摘要： 鄂尔多斯盆地西缘桌子山地区中奥陶统克里摩里组下段以深灰色薄—中层石灰岩夹灰黑色极薄层泥岩为特征,石灰岩单层略显透镜状且基本未受后期成岩作用改造,其中石峡谷剖面垂向上特征变化明显,是研究该组沉积过程的理想剖面。本次研究在详细的野外观察基础上,依据岩石特征和沉积构造进一步确认克里摩里组深水斜坡沉积背景和等深流沉积,同时详细研究了薄—中层石灰岩的充填特征和形态特征,探讨其水动力特征和沉积机制。结果表明: (1)在粉晶石灰岩和灰泥石灰岩中,晶粒呈散点状分布,粉晶之间为灰泥充填,同时岩层内部具有不均一性,粗粉晶、细粉晶和灰泥呈相间分布;(2)石灰岩主要发育和单层内粒度(方解石晶粒)变化有关的沉积构造,包括具有双向递变特征的粒序层、条带状构造和水平层—均匀层—水平层序列;(3)石灰岩层中透镜体发育,包括薄层中的小型连续透镜体、中层(一般小于30^cm)中的长透镜体以及由多个石灰岩层组成的透镜体,后者侧向上尖灭于页岩或地形高处,其内部单个岩层可呈对称性尖灭;(4)剖面上发育单层石灰岩厚度向上变薄的垂向序列以及由该序列组成的石灰岩叠置层。结合已有研究成果认为: 克里摩里组下段薄—中层石灰岩沉积于深水底流发育环境,其水动力具有低速、弱—强—弱周期变化和空间上受限的特征,应为等深流水道沉积,其沉积机制可分为3个阶段,即等深流作用前的清水钙质沉积、等深流作用期间对沉积物的改造和等深流作用后的浑水泥质沉积。

关键词: 等深流沉积, 深水水道, 沉积构造, 中奥陶统, 鄂尔多斯盆地

Abstract: The Lower Member of Middle Ordovician Kelimoli Formation in Zhuozishan area of western margin of Ordos Basin is characterised of dark gray thin- to medium-bedded limestones intercalated with very thin-bedded grayish black shales,while the limestones have slight lenticular shapes and little post-depositional effects,and the Shixiagu section is an ideal one for studying sedimentary processes with its obvious vertical variation. Based on the detail fieldwork,we further ascertained the deep-water slope depositional environments and contour current deposits from sedimentary structures and characteristics of sedimentary rocks. Furthermore,we discussed the hydrodynamics and depositional mechanisms according to their characteristics of fillings and shapes. The results show that: (1)Crystal fragments are punctate distribution with marl between them in calcisiltite and calcilutite as well as the inhomogeneous texture with alternating distribution of calcite coarse microcrystalline,fine microcrystalline and cryptocrystalline within the single bed. (2)Limestones are mainly developed grain-size(crystal grain of calcites)related sedimentary structures which include bigradational grading,banded structure and plane-uniform-plane succession. (3)Lenses are well developed in limestones which include continuous small-scale lenses in the thin bed layers,long lenses in the medium bed layers(usually less than 30^cm)and lenticular shapes formed by multi limestone beds with symmetric pinch in single beds and the hole lens are pinching out in both shale and high topography.(4)Thinning-upward packages and their amalgamated beds are developed in the study section;Combining the previous research findings,we believe that the thin- to medium-bedded limestones in the Lower Member of Kelimoli Formation were deposited in deep-water environments with pervasive bottom currents which have the hydrodynamic characteristics of low velocity,weak-strong-weak periods and dimensional confine,and are supposed to be contour currents channel deposits. Their depositional mechanisms include three stages, i.e. clear water deposition of calcites before contour currents action,re-deposition during contour currents action and the turbid water deposition of mud.

Key words: contour current deposits, deep-water channel, sedimentary structure, Middle Ordovician, Ordos Basin

中图分类号: 

• P512.2