鄂尔多斯盆地西缘桌子山地区上奥陶统拉什仲组深水复合流沉积

Deep-water Combined-flow Deposits of the Upper Ordovician Lashenzhong Formation in Zhuozishan Area,Western Margin of Ordos Basin

深水复合流沉积研究近年来尚处于起步阶段,发现新的实例并探讨复合流沉积构造对环境的指示作用具有重要意义。鄂尔多斯盆地西缘北部内蒙古桌子山地区上奥陶统拉什仲组以深水斜坡至盆地环境下浊流沉积为主,兼有等深流沉积和内波、内潮汐沉积,沉积时具有复杂的水动力条件。在详细的野外观察和测量的基础上,结合已有水槽实验和相关实例的研究成果,对拉什仲组有关复合流沉积构造进行了详细研究。在深水沉积环境中发现了典型的复合流沉积构造,包括复合流层理、准平行层理和小型似丘状交错层理。垂向上可归纳为6种沉积构造序列,分别为:(a)正粒序层—浪成波纹层理—复合流层理;(b)准平行层理—正粒序层—复合流层理;(c)正粒序层—准平行层理;(d)准平行层理—双向交错层理—小型似丘状交错层理;(e)黏土岩中的叠置小型似丘状交错层理和(f)黏土岩中的复合流层理。结合拉什仲组沉积环境和沉积类型,复合流沉积可能为深水环境下浊流、等深流和内波流交互作用形成,依据流体与海底地形的作用,可划分为浊流抑制区(序列a和b)、强交互区(序列c)、内波作用区(序列d)和弱交互区(序列e和f)。该研究对于在地层记录中研究内波、内潮汐与海底地形作用和有关沉积相带划分具有重要意义。

The research of deep-water combined-flow deposits is still in its infancy at present,which has great significance for discovering new case study and discussing the indication effects of combined-flow sedimentary structures on depositional environment.The Upper Ordovician Lashenzhong Formation in Zhuozishan area,Inner Mongolia,in the north of western Ordos Basin,is mainly deposited by turbidity currents concomitant with contour currents as well as internal-waves and internal-tides in deep-water slope and basin floor environment which display typical complex hydrodynamic conditions.Here we studied the deep-water combined-flow sedimentary structures of Lashenzhong Formation in detail based on careful field work,combined with the results of flume experiments and other related case studies in literatures.Several combined-flow sedimentary structures were found in deep-water environment,which include combined-flow-ripple lamination,quasi-planar lamination and small-scale hummocky cross-stratification-like structures.These sedimentary structures can be summarized vertically as 6 sedimentary structure successions:normal graded layer through wave-ripple lamination to combinedflow-ripple lamination(a),quasi-planar lamination through normal graded layer to combined-flow-ripple lamination(b),amphibolous normal graded layer overlaying by quasi-planar lamination(c),quasi-planar lamination through bi-directional cross-lamination to small-scale hummocky cross-stratification-like structures(d),amalgamated small-scale hummocky cross-stratification-like structures enveloped claystone(e)and combined-flowripple lamination enveloped claystone(f).The origin of deep-water combined-flow deposits perhaps is the interactions of turbidity currents,contour currents and internal-wave currents according to the sedimentary types and environment in Lashenzhong Formation.Based on the interactions between sedimentary currents and sea floor topography,four sedimentation zones were suggested:turbidity currents restrained zone(succession a and b),strong interaction zone(succession c),internal-wave action zone(succession d)and weak interaction zone(succession e and f).This study will be helpful for the study of interactions of internal-waves and internal-tides with sea floor topography,and related sedimentation zone division in geological records.