Abstract:

Recently,mixed sedimentation involving clastic,carbonate,and evaporite components has been identified at the base of the Majiagou Formation in the Ordos Basin,offering significant insights into the tectonic-palaeogeographic framework during its early depositional phase. This study systematically examines macroscopic and microscopic petrological features,along with representative depositional sequences,to reconstruct the depositional environment. Integrated geochemical analyses were conducted to investigate the genetic mechanisms,geological significance,and elemental behavior associated with this mixed sedimentary system. Key findings include: (1)The mixed sedimentary assemblage comprises three primary lithological components—clastic rocks,carbonate rocks,and anhydrite.(2)Sedimentary structures indicative of tidal flat environments,including scour surfaces,normal grading,transitional bedding,and brecciated anhydrite,were observed and used to define four subfacies: tidal-dominated channels(F1),muddy/arenaceous dolomitic flat(F2),gypsum-bearing dolomitic flat(F3),and supratidal sabkha(F4).(3)Geochemical data reveal distinct elemental partitioning during sedimentation,with vertical variations reflecting fluctuating terrestrial input driven by sea-level changes. Laterally,rare earth elements exhibit greater mobility compared to terrigenous elements,suggesting longer transport distances. The results indicate that the mixed sedimentation originated from reworking of sandstones derived from the exposed Changcheng System within the central Wushen Banner-Jingbian palaeouplift,triggered by marine transgression during the early stage of Majiagou deposition. This unit thus serves as critical evidence for the existence and persistence of the Wushen Banner-Jingbian palaeouplift during the O₁m₁ to O₁m₂ intervals,which played a critical role in facilitating regional uplift and ensuring sustained supply of terrigenous clastics throughout this period.

Key words: mixed sedimentation, tidal flat facies, geochemical characteristics, Majiagou Formation, Ordos Basin

摘要：

近期在鄂尔多斯盆地马家沟组底部识别出混合沉积,这对于马家沟组沉积初期构造古地理背景研究具有重要价值。本研究厘定了马家沟组底部混积岩的宏观、微观岩石学特征及典型沉积序列,并梳理了其沉积环境; 结合地球化学分析,综合开展了对该套混积岩成因、地质意义以及地球化学响应等方面的研究。 结果表明: (1)鄂尔多斯盆地马家沟组底部混积岩由碎屑岩、碳酸盐岩及硬石膏岩等3种成分混合组成; (2)该混积岩沉积序列中发现大量如冲刷面、正粒序、过渡性层理以及角砾状硬石膏岩等典型的潮坪相沉积标志,基于此划分出潮汐水道(F1)、泥/砂—云坪(F2)、膏云坪(F3)以及潮上萨布哈(F4)等4个亚相; (3)地球化学分析揭示出混积过程中的元素分异规律,表现为垂向上随着海平面从升到降,陆源影响呈现由弱变强的波动特征,横向上稀土元素较亲陆源元素具有更远的扩散距离。研究认为,在马家沟组沉积初期海侵背景下,该套混积岩不仅是乌审旗—靖边古隆起核部出露的长城系砂岩再旋回的产物,同时也是乌审旗—靖边古隆起发育在奥陶系马家沟组沉积早期的关键证据,这表明乌审旗—靖边古隆起在马一段—马二段沉积中期一直扮演着水上隆起并持续供给陆源碎屑的角色。

关键词: 混积岩, 潮坪相, 地球化学特征, 马家沟组, 鄂尔多斯盆地

CLC Number: 

• P588.21