 基金资助:
    国家重点研发计划项目(2017YFC0602806);国家自然科学基金重点项目(U1910205);中国地质调查局项目(DD20190172)

Geochemical Characteristics of Middle Ordovician in Western Margin of Ordos Basin and Its Implication on Paleoenvironment

JIANG Suyang1,2,3,4(), HUANG Wenhui1,3,4, ZHANG Yongsheng2()   

  1. 1. School of Energy Resources, China University of Geosciences, Beijing 100083, China
    2. Key Laboratory of Saline Lake Resources and Environments, Institute of Mineral Resources, CAGS, Beijing 100037, China
    3. Key Laboratory for Marine Reservoir Evolution and Hydrocarbon Abundance Mechanism,China University of Geosciences, Beijing 100083, China
    4. Beijing Key Laboratory of Unconventional National Gas Geology Evaluation and Development Engineering, China University of Geosciences, Beijing 100083, China
  • Received:2018-03-01 Revised:2019-12-16 Online:2020-07-04 Published:2020-07-05
  • Contact: ZHANG Yongsheng

摘要:

鄂尔多斯盆地西缘位于中国东西部构造接合区,其奥陶纪末期构造体制的改变引起了广泛关注。选取老石旦剖面中奥陶统碳酸盐岩作为研究对象,通过地球化学分析,恢复其古环境特征并探索盆地西缘中奥陶世构造体制改变在沉积学与地球化学方面的响应。结果显示:(1)三道坎组和桌子山组碳酸盐岩δ18O为-5.2‰~-8.9‰,平均值为-7.0‰;δ13C为1.3‰~-1.8‰,平均值为-0.1‰。δ18O自下而上不断增加,并在顶部出现小幅降低。δ13C与δ18O变化趋势相似。(2)古环境特征表现为从三道坎组到桌子山组,水温经历了初期震荡,从32.7 ℃缓慢降至15.7 ℃。古盐度Z值呈不断增加的趋势,反映出该沉积期水体加深、盐度不断升高。(3)主量、微量元素指标反映出研究区中奥陶世沉积水体不断加深,从三道坎组到桌子山组陆源碎屑物质含量逐渐降低,指示沉积环境由局限台地向开阔台地转变。结合研究区中奥陶统构造背景,认为中奥陶世鄂尔多斯盆地西缘已经处于前陆盆地的构造体制下,逐渐受到阿拉善地块和华北板块碰撞造山影响,盆地呈现西北高、东南低的古地貌特征,从而导致蒸发浓缩的富钾卤水向东南部汇聚;因此,陕北盐盆东南部次级凹陷是海相钾盐矿床的重点勘探靶区。

关键词: 鄂尔多斯盆地西缘, 中奥陶统, 稳定同位素, 主量、微量元素, 古环境

Abstract:

Western margin of the Ordos basin is located at the tectonic junction between Eastern and Western China, and its end Ordovician tectonic architectural change has attracted wide research attention. Middle Ordovician carbonate rocks in the Laoshidan profile are selected as the research target here. Through geochemical analysis, we aim to reconstruct the paleoenvironment and explore the various sedimentary and geochemical features that can serve as tectonic indicators. Our results indicate that: (1) Sandaokan and Zhuozishan formations have δ 18O=-5.2‰ to -8.9‰ (average -7.0‰) and δ13C=1.3‰ to -1.8‰ (average -0.1‰). δ18O values rise from the bottom upward with a slight drop at the top of Zhuozishan Formation,the same trend is present for the δ 13C values; (2) Paleotemperature of seawater likely fluctuated during the early period and then decreased from 32.7 ℃ to 15.7 ℃ gradually, whereas the Z values maintain an increasing trend, indicating deepening seawater and increasing salinity; (3) Geochemical proxies of major and trace elements are well consistent and indicate a transgressive sedimentary setting in the Middle Ordovician. The petrographic features reveal that the content of terrigenous clastic minerals in the carbonates decreases gradually from the Sandaokan Formation to the Zhuozishan Formation, reflecting a transition from the restricted platform to open platform environment. Considering also the regional Mid-Ordovician tectonic background, the western margin of Ordos basin may have already been dominated by a foreland basin tectonic system,with gradually increasing influence from the collisional orogeny between the Alxa Block and North China Block. This results in the paleogeomorphologic highs in the northwest and lows in the southeast of the Ordos basin, leading to the K-rich brine accumulation in the latter. We suggest that the secondary sag in southeast part of salt basin is the key exploration target area for marine potash deposits.

Key words: western margin of Ordos basin, Middle Ordovician, stable isotope, normal and trace element, paleoenvironment

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