鄂尔多斯盆地西缘上奥陶统拉什仲组包卷层理成因机制探讨*

包卷层理是软沉积物变形构造的一种重要类型,其成因较为复杂。在深水沉积环境中,探讨内波、内潮汐作用与包卷层理成因之间的关系,对于丰富包卷层理形成机制和完善内波、内潮汐沉积鉴别标志均具有重要意义。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阻塞浊流沉积中发育的包卷层理,依据形态可分为倾向型规则包卷层理和回旋状包卷层理2类: 前者具有紧闭背形、开阔向形及背形之下发育砂核等特征,常和双向交错层理伴生; 后者属于层内扭曲变形,多限定在具有削截现象的层系内,常和复合流沉积构造及浪成波纹层理伴生。综合沉积特征、包卷层理特征、伴生沉积构造及相关研究成果可推测: 倾向型规则包卷层理主要由内潮汐形成,在液化过程中出现明显的密度倒置,因瑞利—泰勒不稳定引起变形,在流体持续剪切作用下进一步改造形成; 回旋状包卷层理主要由短周期内波形成,包括浊流反射形成的随机内波和内潮汐裂解形成的内孤立波,表现为同沉积分层液化,但一般不出现密度倒置现象,因开尔文—亥姆霍兹不稳定引起变形,流体剪切也有一定的改造作用。

Genesis mechanism analysis of convolute laminations of the Upper Ordovician Lashenzhong Formation in western margin of Ordos Basin

Convolute lamination,which has very complicated genesis processes,is an important type of soft-sediment deformation structures. In deep-water deposition environments,there are very important significance both in understanding the formation mechanism of convolute laminations and improving the identification of internal-wave and internal-tide deposits by revealing the relationship between them. The convolute laminations were found in blocked turbidity current deposits of Upper Ordovician Lashenzhong Formation have been carried out in Zhuozishan area,Inner Mongolia,in the north of western Ordos Basin. It could be divided into two categories according to the morphology: vergent regular convolute laminations and cyclotron-like convolute laminations. The former type is characterized as tight anticlines,flat synclines and sand nuclear underlying anticline,which is frequently associated with bi-directional cross-beddings. The latter type,which is belonged to intra-stratal contortion,is often restricted to layers with truncation surface and is associated with sedimentary structures of combined-flow deposits and wave-ripple laminations. Based on the comprehensive research on sedimentary characteristics,convolute lamination features,associated sedimentary structures and other related research results,it can be inferred that the vergent regular convolute lamination is mainly induced by internal-tides and the density inversion occured during liquefaction. The deformation driven by Rayleigh-Taylor instability and subsequent modification due to continued horizontal shear imposed by flow. The cyclotron-like convolute lamination is mainly induced by short period internal waves which includes random internal-waves produced by reflected turbidity currents and internal solitary waves distorted from internal-tides. It is characterized as syn-depositional stratified liquefaction rather than density inversion. The deformation occurs induced by Kelvin-Helmholtz instability and further modification of horizontal shear imposed by flow.